The two Allen-Bradley 843ES safety encoders on the telescope must be properly homed to ensure safe operation.

Because the safety encoders are multi-turn and are never normally decoupled from the telescope axis they should not normally require adjustment. However, loss of power, loss of connectivity, or switching the controller to program mode may require that the encoders have the safe home function actuated.

If the encoder was decoupled from the axis then the encoders will have to have the zero offset adjusted.

How to home the telescope safety encoders

In the engineering section of the HMI there are momentary pushbuttons to home the safety encoders.

1. Navigate to the Engineering – Safety Encoders screen on the GIS HMI.

2. Verify that the position value in ‘Target Home Position’ matches the main axis encoder. If they do not match the zero offset must be adjusted (see How to adjust the Telescope Safety Encoder).

3. Press ‘Set Safety Encoder Home Position’ push button.

4. Verify that the value in ‘AB Encoder Current Position’ now matches the main axis encoder.

5. Verify that any warnings associated with 'Encoder not homed' have disappeared.

How to adjust the Telescope Safety Encoders

The adjustment of each safety encoder is affected by two constant values in the Telescope LIC. ALT_ZERO_OFFSET and ALT_SCALE_FACTOR for the Azimuth Axis and AZ_ZERO_OFFSET and AZ_SCALE_FACTOR for the Azimuth Axis.

Normally these should never require adjustment, especially the scale factor term. But if the encoder was removed for repair or replacement then the zero offset term will likely require adjustment.

1. Navigate to the Engineering – Safety Encoders screen on the GIS HMI.

2. Compare the Target Home Position to the main axis encoder position. They should match. If they do not match an adjustment needs to be made to the zero offset.

3. Calculate the correct offset:

4. Modify the constant tag value in the TEL_GLX.