What kind of feedback do piezo motors use?

Piezo motors are known for their extremely small size and high size-to-torque ratio, as well as high-resolution movements (micrometer and even nanometer-level resolution.) The high resolution is attributable to the inherent design of piezo motors themselves, but feedback also helps to ensure highly accurate movement on the order of nanometers.

Incremental linear encoders, such as the LIP 481 from Heidenhain (left), are often used for position feedback with digital motion controllers in piezo motor positioning stages. (Image via Heidenhain) An example of a linear motor positioning stage (right) equipped with digital position feedback. (Image via PI)

As a general rule, piezo motors can be operated either open loop or closed loop. The advantage of closed-loop operation is that it offers higher precision and greater repeatability and linearity. Closed-loop feedback also helps eliminate drift and hysteresis, both factors that can negatively impact precision and accuracy.

There are a few common types of feedback used with piezo motors ranging from strain gauges to optical encoders, depending on the application requirements:

• • • Optical encoders: Incremental optical encoders are commonly used for piezo motor feedback. However, absolute encoders can be used as well, although they are typically more costly than incremental encoders.
    • Capacitive sensors: Along with strain-gauge sensors, these are the most common as they are relatively inexpensive, compact, and provide good quality nanometer-resolution readings.
    • Laser interferometers: These offer sub-nanometer resolution, but they are also more bulky and expensive than either capacitive based sensors or encoders.