Fundamentals ... (cont.)
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Strain Gauge Sensors
A resistive film bonded to the PZT stack changes resistance when strain occurs. Up to four strain gauges (the actual configuration varies with the PZT 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 proportional to the displacement.
Resolution: better than 1 nm (for 15 µm actuator)
Repeatability: to 0.1% of nominal displacement
Bandwidth: to 5 kHz
Advantages:
High bandwidth Vacuum compatible Extremely small (no extra mounting space, no reduction of active cross-section causing reduced stiffness) Cost-effective
Other characteristics: Low heat generation (0.01 to 0.05 W sensor excitation power)
Examples:
Most PI LVPZT and HVPZT actuators are available with strain gauge sensors for closed-loop control (see the "PZT Actuators Selection Guide" see link).
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Linear Variable Differential Transformers (LVDTs)
A magnetic core, attached to the moving part, determines the amount of magnetic energy induced from the primary windings into the two differential secondary windings. The carrier frequency is typically 10 kHz.
Resolution: to 5 nm
Bandwidth: to 1 kHz
Repeatability: to 5 nm
Advantages:
Good temperature stability Very good long-term stability Controls the position of the moving part rather than the position of the PZT stack Cost-effective
Other characteristics: Outgassing of insulation materials may limit applications in vacuum Extra space for mounting required
Examples:
P-780, see link; P-721.10, see link; P-762, see link ; etc., (see "PZT Flexure NanoPositioners" section).
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Fig. 13. Strain gauge sensors. Paper clip for size comparison
Fig. 14. Operating principle of an LVDT sensor
Fig. 15. LVDT sensor, coil and core. Paper clip for size comparison
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