PI Piezo Tutorial: Closed-loop Piezo Motion Control, Position Servo Control (Closed Loop Operation)

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Position Servo Control (Closed Loop Operation)

Digital, Closed-Loop Piezo Controllers

Piezo motion systems can be operated in open and closed loop mode.
Closed-loop provides higher precision, linearity and repeatibility. It also eliminates drift and hysteresis.

Several types of position sensors are used for feedback, ranging from piezoresistive strain gauges to capacitive sensors and incremental linear scales.
PI offers the largest selection of closed loop piezo motion systems and piezo actuators and systems worldwide.

Advantages of position servo controlled piezo operation:

very good linearity, stability, repeatability and accuracy
automatic compensation for varying loads or forces
virtual infinite stiffness (within load limits)
elimination of hysteresis and creep effects

Closed loop position control of a stage driven by a piezo actuator. For optimum performance, the sonsor is mounted directly at the object to be positioned.

Block diagram of a typical PI Closed Loop Piezo Positioning System.

PI Closed Loop Piezo Actuators & Systems are equipped with position measuring systems providing sub-nanometer resolution and bandwidths up to 10 kHz. A servo controller (digital or analog) determines the output voltage to the Piezo by comparing a reference signal (commanded position) to the actual sensor position signal.

Response of a Closed Loop PI Piezo Actuator (P-841.10, 15 µm, stain gage sensor) to a 3 nm peak-to-peak square wave control input signal, measured with servo control bandwith set to 240 Hz and 2 msec setting time. Note the crisp response to the square wave control signal.

Reponse of a Closed Loop PI Piezo Actuator (P-410.015, 15 µm, capacitive position sensor) shows true sub-nm positional stability, incremental motion and bidirectional repeatability.

Open loop vs. closed loop performance of a typical PI Piezo actuator.

For maximum accuracy, it is necessary to mount the sensor as close as possible to the part whose position is to be controlled. Sometimes this is not possible for various reasons. PI offer piezo actuators with integrated sensors as well as external sensors.

Piezo Calibration Data
Each PI Piezo position servo controller is calibrated with the specific Closed Loop Piezo (system) to achieve optimum displacement range, frequency response and settling time. This calibration is done at the factory and is free of charge. A report with plotted and tabulated positioning accuracy data will be supplied with the system. To optimize calibration, information about the specific application is needed.