Archive for category tools

Arduino motor control

We’re using some Arduino  boards for a variety of projects – to run the motion stage (25um resolution), to test LED colors, to run an oven for testing temperature compensation electronics. The add on boards (‘shields’  – since that means something else to me, I have trouble with that name) are a big help. We’ve been using SparkFun as our Arduino parts vendor, and we like ‘em.

Ardumoto 'shield' for the Arduino controls our motion stage prototype. It has capacity to control 2 motors. Photo courtesy Bec Conrad.

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Nixie tube tester

Nixie tube tester by David A. O' Brien

Recently we started working with David O’ Brien, whose list of accomplishments is too long for this single posting. One of his many devices is this great nixie tube tester – runs on a single 9V battery. You can see more of his fabulous work here: David O’ Brien’s website

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Lens testing – our precision MTF test jig

Recently we designed and built a precision motion stage capable of focus adjustments to about 0.5um. Yes, that is 500nm, or about the wavelength of green light. It’s stable, has smooth travel of a few mm. Based on a flexure design, so there’s no bearings to ‘rumble’ or cause misalignment.

we recently built this rig to test the MTF of cinema and medium format lenses

This project shows how we combine off the shelf parts and custom parts to get a job done. Our goal was helping our client specify their unusual custom lens, so that it could be manufactured. Their custom lens had to work with a range of existing medium format photo lenses and with cinematography ‘prime’ lenses. Since the catalog information about these medium format and cinema lenses was not detailed enough for our client’s design needs, we needed to make some measurements.

The common image quality metric is the Modulation Transfer Function, MTF. The MTF function describes the image modulation vs. spatial frequency – perceptually, it’s equivalent to contrast. Good looking images have high contrast in the mid range spatial frequencies. (For more info, check out this excellent tutorial on this topic, Norman Koren tutorial about MTF ).

We used some parts from Thor labs, our PointGrey camera, the excellent ImageJ software, and our own flexure stage design.

Exploded drawing showing the internals of the flexure stage.

Exploded drawing showing the internals of the flexure stage.

We needed an imager with 0.5 micron pixels, so we used a 10X microscope objective in front of the PointGrey camera (which has about 5um pixels) to get the line pair per mm resolution we needed for the tests.

Development Details:

The custom lens design for our client requires an image modulation consistent with the cinema/large format lenses used in the application. The MTF data is infrequently given in these commercial camera lens data sheets, we realized we needed to measure the MTF ourselves. We found an excellent method to calculate MTF from an image of an optical step, which was purposefully slightly misaligned to the imager pixel array. Intuitively, each pixel acts as a pinhole, the array of pixels effectively scans the pinhole, and the resulting data are like scanning the edge of the image bar. The computer program calculates the MTF from the “scanned edge” profile.

We needed an imager with 0.5 micron pixels, so we applied the idea of a scanning micro-densitometer. The micro-densitometer uses microscope optics to magnify the image structure in the film for subsequent analysis. So we reached into our lens bin for a 10X microscope objective.

We found rotating the focus ring on the camera lenses was inadequate for precision focus, and our work was compounded by a lens without a focus ring. The camera this lens is designed for has a translating lens mount, with a bellows for eliminating stray light. Our lens bench does not have the precision required for focus either.

The problem was solved by combining a flexural translation stage with a differential adjusting screw from Thor Labs. This screw advances 250 microns per revolution with the external threads, and the internal differential screw advances 25 microns per revolution. At first use, it was apparent the design was adequate for the task.

We verified the performance of the 10X objective by getting crisp images of a Roncii ruling. Then we measured the on axis MTF of the photographic lenses of interest to learn that part of lens performance.

Our first, hand made, stage was not robust enough for laboratory conditions. We decided to design and build a robust version that interfaced easily to commercial optical bench components. The attached photos show the device almost ready for work.

Here’s a pdf version of this document, which includes Thor Labs part numbers for the lens tube parts needed, MTF-Testing-071510.

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Low noise x1000 amplifier

Here’s the specs: Voltage noise 2.5nV/√Hz, Gain 1000, bandwidth 0.3Hz to 500kHz. I measure 600nVpp from DC to 1kHz at the output.

Here’s the story: Designing and debugging a high precision A/D stage, you will want to know how quiet the voltage reference really is. When I worked on a 20 bit A/D board, I found this amp a great way to prove that a simple voltage regulator was not good enough as a reference voltage (measured data trumps an any preconceived notions). It was a big help getting the shielding and grounding debugged too!

pre production prototype of x1000 low noise amp

Here's the amp on the bench, next to a 9V battery. Click on image to enlarge it.

view of the internal PCB of the x1000 low noise amp

A view inside the prototype x1000 low noise amp. Click on image to enlarge it.

Since most ‘scopes have about 5mV/Div at their highest gain, this AC-coupled amp allows you to ‘see’ to 5uV/Division. For a 5V full scale A/D that’s 1ppm per division on your ‘scope. Trust me, you’ll see things there.  (I read that Pasteur freaked out his dinner hosts, using his microscope to look for germs on his food. He was ‘debugging’ in his day too, with his new favorite toy).

Here’s a pdf of the schematic – it’s based upon the design in the book Low-Noise Electronic System Design, by Motchenbacher and Connelly (a highly recommended text). Since the text came without the amp, I had to make one myself.

SCHEMATIC-low-noise-x1000-amp-MODEL-101

I also have a SPICE model for it, using Linear Tech’s LT-SPICE and I’ll send it to you, if ask.

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Frequency compensated attenuator tests amplifiers

We’re working on a low-noise amplifier product, and to test it we need some attenuation, to generate clean low-level signals. The pdf shows a schematic and describes some test results. It’s a little more tricky than just a couple resistors because with every resistor you get (free!) parasitic capacitance that tends to distort your signals, unless you compensate the network.

Based upon Motchenbacher and Connelly’s excellent book ‘Low Noise Electronic System Design‘. Click on the link for the pdf or on the image for a JPEG version.

pdf: Build-your-own-wideband-attenuator

Build your own wide bandwidth frequency compensated attenuator to test amplifiers.

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Testing Optical Aspheres

Improvements in machining precision, testing and simulation make the use of aspheres available to improve optical system performance.

Most lenses are spherical, in that each curved surface is some part of a sphere (usually a big radius compared to the lens glass diameter). Lately we’ve been working on some systems that require the use of lenses that have an ‘aspheric’ curve. These are more unusual, but if you can solve a problem that is otherwise unsolvable, ‘unusual’ is a good answer. Ok, maybe since I’m the electronics guy, I’m impressed with the precision of these optics and their measurement – I think you’ll be too, when you look into it.

We’ve found some references about designing and testing these asphere elements. Start with the article by Jay Kumler, and  then read the other two about some fancy gear to test these aspheres.

Jay Kumler, Designing and Specifying Aspheres for Manufacturability, by Jay Kumler of Jenoptik-Inc

Interferometric Measurement of Rotationally Symmetric Aspheric Surfaces, by Michael Kuechel of Zygo

Subaperture stitching interferometry of high-departure aspheres by incorporating configurable null optics, by Andrew Kulawiec, Markus Bauer, Gary DeVries, Jon Fleig, Greg Forbes,
Dragisha Miladinovic, Paul Murphy of QED Technologies.

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Photos of mechanical, front panel design

Portable electronic instruments. These photos show knobs, nuts, knurled, and know-how (Pete’s). These are working photos of our differential amplifier project. They show how we can get the electrical boards to line up with the mechanical connectors, and controls. And how an extra layer of plexi can be used to add a nice finish, and provide a solution to the incompatible height of the controls (note the nut is recessed by the plexi).

overall view of assembly, showing how it slides into the case

overall view of assembly, showing how it slides into the case

close up of the BNC connectors

close up of the BNC connectors

BNC aignment of pcb, fron panel, and mounting BNC panel

BNC aignment of pcb, fron panel, and mounting BNC panel

cardboard pcb mock-up, with the real board and assembly behind it

cardboard pcb mock-up, with the real board and assembly behind it - we like to make quick mock ups as we design, just to check everything is going well

Detail of the plexi layer used to 'bury' the nut needed to hold the front panel control. The plexi is an additional part, but the incompatible heights of the controls meant there had to be some way to accomodate all these part variations.

Detail of the plexi layer used to 'bury' the nut needed to hold the front panel control. The plexi is an additional part, but the incompatible heights of the controls meant there had to be some way to accomodate all these part variations.

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Light uniformity testing

For both a clinical test microscope, and a home theater HDTV projection display, the light from the source must be quite uniform.

To test some non-imaging illumination optics, we set up our digital camera, and wrestled with the RAW data files from the camera. Most cameras have some ability to ‘see’ infra-red, so we can also test the pattern from the remote control output, or for other purposes.

Here we test the light uniformity of an LED source using a digital camera and some Thor Labs mounts.

Here we test the light uniformity of an LED source using a digital camera and some Thor Labs mounts.

These graphs were generated by ImageJ from the RAW data files of the Nikon D1x camera.

These graphs were generated by ImageJ from the RAW data files of the Nikon D1x camera.

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Custom front panels

Here’s a vendor we recommend, and a photo of some parts they made for us.

photo of parts made by Front Panel Express for our Diff Amp project

When you make one or 10 of something it can be difficult to make it look ‘real’, that is, to make it look finished. Sometimes the Dymo-Marker labels are OK, say in a sci-fi movie, for your Custom Flux-Capacitor. But other times, you want the prototype to look clean.

We have enjoyed using the services of FrontPanelExpress to make some custom front panels for our projects. They provide free software, that’s simple and easy to use, and you upload the files to their on line ordering … and you get great parts back. A variety of anodized aluminum options and thicknesses.

http://www.frontpanelexpress.com/

They will engrave text, add paint colors into the text -makes a very nice professional looking prototype or short run of parts.

Just imagine – they can easily make a D-shaped hole for the BNC – so it won’t UNSCREW and FALL OUT !!! That alone is worth the price of admission. I can’t find my D-shaped drill bit …

They can do a lot of things that are a pain to do by hand – countersink holes, nice RS-232 type D-cutouts, square holes, etc. Check out their site for some examples.

We also made a Plexiglass panel – this allows us to compensate for the very annoying differences in height of the switches and knobs – and it allows simple paper graphics to be protected. We also considered making a glowing logo, but have not yet done that.

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Image Analysis with (free) ImageJ software

Lately we’ve been able to use our digital camera to perform some nice measurements, through the help of a program called ImageJ.

It’s free, was developed at NIH, is open-source, it has a ton of features and plug-ins, and you can write scripting macros, etc etc. It was developed so that the scientific community would have an open standard to process images. (Without an open standard for image number crunching, there’s no good way to independently reproduce an experiment that makes heavy use of images and image processing.)

You can read about it here at Wikipedia:

link to wiki entry for ImageJ program

http://en.wikipedia.org/wiki/ImageJ

It’s available here:

http://rsbweb.nih.gov/ij/

link to Imagej main website

We were turned onto this image analysis program by a couple of our clients. We recommend it. Today the cool thing was to separate the RGB channels, and allow us to ‘see’ an IR LED without being confused by the camera’s ‘grey scale’ clipping algorithm. Very nice.

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