Reducing Servomotor Instability

As servo drives become more prevalent in industry, they are being applied in a wider range of applications. A common complaint with servos is that they sometimes make an undesired growling noise. This problem can be eliminated by reducing gain on the speed controller. However, lower speed controller gains can lead to an increase in position error and a decrease in needed performance. Here we will look at the problem of servo instability and methods that can be used to eliminate this problem.

Instability in a motor is uncontrolled and unintended motion at the motor shaft. It can occur at low or high frequencies, with the higher frequencies often becoming audible (i.e., growling). Instability is caused by excessive gain in the speed controller of the drive. The gain setting of the speed controller determines how much torque the drive will generate. For this reason, the gain value should be directly proportional to the inertia of the connected load that is seen by the motor shaft.

Some may wonder why they have never had instability problems with AC vector drives. It should be made clear that vector drives can also reach unstable conditions. In practice, most servo applications are more dynamic than speed-only applications. The more dynamic applications require higher gains in the speed controller, which increases the chance of instability.

There are several factors that can contribute to the likelihood of a drive experiencing instability. These include excessive gain in speed controller, a lower-resolution feedback device on the motor, and gearing backlash and mechanical decoupling of the load from the motor.

Most drive engineers are taught that the proper tuning of a speed controller is done with the step response method. This involves running the motor typically at 10-20 percent speed and introducing a step change in the speed set point of 5-10 percent. This step change creates a disturbance to which the drive must respond. By tracing the drive's response to the step change, it can be measured how much the drive overshoots the new speed set point and how quickly the drive returns to a steady state speed value of less than plus or minus 0.5 percent. Many engineers are taught to increase the speed controller gain gradually until they record signs of instability in the step response and then reduce the gain slightly from that point to avoid the instability.

The mistake is that often this test is done at only one speed or at speeds of more than 10 percent of full speed. In some cases, this speed controller gain setting will seem stable at 10 percent speed but may result in instability at lower speeds. Many machines enable servo drives at zero or near-zero speed and may remain at this low speed prior to starting production. This is the point where instability or growling is sometimes reported.

Another factor is that drive tuning is done during commissioning when the machine is new and the mechanics are tight. Over time, the machine will run for hundreds of hours, and the mechanics will loosen up. This additional compliance in the mechanics is different from what the drive was tuned with during commissioning. This is why the instability often occurs several months after the machine is running.