1、 Introduction

With the continuous improvement of processing accuracy and efficiency requirements in the manufacturing industry, laser processing technology has been widely used in industrial production due to its advantages of high precision, high speed, and non-contact processing. As a high-precision driving device, DD motor (Direct Drive Motor) has become a key component for achieving high-precision motion control in laser processing equipment due to its characteristics of direct drive, no transmission clearance, and high dynamic response. Closed loop control technology is an important guarantee to ensure that DD motors can accurately execute motion commands and stably output torque during laser processing.

2、 Overview of DD motor and closed-loop control technology

（1） The working principle and characteristics of DD motor

DD motors typically adopt the structure of permanent magnet synchronous motors or AC synchronous motors, where the stator is equipped with permanent magnets and the rotor contains windings or conductors. When a current is applied to the stator, the generated magnetic field interacts with the magnetic field of the permanent magnet, thereby driving the rotor to rotate. Compared with traditional servo motors, the significant feature of DD motors is their direct drive design, which eliminates the need for traditional gear, belt, and other reduction devices. Therefore, they can achieve extremely high speeds, precise positioning, and smooth low-speed operation. In addition, DD motors also have advantages such as high acceleration, high torque, and high rigidity, which can meet the requirements of laser processing equipment for fast response and high-precision motion.

（2） Basic concepts of closed-loop control technology

Closed loop control is a control method that adjusts the system output through a feedback mechanism. In the closed-loop control system of DD motor, the feedback information is read and compared with the predetermined target value to adjust the current in real time to ensure that the motor accurately executes the motion according to the predetermined requirements. The closed-loop control system mainly consists of a controller, sensors, actuators, and controlled objects. Sensors are used to detect the actual output of the controlled object and convert it into an electrical signal for feedback to the controller; The controller calculates the control signal according to a certain control algorithm based on the deviation between the feedback signal and the target value, and outputs it to the actuator; The actuator drives the controlled object to move according to the control signal, so that its output gradually approaches the target value.

3、 Key elements of closed-loop control for DD motors in laser processing equipment

（1） Encoder feedback system

Encoder is a key sensor for implementing position and velocity feedback in closed-loop control systems. In the closed-loop control of DD motors, commonly used encoders include incremental encoders and absolute encoders. The A/B phase pulse signals output by the incremental encoder represent the direction of rotation and displacement, while the Z phase provides a reference point signal. The control system obtains the relative position and speed information of the motor by calculating the number of pulses and frequency. Absolute encoders can directly output the absolute position information of the motor without the need for position counting and reset operations, and have higher reliability and accuracy. In laser processing equipment, the appropriate encoder type can be selected based on the requirements of processing accuracy and cost.

（2） Control algorithm

The control algorithm is the core of a closed-loop control system, which determines the control performance of the system. Common control algorithms include PID control, fuzzy control, neural network control, etc. PID control is a classic control algorithm with advantages such as simple structure, easy implementation, and strong robustness. It calculates the control signal by linearly combining the proportion, integration, and differentiation of errors, enabling the system's output to quickly and accurately track the target value. Fuzzy control is a control method based on fuzzy logic that does not require precise mathematical models and can handle nonlinear, time-varying, and uncertain systems. Neural network control utilizes the self-learning and adaptive capabilities of neural networks to model and control the system, with high control accuracy and robustness.

（3） Drive performance

A driver is an actuator in a closed-loop control system that converts the control signal output by the controller into the driving current of the motor. The performance of the driver directly affects the operational performance and control accuracy of the motor. In the drive of DD motor, it is necessary to have high-precision current control capability, fast dynamic response capability, and complete protection function. High precision current control can ensure stable torque output of the motor and reduce torque fluctuations; The fast dynamic response capability can enable the motor to quickly respond to changes in control signals, improving the control accuracy of the system; A comprehensive protection function can prevent motor overload, overheating and other faults, extending the service life of the motor.

4、 Closed loop control strategy of DD motor in laser processing equipment

（1） Position closed-loop control

Position closed-loop control is the most basic control strategy for DD motors in laser processing equipment. By providing real-time feedback on the actual position information of the motor through an encoder and comparing it with the target position, the controller calculates a control signal based on the position deviation to drive the motor to gradually approach the target position. Position closed-loop control can achieve high-precision positioning and meet the requirements of laser processing equipment for processing positions.

（2） Speed closed-loop control

Speed closed-loop control is a precise control of the motor speed based on position closed-loop control. By feeding back the actual speed information of the motor through the encoder and comparing it with the target speed, the controller adjusts the control signal based on the speed deviation to stabilize the actual speed of the motor near the target speed. Speed closed-loop control can improve the smoothness of motor motion, reduce speed fluctuations, and thus enhance the quality and efficiency of laser processing.

（3） Torque closed-loop control

Torque closed-loop control is precise control of the torque output of DD motors. In some laser processing processes, such as laser cutting, laser welding, etc., a stable torque output from the motor is required to ensure processing quality. By installing a torque sensor on the motor shaft, the actual torque information of the motor is fed back in real time and compared with the target torque. The controller adjusts the control signal based on the torque deviation to stabilize the actual torque of the motor near the target torque. Torque closed-loop control can improve the torque control accuracy of the motor, reduce torque fluctuations, and ensure the stability of the laser processing process.

5、 The specific application of DD motor in laser processing equipment

（1） Laser cutting

In laser cutting equipment, DD motors are used to drive the movement of the cutting head. Through closed-loop control technology, the position and speed of the cutting head can be precisely controlled to achieve high-precision cutting. For example, the strategy of combining position closed-loop control and speed closed-loop control can enable the cutting head to move quickly and accurately according to a predetermined trajectory, ensuring the flatness and perpendicularity of the cutting edge. Meanwhile, torque closed-loop control can ensure that the cutting head outputs stable torque during the cutting process, avoiding cutting quality problems caused by torque fluctuations.

（2） Laser welding

In laser welding equipment, DD motors are used to drive the movement of the welding head and adjust the welding focal length. Through closed-loop control technology, precise position and speed control of the welding joint can be achieved, ensuring accurate matching between the motion trajectory of the welding joint and the weld seam. Meanwhile, torque closed-loop control can ensure stable pressure output during the welding process, ensuring the quality of the welded joint. In addition, real-time adjustment of laser power can be achieved through closed-loop control technology. According to the requirements of welding materials and welding processes, the laser power can be automatically adjusted to improve welding efficiency and quality.

（3） Laser marking

In laser marking equipment, DD motors are used to drive the movement of the marking head. Through closed-loop control technology, the position and motion trajectory of the marking head can be precisely controlled, achieving high-precision marking. For example, the strategy of combining position closed-loop control and speed closed-loop control can enable the marking head to move quickly and accurately according to the predetermined pattern, ensuring the clarity and accuracy of the marking pattern. At the same time, real-time adjustment of laser marking parameters such as laser power and marking speed can be achieved through closed-loop control technology. Marking parameters can be automatically adjusted according to different marking materials and requirements to improve marking efficiency and quality.

6、 Experiment and Analysis

In order to verify the effectiveness of closed-loop control technology for DD motors in laser processing equipment, a series of experimental studies were conducted. The experiment used a laser cutting device based on DD motor and adopted a closed-loop control strategy for position, speed, and torque. Through comparative experiments, the impact of closed-loop control technology on laser cutting accuracy, speed, and stability was analyzed.

The experimental results show that after adopting closed-loop control technology, the positioning accuracy of the laser cutting equipment is significantly improved, and the flatness and perpendicularity of the cutting edge are significantly improved. At the same time, the cutting speed has also been increased, and the speed fluctuations during the cutting process have significantly decreased. In addition, torque closed-loop control effectively reduces torque fluctuations during the cutting process, ensuring the stability of cutting quality.

7、 Conclusion

The closed-loop control technology of DD motor in laser processing equipment is the key to achieving high-precision and high-efficiency laser processing. By adopting a multi-dimensional closed-loop control strategy based on position, speed, and torque, combined with a high-performance encoder feedback system, advanced control algorithms, and drivers, the positioning accuracy, motion smoothness, and processing efficiency of laser processing equipment can be significantly improved. In the future, with the continuous development of control technology and motor technology, the application prospects of DD motors in laser processing equipment will be even broader, which is expected to bring greater impetus to the development of the manufacturing industry.