Basic gain parameters
In scenarios with extreme performance requirements, the gain can be manually fine tuned. By making more detailed adjustments, optimize the debugging effect.
The servo system consists of 3 control loops, i.e., current loop, speed loop and position loop from the inside out. The basic control block diagram is shown in the following figure.
The more inner the loop, the higher the responsiveness required. Generally, the inner loop bandwidth should be set to at least 4 times the outer loop bandwidth. For example, the current loop bandwidth should be 2,000Hz, the speed loop bandwidth should be set not higher than 500Hz, and the position loop bandwidth should be set not higher than 125Hz. In the debugging process, it should adhere to this principle as much as possible, otherwise it might lead to system instability!
The deError current loop gain of servo drive ensures responsiveness and generally doesn't need any adjustment. Only the position loop gain, velocity loop gain, and other auxiliary gains need to be adjusted. Therefore, when adjusting the gain in position control mode, if users want to improve the position response performance, to ensure system stability, first increase the speed loop gain and ensure that the inner loop bandwidth between loops is 4 times higher than the outer loop bandwidth, and then increase the position loop gain and reduce position tracking error. It must ensure the order of loop gain adjustment from the inside out.
The basic gain parameter adjustment method is as follows.
| Steps | Index codes | Name | Adjustment instructions |
| 1 | 2006-01h | Velocity loop gain 1 |
Parameter function:Determine Max. frequency of speed command that can follow speed-loop change.On the premise that the average of load inertia ratio (2006-0Bh) is set properly, it can be considered that: Max. follow-up frequency of speed loop = 2006-01h
In the case of no noise and vibration, increasing this parameter can speed up the positioning time and bring better velocity stability and followability. If noise is generated, reduce the parameter setting value;When mechanical vibration occurs, the vibration can be suppressed by using the notch filter or torque low-pass filter function in Section 4.3.2 Vibration Suppression". |
| 2 | 2006-02h | Time constant of V-loop integration 1 |
Parameter function: Eliminate velocity loop deviation .Adjustment method:Set values may follow these relations recommended: 500≤2006-01h×2006-02h≤1000 For example, if the velocity loop gain 2006-01h=40.0Hz, the time constant of the velocity loop integral shall satisfy: 12.50ms≤2006-02h≤25.00ms. Reducing the settings can strengthen the integration function and speed up the positioning time, but too small the settings are prone to mechanical vibration.If the settings are too high, the velocity loop deviation can't be returned to zero.When 2006-02h=512.00ms, the integral is Ineffective. |
| 3 | 2006-03h | Position loop gain 1 |
Parameter function:Determines the highest frequency of changes in the position instruction that the position ring can follow. The highest following angular frequency of the position ring =2006-03h
The position loop gain should satisfy: 50.2Hz ≤ 2006-03h ≤ 83.7Hz. Adjust according to the positioning time. Increasing this parameter can speed up the positioning time and improve the ability of the motor to resist external disturbances when it is stationary.Excessively high settings may cause system instability and oscillation. |
| 4 | 2007-03h | Torque filtering 1 |
Parameter function:Eliminate high-frequency noise and suppress mechanical resonance.
For example, when velocity loop gain 2006-01h = 40.0Hz, The time constant of torque command filtering should satisfy: 2007-03h ≤ 1.00ms. When vibration results from 2006-01h increasement, it can be suppressed by adjusting from 2007-03h. Please refer to " Vibration Suppression Low Pass Filter" for specific settings. Excessively large settings may cause deduction of current ring responseTo suppress the vibration during stop, try increasing 2006 -01h and decreasing 2007-03h; The motor vibration in the stop state is too large. It may try reducing the setting of 2007-03h. |
The position loop gain Kp, velocity loop gain Kv, velocity loop integral Ti, and torque low-pass filtering time Tf are the basic loop gain parameters of servo control. A certain relationship needs to be maintained between the 4 basic gain parameters to ensure the balance between stability and high performance of the entire servo system. The mathematical relationship between the 4 basic parameters is as follows:
Below, based on experimental waveforms, we will introduce the role of basic gain parameters in position control mode.
it can be observed that when there are only position gain and velocity gain coefficients without velocity integration, fast positioning can be achieved with steady-state error. Increasing the loop gain can reduce steady-state error; When there is velocity integration, there is no steady-state error, but the tuning time becomes longer.
By comparing,it can be observed that adding torque low-pass filtering can eliminate torque oscillation, but the filtering transition frequency is low, the phase lag increases, and the gain cannot be improved, resulting in longer tuning time.
By comparing,it can be found that by setting the torque low-pass filter reasonably, torque oscillation can be eliminated while ensuring fast position adjustment, achieving a good balance between speed and stability.
The specific function codes for gain class and torque control parameters are shown in the table below:
| 0x2006- Gain class parameter | 0x2007- Torque Control Parameter | |||
| Subindex | 0x01- Speed Proportional Gain 1 | 0x02- Speed Integral Gain 1 | 0x03 Position Proportional Gain 1 | 0x03- Torque Filter 1 |
| Data type | UINT16 | |||
| Accessibility | Readable/writable | Readable/writable | Readable/writable | Readable/writable |
| Unit | Hz | ms | Hz | ms |
| Default | 25 | 31.83 | 40 | 0.79 |
| Min. | 1 | 15 | 1 | 0 |
| Max. | 20000 | 51200 | 20000 | 3000 |
| Setting and effective mode | Stop setting/immediate enable | Stop setting/immediate enable | Stop setting/immediate enable | Stop setting/immediate enable |
| Related mode | - | |||
| Note | - | |||
Feedforward control
Speed feedforward
In position control mode, the theoretical speed command required for the action is directly calculated through internal position commands, and added to the speed command calculated by the position feedback loop. It is applied to the input of the speed regulator command, which can significantly reduce position tracking errors and improve response performance compared to simple feedback control. Therefore, using the speed feedforward function can improve the speed command response and reduce the position deviation when the speed is fixed.
In theory, the relationship between position deviation and position loop gain, as well as velocity feedforward gain, is shown below. If the speed feedforward gain is set to 100%, theoretically the position deviation will become zero, but an excessively large feedforward gain coefficient will cause excessive speed overshoot during acceleration and deceleration.
When the update cycle of the position command is less than the servo control cycle, the differential operation of speed feedforward will cause significant differential errors, which will be converted into high-frequency torque command components, thereby inducing electromagnetic noise during operation. In this case, please use a position command filter (FIR filter or sliding mean filter), or increase the speed feedforward filter value.
Operational steps for speed feedforward function:
A) Set the source of speed feedforward signal
Set 2013-0Dh(Speed Feedforward Control Selection) as a non-0 value, enable speed feedforward function, and the speed feedforward signal source can be selected internally and externally, shown as in the following table.
| Index codes | Name | Settings | Remarks |
| 2013-0Dh | Selection of speed feedforward control | 0: No speed feedforward | - |
| 1: Internal speed feedforward | Use the velocity data corresponding to position command as the source of the velocity feedforward signal. | ||
| 2: Use 60B1h as speed feedforward input | Use 60B1h speed bias (instruction unit/second) as the source of the speed feedforward signal. By using bit 6 of 607Eh (Polarity), polarity of the velocity feedforward signal can be changed at this time. |
B) Set speed feedforward parameters
Including Velocity loop feed forward (2006-09h) and speed feedforward filtering time (2007-07h).
| Index codes | Name | Adjustment instructions |
| 2007-07h | Velocity feedforward filtering time |
Reducing filtering time can suppress speed overshoot of acceleration and deceleration; Increasing the filtering time can suppress noise in situations where the update cycle of the position command is longer than the control cycle of the driver, and the pulse frequency of the position command is uneven, thus suppressing the jitter of the positioning completion signal; |
| 2006-09h | Velocity loop feed forward |
Torque feedforward
0Position control mode, using torque feedforward can improve dynamic velocity response and reduce position deviation during fixed acceleration/deceleration; To use torque feedforward, it is necessary to set the correct load torque inertia ratio. Please refer to the mechanical load identification results in Section 4.2.1. The torque feedforward gain is set to a non-zero value, and the torque feedforward function is enabled. By increasing the torque feedforward gain, the position deviation during constant acceleration /deceleration can be controlled to around 0, and the trapezoidal motion curve can be perfectly tracked without external torque interference.
Operation steps for torque feedforward function:
A) Set the source of torque feedforward signal
Set 2014-0Ch (torque feedforward control selection) as non-zero value, enable the torque feedforward function. The feedforward signal source can be selected from internal and external sources, as shown in the table below.
| Index codes | Name | Settings | Remarks |
| 2014-0Ch | Selection of torque feedforward control | 0: No torque feedforward | - |
| 1: Internal torque feedforward |
Use speed command as the source of torque feedforward signal. In position control mode, the speed command comes from the output of the position controller. |
||
| 2: Use 60B2h as torque feedforward input |
Use 60B2h (Torque offset , 0.1%) as the source of torque feedforward signal. By using bit 5 of 607Eh (polarity), the polarity of the torque feedforward signal can be changed at this time. |
B) Set torque feedforward parameters
Including torque feedforward proportional gain (2006-0A) and torque feedforward filtering time (2007-08).
| Index codes | Name | Adjustment instructions |
| 2006-0Ah | Torque feedforward proportional gain |
Increasing the proportional gain can improve response, but overshoot may occur during acceleration/deceleration; Reducing filtering time can suppress overshoot during acceleration and deceleration; Increasing the filtering time can suppress noise; Adjustment method:When adjusting, first, keep the filtering time at the Default; Then, gradually increase the proportional gain setting value from 0 until the torque feedforward effect is achieved at a certain setting value.When adjusting, the settings of 2006-0Ah and 2007-08h should be repeatedly adjusted to find a balanced setting |
| 2007-08h | Torque feedforward filtering time |
(3) Two-degree-of-freedom control
In non torque control mode, two-degree-of-freedom control coefficient can be used to improve the control effect, set to 100%, which is the normal PI control mode; Setting it other than 100%, i.e.,two-degree-of-freedom control, which can be used to increase resistance to external forces and improve velocity response waveforms.
The following figure shows the improvement of the two-degree-of-freedom control coefficient on slow speed increase and slow positioning completion.
two-degree-of-freedom control enhances the anti-interference ability of the velocity loop and improves its ability to follow speed commands by adjusting the velocity loop control method.
| Index codes | Name | Adjustment instructions |
| 2006-0Dh | Two-degree-of-freedom feedforward coefficient |
Parameter function: The control method for changing the speed loop in non torque control mode. Adjustment method: The setting of 2006-0Dh is too small, resulting in slow speed loop response;When overshoot exists in velocity feedback, gradually reduce 2006-0Dh from 100.0 until the two-degree-of-freedom control achieves effect at a certain set value.When 2006-0Dh=100.0, the velocity loop control method remains unchanged and deErrors to proportional integral control. |