Radiotherapy (RT) is one of the main methods of cancer treatment. With the rise and application of computer technology, radiotherapy equipment has developed significantly, and precise dose irradiation of the lesion target area has become the main direction of modern radiotherapy technology development. Conformal radiotherapy is an external irradiation technology adapted to this development. Compared with conventional radiotherapy equipment, conformal radiotherapy equipment adds a multi-leaf collimator—MLC, also known as a multi-leaf grating or multi-leaf aperture. [b]Working principle of MLC[/b] It consists of dozens of pairs of baffles (or blades), as shown in the figure. Each baffle is independently driven by a miniature motor. It is located at the X-ray exit of the accelerator head. Its main principle is to modulate the shape of the beam by using a microcomputer to precisely control multiple blades in the multi-leaf collimator. By utilizing the relative movement of each pair of blades in the MLC, the shape of the beam is made approximately the projection of the lesion target area on the irradiation plane, resulting in an irradiation field with uneven irradiation intensity. Individualized block therapy can be performed for patients with different lesion shapes, or dynamic conformal therapy can be performed through the treatment planning system. That is, as the accelerator gantry rotates, the block bars are continuously opened and closed, so that the shape of the X-ray field is always consistent with the shape of the tumor to be irradiated. This eliminates the randomness and non-repeatability of manual blocks in the past when using accelerators and Cobalt-60 machines for fixed treatment, as well as the drawbacks of the irradiation field not changing with the shape (lesion shape) during dynamic rotational treatment. It effectively protects the surrounding normal tissues and organs, making radiotherapy more rational and scientific, and greatly improving the reliability and safety of tumor cure. The motion control system of the MLC can be used to obtain its technical requirements: ① Displacement accuracy. The displacement accuracy of a servo-driven feed system refers to the degree of conformity between the calculated blade feed displacement and the actual blade displacement. Each time the interpolator or computer interpolation software issues a command, the servo-driven feed system converts it into a corresponding blade displacement. ② The blade movement following the displacement command has a small following error; that is, the blade must form the shape of the irradiation field in a timely manner as the accelerator head rotates. In other words, the servo-driven feed system has a fast speed response. Control system structure selection: Single-chip microcomputer systems are inexpensive, but have slow processing speed, few bits, slow response, and poor network communication capabilities. PLC systems have weak multi-axis linkage processing capabilities, making it difficult to achieve multi-axis trajectory interpolation. Furthermore, high-performance PLC systems are expensive, making selection very difficult. Therefore, neither of the above two solutions meets the requirements of this system. The two-level control system, consisting of a host computer and a multi-axis motion controller, involves the multi-axis motion controller performing functions such as servo calculations, trajectory interpolation, and I/O control, and is capable of controlling more than 8 axes. On the other hand, the host computer can simultaneously receive and process data from tumor image processing. The two systems communicate via an RS-232 serial communication interface, which offers fast processing speed and enables the implementation of complex algorithms. In summary, considering the characteristics of MLC, an open CNC system based on a PC is adopted as its control system, namely a host computer + a multi-axis motion controller (PMAC). The main functions of the PMAC in the MLC device are defined as follows: ① Processing the feedback blade position and velocity data, applying corresponding algorithms and principles to control the blade position and velocity; ② Driving and controlling the motors to form the desired blade shape. Therefore, the motion control system of the MLC device can be designed, as shown in Figure 2. After receiving the MLC blade drive file, the host computer completes functions such as setting the blade position/velocity signal, interactive communication with the PMAC multi-axis control card, displaying the position/velocity signal during blade operation, and monitoring the overall system status. The PMAC multi-axis control card is used to perform motion inverse kinematics calculations, collect the blade position/velocity signals detected by the position/velocity sensors, and complete the control calculations for each position servo system. The control command signals output by the PMAC multi-axis control card control the MLC blade movement. Multi-axis motion control cards utilize high-performance microprocessors such as DSPs (Digital Signal Processors) and large-scale programmable devices to achieve multi-axis coordinated control of multiple servo motors. This integrates the underlying software and hardware of motion control, enabling various position and speed control functions required for servo motor control. Parameters can be set and motion and PLC programs written by selecting hardware settings (through options and aids) to achieve specific applications. Communication between the PMAC CPU and the axes is achieved through specially designed gate arrays (ICS, i.e., DSPGATE). Each ICS can control four analog output channels, four encoders as inputs, and four analog drive inputs from accessories. A board can simultaneously use one to four of these gate arrays to determine the hardware configuration, the number and type of inputs and outputs. Up to 16 PMAC boards can be cascaded synchronously for shared use, controlling a total of up to 128 axes. It features flag signal inputs and a general-purpose I/O interface. In the MLC device, three PMAC cards were used to implement closed-loop PID position servo control of 20 blades. Each board can expand up to 8 analog-to-digital conversion channels, serving as the interface between the PMAC control card and the position/speed sensor to realize the feedback of position/speed detection signals. This article only takes the zero-position calibration in the MLC blade initialization module as an example to introduce its implementation process. A sensor is installed at the rear end of each blade (as shown in Figure 1). Before the irradiation begins, each pair of blades should be in a closed state. Therefore, each pair of blades is first opened to its maximum displacement, and then each pair of blades is controlled to move from the maximum displacement to the closed state. When the blades are closed, the sensor is activated, and at this time the sensor will generate a pulse. The PMAC can use this pulse to calibrate the zero position. Taking one pair of blades as an example, the specific implementation method is that during calibration, blade 1 moves to the left, and blade 1-1 moves to the right until they hit their respective left and right limits. Then, blade 1 and blade 1-1 perform a zero-return movement. The program flowchart is shown in Figure 3. [b]The implementation process of part of the program is as follows:[/b] First, a timer is set, and its internal value is set to E The nabled attribute is true, and an integer variable s with an initial value of is set. Conclusion This paper applies the motion control theory of CNC system to the precise motion control of multi-leaf collimator in conformal radiotherapy, and improves its motion control method. It makes full use of the characteristics of PMAC multi-axis motion control card, which can control multiple MLC blades, enhance the similarity between the irradiation field and the lesion contour projection, and meet the position control accuracy, so that the MLC blades can accurately reach the corresponding position according to the shape of the lesion, thus improving the reliability and safety of tumor radiotherapy. References Mei Weidong , Liu Jin. New technology for radiotherapy - multi-leaf collimator. Medical Equipment, ; (1): 3 Li Guangming, Li Shuxiang, Lü Qingwen, et al. Analysis of the optimal position of the blades of multi-leaf collimator. Chinese Journal of Medical Physics, 2001; 18 (1): 5 Yang Peng, Liang Lihua, Song Jiguang, et al. Based on PMA Design of a 6-DOF ship motion simulator control system. Manufacturing Informatization, 2006; 28(11): 51-51. —Wang Zhenglin. Process Control and Simulink Applications. Beijing: Electronic Industry Press. Click here to download materials: Research on motion control system of multi-leaf collimator device based on PMAC. Editor: He Shiping