What is a servo drive ?

The main function of the servo drive is to control the servo motor. It is used in conjunction with the servo motor and is generally not used alone.

Servo drive, also known as "servo controller" and "servo amplifier", is a controller used to control servo motors. Its function is similar to that of a frequency converter acting on an ordinary AC motor. It is a part of the servo system and is mainly used in high Accurate positioning system. Generally, the servo motor is controlled by three methods of position, speed and torque to achieve high-precision transmission system positioning. It is currently a high-end product of transmission technology.

Servomotor (servomotor) refers to the engine that controls the operation of mechanical components in the servo system, and is an auxiliary motor indirect speed change device. The servo motor can control the speed and position accuracy very accurately, and can convert the voltage signal into torque and speed to drive the control object. The rotor speed of the servo motor is controlled by the input signal and can respond quickly. It is used as an actuator in an automatic control system, and has the characteristics of small electromechanical time constant, high linearity, and starting voltage. Converted into angular displacement or angular velocity output on the motor shaft.

Servo motors are generally controlled by three loops. The so-called three loops are three closed loop negative feedback PID adjustment systems. From the inside to the outside are the current loop, speed loop, and position loop.

1. Current loop: The innermost PID loop is the current loop. This loop is carried out completely inside the servo drive. The output current of each phase of the drive to the motor is detected by the Hall device, and negative feedback is given to the current setting for PID adjustment. To achieve the output current as close as possible to the set current, the current loop is to control the motor torque, so in the torque mode, the driver has the smallest calculation and the fastest dynamic response.

2. Speed loop: Negative feedback PID adjustment is performed through the detected signal of the motor encoder. The PID output in the loop is directly the setting of the current loop, so the speed loop control includes the speed loop and the current loop. In other words In other words, any mode must use a current loop. The current loop is the basis of control. While the speed and position are controlled, the system is actually controlling the current (torque) to achieve the corresponding control of the speed and position.

3. Position loop: It is the outermost loop. It can be built between the drive and the motor encoder or between the external controller and the motor encoder or the final load, depending on the actual situation. Since the internal output of the position control loop is the setting of the speed loop, the system has performed all three loop calculations in the position control mode. At this time, the system has the largest amount of calculation and the slowest dynamic response speed.

Servo motor control mode selection

1. Torque control:

The torque control method is to set the external output torque of the motor shaft through the input of the external analog quantity or the direct address assignment. For example, 10V corresponds to 5Nm, when the external analog quantity is set to 5V, the motor shaft The output is 2.5Nm: If the motor shaft load is lower than 2.5Nm, the motor will rotate forward, when the external load is equal to 2.5Nm, the motor will not rotate, and when the external load is greater than 2.5Nm, the motor will reverse (usually generated under gravity load).

The set torque can be changed by instantly changing the analog setting, or by changing the value of the corresponding address through communication. The application is mainly used in winding and unwinding devices that have strict requirements on the force of the material, such as wire-raising device or fiber-optic equipment. The torque setting should be changed at any time according to the change of the winding radius to ensure that the material is not stressed. Will change as the winding radius changes.

2. Position control:

In the position control mode, the rotation speed is generally determined by the frequency of externally input pulses, and the rotation angle is determined by the number of pulses. Some servos can directly assign speed and displacement through communication.

Since the position mode can have very strict control over speed and position, it is generally used in positioning devices.

3. Speed mode:

Rotation speed can be controlled through analog input or pulse frequency, and positioning can also be carried out in speed mode when the outer loop PID control with upper control device is provided, but the position signal of the motor or the position signal of the direct load must be sent to the upper Feedback for calculation purposes.

The position mode also supports direct load outer loop detection position signal. At this time, the encoder at the motor shaft end only detects the motor speed, and the position signal is provided by the direct detection device at the final load end. This has the advantage of reducing the intermediate transmission process. The error increases the positioning accuracy of the entire system.

4. Full closed loop control mode: Full closed loop control is relative to semi-closed loop control.

First of all, let's understand the lower half closed-loop control. The semi-closed loop is an exponential control system or PLC that issues a fast pulse command. The servo accepts the instruction and executes it. During the execution, the encoder of the servo itself feedbacks the position to the servo, and the servo itself performs deviation correction. The error of the servo itself can be avoided, but the mechanical error cannot be avoided because the control system does not know the actual position .

The full closed loop means that the servo accepts the speed controllable pulse command from the upper controller, and the servo receives the signal to execute. During the execution, there is a position feedback device on the mechanical device, which directly feeds back to the control system, and the control system judges through comparison If the deviation from the actual situation is detected, the servo command is given to correct the deviation, so that the control system completes the speed loop control of the servo through the frequency-controllable pulse signal, and then completes the position loop control of the servo through the position sensor (grating ruler, encoder). A control mode that organically combines the servo motor, motion controller, and position sensor is called full closed loop control.

PID is an important parameter in the control system. It refers to the control mode and the response mode between output and input. The English letters are proportional (P), integral (I), and derivative (D).

PID control compares the collected data with a reference value, and then uses this difference to calculate a new input value. The purpose of this new input value is to allow the system data to reach or maintain the reference value. Different from other simple control calculations, PID control can adjust the input value according to historical data and the occurrence rate of differences, which can make the system more accurate and stable. It can be proved by mathematical methods that a PID feedback loop can maintain the stability of the system when other control methods cause the system to have stable errors or process repetitions. Simply put, PID control is feedback control by measuring the variable of interest and comparing it with the expected value. Then use this error to correct and adjust the control system.

Gain adjustment of three loops

1. First, the current loop: the input of the current loop is the output after the speed loop PID adjustment, let’s call it the “current loop setting”, and then the current loop setting is compared with the “current loop feedback” value The latter difference is PID adjusted in the current loop and output to the motor. The "current loop output" is the phase current of each phase of the motor. The "current loop feedback" is not the feedback of the encoder but is installed inside the drive in each phase. The Hall element (magnetic field induction becomes current and voltage signal) feedback to the current loop.

2. Speed loop: The input of the speed loop is the output of the position loop PID adjustment and the feedforward value of the position setting. We call it "speed setting". This "speed setting" is compared with the "speed loop feedback" value After the difference value is PID adjusted in the speed loop (mainly proportional gain and integral processing), the output is the "current loop setting" mentioned above. The feedback of the speed loop comes from the feedback value of the encoder through the "speed calculator".

3. Position loop: The input of the position loop is an external pulse (usually, the servo that writes data directly to the drive address is an exception). The external pulse is processed by smooth filtering and electronic gear calculation as the "position loop setting". The set value and the pulse signal feedback from the encoder are calculated by the deviation counter after the PID adjustment of the position loop (proportional gain adjustment, without integral derivative link), the combined value of the output and the feedforward signal of the position setting is constituted The above mentioned speed loop setting.

The feedback of the position loop also comes from the encoder installed at the tail of the servo motor. It has no connection with the current loop. It samples the rotation of the motor instead of the motor current, and has no connection with the input, output, and feedback of the current loop. The current loop is formed inside the drive. Even if there is no motor, the current loop can form a feedback operation by installing an analog load (such as a light bulb) on each phase.