I. Servo System
A servo system is an automatic control system that uses the action of a component (such as a joystick) to bring the system to or near a predetermined value, and compares the desired state (the desired value) with the actual state, adjusting the control components according to their difference (sometimes the rate of change of this difference). Looking back at the development of servo systems, from the earliest hydraulic and pneumatic systems to today's electrified systems, servo systems composed of servo motors, feedback devices, and controllers have been around for nearly 50 years.
Today, with the continuous maturation of technology, AC servo motor technology, with its excellent cost performance, is gradually replacing DC motors as the dominant actuator in servo systems. The maturity of AC servo system technology has also led to rapid and diversified market development, making it one of the supporting technologies for industrial automation.
II. Servo System Composition
A servo drive mainly consists of a servo control unit, a power drive unit, and a communication interface unit. The servo control unit includes a position controller, a speed controller, a torque controller, and a current controller.
Servo drives can be broadly divided into two modules: a power board and a control board. The power board is the high-voltage section (high power, high current, low frequency electrical component), comprising two units: an IPM (Integrated Power Module) for driving the motor and a switching power supply unit to provide digital and analog power to the entire system. The control board is the low-voltage section, the core of motor control, and the platform for the core control algorithms of the servo drive technology. The control board outputs pulse width modulation (PWM) or pulse frequency modulation (PFM) signals through corresponding algorithms as drive signals for the drive circuit, thereby changing the inverter's output power to control the AC servo motor. The inverter is a converter that transforms DC power (from batteries or accumulators) into fixed-frequency, fixed-voltage or frequency- and voltage-modulated AC power (typically 220V, 50Hz sine wave).
In servo drive components, IGBTs and DSP chips account for over 50% of the total material cost. IGBTs, or Insulated Gate Bipolar Transistors, are composite, fully controllable, voltage-driven power semiconductor devices composed of bipolar transistors and insulated gate field-effect transistors. They combine the advantages of high input impedance of MOSFETs and low on-state voltage drop of power transistors, and are used in power drive units to assist in the conversion of DC to AC. Foreign companies occupy over 90% of my country's IGBT market, making domestic production of IGBTs in the short term quite difficult.
III. Development Trends of Servo Systems
1. Communication
Servo technology will continue its rapid shift from DC servo systems to AC servo systems. Currently, almost all new products in the international market are AC servo systems. In industrialized countries, the market share of AC servo motors has exceeded 80%. The number of domestic manufacturers producing AC servo motors is also increasing, gradually surpassing the number of manufacturers producing DC servo motors. It is foreseeable that in the near future, except for certain micro-motor applications, AC servo motors will completely replace DC servo motors.
2. Fully digital
Servo control units employing new high-speed microprocessors and dedicated digital signal processors (DSPs) will completely replace those based primarily on analog electronic devices, thus realizing a fully digital servo system. This full digitalization transforms traditional hardware servo control into software servo control, making it possible to apply advanced algorithms from modern control theory (such as optimal control, artificial intelligence, fuzzy control, and neural networks) within the servo system.
3. Employ new power electronic semiconductor devices
Currently, servo control systems increasingly utilize high-frequency power semiconductor devices for their output circuitry, primarily high-power transistors (GTRs), power MOSFETs, and insulated-gate transistors (IGBTs). The application of these advanced devices significantly reduces power consumption in the servo unit's output circuit, improves system response speed, and reduces operating noise. Particularly noteworthy is the emergence of a new type of module in servo control systems that integrates control circuit functions with high-power electronic switching devices, called Intelligent Power Modules (IPMs). These devices integrate input isolation, energy-saving braking, over-temperature, over-voltage, and over-current protection, as well as fault diagnosis, all within a small module. Their input logic levels are fully compatible with TTL signals and can directly interface with microprocessor outputs. Their application significantly simplifies servo unit design and enables the miniaturization and micro-miniaturization of servo systems.
