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The development of control system for ultra-high speed NC machine tools (I)

high speed NC machining technology is a comprehensive technology, and there are still many problems to be solved in practical application. These problems include: the dynamic and thermal characteristics of high-speed machine tools, the relationship between tool materials, geometry and durability, the tools, fixtures and process parameters of high-speed machine tools, cooling and lubrication, cutting exclusion and safe operation, CNC high-speed and high-precision control systems abroad, etc. Recently, in order to solve the comprehensive technical problems of numerical control machining in the aviation field, the Aviation Industry Corporation has established the aviation science and Technology Key Laboratory of numerical control manufacturing technology (lancmt) in the Beijing Institute of Aeronautical Technology, which lists the high-speed consequence as one of the main research contents

this paper introduces the control system of ultra-high speed CNC machine tools from the aspects of spindle control, servo system, CNC system, etc

1. The vector controlled PWM AC variable frequency controller

motorized spindle is the key component of high-speed CNC machine tools. At present, the highest level motorized spindle products in the world, such as the products of Fisher company in Switzerland, nmax = 40000r/min, P = 40kW; The products of French forest line company orb17, nmax = 40000 r/min, P = 40kW, m = 9.5n m, etc. Ceramic ball bearings, magnetic bearings and aerostatic bearings are mostly used. The high-level motorized spindle needs only 1.5s from static to maximum speed, and the acceleration reaches 1g. These parameters require the spindle controller to have high dynamic quality, precision, reliability and maintainability. Vector controlled PWM AC variable frequency system is the best choice for this control

vector control includes coordinate transformation, vector operation (nonlinear complex operation) and parameter detection. The necessary condition to control the instantaneous value of AC motor is high-speed operation. The 32-bit DSP with special CPU improves the operation speed, and it only needs nanoseconds to execute an instruction, so as to achieve the goal of fast torque response. Another factor of high speed is the use of solid-state drive circuit. The fully digital h/w current control system, the motor speed adaptive identification system and the voltage and current test signals are processed by sampling data to obtain the motor dynamic parameters with high reliability. The performance and specification requirements of the PWM frequency converter with switch control are as follows: vector control is adopted, and there is more than 150% high starting torque at 1Hz; 1GBT intelligent power module is adopted, with high carrier frequency (> 15KHz); Using 32-bit DSP (digital signal processor) and MPU chip, the complex vector operation and PWM control of full signal digital processing are realized by dual CPUs; Fault self diagnosis monitoring and display; Parameter self detection and off-line self setting function; Adaptive speed identification based on neural network; Two speed control modes: constant torque and constant power; Output frequency range: 0.1 ~ 400Hz; The acceleration and deceleration time is equal to 0.1 ~ 300s, etc

2. Rapid response, high positioning accuracy, instantaneous variable structure, real-time control servo system and linear motor drive

ultra high speed machining requires not only very high spindle speed, but also very high feed speed and acceleration. The feed speed is generally greater than 30m/min, and the acceleration reaches 1g. Under the ball screw driving mode, the limit value is about 60m/min and 1g, while the linear motor can reach more than 160m/min and more than 2.5G, and the positioning accuracy can be as high as 0.5 ~ 0.05 M. It adopts a fast, precise, high-speed and durable linear motor, which avoids the shortcomings of insufficient reverse clearance, inertia, friction and stiffness in the ball screw (gear and rack) transmission, realizes the direct drive without contact, and can obtain the unanimously recognized high-precision and high-speed displacement movement (extremely high positioning accuracy and repeated positioning accuracy in high-speed displacement), The excellent stability of the large load friction and wear testing machine (1000KN) was obtained. However, in order to meet the requirements of good mechanical properties at room temperature and high temperature, a servo drive system with high performance and high sensitivity is required

at present, all digital AC drive system has been widely used as a product, which lays a foundation for high sensitivity and variable structure control of servo control. The full closed-loop control of current loop, speed loop and position loop is carried out with a special CPU. The feedforward control is adopted, and the servo tracking prediction is used for forward compensation to reduce the tracking error, speed up the response speed, increase the nonlinear compensation control function, and compensate the error caused by the static friction and viscous resistance of the driving machinery; The robust control theory is used for self-tuning control to overcome the error caused by torque inertia and load change; In order to ensure high positioning accuracy in high-speed motion, magnetic high-resolution absolute position encoder is applied, such as 1million lines per revolution, resolution of 0.01 M and other advanced technologies. In order to achieve the advantages of fast response speed, strong anti-interference ability and high positioning accuracy in high-speed machining, a variable structure servo control mode is used at present. This control method can change the system structure in the process of system transient change, which is determined by the current state of the system. The system is insensitive to the changes of system parameters and external disturbances, and can improve the dynamic characteristics of the system, so that the system can locate or track the given curve quickly and accurately

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