Research On DSP Based Capacitive Displacement Measurement System Using Comb Electrodes

Because the advantage of simple structure, high sensitivity and non-contact detecting, capacitive sensors are widely used in fields of precision measurement for linear, angular and spatial displacement. However, there are also shortages. For example, gap typed sensors involve with small scale and poor linearity. And grating typed sensors shows multi coupling factors in the capacitance variation signal, as well as the lack of direction identification. Hence, the development of a capacitive displacement measurement system features simple structure, good linearity, high accuracy and large scale shows important practical significance.Thus, a novel large-scale displacement measurement strategy is proposed based on the existing grating electrode structure in this research, including a pair of interlocking comb-type emitter electrodes and double-channel electrical orthogonal sensing electrodes. Driving the interlocking emitter electrodes with two high frequency AC voltage sources with equal amplitude and opposite phase, a continuous distributed AC electric field features alternating phase is generated on the surface of the emitter. Hence, through capacitance coupling, the relative displacement between emitter electrodes and double-channel sensing electrodes is converted to a pair of quadrature amplitude modulation signals.Simulation model for the emitter electrodes and the sensing electrodes is firstly established using ANSOFT MAXWELL. Hence the capacitive coupling characteristic is analyzed, as well as the synthesizing of modulation function for single sensing electrode. The inter-channel’s relationship is also investigated for both amplitude and phase angle. And the quasi sine/cosine incremental displacement output pattern is theoretical proved, which indicates that the proposed scheme is capable of displacement interpolation and direction identification. For the interference introduced by air gap fluctuation, a normalize algorithm is carried out in relating to the instantaneous output values of the double sensing channels. Hence the air gap interference can be efficiently reduced during displacement measurement process.The signal processing system for the double-channel sensing electrodes is designed with a high speed digital signal processor (TMS320F28335). In which real-time displacement measurement is achieved by the signal processing of the sensing electrodes, including band-pass filtering, amplifying, amplitude demodulating, data acquiring and software analyzing. Moreover, a test bench is built by employing a precision micro table and a reference displacement sensor. Thus the overall performance of the presented measurement system is experimentally evaluated.With an air gap between 0.2～1.1mm, the displacement resolution ranges from 82.3nm to 4.2nm, the sensitivity ranges from 3.6 mV/μm to 72.6mV/μm, nonlinear is below 2‰, the incremental pulse repeatability is±1μm, the continuous measurement repeatability is better than 2μm, and the effective measurement range is 100mm. Experimental results indicates that the proposed strategy emerges advantages of high repetition accuracy of incremental pulse, convenience of interpolation and direction identification, high suppression capability to air gap fluctuation, which ensures a good prospect in precision displacement measurement applications.