When the motor is homed the step counter starts to decrease until but never stops. The full stroke of the motor is about 1500 steps; so no matter what the starting point is, if you see a value of -2000 steps or less it means that the homing is not having success.
The Newport NSA12 linear actuator has a stroke of 10mm and a single negative limit switch (corresponding to the fully-retracted position) that is used for homing.
When the motor is fully-elongated the actuator may detach from the rotor. In this case the motor is not able to retract unless manually "helped". With the motor rotating in the retracting direction one has to push the actuator toward the motor to let the the actuator hook again up to the rotor.
It is to be noted that similar symptoms could be experienced when the motor is actually completely retracted but the limit switch is broken (not very likely in our statistics). The 2 cases can be discriminated by checking the actual elongation of the actuator: if it is fully retracted (only the spheric tip is visible) then the limit switch is broken. If instead the actuator is fully elongated the procedure below can be attempted.
The image shows a detail of the actuator spherical tip touching the stage. In this example the actuator is almost completely retracted:
Procedure to recover from a motor stuck in the fully elongated position.
1. Access the LGSW from the side of the connectors and the cooling pipes (opposite to M1). The operator can stand on the inclined beam of the instrument gallery, very close to the RFBG cable chain.
2. Remove the front upper panel of the LGSW. See photo. A 2.5mm Allen key is needed.
3. Identify the motor. The one circled in the photo is the yellow Y.
4. Using the software start the motor homing procedure. The step counter starts to decrease, the operator can feel the motor that is rotating, but the stage does not translate.
5. Separate the stage from the actuator (move against the spring) and then let the stage come back and press against the actuator. See photo and movie
). Please, don't touch the lens, use your finger on the corner of the mount as shown in the picture.
6. When the actuator is pushed against the motor rotor it should hook up again to it. The actuator should start to retract and the homing should succeed.
7. Put back in place the upper cover, tight the screws.