C2.2 - High Performance Capacitive Sensor Electronic Interfaces for Displacement Measurement in Industrial Applications

SENSOR+TEST Conferences 2009
2009-05-26 - 2009-05-28
Congress Center Nürnberg
Proceedings SENSOR 2009, Volume I
C2 - Sensor Electronics II (Capacitive Sensors)
S. Nihtianov - Technical University Delft, Delft, Netherlands
281 - 286


Capacitive sensors are popular in industry because they have a simple construction and they perform well. As the name of the sensor suggests, the electrical model of the capacitive sensor is capacitance, the value of which is related to the value of the input variable. Naturally, the main function of the capacitivesensor electronic interface is to measure capacitance. This can be done in a number of ways by using different excitation signals.
With harmonic excitation signals (u or i), the reactance of the unknown capacitance can be measured. By knowing the frequency, the excitation, and the measured signal values, the value of the unknown capacitance can be found. Another way to measure capacitance is to make it part of the frequencydefining circuit of an oscillator, together with another passive component. For a first-order (relaxation) oscillator this is usually a resistor, whereas for a second-order (harmonic) oscillator this can be an inductor. In this particular case the information carrier of the measured capacitance value is the frequency/period of the generated signal. Altematively, the sensor capacitance Cx can be extracted by measuring the charge Qx=Cx chef, which is stored in it. Here Uref is a reference voltage used to (re)charge the sensor capacitance.
There are three main performance parameters used to evaluate the level of performance of capacitivesensor electronic interface: (1) resolution; (2) measurement time (i.e., the time from the moment we want to know the value of the measurand and the moment when the result is available); and (3) stability. Additionally, in some applications, other parameters are also important, such as power consumption, linearity, and dynamic range. The required accuracy is achieved by calibration with an accurate reference.
This paper addresses the high-end industrial applications of capacitive sensors for measuring very small displacements in the nanometer and the sub-nanometer range, for which very high performance is required with respect to resolution, measurement speed, and drift. First, the general features and challenges of a typical industrial working environment are briefly discussed. Then, the basic limitations of the capacitive-sensor and the interface electronics are addressed. Lastly, methods for improving the longterm stability, resolution, and immunity to interference are presented.