INSTRUCTIONS FOR SLOWING THE HALF-WAVE PLATE MOVES

Background information:

The half-wave plate is moved by a stepper motor that is controlled by an IMC controller (the "indexer"). The control computer talks directly to this indexer via a serial line. The command syntax for the IMC controller is in the IMC manual. This manual was scanned and analyzed using OCR software and is available here. (But you shouldn't need to read this manual to make the change of speed discussed here.)

One revolution of the stepper motor corresponds to 200 steps. There is a 15:1 gear ratio so that one rotation of the half-wave plate corresponds to 15 rotations of the stepper motor. Thus, there are 250 steps required for a 30 degree move of the half-wave plate.

During Jan-April 2003, we used the following speed settings for the stepper motor: initial speed = 100 steps per second; maximum speed = 150 steps per second.

At the beginning of winter, the (nominally) 30 degree moves were quite accurate. However, these moves have become less accurate with time. Right now, these moves are averaging about 28-29 degrees. One possible explanation for this change is an increase in the amount of friction in the half-wave plate drive train.

If there is indeed an increasing level of friction in this system, then it is plausible that by slowing the half-wave plate motion we will prevent further increases.

The initial speed and maximum speed are set during the initialization of the indexer. Indexer initialization occurs when "resetting" the system, and also during normal operation (once per polarimetry file).

Sometimes, at low speeds, a stepper motor can resonate. So you should listen to the motor both before and after the reduction in speed. You can easily hear it move. If the motor gets a lot louder at a certain speed, then it is probably having a resonance problem at that speed. We should not run it at a speed that causes a resonance problem.

How to change the motion parameters:

The parameters used for the initialization are stored in the following file:
```
~viper/sparo2003/indexer/indexer.tcl
```
In that file, you will find the following lines:
```
    tellindexer "I 100"
```
To slow the motor to a speed of, say 70 steps per second, you should replace the values 100 and 150 with the value 70, so that these lines will read as follows:
```
    tellindexer "I 70"
    tellindexer "V 70"
```
The minimum values for I and V are 20 steps per second.

In addition, sometimes stepper motors have a tendency to resonate more at lower speeds. To prevent this, the IMC controller uses "variable resolution". You should make sure that the indexer is set in variable resolution mode by adding the following line to the initialization code:
```
    tellindexer "H 1"
```

You can add this line right after you set the speeds, so the complete list of initialization commands will look like this:

    tellindexer " "
    tellindexer "K 10 10"
    tellindexer "I 70"
    tellindexer "V 70"
    tellindexer "H 1"
    tellindexer "Y 0 8"
    tellindexer "O"
    tellindexer "D 0"

How to reinitialize and test:

After changing the speed parameters, you should reinitialize the indexer by doing the following:
```
Go to viper home directory
type vcs
at the vcs prompt, type vcs::tell indexer "init"
```
next, practice a few 30 degree moves, by doing the following:
```
at the vcs prompt, type vcs::tell indexer "move 30.0"
```
30 degrees is 250 steps, so it should now take about 3.5 seconds for a 30 degree move, for a velocity of 70 steps per second.
When you start to take polarimetry data again, please pay attention to how the half-wave plate angles look for the first one or two polarimetry files after the change. If for some reason the angles look really bad, you can go back to the original speed settings. It is possible that the motor motion may be erratic at the lower speeds due to resonances. But it is unlikely that such erratic motion could do any permanent damage.

Last updated April 30, 2003.