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Position Servo Control (Closed Loop Operation)
Click here for Closed-Loop Piezo ControllersPI offers the largest selection of closed loop piezo actuators and systems worldwide. The advantages of position servo control are:
- 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.
PI closed loop piezo actuators provide sub-nanometer resolution, repeatability and linearity to 0.03% .
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. See "Piezo Control Electronics" section for details.
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