Servo motors are a type of control motor, categorized as DC servo motors and AC servo motors. Due to their advantages, such as small size, light weight, high torque output, low inertia, and excellent control performance, AC servo motors are widely used in automatic control systems and automated inspection systems as actuators, converting control electrical signals into mechanical rotation of shafts.

Due to the high positioning accuracy of servo motors, modern position control systems are increasingly using AC servo motors as their primary components. 

AC Servo Motor And Driver System Structure

The gear measurement center consists of four motion axes: three linear axes and one rotary axis. For this parameter setting, the AC servo motor servo system is used on the linear axis with an inertia of approximately 250 kg.

The system's position detection unit uses a Renishaw incremental long grating. The output signal is a square wave voltage signal, with 2500 pulses per 1 mm. After a four-dimensional resolution circuit, a resolution of 0.1 μm (10,000 pulses per 1 mm) can be achieved.

Select Tonghang series AC servo motor (its main performance indicators are: power supply voltage is three-phase 200V, rated power of the adapted motor is 400W, encoder type is 2500p/r, resolution is The 10000 servo drive also uses a Tonghang product line. This drive accepts external pulse position command input and square wave encoder feedback, and features four internal control modes: position control, speed control, torque control, and full closed-loop control. Its basic parameters are as follows:

• Input analog voltage range: -10V-+10V;
• Maximum input pulse frequency: 2Mpps.

Depending on the application, the drive can be configured for position command control, pulse position command control, speed command control, or torque command control. Pulse position command control is used in this paper.

The servo drive receives the position command signal (pulse/direction) from the servo control card and feeds it into a pulse train. After frequency multiplication by the electronic gear, it is compared with the feedback pulse signal in a deviation reversible counter to generate a deviation signal.

The feedback pulse is generated by the encoder detecting the actual number of pulses generated by the motor and then multiplied by four. The position deviation signal is adjusted by the composite feedforward controller in the position loop to generate the speed command signal.

The deviation signal, resulting from the comparison between the speed command signal and the speed feedback signal, is adjusted by the proportional-integral controller in the speed loop to generate the current command signal. This signal undergoes vector conversion in the current loop and is then converted by the current loop. SPWM outputs torque and current to control the operation of the AC servo motor.

Position control accuracy is determined by the number of pulses generated per revolution by the photoelectric encoder. Photoelectric encoders are classified into incremental and absolute types.

This article uses a servo driver with an incremental photoelectric encoder. Its simple construction, ease of use, long lifespan, high resolution, and wide range of practical applications are key features. The servo driver wiring is as follows: PULS1 and PULS2 connect to the control card output pulse signal, SIGN1 and SIGN2 connect to the direction signal, COM+ and COM- connect to the +24V power supply (positive and negative terminals), and SRV ON connects to COM-. This completes the basic wiring for position control mode. The motor and driver form a closed-loop servo control system, and users can adjust the driver parameters to adjust the servo system performance.

Servo Drive Research

Before adjusting drive parameters, let's explain in detail the principles of adjusting drive parameters. When adjusting servo drive parameters, follow the order: current loop -> speed loop -> position loop. The following explains and analyzes the principles of each control loop.

1. Current Loop
Compared to DC motors, the current loop control of AC servo motors requires vector control. Therefore, a three-phase current calculation loop is included within the current loop. Its purpose is to ensure that the direction of the resultant magnetic field generated by the three-phase currents is always perpendicular to the rotor magnetic field, achieving maximum thrust, thus achieving the same effect as a DC motor.

Under ideal vector control, the current loop model for an AC motor is identical to that for a DC motor. With appropriate current negative feedback and current controller parameters, the input voltage and motor torque are proportional. In actual testing, the actual current value tracks the command current value very well, and the model can be approximated as a proportional link, so the current loop does not require user adjustment.

2. Speed Loop
The speed loop control method used in servo drives is PI, or proportional-integral control.

The figure shows the principle of the velocity loop controller. The servo performance of the position loop can be modified by adjusting the proportional gain K and the integral gain K. The PI algorithm is an error tracking algorithm with excellent positioning tracking capabilities. The velocity loop of a position servo system typically uses the PI algorithm and requires a certain amount of overshoot.

3. Position Loop
The servo drive's position loop has proportional, integral, differential, velocity feedforward, and acceleration feedforward control modes. As shown in the figure.

Kp, KpI, and Kpd are the proportional gain, integral gain, and differential gain of position control, respectively. Kpvfn is the velocity feedforward coefficient, and Kpafr and Kpafr2 are the acceleration feedforward coefficients. This servo system requires minimal velocity tracking error to ensure dynamic measurement accuracy, so a P controller with velocity feedforward is used. Because overshoot is undesirable in position control, an I controller is generally not used. Kp, Kpn, and Kpvfr are the proportional, differential, and velocity feedforward, respectively, and are adjustable parameters for position control. Since constant speed motion is primarily employed in practical applications, acceleration feedforward is not used.

Servo system experiment

Based on the above analysis, the servo drive parameters need to be adjusted. The user parameters that need to be set are as follows:
Pr02: Set to "7" to select full closed-loop control for the servo motor.
Pr15: Set to "100" to set the velocity feedforward value. A higher setting results in a faster response with smaller position deviations, especially when high-speed response is required.
Pr16: Set to "3000" to set the time constant of the primary delay filter for the velocity feedforward.
Pr18: Set to "200" to set the position loop gain. Increasing this gain value can improve the servo rigidity of position control, but excessively high gain can cause oscillation.
Pr19: Set to "150" to set the velocity loop gain. Increasing this gain value can improve the response speed of velocity control.
Pr 1A: Set to "10" to set the speed loop integral time constant. Reducing this parameter speeds up the integral action. Unit: ms.

Pr1B: Set to "4" to set the speed detection filter type. A higher setting reduces motor noise.

Pr1C: Set to "20" to set the torque filter time constant. Setting the torque filter parameters can reduce machine vibration.

Pr20: Set to "250" to set the ratio of the mechanical load inertia to the motor rotor inertia.

Pr40: Set to "0" to input via the optocoupler circuit (X5 connector, PULS1: pin 3, PULS2: pin 4, SIGN1: pin 5, SIGN2: pin 6).

Pr41: Set to "0" to set the command pulse type, which determines the corresponding rotation direction and pulse form. Used together with Pr42.
Pr42: Set to "3", that is, the command pulse type sent from the controller to the driver uses the pulse/direction method.
Pr44: Set to "16384", that is, it is used to set the number of pulses output from the feedback signal interface per motor rotation.
Pr45: Set to "0", it works together with Pr44.
Pr46: Set to "1", that is, it is used to set whether the logic level of the B phase signal output from the feedback signal interface is inverted. The value 1 is inverted.
Pr48: Set to "1", Pr49: Set to "1", Pr4A: Set to "1", Pr4B: Set to "10". These parameters are the parameters for command frequency division and can realize the electronic gear function of any speed ratio. The relationship between these four parameters is as follows: F = f x (Pr4B) / Pr48 (or Pr49) * 2 Pr4A = 50 000 (1) Where F: The number of internal command pulses required for the motor to rotate one circle.

F: Encoder resolution

The corresponding control card internally sets the pulse equivalent to 10,000.
The driver debugging software verifies the adjusted parameters. The driver provides three methods for control loop debugging, allowing it to generate position, speed, or torque commands to obtain servo curves under specific motion conditions.
The following sample curves test the driver in two motion modes: uniform motion and start-to-stop motion. The red line shows the position deviation curve, the pink line shows the output torque curve, the green line shows the command speed curve, and the blue line shows the actual speed curve. As can be seen from the graph, there is no significant difference in position, torque, or speed during motor acceleration and deceleration.

During debugging:
Since the current loop does not require user adjustment, torque control is unavailable.

Velocity deviation curve of uniform motion when Pr19=150, Pr1A=10

Velocity deviation curve of uniform motion when Pr19=150, Pr1A=5

When adjusting the speed loop, if the ratio is set to 200, the system will oscillate under the disturbance of other axis motion. The figures show the speed deviation curves for the motor's uniform motion and startup process, respectively. Figure 11 shows the position deviation curve for the startup process. At this point, the motor driver parameter settings (Pr19: 150, Pr1A: 10) are set. Other parameters remain the same as those adjusted above. The speed loop has a moderate amount of lag, and the speed response curve is relatively ideal. However, the speed loop has significant lag when the motor driver parameter settings (Pr19: 150, Pr1A: 5) are set in the figure. The speed loop response is significantly faster when the motor driver parameter settings (Pr19: 170, Pr1A: 5) are set in the figure, but there is noticeable vibration.

Velocity deviation curve of uniform motion when Pr1A=5,Pr19=170

Speed deviation curve during startup when Pr19=150, Pr1A=10

Position deviation curve during startup when Pr19=150, Pr1A=10

Position deviation curve of uniform motion when Pr18=200

When Pr15 is set to "100," Pr18 is set to "200," and Pr16 is set to "3000," while other parameters remain unchanged, the tracking error during constant-speed tracking and the start-to-stop motion process reaches approximately 2 pulses, achieving satisfactory results and reducing the operating speed tracking error, as shown in the figure. However, when Pr15 is set to "100," Pr18 is set to "190," and Pr16 is set to "3000," while other parameters remain unchanged, the tracking error during constant-speed tracking and the start-to-stop motion process reaches approximately 10 pulses, with slight oscillation, as shown in Figure 15. This indicates that setting Pr15 to "100," Pr18 to "200," and Pr16 to "3000" is ideal.

Position deviation curve of uniform motion when Pr18=240

Position deviation curve of the motion process that stops after starting

Conclusion

Tonghang has built an Servo system and conducted experimental research. This demonstrates the servo system's operating principles and provides a reference method for adjusting servo loop parameters. Simulation analysis allows users to intuitively visualize the impact of parameter changes on the response curve before actual operation, and to determine whether the adjusted parameters are appropriate based on the response curve. Our research team is committed to collaborating with customers to provide optimal solutions.

Our AC servo motor servo system has been deployed in gear measurement centers, eliminating hysteresis and oscillation issues associated with lead screw transmissions, improving measurement accuracy and repeatability, and enhancing instrument performance. We also address specific industry challenges and enhance efficiency, providing one-stop Servo solutions tailored to customer needs.

Servo drives, also known as "servo controllers" or "servo amplifiers," are controllers used to control servo motors. Their function is similar to that of a frequency converter on a conventional AC motor. They are part of a servo system and are primarily used in high-precision positioning systems. They typically control servo motors using position, speed, and torque to achieve high-precision positioning in transmission systems. They are a high-end product in transmission technology.

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