Parallel-Plate (Flat-Plate) Capacitive Sensors

Monday, November 27th, 2017 - Capacitive Transducers

Parallel-Plate (Flat-Plate) Capacitive Sensors

The simplest capacitive sensor consists of two parallel metal plates separated by a distance d (Figure 1). To eliminate the fringe field or edge effect of the electrodes, the guard electrodes are added and kept at the same potential as the sensing electrode.

Most capacitive sensors have the parallel-plate or configuration. They are widely used to detect motion, distance, pressure, acceleration, fluid level, chemicals, and biocells. The sensing principles are well explained by : parallel-plate (flat-plate) capacitive sensors,capacitive sensors,four basic principles underneath the parallel capacitive sensor design,parallel capacitive sensor,sensing principle and characteristics of capacitive sensors,sensing principles used in parallel capacitive sensorsany variation in the dielectric constants (εr), the plate area (A), or the plate spacing (d) will cause a change in capacitance (C). Figure 2 shows the four sensing principles used in parallel capacitive sensors.

Spacing-Variation-Based Sensors

Sensing Principle and Characteristics of Capacitive Sensors

A parallel capacitive sensor with a guard electrode

Figure 1. A parallel capacitive sensor with a guard electrode

In this design type, spacing d between the two parallel plates varies, causing the capacitance C to change. An inverse relationship exists between the spacing d and the capacitance C. This gives a large change in capacitance value with a small spacing change, but it also displays the nonlinearity. A bridge or signal conditioning circuit is often required to compensate for this nonlinearity.

Four basic principles underneath the parallel capacitive sensor design

Figure 2. Four basic principles underneath the parallel capacitive sensor design

Compared to other capacitive sensors, spacing-variation-based sensors are generally more sensitive, but only suitable for a small displacement range (usually the spacing variation is less than the electrode size and in the range of micrometers).  They are used for industry standards for ultra-high-precision measurements. Areavariation-based capacitive sensors, on the other hand, are less sensitive, but suitable for a larger displacement range.

 I hope this information about capacitive sensors is useful.