The Open laser focus position-control system in Laser Engraver

The control accuracy of the laser cutting focus position automatic tracking system directly affects the quality of laser cutting processing.

The control system consisting of an inductive LVDT sensor or a capacitive sensor and a single-chip microcomputer has disadvantages such as low anti-interference ability, poor dynamic response quality, and poor openness. This paper introduces a control system composed of digital optical encoder and motion controller. It uses a universal motion controller as the control system to improve the control quality, openness, stability and reliability of the system, which can effectively solve the above problems.

Studying the laser cutting focus position automatic tracking system can be considered in two aspects:

(1) How to detect the relative position between the laser focus and the processed object stably, reliably and conveniently

Laser processing is non-contact processing, and the focus position cannot be directly detected, and the focus position is determined by the distance between the focusing mirror and the surface of the processing object. Therefore, a common method is to detect the distance between the focusing mirror and the surface of the processing object, thereby indirectly detecting the relative position of the laser focus and the surface of the processing object.

Commonly the detection methods are divided into contact type and non-contact type:

The contact sensor consists of a mechanical transmission and some linear displacement sensors (usually inductive sensors) that convert the relative displacement of the focusing mirror and the surface of the object to a voltage for use by the control system.

The non-contact sensor is equipped with a capacitive and inductive eddy current sensor on the optical head, which uses the change of the capacitance or inductance of the sensor on the optical head to detect the relative distance between the focusing mirror and the surface of the processing object.

These two detection methods are for different applications. Capacitive non-contact sensors are mainly used in 3D laser metal processing applications because it is not convenient to use contact sensors. In other cases, it is more suitable to use a contact sensor.

However, both sensors are detected by analog signals, and during the laser cutting process, ionization is generated in the processing area to form electromagnetic interference, which has an influence on the detection result. Meanwhile, the response frequency of the inductive LVDT sensor is low. Affecting the dynamic characteristics of the control system, these are the issues that are urgently needed to be solved.

(2) After detecting the change of the laser focus and the position of the processing object, how to quickly compensate for the deviation, that is, the design problem of the position following system.

The usual separate focus tracking system is implemented using the minimum system control stepper motor of the micro-controller. Because the performance of the single-chip microcomputer is relatively simple, it is difficult to implement a more complicated control strategy, and the dynamic characteristics of the ordinary stepping motor are relatively poor, which is difficult to meet the rapid requirements of laser focus tracking.

In order to overcome the above shortcomings, there introduces a laser focus automatic tracking system based on motion controller, which uses optical encoder as displacement sensor and uses the master-slave tracking (electronic gear) function of motion controller to achieve fast compensation of focus position error.

2 control system hardware design

The control system consists of a laser focus position sensor - an optical encoder, a controller - a motion controller and an actuator AC servo system.

The optical code disc is the most widely used displacement sensor in the semi-closed loop numerical control system. Compared with the inductive displacement sensor, it has the advantages of good stability, good dynamic characteristics, strong anti-interference ability, and easy connection with the position controller. However, since the optical encoder is an angular displacement sensor, it is necessary to change the mechanical component to detect the relative displacement of the laser focus and the surface of the work piece. We use the gear rotation associated with the rack drive and the optical encoder to achieve this conversion. The specific structure is not discussed here.

The motion controller is a single-axis motion controller with position master-slave tracking function. In addition to the function of the general motion controller, it also has an automatic tracking function for the position of an optical encoder signal. Automatic tracking of the laser focus position during machining can also achieve various operations during jog adjustment.

Since the motion controller can only output two pulse signals or pulse and direction signals, the drive can only use an all-digital servo system with pulse input.