Development of Double Equipotential Ring Capacitance Sensor in Ultra-Precise Optical Measurement

Measurements based on all sorts of optical methods in mechanical engineering area had been extensively studied, and in these measurements, many sensors, such as capacitance, are often applied to detect the focusing displacement of objective. For improving the edge effect of the plate capacitance, an equipotential ring is applied. Following the Abbe rule, a plate capacitance sensor with double equipotential rings and a central hole is introduced. The axis of the optical system perforates through the hole of the sensor, thus no Abbe error occurs. If the capacitance sensor is manufactured with machine tools, the gap between the equipotential ring and the orbicular electrode can not be narrow enough, meanwhile, the assembly becomes difficulty. In order to get narrow gap as well as thin electrodes, PCB process is tried to make the capacitance electrodes, but the gap between the equipotential ring and the electrode can only be about 0.1mm. When MEMS technique is introduced, the gap can be about 2μm. Although the cost is by far the higher than that made with PCB, the linear error as well as the measurement accuracy can be promoted greatly. After the circuit is designed, test of the capacitance sensor made with MEMS is completed, and the result that linear error is less than 0.05% and the sensitivity is 1mV/nm is presented with displacement of 30μm.