Robotic Foucault Tester with USB Camera and Motor Control

The Test Stand

The test stand is made of four levels. Separating three layers are linear bearings that slide about 6”. The lower level is the 'ground board' and allows the rest of the stand to be angled up or down from a knob in the back end (opposite where the camera is pointing. The stand needs to be tall enough so that the mirror isn’t angled down risking a fall or adding stress if a safety clip is used.

Between the second and third level the sliders move the camera forward and backwards using a 1/4” x 20 threaded rod. Between the third and fourth levels the sliders move the camera side to side using a 1/4” x 28 threaded rod. The RTAFT software controls the stepper motors through the Arduino and keeps track of their motion as well as resetting the platform to its starting position at the end of the test.

The Motors

The motors are 12v .4A Unipolar stepper motors with a 1.8 degree step angle, jameco part #155433. The test is preformed using the motors in half step mode for 0.9 degrees per step.

The Camera

The Camera used here is a UVC (USB) web camera with 5-50mm manual zoom lens that has manual brightness and focus control. You cannot use auto focus or exposure with this system. Camcorder cameras with manual controls may be used also as long as you can get the video stream into RTAFT at around 320x240 to 640x480 resolution, depending on your display size. Usually this option requires a composite video USB capture dongle, which we have used for years. We have successfully tested mirrors down to f/4.4 (we haven't tried any faster than that yet with the USB camera)

If you have an older camera you may use most USB video capture adapters as long as they will allow the incoming image to be made small enough to fit in the robo window and leave room for the rest of the application. Most HDMI capture cards do not allow the video to be small enough. Once the camera is selected there is a menu option in robo to select the video size. The sizes are controlled by the camera or capture dongle.

The Laser and LED

The white tube next to the camera contains a 5v laser (USB powered) that is used for initial alignment. The laser's beam returns from the mirror about an inch from the other side of the camera. We will place a small card there with a bulls-eye on it in the future.

You will notice that there are two knife blades. The one in front of the lens is used for testing the mirror, the lower one is used to make a narrow slit that a green LED is used to illuminate the mirror during the test. Make the bottom of the lower mirror save by taping the blade side or you will bleed on your tester.

The LED is also powered by 5V and has an inline potentiometer (small black box with the knob) that allows some brightness control illuminating the mirror. Do not use Pulse Width Modulation (very fast blinking of the LED) to control the visual brightness of the LED because the video capture will grab frames when the LED is fully on or off but not actually dimmer.

We found that the camera was sensitive to the laser being on during the test. An on/off switch was added to the POT box. Now we turn off the laser before any test begins. The box has 5v coming in from the Arduino and two 5v outputs, one attached to the potentiometer that goes to the illumination LED and another 5v output that goes through the on/off switch and goes to the alignment laser.

USB Motor Controller

The motor controller is an SMT Arduino R3 Uno (this clone did not require any soldering) with an Adafruit motor controller board v2.3 (shield) added to it. These are both commercially available items and reasonably priced. The shield comes with some header pins that will need to be soldered to the shield so that it will connect to the Ardunio.

The combination of a SMT Arduino R3 and the Adafruit R2.3 motor controller have worked in every test that we have thrown at it.

If you have access to a 3D printer the STL (link in the downloads near the end of this page) file will allow you to print a case for the Arduino with the motor shield attached. You may also access it in TinkerCad. The photo shows it with the USB and 12v power connected to a barrel connected added to the side. Note that the jumper above the 12v power input is removed.

Plug in the USB cable to the computer and Arduino (with or without the motor shield attached)

Install the Arduino development environment - I'm using v2.1.1 when I was working on this project

Click yes to everything so that it can also install the USB drivers.

From within the Arduino IDE:

Install the Adafruit Motors Shield V2.3 Library (from the 'Sketch' menu → Include Library → Manage Libraries and search for "Adafruit Motors') and install it and allow its dependencies.

There is a jumper on the motor board next to the power inout that must be removed if it is there. The motor needs 12v DC power at at least 1A, although I recommend 1.5A or higher (I use 2A). A power supply. The Arduino will get its power from the USB connection.

Select the Arduino Uno board from within the Arduino software. (Tools → Board -> Arduino Uno)

Select the Arduino's Com port, usually it's the last one in the list. (Tools → Port → Port x). The Arduino's com port is only available if the Arduino is plugged into the USB port of the computer.

Unzip and load the Arduino RoboMultiStepper sketch (source code) from this page into the Arduino IDE

Click on the check in the upper left corner to make sure that the code compiles.

If it does click on the arrow next to the check to compile and install the code on the Arduino Uno.

If this fails check your serial port and any connections between the motor controller and the motor.

We tested using a 5v laser module and the green illumination LED from the Arduino Uno v3 and it appears to work well. Draw power from the 5v and GND pins near the 12V input on the motor shield. This power is passed through from the Arduino's USB input.

⚠️ Avoid the Ardiono Uno R4 or the DIP Arduino R3 with this project.

Alternate Motor Controller - The L293D

⚠️ This controller is not reliable ⚠️

After much testing we have concluded that the L293D motor controller shield does not work reliably with the Robo Foucault tester. Do not use the L293D motor controller, use the Adafruit v2.3 motor controller.

If you want to give the L293D a try, and there may be a tweak that will get this to work, contact me and I'll give you the Arduino software that we were testing.

The Mirror Test

When you test a mirror you will select a number of zones. Fewer zones test faster but result in less detail. The images show a 12" Zerodur mirror with a slightly turned up edge. The 7 zone test (second test image) doesn't quite show the turned up edge like the 15 zone test does (the third test image).

Other than the number of zones nothing else changed between the tests. The resulting FigureXP plot and results go from a 1/9th wave mirror to a 1/5th wave mirror due to the more zones 'seeing' the edge better.

You may set up two (or more) mirror profiles for the same mirror and only change the zone count. For frequent test use the low zone count and occasionally during polishing and frequently during figuring run a high zone test.

Before the test starts get the mirror's face fully illuminated as evenly as possible from the illumination LED between the blade slit. If you turn on the mirror brightness meter the average for each side should be between 75 and 100. Your setup may vary considerably from this due to camera sensitivity, LED brightness and color and testing area conditions.

Then cut out the knife so that the shadow on the right is thin and symmetrical top to bottom. Cut the knife in so that Zone 1, the inner zone's blue and red bars are about the same height and about 1/3 to 1/2 of the way their scale. The mirror's image will be similar to how it looks in the the first test image (that is testing zone 5).

Since this test is against a horizontal strip of the mirror you may want to mark your mirror on the side ('A' axis) and preform multiple test with a 45 degree or 90 degree rotation between each. After figuring allow the mirror to cool off for at least 10 minutes and release any friction heat absorbed before testing.

👉 For FigureXP to open after the test you must first associate *.fig files with FigureXP. We recommend associating Sixtests with its *.set files also. The Sixtests output is good but the app doesn't like opening automatically from RTAFT. We're working on this.

It is important that the mirror being tested is significantly polished before it is tested. The mirror shown below is polished enough to test although not fully polished. We recommend using a laser on the surface (never look onto the return beam) and see if it is visible on the mirror's surface. If it is there is either something on the surface, like dust or cerium, or the surface itself is rough at the microscopic level. If the mirror is fully polished the laser will be seen only on the bottom of the mirror where it passed through, on internal bubbles and on the ceiling where it bounced off. You will find that moving the laser from the mirror's edge towards the center often shows different levels of polish. If the laser light scatters so will star light reducing contrast by making stars dimmer and the background brighter.

The Mirror Brightness Values

The mirror brightness display is only available during the mirror's setup. It turns off during the test. The most basic part displays a box in the lower right of the mirror's video image. In it there are two numbers. The one on the left is the average of the pixels tested on the mirror's left side. The right numbers are the average for the right side.

If both numbers are within the range that you set in the setup the box's background becomes green. The intent is to make it easier to adjust the illumination LED's brightness for more consistent results.

The total range for each pixel is 0 (zero), which is black, to 255, which is white. If the pixel squares or values are enabled they will display in white for values 128 or less and in black for values 129 and greater.

When you start the test the illumination meter is disabled. Additionally you may enable and disable it when not testing from the Test Options → Mirror Brightness Value menu.

Testing RTAFT and the Motor Controllers

Here are seven of the nine (sorry Star Trek) Arduino boards that I tested and six of the eight motor controllers. Only one combination proved to be reliable.

Each Arduino was tested with both motor controllers. First as a bench test where the motors and power were connected to an Arduino and motor controller but not to a full Foucault stand. RTAFT was started and dozens of motor tests were run from within the Arduino (RTAFT menu option), from RTAFT (another RTAFT menu option) and through RTAFT using the keyboard.

Pass or fail was noted and another combination was tested to confirm the results. In the case of using the R4 Arduino and the DIP Arduino R3 the failures were consistent. If the bench testing was successful that Arduino and motor controller were tested using the Foucault stand with a mirror of known zone values.

While in RTAFT we tested the settings options and verified that they were being saved and loaded in the ini file. After each mirror test we compared the results with what the parallel port RTAFT had for this same mirror as well as consistency with previous Robo 2 tests both with raw numbers and in FigureXP.

In the end the SMT Arduino Uno worked reliably with the Adafruit Motor Controller V2.3 was the only combination that worked every time.

RTAFT is developed on a Windows XP VMWare virtual PC & bench tested on a Windows 11 PC with an AMD Ryzen CPU. The Foucault mirror testing notebook is running Windows 10 with an Intel i7 CPU. Both AMD and Intel CPUs had the same successes and failures with the Arduino combinations.

The Downloads Are Here

Download the RTAFT Windows installer v2.0.5

Download the Arduino IDE.

Arduino source code (sketch) for the Adafruit Motor Controller V2.x shield.

Download the 3D printer file (STL) for a case for the Arduino with the Adafruit v2 motor shield.

Download the PDF to create the robo Foucault camera stand and motor controller. Create your own. This is just a starting point.

The Basics of Using RTAFT and the Robo Foucault Tester.  Some notes to get you started.

Here are Some of the changes in the new version.

Contact me if you need the RTAFT VB6 source code. You will need the VB 6 compiler with SP3 and a Windows XP machine to run the compiler on (a virtual PC works). Include in the email the reason for the request, your real name, and the astronomy club that you are a member of. I don't want to give the source code to scammers and I would like to limit the different versions of Robo that are in the wild.