Introduction to Servo Drivers
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 transmission system positioning. Currently, they represent a high-end product in transmission technology.
Servo drives are an important component of modern motion control and are widely used in automated equipment such as industrial robots and CNC machining centers. Servo drives for controlling AC permanent magnet synchronous motors, in particular, have become a research hotspot both domestically and internationally. Current AC servo drive designs commonly employ a three-loop control algorithm based on vector control, encompassing current, speed, and position. The rationality of the speed closed-loop design within this algorithm plays a crucial role in the overall performance of the servo control system, especially in terms of speed control performance.
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 (Mean Transmission/Turning) method. While the M/T method offers a certain level of measurement accuracy and a relatively wide measurement range, it has inherent drawbacks, primarily 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.
Servo driver principle
Servo drives all use digital signal processors (DSPs) as the control core, which can implement relatively complex control algorithms and achieve digitalization, networking and intelligence. Power devices generally use drive circuits designed with intelligent power modules (IPMs) as the core. The IPM integrates the drive circuit and has fault detection and protection circuits such as overvoltage, overcurrent, overheating and undervoltage. A soft start circuit is also added to the main circuit to reduce the impact on the driver during the startup process.
Servo driver working principle diagram
First, the power drive unit 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 AC servo motor. The entire process of the power drive unit can be simply described as an AC-DC-AC process, with the main topology of the rectifier unit (AC-DC) being a three-phase full-bridge uncontrolled rectifier circuit. Servo drives generally have three control modes: position control, torque control, and speed control. Position control typically determines the rotation speed by the frequency of externally input pulses and the rotation angle by the number of pulses. Some servos can also directly assign speed and displacement values via communication. Because position control allows for very strict control over both speed and position, it is generally used in positioning devices.
Torque control is a method that sets the output torque of the motor shaft by inputting an external analog signal or directly assigning a value to an address. The torque can be changed in real-time by altering the analog signal setting, or by changing the corresponding address value via communication. Its main applications are in winding and unwinding devices with strict requirements on material handling, such as winding machines or fiber optic drawing equipment. The torque setting must be adjusted continuously according to changes in the winding radius to ensure that the stress on the material does not change with the winding radius.
Speed mode can control rotational speed through analog input or pulse frequency. With outer-loop PID control from a higher-level control unit, speed mode can also be used for positioning, but the motor position signal or the position signal from the direct load must be fed back to the higher-level unit for calculation. Position mode also supports direct load outer-loop position signal detection. In this case, the encoder at the motor shaft end only detects the motor speed, and the position signal is provided by the detection device at the direct final load end. This approach reduces errors in the intermediate transmission process and increases the overall positioning accuracy of the system.
Servo driver applications
Servo drives are widely used in injection molding machines, textile machinery, packaging machinery, CNC machine tools, and other fields.
Servo driver selection
1. Choosing a suitable servo driver requires consideration of various aspects, mainly based on system requirements. Before selection, first analyze the following system requirements, such as size, power supply, power, and control method, to set the direction for selection.
2. The types of motors supported by the driver are generally brushed DC, sine wave, trapezoidal wave, etc. Also, the continuous output current of the driver must be greater than the rated current of the motor. Consider whether the driver can handle the motor based on the motor's back electromotive force and maximum speed.
3. Feedback elements and sensors come in a wide variety. The choice of feedback sensor depends on whether a closed-loop system is required, and includes encoders, tachometers, resolvers, etc. If the system has feedback elements, the choice of driver must consider whether the driver supports this type of feedback, the type of feedback, and the form of the feedback signal output.
4. Servo drives have three control modes: torque, speed, and position. The command formats differ depending on the mode. Torque and speed modes are controlled via analog commands, while position mode uses pulse + direction control. There are also bus-based control methods, such as EtherCAT.
5. Accuracy requirements: The accuracy of a system is affected by many factors, and the servo driver is an important one. Generally, servo drivers are divided into digital servo drivers and linear servo amplifiers. Linear amplifiers are suitable for low noise, high bandwidth and distortion-free applications when the current crosses zero.
6. Power supply and operating environment: Power supply options include DC and AC, and sometimes the driver's power requirements must also be considered. Operating environment considerations mainly include temperature effects, operating conditions, and whether a protective shield is needed.
