Question
Why is the Zero location of the pRESP Platform inconsistent or off by around 0.5mm or less?
Answer
There are 2 bugs in the OES controller firmware that could influence the identification of the zero location on pRESP,Platform or cRESP units. These bugs created an error in the home position of +/- 0.5 mm.
The possible bugs are:
1. The OES controller reports 2 consecutive motor steps as 0 even though, only one location is truly zero
2. The 0 location is defined differently according to the starting position of the motor shaft when power is applied. If the motor shaft starts in the "top half" of the rotation (as defined by +/- 90 degrees from top dead center when assembled in the resp motion housing) then zero is defined at location A. If the motor shaft starts in the "bottom half of the rotation (as defined by +/- 90 degrees from bottom dead center) then the zero is defined at location B. Location B can be 2 encoder steps away from location A.
Since there are 2 encoder counts defined as zero for each power up homing routine and there can be 2 different locations defined as zero depending on the start position of the shaft, the inconsistency in the zero location can be as great as 4 encoder counts, which would be easily observable on the translation stage.
To minimize variability, the unit can be powered on twice before the home location is observed. The first power on guarantees that the motor shaft will be in the “top half” at power on. The second power on then identifies the best reference for a zero location (although there will still be 2 motor positions identified as zero)