Piezo Calibration Data Control of Piezo Act
Each PI piezo position servo-controller is calibrated with the specific closed-loop piezo posi-tioning system to achieve opti-mum displacement range, fre- quency response and settling time. The calibration is perfor- med at the factory and a report with plotted and tabulated po-sitioning accuracy data is sup-plied with the system (see p. 2-8, p. 3-7). To optimize calibra- tion, information about the spe- cific application is needed. For details see p. 6-53 in the “Piezo Drivers & NanopositioningControllers” section. Digital controllers can automat-ically read important calibra- tion values from an ID-chip integrated in the piezo me-chanics. This feature facilitatesusing a controller with various stages/actuators and vice versa. u ators an d Stages
Open- and Closed-LoopResolution Position servo-controlled piezodrives offer linearity and re- peatability many times better than that of open-loop systems.The resolution (minimum in-cremental motion) of piezo actuators is actually better for open-loop than for closed-loop systems. This is because piezo resolution is not limited by fric- tion but rather by electronic noise. In open-loop, there is nosensor or servo-electronics toput additional noise on the con- trol signal. In a servo-controlled piezo system, the sensor and control electronics are thus of considerable importance. Withappropriate, high-quality sys-tems, subnanometer resolution is also possible in closed-loop mode, as can be seen in Fig. 30 and 31. Capacitive sensors attain the highest resolution, linearity and stability. Fig. 30. Response of a closed-loop PI piezo actuator (P-841.10, 15 µm, strain gaugesensor) to a 3 nm peak-to-peak square-wave control input signal, measured with servo-control bandwidth set to 240 Hz and 2 msec settling time. Note the crisp, backlash-free behavior in the nanometer range. Fig. 31. PI piezo actuators with capacitive position sensors can achieve ex-tremely high resolutions, as seen in the above result of a 250 picometerstep by a S-303 phase corrector (Controller: E-509.C1A servo-controller and E-503 amplifier). The measurements were made with an ultra-sensitive capacitive sensor having a resolution of ±0.02 nm. L Ctrl Input/V Fig. 32. Open-loop vs. closed-loop performance graph of a typical PI piezo actuator. © PI 1998-2005. Subject to change w/o notice. Cat 118 05/09.17 4-32