Messed around with the motors a little more this morning, and it now seems to me that disabling/enabling the motors does cause the pots to jump a bit, but there seems to be enough damping from the internal mechanisms to keep the slit features from moving with them....
The end result is that there is a range of a few tens of mV for a given mechanism position. There is also backlash in the mechanism motions, but that seems to be pretty well controlled with the present motion scheme.
I now think that my problems yesterday resulted from pot readings that were affected by the motor enable/disable cycle that comes with data taking. These pot reading may have messed with the logic of the motor positioning algorithm, which I didn't understand very well at the time. Bottom line is that if we get a source on the slit, it should not fall off just because the holding current is cycled on and off, even if the pot says the slit moved in the process.
Today I took a series of frames with each of the slits centered on the FCLS source. The source was positioned at the place slit 4 landed on a particular move, but given that the position of slit 4 from one move to the next may vary on the order of a slit width, this FCLS position isn't especially significant. It's about 1.2mm off the HIPO foresight position of 45.75, 31.112 (maybe 4.8" of arc).
Here is a list of slit positions taken to coincide with the 10µm FCLS source positioned at TAAS (X, Z) of (44.7, 30.6), which maps to pixel (452, 487) in this configuration:
Slit V ctr.@(452,487) indicated angle after dis/en cycle
1 6.2634 59.966 6.2505V/60.547deg
2 5.5998 89.500 5.5886V/90.0deg
3 4.9304 119.550 4.9241V/120.0deg
4 4.2646 149.550 4.2543V/150.0deg
5 3.5969 179.750 3.5953V/178.75deg
6 2.9280 209.700 2.9252V/210.0deg
7 2.2604 239.800 2.2526V/240.0deg
Note that in every case the disable/enable cycle caused the pot reading to change, but the slit as observed on the display did not move and the source position in the slit was maintained. A number of times the disable/enable cycle cause the volt_err dialog to pop up, but I just said 'yes' to setting the angles based on the voltages read.
We might be able to use the able above to refine placement of the slits in the init file. They should be exactly 30 degrees apart, but they are not. The delta from 30deg centers off the position of slit 1 is:
Slit Delta from 667mV centers Delta from 30deg
2 -0.0028V 0.466deg
3 0.0003 0.416
4 -0.0012 0.416
5 -0.0002 0.216
6 0.0021 0.266
7 0.0030 0.166
Might try massaging the init file with these numbers to see whether we get correct slit registration (modulo pot backlash/read errors, or course).
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