A servo system is an actuator that converts numerical control information into machine tool feed motion.
CNC machine tools combine the advantages of traditional automatic machine tools, precision machine tools, and universal machine tools, integrating high efficiency, high precision, and high flexibility. The improvement of CNC machine tool technology primarily relies on improvements in feed and spindle drive characteristics, as well as the expansion of functionality. Therefore, CNC machine tools place high demands on the position control, speed control, servo motors, and mechanical transmission of the feed servo system .
Since different CNC machine tools perform different machining tasks, their requirements for the feed servo system are also different, but they can generally be summarized in the following aspects.
1. CNC machine tools can operate reversibly.
Reversible operation requires the ability to move flexibly in both directions. During machining, the machine tool table is in a random state, and may move forward or backward at any time according to the requirements of the machining trajectory. Simultaneously, it is required that there should be no backlash or loss of motion when changing direction. From an energy perspective, reversible energy conversion should be achieved; that is, during machining, the motor absorbs energy from the power grid and converts it into mechanical energy; during braking, the mechanical inertial energy of the motor should be converted into electrical energy and fed back to the power grid to achieve rapid braking.
2. The CNC machine tool has a wide speed range.
To adapt to different machining conditions, such as variations in the material, type, size, and location of the parts being machined, as well as the type of cutting tools and cooling methods, CNC machine tools require stepless feed rate variation over a wide range. This necessitates servo motors with a wide speed range and excellent speed control characteristics. After mechanical transmission , the range of motor speed variation can be converted into the range of feed rate variation. Currently, the most advanced level allows for continuous adjustment of the feed rate within the range of 0–240 μm/min with a feed pulse equivalent of 1 pm.
For general CNC machine tools, a feed speed range of 0 to 24 m/min is sufficient to meet machining requirements. Typically, at this speed, and when the speed decreases to zero (i.e., when the table stops moving), the motor needs electromagnetic torque to maintain positioning accuracy. Since a position servo system consists of two main parts: a speed control unit and a position control loop, excessively pursuing a speed control system with the same large speed range as a position servo system while also requiring reliable and stable operation is unrealistic. Generally speaking, for a position control system with a feed speed range of 1:20000, with a total open-loop position gain of 20⁻¹, ensuring the speed control unit has a speed range of 1:1000 is sufficient, making the speed control unit circuit both simple and reliable. Of course, in today's advanced experimental systems, the speed control unit's speed range has reached 1:100000.
This requires the servo system to have excellent static and dynamic load characteristics, meaning that the feed rate should remain constant under different load conditions or when cutting conditions change. A system with good rigidity will have minimal impact on speed due to changes in load torque. Typically, the static speed drop should be less than 5% and the dynamic speed drop should be less than 10% when subjected to changes in rated torque.
