A servo motor is an engine that controls the operation of mechanical components in a servo system ; it is a type of auxiliary motor with indirect speed change.
Servo motors enable highly accurate speed and position control, converting voltage signals into torque and speed to drive the controlled object. The rotor speed of a servo motor is controlled by the input signal and can respond quickly. In automatic control systems, they are used as actuators and possess characteristics such as a small electromechanical time constant, high linearity, and low starting voltage. They can convert received electrical signals into angular displacement or angular velocity output on the motor shaft. Servo motors are broadly classified into DC and AC servo motors. Their main characteristic is that they do not rotate when the signal voltage is zero, and their speed decreases uniformly as the torque increases.
Servo control systems generally fall into three control modes: speed control, torque control, and position control. Speed control and torque control use analog signals, while position control is achieved by sending pulses.
(1) If there are no requirements for the speed or position of the motor, and only a constant torque needs to be output, then torque mode is of course used.
(2) If there are certain accuracy requirements for position and speed, but not much concern for real-time torque, torque mode is not very convenient, and speed or position mode is better. If the host controller has a good closed-loop control function, speed control will be more effective. If the requirements are not very high, or there are basically no real-time requirements, position control does not place high demands on the host controller. In terms of the response speed of the servo drive, torque mode has the least amount of computation and the fastest response of the drive to the control signal; position mode has the largest amount of computation and the slowest response of the drive to the control signal. When there are relatively high requirements for dynamic performance during motion, the motor needs to be adjusted in real time. So if the controller itself has a very slow calculation speed (such as PLC or low-end motion controller), position mode control is used. If the controller has a relatively fast calculation speed, speed mode can be used to move the position loop from the drive to the controller, reducing the workload of the drive and improving efficiency (such as most mid-to-high-end motion controllers); if there is a better host controller, torque mode control can also be used to move the speed loop away from the drive. This is generally only possible with high-end dedicated controllers, and at this time, there is no need to use a servo motor at all.
In other words:
1. Torque Control: Torque control is achieved by setting the output torque of the motor shaft through external analog input or direct address assignment. For example, if 10V corresponds to 5Nm, then when the external analog input is set to 5V, the motor shaft output will be 2.5Nm. If the motor shaft load is below 2.5Nm, the motor rotates forward; if the external load is equal to 2.5Nm, the motor does not rotate; and if the load is greater than 2.5Nm, the motor rotates in reverse (typically occurring under gravity loads). The torque setting can be changed in real-time by altering the analog input or by changing the corresponding address value via communication.
The main applications are in winding and unwinding devices where there are strict requirements on the stress on the material, such as wire winding devices or optical fiber drawing equipment. The torque setting must be changed at any time according to the change of the winding radius to ensure that the stress on the material does not change with the change of the winding radius.
2. Position Control: Position control typically determines the rotation speed by the frequency of externally input pulses and the rotation angle by the number of pulses. Some servo systems can also directly assign speed and displacement values via communication. Because position control allows for very strict control over both speed and position, it is generally used in positioning devices.
Application areas include CNC machine tools, printing machinery, etc.
3. Speed Mode: Rotation speed can be controlled via analog input or pulse frequency. With outer-loop PID control from a higher-level control device, speed mode can also be used for positioning, but the motor's position signal or the position signal of the direct load must be used as feedback signals to the host computer for calculation and control. Position mode also supports direct load outer-loop detection of position signals. In this case, the encoder at the motor shaft end only detects the motor speed, and the position signal is provided by the detection device at the direct final load end. This has the advantage of reducing errors in the intermediate transmission process and increasing the positioning accuracy of the entire system. Simply put: a regular motor will continue to rotate for a while due to its inertia after power is cut off before stopping. Servo motors and stepper motors, however, stop and start instantly, with extremely fast response. However, stepper motors are prone to step loss.
