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High Resolution Sensors

The following description explains the features of different high resolution sensor types used for closed loop control of Piezo actuators. The given bandwidth is open loop bandwidth of the sensor. Closed loop bandwidth of a Piezo/sensor/servo controller system is limited by the mechanical and electrical properties of the system.

Strain Gage Sensors
Based on the principle of electrical resistance.

A resistive film bonded to the Piezo stack changes resistance when strain is applied. Up to four strain gages (the actual configuration varies with the Piezo construction) form a Wheatstone bridge driven by a DC voltage (5 to 10 V). When the bridge resistance changes, electronics converts the resulting voltage change into a signal analogous to displacement.

Resolution: better than 1 nm (for 15 µm actuator)
Repeatability: up to 0.1% of nominal displacement
Bandwidth: up to 5 kHz

Advantages:

• high bandwidth
• high-vacuum compatible
• extremely small (no extra mounting space/no reduction of active cross section causing reduced stiffness)
• cost effective

Other Features

• low heat generation (0.01 to 0.05 W sensor excitation power)
• long term position accuracy (> 1 h) may be affected by the bond between sensor and Piezo ceramics
• measuring principle (ceramic stack strain rather than top piece position) can lead to reduced accuracy with heavy loads (when buckling of the stack occurs)

Examples:
Most PI LVPZT and HVPZT actuators are available with strain gage sensors for closed loop control (see "Piezo Actuators" section).

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