Reducing servo motor reducer Instability

As servo motor reducer drives become more prevalent in industry, they are being applied in a wider range of applications. A common complaint with servo motor reducer is that they sometimes make an undesired “growling” noise. This problem can be eliminated by reducing the gain on the speed controller. However, lower speed controller gains can lead to an increase in position error and a decrease in needed performance. This paper looks at the problem of servo instability and methods that can be used to eliminate this problem. It is best to begin by defining instability. 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. Remember the phrase “load seen by the motor shaft”, as we will examine what this means later in this paper and how it affects stability. 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:

• servo motor reducer excessive gain in speed controller

• servo motor reducer lower resolution feedback device on motor (i.e. resolver or pulse encoder)

• servo motor reducergearing backlash and mechanical “decoupling” of the load from the motor