Servo motor noise and instability analysis

卓联微科技

Servo motor noise and instability analysis [Copy link]

When users use servo motors on some machines, they often experience excessive noise and unstable operation of the motor-driven load. When this problem occurs, many users' first reaction is that the servo motor is of poor quality, because sometimes when a stepper motor or variable frequency motor is used to drive the load, the noise and instability are much smaller. On the surface, it seems that the servo motor is indeed the cause, but after a careful analysis of the working principle of the servo motor, we will find that this conclusion is completely wrong.

The servo system includes: servo drive, servo motor and a feedback sensor (generally servo motors come with optical encoders). All these components operate in a closed-loop control system: the drive receives parameter information from the outside, and then transmits a certain current to the stepper servo motor, which is converted into torque to drive the load. The load moves or accelerates and decelerates according to its own characteristics. The sensor measures the position of the load, so that the drive device compares the set information value and the actual position value, and then changes the motor current to keep the actual position value consistent with the set information value. When the load changes suddenly and causes the speed to change, the encoder will immediately respond to the servo drive after notifying the speed change. The drive changes the current value provided to the servo motor to meet the load change and return to the set speed.

The servo system is a fully closed-loop system with a very high response. The time lag response between load fluctuation and speed correction is very fast. At this time, what really limits the system response effect is the transmission time of the mechanical connection device.

Example

A machine uses a servo motor to drive a constant speed and large inertia load through a V-belt. The entire system needs to obtain a constant speed and a faster response characteristic. The action process is analyzed:

When the driver sends current to the motor, the motor immediately generates torque. Initially, the load will not accelerate as fast as a stepper motor because the V-belt is elastic. The servo motor will reach the set speed earlier than the load, and the encoder installed on the motor will weaken the current and then weaken the torque. As the tension of the V-belt continues to increase, the motor speed will slow down, and the driver will increase the current again, and the cycle will repeat.

In this case, the system is oscillating, the motor torque is fluctuating, and the load speed is also fluctuating. The result is of course noise, wear, and instability. However, this is not caused by the servo motor. This noise and instability come from the mechanical transmission device, which is caused by the mismatch between the servo system reaction speed (high) and the mechanical transmission or reaction time (long), that is, the servo motor responds faster than the time required for the system to adjust the new torque.

To find the root cause of the problem and solve the above example problem, you can:

1. Increase mechanical rigidity and reduce system inertia to reduce the response time of mechanical transmission parts, such as replacing the V-belt with direct screw drive or using a gear box instead of the V-belt.

2. Reduce the response speed of the servo system and reduce the control bandwidth of the servo system, such as reducing the gain parameter value of the servo system.

The above is just one of the reasons for the unstable noise. There are different solutions for different reasons. For example, for the noise caused by mechanical resonance, resonance suppression and low-pass filtering can be adopted in the servo. In short, the reasons for noise and instability are basically not caused by the servo motor itself.

bigbat

In this case, you can add a PID algorithm to solve the control problem. Reasonable adjustment parameters will usually improve the problem. You can also increase the power of the servo motor to solve the problem.

freebsder

We have also been adjusting the motors recently and it feels like we are doing something different.

Gen_X

This analysis is very useful! Thumbs up!

Sometimes the system is stable, but external electronic or mechanical interference "shock" can cause the system to briefly enter "resonance", thereby generating noise. This is the most difficult to deal with because it is very random. It may not occur for 10 days, or it may appear 20 times in 10 minutes. There is no regularity and it is difficult to capture the frequency.