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1、A Low-Cost, Smart Capacitive Position SensorAbstractA 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 m over a 1 mm range. Since
2、 the output of the oscillator can directly be connected to a microcontroller, an A/D converter is not needed.I. INTRODUCTIONThis paper describes a novel high-performance, low-cost, capacitive displacement measuring system featuring:1 mm measuring range,1 m accuracy,0.1 s total measuring time.Transla
3、ted to the capacitive domain, the specifications correspond to: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 displaceme
4、nt-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 proces
5、sor 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 s
6、elected and directly read out by a microcontroller. The microcontroller,in return, calculates the displacement and can send this value to a host computer (Fig. 1) or a display or drive an actuator.Electronic CircuitPersonal ComputerActuatorDisplayFig. 1. Block diagram of the systemFig. 2. Perspectiv
7、e and dimensions of the electrode structure. THE ELECTRODE STRUCTURE 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 electr
8、odes 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 . According to Heerens 3, the relative deviation in the capacitance Cx between the two ele
9、ctrodes caused by the finite guard electrode size is smaller than: e-(x/d) (1)where x is the width of the guard and d the distance between the electrodes. This deviation introduces a nonlinearity.Therefore we require that is less than 100 ppm.Also the gap between the small electrode and the surround
10、ing guard causes a deviation: e-(d/s) (2)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 between the two electrodes (Fig. 3). Ass
11、uming 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.dl/2l/2Fig. 3. Electrodes with angle .To
12、 keep the calculations simple, we will assume the electrodes to be infinitely large in one direction. Now the problem is a two-dimensional one that can be solved by using polar-coordinates (r, ). In this case the electrical field can be described by: (3)To calculate the charge on the small electrode
13、, we set to 0 and integrate over r: (4)with Bl the left border of the small electrode: (5)and Br the right border: (6)Solving (4) results in: (7)For small s this can be approximated by: (8)It appears to be desirable to choose l smaller than d, so the error will depend only on the angle . In our case
14、, a change in the angle of 0.6will cause an error less than 100 ppm.With a proper design the parameters o and l are constant,and then the capacitance between the two electrodes will depend only on the distance d between the electrodes.ELIMINATION OF PARASITIC CAPACITANCESBesides the desired sensor c
15、apacitance C, there are also many parasitic capacitances in the actual structure (Fig.2). These capacitances can be modeled as shown in Fig.4. Here Cpl represents the parasitic capacitances from the electrode E1 and Cp2 from the electrode E2 to the guard electrodes and the shielding. Parasitic capacitance Cp3 results from imperfect shielding and forms an offset capacitance. Wh
