Capacitive position transducers are becoming increasingly common in manufacturing industries. These capacitive transducers include a pair of substrates that move relative to each other along a measurement axis. A set of electrodes is carried by each of the substrates, with the electrodes of one substrate being positioned close to the electrodes on the other substrate to form a capacitor therebetween. The electrodes on each of the substrates are connected to conventional circuitry for providing an indication of the relative position between the two substrates as a function of the capacitance between various electrodes in each set. The capacitive position transducer may be of the incremental type, in which the circuitry provides only an indication of incremental movement from a known point, or an absolute position type, in which the circuitry provides an indication of the relative position between the two substrates regardless of whether their initial relative position is known. Capacitive position transducers of these types are disclosed in U.S. Pat. Nos. 4,420,754 and 4,879,508.
.Capacitive position transducers are often used as the principle component of electronic calipers for measuring thickness and other physical parameters. Although these calipers are sometimes used in dry, relatively, clean environments, such as inspection rooms or engineering offices, such calipers are often used to measure dimensions of work pieces in machine shops and other relatively dirty environments. When used in these environments, such calipers can become contaminated by particulate matter and fluids, such as cooling or cutting fluids. The liquid or particulate contaminants may find their way between each set of electrodes and affect the capacitance between the electrodes in a manner that is not related to the relative position between the substrates along the measurement axis. Contaminants between the electrodes of a capacitive position transducer degrade performance because the particulate or fluid may be a dielectric having a dielectric constant that is different from the dielectric constant of air that is in an air gap between the sets of electrodes. The capacitance between electrodes on opposite sides of the contaminate will thus be greater than the capacitance between other electrodes having the same relative geometry which do not have contaminants therebetween. Under these circumstances, the capacitance between the electrodes will not provide an accurate indication of the relative position between the substrates.
One approach to minimizing the adverse effects of contaminants in capacitive position transducers is described in U.S. Pat. No. 5,172,485 to Gerhard et al. Gerhard et al. teach coating the electrodes of each substrate with a thin layer of dielectric material and then mounting the substrate so that the dielectric material coating the electrodes of one substrate slides along the dielectric material coating the electrodes of the other substrate. In theory, the sliding contact between the dielectric layers eliminates any air gap for contaminants to fill. However, as a practical matter, the sliding contact approach described in the Gerhard et al patent cannot entirely eliminate an air gap. In fact, if there was no air gap at any area between the electrodes, very high frictional loads and wear would result. The absence of an air gap in any area between the electrodes also implies a perfect level of surface finish and flatness which is never achieved in practice. A practical best case design would still result in an air gap of 0.002-0.004 mm in some areas. For these reasons, the sliding contact approach described in the Gerhard et al. patent normally requires that the substrates be resiliently biased toward each other so that deviations from exact surface flatness and alignment can be accommodated by allowing the substrates to move apart. However, the compliant nature of the suspension for the substrates allows the substrates to be forced apart from each other by particulate contaminants which inevitably collect between the electrodes when the capacitive position transducer is used in a dirty environment. If the fluid contaminants were collected in the gap with a uniform thickness, the contaminants probably would not have an adverse affect on accuracy. However, the fluid contaminants are in fact collected between the electrodes in a highly non-uniform manner so that the thickness of the contaminant layer varies between zero and a substantial value. However, the particulate contaminants space the electrodes apart from each other by the maximum thickness of the contaminant layer. At areas between the electrodes where the fluid contaminant layer is thinner or not present, gaps are created which are filled with air. Thus, the sliding contact approach used to eliminate the air gap altogether has not proven to be adequate under most circumstances when used in contaminated environments.