CNC system servo motor control

In recent years, servo motor control technology is developing in three directions: AC, digital and intelligent. As the actuator of CNC machine tools, servo systems integrate power electronic devices, control, drive and protection. With the progress of digital pulse width modulation technology, special motor material technology, microelectronics technology and modern control technology, it has experienced a development process from stepping to DC and then to AC. This article briefly discusses its technical status and development trends.

1. CNC machine tool servo system

(I) Open-loop servo system. The open-loop servo system does not have a detection feedback device and does not constitute a motion feedback control loop. The motor works according to the command pulse issued by the CNC device. There is no detection feedback and processing correction process for the motion error. The stepper motor is used as the driving device. The position accuracy of the machine tool depends entirely on the step angle accuracy of the stepper motor and the transmission accuracy of the mechanical part, which is difficult to achieve relatively high precision requirements. The speed of the stepper motor cannot be very high, and the speed of the moving parts is limited. However, the stepper motor has a simple structure, high reliability, low cost, and its control circuit is also simple. Therefore, the open-loop control system is mostly used for economical CNC machine tools with low precision and speed requirements.

(ii) Fully closed-loop servo system. The closed-loop servo system is mainly composed of a comparison link, a servo drive amplifier, a feed servo motor, a mechanical transmission device, and a linear displacement measuring device. It has the function of detecting and feedback-correcting the movement of the moving parts of the machine tool, and uses a DC servo motor or an AC servo motor as a driving component. A grating or inductive synchronizer directly mounted on the workbench can be used as a position detection device to form a high-precision fully closed-loop position control system. The linear displacement detector of the system is installed on the moving part, and its accuracy mainly depends on the accuracy and sensitivity of the displacement detection device, and the processing accuracy it produces is relatively high. However, various nonlinear factors such as the stiffness, friction damping characteristics, and reverse clearance of the mechanical transmission device have a great influence on the stability of the system, making the installation and debugging of the closed-loop feed servo system more complicated. Therefore, it is only used on high-precision and large CNC machine tools.

(iii) Semi-closed-loop servo system. The working principle of the semi-closed loop servo system is the same as that of the full closed loop servo system. It also uses a servo motor as a driving component. The pulse encoder, brushless rotary transformer or tachometer generator installed in the motor can be used as a position/speed detection device to form a semi-closed loop position control system. The feedback signal of the system is taken from the motor shaft or screw. The mechanical transmission device in the feed system is outside the feedback loop. Its stiffness and other nonlinear factors have no effect on the stability of the system, and installation and debugging are relatively convenient. The positioning accuracy of the machine tool is related to the accuracy of the mechanical transmission device. The CNC device has functions such as pitch error compensation and clearance compensation. When the accuracy of the transmission device is not too high, the compensation function can be used to improve the processing accuracy to a satisfactory level. Therefore, the semi-closed loop servo system is widely used in CNC machine tools.

2. Superior control performance of servo motor

(I) Good low-frequency characteristics. Stepper motors are prone to low-frequency vibration at low speeds. AC servo motors will not have this phenomenon and run very smoothly. The AC servo system has a resonance suppression function that can cover the lack of mechanical rigidity, and the system has a frequency analysis function that can detect the resonance point of the machine, which is convenient for system adjustment.

(ii) High control accuracy. The control accuracy of the AC servo motor is guaranteed by the rotary encoder at the rear end of the motor shaft. For example, for Panasonic's fully digital AC servo motor with a 17-bit encoder, the driver receives 217=131072 pulses for each motor rotation, that is, its pulse equivalent is 360°/131072=9.89 seconds. It is 1/655 of the pulse equivalent of a stepper motor with a step angle of 1.8°.

(iii) Strong overload capacity. Stepper motors do not have overload capacity. In order to overcome the inertia moment of the inertial load at the moment of starting, it is necessary to select a motor with a rated torque much larger than the load torque, resulting in a waste of torque. AC servo motors have a strong overload capacity. For example, the maximum torque of the servo motor in the Panasonic AC servo system is three times the rated torque, which can be used to overcome the inertia moment at the moment of starting.

(IV) Fast speed response. It takes 200 to 400 milliseconds for a stepper motor to accelerate from standstill to rated speed. The speed response of AC servo system is fast. For example, Panasonic MSMA 400W AC servo motor only takes a few milliseconds to accelerate from standstill to its rated speed.

(V) Good torque-frequency characteristics. The output torque of stepper motor decreases with the increase of speed, and the torque will drop sharply at higher speed, so its maximum operating speed is generally 300 to 600RPM. AC servo motor has constant torque output, that is, it can output rated torque within its rated speed (generally 2000RPM or 3000RPM).

III. Outlook of servo motor control

(I) The development of servo motor control technology promotes the high speed and high precision of processing technology. Since the 1980s, CNC systems have gradually used servo motors as drive devices. AC servo motors have a brushless structure, which requires almost no maintenance and is relatively small in size, which is conducive to the improvement of speed and power. At present, AC servo systems have replaced DC servo systems to a large extent. In contemporary CNC systems, the development trend of servo technology has been that AC servo replaces DC servo and software control replaces hardware control. This has led to the development of AC digital drive systems used in servo feed and spindle devices of CNC machine tools. With the development of microprocessors and fully digital AC servo systems, the computing speed of CNC systems has been greatly improved and the sampling time has been greatly reduced. After hardware servo control has been changed to software servo control, the performance of the servo system has been greatly improved. For example, the servo control loop of the OSP-U10/U100 network CNC system is a high-performance servo control network, which realizes decentralized configuration and network connection for each servo device and component that performs autonomous control, further exerting its control ability and communication speed for machine tools. The development of these technologies has improved the performance of servo systems, increased reliability, facilitated debugging, and enhanced flexibility, greatly promoting the development of high-precision and high-speed machining technology.

In addition, the development of advanced sensor detection technology has also greatly improved the dynamic response performance and positioning accuracy of AC motor speed control systems. AC servo motor speed control systems generally use brushless rotary transformers, hybrid photoelectric encoders and absolute encoders as position and speed sensors, and their sensors have a response time of less than 1μs. The servo motor itself is also developing in the direction of high speed. With the above-mentioned high-speed encoder, it can achieve a fast feed of 60m/min or even 100m/min and an acceleration of 1g. In order to ensure that the motor rotates more smoothly at high speed, the magnetic circuit design of the motor has been improved, and with the high-speed digital servo software, it can ensure that the motor appears smooth and without creeping even when it rotates less than 1μm.

(II) The direct drive feed technology of AC linear servo motor has matured. There are two types of feed drives for CNC machine tools: "rotary servo motor + precision high-speed ball screw" and "linear motor direct drive". The traditional ball screw process is mature and has high processing accuracy. The cost of achieving high speed is relatively low, so it is currently widely used. The maximum moving speed of high-speed machining machine tools driven by ball screws is 90m/min and the acceleration is 1.5g. However, the ball screw is a mechanical transmission. There are elastic deformation, friction and reverse clearance between mechanical components, which will cause motion lag and nonlinear errors accordingly. Therefore, it is difficult to further improve the moving speed and acceleration of the ball screw pair. Since the 1990s, linear motor direct drive feed drive has been applied in high-speed and high-precision large-scale processing machine tools. It has the advantages of higher rigidity, wider speed range, better acceleration characteristics, smaller motion inertia, better dynamic response performance, smoother operation, and higher position accuracy than ball screw drive. Moreover, linear motor direct drive does not require intermediate mechanical transmission, which reduces mechanical wear and transmission error and reduces maintenance work. Compared with ball screw drive, linear motor direct drive has a 30-fold increase in speed, a 10-fold increase in acceleration, a maximum of 10g, a 7-fold increase in rigidity, and a maximum response frequency of 100Hz, and there is still a lot of room for development. At present, in the field of high-speed and high-precision processing machine tools, the two drive modes will coexist for quite a long time, but from the development trend, the proportion of linear motor drive will become larger and larger. Various signs indicate that the application of linear motor drive in high-speed and high-precision processing machine tools has entered a period of accelerated growth.