For Homodyne Interferometers using Single Frequency Lasers,
Linear and Rotary Encoders, and Ring Laser Gyros
and
Heterodyne Interferometers using Two Frequency Lasers
µMD2 is intended for use in hobbyist, experimental, research, and other
applications where a bug in the hardware, firmware, or software, will not
have a significant impact on the future of the Universe or anything else.
While every effort has been made to avoid this possibility, µMD2 is an
on-going development effort. We will not be responsible for any consequences
of such bugs including but not limited to damage to the construction crane you
picked up on eBay for $1.98 + shipping, financial loss from ending up in
the Antarctic when the compass orientation provided by
your home-built ring laser gyro was off
by 1,536 degrees, or bruising to your pet's ego from any number
of causes directly or indirectly related to µMD2. ;-)
Thanks to Jan Beck for selecting the Teensy 4.0 and writing and testing
the initial version of the firmware and providing support for enhancements
and bug fixes. He was also instrumental in developing the initial
µMD1 firmware and GUI. And for getting me interested
in actually getting involved in that project. If anyone had told me
six months ago that I'd be writing code in C, MIPS assembly language,
and Visual Basic - and enjoying it (sort of) - I would have suggested
they were certifiably nuts. ;-) Jan maintains the master GUI source code
as well as slightly different versions of both the µMD1 and
µMD1 firmware and a development blog on these and other projects.
Introduction
Note: Links to Web pages external to this document will open in a single
separate tab or window depending on your browser's settings.
µMD2 is an inexpensive system for precision readout of displacement
(change in position), angle, straightness, and more
in metrology applications using (homodyne) interferometry
with single frequency lasers,
as well as devices like linear and rotary
encoders which produce Quad-A/B signals. There is also limited support
for use in heterodyne interferometry using two frequency lasers. The
later is currently a "works in progress", so for now,
Micro Measurement Display 1 (µMD1)
is recommended for this with some exceptions that will be noted below.
While targeted for
experimenters, hobbyists, and researchers, there is no reason why
µMD2 can't also be of value in science and industry.
The hardware platform is a readily available inexpensive
microcontroller development board which communicates via
USB to a Windows PC, laptop, netbook, or tablet.
The graphics below show a rotary optical encoder which uses a pair of LEDs and
photodiodes physically offset by 90 degrees to generate Quad-Sin-Cos analog
signals which are then thresholded to yield Quad-A-B digital signals.
The specific type of sequence is called a "Gray" code (not based on color
but attibutable to someone named Frank Gray) and has the property that any
possible allowable change in value is a change in only a single bit.
This eliminates the ambiguity with sensors using the normal binary order
where two bits can change at not quite the same time.
Rotary Encoder Digital Signals as a Function of
Orientation
(The animated encoder graphic seems to be all over the Web. If anyone knows
who the original copyright holder is, I will acknowledge them.)
Many other types of encoders produce similar signals. They may use
optical, mechanical, or magnetic sensing, among others.
The input to µMD2 are the digital signals which can have change rates
of many 10s of MHz. The decoding is done in hardware and it is unlikely the
upper limit will be reached by any practical application.
There is no support for analog inputs now and probably never will be.
However, there are hardware quadrature interpolators available that go in-line
with the analog signals to multiply the effective resolution by anywhere
from 2 to 1,000 or more, so they are a possible option were higher
resolution is required. Those made by Renishaw are relatively
inexpensive on eBay. There are also ICs available that perform
this function.
For use with interferometers, the only difference is that the origin of
the analog signals are from a different type of detector:
Typical Homodyne Interferometer Setup using µMD2
This also applies to devices like Ring Laser Gyros (RLGs) which have
similar types of detection schemes.
For homodyne, the native resolution of single-pass interferometers
is 1/8th wavelength,
usually 633/8 or ~79 nm. But depending on the optics, this may be doubled
or quadrupled without requiring interpolation. Specifically, the Linear
Interferometer (LI, shown above) and Single Beam Interferometer (SBI)
have a native resolution of ~79 nm; the Plane Mirror Interferometer (PMI) and
High Stability Plane Mirror Interferometer (HSPMI)
have a native resolution of ~40 nm,
and the High Resolution Plane Mirror Interferometer (HRPMI)
has a native resolution of ~20 nm. It is theoretically possible to convert
a PMI or HSPMI into an HRPMI with the addition of a cube corner but the
alignment could be quite hairy.
The heterodyne setup is similar except for the use of a two frequency laser:
Typical Heterodyne Interferometer Setup using µMD2
For heterodyne, the native resolution of single-pass interferometers
is 1/4th wavelength for µMD2,
usually 633/4 or ~158 nm. But depending on the optics, this may be doubled
or quadrupled without requiring interpolarion. Specifically, the Linear
Interferometer (LI, shown above) and Single Beam Interferometer (SBI)
have a native resolution of ~158 nm; the Plane Mirror Interferometer (PMI) and
High Stability Plane Mirror Interferometer (HSPMI)
have a native resolution of ~80 nm; and the High Resolution Plane Mirror
Interferometer (HRPMI) has a native resolution of ~40 nm.
Phase-based interpolation in the µMD2 firmware increases the end-point
resolution to well below 1 nm.
However, for stationary or slowly varying displacement (order of less than one
wavelength per second), the use of a large value for the moving average
in the GUI should result in a similar improvement in resolution as the
sampling can be set up to be essentially random.
Note that µMD2 refers specifically to the combination of the
Teensy-based hardware and firmware. It's possible there could be
µMD3, .... µMDn - or µMD0 - in the future using
the same GUI. :) µMD1 is already taken for the original version
supporting heterodyne interferometry using two frequency lasers.
This document provides installation and operating instructions for the
µMD2 hardware and software using the SG-µMD2 PCB.
The PCB layout includes the line receivers and associated
components for up to 3 measurement axes, eliminating hand wiring.
It may also have small OLED display for local monitoring of the
encoder or interferometer counting (primarily for diagnostics).
It is assumed the reader is
familiar with homodyne and/or heterodyne
interferometry. Linear and rotary encoders generate signals to those from
homodyne interferometers.
If not, back up and start with Sam's Laser FAQ: Interferometers for Precision Measurement in
Metrology Applications.
Mote: Throughout this document, "µMD2 board" or simply "board" refers
to the SG-µMD2 PCB. "µMD" refers to the Windows Graphical
User Interface (GUI), which is identical for µMD1 and µMD2.
Specifications
Laser/encoder compatibility:
Homodyne (possibly with additional electronics): Melles
Griot / Pacific Lasertec 05-STP-901/91x, Teletrac/Axsys/General Dynamics 150,
Thorlabs HRS015, and various models from REO, SIOS, and others, as well
as home-built single frequency HeNe lasers. Multi-longitudinal lasers
(not single frequency) may also be used if the Path Length Difference
(PLD) between the REF and MEAS beams including traversals through
the interferometer optics is less than a few cm.
Suitable DC power supplies will be required for the laser and/or
detector electronics. The µMD2
board is powered from USB. While all the common metrology lasers operate
at 633 nm, the wavelength is a settable parameter in the software, so
devices with other wavelengths can be accomodated.
Linear and rotary encoders with digital Quad-A-B formats (or Analog
Quad-Sin-Cos formats if converted to digital).
Interpolation may be done externally. The "wavelength" for lasers or
encoder rulings may be set from 1 nm to 10 mm in 0.001 nm increments
in the GUI.
Heterodyne: HP/Agilent/Keysight 5501A/B, 5517A/B/C/D,
5518A, 5519A/B, Excel 1001A/B/F, Zygo 7705, and Zygo 7701/2/12/14/22/14
as well as clones or home-built
two frequency HeNe lasers with a REF/split frequency from below 1 MHz to
more than 20 MHz. If the laser does not have a built-in optical receiver, a
10780A/B/C or Zygo 7008 or equivalent will be needed. Suitable DC power
supplies will be required for the laser and/or optical receiver. The
µMD2 board is powered from USB. While all the common metrology lasers
operate at 633 nm, the wavelength is a settable parameter in the software, so
devices with other wavelengths can be accomodated.
Signal compatibility: Differential inputs including RS422 and ECL
or PECL, and single-ended inputs like TTL and CMOS. Analog inputs may be
accepted directly under some conditions (generally if their amplitudes and
offsets are closely matched), but more likely will require
conditioning (offset/gain/threshold adjustment).
Interferometer optics support: Linear Interferometer (LI, 10702A),
Plane Mirror Interferometer (PMI, 10706A/B), Single Beam Interferometer
(SBI, 10705A), High Resolution Plane Mirror
Interferometer (HR-PMI, 10716A), Angular Interferometer (10770A),
Long Range Straightness Interferometer (LRSI, 10775A),
Short Range Straightness Interferometer (SRSI, 10774A), including
older versions as well Excel, Zygo, and other clone optics, and other
configurations like differential interferometers. Relevant
measurement parameters may be set for older, non-standard, and/or
for home-built interferometers, and to accomodate other
interferometer configurations which have a different multiplier
coeficient with respect to displacement or angle including
squareness and flatness.
Display: Graphical User Interface (GUI) runs on Windows PC
(not included). Click for a sneak peek at the µMD2
Main Window. Note that there is no requirement that the GUI must
be used. The raw data from the firmware can be input to another
application of the user's choice via the USB port as it is simply
a space-delimited text stream. There is no communication in the
other direction (to the firmware). Details are left as an exercise
for the user but the format is included later in this document. ;-)
The GUI is identical to the one used with µMD1.
Number of axes: Up to three independant axes are supported by
the firmware. The GUI can display their values with a "primary" axis
selected among them for the main readout and graphing. The values for
the other axes will also be displayed (but not shown in the graph).
There are no current plans for anything beyond this.
Measurement modes: Displacement, Velocity, Angle, Straightness
Long, Straightness Short, and Frequency (Discrete Fourier Transform (DFT)
of the displacement data up to about 100 Hz). Flatness, squareness,
and other measurements that can be framed in terms of the modes, above,
should be possible. And Encoder Angle mode enables correct reading of
rotary encoders and ring laser gyros. All 3 axes must use
the same parameters.
Units: nm, µm, mm, cm, m, in, ft for displacement,
Straightness Long, and Straightness Short, with /s for Velocity;
arcsec, arcmin, degree for Angle measurements.
Plotting/graphing: A real-time graph with automatic
vertical scaling and selectable horizontal compression
displays the trend of the selected parameter (e.g., displacement). The
DFT is able to display a rough analysis of the velocity data in
the frequency domain. The plot may be turned off (freeing up
desktop space and reducing CPU load if desired.
Environmental compensation: Manual entry of temperature, pressure,
and humidity. Digital sensor inputs are not supported due to limited
interest and too many hassles in getting them to work correctly with
µMD1. But there is nothing fundamental preventing sensors to
be added with firmware developed by the user. And there is space on
the SG-µMD2 to install you favoriate varieties. ;-) However,
uMD Central will not provide debugging support under any foreseeable
circumstances up to and including the exchange of large quantities of
folding money or Bitcoin. ;-)
Resolution:
Homodyne: Basic resolution is 1/8, 1/16 or 1/32
wavelength for Displacement at 633 nm depending on the interferometer
optics, or 4 counts per cycle. With a Plane Mirror Interferometer (double
pass), the resolution is ~40 nm.
Heterodyne: Basic resolution is 1/4, 1/8 or 1/16
wavelength for Displacement at 633 nm depending on the interferometer
optics. With a Plane Mirror Interferometer (double pass), the resolution
is ~80 nm.
Phase interpolation implemented in firmware (V261.28 or above) provides
sub-nm end-point resolution. For an amplitude of 300 nm, the frequency
response is over 20 Hz. This means that the displacement will settle in
to the correct value with sub-nm accuracy within a small fraction of a
second. Super-sampling in firmware should be available soon to provide
resolution for larger displacements of a few nm or better. However, your
mileage may vary as even being able to provide a stable enough setup to
be able to make sense of such small changes is a challenge. Interpolation
currently only works for lasers with a split frequency of less than
around 8 MHz (which excludes Zygo lasers except the 7705) but we're
working on that. ;-)
An adjustable weighted average to smooth data may be applied to the readout
and graph.
There is no interpolation for homodyne but external in-line
interpolator hardware may be used to improve resolution.
For heterodyne, there is now firmware that implements phase-based
interpolation with sub-nm end-point resolution. "End-point" means that
the displacement is not varying by more than about 10 µm per second.
Even without interolation, for stationary or slowly varying displacement,
the use of a large value for the moving average
in the GUI should result in a similar improvement in resolution as the
sampling can be set up to be essentially random. To take advantage of this,
the and "#define Interpolation" and "#define REF_Sync" must be commented out
in the firmware (V2.61 or above). Without doing this, sampling syncs
with REF and displacement will tend to be in increments of 1/2, 1/4, or 1/8
wavelength depending on the interferometer optics.
Maximum range: Displacement data is stored as 64 bit
signed integers for the displacement counts (1/2, 1/4, or 1/8 wavelength
depending on the type of interferometer). These will not overflow during
the life of a multiplicity of multiple universes. ;-)
Slew rate: For all practical purposes, the count rate is limited
by the signal source and interface. The Teensy Quad-A-B processing is
done in fast hardware and has an upper limit of many 10s of MHz.
Data logging: Capture allows displacement data to be saved to a
file at the sample rate. The current default is 1,000 s/s. (All other
modes are a function of this data.) This value is in an obvious location
in the firmware and can be changed if desired over a relatively wide range.
But there could be side-effects and any request to explain
strange behavior if the value is changed may be ignored by Tech support
since they probably don't understand it either. :)
Error detection:
Homodyne: None.
Heterodyne: REF dropout (REF or Head Error), beam dropout (MEAS
or Path Error), LOS (Loss Of Signal Error, both REF and MEAS disappear),
The detection duration sensitivity is about 1/300th of a second or more.
Also Slew (Rate+) Error (stage speed too fast in the direction that increases
MEAS frequency, Slew (Rate-) Error (stage speed too fast in the direction
that decreases MEAS frequency.
Test capability: Software-based displacement function
generator can be used to simulate the behavior with no laser or
interferometer or even the data acquisition hardware hooked up.
Includes constant, ramp, triangle, and sinusoid functions with
adjustable frequency/slope, amplitude, and offset. For testing,
the error detection may be turned off.
Hardware platform: Teensy 4.0 which is based on the ARM
Cortex-M7 600 MHz CPU, plugged into the SG-µMD2 PCB.
The board is powered from USB, which is also used for
firmware download and communications
with the PC-based GUI. The Teensy 4.0 is available from many suppliers
for around $20. The SG-µMD2 PCB is now available as
a kit of SG-µMD2 parts from me direct or on eBay, and
includes lifetime tech support. ;-)
Local display: Optional 0.96" 128x64 pixel OLED mounts on
SG-µMD2. This is only intended as a confirmation of activity
of the firmware and axis inputs with minimal formatting: Just the
firmware version, a unique sequence number, and the raw hardware
counts or displacement for axis 1,2,3. It does NOT currently nor
likely ever will duplicate the functions of the GUI, sorry.
However, user enhancements to the firmware are possible and are encouraged
as long as we won't be involved in their debugging. ;-)
Firmware development environment: Arduino IDE, which is a free
download from www.arduino.cc.
The Quadencoder library may need to be downloaded separately.
Firmware is written in C. But as a user, these are
only required to compile and upload the firmware to the SG-µMD2 board.
Where I provide the board as part of µMD2, a version of the
firmware will be preloaded. But updates in case of bug fixes (Arrrrgggg!)
will require being able to compile and upload. And there will probably
be at least every 1/2 microsecond initially.
GUI development environment: Microsoft Visual Studio. The original
development was done under VS 2013, abut later
versions work. This isn't of any consequence for the user
since the Windows executable is provided. The µMD2 GUI
may be test driven in "Test Mode" with simulated data
by downloading µMD2.exe.
(However, this version is not guaranteed to be the most up-to-date.)
PC requirements: PC/laptop/netbook running Windows XP/Vista/7/10
higher with MS Net Framework 4.0 or higher for the GUI. It's possible
the Arduino IDE requires Win 7 or higher, but this hasn't been tested.
These will run on almost any PC with a free USB port that isn't totally
ancient. The firmware uploader can even be run from a USB thumb drive
if there isn't enough free disk space to load UECIDE. The GUI requires
minimal disk space or memory. For a full development environment using
Microsoft Visual Studio, 0.5 GB of free main memory (after Windows hogs
the rest of it) and 2 GB free disk space is recommended. But again, as
a user, this is not needed.
Firmware and GUI cost: These will be free for hobbyist,
experimenter, research, and other non-commercial users. The cost
for commercial users has not been determined but should be very modest
at most.
Support: Lifetime Tech support will be provided via email and
the µMD2 Web site (this manual). "Lifetime" is defined to be the
life of the user or the duration of the user's interest in precision
measurement. This definition is subject to change without notice. ;-)
However, while the source code may be provided and bug reports are welcome,
we will NOT support unauthorized changes to either the firmware or GUI unless
they are deemed by our experts to be critical to the future of the
Universe! ;-) And as multiple words of advice, all but the most trivial
modification of the firmware is guaranteed
to create multiple gray hairs regardless of your age even though
a number of the lines of code have comments, which may or may
not mean anything intelligible. :) Furthermore, at some point I will have
purged my brain of detailed knowledge of the firmware's finer details
due to its limited memory capacity (my brain, not the Teensy), so
don't expect assistance in implementing anything beyond changes to
comments as the backups are not accessible. ;-)
Specifications are subject to change without notice. :-)
Computer and Operating System Requirements
Everything that follows assumes the use of Windows. If you're
really smart and run Linux instead, sorry. ;-) (However, the GUI
may run on Linux under something called "wine" so there may be hope.) The
µMD2 GUI is known to work under Win XP, Vista, 7, and 10,
and should be fine on
Win 8 as well. Microsoft Net Framework 4.0 or higher is required
to run µMD2. Net 4.0 or a more recent version is probably on
your computer already but is available free from the Microsoft Web site.
Win 7 or higher may be required to compile and upload the firmware.
Minimal System Constructed on Solderless Breadboard
Since the external interface to the Teensy is very simple, a version of
µMD2 can be put together on a solderless breadboard (or prototyping
board). While this may not run at the maximum speed supported
by the Teensy, it would be a quick way of getting something working.
Some typical basic interface circuits for the homodyne version
are shown below:
Minimal Parts Count Input Circuits
CAUTION: Teensy pins are NOT 5V tolerant - they will be unhappy if
a 5 V signal is connected to them directly. Thus any signal
that may go higher than 3.3 V should NEVER
be connected to them, even for an instant. Bad things may happen.
In the case of the Quad-Sin-Cos decoder,
the LEDs provide enough voltage drop so the input to the Teensy
will not be greater than 3.3 V since the outputs of the LM393 are
open collector. For the Quad-A-B input, the resistor voltage
dividers reduce the output of the UA9637 under 3.3 V (exactly as on
the SG-µMD2 PCB.
The LM393 is an 8 pin DIP so it, along with the Nano and other parts, will
easily fit on the typical 250 column 2-1/4 x 3-1/4 inch
solderless breadboard with room to
spare. For the Quad-Sin-Cos Decoder version, the photodiodes can be
somewhat remote connected with twisted pairs. The 1M ohm positive
feedback resistor adds a small amount of hysteresis to filter
out fuzzy noise on the signals which appears to be capable of inducing
oscllation and totally messing up the displacement due to bogus
excessive slew rate. Or something. :) For the Quad-A-B Differential version
(RS422), the input cables can be long and also be twisted with a single
100 ohm resistor between the two inputs for termination, but
there needs to be a common ground connection somewhere.
While the UA9637 is essentially a voltage comparator with built-in
hysteresis, it CANNOT be used with photodiode inputs because its
bias current is too high. The difference in resistor values for
the red and green LEDs balances their brightnesses, mostly. Of course,
the LEDs aren't *really* essential. ;-)
Tne value of 100K for the photodiode load resistors
has been selected as suitable for the typical ~1 mW SLM laser to result
in the signal going from near 0 to around 3 V. It's not necessary to
have them be adjustable since the threshold settings are adjustable.
But you can if you wish, and of course change their value to reflect
the real world situation of your laser power and losses in the Quad
decoder. If the source is differential analog (typically each 1 V p-p),
the signals can go directly to the comparator inputs dispensing with
the load resistors and trim-pot.
A photo of such a setup along with a parts list
will be forthcoming. The parts list references the designations
on the schematic.
It is not anticipated that later versions will differ in any major way.
This is supposed to be bare bones after all. ;-)
For this simple setup, a wireless breadboard with 25 columns should be
adequate. The Nano with soldered pins can plug in at the right side with
the USB connector facing out, and the LM393 and associated components
can be placed next to it. Those connections that cannot be made with
component leads should use pieces of #22-24 AWG solid wire.
The "cables" for the photodiodes can simply be twisted pairs.
Keep them short but a foot or so should be fine. If the source
is something else like a commercial optical detector or signals
from a self-contained laser/interferometer, a different type of
termination than the load resistors may be needed.
The current PCB version is V1.00, which is shown below. Clicking on these
will bring up higher resolution versions in a single separate tab or window
depending on your browser settings:
SG-µMD2 V1.00 PCB Layout Diagram (Left) and Photo of Populated PCB
for Three-Axis Homodyne with OLED Display (Right)
The diagrams below show layout diagrams of the V1.22 SG-µMD2 PCBs
populated for various interferometer configurations:
Populated SG-µMD2 V1.23 PCB Layout Diagrams: 1-Axis Heterodyne or
Homodyne (Left), 2 or 3-Axis Heterodyne or 2-Axis Homodyne (Center), and
3-Axis Homodyne
(Right)
V1.22 and V1.23 are virtually identical. Kits are currently shipping with
the V1.23 PCB. You will almost certainly not see a V1.00 PCB.
It is not anticipated that later versions
will differ in any major way, but there will be some changes so a different
assembly manual will be provided for each where appropriate since some
part numbers and locations may have changed. These manuals include
complete parts lists and Heathkit™-style step-by-step assembly
instructions.
For differential RS422-type signals, it's usually not necessary to run the
return to the board even if there is no common ground connection between the
board and their source. The terminating
resistors will provide the ground reference. In fact, under some conditions
where everything is tied together with a common ground, the return connections
could add noise due to a ground loop. The line receivers only care about
the difference between the voltage levels for each signal
pair as long as the absolute voltage levels are
within their common mode and absolute voltage specifications.
For cables of a few feet or less, it's almost certain that no
connections are required for the Returns. But for long runs,
shielded cable or twisted pairs may be desirable. This won't
apply to most hobbyist/experimenter applications. :) There has to be
a common Ground somewhere though, usually via the power supply.
For single-ended signals like TTL, there are locations on the µMD2 PCB
for a voltage divider to set a threshold reference. Check the schematics.
A trace with runs from there across the board with pads near each axis
so it's easy to wire it to the
unused UA9637 inputs. For TTL, the threshold should be set at around 1.4 V.
For analog 0-1 V signals, set it at 0.5 V. If they aren't all the same,
some creativity will be required. :)
Using the 5 pole screw terminal blocks, attaching cables is a cinch. Each
of the pairs of outer two pins is the signal and its complement. The
center pin is GND. Strip
the wires to about 3/16" or 4 mm and insert so that no bare wire is exposed,
tighten the screw. If connectors are preferred, 5 pin Molex/AMP headers
can be installed in place of the terminal blocks (not included in the kits
but readily available). For other connector types, adapters can be easily
constructed. Or 2 pin connectors can be substituted if desired.
Arduino Pin Assignments
There's probably little need to know this unless something goes wrong. ;-)
CAUTION: Teensy pins are NOT 5V tolerant - they will be unhappy if
a 5 V signal is connected to them directly. Thus any signal
that may go higher than 3.3V should NEVER
be connected to them, even for an instant. Bad things may happen. 3.3V
is acceptable through a current limiting resistor (just to be doubly safe,
for the micro that is). Hooking raw power to what may be a logic output
(accidentally or otherwise) is never a good thing! P.S. "Unhappy" and
"Bad things may happen" could mean that you'll ruin the Teensy board.
The pin assignments for REF, MEAS1, MEAS2, and MEAS3 for heterodyne are
preliminary and may change without any notice of any kind, even to the
developers.
The current default firmware loaded with SG-µMD2 PCBs shipped alone or
with Hobby Special and DIY Displacement Measuring systems kits is heterodyne
V2.20. If you are totally all thumbs when it comes to dealing with computers,
I will consider loading a different version. ;-) But if there is even an
upgrade which is around 99.9999999....% likely, you will have to deal with
it eventually.
The (non-Alpha) versions of the firmware are absolutely guaranteed to be new
and improved in terms of features, capabilities, and performance. This
probably means there will be some new and improved bugs as well. The Alpha
versions are even more likely to have some juicy bugs. Please contact us via
the link at the top of this page should any dare to appear (or for any
other legitimate reason). ;-) And I don't believe there are any Alpha
versions, so that comment doesn't apply.
There is separate firmware for homodyne and heterodyne and whatever version
is pre-loaded will likely be out of data anyhow. :( :) For this reason, and
because it may be necessary to configure the firmware, one of the following
should be compiled and loaded:
Note: For all new firmware, sub-versions will have the format: .XX
where XX is a number from 01-99 (rather than letters as it was before).
It's just simpler to format. ;-)
Standard heterodyne firmware:µMD2 Firmware Version 2.20
(16-Apr-2022). Pin assignments for REF, MEAS1, MEAS2 and MEAS3 have changed
and match the SG-µMD2 V1.2 and later PCBs. This will probably be
replaced by Firmware Version 264.06 or higher.
V2.20 includes support for the optional OLED display of REF and MEAS(s).
The "#define OLED..." needs to be uncommented out and the OLED library
must be installed.
Heterodyne Special REF OLED display firmware:µMD2 Firmware version 2.47r
(25-Jun-2021. (Firmware version 2.47r is only included here for
historical purposes as V264.06 (above) replaces it providing both the
frequency counter display capability and fully functional phase interpolation.)
v2.47r provides optional display of REF frequency on OLED, either by itself in
large numbers or in small numbers along with the other annotations. This can
be used as a stand-along frequency counter from 10 Hz to >20 MHz, which
is how it is set up here and should work fine for monitoring Zeeman lasers
with an HP 10780 or SG-OR3 optical receiver. Use for µMD2 displacement
at your own risk. ;-) This is actually the pre-Alpha interpolation firmware
which has bugs. Definitely avoid the temptation to enable the
interpolation options (which will also require a few jumpers on the
SG-µMD2 PCB prior to PCB artwork V1.22). I cannot
guarantee that the Sun will not
go nova at a random time under some unspecified conditions if that
code is run. :( :) You were warned. ;-)
Heterodyne firmware:µMD2 Firmware Version 2.09
(9-Feb-2022). Original pin assignments for REF, MEAS1, MEAS2, and MEAS3
using V1.00 PCB.
Heterodyne firmware V2.20 and V3.xx: These free up D13 to be the used
for status or the heartbeat indicator, as well as shortening PCB trace
lengths. V2.20 and V2.09 are otherwise functionally equivalent.
These also apply to V2.47r and other newer versions should they appear.
Heterodyne firmware V2.09: Original released version. About the only
reason to use V2.09 would if you have already wired the jumpers on the
SG-µMD2 PCB with these pin assignments.
The status LED on D13 is the heartbeat/status indicator for homodyne
depending on firmware version. Currently it pulses at between a
1 and 2 Hz rate. Since heterodyne firmware V2.09 MEAS2 uses that pin,
it is not available. The availability of that critical LED is the
major reason for the firmware upgrade. This is all fully subject
to change without any notice. ;-)
Most of these details are really only relevant if there is a desire to
modify the firmware, which is not advised since no support will be provided
if even 1 character in a comment field is changed without prior approval
from µMD2 Central. :) This approval process normally requires a minimum
of 3 years, 7 months, 24 days, 11 hours, 35 minutes, and 22.37 seconds, but
often takes a lot longer. :-) For wiring using the firmware provided,
only the pin assignments matter.
Note that if updating firmware, the GUI may also need to be updated
and vice-versa. However, given the stability of the GUI, this is unlikely.
And none of the 5,263,782.5 known bugs are significant enough
to recall the µMD2 crew back from their extended vacation any time soon.
Here is the communications format between the firmware and GUI. This
information is of little relevance if using the GUI, but will be
useful if writing your own application software or for data analysis.
Each of the values is sent as an ASCII string representing a signed (if
needed) decimal number separated by spaces at the sampling rate.
The firmware maintains a FIFO buffer so that if the USB
data is delayed for some reason, no data should be lost (hopefully):
The same format is used for both homodyne and Heterodyne modes but
not all fields will be filled in or relevant for homodyne.
Standard (Single Axis) Data (8 values):
0: Axis 1 Count/REF Frequency Count* = REF frequency/Sample Frequency
1: MEAS Frequency Count 1* = MEAS 1 frequency/Sample Frequency
2: Displacement 1 (in 1/2, 1/4, or 1/8 wavelength)
3: Velocity Count 1 = (Displacement 1 - Previous Displacement 1)/Sample
Frequency
4: Phase 1 = Signed fractional offset between Displacement increments in
2's complement format with a range of -1/2 to +1/2. There are two
versions:
* 8 bit Phase: The typical range of values sent is -128 to +127. This
is supported by µMD2 and and an option in µMD2.
* 16 bit Phase: The typical range is -32768 to 32767. This is the
default for µMD2.
As a practical matter, which one is used is probably irrelevant. ;-)
5: Sequence Number (Unique serial number for each sample)
6: LowSpeedCode (See below)
7: LowSpeedData (see below)
The following 8 values will also be sent when Multiple Axis Mode is active:
8: Axis 2 Count/MEAS Frequency Count 2*
9: Displacement 2
10: Velocity Count 2
11: 0/Phase 2
12: Axis 3 Count/MEAS Frequency Count 3*
13: Displacement 3
14: Velocity Count 3
15: 0/Phase 3
LowSpeedCode (specifies contents of LowSpeedData):
0-99: GUI Data/Control:
0: No Data
1: Laser Power
2: Signal Strength
3: Temperature 1 (XXX.YY, °C, 0 to 70.00)
4: Temperature 2 (XXX.YY, °C, 0 to 70.00)
5: Pressure (XXX.YY mBar, 500.00 to 2000.00)
6: Humidity (XXX.Y percent, 0 to 100.0)
8: Sample Frequency (XXX.YY Hz)
10: Firmware Version (XXX.YY)
20: Homodyne Interferometer (if non-zero)
Low byte: # homodyne axes
Next byte: counts/cycle (4 for quadpulse)
(Not all of these are currently implemented.)
100-199: Diagnostics
200-255: Reserved
* The REF and MEAS Counts are the incremental change since the last sample,
NOT the total value.
(If you purchased the from me directly, or via eBay, the latest released
version of the firmware has been preloaded, so the following step should not
be necessary unless a later release is available or you would like to hack
the firmware - which is not really recommended.)
The Arduino IDE will work with all versions of the firmware.
As of Winter 2025, the current version of the Arduino IDE
is V2.3.4 (but older versions like V1.8.13 also work). It can be downloaded free from the
Arduino Web Site.
Save it to your Downloads directory or any other convenient
location. It will probably be an executable, so just run it.
There is no actual installation, it will just unpack
and copy files to the selected Arduino program directory.
(With Windows, this is typically located under "Program Files"
or something similar but has no relation to where the actual
sketches and contributed libraries may be located.) It can
be named anything but "Arduino IDE" or "Arduino V1.8.13" are suggestions.
Go to Download
Teensyduino.... and follow the instructions there to add the link
under "File", "Preferences". If this is your first time using an Arduino board, it may
ask to install the device driver. When it has finished, start the
Arduino IDE and go to "Tools", "Board", "Teensyduino", and select "Teensy 4.0".
Note: The current version of the Arduino IDE (V2.3.4 as of Winter 2025)
may not work with the latest version of the "Teensy (for Arduino IDE 2.0.4 or Later)..."
V1.59 in the Boards Manager. I could not get sketches to compile successfully
with that. So if V1.59.0 was installed automatically with the Arduino IDE and
compiling even the Blink sketch produces errors, back it off the V1.58.2. Click on
the Boards Manager (along the left side - the icon below the file folder icon). Put
"teensy" in the search box and use "All" for the Type. The current version will be
displayed and there will an option to select V1.58.2. However, this may not be an
issue with a clean installation of the Arduino IDE.
If the OLED display is enabled ("#define OLED 1" is present and not
commented out near the start of
the firmware), an additional library will be required.
Go to "Tools", "Manage Libraries", search for "u8x8" and install the
library that it finds.
Plug the board into any available USB port. Windows should recognize it.
Go to "Tools", "Port", and select the correct port. With the µMD2
firmware, it will be named something like "COM7 Serial (Teensy 4.0)".
Note: Plugging in a virgin :) Teensy (one probably having only the Blink
sketch or another one that doesn't open a COM port) may result in Windows
issuing notifications about installing a new device. It's best to wait until
that completes before attempting to upload a sketch. Before then, the COM
Port in "Tools","Port" may be something strange. This should not be an
issue with Teensys having µMD2 firmware pre-loaded.
Windows seems to like to reassign Teensy COM ports for unfathomable reasons,
so check COM port setting for uploads and the µMD2 GUI.
If you purchased the from me directly, or via eBay, the latest released
version of the firmware has been preloaded, so the following step should not
be necessary unless a later release is available or you would like to hack
the firmware - which is not really recommended.
The firmware (via the links, above)
is provided as a source file which probably has an extension
of ".pde" or ".ino" (though the specific name doesn't matter - it's just a
text file). However, the name may NOT contain any dashes "-" due to the
peculiar restrictions of Java or something. Make a directory with the name of
the firmware (without the extension) and put the firmware file there.
For example, if the file is named umd2_FW_v123.ino, make a directory
called umd2_fw_v123. and put umd2_FW_v123.ino in it. Note that case matters so
the name of the directory and name of the firmware file (without the
extension) must match case character-by-character exactly. Thus
Interferometer.pde is not the same as interferometer.pde
In my experience so far, compiling and uploading firmware has been a bit
fiddly for the Teensy 4.0, mostly related to the USB (COM) port assignments
and whether or not the software or hardware reset button needs to be
depressed. These were never issues with the Atmega boards.
More to come on this topice as it becomes available.
Plug the board into a USB port. I've occasionally seen problems
using a USB port replicator though these generally are acceptable. But if
the board doesn't come up or behavior is more erratic than
usual, go to a direct USB port.
Use Ctrl-O to open the firmware file.
Select the directory, double click on it, and then
open the firmware file itself. The source code should
appear in the same window unless a file is already open, in which
case a new window will appear.
(Or for testing, just go to Files->Examples->01.Basics->Blink and upload that.
It's easy to make change and then see immediately that thay are taking
effect without risking the rest of the Universe.)
Use Ctrl-U to compile and upload the firmware to the board. This
typically takes a minute or two, unlike UECIDE which may finish
almost before the command is issued. :( :)
Once complete, status lines will appear at the bottom of the
Arduino window, the board will be automatically reset
and start running the firmware. No other status messages will appear
unless there are errors.
Windows should recognize that the COM port dropped out momentarily and
reappeared. The µMD2 firmware will be spitting out sequences of numbers
at the sampling rate. These may be viewed by going to:
Tools->Serial Monitor. With no interferometer hardware, they will
be rather boring with only the sixth value incrementing sequentially
(Sequence Number) and the 7th and 8th values
cycling among some obscure numbers (Low Speed Code and Low Speed Data).
To Windows, the board appears as a COM port. Thus any application
that processes COM port data can be used in place of the µMD2 GUI,
should this be desired.
If the firmware crashed somehow, Windows may display a
message saying something about the USB port not working. But that
shouldn't happen with any firmware downloaded from here. :)
And on rare occasions a cosmic ray or hardware glitch may result in
the upload failing with a checksum or other error. Just
try again. If the selected COM port is incorrect, cancel and retry.
Sometimes the Arduino IDE decides to change the COM port for no good
reason I can fathom. Go to Tools->Port, and make sure it has
selected the COM Port with "(Teensy)" after it.
Once loaded, the firmware is retained in non-volatile memory so this only
needs to be done at most once - or until a firmware update is available!
The firmware may also be compiled without uploading by using Ctrl-R. Since
you haven't messed with the code, it should compile without errors.
This is slightly faster for testing and doesn't use the board at all so it
can be off doing whatever it pleases. :)
Important: Terminate any instances of the µMD2 GUI before uploading
the firmware and put the board into program mode (again if necessary)
AFTER doing this even if LED1 is flashing.
With µMD2 firmware loaded, the simplest way to test that the board
is working without running the GUI is to use the "Serial Monitor" (under
"Tools" in the Arduino IDE.) With the board reset, then open the Serial
Monitor. For only one axis enabled, the terminal
window should begin spewing forth data similar
to the following:
If there is a display like one of these with the 6th number incrementing
by 1, the board is probably working.
(Should you care, the 6th and 7th values are the "Low Speed Code" and
"Low Speed Data", respectively. 10,124 is the firmware version 1.24;
8,100000 is the sample rate of 1,000 x 100, and 20,4099 specifies 3 homodyne
axes + a homodyne multiplier of 4 x 256.) Refer back to the main µMD2
manual for more info.
If the OLED is present, V2.47r firmware and above has the option to display
the REF frequency and can be used as a stand-alone frequency counter for this
or any other application which can provide a signal to the REF RS422 inputs.
It has been tested with HP 5517B/C/Ds lasers up to at least 4 MHz but should
respond well beyond 20 Mhz.
For the REF frequency display, the following statements near the beginning
of the firmware should be set as shown below:
#define OLED 1 // Enables OLED
// #define OLED_Displacement 1 // Disables display of displacement counts
#define OLED_REF 2 // Enables display of REF frequency
This will display the REF frequency in large digits on the OLED. Other
combinations of the #defines for these three parameters may result in
strange behavior but feel free to fix that. ;-)
Only power to the SG-µMD2 PCB is required for the REF frequency
display. Power is most conveniently provided by a USB port, USB charger cube,
or 5 VDC power supply to +5V on the board or via the USB connector.
(A USB backup battery pack would also work but these may shut down
if the current is too low, as is likely here.) If connected to a USB port,
µMD2 will still behave normally for measurements.
The GUI has been stable for several years with only a
few updates mostly relating to environmental compensation, and the
last one to upgrade the copyright to 2024. ;-) It is compatible
with all versions of the board/firmware: µMD0, µMD1, and
µMD2.
Added option to use 16 bit phase values for interpolation and to display
firmware version in XXX.YY format.
This version may have broken the number converson - again. If you have a
problem, use the one below unless you need interpolation, in which case
feel free to complain. ;-)
Possible bug fix: Forces number conversion to use USA format to prevent
the GUI from crashing if the PC was set up in a different country. This
fix may have gotten corrupted in the previous update.
Changed copyright from 1994-2024 to 1994-2025. ;-)
Bug fixes: Forces number conversion to use USA format to prevent
the GUI from crashing if the PC was set up in a different country.
Eliminates the gross scale error in Frequency plots (but still may
be off by 20 percent depending on sample rate, and more if using the
ultra-low sample rate for hobbyist laser support).
Additions: Adds homodyne support for selectable number of
counts/cycle defined in the firmware. Updates copyright date. ;-)
Save the µMD2 GUI .exe file into any convenient directory. (There's
a small chance that the first time it's run, an error is produced since
there is no configuration file associated with it. Simply continue and the
GUI will come up. When it is closed using "Finish", valid settings will be
saved so that the error should ot appear again.)
When started, the µMD2 GUI (henceforth simply called the "GUI")
comes up in Displacement mode with graphing enabled. The only action
required by the user is to select the USB COM port. Once selected,
the readout and graph will begin displaying the interferometer data.
Even if there is no encoder or interferometer hardware attached to the
board, it is
possible to confirm that the Teensy is talking to the GUI. Go to "USB Port"
and select the same COM as used to upload the firmware. The graph should
immediately start scrolling to the left indicating that it is accepting
valid data, even if it is all 0s. With homodyne, there will be no error
detection, but the fact that it started scrolling means the communications
link is working.
Important:
DO NOT reset the board while the µMD2 GUI is running.
The GUI will need to be aborted, the board may need to be reset again,
and only then can the GUI be restarted.
The GUI can be started at any time but the firmware must
be running before the USB COM port is selected or else the Universe
may implode. :) Confirmation of this issue is left as an exercise for
the user. ;-) There is usually no need to restart the firmware when
restarting the GUI. However, it is recommended, especially if it
has been running for a long time. If the GUI behaves strangely,
exitting the GUI and resetting the firmware may be required.
IMPORTANT: The pushbutton on the Teensy PCB is NOT for reset, it initiates
a load of the last compiled firmware. Go figure. :( :) So, the only real
way to reset the firmware is to unplug the USB cable. A hard reset button
may be added to future versions of the SG-µMD2 PCB.
For the type fonts in the GUI to be correctly sized,
the display scaling may need to be set to 100% in Windows.
If it works OK, don't worry, be happy. :-) But if some characters appear a
bit too large in Windows 10: right click on the Desktop, "Display
Settings", "Scale and Layout". There is a similar option in Window 7.
For XP, you're on your own. :) This is not a bug in the GUI, it appears to
be an issue in Visual Studio.
When installed on slow pre-Jurassic computers :), avoid running other
applications at the same time and disable the screen saver as well as the
dim or turn off screen power saving options to avoid random errors due to
excessive latency.
For homodyne mode with µMD2, the default sample rate is
fixed at 1,000 samples/second. (1 ks/s/). This can be changed
in the firmware. While doing so is strongly discouraged, setting
to one of those used by µMD1 (457.76, 610.35, or 732.42 s/s)
will not cause problems and the DFT should then return reasonable
results, hopefully.
The data includes counts for each axis including displacement, and velocity;
a unique sequence number to identify samples; as well as other
low speed data such as environmental
sensors and diagnostics. (More info can be found a few paragraphs above.)
The GUI displays are updated at approximately 60 Hz.
All the screenshots below except for ring laser gyro
use simulated data, which was more convenient for developing
this manual! However, it also means you can play around and recreate
these displays before building your interferometer.
This graphic below shows the typical GUI main window at startup.
µMD2 Main Window in Displacement Mode with Graphing Enabled (Startup Settings)
Main Window Controls
The first set are the selection buttons at the top of the window. Note that
except for USB Port, these require only a single click to activate:
Finish is the same as close or exit. :)
Open Log File is used to select the (optional) file for data
capture. This is a text file that will accumulate raw data from the
USB COM Port when capture is enabled. The button's label will change
to "Close Log File".
The log file is closed and its name and path are saved upon exiting the GUI.
Inteferometer Configuration opens a window to allow
settings for the type of
interferometer, units for display, paremeters for straightness and
angular optics to be entered. Although it may be possible to set
the interpolation checkbox, it will only have an effect with firmware V3.15
or higher and 261.28 or higher, and only if it is also enabled there.
Environmental Compensation opens a window to enable
the temperature, pressure, and humidity to be entered manually.
Test Mode opens a window which
provides a means of exercising the GUI with a
software function generator, and also determines whether error detection
and diagnostic readouts are enabled, neither of which are valid in
Homodyne mode.
Help is a menu that may select "Information" which presently
tells the user to read this manual ;-) or "About" which lists the
GUI and firmware versions, and has links to the authors.
USB Port provides selection of the USB COM port for the
interferometer and monitor.
The first COM port to be opened is assumed to be for the interferometer
board. Make sure the selection matches the one labaled "Teensy" in the
Arduino IDE.
With the Teensy in particular, Windows may get confused about which COM
Ports are available and pop up multiple ones with the same number. In
that case, it's the first instance that should be valid.
If a second COM port is opened, the current averaged displacement values
will be sent to it as a line of text at a rate of once every
(Graph Schollrate * 16 samples). For example, if the Scrollrate is 10, then
a line will be sent every 160 samples.
One use would be for another device to detect motion limits.
The format depends on the number of active axes:
The values are signed long integers of the designated displacement(s)
in nanometers including averaging and axis flip. (Same as the readouts.)
The "*" can be used by the parser at the receiving end to
detect the end of line.
To indicate that the Monitor COM port is active, a "+" will appear
to the left of the "Graph Averaging On" label, flashing at the rate
of the data lines being sent. Its color is subject to change without
notice. :) Note that the graph does NOT need to be visible for the
data to be transmitted.
Note: Once a USB port has been opened, the selection
cannot be changed or closed. Code to close an open USB port is purported to
do bad things and has been disabled. If an incorrect USB port was selected
by accident, exit the GUI and restart. Any open USB port will be closed
upon exiting.
µMD2 Main Window with Test Mode Function Generator Triangle Displacement Waveform
These may all be accessed via Alt-first letter.
The next set are the buttons, checkboxes, and other widgets on the
main window:
Displacement selects basic measurement of position or distance.
It is probably the mode that will be used most often. The displacement
data is also what all other measurements are based on.
Velocity selects measurement of linear velocity defined as
rate of change of displacement.
Angle selects angle mode and assumes the use of special
angular interferometer optics, either commercial from HP or similar,
or home-built.
When the "Encoder" checkbox in Interferometer Configuration is checked
µMD2 will use the constant small angle increment WTIHOUT the Trig
calculation and the text will change to "Encoder Angle".
This is useful with rotary encoders and ring laser gyros.
Straightness Long selects straightness mode using long range
straightness optics.
Straightness Short selects straightness mode using short range
straightness optics.
Flatness measurements using the angular interferometer optics with
the 55282A Flatness Accessory Kit (10773A Flatness Mirrors and 10759A
Foot Spacing Kit) should be possible in µMD2
but I have been unable to determine
how exactly it's supposed to work. There is no explicit Flatness
button. Flatness seems to be a linear
deviation based on the combination of the measured angle and foot spacing
dimensions (2, 4, or 6 inches). For small deviations, it may be as
simple as using the Encoder Angle option with a specific Angle Reflector
Spacing value. However, the result would still be in angular units.
Alternatively, using "Straightness Short" with a custom value for
"Straightness Short Coefficient" set to the
ratio of the foot spacing to the angle reflector spacing
may be better. For example,
using the 2 inch (50.8 mm) foot Spacing plate and standard 32.61 mm
10771A Anguler Reflector, the Straightness Short Coefficient would
be set to 1.558, and 3.116 for the 4 inch and 4.673 for the 6 inch.
But for maximum precision, the calculation may need to
be done using an external application like Excel or Matlab operating
on the raw data. However, it would appear that the error for a 1 degree
angle (which would be HUGE) is less than 1 part in 106.
If anyone understands the Flatness measurement, or
has used µMD2 for Flatness, or doesn't agree with these
conclusions - or does, please contact me via email.
Disable Graph toggles whether to show the graph or not and will
change to "Enable Graph" when the graph is off. The data for
the graph is still saved when disabled but certain functions like the
DFT (Frequency) computation are not performed. Disabling the graph
both saves space on the computer screen and reduces computational load.
Enable Capture toggles the acquisition of raw data from the
USB COM port. The log file must first be opened for this button to have
any effect. Then, the label will change to "Disable Capture" and the
button will turn green signifying that capture is enabled and running.
Once the log file is open, capture may be turned on and off at will
with data appended to the open log file. Prior to each segment, the
line "Sample Frequency = XXX.XX Hz" is stored.
There are three options to select what data is saved:
For all conditions using interferometer data EXCEPT where the Test Mode
function generator is set to Constant and is OFF, only
Displacement and Sequence Number are saved.
The format is: "D: Displacement N: Sequence Number" where
Displacement and Sequence Number are values in decimal. The Displacement
an integer multiple of wavelength times the multiplier depending on the
type of interferometer. Example:
D: 51643 N: 15345
Assuming the use of a Plane Mirror Interferometer (10706A) with a
Homodyne Multiplier of 4 ("#define HomodyneMuliplier 4" in the firmware),
the Displacement would be 51643 * 39.56 nm or
approximately 2.043 mm.
When the Test Mode function generator is set to Constant and OFF,
ALL interferometer data is captured without any annotation.
The format in Single Axis Mode (8 values) is: "REFFrequencyCount
MEASFrequencyCount Displacement VelocityCount Phase SequenceNumber
LowSpeedCode LowSpeedData". Example:
1534 1532 51643 2 144 15345 0 0
The format in Mulitiple Axis Mode (16 Values) is: "REFFrequencyCount
MEAS1FrequencyCount Displacement1 Velocity1Count Phase1 SequenceNumber
LowSpeedCode LowSpeedData MEAS2FrequencyCount Displacement2 Velocity2Count
Phase2 MEAS3FrequencyCount Displacement3 Velocity3Count Phase3".
With Homodyne mode, only Displacement1, Velocity1Count, SequenceNumber.
LowSpeedCode, and LowSpeedData are valid. The others will probably be 0s.
Where the Test Mode function generator is ON and thus it is desired
to capture the simulated Test Mode data, Axis 1 data is stored.
The format regardless of mode is: "R: REFFreqeuncyCount M: MEASFrequencyCount
D: Displacement V: VelocityCount P: Phase N: SequenceNumber T" where the
"T" signifies that this is Test Mode data. Example:
R: 0 M: D: 51643 V: 2 P: 0 N: 15345 T
Caution: The log file can grow rapidly - especially where all the data is
stored - so it's probably not the sort of thing to do for hours on end
unless you have stock in a disk drive manufacturer! :)
All these formats have a fixed number of fields that are space-delimited.
Thus importing the log file into programs like Excel or Matlab should be
straightforward (if the size of the file doesn't break it).
Averaging adjust the "strength" of a moving average from 0 (no
averaging) to 999 (1000 samples of averaging). It is a logarithmic
scale so each third of movement represents approximately a factor of 10.
The color also changes from black to violet to remind you that averaging
is taking place. :)
The effects differ for homodyne and heterodyne:
Homodyne: Since there is no hardware or firmware interpolation,
averaging will only make a difference with very noisey or jittery signals.
Heterodyne: For firmware with no interpolation
the sampling is random with respect to REF and MEAS, so averaging should
have the effect of increasing the effective resolution for slowly varying
displacements. (This applies to firmware V2.20, or V2.47r with both
Interpolation and REF_Sync disabled.)
For Firmware V3.15 or higher with super-sampling (which is similar to
interpolation), some averaging is recommended but it can be much smaller.
A setting of 0.900 is usually acceptable. For Firmware V261.28 or higher
with phase-based interpolation, averaging may be useful in some instances
for very small p-p displacement (<50 nm) but is generally not required.
The Averaging coeficient is saved upon exiting the GUI.
Reset Display zeros the displacement and clears any errors
that may have occurred.
Suspend freezes the readout and graph and the button label
changes to "Resume" and its color changes to yellow. Pressing the
button then restarts the readout and
graph at exactly the point they left off. Thus, any change in position
or other parameter of the interferometer is not detected while suspended.
Suspend is useful when making adjustments to the inteferometer beam path
and/or laser.
Graph Averaging On checkbox determines whether averaging is
also applied the graph. Thus the graph can show raw data even if the
readout is averaged. (Does not apply to Frequency mode.)
Time Compression applies a sub-sampling factor to change the
rate of the graph scrolling. It does NOT average intermediate samples -
they are lost. (Applies to all except Frequency mode.)
The Time Compression factor is saved upon exiting the GUI.
Displacement, Velocity, Straightness Long, and Straightness Short
Range select the vertical axis scaling for their respective modes
including Auto (which is the default).
The format is slightly modified when Frequency mode is selected. This
graphic shows the actual DFT of the triangle waveform in the one above.
Note that the DFT coefficients go as 1/N rather than 1/N-squared because
it's actually using the velocity data, which is a squarewave.
µMD2 Main Window in Frequency Mode with Graphing Enabled
Frequency selects frequency mode which displays the frequency
content of the velocity data up to about 100 Hz on the graph.
(The 0 to 30 Hz range is shown.)
It uses a Discrete Fourier Transform algorithm operating on the
velocity data from the graph. Thus, the Velocity button
is also highlighted when
Frequency is selected to indicate this. The actual algorithm computes
the power spectral density and takes its square root.
The horizontal scale is approximately accurate for real data. The vertical
scale is somewhat arbitrary. The Main Readout shows Displacement
data when in frequency mode.
There may still an error of up to 25 percent is the actual values since they
depend on the sample rate being used, which varies depending on the sample
rate. The uMD technical department has not gotten
around to provided corrected labels, can you believe that? :)
Selecting Frequency sets Time Compression to 1 since the graph data is
used for the DFT calculation. There will also be a delay in the Frequency
plot accuracy depending on the DFT Frequency Range and input
signal as data is accumalated.
DFT Frequency Range determines the span for Frequency mode.
(Does not apply to other modes.)
The DFT Frequency Range is saved upon exiting the GUI.
DFT Amplitude Range selects the vertical axis scaling for
Frequency mode, including Auto (which is the default).
Main Window Indicators
Next are the various fields for displaying information in the Main Window:
µMD2 Main Window Showing Most Indicator Fields
Main Readout always shows the value of the measurement for the
mode that is selected except when in Frequency mode, in which case
it shows Displacement.
The units for the Main Readout, as well as for the graph vertical axis
(all except Frequency mode) are selected in the Interferometer
Configuration window. The options are nm, µm, mm, m, in, and ft for
all but angle, which has arcsec, arcmin, and degree. For velocity,
"/s" is added. The same units also apply to the graph
"Range" selection and vertical axis of the graph.
WL is the wavelength after environmental compensation
(if enabled).
For encoder applications with environmental compensation
OFF and "LI / Other" selected in "Interferometer Configuration", the
resolution will be WL / (HomodyneMultiplier * 2).
Simulated Data will be present whenever the Test Mode function
generator is ON to remind you that it's too pretty (or ugly) to be
interferometer data. :)
Error Detection Off will be present when the Error Detection
checkbox is not checked in the Test Mode window. Error detection is
disabled for homodyne mode anyhow.
Log File displays the name of the current open file for data
capture. The Capture button will only have an effect if a log file
is open.
These entries provide for selection of the type of interferometer used for
displacement/velocity measurements, the measurement units to be used
for the Main Readout and graph, parameters for the strightness and
angular optics, and whether to enable use sub-count interpolation.
All interferometer configuration parameters are saved when exiting the GUI.
µMD2 Interferometer Configuration Window
Linear coefficienets:
These settings only affect linear measurements.
LI / Other (1X) is used for the 10702A Linear Interferometer
or other interferometers with a resolution of 1/2 wavelength like the
Single Beam Interferometer (10705A). It will typically also be used
for linear and rotary encoders and other directly sources of Quadrature
signals which along with the "#define HomodyneMuliplier n" in the firmware
determine the effective resolution.
PMI (2X) is used for the 10706A Plane Mirror Interferometer,
10706B High Stability Plane Mirror Interferometer, or other interferometer
with a resolution of 1/4 wavelength.
HR PMI (4X) is used with the 10716A High Resolution
Plane Mirror Interferometer or other interferometer with a resolution
of 1/8 wavelength.
Units (as appropriate)
nm, µm, mm, cm, m select Metric units which apply to
displacement, velocity, and straightness measurements in the readouts
and graph.
in, ft select English units which apply to displacement,
velocity, and straightness measurements in the readouts and graph.
arcsec, arcmin, degree select the units to be used for angle
measurements in the readouts and graph.
Known quirk: If neither the COM Port or Test Mode is active, switching
among the Units buttons will only change the Units lable and/move the
decimal point/precision without affecting the readings. Just thought
you should know. ;-)
Parameters (as appropriate):
Angle Reflector Spacing enables the precise spacing parameter of
the angular optics to entered. The default of 36.610 mm is for the 10770A
Angular Interferometer with 10771A Angular Reflector. This
may also be set to some other value to accomodate an angle interferometer
with a different coefficient. (Range 1 to 100 mm in steps of 0.001 mm.)
Straightness Long Coefficient enables the precise multiplier for
the straigntness long interferometer to be selected. The default of 360 is
for the 10775A Long Range Straightness Interferometer with Cal 1.00. This
may also be set to some other value to accomodate an interferometer
configuration with a different multiplier coefficient relative to
displacement. (Range 100 to 1000 in steps of 0.01.)
Straightness Short Coefficient enables the precise multiplier for
the straigntness short interferometer to be selected. The default of 36
is for the 10774A Short Range Straightness Interferometer with Cal 1.00.
This may also be set to some other value to accomodate an
interferometer confirguration with a different multiplier coefficient
relative to displacement. (Range: 1 to 100 in steps of 0.001.) May
also be used for Flatness.
Miscellaneous:
Reverse Polarity flips the sign of the checked
axes. The measured direction of motion depends on the type
of laser (5501A/B versus 5517) and type and orientation
of the interferometer. Applies to all modes and graph. Frequency
will not change in the steady state, but since it uses the velocity graph
data, there will be a transient if the polarity of the primary axis
is changed on the fly.
Encoder Checkbox enables linear angle calculation with respect
to half wavelengths / Angle Reflector Spacing if checked. Useful
for devices like rotary encoders and ring laser gyros.
The combination of the Angle Reflector Spacing and Wavelength then
define the encoder's resolution. The text for angle mode will
change to "Encoder Angle" if the Encoder Checkbox is checked.
This is also equivalent to using the small angle approximation
for angle, but all the time. :)
The graphic below shows µMD1 being used to test a home-built ring laser gyro.
It is being rotated by hand on a Lazy Susan turntable. A combination
of Angular Reflector Spacing and the other resolution parameters can be
used to adjust the scale factor to read correctly.
Screenshot of Sam's RLG Angle Readout using µMD1
Interpolation Checkbox This has no effect in homodyne mode. For
heterodyne mode with compatible firmware, it will enable the fractional part
of the displacement (Phase).
This provides for entry of temperature, pressure, and humidity, or to
select the use of environmental sensor data if available. All environmental
compensation parameters are saved when exiting the GUI.
µMD2 Environmental Compensation window
Temperature Value may be entered in degrees C, F, or K as
specified by the Temperature Units Select. Any change results in
the compensation factor being recalculated.
Temperature Units Select may be used to specify degrees C, F, or K.
When selected, the temperature value is reset to 20, 68, or 293
degrees, for C, F, and K, respectively.
Temperature Auto Checkbox enables the use of a temperature sensor
if available. If the Auto checkbox is checked but no sensor is available,
the value will not change.
If the Temperature Auto checkbox is checked, nothing should change since
the homodyne firmware doesn't send any sensor data.
Test Mode sends 34 °C.
Pressure Value may be entered in mm/Hg or mBar (hPa) as
specified by the Pressure Units Select. Any change results in
the compensation factor being recalculated.
Pressure Units Select may be used to specify mm/Hg or mBar. When
selected, the pressure value is reset to 760 or 1,000 for mm/Hg and mBar,
respectively.
Pressure Auto Checkbox enables the use of a pressure sensor
if available. If the Auto checkbox is checked but no sensor is available,
the value will not change.
If the Pressure Auto checkbox is checked, nothing should change since
the homodyne firmware doesn't send any sensor data.
Test Mode sends 567 mm/Hg.
Humidity Value may be entered in increments of 1%. Any change
results in the compensation factor being recalculated.
Humidity Units Select Only relative humidity is supported.
Sorry, live with it. :) When selected, the humidity value reverts back
to 50 percent.
Humidity Auto Checkbox enables the use of a humidity sensor
if available. If the Auto checkbox is checked but no sensor is available,
the value will not change.
If the Humidity Auto checkbox is checked, nothing should change since
the homodyne firmware doesn't send any sensor data.
Test Mode sends 89%.
Wavelength Correction Buttons: turn Environmental Correction on
and off. Changing their state also resets the display. The default is ON
(using 20 °C, 760 mm/Hg, and 50% RH).
Vacuum Wavelength is the nominal wavelength which may be found
in the spec sheet for the specific laser. For reference, the default value
is 632.991372 nm (should it get changed accidentally or otherwise).
For use with encoders, it can be set to almost anything.
Wavelength Correction On and Off determine if environmental
compensation is enabled. The corrected wavelength will be displayed
as WL in the Main Window.
For use with encoders, it should be set to Off.
Note 1: With the current GUI, the Factors for Temperature, Pressure, and
Humidity will change based on the entered values
However, their exact value is not guaranteed to
be meningful and should only be considered for trends. This was supposed
to be fixed when the environmental calculations were corrected using the
NIST formulas, but the staff assigned to do the work took a couple decades
off, sorry.
Note 2: With Environmental Compensation turned OFF, the vacuum wavelength
is used in calculations. Therefore, Environmental Compensation should
be turned ON for laser interferometers even if using standard values.
The primary use of Test Mode is to exercise the GUI without the need for
a laser or actual interferometer hardware. :) The only other user function
that is useful would to disable error detection suring setup and alignment
of the laser and optics.
µMD2 Test Mode Window
Simulated Data On/Off controls the software function generator.
When enabled, its data overrides the data from the USB port. The waveform
is not affected by the Interferometer Configuration.
Waveform - Constant, Ramp, Triangle, Sine select the function.
The Waveform is saved upon exiting the GUI.
Frequency selects the function generator frequency with range
from 0.1 to 1 kHz. The slider increment is 1 percent of the selected range.
The entered frequency for the triangle and sine waveforms roughly
corresponds to real-time but this is not guaranteed.
Amplitude selects the function generator amplitude with ranges
from 0.01 to 1. The slider increment is 1 percent of the selected range.
Offset selects the function generator offset with a range
of +/-1 to +/-100. The slider increment is +/-1 percent of the selected
range.
REF is used as the REF frequency for the function generator
to compute MEAS and DIFF, irrelevant for Homodyne mode.
The REF value is saved upon exiting the GUI.
Units apply to amplitude with the same options as for the
interferometer - nm, µm, mm, cm, m, in, ft. These apply only
to the output of the function generator and do NOT affect the Main Window
units settings. There are no angle units.
Note that while Test Mode will operate in Angle mode, the values it sends
are always linear displacements. Thus, what this really means may be too
difficult for the average human brain to evaluate and may result in an
overload and an unintentional brain reboot endless loop. :)
The Units selection is saved upon exiting the GUI.
Multiple Axis Mode Checkbox enables the function generator to
send data to any combination of up to 3 axes. When checked, axis select
checkboxes appear below it to enable the selected axes. More on multiple
axis options below.
Axis 1, Axis 2, and Axis 3 checkboxes select the desired
axes for use with the Test Mode function generator.
Diagnostic Readout Check box enables the display to show
a variety of criptic values including the sample
rate, percent of cycles (or something) spent in the firmware processing
and percent spent in communications. Only the sample display is
currently meaningful, always set at 1,000 s/s.
Under some conditions, this may cause the current values of displacement
from the SG-µMD2 firmware to be displayed instead of resetting the
GUI readouts to 0s when the COM Port is opened, which may be a feature or
a bug depending on your point of view. Never mind that diagnostics
and reset should be totally unrelated to anything. Live with it or
figure out how it works and then tell us. ;-)
The Diagnostic Readout state is saved upon exiting the GUI.
Reset Display replicates the function of the reset button in the
Main Window.
While the firmware handles up to three measurement axes, only limited
support is provided in the GUI due to the software complexity and lack of
demand at the present time. (Though it's unlikely any amount of demand
will make the slightest difference.) What is present should be enough to get
started. It's possible this could chenge in the future but don't hold
your breath in anticipation. It may never happen.
The "#define Homodyne n" or "#define Heterodyne n" line in the firmware
specifies how many axes will be displayed. The value for "n" may be 1 or 3.
Setting it to 2 will do the same thing as 3.
µMD2 Main Window with Multiple Axes Active
All GUI functions apply to the primary axis, which defaults on startup
to Axis 1. The primary axis is what the Main Readout, REF/MEAS/DIFF
frequency displays, frequency analysis, averaging, and graph
apply to. Clicking on the Axis 1, Axis 2, or Axis 3 labels will
select it to be the primary axis and change the color of the selected
axis label to identify it as the primary axis. The units of the primary
axis are also used for the others. Error detection (if enabled)
only applies to the primary axis. Averaging is NOT applied to the
Axis 1, Axis 2, or Axis 3 readouts, even the one that is the same as
the primary axis.
Note that in Multiple Axis Mode, the communications format sends somewhat
more data over the USB COM port and the GUI must perform more computation.
Thus this may cause problems for a wimpy pre-Jurassic PC operating on
the hairy edge of what's possible. Once Multiple Axis Mode is entered
using the interferometer hardware, the only way to return to Single Axis
Mode is to restart both the firmware and GUI. This is because neither has
any way to know whether dropouts of measurement clock signals are due to
a beam path being momentarily interruprted or an axis actually being shut
off (whatever that might mean).
Test Mode is also capable of exercising all three axes singly or in combination
but the function generator data will be the same for all. This is controlled
by the Multiple Axis Mode checkbox. When enabled, Axis 1, Axis 2, and
Axis 3 checkboxes will appear below it with Axis 1 being the default
on startup. Turning Multiple Axis Mode off will put the GUI back in
Single Axis Mode. But this will be overidden when Test Mode is turned
off if the USB port is enabled and the firmware is running with multiple axes.
Multiple Axis GUI support is currently under development. Specifications and
behavior are subject to numerous changes without notice. ;-) However, no
major features beyond what are described above are anticipated to be
implemented in the GUI.
Help Menu
Information has only one purpose at present: To direct you to
this µMD2 Installation and Operation Manual! ;-)
µMD2 Information Window
About lists the GUI build and firmware versions, and provides links
our (Jan and Sam) Web sites.
µMD2 About Window
Troubleshooting
Naturally, all is expected to go smoothly. But if it doesn't, here are
some common problems. Some of these may be bugs in the firmware or GUI
as hard as that is to believe. So, if you find something that cannot
be solved based on what's below, contact us for a timely response:
GUI won't start and generates error about value out of range:
Ignore the error by hitting "continue" as there is no configuration file. The
GUI should come up. Close it using "Finish" and it will save a valid
configuration file and the error will not appear again except possibly
if a new GUI Build is run since each one has a unique configuration
file. (This error should not occur but at least be prepared!)
GUI starts but then generates error about value out of range:
If you are in a location where the interpretation of "." and "," differs
(e.g., Brazil), numbers with decimal points may get screwd up. One specific
instance is that for the wavelength in the Environmental Compensation window,
the decimal point gets ignored so that 632.993172 is interpreted
as 632991372, which is obviously out of range. Sorry, you'll just
have to move your entire operation to the USA (or at least set the
environment that way for now). ;-)
There is no valid USB Port available: The firmware is not running
or has crashed. (Hard to believe!) Exit the GUI and assuming the firmware
has been loaded, press the RESET button. With the present firmware, after
about 1 second, LED3 should be on at reduced brightness when the firmware
has started. It's actually flashing at the sample frequency but this may
not be obvious except by waving your eyeballs.
GUI generates error when the USB Port is selected: This means
either the firmware is not producing the proper communications sequence
or more likely, the board has simply gotten out of sync. Selecting the
USB Port a second time will usually clear it.
OLED (when present) displays one or more columns of garbage
pixels along the left edge that cannot be overwritten: The cause may be
that the OLED panel is actually 132 columns wide (not 128) and these are
not being cleared due to an outdated graphics library, probably U8g2.
The OLED is NOT defective. Confirm that the Arduino libraries are the
latest versions.
Graph is not scrolling after USB Port is selected: Similar to
above. First try selecting the USB Port again, but may require exiting
the GUI, pressing RESET, and reatsrting the GUI. Attempting to run
with an incompatible version of the firmware or firmware not properly loaded
or in a peculiar state can also produce this error.
There is no change in displacement even when there should be:
Assuming the Quad-A-B signals are coming from the correct sources (laser and
optical receiver, respectively), this usually means the interferometer
is installed or assembled incorrectly (e.g., backwards), is misaligned.
Displacement takes off up or down out of control:
In rare cases, such behavior can be due to noisy signals as a
result of crosstalk or improper termination. If a 50 MHz or higher bandwidth
oscilloscope is available, check the quality of the signals.
And while the default value of 150 ohms for the terminating
resistors is generally satisfactory, depending on your wiring run length
and other factors, using a different (probably lower value) may be necessary.
The simplest woudl be to add 100 ohm resistors (which are included) across
the differential signals.
Ironically, a very short run like a few inches may also cause problems like
this. Substituting a UA9639 for the UA9637 (if used) may also help.
It is a slower part and will tend to filter out any glitches. This
wouldn't be a preferred solution though and might earn you an "F" in
my CSE371 Intro to Computer Design course. :( :)
Displacement or velocity values are off by an integer factor: The
Interferometer Configuration does not match you interferometer hardware.
Angle or Straightness values are not accurate: The parameters of
your interferometers are non-standard. They may be adjusted over a wide
range in Interferometer Configuration.
Update of GUI readouts and other indicators is sporadic or slow:
While µMD2 should run on just about any computer that still runs,
this cannot be guaranteed for a 20 year old machine. Time to Upgrade.
However, CPU intensive applications like antivirus scans can hog resources.
The USB subsystems on some computers are more efficient than others and
there's no way to predict performance based on make or model.
Sensor values are not updating in real-time: There are no hardware
sensors so this is normal. Enter the environmental settings manually.
Sub-Wavelength Interpolation for µMD2
The goal is to implement interpolation to achieve a usable resolution of
a 1 or 2 nm or better. The principal requirement is that it should be
totally internal to the Teensy with
no fancy interpolation hardware, FPGAs, or GPUs. ;-) (Actually, I don't
believe there is any commercially available external hardware for heterodyne
interpolation, each company has their own $$$ scheme to do it.)
Note: The LEDs in the unused third block for heterodyne may be used as
status indicators for interpolation and/or averaging. If you are into
flashy displays, populate LD5 and LD6 with the LEDs of your choice of
decorator colors, along with the associated resistors but NOT the UA9637/9.
Approach 1 - Interpolation based on phase measuremet of REF and MEAS
waveforms
The algorithm is conceptually very simple: For each sample period (currently
1 ms), multiple cycles of the REF and MEAS digital inputs
to the Teensy are captured at high speed and stored as
digital waveforms (Os and 1s) similar to what would be done in a
Digital Storage Oscilloscope (DSO). This is currently a 4 instruction loop
implemented in assembly code with
a sample rate around 40 MHz. (The I/O on the Teensy actually
sucks big time! While the CPU clock speed is 600 MHz, it takes 15 cycles
to implement that 4 instruction loop!) The locations of the edges (rising
or both rising and falling) are then located and averaged over multiple
cycles to estimate the relative phase between REF and MEAS.
That averaging greatly reduces the
jitter of the edge positions caused by noise in the REF and MEAS
analog signals and optical receiver electronics.
The same Teensy hardware counters used for the normal µMD2 firmware
keep track of the number of
REF and MEAS cycles since the start of the Universe (or at least since
boot). ;-) The objective of the interpolation firmware is to determine
the relative phase shift between REF and MEAS and use that for the
fraction of the displacement called "Phase".
I thought that would be simple or at least straightforward,
but was not able to get it to work for a long time and almost gave up
due to excessive brain temperature rise and shutdown.
Not because the averaging is challenging - a version has been operational
since mid-2021 - but there were giant size glitches where REF and
MEAS transitions when nearly coincident and due to noise, the computed
displacement and phase were inconsistent as shown in the screenshot
below:
Firmware V247r Screenshot with Interpolation Showing Monster Glitches
And it's a lot worse than it looks because they are almost always multiple
glitches next to one-another that may go up or down but hidden by the
sub-sampling of the uMD plot.
(However, the decaying exponential in the plot above IS correct
since it used the capacitor voltage on a 555 timer as
the PZT drive signal while those below use a triangle wave from
a function generator.)
However, as of January 2025 (new year, perhaps that's the reason) or more
likely using a more systematic approach, phase interpolation
is now operational in firmware V261.05 and above. The glitches were removed
through the use of an insanely convoluted state machine implemented in firmware
that identifies illegal transitions and corrects them. The basic idea is that
for slowly varying Displacement, a change in phase by more than ±
180 degrees MUST be accompanied by a corresponding change in the Displacement
count for it to be legal. So, using the averaged phase as the reference since
that should be very accurate, the Displacement count is corrected up or down
by one count to make the overall transition legal.
The screenshots below show the displacement of a mirror on a PZT
attached to a High Stability Plane Mirror Interferometer (HSPMI)
driven by a triangle waveform with several amplitudes from 300 nm
to 1 nm p-p:
Firmware V261.30 Screenshots with Interpolation Enabled for Motion with
Peak-to-Peak Amplitudes of Approximately 300 nm, 40 nm, 5 nm, and 1 nm
No, these displays are not of the electrical signal but the actual displacement
measured using µMD2!
At 1 nm, there is some "wandering" possibly due to the relative sampling
of the REF and MEAS edges with respect to the 600 MHz CPU clock. This was
present with µMD1 interpolation as well (though it was much worse).
But even that doesn't make much sense. The jitter in sampling would
then be order of 1.66 ns. That
is only around 188th of the REP/MEAS period so at most it could result
in an error 1.66/188*80 (the integer resolution) which is less than 1 nm,
or perhaps twice that. Of course if it is the instruction timing and not
the clock, it could be several times as large. So still a possibility.
I did mostly rule out ripple in the HeNe laser power supply but using a
very quiet heavily filtered linear supple as a test. And there was no
noticeable difference in the wandering.
The µMD Tech Support Department was never able to locate the cause of that
either, despite the billions of nanoseconds devoted to it. But there are
also thermal effects and vibration. It is almost impossible to really measure
these nm-scale changes without outside disturbances dominating. In fact, to
record these data, a PZT (piezo-beeper element) had to be secured directly to
the HSPMI. And even then any vibration like tapping the table or even
walking nearby still shows up in the display.
Note that this approach only works for very slow changes in displacement -
on the order of 0.01 mm/second or less - those where the discplament count
doesn't change by more than ± 1. Above that, the following can be
used (and will probably be added to substitute for the phase interpolation
which would be worse than useless at higher slew rates.
Approach 2 - Supersampling in the firmware
Rather than implement actual interpolation as above which was causing my
brain to overheat and shut down (not to mention the hair loss), this does
a moving average, but in the firmware rather than GUI. The results in a
much better frequency response. Since there are no critical boundary
condition issues and race conditions (though boundary noise is still
present, though to a lesser degree).
This is implemented in firmware V3.15 (and probably higher to come) and should
be quite usable. The parameter for the moving average may be set as a
(#define NumbSamples) between 1 and 1000. 1 is essentially no averaging
while 1000 would result in way too much smoothing. 100 is the default
setting and is a good compromise - but experimentation is welcome.
The uMD Research Department is spending all their time (around 0.01 hours
per day) on this effort so stay tuned. ;-)
Here is a screenshot of displacement with a p-p amplitude of around 9 nm:
Firmware V3.15c Screenshot with Averaging Enabled
So the typical "jiggly" amplitude is under 1/2 nm. However, under some
entirely unspecified random conditions, they could be much larger, which
is what that 0.01 hours is devoted to solving.
