A servo system , also known as a follow-up system, is an automatic control system that can track input command signals to achieve precise position, speed, or force output. Most servo systems have a feedback loop, making them a type of feedback control system. According to feedback control theory, a servo system continuously detects changes in the output of the controlled object under various disturbances, compares this change with the command value, and uses the deviation between the two to automatically adjust the system to eliminate the deviation, ensuring that the output of the controlled object always tracks the input command value.
A servo system controls actions based on the deviation between the input command value and the output physical quantity. Therefore, the operation of a servo system is a dynamic transitional process in which deviations are constantly generated and eliminated.
Examples of servo control are ubiquitous. For instance, when a worker operates a machine tool, they must constantly observe the machining process, process the feedback from their eyes, and decide on the next step. Then, by manually cranking a handwheel, they drive the workpiece or cutting tool on the worktable to execute the brain's decision, eliminating deviations during machining and ultimately producing a workpiece that meets requirements. In this example, detection, feedback, and control functions are performed by the human. However, in a servo system , these functions are achieved through sensors, control, and information processing devices. For example, in a CNC machine tool servo system, position sensors, CNC devices, and servo motors respectively replace the functions of the human eye, brain, and hand.
Many mechatronic products (such as CNC machine tools and industrial robots) require tracking and control of output. Therefore, servo systems are an important component of mechatronic products and often the main body for achieving certain product functions. Servo systems rely on the integrated application of mechanical and electronic technologies; their functions are realized through electromechanical integration. Thus, a servo system itself is a typical mechatronic system.
Classification of servo systems
Classification by Regulation Theory
1. Open-loop servo system
Open-loop servo systems, also known as systems without position feedback, primarily use power stepper motors or hydraulic pulse motors as their drive components. The working principle of these two types of drive components is essentially the conversion of digital pulses into angular displacement. They do not rely on position detection elements for positioning; instead, they depend on the drive device itself. The angle rotated is proportional to the number of command pulses, and the movement speed is determined by the frequency of the feed pulses.
Open-loop servo systems are simple in structure and easy to control, but they have poor accuracy, are unstable at low speeds, and have low torque at high speeds. They are generally used in lightly loaded or economical CNC machine tools.
2. Closed-loop servo system
A closed-loop servo system is an error-controlled follow-up system. The error in a CNC machine tool's feed system is the difference between the position command output by the CNC and the actual position of the machine tool's worktable (or tool post). Since the motion actuators in a closed-loop system cannot reflect the position of the movement, a position detection device is required. This device measures the actual displacement or actual position and feeds the measured value back to the CNC device. This value is compared with the command to calculate the error, thus forming the closed-loop position control.
Because the closed-loop servo system is a feedback control system with a highly accurate feedback measurement device, errors in the system's transmission chain, errors in each component within the loop, and errors caused during motion can all be compensated, thereby greatly improving the following accuracy and positioning accuracy.
3. Semi-closed-loop system
The position detection element is not directly installed on the final moving part of the feed coordinate system, but is instead transferred through an intermediate mechanical transmission component; this is called indirect measurement. In other words, a portion of the coordinate motion transmission chain is outside the position closed loop, and the transmission error outside the loop is not compensated by the system. Therefore, the accuracy of this type of servo system is lower than that of a closed-loop system.
The control structures of semi-closed-loop and closed-loop systems are identical; the difference lies in that the closed-loop system includes more mechanical transmission components, and transmission errors can be compensated for. Theoretically, very high accuracy can be achieved. However, due to the influence of mechanical deformation, temperature changes, vibration, and other factors, system stability is difficult to adjust. Furthermore, after a period of machine tool operation, wear and deformation of mechanical transmission components, as well as changes in other factors, can easily alter system stability and accuracy. Therefore, semi-closed-loop systems are currently more commonly used. Fully closed-loop servo systems are only used on high-precision CNC machine tools with high-density transmission components, stable performance, and minimal temperature variations during operation.
Based on whether DC or AC servo motors are used, or classified according to the drive components used.
1. DC servo system
The servo motors commonly used in DC servo systems include low-inertia DC servo motors and permanent magnet DC servo motors (also known as high-inertia, wide-range DC servo motors). Low-inertia servo motors minimize the armature's rotational inertia, thus achieving the best speed. Low-inertia servo motors are generally designed with high rated speed and low inertia, so in application, they must be connected to the lead screw via intermediate mechanical transmission (such as a gear pair).
2. AC servo system
AC servo systems utilize AC asynchronous servo motors and permanent magnet synchronous servo motors. Due to the inherent inertia of DC servo motors, their application environments are limited. AC servo motors do not have these drawbacks, and their rotor inertia is smaller than that of DC motors, resulting in better dynamic response. Furthermore, under the same volume conditions, the output power of AC motors can be 10% to 70% higher than that of DC motors. Additionally, AC motors can be manufactured with larger capacities than DC motors, achieving higher speeds and voltages.
Classified by feed drive and spindle drive
1. Feed servo system
A feed servo system is a general concept of a servo system, which includes a speed control loop and a position control loop. The feed servo system completes the feed motion of each coordinate axis and has positioning and contour tracking functions.
2. Spindle servo system
Strictly speaking, a typical spindle control system is simply a speed control system. Its main function is to achieve the spindle's rotational motion, provide torque and power during the cutting process, and ensure adjustable speeds, achieving stepless speed changes within a given range. A spindle with C-axis control, like a feed servo system, is a position servo control system in the general sense.
The structure of a servo system
Mechatronics servo control systems come in many structures and types, but from the perspective of automatic control theory, a servo control system generally includes five parts: a controller, a controlled object, an execution element, a detection element, and a comparison element. The following figure shows the block diagram of the servo system's composition principle.
1. Comparison Section
The comparison stage compares the input command signal with the system's feedback signal to obtain the deviation signal between the output and the input. It is typically 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.
Technical requirements of servo systems
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 system's efficiency. Response speed is related to many factors, such as the computer's operating speed, the damping of the moving system, and its mass.
4. Operating frequency
Operating frequency typically refers to the range of input signals that a system is allowed to operate within. When a signal at the operating frequency is input, the system can function normally according to technical requirements; however, when other frequency signals are input, the system cannot function normally.
