Act u ators an d Sensors

Bandwidth: Up to 10 kHz Advantages:
Fig. 16. Capacitive sensors canattain resolution 10,000 times better than calipers. Fig. 14. LVDT sensor, coil and core. Paperclip for size comparison. ff. for details. Linear Variable DifferentialTransformers (LVDTs) Controls the position of the moving part rather than the position of the piezo stack Resolution: Capacitive Industrial position sensors Better than 0.1 nmpossible Capacitive sensors are themetrology system of choice forthe most demanding applica- tions. Two-plate capacitive sensorsconsist of two RF-excited plates that are part of a capaci- tive bridge (Fig. 17). One plateis fixed, the other plate is con-nected to the object to be posi- tioned (e.g. the platform of a stage). The distance between the plates is inversely propor- tional to the capacitance, fromwhich the displacement is cal-culated. Short-range, two-plate sensors can achieve resolution on the order of picometers. See the “Capacitive Displa- cement Sensors” section pp. 5-2 LVDTs are well suited for directmetrology. 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 (Fig. 15). The carrier frequency is typically 10 kHz. Resolution: to 5 nm Cost-effective Bandwidth: to 1 kHz Other characteristics: Repeatability: to 5 nm Outgassing of insulationmaterials may limit applica- tions in very high vacuum Highest resolution of allcommercially available sen- sors Advantages: Generates magnetic field Ideally suited for parallel metrology Examples Non-contacting P-780, p. 2-32;P-721.LLQ, p. 2-20. Excellent long-term stability Excellent frequency response No magnetic field Excellent linearity Other characteristics: Ideally suited for integrationin flexure guidance systems,which maintain the neces-sary parallelism of the pla- tes. Residual tip/tilt errors are greatly reduced by the ILS linearization system (see p. 5-6) developed by PI. Good temperature stability Very good long-term stability Non-contacting Examples P-733 parallel kinematic nano-positioning system with paral- lel metrology, see p. 2-64. P-753 LISA NanoAutomation
® actuators, see p. 2-26; addition- al examples in the “Nanoposi- tioning & Scanning Systems” section.
Fig. 15. Working principle of an LVDT sensor Fig. 17. Working principle of two-plate capacitive position sensors © PI 1998-2005. Subject to change w/o notice. Cat 118 05/09.17 4-20