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    Servo System Types and Functions

    Nowadays, with the continuous development of technology, AC servo motor technology gradually replaced the DC motor as the leading executive motor with its excellent cost performance. The mature technology of AC servo system also makes the market develop rapidly and become one of the supporting technologies of industrial automation.

    Main functions

    1. Control of high power load with small power command signal.
    2. In the absence of mechanical connection, the input shaft controls the output shaft at the far distance to realize the long distance synchronous drive.
    3. Make the output mechanical displacement accurately track electrical signals, such as recording and indicating instruments.


    • From the nature of the components of the system, there are electrical servo system, hydraulic servo system, electro-hydraulic servo system and electrical-electrical servo system.
    • From the physical properties of the system output, there are speed or acceleration servo system and position servo system.
    • From the characteristics of the components and the signal function in the system, there are analog servo system and digital servo system.
    • According to the structure characteristics of the system, there are single servo system, multi return servo system, open loop servo system and closed loop servo system. The servo system is divided by its driving elements, such as stepping servo system, DC servo motor┬ásystem, AC servo motor┬ásystem.
    • According to the control methods, they are divided into open loop servo system, closed loop servo system, and semi-closed loop servo system . In fact, the numerical control system is divided into open loop, closed loop and semi closed loop 3 types, which are closely related with the three ways of servo system.

    Open loop system
    The open-loop system consists of 3 parts, the drive circuit, the execute component and the machine tool. The frequently-used execute component is the stepping motor. Usually, the stepping motor is the open loop system of the execute component. In this system, stepping motor is used as actuating element if it is driven by high power. The main task of the drive circuit is to convert the instruction pulse into the signal needed to drive the actuator.

    Closed loop system
    The closed loop system consists of 5 parts, the execution unit, the detection unit, the comparison link, the drive circuit and the machine tool. In the closed loop system, the detection element detects the actual position of the moving parts of the machine tool and converts it into electrical signals and feedbacks to the comparison link. The common detecting elements include rotary transformer, inductosyn, grating, magnetic grating and encoder. Usually, the servo system consisting of the testing elements mounted on the screw is called the semi- closed loop system. The servo system composed of the test elements installed on the worktable is called the closed loop system. Due to the existence of the driving error between the guide screw and the operation platform, the accuracy of the semi-closed loop servo system is lower than that of the closed-loop servo system. Comparison of link function is to compare the signal and feedback signal. The difference value controls the execution element and drives the working platform to continue to move until the following error is zero as the following error of the servo system. The closed-loop (semi closed loop) system can be divided into 3 types, pulse comparison servo system, phase comparison servo system and amplitude comparison servo system according to the signal form and the feedback detection mode in the comparing element.
    Because the output signal of the comparing element is relatively weak so it needs to be amplified, and the drive circuit is set up for this purpose.
    The function of the execute component is to convert the displacement signal into mechanical displacement according to the control signal, that is, the following error signal from the comparing element. The commonly used components are DC wide speed regulating motor, and alternating current motor. The execute component is an essential part of the servo system, and the drive circuit varies with the execute component.

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