Capacitive Position Sensors and Their Applications

Sub-nanometer resolution:

Capacitive Position Sensors and Their Applications

Physik Instrumente (PI): Many application areas today require the highest of measurement accuracies, with resolutions in the nanometer or subnanometer realm. Examples range from semiconductors and biotechnology through quality assurance and automotive to metrology and microscopy. Even in “normal” machining, more and more automated tasks require high-accuracy positioning, which means that suitably high-precision measuring equipment is essential. This places high demands on the sensors used. In addition, the sensors must be as easy to integrate in the application as possible, and usually must also exhibit excellent dynamic performance.

The accuracy and repeatability of the sensors used is a major determinant of the performance of any positioning system. Anyone requiring the maximum possible accuracy—in the nanometer or even picometer realm—will be using capacitive sensors. One specialist in the area of nanopositioning systems manufacture is PI (Physik Instrumente), a company which has long been a moving force in the development of high-precision capacitive position sensors, mostly for use in its own products. Today, capacitive position sensors of various different types, some with resolutions down into the subnanometer realm, have their own place in the product palette. Capacitive sensor metrology has more to offer than just high accuracy: The sensors provide non-contact measurements of the actual position of the moved object directly—accuracy, stability and bandwidth are substantially better than those obtained with conventional LVDT or SGS sensors. As a bonus, the non-contact operation means no wear and tear, no hysteresis and no effect on the application, which is already concerned with highly sensitive displacements and/or layer-thickness matters.

Capacitive sensor working principle: Guard-ring design for best linearity

Capacitive sensor metrology is based on a simple physical principle. When there is a potential difference between the plates of a capacitor, a homogeneous electric field is created between them. If the gap between the “plates” changes, the output signal from the metrology electronics changes proportionally. Prerequisite for the proportionality is, however, that the electric field between the electrodes be really homogeneous. In order to assure this precondition and to eliminate degradation of the measurements by possible edge effects, PISeca™ sensors have a guard ring surrounding the measuring head. The guard is actively held at the same potential as the sensor surface. This design provides optimum shielding against interference from external electric fields and also delimits the measurement surface exactly. In this way, an extremely uniform electric field is created, enabling very high linearity of the measurement results. Typical linearity values are below 0.01% of the nominal measurement range. That means that for a range of 100 µm, the maximum deviation between measured and true values would be 0.01 µm. Linearity errors, of course, have no influence on resolution or repeatability of measurements. In principle—given proper readout electronics—linearities to 0.003% are possible.

For different applications one- or two-plate sensors?

Capacitive position sensors are available in two basic types, namely single-plate and two-plate systems. Single-plate sensors are especially easy to integrate into applications. They measure displacement directly against an existing surface in the application, provided that surface fulfills certain requirements. The most important are that it be conductive, grounded and of sufficient size. Because the character of the target surface influences the electric field, roughness and other irregularities should be avoided. To calibrate the sensors under ideal conditions at the factory, an extremely flat, conductive surface considerably larger than the sensor head is used as target. Measurements against convex or concave surfaces are, of course, also possible. In such cases, however, the measured value will be an average. Standard, mass-produced, single-electrode sensors are made for measurement ranges of 20, 50 and 100 µm; custom versions are available with measuring ranges as high as 1 mm or more.

While these single-electrode sensors operate with resolutions in the nanometer realm, with 2-plate sensors resolution values of 0.01 nm can be realized. This is because both capacitor plates are well-defined and can be of very high quality. Standard measurement ranges for these sensors are maximally 15, 50 or 100 µm, with extended ranges of up to 300 µm possible. There is also a choice of different versions when it comes to the ultra-low-noise, one-, two- or three-channel excitation and read-out electronics, especially designed for these sensors. Different electronics are available for the one- and the two-plate sensors; if desired, piezo amplifier modules, displays or PC interfaces can be added to the systems. Bandwidth and measurement range can be set at the factory to values optimal for the application; with single-plate sensor systems, the user also has the possibility of adjusting these parameters on-site so as to obtain the best results. With such a selection, it should be no surprise that these ultra-precise sensors find there way into the most diverse of applications.

From displacement to thickness measurement

A typical application for two-plate sensors is displacement measurement in nanopositioning. The high measuring frequency, up to 10 kHz, makes servocontrol possible even in dynamic operation. When used in multi-axis positioning systems, all degrees of freedom can be monitored simultaneously and any guidance errors actively compensated. Where the highest resolution is required, the two-plate sensors give the best results. In some applications, however, their use is not possible. This can occur, for example, if there is insufficient space for two electrodes, or if the target also moves perpendicular to the measuring direction and would thus make constant alignment of the two electrodes impossible. In such situations, single-electrode sensors are the obvious choice. They, too, can achieve resolutions 1 nm or better.

Other typical applications for single-electrode sensors are where “bumpy” motion must be detected and compensated, for example in constant-height scans or white-light interferometry. Because of their excellent dynamic performance, vibration and oscillation can be measured, as can the flatness of rotating workpieces. The thickness of non-conductors riding on moving, conductive surfaces (e.g. rollers or drums) can also be measured. In all these situations, the non-contact working principle and high dynamics are of great importance. With sensors at two measuring points, angular measurements are possible. The amount of tilt of the moved object is determined by the displacement differential, and could be compensated, if required. Single-electrode capacitive sensors are also often used as high-resolution force sensors for non-contact measurements in the micronewton range. The known stiffness of the system then makes possible the calculation of the force.