Study On Capacitance Tracking Focus Of Laser Cutting System

It is necessary to control the focus position with the tracking focus device for controlling the shape and coarseness of the incision in laser cutting applications. The tracking focus device commonly consists of such five parts, the displacement sensor, the measuring circuit, the A/D & D/A, the central controller, the servo electromotor. It can be divided into two kinds by the type of the displacement sensor ordinarily. One is capacitive, the other is inductive. The former will be studied in this thesis. The technical indexs of the tracking focus device primarily include the position precision, the repetition, the sensitivity, the linearity, the resolving power, the hysteresis, the excursion, etc. These indexs are tied up with the sensor and the measuring circuit closely. Therefore, the performance of the tracking focus device is affected deeply by the stability of the sensor structure and the precision of the measuring circuit. The factors which impact the stability of the sensor structure mainly include the edge effect, the environmental temperature, the leaking resistance, etc. The measures taken by plating a thin metal layer on the silicon or the porcelain, which has a small temperature coefficient and nicer insulation, and adopting the protecting ring can eliminate the effect on the stability of the sensor structure by above factors. Limited by the operating area of the sensing electropolar plate and the operating span between the two electropolar plate, the capacitance of the sensor is very small. Consequently, the variational quantity of the capacitance is too tiny to be measured. The measuring circuit of low capacitance by charging & discharging can solve the technical problem. When the variational quantity of the capacitance reaches 0.01pF, the output voltage will reach about 15mV. The measuring circuit consists of such 7 parts, the regulated power supply, the signal generator, the driver by operating & applifying, the referenced capacitance & the sensing capacitance, the signal transformer, the filter and the differential amplifier. The sensor of the tracking focus device in the traditional applications only has a single sensing electropolar plate ordinarily. For this reason, none but sampling repeatedly can increase the position precision at a cost of victimizing the cutting velocity. Additionally, this way can not eliminate the negative impact on the sensor operation caused by the ejectamenta. Adopting a new technology named sensing synchronously with multiple channels, which is of deviding the single sensing electropolar plate into several cells symmetrically, can both increase the position precision without decreasing the processing velocity and eliminate the system error caused by the ejectamenta. The measures taken by making use of the accessorial gas to blow plasma off and enlarging the inner ring's diameter of the sensing electropolar plate can eliminate or lessen the negative impact on the sensor operation caused by the laser plasma.