Piezo motors can provide motion in extremely small increments, smaller than one nanometer. This is due to solid state effects in the piezo material, where small changes in the electric field affect the shape of the piezo elementary cells with virtually unlimited resolution. For more information on the piezo effect and types of piezo actuators and motors read Piezo Motion Control Tutorial.
Changing the electric field by controlling the voltage, or charge, across a piezo ceramic actuator is one thing, but knowing the exact amount of the positional change caused by it, is a different story. For exact closed-loop position control, a position sensor is required, and it has to be sensitive enough to see the minute changes piezo material is capable of. For classical piezo actuators the motion is limited to a few hundred microns, and sensors such as capacitive gauges or strain gauges provide great results.
Piezo motors, on the other hand, can provide much longer travel ranges, and require different feedback sensors. Incremental encoders are often used, laser interferometers are recommended, when the highest accuracy is required. The interferometer can measure the motion directly at the point of interest and for that reason alone, is superior to any internal sensor. It is also not affected by thermal errors caused by direct heat transfer of the motor to an internal sensor.
To facilitate the use of interferometers with PI’s V8 piezo linear motor, PI designed a new interface for its motor controller to process the high-resolution signals of Zygo’s ZMI interferometer series.
The interferometer provides an output signal with a resolution of ~154.5 picometers.