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.
Main function
1. Using low-power command signals to control high-power loads;
2. In the absence of a mechanical connection, the input shaft controls the output shaft located at a remote distance, achieving remote synchronous transmission;
3. To enable the output mechanical displacement to accurately track electrical signals, such as in recording and indicating instruments.
Main categories
From the perspective of the nature of the system components, there are electric servo systems, hydraulic servo systems, electric-hydraulic servo systems, and electric-electric servo systems, etc.
From the perspective of the physical properties of the system output, there are velocity or acceleration servo systems and position servo systems, etc.
Based on the characteristics of the components and the signal interaction features within the system, there are analog servo systems and digital servo systems;
From the perspective of system structure, there are single-loop servo systems, multi-loop servo systems, open-loop servo systems, and closed-loop servo systems.
Servo systems can be classified according to their driving components, such as stepper servo systems, DC motor servo systems, and AC motor servo systems. They can also be classified according to their control methods, such as open-loop servo systems, closed-loop servo systems, and semi-closed-loop servo systems. In fact, CNC systems are also divided into three types: open-loop, closed-loop, and semi-closed-loop, which are related to these three control methods of servo systems.
1. Open-loop system
An open-loop system mainly consists of three parts: a drive circuit, an actuator, and a machine tool. The commonly used actuator is a stepper motor; an open-loop system using a stepper motor as the actuator is usually called a stepper servo system. In such systems, stepper motors are used as actuators for high-power drives. The main task of the drive circuit is to convert command pulses into signals required to drive the actuator.
2. Closed-loop system
A closed-loop system mainly consists of five parts: an actuator, a detection unit, a comparison circuit, a drive circuit, and a machine tool. Its block diagram is shown in Figure 2. In a closed-loop system, the detection element detects the actual position of the moving parts of the machine tool and converts it into an electrical signal, which is then fed back to the comparison circuit. Common detection elements include rotary transformers, inductive synchronizers, optical gratings, magnetic gratings, and encoder disks. A servo system with a detection element mounted on the lead screw is usually called a semi-closed-loop system; a servo system with a detection element mounted on the worktable is called a closed-loop system. Due to the transmission error between the lead screw and the worktable, the accuracy of a semi-closed-loop servo system is lower than that of a closed-loop servo system. The comparison circuit compares the command signal and the feedback signal; the difference between the two is used as the following error of the servo system. This difference is then used by the drive circuit to control the actuator to continue moving the worktable until the following error is zero. Based on the form of the signal entering the comparison circuit and the feedback detection method, closed-loop (semi-closed-loop) systems can be divided into three types: pulse comparison servo systems, phase comparison servo systems, and amplitude comparison servo systems.
Since the signal output by the comparison circuit is relatively weak and insufficient to drive the actuator, it needs to be amplified, and the drive circuit is designed for this purpose.
The function of an actuator is to convert an electrical signal representing displacement into mechanical displacement based on a control signal, specifically a tracking error signal from the comparison circuit. Commonly used actuators include DC wide-range speed-adjustable motors and AC motors. Actuators are an essential part of a servo system, and the drive circuit varies depending on the actuator used.
