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A Low-Cost, Smart Capacitive Position Sensor
Abstract
A
new
high-performance,
low-cost,
capacitive
position-measuring system is described. By using a highly linear oscillator, shielding and a three-signal approach, most of the errors are eliminated. The accuracy amounts to 1 μm over a 1 mm range. Since the output of the oscillator can directly be connected to a microcontroller, an A/D converter is not needed.
I. INTRODUCTION
This paper describes a novel high-performance, low-cost, capacitive displacement measuring system featuring: 1 mm measuring range, 1 μm accuracy,
0.1 s total measuring time.
Translated to the capacitive domain, the specifications correspond to:
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a possible range of 1 pF;
only 50 fF of this range is used for the displacement transducer;
50 aF absolute capacitance-measuring inaccuracy.
Meijer and Schrier [l] and more recently Van Drecht,Meijer, and De Jong [2] have proposed a displacement-measuring system, using a PSD (Position Sensitive Detector) as sensing element. Some disadvantages of using a PSD are the higher costs and the higher power consumption of the PSD and LED (Light-Emitting Diode) as compared to the capacitive sensor elements described in this paper.
The signal processor uses the concepts presented in [2],but is adopted for the use of capacitive elements. By the extensive use of shielding, guarding and smart A/D conversion,the system is able to combine a high accuracy with a very low cost-price. The transducer produces three-period-modulated signals which can be selected and directly read out by a microcontroller. The
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microcontroller,in return, calculates the displacement and can send this value to a host computer (Fig. 1) or a display or drive an actuator.
PersonaElectro?C CxDisplay ActuatoCref Fig. 1. Block diagram of the system
Fig. 2. Perspective and dimensions of the electrode structure
Ⅱ. THE ELECTRODE STRUCTURE
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The basic sensing element consists of two simple electrodes with capacitance Cx, (Fig. 2). The smaller one (E2) is surrounded by a guard electrode. Thanks to the use of the guard electrode, the capacitance Cx between the two electrodes is independent of movements (lateral displacements as well as rotations) parallel to the electrode surface.The influence of the parasitic capacitances Cp will be eliminated as will be discussed in Section Ⅲ.
According to Heerens [3], the relative deviation in the capacitance Cx between the two electrodes caused by the finite guard electrode size is smaller than:
δ electrodes. This deviation introduces a nonlinearity.Therefore we require that δ is less than 100 ppm.Also the gap between the small electrode and the surrounding guard causes a deviation: 资料内容仅供您学习参考，如有不当或者侵权，请联系改正或者删除。 δ with s the width of the gap. This deviation is negligible compared to (l), when the gap width is less than 1/3 of the distance between the electrodes. Another cause of errors originates from a possible finite skew angle α between the two electrodes (Fig. 3). Assuming the following conditions: the potentials on the small electrode and the guard electrode are equal to 0 V, the potential on the large electrode is equal to V volt, the guard electrode is large enough, it can be seen that the electric field will be concentric. ? d l/l/
电容触摸屏控制设计外文文献及中文翻译
