There are many control terms for servo systems : closed-loop control, semi-closed-loop control, open-loop control, speed loop, current loop, and position loop. Many people may not understand these terms clearly. Today, we will explain in detail what the control modes of a servo system are and how to systematically master this area.
Control types of control systems
Open-loop control: There is no detection device, or no feedback of the machine tool's displacement to the controller. In position control, the command signal is mostly in the form of a pulse.
Semi-closed-loop control: The detection device is installed on the servo motor and indirectly feeds back the displacement of the machine tool to the controller, without considering mechanical errors.
Full closed-loop control: The detection device is installed on the machine tool body and directly feeds back the machine tool's displacement to the controller. For the latter two types, the command signal is mostly in the form of analog voltage for position control.
Control mode types
Three control methods for servo amplifiers
1. Torque Control: The torque of the motor shaft is set by external analog input or direct address assignment.
The magnitude of the output torque is primarily used in applications requiring strict torque control. — Current loop control
2. Speed Control: Control of rotational speed through analog input or pulse frequency. —Speed Loop Control
3. Position control: The most commonly used control method in servo systems . Position control is generally achieved through externally input pulses.
The frequency of the pulses determines the rotational speed, and the number of pulses determines the rotational angle. Therefore, generally...
It is used in positioning devices.
Control Theory Concepts
Position control mode
Distinction between position control:
1. Semi-closed loop control I "Motor shaft end detection"
2. Semi-closed-loop control II "Reducer end detection"
3. Fully closed-loop control "mechanical position detection"
Using an AC servo motor for position control generally refers to obtaining feedback signals from a position detector (encoder, angle detector, etc.) integrated with the servo motor shaft. When the feedback signals match the position command, the motor shaft is stopped, and a servo lock function is executed.
Suitable for position control
AC servo motors are used for position control and can be divided into two main categories based on their applications.
The role of the position ring
★ Positioning commands are typically input in the form of pulse trains. The total number of pulses represents the positioning value, and the positioning speed is the number of pulses per unit time (second). (PPS: Pulse/Second)
★Location is complete when the number of input pulses matches the number of feedback pulses.
★In the position control unit, because there is an increment for the input pulse and a decrement for the feedback pulse, there is a counter (also called a deviation counter).
During position control: The industrial computer and the host controller send position command signals (pulse + direction) to the pulse train. After frequency division by the servo electronic gear, the signal is compared with the feedback pulse signal in the deviation reversible counter to form a deviation signal. The feedback pulse is the number of pulses actually generated by the motor detected by the photoelectric encoder. After being quadrupled in frequency, the position deviation signal is adjusted by the composite feedforward controller of the position loop to form a speed command signal. The deviation signal after comparing the speed command signal with the speed feedback signal is adjusted by the proportional-integral controller of the speed loop to generate a current command signal. After being vector-transformed in the current loop, the torque current is output by SPWM to control the operation of the AC servo. The position control accuracy is controlled by the number of pulses generated per revolution of the photoelectric encoder, which can be either absolute or incremental. Incremental accuracy is easier to control, has a longer average lifespan, and higher resolution, but it does not retain position after power failure.
Speed control mode
Like other general variable speed devices (frequency converters, DC motors, etc.), AC servo motors operate at a certain speed.
It is convertible. Its special features include:
a. Slow start and stop function: The impact generated during acceleration and deceleration, and the rate of change of acceleration and deceleration.
b. Wide speed control range: Control range from low speed to high speed (1:1000~5000) within the speed control range, constant torque...
characteristic.
c. Small speed variation rate: Even if the load changes, the speed will not change much.
Torque control mode
AC servo motors use current for high-precision output torque control. Common applications include position or speed control based on speed deviations, where the current is controlled directly from the outside. They can also be used to control the motor's output torque, such as in tension control applications like fabric winnowing machinery.
A. During tension control, a larger radius of the winding roller results in a relatively increased load torque. This, in turn, increases the output torque of the servo motor.
B. When the material is cut during winding, the load becomes lighter instantly, but the motor rotates at high speed, and the output torque of the servo motor decreases at this time.
Servo performance evaluation
To evaluate the responsiveness of a servo (how the output behaves after a command is input, i.e., how the motor rotates), the term loop gain is used. It's a technical term explaining the ratio of input to output, but now it's directly expressed as gain.
1. Relative motion: It is necessary to calculate the position value and control the position amount of jogging or reciprocating motion.
2. Absolute Motion: Directly issues position values; suitable for XY axes, tables, etc., and applicable to most machines. It is relatively simple in instruction programming.
