A computer-controlled multi-purpose fixed- KE tester for Foucault and related tests

The device described here is a highly experimental setup, partly to gain experience of driving stepper motors, partly to try out variations on the Foucault family of tests, such as the Caustic test of Platzeck-Gaviola or the wire test - in particular the "lateral wire test".

Using remote controlled steppers would offer hands-free operation with little risk of moving the setup during a measurement series. Commonly are used either fixed-source or moving-source testers, implying the knife-edge or other reading apparatus moves - I liked to try the third possibility of having the light source moving, but the knife-edge fixed! This, I believe, would simplify using a CCD/video camera as detector, or a reading microscope/telescope.

I had a couple of junked hard disk drives, circa -88, with bipolar stepper motors of 400 steps/turn. Careful examination showed that the head assembly moved 0.03 mm per full step, over a range of 20 mm (unfortunately a bit less than desirable for Gaviola test of large mirrors, but adequate for Foucault and wire test for 16" f/4.5 [a dormant project that I hope to revive] or larger). But the important thing is they offer ready-made precision mechanics.

How about driving the steppers? Another project that is slowly progressing is making a Bartels type scope-drive - first for my old SP-C6 6" f/5 German EQ mount with stepper motors. I have been planning to make the interface electronics for one year, but for lack of components, progress has been slow. Then one day I searched for circuit specs and found that an old junked OKI printer contained six Darlington driver ICs of 4 section each (LB1205), with TTL-compatible inputs that could be driven directly from the printer port outputs, and protection diodes connected to a separate pin. They were not easy to remove, though, and (so far) one was killed, but they do the job well (I won't go into details here, there are variations described on Mel's project page).However, this interface won't drive bipolar motors. Checking "Jones On Stepping Motors" again, I saw an idea that I slightly modified as a stand-alone "slave driver":

As I had lots of resistors and small signal transistors I used 3 BC237 and 3 180ohm 1/4 watt resistors in parallel instead of each one transistor showed, but one of each, higher rated, would have been even nicer. As is, it will drive the 20 ohm windings with about 100 mA, less than the nominal 300 mA but perfectly adequate as speed is no concern here (and anyway, they are essentially current driven here). You may note that no protection diodes are added. Excursions to the negative side are limited ny the driver transistors starting to conduct, positive excursions via the diodes here and the protection diodes to the Bartels interface card (they should go to the 12V supply directly, if you have the 30 volt zener diode(s) of the original circuit, short-circuit them for this application).

Anyway, I wrote simple driving software to let the arrow keys move the steppers one half-step a time, displaying the position on the laptop screen and keeping counts to keep the motion within the allowed range of 20 mm. I used Turbo Pascal for DOS as I have been fond of it ever since my CP/M days (if anyone would like the gory details, mail me). So far, for instance pressing key "2" will save the readings to the screen, but no doubt I will output a file for direct input to Jim Burrows' SIXTESTS (that can handle any test I could think of) or Dave Rowe's FIGURE (for Foucault) - both run under DOS (SIXTESTS also has a Windows version).

The mechanics: I stripped the disk drives of anyting not needed, sawing off pieces of the cast frame where needed. On the moving stage, ball bearings taken from other disk drives were screwed, each carrying a lever (on one, the metal frame originally carrying the r/w heads were partially preserved). The two (wooden) levers, at approx. right angles to each other, connect via a third, smaller ball bearing. The light source is at 1/10 of the total distance from the driven bearing, thus it will move 1/10 as much sideways as the free bearing is moved - meaning I have a 2 mm range at 0.0015 mm per half-step. The range of angular movement is a little over 3 deg, and I believe I can safely disregard sine/cosine errors.

The light source was taken from a cheap laser pointer - it has no feedback PIN diode, just a 75 ohm (?) in series with the 4.5 V supply, drawing a little over 20 mA. When running from fresh 3V batteries, it will laser at about 15 mA, so I have to insert a series resistor to limit the current. The diode does not laser but acts as a very small, quite bright "pinhole" source. I might, of course, attach other light sources to try.

So far, I have tried plain masked Foucault, and the "lateral wire test" as described by John Francis, where the wire is fixed but appears to move over the face of the mirror and pinstick when stepped sideways - this test shows great promise.

Nils Olof Carlin