| | | mode with an analog controller. All errors contributed by the mechanics, PZT drive, sensors and electronics are included in the resulting linearity of better than 0.02%. Even higher linearity is achievable with PI digital controllers like the E-710. Stability of the measurement is limited mainly by thermal and | | electronic drift. For accuracy and repeatability reasons, it is thus necessary to maintain constant environmental conditions. The exceptional long-term stability of the PI capacitive position sensor and electronics design is shown in Figure 5. | | |
| | | Signal/Displacement Proportionality When a voltage is applied to the two plates of an ideal capacitor, it creates a homogenous electric field. Apart from constant factors, the electrical capacitance of the set-up is determined by sensor area and plate distance. Thus, a change in displacement leads directly to a change in capacitance. This value is matched to a reference capacitance in a bridge circuit. Design of the signal conditioner electronics is such that the output signal is proportional to the gap change. The planes of the sensor surface ("probe") and the target form the two capacitor plates. The target should not be below a certain size because | | of boundary effects. This is important for applications with, say, a rotating drum as target. For metallic materials, the thickness of the target has no influence on the measurement. Guard Ring Geometry/Design The proportionality referred to is based on the homogeneity of the electric field. To eliminate boundary effects, the superior PI design uses a guard-ring electrode that surrounds the active sensor area and is actively kept at the same potential (see Fig. 7). This design shields the active sensor area and provides for excellent containment of the measurement zone. Thus optimum measuring linearity over the full range is achieved within the specified accuracy. | | |
| | | Fig. 6: Capacitive sensor working principle. The capacitance C is proportional to the active sensor area A, e0 is constant, er is the dielectric constant of the material between the plates, generally air | | |
| | | PI's nanometrology calibration laboratories offer optimum conditions for factory calibration. As references, ultra-high-accuracy incremental sensors like laser interferometers are used. PISeca™ systems are calibrated at PI with a NEXLINE® positioning system having a | | closed-loop resolution better than 0.01 nm in a test stand with friction-free flexure guidance and an incremental reference sensor featuring a resolution better than 0.1 nm (Fig. 8 and 9). | | |