Overview

The servo drive is required to run the motor in least time delay and as faithful as possible against references from the host controller or internal setting. Gain adjustment needs to be performed to meet the requirements.

Figure 7-1 Gain adjustment flowchart

Servo gain is adjusted by setting multiple parameters (including position loop gain, speed loop gain, filter and inertia ratio) that affect each other. Ensure these parameters have a balanced relationship during setting.

The following table shows the general gain adjustment description:

Inertia ratio

The inertia ratio [P3-1] is:

The inertia ratio is an important parameter of the servo system, and quick commissioning could be implemented with the correct setting of this parameter.

It could be set manually or auto-tuned automatically by the servo drive itself.

Before performing online auto-tuning, the following conditions should be met:

The maximum speed of the motor should be greater than 300rpm.

The actual inertia ratio is between 0.00 and 100.00.

The load torque is stable without dramatic change.

The backlash of the load transmission mechanism is within a certain range.

The movement travel of the motor should meet the following requirements.

The movement travel of above one revolution in either forward or reverse direction is available between the mechanical limit switches. To make sure the limit switches have been installed, the required movement travel is reserved to prevent the condition that the motor senses the limit switches, it maybe cause accidents during auto-tuning.

[P3-5] (Motor revolutions for an inertia auto-tuning) is met: Ensure the movement travel for the motor in the stop position is larger than [P3-5]. if not, decrease [P3-6](maximum speed of inertia identification).

During inertia auto tuning, if vibration occurs, the auto tuning should be stopped Immediately.

Relevant function code:

Gain adjustment

In order to optimize the response of the servo drive, it is necessary to adjust the servo gain. The servo gain is defined by several parameters. Therefore, the adjustment of the servo gain should take into account the relationship between the various parameters.

In general, high-rigidity machinery could improve the response performance by increasing the servo gain. However, for machines with low rigidity, when the servo gain is increased, vibration may occur, which in turn affects the increase in gain. Therefore, selecting a suitable servo gain parameter could achieve stable performance while achieving a high response.

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

Automatic Gain Tuning

Automatic gain tuning means that the servo drive automatically produces the matching gain parameters based on the setting of[P3-2] (rigidity level selection) to achieve fast response and stability.

Servo rigidity 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 greater the position loop gain and speed loop gain, the faster the system response speed.

The setting range of [P3-2]is 0–31. The level 0 indicates he weakest stiffness and lowest gain and level 31 indicates the strongest stiffness and highest gain. The following table lists the stiffness levels for different load types.

When the function code [P3-3]is set to 0, the gain parameter is stored in the first gain by modifying the rigidity level.

During actual commissioning, it could be used for automatic rigidity level determination with the debug tool. It is used to select a suitable rigidity level as the operating parameter. The operation steps are as follows:

Confirm that the servo is in the ready state, the panel displays rdy, and the communication line is connected.

Open the debugging tool software, go to the run interface, set the corresponding parameters, and click "Servo On".

Click the "Forward" or "Reverse" button to confirm the travel range of the servo operation.

After the "auto tuning" lights up, click "auto tuning" to perform inertia recognition

After the auto gain tuning is completed, click "Save Inertia Value".

Click "Next" on the lower right to go to the parameter adjustment interface, and click "Parameter Measurement" to start parameter measurement.

After the parameter measurement is completed, the debug tool would pop up a confirmation window for parameter writing and saving.

Related function codes:

Manual Gain Adjustment

When the automatic gain adjustment result is not satisfactory, execute fine manual gain adjustment to achieve better result.

The servo system consists of three control loops, namely, position loop, speed loop, and current loop from external to internal. The following figure shows the basic control block diagram.

Figure 7-2 Basic control block diagram of manual gain adjustment

The most internal loop must have the highest response. The default current loop gain of the servo drive ensures the response, and need not be adjusted. You only need to adjust the position loop gain, speed loop gain and other auxiliary gains. When executing gain adjustment in the position control mode, increase the speed loop gain as well after increasing the position loop gain, and ensure the response of the position loop is lower than that of the speed loop to keep the system stable.

This servo drive has two sets of position loop and speed loop gain parameters. The user could switch between 2 sets of gain parameters according to the method corresponding to the setting value of the second gain switching mode in [P2-7]. Usually use external DI to switch.

Relevant parameters:

Speed loop gain

Within the range of no vibration or noise in the mechanical system, the greater the speed loop gain setting value, the better the response of the servo system and the better the speed followability.

When noise occurs in the system, the speed loop gain should be reduced.

Relevant function code:

Speed loop integral time constant

Speed loop integral time constant is used to eliminate speed loop deviation. Decreasing the integral time constant of the speed loop could increase the speed of the speed following, but the set value is too small would easily cause speed overshoot or vibration. when the time constant is set too large, the integral effect would be weakened, resulting in the deviation of the speed loop.

Relevant function code:

Position loop gain

Determine the highest frequency that the position loop could follow changing position reference. Increasing this parameter could speed up the positioning time and improve the ability of the motor to resist external disturbances when it is stationary. However, if the value is set too high, the system may be unstable .

Relevant function code:

Torque reference filter time

Choosing an appropriate torque filter time constant could suppress mechanical vibration. The larger the value of this parameter, the stronger the suppression ability, but if the setting value is too large, the response frequency of the current loop would decrease, causing vibration.

Relevant function code:

Feedforward gain

Summary: During position control and full-closed control, the speed control reference required for the operation is calculated from the internal position reference, and get the speed feedforward by comparing speed reference with the position feedback to get the speed feedforward, which could reduce more position deviation and improve response.

additionally, the torque reference required for operation is calculated from the speed control reference, and the torque feed-forward obtained by comparing the torque reference with the speed feedback could improve the response of the speed control system.

Relevant function code(speed Feedforward Gain):

Relevant function code (torque Feedforward Gain):

Vibration Suppression

Suppression of Mechanical Resonance

Resonance may occur at vicinity of the mechanical resonance frequency when the servo gain is increased, making the gain couldnot be increased further.

Mechanical resonance could be suppressed in the following two methods:

Torque reference filter

By setting the filter time constant so that it suppresses gain near the resonance frequency. The cutoff frequency of the torque reference filter could be calculated using the following formula. Filter cutoff frequency (Hz).

fc = 1 / [2π × setting parameter value × 0.001]

Notch:

The notch reduces the gain at certain frequencies to suppress mechanical resonance. After resonance is suppressed with correct setting of the notch, attempt to increase the gain gradually. The following figure shows the resonance

suppression principle of the notch.

Figure 7-3 Resonance suppression principle of the notch

There are 2 sets of notches in the servo drive. Each notch has 3 parameters, which are the frequency, width level and depth level.

Notch Width Level

The notch width level indicates the ratio of the notch width to the notch center frequency:

Where:

 : Notch center frequency, that is, mechanical resonance frequency

 : Notch width, indicating the ratio of -3dB attenuation frequency band with respective

to the notch center frequency

The corresponding relationship is shown in the figure below. Generally keep the default value of 4.

(b)Notch Depth Level

The notch depth level indicates the ratio of input to output at center frequency.

The input is completely shut with depth level 0 and fully received with depth level 100

at the center frequency. A smaller notch depth level indicates larger notch depth, which

produces stronger resonance suppression and makes the system instable. Pay attention

to this during use. The specific relationship is shown below:

Figure 7-4 Frequency characteristic curve of notch

Relevant function code: