Repairing PM500-1A actuator (25mm travel, 25mm/sec speed, 0.05micron precision)

Sergei P. Skorobogatov
sps32 (at)

Manufacturer: Newport Corporation, USA

Date of production: July 1991

General information:<wo=Motorized%20Actuators

This is a Newport ultra-precision compact actuator that can be used as a direct replacement for manual actuators and micrometers in various motion control applications. Unfortunately, as it always happens when you get a second-hand mechanical or optical equipment, it was in a poor working condition and cannot be used without prior repair. The actuator responded to the controller commands moving the shaft forward and backward, but it did not stop at the desired position and continue the move until it reaches the limit switch. If the motor is switched off, the controller always return the same position disregard of the shaft displacement during the manual move by the handle.

PM500-1A PM500-1A PM500-1A
To locate core of the problem, the actuator was opened first. Then an oscilloscope was connected to 'SIN' and 'COS' signals coming from the encoder on the actuator's cable to monitor the response. In fact, the signals were very close to the noise level instead of 1V swipe required by the controller. That means something wrong happened to the encoder. For further investigation, the detector part of the encoder was dismount from the moving part. By inserting a piece of paper in the gap between light emitting diodes and photodiodes changes in the signals were observed. That proved that the encoder was working and only alignment is required.

PM500-1A PM500-1A PM500-1A
The detector was placed back into its original position, but the screws were not tighten. That allowed to move it slightly so that the surface of the origin plate was parallel and very close to the reference plate. The result was observed on the oscilloscope while the actuator was moved manually. Once the correct position of the detector was found the screws were tighten. Final testing with controller shown reliable and smooth operation.

Sergei Skorobogatov <Sergei.Skorobogatov (at)> <Sergei.Skorobogatov (at)>
created 20-10-2004 -- last modified 07-11-2004 --