Research Of Small Capacitance Measuring System In Laser Cutting Capacitance Z Floating Device

The laser cutting capacitive Z floating tracking focus device is used for real-time detect surface height of unit in the laser cutting process, then the cutting head can make appropriate adjustments according to the undulating surface of unit to ensure the focus is always in ideal location and maintain the defocus constant in cutting process, so as to control the incision shape to improve the roughness of cut surface. At present, foreign countries have developed a laser cutting head with auto-focus system, however the price is expensive.So it is of great significance to develop a high cost-effective laser cutting equipment with Z floating detection device to satisfy the Urgent request about relevant technology in the laser industry and huge advantage in mid-grade and low-grade market of domestic equipment.The Z floating tracking focus device consists of such four parts, the displacement sensor, the measuring circuit, the central controller, the servo electromotor. The main technical indicator to measure the performance of Z floating depends on accuracy, linearity, sensitivity, repeatability, resolution, hysteresis, and drift, which is related to the structural design of the sensor and the condition of tiny capacitance detection signal circuit. So the stability of sensor structure and the accuracy of signal detection circuit has a large effect on the performance of Z floating. The issue about research on capacitance sensor structure on laser cutting Z floating and tiny capacitance measuring system is as follows:First, it can be analysed by the working principle of Z floating parallel plate capacitance sensor and capacitance sensor characteristics synthesized considered when on the high and low frequency status respectively, then 2MHz excitation signal frequency and the charge-discharge detection circuit network can be resolved.Secondly, we analysis the various factors that affect the capacitance sensor performance in actual environment when the laser cutting work, such as Z floating edge effects of capacitance sensor, parasitic capacitance, materials'insulation and the plasma produced during cutting and residual. we abandon the traditional design of which the material of cutting head is copper totally and find out solution to eliminate or minimize various effects by structure design, finally designed "three-layer shied " structure: the outer shield, the guard ring and the sensor sensing plate.Finally, we design the whole circuit of tiny capacitance system including parameter and device selection, circuit modeling, simulation and hardware set up, after a circuit debugging and system calibration, we respectively experiment with the sensor modules combination of equivalent capacitance and the Z floating sensor module, analyse all kinds of interference and error factors and assess the small capacitance detection system performance.Experiments show that the Z floating tiny capacitance detection system can satisfy design requirements of Z floating tracking focus device .