"Servo" English servo - the meaning of the Greek "slave". People want to use the "servo mechanism" as a handy tame tool, subject to the requirements of the control signal. Before the signal arrives, it will not move; after the signal arrives, it will turn immediately; when the signal disappears, it can stop immediately. Because of its "servo-slave" performance, it is named - servo control system.
Servo definition:
(1) Servo system: An automatic control system that changes the output of an object such as the position, orientation, state, and moment of an object and can follow any change in the input amount (or a given value).
(2) In an automatic control system, a system that can respond to a control signal with a certain degree of accuracy is called a follower system, also called a servo system. In order to ensure the accuracy of this instant response, there is generally a sensor feedback comparison of position, speed, and torque, also known as closed loop control.
The main task of the servo is to amplify, transform and control the power according to the requirements of the control command, so that the closed-loop control of the torque, speed and position of the output of the driving device is very flexible and convenient.
To put it simply, the position, time and force of the motion system are “obedient” at all times, at what time and at what position, and how much force is output at this position is called servo control.
Among them, if the motion system is driven by a motor, then the position of the motor corresponds to the working current input by the motor, which is the problem to be solved by the servo control system. At what position, how much voltage and current (including phase) is input to the motor, this Called position loop and current loop control. The amount of change in position over time is the speed loop, and the change in the speed loop is acceleration and jerk. From physics we know that the acceleration corresponds to the force (such as gravitational acceleration G), and the output force of the motor is driven by the input voltage (including phase), then from the motor input is the current loop = force, from the motor The feedback of the sensor is acceleration=force. Through the feedback to the sensor on the motor, the position and acceleration information is obtained and compared with the control input to form a closed-loop servo control system.
A rotary encoder is a rotary position sensor that outputs an incremental pulse signal (representing a change in position) or an angular position signal of an absolute value. The first derivative of this signal for time is velocity and the second derivative is acceleration. Therefore, rotary encoders are the best feedback sensor choice for servo systems.
Servo motor:
The motor is the most commonly used motion actuator. The motor driver has closed-loop control of position, speed and torque. It is called “servo motor”. The commonly used servo motor is an AC permanent magnet synchronous motor, and sometimes the AC permanent magnet synchronous motor is called a servo motor.
AC permanent magnet synchronous motor:
That is, the rotor is made of permanent magnet material, so after the rotation, as the rotating magnetic field of the stator of the motor changes, the rotor also changes the speed of the response frequency, and the rotor speed = the stator magnetic force pushing speed, so it is called "synchronous". AC permanent magnet synchronous motors are equipped with encoders for their synchronism-driven requirements. These encoders not only provide angular position signals (such as incremental pulse signals or absolute digital signals), but also provide rotor position changes. Phase signals (such as UVW or single-turn sine and cosine C, D signals), angular position signals as closed-loop feedback of position and velocity, and rotor commutation signals for closed-loop feedback of motor current loop-torque thrust input to obtain synchronous rotor rotation . Therefore, the AC permanent magnet synchronous motor has obtained the "servo" characteristic naturally because of the direct addition of the encoder, the feedback information of the position, speed and torque, and the closed loop control.
In fact, not only AC permanent magnet synchronous motors can have servo characteristics, AC asynchronous motors through their controllers (inverters) and sensor feedback (such as encoders), can also be commanded by the controller to achieve position, speed, and even output torque The closed-loop control and the follow-up response can also achieve the "servo" system characteristics.
Whether it can be called "servo" is whether its follow-up response and control accuracy in terms of position, speed, and output force can meet the requirements of use, and it does not lie in what motor actuator is used.
Under the premise of developing the frequency conversion technology, the servo drive controller has more precise control technology and algorithm operation than the general frequency conversion in the current loop, speed loop and position loop inside the drive. It is also more powerful than the traditional inverter. Many, the main point is to make precise position control. The speed and position are controlled by the commands sent by the host controller (of course, there are some frequency conversions - the controller integrates the control unit or directly sets the position and speed parameters in the drive by means of bus communication, or PG card) The internal algorithms of the drive and faster and more accurate calculations and better-performing electronics make it superior to the frequency converter.
The variable frequency controller and the motor form an open loop control of speed change control, and the stepping motor and the driver form an open loop control of a position (step) change if a sensor (for example, a code is added to the variable frequency motor system or the stepping motor system) Therefore, an external command controller (such as a PLC or a control card integrated in the motor driver) can also achieve a double closed loop of position and speed, and at the same time guarantee the response of the motor output force and stop positioning. A "servo" control system.
The servo control system is not only a motion actuator motor, but also a mechanical transmission system, such as a gearbox, a push screw, a gear drive, etc. These mechanical transmission systems have machining and assembly errors, while also having temperature changes and wear. Errors caused by other on-site environmental factors, in order to prevent the influence of control accuracy caused by these errors, sensors are sometimes added to the motion terminal as feedback position and speed information to the servo control system to correct the error. Such a control method is called "full closed loop" control, such as the addition of a linear encoder or a rotary encoder. In order to ensure the long-term accuracy of the position control, it is necessary to add a zero position sensor or a terminal position absolute value encoder in the control-execution system. The absolute value encoder has no need to worry because of the unique coding of each mechanical position in the sensor. The signal is affected by external interference and the position information after power failure is lost.
Whether it is AC permanent magnet synchronous motor (also known as servo motor directly), or mechanical actuators such as variable frequency motor and stepping motor, it needs to be corrected by controller, mechanical transmission system and terminal sensor to form a complete "Servo" control system. The control accuracy (position accuracy and follow-up time response) of the servo system is composed of actuator motor, motor driver, mechanical transmission execution, system total controller, etc., and the intrinsic requirements of AC permanent magnet synchronous motor and driver due to its "synchronization" requirements. Designed with the highest servo control accuracy. However, to ensure the control accuracy and control reliability of the motion execution terminal, it is necessary to balance the accuracy of the mechanical transmission system with the accuracy and reliability of the end position sensor (such as absolute encoder).
For example, the closed-loop control of the elevator car lift, the encoder has been installed on the elevator lifting host (such as ERN1387 from HEIDENHAIN, Germany), providing incremental A, B sine and cosine signals, 2048 pulse cycles per week, while providing a single The C and D sine and cosine signals of one cycle, the C and D sine and cosine signals of a single circle are divided by rough position, which can provide the commutation information of the motor UVW; and the sine and cosine signals of 2048 cycles per week go further. Subdivided to obtain high-resolution position changes. This high-resolution position change information is mainly used for short-term acceleration calculations. Because accurate acceleration feedback is needed when the time variable is small, it is necessary to compare More positional change information, which requires a very high resolution of the encoder and accurate positional accuracy, which ensures accurate acceleration feedback to control the motor input current.
However, due to the mechanical error on the mechanical system of the elevator, the elevator still needs to be fed back by the external leveling sensor when each layer stops, so that accurate positioning can be obtained, for example, using a flat-level photoelectric switch, or directly using a flat-layer absolute multi-turn encoder. In order to form a position-accurate closed-loop servo system.
In fact, the encoder required by the servo system may have two (or only one), one is on the high-speed end of the motor, for the commutation and acceleration feedback of the motor, this feedback enters the motor driver, which determines the commutation of the control current of the motor. With the size (torque ring), the other is for accurate positioning of the low-speed end of the position terminal. Encoders on the motor side require high resolution, high-resolution incremental encoders are often used to obtain fine variations in acceleration; while encoders in motion terminals require accurate and reliable position, commonly used multi-turn encoders ( Also useful as a linear scale).
If only one encoder is used (for example, only the motor end encoder), then it is necessary to rely on the high precision of the mechanical transmission part in position, and the current high-precision mechanical transmission is almost in the hands of Japanese and German manufacturers. monopoly. Adding a sensor (encoder) to the terminal is one way to avoid this monopoly.
In the inverter control system, since the motor commutation signal is not required, the encoder can be directly mounted on the motion terminal, also called the low speed end.
We have two concepts, one is the servo system and the other is the servo motor. These two are not the same concept. The servo motor is a special actuator. Its motor drive design is closed-loop control of the position, speed and torque from the beginning, but the motor is not part of the actuator, it does not represent the entire servo system.
Can closed-loop control be called a servo system? No, but the guarantee of fast control and precision in space (position) accuracy and time response. However, the accuracy of the follow-up "fast" and position is relative, there will always be a certain deviation, which is also one of the characteristics of the servo. The real servo is to eliminate the influence of this deviation on the control result.
