Differential Capacitance Measurement Method With Continuously Variable Resolution For Capacitance Sensor

Capacitance sensor is a class of widely used sensors. Capacitance detection is an important research field of sensor technology. Most of the capacitive sensor detection methods are concerned with the amount of change in capacitance. With the progress of modern technology, especially the rapid development of integrated technology, capacitive sensor move toward miniaturization. Capacitance relative variation of capacitance sensors get smaller, even less than a million. On the premise of ensuring resolution, to measure so small changes, requirement of the detection circuit is very high. Even using relative method and offsetting the impact of the background capacitance, it still cannot meet the needs of many applications, such as pressure touch detecting sensors or other large dynamic applications. Conventional capacitance measurement methods are restricted by accuracy of AD, range and resolution is usually a contradiction. It is often necessary to require resolution in exchange for the loss of range or require range at the expense of resolution. Although the use of automatic range switching and logarithm quantization can bargain range and resolution in some extent, but they has grate error at the junction of switching range, non-uniform quantization and other issues. In order to achieve a large dynamic, wide-range capacitance measurement difference, while ensuring constant accuracy over the whole range, researching and developing a capacitance difference detection method with continuously variable resolution has great significance. To solve this problem, we do the following work:(1) Based on the conventional capacitive digital converter(CDC), this paper propose a method of measuring the differential charge based on switched capacitor, enabling to measure the differential capacitance with continuously variable resolution. Fatherly, this method achieve an automatic resolution optimization function to any tested capacity, getting the measurement effect that any measured capacitance are always close to full-scale state,and maintain high resolution and lower error.(2) This paper designs the key capacitance difference detecting circuit, and a variety of non-ideal factors affecting measurement performance at the actual application are remodeled. Meanwhile, the symmetry of the excitation source, the non-ideal parameter of OP-AMP, the electronic switch Error leakage and other factors are analyzed to illustrate the mechanism and rules causing errors one by one. On this basis, a comprehensive calibration compensation method is proposed. It can eliminate measurement errors due to non-ideal factors mentioned above.(3) In order to verify the correctness and validity of the detection principle and calibration methods, a wide range capacitive difference detection common circuit module with a continuously variable resolution is been designed. The author completes the circuit design, component selection, software design and final performance test.The test results show that when the sensor capacitance variation span up to 100 times, ?the measuring device with variable resolution for differential capacitance remains relative resolution of 0.03%(equivalent to traditional methods at 80% of full scale). Meanwhile, the error of measuring capacitance is always less than 5%, verifying the effectiveness of the detection system.The method not only to meet the measuring demand in small relative variation change application of capacitive sensor, but also greatly expand the measurement dynamic range of capacitive sensors under the premise of ensuring the accuracy of the touch, and have wide application prospects in the machine/pressure sensation, stress/strain testing, industrial automation, automotive electronics and other fields.