| The key to laser cutting technology is the detection and control of the cutting head height,which generally requires the laser focal point to be located 1/3 of the thickness below the workpiece surface.When the cutting head moves horizontally,it often needs to move up and down and quickly reach the specified position,while keeping the nozzle and workpiece spacing unchanged,in addition,the workpiece surface will be undulating and uneven,making the focal point position deviate,so the cutting head needs to be detected quickly and accurately height.Using the capacitance between the nozzle and the workpiece for height detection is a noncontact ranging method with the advantages of small size and easy installation,but the plate spacing in the measuring range of the capacitance sensor is very small,and the electric capacity is within a few tens of picofarads,and there is plasma,parasitic capacitance and other interference leading to measurement errors,making capacitance ranging more difficult.In this regard,this paper designs a capacitive laser cutting head height adjustment controller,and improves the response speed and control accuracy of the height adjustment controller by designing high-frequency oscillation circuits and motor control algorithms.The main aspects of research in this paper are as follows:(1)Analyze the ranging principle of capacitive laser cutting head,calculate the electric capacity of 1.4pf～71.2pf when the nozzle is 0.1mm～5mm from the workpiece,consider the influence of parasitic capacitance,give the equivalent model of the sensor,and lay the theoretical foundation for the signal conditioning circuit design.(2)Design the hardware platform of the heightening controller,choosing servo,screw slide,nozzle,steel plate and controller to build.Among them,the controller circuit design includes capacitive signal measurement circuit and control circuit with STM32F405 as the core.Adopting FM type measurement circuit,the nozzle is connected to the capacitor three-point oscillation circuit,which outputs a sine wave with a frequency of about 3MHz,and after amplification and waveform conversion,a square wave with stable frequency,large FM range,fast response and high resolution is output,which is connected to the controller through the IO port for signal processing.(3)Study the automatic height adjustment method of cutting head.The controller uses the external pulse counting method to measure the frequency,and demodulates the frequency into distance value according to the fitting curve and analyzes the measurement range.The motor control is divided into three segments according to the measurement range: rapid descent,automatic following,and emergency stop.When it is in the automatic following state,the relative error between the measured distance value and the set distance value is used as the target value,the encoder feedback motor position is used as the following value,and the PID algorithm is selected to realize the height following of the cutting head to the workpiece.When it is in the fast descending state,it descends 0.2mm each time until the nozzle is close to the automatic following interval.There are three cases of motor emergency stop: the nozzle touches the workpiece,the distance from the nozzle to the workpiece is less than the measurement range,and the end key is pressed.Finally,this paper tests two aspects of capacitive sensor performance index and height following error respectively.After testing,when the control accuracy of the height controller is within 0.1mm,the capacitance sensor range is 0.1mm～3.3mm. |
