OSIRIS analyser towers docs

doc/specific_iocs/motor_extensions/OSIRIS-analyser-towers.md

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+# OSIRIS Analyser Towers
+
+OSIRIS has a set of analyser towers with an unusual motion control setup. The purpose of the analyser towers
+is to focus beam from the sample onto a detector. The towers are motion-controlled to allow this focus to
+be adjusted.
+
+There are 4 analyser towers in total; these are labelled Towers 4, 3, 2, 1 looking left-to-right from the sample
+position. Each tower has two plates, an upper and a lower plate, which can drive independently.
+
+![](OSIRIS_analyser_diagram.png)
+
+````{seealso}
+User documentation and maintenance documentation for this system in 
+```
+\\isis\shares\ISIS_Experiment_Controls\Manuals\OSIRIS_Analyser_towers
+```
+````
+
+## Setup notes
+
+- This setup is unusual in that only two galil axes are used at a time (always an upper/lower pair for one tower).
+To control other towers, the corresponding motors will be physically unplugged and a new tower plugged in to the
+Galil controller.
+  - **After plugging in a new tower, the motors must be homed before the positions are meaningful**
+- All axes are open loop - there are physically no encoders present.
+- The motion is not fully reliable. During commissioning, drift (loss of motor steps) was observed on various motors.
+As there are no encoders present, the only remedy for this is to rehome regularly.
+- Homing is set up to **physically** home in the positive direction, to the upper limit switch.
+  - This represents maximum upwards tilt of the analysers
+  - However, because the limit switches were wired the wrong way, this actually uses the `Home_RevLimit.gmc` homing routine, and `DIR` is swapped at the motor record level to "Reverse".
+
+For Galil settings used during commissioning, see [here](https://github.com/ISISComputingGroup/ControlsWork/issues/829#issuecomment-3836073060).
+
+## Commissioning positions
+
+Commissioning was done against a set of "reference" positions, checked using a surveying setup (by Accelerator
+engineering group). The reference positions represent the positions "as designed" to focus beam onto the detector.
+Limit switch positions are set to give approx +/- 1 degree of motion from the reference/design position.
+
+| Tower    | Upper limit switch | Reference position | Lower limit switch |
+|----------|--------------------|--------------------|--------------------|
+| T4 Lower | 0                  | -0.90              | -2.38              |
+| T4 Upper | 0                  | -1.60              | -2.60              |
+| T3 Lower | 0                  | -1.02              | -2.07              |
+| T3 Upper | 0                  | -0.97              | -2.10              |
+| T2 Lower | 0                  | -1.04              | -1.96              |
+| T2 Upper | 0                  | -0.87              | -2.14              |
+| T1 Lower | 0                  | -1.20              | -2.25              |
+| T1 Upper | 0                  | -1.10              | -1.99              |
+
+This photo shows the motion setup from the back (T1 and T2 shown):
+
+![](OSIRIS_analyser_back.jpg)