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.

Figure 7-1 Gain adjustment process

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.

✎Note: Before adjusting the gain, it is recommended to perform a jog trial run first to ensure that the servo motor can operate normally! The gain adjustment process description is shown in the table below.

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.

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, the host computer debugging software 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.

Figure 7-2 Basic block diagram of servo loop gain

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 are below.

(1) 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

(2) 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

(3) 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

(4) 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

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.

(1) 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:

(2) 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.

(1) 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.

(2) 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.

Figure 7-3 Notch characteristics, notch width, and notch depth

Figure 7-4 Frequency characteristics of notch filter

Table 7-11 Notch filter function code parameters