Servo systems are mechatronic systems and should be designed using mechatronic methodologies. There is no single, fixed answer or a unified approach to servo system design. Different requirements necessitate different design methods, resulting in servo systems with varying structures. Even for servo systems with the same requirements, different designers may employ different design methods, leading to different design solutions.
The structural complexity of servo systems determines the complexity of their design process. In practice, designing a servo system is rarely a one-time success; it often requires multiple modifications and debugging sessions to achieve satisfactory results. The following is a brief introduction to the general steps and methods of servo system design.
The structure of a servo system
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.
1. Comparison Section
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.
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 comparator in order to control the actuator to act as required.
3. 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, thereby driving the controlled object to work. In mechatronics systems, actuators generally refer to various motors or hydraulic/pneumatic servo mechanisms, etc.
4. Controlled object
5. Testing process
The detection stage refers to the device that can measure the output and convert it into the dimensions required by the comparison stage. It generally includes sensors and conversion circuits.
Servo System Design Requirements
1. Stability
The stability of a servo system refers to its ability to return to its original stable state after the disturbance signal disappears, or to reach a new stable operating state under the action of input command signals.
Stability is a fundamental requirement and a basic condition for ensuring the normal operation of a servo system.
2. Accuracy
The accuracy of a servo system refers to the degree to which its output accurately reproduces the input command signal.
Errors in any component within a system will affect its accuracy. These include the sensitivity and accuracy of sensors, the zero-point drift and dead-zone error of servo amplifiers, backlash and transmission errors in mechanical devices, and the nonlinear factors of various components. In a servo system, this manifests as dynamic error, steady-state error, and static error. A servo system should achieve a given level of accuracy under relatively economical conditions.
3. Rapid Response
Fast response refers to the ability of a system's output to quickly follow changes in the input command signal. It mainly depends on the system's damping ratio and natural frequency. Fast response 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
Changes in the parameters of each component in a system can affect its performance. The system should be insensitive to these changes, meaning its performance should be unaffected by parameter variations. Specific measures include: for open-loop systems, rigorous selection of each component; for closed-loop systems, the selection criteria for components in the output channel can be appropriately relaxed, while rigorous selection of components in the feedback channel is essential to improve system sensitivity.
Servo System Design Steps and Methods
1. Design requirements analysis and system solution design
First, the design requirements of the servo system are analyzed to clarify its application scenarios and purposes, basic performance indicators and other performance indicators. Then, several technical solutions are proposed based on existing technical conditions. After evaluation and comparison, a more reasonable solution is selected.
The scheme design should include the following: selection of control method; selection of actuator; selection of sensors and their detection devices; selection of mechanical transmission and actuators, etc. Scheme design is the first step in system design. The selection of each component is only preliminary and needs to be further modified and determined in the detailed design stage.
2. System Performance Analysis
Even though the specific structural parameters have not yet been determined after the design is completed, a preliminary analysis of its basic performance should be conducted based on the basic structural form.
First, draw the system block diagram, list the approximate transfer function of the system, and simplify the transfer function and block diagram (generally, it should be simplified to a system of second order or lower). Then, based on this, conduct a preliminary analysis of the system's stability, accuracy, and fast response. The most important of these is the stability analysis. If the design requirements cannot be met, the scheme should be modified or a correction mechanism should be added.
3. Selection of actuators and sensors
The design of the scheme only involves the preliminary selection of the actuators and sensors. This step should be based on the specific speed, load and accuracy requirements to determine the parameters and models of the actuators and sensors.
4. Mechanical System Design
Mechanical system design includes the design of the specific structure and parameters of mechanical transmission mechanisms and actuators. During the design process, attention should be paid to eliminating various transmission clearances, maximizing system rigidity, minimizing inertia and friction, and especially preventing the "creeping" phenomenon when designing the guide rails of the actuators.
5. Control System Design
The control system design includes detailed design of signal processing and amplification circuits, correction devices, servo motor drive circuits, etc. If computer digital control is used, it should also include the design of interface circuits and controller algorithm software. In the control system design, attention should be paid to the selection of parameters for each component and their matching with the mechanical system parameters to ensure the system has sufficient stability margin and fast response, and meets accuracy requirements.
6. System performance review
Once all structural parameters are determined, the precise transfer function of the system can be re-listed. However, actual servo systems are generally high-order systems, so appropriate simplification is necessary before performance review. If the performance is found to be unsatisfactory after review, the parameters of the control system can be adjusted, the algorithm modified, or even redesigned until satisfactory results are achieved.
7. System Testing Experiment
The above design and analysis are still in the theoretical stage. The performance of the actual system needs to be determined through testing experiments. Testing experiments can be conducted on a model experimental system or on a prototype. Through testing experiments, some problems will often be discovered, and measures must be taken to solve them.
8. System design finalized.
After going through the above 7 steps and repeating them multiple times to obtain a satisfactory result, the design scheme can be finalized. Then, the design drawings and technical documents such as the design calculation instructions can be organized and put into formal production.
