Servo drives, also known as servo controllers or servo amplifiers, are essential components used to regulate the operation of servo motors. They function similarly to frequency converters used for standard AC motors and form a critical part of the servo system. These devices are primarily utilized in high-precision positioning systems where accuracy and responsiveness are crucial. Servo motors can be controlled through three main modes: position, speed, and torque, enabling highly accurate motion control. As an advanced technology in the field of transmission, servo drives have become a key element in modern industrial automation.
The working principle of modern servo drives relies on digital signal processors (DSPs) as their core control units. This allows for more complex control algorithms, leading to greater levels of digitalization, networking, and intelligence. The power stage typically uses intelligent power modules (IPMs), which integrate driving circuits and include built-in fault detection and protection features such as overvoltage, overcurrent, overheating, and undervoltage protection. Additionally, soft-start circuits help reduce stress during startup. The power drive unit first rectifies incoming three-phase or commercial power using a full-bridge rectifier, converting it into DC power. This DC power is then converted back to AC via a three-phase sinusoidal PWM inverter, which powers the permanent magnet synchronous motor. This entire process can be described as AC-DC-AC conversion. The rectifier section commonly uses a three-phase full-bridge uncontrolled rectifier circuit.
With the widespread adoption of servo systems, the use, debugging, and repair of servo drives have become significant technical challenges. Many industrial control service providers are conducting in-depth research to improve performance and reliability. Servo drives play a vital role in modern motion control systems, especially in applications like industrial robots and CNC machining centers. Particularly, servo drives designed for controlling AC permanent magnet synchronous motors are a major focus of research globally. Vector control-based closed-loop algorithms for current, speed, and position are widely used in current AC servo drive designs. The effectiveness of the speed control loop significantly influences the overall performance of the servo system, especially in terms of dynamic response and stability.
Servo drive systems must meet several basic requirements. First, they need a wide speed range to accommodate various operational needs. High positioning accuracy is essential for maintaining product quality. The system must also offer sufficient rigidity and stable speed performance. Fast response times without overshooting are important to ensure efficient operation, especially when starting and stopping. Low-speed operation with high torque and strong overload capacity are also required, allowing the system to handle heavy loads for extended periods. Lastly, high reliability is crucial, as these systems often operate in harsh environments with varying temperatures, humidity, and vibrations.
Motors used in servo systems must run smoothly across all speeds, with minimal torque fluctuations, even at very low speeds. They should also have long-term overload capability to support low-speed, high-torque operations. To enable fast response, the motor should have a small moment of inertia and high stall torque. It must also be capable of handling frequent starts, stops, and reversals without degradation.
In terms of software design, a servo driver's program typically includes a main program, interrupt service routines, and data exchange programs. The main program handles system initialization, interface control signals, and register settings within the DSP. Once initialized, the system enters a waiting state, ready to respond to interrupts that adjust the current and speed loops. Initialization processes involve setting up the DSP core, configuring the current and speed loops, initializing PWM, ADC, QEP, and other key functions. The PWM interrupt routine is responsible for sampling phase currents, calculating rotor orientation angles, and generating control signals. Fault detection and protection mechanisms are also implemented through dedicated interrupt programs. Encoder zero-pulse capture ensures precise feedback for vector control. Data exchange programs manage communication with the host computer, parameter storage, and display functions. Overall, the software architecture is carefully structured to ensure reliable and efficient operation of the servo drive.
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