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    2021-10-21 17:39:00
    Introduction to Servo System


    Topics covered in this article:

    What is a servo system?

    Robot and servo system

    The structure of the servo system

    Features and functions of the servo system

    Types of servo system

    Actuator components of the servo system

    What is robot servo system?

    The development trend of servo system


    What is a servo system?

    Servo system is a product developed on the basis of frequency conversion technology. It is an automatic control system that takes a mechanical position or angle as the control object. In addition to speed and torque control, the servo system can also perform precise, fast, and stable position control.

    The generalized servo system is a control system that accurately tracks or reproduces a given process, and can also be called a follow-up system.

    The narrow sense servo system is called the position follow-up system. Its controlled quantity (output quantity) is the linear or angular displacement of the space position of the load machine. When the position given quantity (input quantity) changes arbitrarily, the main task of the system is to make the output quickly and accurately reproduce the change of a given amount.

    Robot and servo system

    If we talk about the current hot technologies, the robot is definitely one of them. As typical mechatronics and technology-intensive product, how does the robot operate?

    Robot control is divided into two categories: mechanical body control and servo mechanism control. The servo control system is an important part of realizing robot mechanical body control and servo mechanism control. Therefore, to understand the operation process of the robot, the servo system must not be bypassed.

    The structure of the servo system

    servo system.jpg

    servo system

    There are various structures and types of mechatronics servo control systems. From the perspective of automatic control theory, servo control systems generally include five parts: controller, controlled object, execution link, detection link, and comparison link.

    1. Comparison link

    The comparison link is the link that compares the input command signal with the feedback signal of the system to obtain the deviation signal between the output and the input and is usually implemented by a special circuit or computer.

    2. Controller

    The controller is usually a computer or PID (proportional, integral, and derivative) control circuit. Its main task is to transform the deviation signal output by the comparison element to control the actuator to act as required.

    3. Implementation link

    The function of the execution link is to convert various forms of input energy into mechanical energy according to the requirements of the control signal to drive the controlled object to work. The executive components in the mechatronics system generally refer to various motors or hydraulic and pneumatic servo mechanisms.

    4. Controlled object

    The controlled object is a mechanical arm or a mechanical working platform.

    5. Testing link

    The detection link refers to a device that can measure the output and convert it into the dimension required by the comparison link, which generally includes a sensor and a conversion circuit.

    Features and functions of the servo system

    The servo system is essentially different from the feed system of the general machine tool. It can accurately control the motion speed and position of the executing part according to the instruction signal. The servo system is the link between the numerical control device and the machine tool, and an important component of the numerical control system. It has the following features:

    1. It must have a high-precision sensor that can accurately give the electrical signal of the output.

    2. Both the power amplifier and the control system must be reversible.

    3. Enough speed range and strong enough low-speed load performance.

    4. Fast response-ability and strong anti-interference ability.

    Types of servo system

    1. According to the control principle: there are three types of open-loop, closed-loop, and semi-closed loop.

    2. According to the nature of the controlled quantity: there are servo system forms such as displacement, speed, force, and moment.

    3. According to the drive mode: there are electrical, hydraulic, and pneumatic servo drive forms.

    4. According to the actuator: there are stepper motor servo, DC motor servo, and AC motor servo form.

    servo motor.jpg

    servo motor

    Actuator components of the servo system

    1. Types and characteristics of actuators

    (1) Electric actuator

    Electrical actuators include direct current (DC) servo motors, alternating current (AC) servo motors, stepper motors, and electromagnets, etc., which are the most commonly used actuators.

    (2) Hydraulic actuator

    Hydraulic actuators mainly include reciprocating cylinders, rotary cylinders, hydraulic motors, etc., of which cylinders are the most common. In the case of the same output power, hydraulic components have the characteristics of lightweight and good speed.

    (3) Pneumatic actuator

    Pneumatic actuators are no different from hydraulic actuators except for using compressed air as the working medium. Although the pneumatic drive can obtain greater driving force, stroke and speed, it cannot be used in occasions that require high positioning accuracy due to poor air viscosity and compressibility.

    2. Commonly used control motors

    The control motor is the power component of the electrical servo control system. It is an energy conversion device that converts electrical energy into mechanical energy. Commonly used control motors in mechatronics products refer to servo motors that can provide correct or more complex actions.

    Control motors include rotary and linear drive motors. Through the control of voltage, current, frequency (including command pulse), etc., constant speed, variable speed drive, or repeated start and stop the incremental drive and complex drive are realized, and the drive accuracy varies with the drive object.

    (1) Servo drive motors generally refer to Stepping Motor, DC Servo Motor, AC Servo Motor.

    (2) The control methods of commonly used servo-controlled motors mainly include open-loop control, semi-closed-loop control, and closed-loop control.

    The drive system of the closed-loop system has a position (or speed) feedback links. The open-loop system has no position and speed feedback links.

    What is robot servo system?

    Under normal circumstances, what we call a robot servo system refers to the precision servo system used in multi-axis motion control. A multi-axis motion control system is composed of a high-level motion controller and a low-level servo driver. The motion controller is responsible for the decoding of motion control commands, the relative movement of each position control axis, acceleration and deceleration contour control, etc. The function is to reduce the path error of the overall system motion control. The servo drive is responsible for the position control of the servo motor, and its main function is to reduce the following error of the servo axis.

    The robot's servo system consists of three parts: servo motor, servo driver, and command mechanism. The servo motor is the actuator, which realizes movement. The servo driver is the power supply of the servo motor. The command mechanism is for pulse or speed. It works normally with the servo drive.

    robot.jpg

    robot

    Robots have higher requirements for servo motors than the other two parts. First of all, the servo motor is required to have a fast response. The time for the motor from obtaining the command signal to completing the working state required by the command should be short. The shorter the response time to the command signal, the higher the sensitivity of the electrical servo system and the better the fast-response performance. Generally, the size of the electromechanical time constant of the servo motor is used to illustrate the fast response performance of the servo motor. Secondly, the starting torque inertia ratio of the servo motor should be large. In the case of driving a load, the servo motor of the robot is required to have a large starting torque and a small moment of inertia. Finally, the servo motor must have the continuity and linearity of the control characteristics. With the change of the control signal, the speed of the motor can continuously change, and sometimes the speed needs to be proportional or approximately proportional to the control signal.

    Of course, in order to match the body shape of the robot, the servo motor must be small in size, small in mass, and short in axial size. It also should withstand harsh operating conditions and can carry out very frequent forward and reverse and acceleration and deceleration operations.

    A servo driver is an actuator that can directly or indirectly drive the robot body to obtain various motions of the robot by using the torque and force generated by various motors. It has the advantages of a high torque-to-inertia ratio, no brushes, and reversing sparks. It is widely used in robots.

    The development trend of servo system

    As the application of servo systems becomes wider, users have higher requirements for servo drive technology. In general, the development trend of the servo system can be summarized as the following aspects:

    Integration: More and more new power semiconductor devices with high switching frequency are used in the output devices of the servo control system. This device integrates functions into a small module to form a high-precision full closed-loop adjustment system. These functions include input isolation, energy consumption braking, over-temperature, over-voltage, over-current protection and fault diagnosis, etc. The high degree of integration has significantly reduced the volume of the entire control system.

    Intelligence: The intelligence of the servo system is manifested in the following aspects: all operating parameters of the system can be set by software through man-machine dialogue. They all have fault self-diagnosis and analysis functions and parameter self-tuning functions, etc. The servo unit with a self-tuning function can automatically tune the parameters of the system through several trial runs and automatically realize its optimization.

    Networking: Servo system networking is an inevitable trend in the development of integrated automation technology. It is a product of the combination of control technology, computer technology, and communication technology.

    Simplification: The "simplification" mentioned here is not simple but simplification. According to the user's situation, the servo function used by the user is strengthened to make it specialized and refined, and some functions that are not used are streamlined, thereby reducing the servo system cost, creating more benefits for customers.

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