p l i c A t i o n
Capacitance Sensing Theory
Capacitive sensors can be divided into two categories based upon their performance and intended use. High resolution capacitance sensors are typically used in displacement and position monitoring applications where high accuracy, stability and low temperature drift are required. Quite frequently these sensors are used in process monitoring and closed-loop feedback control systems. Proximity type capacitive sensors are much less expensive and are typically used to detect the presence of a part or used in counting applications. The following paper describes characteristics of high resolution capacitance systems, their operating principle and application. Capacitance sensors are used throughout a variety of industries to provide highly stable, accurate measurements of displacement, vibration, position, thickness and runout. Most capacitance probes are passive by design, allowing them to be operated in high shock and vibration environments, and at extreme temperatures. Capacitance probes are typically modeled as a parallel plate capacitor. If two conductive surfaces are separated by a distance and a voltage is applied to one of the surfaces, an electric field is created. This occurs due to the Figure 1: Typical Capacitance Sensor different charges stored on each of the surfaces. Capacitance refers to the ability of the surfaces to hold a charge. In a typical capacitive sensor system the probe is one of the plates and the target being measured is the other plate. If a constant current is applied to a capacitive probe the capacitance change can be monitored as a linear voltage charge related to the distance between the plates. This distance, or gap, is a function of the area of the capacitance sensor according to the following formula: Capacitance = Area X Dielectric/Gap. From this relationship you can see that capacitance is directly proportional to the area of the sensor and the dielectric property of the material between the sensor and target (typically air). The greater the area of the capacitance sensor the larger the measurement range, or gap. If we assume the area and the dielectric between the plates remain constant for a specific capacitive probe, any change in capacitance is inversely proportional to the change in distance between the capacitive probe and target being measured. This change in capacitance is processed within MTII?s Accumeasure TM capacitance amplifier and converted to a voltage for monitoring. The amount of voltage output change for a given distance change is commonly referred to as the sensitivity of the capacitance system. For example, if a distance change of 1mm corresponds to a voltage output change of 10 volts the sensitivity would be 1mm/10 volts, or 0.1mm/volt.
i. Capacitance Sensors Electric Field
A typical high performance capacitance sensor consists of three basic elements, the sensor tip, the guard and the ground shell. Figure 1 illustrates a typical capacitance probe manufactured by MTI Instruments. MTI Instruments, Inc. 325 Washington Avenue Extension Albany, NY 12205 PH: +1-518-218-2550 OR USA TOLL FREE: 1-800-342-2203 FX: +1-518-218-2506 email@example.com www.mtiinstruments.com MTII appnote: capacitancesensingtheory.pdf - Page 1 of 13