During the laser cutting process, the distance between the cutting head and the workpiece and the perpendicularity between the nozzle and the workpiece surface are two important factors. It directly affects the processing quality, so in order to improve the cutting quality and reduce the generation of waste products, it is necessary to install some special sensors on the cutting head to ensure that it can produce stable and consistent cutting quality and can increase the process safety.

When the laser cutting head changes the shape of the workpiece and the surface is uneven, the sensor automatically detects the change and automatically adjusts the height according to the change, so that it always keeps the same distance from the workpiece surface, which can process the material faster without continuous supervision (it is also important to see). The displacement sensor used at home and abroad today is basically capacitive, and its structure and shape are compatible with the processing head, It is also equipped with detection signal processing unit.

Displacement measurement is a basic measurement work. According to whether the sensor is in contact with the measured workpiece, the displacement sensor can be divided into contact type and non-contact type. Compared with the contact type sensor, the non-contact type sensor has fast dynamic response speed, low hysteresis error, or even zero while ensuring high resolution.

Non-contact sensor, sometimes called proximity sensor, should be used as a proximity switch, that is, when the detected object is close to a certain distance from the sensitive probe, the switch signal will be given. The current proximity sensor has developed to not only detect the presence of an object, but also give the distance between the object and the sensitive probe, and provide information about the shape and spatial position of the workpiece, Because it can be used to measure the displacement of moving objects, this type of sensor is also called non-contact displacement sensor. The commonly used displacement sensors include magnetostrictive displacement sensor, eddy current displacement sensor, capacitive displacement sensor, inductor displacement sensor, etc.

According to the principle of various sensors, the capacitance sensor has high sensitivity and the following characteristics:

1. Simple structure, strong adaptability, easy manufacturing, easy to ensure high accuracy, can be made into small size sensors to achieve special measurement, can work in high and low temperature, strong radiation, strong magnetic field and other harsh environments, can withstand high pressure, high impact, overload, etc.

2. The dynamic response is good. Because the electrostatic attraction between the plates is very small, the action energy required is very small, and the movable part can be made very small and thin, so its natural frequency is very high, and the dynamic response time is short. It can work at a frequency of several megahertz, especially suitable for dynamic measurement.

3. The larger relative variation is only limited by the linear region, and its value can reach 99% or more, which can ensure the resolution and measurement range of the sensor.

4. Low heat generation and low temperature coefficient. Since the capacitance value of the capacitance sensor is independent of the electrode material, the material with low temperature coefficient can be selected, which can ensure good stability under the stable external temperature.

To sum up, it can be seen that these advantages are very suitable for laser processing. For laser cutting heads, it is appropriate to use capacitive displacement sensors.

Structure analysis of capacitance sensor:

The laser uses displacement capacitance sensor and nozzle body to compound. The sensor is composed of two different metal conical shells sleeved together. The inner and outer shell layers are ceramic insulating media. The shell layer is grounded and insulated from the inner layer. The sensor works as a shield. The lower part of the inner shell layer on the conical side is connected with a circular metal sheet which is cut with the outer layer for insulation. The circular metal sheet and the metal workpiece constitute the two plates of a capacitance sensor, A channel is led out from the inner shell layer and connected to the signal acquisition system. The sensor passes through this channel in turn when it works. The inner metal layer makes the emitter plate (annular metal sheet) charged. The upper end of the whole probe is connected to the laser processing machine. During the work, the laser beam penetrates through the inner metal shell. During the laser cutting process, the change of the distance between the nozzle and the workpiece has a great impact on the cutting quality. If the distance is too small, The huge recoil pressure will act on the lens, causing the lens to be impacted or attached by the slag particles, causing damage to the lens. In addition, the nozzle end is very vulnerable to the erosion of the slag, affecting the roundness of the nozzle hole, and damaging the symmetry of the cutting direction. Too small working space will make the air flow at an over-expanded supersonic speed, and the nozzle gap is prone to generate shock waves. When the spacing becomes small, the shock wave occurrence area will expand towards the outer diameter of the nozzle, The gas pressure will fluctuate, and the non-uniformity of the pressure will make the gas density change, forming a density gradient field. The area covered by the gas density change will increase with the increase of the gas pressure, and its strength will also increase with the increase of the pressure. When the nozzle gap is too large, the gas nozzle will be too far away from the workpiece, and the air pressure in the nozzle gap will decrease, resulting in overheating of the workpiece surface, increasing of the melting zone, and widening of the slit.