Servo drives , also known as servo controllers or servo amplifiers, are controllers used to control servo motors. Their function is similar to that of a frequency converter for a regular AC motor. They are part of a servo system and are primarily used in high-precision positioning systems. Generally, they control the servo motor through position, speed, and torque to achieve high-precision positioning of the transmission system . Currently, they represent a high-end product in transmission technology.
Servo drives are used in automated equipment such as industrial robots and CNC machining centers.
In the speed closed loop of a servo drive , the real-time speed measurement accuracy of the motor rotor is crucial for improving the dynamic and static characteristics of the speed loop's speed control. To achieve a balance between measurement accuracy and system cost, incremental photoelectric encoders are generally used as speed sensors, and the commonly used speed measurement method is the M/T speed measurement method.
Although the M/T speed measurement method has a certain measurement accuracy and a wide measurement range, this method has its inherent defects, mainly including: 1) At least one complete encoder pulse must be detected within the speed measurement cycle, which limits the minimum measurable speed;
2) The timer switches of the two control systems used for speed measurement are difficult to keep strictly synchronized, and speed measurement accuracy cannot be guaranteed in measurement situations with large speed variations. Therefore, the traditional speed loop design scheme using this speed measurement method is difficult to improve the speed tracking and control performance of the servo driver .
Working principle
Currently, most mainstream servo drives use digital signal processors (DSPs) as their control core, enabling the implementation of complex control algorithms and achieving digitalization, networking, and intelligence. Power devices generally employ drive circuits designed around intelligent power modules (IPMs). The IPM integrates the drive circuitry and includes fault detection and protection circuits for overvoltage, overcurrent, overheating, and undervoltage. A soft-start circuit is also added to the main circuit to reduce the impact on the driver during startup. The power drive unit first rectifies the input three-phase power or mains power through a three-phase full-bridge rectifier circuit to obtain the corresponding DC power. The rectified three-phase power or mains power is then frequency-converted by a three-phase sinusoidal PWM voltage-type inverter to drive the three-phase permanent magnet synchronous AC servo motor. The entire process of the power drive unit can be simply described as an AC-DC-AC process.
The main topology of the rectifier unit (AC-DC) is a three-phase full-bridge uncontrolled rectifier circuit.
Basic requirements
Requirements of servo feed system
1. Wide speed range
2. High positioning accuracy
3. It has sufficient transmission rigidity and high speed stability.
4. Fast response, no overshoot
In order to ensure productivity and processing quality, in addition to high positioning accuracy, good fast response characteristics are also required. That is, the response to tracking command signals must be fast, because the CNC system requires sufficient acceleration and deceleration when starting and braking to shorten the transition time of the feed system and reduce contour transition error.
5. High torque at low speeds, strong overload capacity
Generally speaking, servo drives have an overload capacity of more than 1.5 times for several minutes or even half an hour, and can be overloaded by 4 to 6 times in a short period of time without damage.
6. High reliability
The feed drive system of CNC machine tools is required to have high reliability, good working stability, strong adaptability to environmental conditions such as temperature, humidity, and vibration, and strong anti-interference ability.
Requirements for motors
1. The motor can run smoothly from the lowest speed to the highest speed with little torque fluctuation. Especially at low speeds such as 0.1 r/min or lower, it still maintains a stable speed without creeping.
2. The motor should have a large and long-term overload capacity to meet the requirements of low speed and high torque. Generally, DC servo motors are required to withstand overloads of 4 to 6 times the rated load for several minutes without damage.
3. To meet the requirements of rapid response, the motor should have a small moment of inertia and a large stall torque, as well as the smallest possible time constant and starting voltage.
4. The motor should be able to withstand frequent starting, braking and reversing.
