Structural composition and design requirements of servo system

A servo system, also known as a follow-up system, is an automatic control system that can track the input command signal to obtain accurate position, speed or force output. Most servo systems have a detection feedback loop, so the servo system is a feedback control system. According to feedback control theory, the servo system needs to continuously detect the changes in the output of the controlled object under various disturbances, compare it with the command value, and use the deviation value of the two to automatically adjust the system to eliminate the deviation so that the output of the controlled object always tracks the input command value.

The servo system controls its actions based on the deviation between the input command value and the output physical quantity. Therefore, the working process of the servo system is a dynamic transition process in which the deviation is constantly generated and eliminated.

Examples of servo control can be found everywhere. For example, when a worker operates a machine tool for processing, he must always observe the progress of the processing process with his eyes, process the feedback information from his eyes through his brain, decide how to operate next, and then drive the workpiece or tool on the workbench by turning the hand wheel to execute the brain's decision, eliminate the deviations in the processing process, and finally process the workpiece that meets the requirements. In this example, the functions of detection, feedback and control are realized by people, while in the servo system, these functions must be realized through sensors, control and information processing devices. For example, in the servo system of CNC machine tools, the position detection sensor, CNC device and servo motor replace the functions of human eyes, brain and hands respectively.

Many mechatronics products (such as CNC machine tools, industrial robots, etc.) need to track and control the output, so the servo system is an important part of mechatronics products, and is often the main body to achieve the purpose and function of certain products. The servo system is inseparable from the comprehensive application of mechanical technology and electronic technology, and its function is achieved through the combination of electromechanics. Therefore, the servo system itself is a typical mechatronics system.

Servo system structure

The structures and types of mechatronic servo control systems are diverse, but 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

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

2. 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 comparison element to control the actuator to act as required.

3. Execution

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 actuators in the mechatronics system generally refer to various motors or hydraulic and pneumatic servo mechanisms.

4. Accused Person

5. Testing

The detection link refers to a device that can measure the output and convert it into the dimension required by the comparison link, generally including sensors and conversion circuits.

Servo system design requirements

1. Stability

The stability of the servo system refers to the ability of the system to return to its original stable state after the disturbance signal on the system disappears, or to reach a new stable operating state under the action of the input command signal.

Stability is a basic requirement and the most basic condition to ensure the normal operation of the servo system. The servo system should be stable within its working range. Its stability mainly depends on the structure of the system and the parameters of its components. It can be controlled by various methods provided by automatic control theory.

2. Accuracy

The accuracy of a servo system refers to the degree to which its output replicates the input command signal.

The errors of each component in the system will affect the accuracy of the system, such as the sensitivity and accuracy of the sensor, the zero drift and dead zone error of the servo amplifier, the backlash and transmission error in the mechanical device, the nonlinear factors of each component, etc. The servo system will show dynamic error, steady-state error and static error. The servo system should achieve a given accuracy under relatively economical conditions.

3. Rapid responsiveness

Rapid responsiveness refers to the ability of the system output to quickly follow changes in the input command signal. It mainly depends on the system's damping ratio and natural frequency. Rapid responsiveness can be improved, but it has an adverse effect on the system's stability and maximum overshoot. Therefore, both should be optimized during system design to make the system's output response speed as fast as possible.

4. Sensitivity

The parameter changes of each component of the system will affect the performance of the system. The system should be less sensitive to these changes, that is, the performance of the system should not be affected by parameter changes. The specific measures are: for open-loop systems, each component should be strictly selected; for closed-loop systems, the selection criteria for components in the output channel can be appropriately relaxed, and each component in the feedback channel must be strictly selected to improve the sensitivity of the system.