Three Control Methods of Servo Motor

Figure 1: AC servo motor controller and servo motor.

There are three types of servo motor control modes (servo motor control techniques): pulse, analog and communication control, and how to determine the servo motor control mode in different application scenarios?

1. Servo Motor Pulse Control

In some small stand-alone equipment, the use of pulse control to achieve motor positioning should be the most common application method. This control method is simple and easy to understand. The basic control idea is that: the total amount of pulses determines the motor displacement, and the pulse frequency determines the motor speed.
If the pulse control method is selected to realize the control of the servo motor, open the manual of the servo motor, and you generally will see a table like the following:

Table 1: Three pulse control methods for servo motors.

These three in the table are all pulse controls, but their implementation methods are different.

1.1 The First Pulse Control Method

The servo drive receives two (A, B) high-speed pulses, and determines the rotation direction of the motor through the phase difference between the two pulses. As shown in the figure above, if phase B is 90 degrees faster than phase A, it is forward rotation; if phase B is 90 degrees slower than phase A, it is reverse rotation.

During operation, the two-phase pulses of this control are alternating, so we also call this control method differential control. This control method has higher anti-interference ability, and is preferred in some application scenarios with strong interference.
However, in this way one motor shaft needs to occupy two high-speed pulse ports, which is not very convenient under the situation that the number of high-speed pulse ports is not enough.

1.2 The Second

The servo drive still receives two high-speed pulses, but the two high-speed pulses do not exist at the same time. When one pulse is in the output state, the other must be in an invalid state. When choosing this control method, please ensure that there is only one pulse output at the same time.
One pulse output runs in positive direction and the other runs in negative direction. As in the above case, in this method one motor shaft also requires two high-speed pulse ports.

1.3 The Third

It is only necessary to give a one way pulse signal to the servo drive, and the forward and reverse operation of the motor is determined by a one way direction IO signal. This control method is simpler to control and occupies the least resources of the high-speed pulse port. In general small systems, you can give preference to this method.

2. Servo Motor Analog Quantity Control

In the application scenario that needs to use the servo motor to realize speed control, we can choose the analog quantity control to realize the speed control of the motor, and the value of analog quantity determines the running speed of the motor. There are two ways to choose the analog quantity: current or voltage.

In the voltage mode, it is only necessary to add a certain voltage to the control signal terminal, which is simple to implement. In some scenarios, a potentiometer can be used to achieve this control. However, if the voltage is selected as the control signal, in a complex environment the voltage is easily disturbed, resulting in unstable control.
The current mode requires a corresponding current output module, but the current signal has strong anti-interference ability and can be used in complex scenarios.

Figure 2: Servo motor controlled by the output of the analog output modules.

3. Servo Motor Communication Control

Common ways to realize servo motor control by communication are CAN, EtherCAT, Modbus, and Profibus. This method is currently the preferred control method for some complex and large system application scenarios. With the communication method, the size of the system and the number of motor shafts are easy to adjust, there is no complicated control wiring, and the built system has high flexibility.

4. Loops of Servo Motor

The servo generally has 3 loops. The so-called three loops are three closed-loop negative feedback PID adjustment systems. The following is the three-loop control block diagram of the servo motor servo system:

Figure 3: Three-loop control block diagram of servo motor servo system.

4.1 Current Loop

The innermost PID loop is the current loop, which is completely carried out inside the servo drive. The Hall device detects the output current from the servo drive to each phase of the motor, and the negative feedback is used to perform PID adjustment for the current set, so that the output current is as close as possible to equal to the set current. The current loop controls the motor torque, so in the torque mode, the servo drive has the smallest operation and the fastest dynamic response.

4.2 Velocity Loop

The second loop is the speed loop, which performs negative feedback PID adjustment by detecting the signal of the motor encoder. The PID output in the loop is directly the setting of the current loop, so the speed loop and the current loop are included in the speed loop control. In other words, any mode must use the current loop. 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.

4.3 Position Loop

The third loop is the position loop, which is the outermost loop. Depending on the actual situation, it can be constructed between the servo drive and the motor encoder, or between the external controller and the motor encoder or the final load. Since the internal output of the position control loop is the setting of the speed loop, the system in position control mode performs all 3 loops. At this time, the system has the largest amount of operations and the slowest dynamic response speed.

Figure 4: German REXROTH servo CNC system.

5. Summary

The servo motor speed control and servo motor torque control are controlled by analog quantity. Position control is controlled by sending pulses. To select which servo motor control method is determined by your requirements and which motion function to satisfy.

1. If you have no requirements on the speed and position of the motor, and only require constant torque output, of course, select the servo motor torque control mode.
2. If you have certain accuracy requirements for position and speed, and are not very concerned about real-time torque, using the servo motor torque control mode is not convenient, and it is better to use the speed or position mode.
3. If the upper controller has a better closed-loop control function, it will be better to use speed control.
4. The position control method does not have high requirements on the upper controller. When you don’t have a good upper controller, and there are no high requirements, no real-time requirements, it is appropriate to select this way.
5.If there is a better upper controller, you can also use the servo motor torque control mode.

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