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| | | Piezo • Nano • Positioning | | |
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| | | Tutorial | | |
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| | | Resolution / Bandwidth | | |
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| | | Resolution in nanopositioning relates to the smallest change in displacement that can still be detected by the industrial measuring devices. For capacitive sensors, resolution is in principle unlimited, and is in practice limited by electronic noise. PI signal conditioner electronics are optimized for high linearity, bandwidth and minimum noise, enabling sensor resolution down to the picometer range. Electronic noise and sensor signal bandwidth are interdependent. Limiting the bandwidth reduces noise and thereby improves resolution. The working distance also influ- | | ences the resolution: the smaller the working distance of the system, the lower the absolute value of the electronic noise. Figure 1 shows measurements of nanometer-range actuator cycles taken with a D-015, 15 urn capacitive position sensor and a laser interferometer. The graphs clearly show the superior performance of the capacitive position sensing technique. Figure 2 illustrates the influence of bandwidth upon resolution: the PISeca™ single-electrode sensors show excellent resolution down to the sub-nanometer range, even at high bandwidths. | | |
| | | Fig. 1: Piezo nanopositioning system making 0.3 nm steps, measured with PI capacitive sensor (lower curve) and with a highly precise laser interferometer. The capacitive sensor provides significantly higher resolution than the interferometer | | |
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| | | Fig. 2: Resolution significantly below 1 nm is achieved with a 20 um PISeca™ single-electrode sensor (D-510.020) and the E-852 signal conditioner electronics. Left: 0.2 nm-steps under quasi-static conditions (bandwidth 10 Hz), right: 1 nm-steps with maximum bandwidth (6.6 kHz) | | |
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| | | Linearity and Stability of PI sensors | | |
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| | | The linearity of a measurement denotes the degree of constancy in the proportional relation between change in probe-target distance and the output signal. Usually linearity is given as linearity error in percent of the full measurement range. A linearity error of 0.1% with range of 100 urn gives a maximum error of 0.1 um. Linearity error has no influence whatsoever upon resolution and repeatability of a measurement. Linearity is influenced to a high degree by homogeneity of the electric field and thus by any non-parallelism of the probe and target in the application. PI capacitive position sensor electronics incorporate a propri- | | etary design providing superior linearity, low sensitivity to cable capacitance, low background noise and low drift. The Integrated Linearization System (ILS) compensates for non-parallelism influences. A comparison between a conventional capacitive position sensor system and a PI ILS system is shown in Figure 3. When used with PI digital controllers (which add polynomial linearization) a positioning linearity of up to 0.003 % is achievable. Figure 4 shows the linearity of a P-752.11C piezo flexure nano-positioning stage with integrated capacitive position sensor operated in closed-loop | | |
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| | | Fig. 3: Linearity of conventional capacitive position sensor system vs. PI ILS (integrated linearization system), shown before digital linearization | | |
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| | | Fig. 4: Linearity of a P-752.11C, 15 um piezo nanopositioning stage operated with E-500/E-509.C1A control electronics. The travel range is 15 um, the gain 1.5 um/V. Linearity is better than 0.02 %; even higher linearity is achievable with PI digital controllers | | |
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