A feedback control system is used to accurately follow or reproduce a process. In many cases, a servo system specifically refers to a feedback control system where the controlled variable (the output of the system) is mechanical displacement or displacement velocity and acceleration. Its function is to make the output mechanical displacement (or rotation angle) accurately track the input displacement (or rotation angle).
A servo control system is an operating system that automatically controls the mechanical motion of a testing device according to predetermined requirements. In many cases, a servo system specifically refers to a feedback control system where the controlled variable (the system's output) is mechanical displacement, displacement velocity, or acceleration. Its function is to ensure that the output mechanical displacement (or rotation angle) accurately tracks the input displacement (or rotation angle). The structural composition of a servo system is not fundamentally different from other forms of feedback control systems.
Key indicators:
The main indicators for evaluating the performance of a servo control system are system accuracy, stability, response characteristics, and operating frequency, with particular emphasis on bandwidth and accuracy.
Frequency bandwidth, or simply bandwidth, is defined by the system's frequency response characteristics and reflects the tracking speed of the servo system. A larger bandwidth indicates better speed. The bandwidth of a servo system is primarily limited by the inertia of the controlled object and the actuator. Greater inertia results in a narrower bandwidth. Generally, the bandwidth of a servo system is less than 15 Hz, while the bandwidth of a large-scale equipment servo system is below 1-2 Hz. The accuracy of a servo system is mainly determined by the accuracy of the measuring elements used. Therefore, high-precision measuring elements, such as precision potentiometers, synchros, and rotary transformers, must be used in servo systems. Furthermore, additional measures can be taken to improve system accuracy, such as connecting the measuring axis of the measuring element (e.g., a synchro) to the rotating shaft via a reducer, thus amplifying the rotation angle and improving relative measurement accuracy. Servo systems employing this approach are called fine/coarse measurement systems or dual-channel systems. The angle measurement circuit meshing with the rotating shaft via a reducer is called the fine reading channel, while the angle measurement circuit directly taken from the rotating shaft is called the coarse reading channel.
Structural composition:
The structure and types of mechatronic servo control systems are diverse, but from the perspective of automatic control theory, a servo control system generally includes five parts: controller, controlled object, execution link, detection link, and comparison link.
The comparison stage is a stage that compares the input command signal with the system feedback signal to obtain the deviation signal between the output and the input. It is usually implemented by a dedicated circuit or computer.
Controller:
The controller is usually a computer or a PID control circuit. Its main task is to transform and process the deviation signal output by the comparator in order to control the actuator to act as required.
Execution phase:
The function of the actuator is to convert various forms of energy into mechanical energy according to the requirements of the control signal, and drive the controlled object to work. The actuator in a mechatronics system generally refers to various motors or hydraulic, pneumatic servo mechanisms, etc.
There are many ways to classify servo systems, but the three most common methods are as follows:
(1) Classified according to the characteristics of the controlled variable parameter.
(2) Classified by the type of driving element.
Servo control systems can be classified into electromechanical servo systems, hydraulic servo systems (hydraulic control systems), and pneumatic servo systems according to the type of control components used.
(3) Classified by control principle.
Servo systems can be divided into open-loop control servo systems, closed-loop control servo systems, and semi-closed-loop control servo systems.
The four common servo control systems are as follows:
(1) Hydraulic servo control system
A hydraulic servo control system uses an electric motor as its power source and a hydraulic pump to convert mechanical energy into pressure, which then drives hydraulic oil. By controlling various valves to change the flow direction of the hydraulic oil, it drives the hydraulic cylinder to perform movements of different strokes and directions, fulfilling the different action requirements of various equipment. Hydraulic servo control systems are classified into mechanical-hydraulic, electro-hydraulic, and pneumatic-hydraulic systems according to the different methods of obtaining and transmitting deviation signals, with mechanical-hydraulic and electro-hydraulic control systems being the most commonly used. Based on the different controlled physical quantities, hydraulic servo control systems can be divided into position control, speed control, force control, acceleration control, pressure control, and other physical quantity control. Hydraulic control systems can also be divided into throttling control (valve-controlled) and volumetric control (pump-controlled) types. In mechanical equipment, mechanical-hydraulic servo systems and electro-hydraulic servo systems are the main types.
(2) AC servo control system
AC servo control systems include AC servo systems based on asynchronous motors and AC servo systems based on synchronous motors. Besides being characterized by good stability, high speed, and high precision, they possess a range of advantages. Their performance can be measured by factors such as speed range, positioning accuracy, speed stability, dynamic response, and operational stability.
(3) DC servo control system
The working principle of an AC servo control system is based on the law of electromagnetic force. Related to electromagnetic torque are two independent variables: the main magnetic flux and the armature current. These control the excitation current and armature current respectively, facilitating torque and speed control. On the other hand, from a control perspective, a DC servo control system is a single-input, single-output, single-variable control system. Classical control theory is fully applicable to this type of system. Therefore, due to its simple control and excellent speed regulation performance, it once dominated the feed drive of CNC machine tools.
(4) Electro-hydraulic servo control system
It is a feedback control system consisting of an electrical signal processing device and a hydraulic power mechanism. The most common types are electro-hydraulic position servo systems, electro-hydraulic speed control systems, and electro-hydraulic force (or torque) control systems.
These are four commonly used servo systems. Their working principles, performance, and application ranges differ, each with its own characteristics, advantages, and disadvantages. Therefore, when selecting or purchasing, it is necessary to calculate and choose the appropriate product based on the system's requirements, the parameters to be controlled, and the performance to be achieved.
Technical requirements:
1. System accuracy
Servo system accuracy refers to the degree to which the output reproduces the input signal with the required precision. It is expressed in the form of error and can be summarized into three aspects: dynamic error, steady-state error, and static error.
2. Stability
The stability of a servo system refers to its ability to return to its original stable state after the disturbance acting on the system disappears; or its ability to reach a new stable operating state after a new input command is given to the system.
3. Response characteristics
Response characteristics refer to the speed at which the output changes in response to input commands, and determine the efficiency of the system. Response speed is related to many factors, such as the computer's operating speed, the damping of the motion system, and its mass.
4. Operating frequency
Operating frequency typically refers to the range of frequencies of input signals that the system is allowed to operate. When a signal at the operating frequency is input, the system can operate normally according to technical requirements; however, when other frequency signals are input, the system cannot operate normally.
