Overview

The servo drive needs to make the motor faithfully operate in accordance with the instructions issued by the upper controller without delay as much as possible. In order to make the motor action closer to the instruction and maximize the mechanical performance, gain adjustment is required. The process of gain adjustment is shown in Figure 7-1.

The servo gain is composed of multiple sets of parameters such as position loop, speed loop, filter, load inertia ratio, etc., and they affect each other. In the process of setting the servo gain, the balance between the setting values of each parameter must be considered.

Table 7-1 Description of gain adjustment process

Inertia recognition

Load inertia ratio P03-01 refers to:

The load inertia ratio is an important parameter of the servo system, and setting of the load inertia ratio correctly helps to quickly complete the debugging. The load inertia ratio could be set manually, and online load inertia recognition could be performed through the host computer debugging software.

The related function codes are shown in the table below.

Table 7-2 Related parameters of gain adjustment

Gain adjustment

In order to optimize the responsiveness of the servo drive, the servo gain set in the servo drive needs to be adjusted. Servo gain needs to set multiple parameter combinations, which will affect each other. Therefore, the adjustment of servo gain must consider the relationship between each parameter.

Under normal circumstances, high-rigidity machinery can improve the response performance by increasing the servo gain. But for machines with lower rigidity, when the servo gain is increased, vibration may occur, and then affects the increase in gain. Therefore, selecting appropriate servo gain parameters can achieve higher response and stable performance.

The servo supports automatic gain adjustment and manual gain adjustment. It is recommended to use automatic gain adjustment first.

Automatic gain adjustment

Automatic gain adjustment means that through the rigidity level selection function P03-02, the servo drive will automatically generate a set of matching gain parameters to meet the requirements of rapidity and stability.

The rigidity of the servo refers to the ability of the motor rotor to resist load inertia, that is, the self-locking ability of the motor rotor. The stronger the servo rigidity, the larger the corresponding position loop gain and speed loop gain, and the faster the response speed of the system.

Before adjusting the rigidity grade, set the appropriate load inertia ratio P03-01 correctly. VD2L drive does not support automatic gain adjustment!

The value range of the rigidity grade is between 0 and 31. Grade 0 corresponds to the weakest rigidity and minimum gain, and grade 31 corresponds to the strongest rigidity and maximum gain. According to different load types, the values in the table below are for reference.

Table 7-3 Experience reference of rigidity grade

When the function code P03-03 is set to 0, the gain parameters are stored in the first gain by modifying the rigidity grade.

When debugging with the host computer debugging software, automatic rigidity level measurement can be carried out, which is used to select a set of appropriate rigidity grades as operating parameters. The operation steps are as follows:

• Step1 Confirm that the servo is in the ready state, the panel displays “rdy”, and the communication line is connected;
• Step2 Open the host computer debugging software, enter the trial run interface, set the corresponding parameters, and click "Servo on";
• Step3 Click the "forward rotation" or "reverse rotation" button to confirm the travel range of the servo operation;
• Step4 After the "start recognition" of inertia recognition lights up, click "start recognition" to perform inertia recognition, and the load inertia can be measured.
• Step5 After the inertia recognition test is completed, click "Save Inertia Value";
• Step6 Click "Next" at the bottom right to go to the parameter adjustment interface, and click "Parameter measurement" to start parameter measurement.
• Step7 After the parameter measurement is completed, Wecon SCTool will pop up a confirmation window for parameter writing and saving.

✎There may be a short mechanical whistling sound during the test. Generally, the servo will automatically stop the test. If it does not stop automatically or in other abnormal situations, you can click the "Servo Off" button on the interface to turn off the servo, or power off the machine! ✎For the detailed operation of the host computer debugging software, please refer to "Wecon Servo Debugging Platform User Manual".

Table 7-4 Details of self-adjusting mode selection parameters

Manual gain adjustment

When the servo automatic gain adjustment fails to achieve the desired result, you can manually fine-tune the gain to achieve better results.

The servo system consists of three control loops, from the outside to the inside are the position loop, the speed loop and the current loop. The basic control block diagram is shown as below.

The more the inner loop is, the higher the responsiveness is required. Failure to comply with this principle may lead to system instability!

The default current loop gain of the servo drive has ensured sufficient responsiveness. Generally, no adjustment is required. Only the position loop gain, speed loop gain and other auxiliary gains need to be adjusted.

This servo drive has two sets of gain parameters for position loop and speed loop. The user can switch the two sets of gain parameters according to the setting value of P02-07 the 2nd gain switching mode. The parameters are below.

Speed loop gain

In the case of no vibration or noise in the mechanical system, the larger the speed loop gain setting value, the better the response of servo system and the better the speed followability. When noise occurs in the system, reduce the speed loop gain. The related function codes are shown as below.

Table 7-5 Speed loop gain parameters

Speed loop integral time constant

The speed loop integral time constant is used to eliminate the speed loop deviation. Decreasing the integral time constant of the speed loop can increase the speed of the speed following. If the set value is too small, is will easily cause speed overshoot or vibration. When the time constant is set too large, the integral action will be weakened, resulting in a deviation of the speed loop. Related function codes are shown as below.

Table 7-6 Speed loop integral time constant parameters

Position loop gain

Determine the highest frequency of the position instruction that the position loop can follow the change. Increasing this parameter can speed up the positioning time and improve the ability of the motor to resist external disturbances when the motor is stationary. However, if the setting value is too large, the system may be unstable and oscillate. The related function codes are shown as below.

Table 7-7 Position loop gain parameters

Torque instruction filter time

Selecting an appropriate torque filter time constant could suppress mechanical resonance. The larger the value of this parameter, the stronger the suppression ability. If the setting value is too large, it will decrease the current loop response frequency and cause needle movement. The related function codes are shown as below.

Table 7-8 Details of torque filter time constant parameters

Feedforward gain

Speed feedforward could be used in position control mode and full closed-loop function. It could improve the response to the speed instruction and reduce the position deviation with fixed speed.

Speed feedforward parameters are shown in Table 7-9. Torque feedforward parameters are shown in Table 7-10.

Torque feedforward could improve the response to the torque instruction and reduce the position deviation with fixed acceleration and deceleration.

Table 7-9 Speed feedforward parameters

Table 7-10 Torque feedforward parameters

Model Tracking Control Function

Model tracking control is suitable for position control mode, which adds a model loop outside the three loops. In the model loop, new position commands, speed feedforward and torque feedforward and other control quantities are generated according to the user's response requirements to the system and the ideal motor control model. Applying these control quantities to the actual control loop can significantly improve the response performance and positioning performance of the position control, the design block diagram is as follows:

The usage method and conditions of model tracking control:

1. Correctly set the inertia ratio of the system P3-1, which can be obtained by monitoring the real-time load inertia ratio of U0-20.

2. Set the load rigidity level P3-2, set an appropriate value, it does not need to set a high rigidity level (recommended value 17~21 under rigid load).

3. Set P2-20=1 to enable the function of model tracking control.

4. Adjust the P2-21 model tracking control gain from small to large, and gradually increase in steps of 1000 until the responsiveness of the system meets the actual demand. The responsiveness of the system is mainly determined by this parameter.

5. After the responsiveness meets the requirements, user can adjust the parameters appropriately to increase the load rigidity level P3-2.

Please refer to the following for an example of the procedure of adjusting servo gain. 

Gain switching

Gain switching function:

● Switch to a lower gain in the motor stationary (servo enabled)state to suppress vibration;

● Switch to a higher gain in the motor stationary state to shorten the positioning time;

● Switch to a higher gain in the motor running state to get better command tracking performance;

● Switch different gain settings by external signals depending on the load connected.

Gain switching parameter setting

①When P02-07=0

Fixed use of the first gain (using P02-01~P02-03), and the switching of P/PI (proportional/proportional integral) control could be realized through DI function 10 (GAIN-SEL, gain switching).

② When P02-07=1

The switching conditions can be set through parameter P02-08 to realize switching between the first gain (P02-01~P02-03) and the second gain (P02-04~P02-06).

P02-08	Content	Diagram
0	Fixed use of the first gain	--
1	Switching with DI	--
2	Large torque command
3	Large actual torque
4	Large speed command
5	Fast actual speed
6	Speed command change rate is large
7	Large position deviation
8	Position command
9	Positioning completed
10	Position command + actual speed	Refer to the chart below

Description of related parameters

Mechanical resonance suppression

Mechanical resonance suppression methods

When the mechanical rigidity is low, vibration and noise may occur due to resonance caused by shaft twisting, and it may not be possible to increase the gain setting. In this case, by using a notch filter to reduce the gain at a specific frequency, after resonance is effectively suppressed, you can continue to increase the servo gain. There are 2 methods to suppress mechanical resonance.

Torque instruction filter

By setting the filter time constant, the torque instruction is attenuated in the high frequency range above the cutoff frequency, so as to achieve the expectation of suppressing mechanical resonance. The cut-off frequency of the torque instruction filter could be calculated by the following formula:

Notch filter

The notch filter can achieve the expectation of suppressing mechanical resonance by reducing the gain at a specific frequency. When setting the notch filter correctly, the vibration can be effectively suppressed. You can try to increase the servo gain. The principle of the notch filter is shown in Figure 7-3.

Notch filter

The VD2 series servo drives have 2 sets of notch filters, each of which has 3 parameters, namely notch frequency, width grade and depth grade.

Width grade of notch filter

The notch width grade is used to express the ratio of the notch width to the center frequency of the notch:

In formula (7-1),  is the center frequency of notch filter, that is, the mechanical resonance frequency;  is the width of notch filter, which represents the frequency bandwidth with an amplitude attenuation rate of -3dB relative to the center frequency of notch filter.

Depth grade of notch filter

The depth grade of notch filter represents the ratio relationship between input and output at center frequency.

When the notch filter depth grade is 0, the input is completely suppressed at center frequency. When the notch filter depth grade is 100, the input is completely passable at center frequency. Therefore, the smaller the the notch filter depth grade is set, the deeper the the notch filter depth, and the stronger the suppression of mechanical resonance. But the system may be unstable, you should pay attention to it when using it. The specific relationship is shown in Figure 7-4.

Table 7-11 Notch filter function code parameters

Low frequency vibration suppression

Low-frequency vibration suppression is suitable for working conditions where the motor vibrates during deceleration and shutdown after the position command is sent, and the vibration amplitude gradually decreases. The use of the low-frequency vibration suppression function is effective in reducing the time to complete positioning due to vibration effects.

VD2L drive does not support low frequency vibrartion suppression.

Vibration frequency detection:

• Users can measure vibration by measuring equipment such as laser displacement．
•  If no measuring equipment, the user can also read the position deviation waveform to confirm the vibration frequency through the "waveform" function of the PC debugging software.
• Low-frequency vibration detection needs to be coordinated by the two parameters of completion positioning threshold and vibration detection amplitude. When the vibration amplitude is greater than (P5-12*P4-14 detection amplitude ratio), the low-frequency vibration frequency can be recognized and updated to U0-16 monitoring quantity. For example, when the vibration amplitude is greater than (P5-12*P4-14*0.001) detection amplitude ratio. For example, in P05-12=800, P04_14=50, the vibration amplitude is greater than P5-12*P4-14*0.001=800*50*0.001=40 pulses, stop vibration frequency can be identified in U0-16.

Debugging method:

• Set the appropriate positioning completion thresholds P5-12 and P4-14 to help the software detect the vibration frequency.
• Run the position curve command to obtain the vibration frequency, and obtain the frequency through the speed curve of oscilloscope or U0-16.
• Set P4-12 vibration frequency and enable low frequency vibration suppression function P4-11.
• Run again to observe the speed waveform and determine whether to eliminate the vibration. If the vibration is not eliminated, please manually modify the vibration frequency and try again.

Note: If there is a speed substantial vibration and the vibration increases during the debugging, it may be that the low-frequency vibration suppression is not suitable for the current working conditions, please immediately close the servo, or power down!

Type A vibration suppression

Type A vibration suppression is suitable for durational vibration during motor operation or shutdown. Use Type A suppression to help reduce vibrations at specific frequencies that occur during motion (For the situation where the vibration continues to maintain and the vibration amplitude is almost constant after the command is completed.) As shown in Figure 7-14.

VD2L drive does not support type A vibration suppression.

Vibration frequency detection:

The vibration frequency can directly obtain the value of the current vibration frequency from the software oscilloscope vibration frequency, combined with real-time speed waveform to observe the current vibration situation.

Debugging method:

• Please set the correct inertia ratio parameter P3-1 when using type A vibration suppression,
• Run the position curve command, observe the servo host computer software waveform interface (sine wave) to obtain the vibration frequency.
• Set P4-20 vibration frequency and enable type A vibration suppression function P4-19. ( Type A vibration frequency takes effect when P4-19 is set to 1 for the first time. If change A-type vibration frequency P4-20, please set P4-19 to 0 again, then set to 1)
• Set P4-22 damping gain, gradually increasing from 0, each time increasing about 20.
• Observe the size of the vibration speed component, if the amplitude speed component is getting larger, it can be the vibration frequency setting error, if the vibration speed component is getting smaller, it means the vibration is gradually suppressed.
• When the vibration is suppressed, there is still a small part of the vibration speed component, users can fine-tune the P4-23 phase correction, the recommended value of 150~300.

Note: If there is a speed substantial vibration and the vibration increases during the debugging, it may be that the low-frequency vibration suppression is not suitable for the current working conditions, please immediately close the servo, or power down!