Servo systems are used in applications requiring high precision and fast positioning. When simple adjustments to stiffness and inertia ratio are insufficient to meet on-site requirements, how can servo gain be adjusted? Leadsai summarizes various application examples, introducing the principles and application experience of servo gain adjustment from aspects such as the three-loop control inherent in servo control, the relationship between the three-loop bandwidths, the steps for adjusting gain parameters, the principles of each gain parameter and their impact on servo operation, and commonly used filters.
The Leadshine servo drive consists of three loops: a current control loop, a speed control loop, and a position control loop. The control block diagram is as follows:
The bandwidth of each layer must be greater than the bandwidth of the outer layer; otherwise, the entire control system will be unstable and cause system oscillations. Therefore, the bandwidth relationship of the three loops above is as follows:
Current loop bandwidth > velocity loop bandwidth > position loop bandwidth
Whether the selection of position and velocity bandwidth is appropriate depends on the rigidity of the machine and the application environment. In general applications, adjusting the inertia ratio and rigidity can meet the on-site response and positioning requirements.
Mechanical loads connected by belts, chains, or gear reducers have low rigidity and can be set to a low bandwidth. The rigidity is generally around 7 to 10, for example, multi-joint robots, etc.
Direct-drive ball screws have high mechanical load rigidity and can be set to a large bandwidth. The rigidity is generally around 14~17. For example, they are used in high-precision machining machinery such as machine tools and pick-and-place machines.
The mechanical load stiffness of the ball screw driven by the reducer is moderate, and it can be set to medium bandwidth. The stiffness is generally about 11~14. For example, it is used in general working machines, handling machinery, etc.
If adjusting the inertia ratio and stiffness still fails to meet the application requirements, and fine-tuning of the position and velocity loop parameters is necessary, it's important to note that changing one parameter will require readjusting the others. Avoid making significant changes to any single parameter. Generally, the following steps can be followed:
Leadshine AC servo drives and motors
The gain adjustment steps for position control are as follows:
1. Set an appropriate rotational inertia ratio;
2. Set the velocity loop integral time constant to a large value;
3. Increase the speed loop gain; if mechanical vibration occurs, slightly decrease it.
4. Decrease the integral time constant of the velocity loop; if there is mechanical vibration, increase it slightly.
5. Increase the position loop gain; if there is mechanical vibration, slightly decrease it.
6. If the gain cannot be increased due to resonance in the mechanical system, thus failing to meet the requirements of the servo application, the resonance in the mechanical system can be suppressed by adjusting the torque low-pass filter or notch filter; then repeat the above steps to improve servo performance. It is recommended to use a torque low-pass filter first, and only consider a notch filter if the torque low-pass filter is ineffective.
7. If a shorter positioning time and a smaller position tracking error are required, the velocity feedforward can be appropriately increased, i.e., the velocity feedforward gain, but it should not exceed 80%.
The gain adjustment steps for speed control are as follows:
1. Set an appropriate moment of inertia ratio;
2. Set the velocity loop integral time constant to a large value;
3. Increase the speed loop gain; if mechanical vibration occurs, slightly decrease it.
4. Decrease the integral time constant of the velocity loop; if there is mechanical vibration, increase it slightly.
5. If the gain cannot be increased due to resonance in the mechanical system, thus failing to meet the requirements of the servo application, the resonance in the mechanical system can be suppressed by adjusting the torque low-pass filter or notch filter; then repeat the above steps to improve servo performance. It is recommended to use a torque low-pass filter first, and only consider a notch filter if the torque low-pass filter is ineffective.
Velocity loop gain:
The speed loop gain directly determines the speed loop's response bandwidth. Without resonance or noise in the mechanical system, increasing the speed loop gain results in a faster speed response and better speed tracking. However, excessively high speed loop gain can cause mechanical resonance.
Speed loop bandwidth (Hz) = (1+G)/(1+JL/JM) * Speed loop gain (Hz)
Where: G is the moment of inertia ratio, JL is the load moment of inertia referred to the motor shaft, and JM is the motor rotor moment of inertia. When the set value G = JL/JM, the speed loop gain is the speed loop bandwidth.
Velocity loop integral time constant:
The velocity loop integral time constant can effectively eliminate steady-state velocity errors and quickly respond to minute velocity changes. When the mechanical system does not generate resonance or noise, reducing the velocity loop integral time constant can increase system rigidity and reduce steady-state errors. If the load inertia ratio is large or the mechanical system has resonance factors, the velocity loop integral time constant must be increased to reduce the effect of integration; otherwise, the mechanical system is prone to resonance. If the inertia ratio parameter G is set to JL/JM, the velocity loop integral time constant is:
The velocity loop integral time constant (ms) = 4000 / (2 * pi * velocity loop gain (Hz)) where: pi is the mathematical constant π.
Position loop gain:
The position loop gain directly determines the response speed of the position loop. Increasing the position loop gain reduces position tracking error and shortens positioning time, provided the mechanical system does not generate resonance or noise. However, excessively high position loop gain can also cause mechanical system jitter or positioning overshoot. The position loop bandwidth should not exceed the velocity loop bandwidth, as follows:
Position loop bandwidth (Hz) <= Velocity loop bandwidth (Hz) / 4
If the inertia ratio parameter G is set to JL/JM, the position loop gain can be calculated:
Position loop gain (1/s) <= 2*pi*velocity loop gain (Hz)/4
Torque low-pass filter:
Low-pass filters offer excellent attenuation of high frequencies and effectively suppress high-frequency oscillations and noise, but they have no effect on suppressing mid-to-low frequency resonances. For example, when a lead screw connects to a load, increasing rigidity can improve the system response. However, when the rigidity is increased to a certain extent, high-frequency resonance may occur, causing current oscillations. In such cases, using a torque low-pass filter can be quite effective.
The smaller the setting value, the better the system's responsiveness can be controlled, but it is limited by mechanical conditions; the larger the setting value, the better it can suppress high-frequency resonance, but too large a setting value will reduce the response bandwidth and phase margin, causing system oscillation.
Notch filter:
The servo driver contains two notch filters that can be used simultaneously to suppress two different frequency resonances. The system's resonant frequency can be roughly calculated by observing the current waveform. If the resonant frequency is known, the notch filter can directly eliminate the resonance phenomenon. If the resonant frequency is uncertain, the notch filter frequency setting can be gradually reduced from high to low until the frequency setting with the minimum current oscillation is the optimal value.
If the resonant frequency shifts with time or other factors, and the shift range is large, then a notch filter is not suitable.
A notch filter's performance is determined not only by its frequency but also by its depth and quality factor. A deeper notch filter results in better suppression of mechanical resonance but also leads to a larger phase delay, which can sometimes increase system vibration. Similarly, a wider notch filter results in a smaller quality factor and better suppression of mechanical resonance but also leads to a larger phase change area, which can sometimes also increase system vibration.
Common debugging steps for Leadshine servos:
Determine the wiring method
Checking the enabling method, when enabled externally, parameter Pr4.00 uses the default value. When enabled internally, the value of parameter Pr4.00 changes to 383.
Pr4.10 is the driver alarm output. When parameter 410 is set to 0101, the normal output voltage is 0V, and the alarm output is 24V. When multiple driver alarm points are connected in parallel, the measured output is 0V, which cannot reflect the alarm status.
When the parameter is set to 8181, the normal output voltage is 24V, and the alarm output is 0V. When multiple driver alarm points are connected in parallel, the measured output is 0V, which can reflect the alarm status.
Determine the pulse input method and the parameters Pr0.06 and Pr0.07.
Test load inertia
The driver panel monitoring parameter D16 can display the load inertia. Test method:
Operating speed ≥ 1000 rpm, interval time ≥ 1000, number of repetitions ≥ 1, acceleration/deceleration time and stroke set according to application requirements. Then click "Start" and observe the load inertia value detected by d16 Jrt on the driver panel. Subtract 100 from the average load inertia value detected and enter it into Pr004 (100 refers to the load inertia of the motor rotor itself).
Equipment rigidity
Increase rigidity as much as possible when the equipment allows, and reduce rigidity when the equipment vibrates.
The rigid adjustment is as follows:
A: When parameter Pr0.02 is 1 or 2, directly adjust the rigidity Pr0.03. The parameters associated with rigidity are shown in the table below:
The switching between the first and second gains is set according to parameter Pr1.15:
B: When parameter Pr0.02 is 0, rigidity Pr0.03 is unbound from the first and second gain parameters associated in the table above. The position loop gain, velocity loop gain, velocity loop integral time constant, and torque filter can be adjusted separately according to the above steps.
Electronic gear ratio
The associated parameters for the electronic gear are Pr0.09 and Pr0.10.
The driver's default pulse count is 10,000. When the actual pulse count is 10,000, both parameters are 1.
When the actual number of pulses is 1000, Pr0.09 is 10 and Pr0.10 is 1, which is the ratio of the default number of pulses to the actual number of pulses.
Panel operation method
Steps for saving parameters:
