Mechanical decoupling between motor and load
Decoupling happens when a section of the mechanical linkage changes in a way that causes the motor to sense variations in the inertia of the load. Examples of decoupling include twisting of a shaft, flexing of a mechanical coupling, elasticity of a timing belt, and gear backlash.
As the gain of the speed controller is increased, the drive's commands and the motor's responses become more rigid or synchronized. The stiffness of this command-response between the drive and motor is very similar to mechanical stiffness. In fact, once this drive-motor stiffness surpasses the stiffness of any of the mechanical linkages, those linkages decouple. The image below shows three separate decoupling events. The 29-53Hz valley-peak is one, the 180-210Hz valley-peak is another, and the 320-350Hz valley-peak is the third.
This Bode plot shows three separate decoupling events.
As the frequency increases logarithmically from left to right, a valley is observed at 29Hz. This is known as the natural or locked rotor frequency. At 53Hz, a peak -- known as a pole frequency -- is shown. If these were the only peak and valley in the entire Bode plot, this system would be known as a two-mass system, where the two masses would be the inertia of the motor's rotor and the inertia of the load. The plot line below 29Hz would represent the characteristics of the motor, and the plot line above 53Hz would represent the characteristics of the load. The plot section between 29 and 53Hz represents the decoupled region. The drive is unable to control these frequencies, so ideally it is best if these decoupled frequency regions are kept to a minimum.
If you are having trouble visualizing the concept of decoupling, perhaps this analogy will help. Imagine you have your hand outstretched holding one end of a rubber band. At the other end of the rubber band is a one-pound ball hanging from gravity. If you gently move your hand up and down, you will sense variation in the load as if the weight of the ball were changing. When the band is being stretched, the mass seems higher, and when the band is contracting, the mass seems less. This is similar to what the motor experiences when a shaft twists or a coupling flexes or a belt stretches. These changes are linear and don't seem so abrupt to your senses. However, gear backlash is nonlinear, and the above analogy is not adequate.
To imagine gear backlash, we start with the same scenario of a one-pound ball at the end of a rubber band. However, this time, the rubber band is being cut so that we instantly sense a change from one pound to zero. Our hand might actually jerk up for an instant until we adjust our arm muscles to the fact that there is no longer a weight to hold up. Just as we adjust to this no-load condition, the band is magically restored to its original condition, and we instantly sense the one-pound ball again. This time, our hand might drop down until we adjust our arm muscles to compensate for the new weight.
No matter if the decoupling is linear or nonlinear, the result is the load seen by the motor shaft changes. Stability of a drive controller exists when the gain of the speed controller properly matches the inertia of the connected load. When sections of the mechanical load decouple, the motor shaft senses less inertia. The controller gain is no longer properly matched, because the perceived inertia is less. If enough of the load decouples, the gain-to-inertia ratio can reach a level that creates instability. Nonlinear decoupling (gear backlash) is the worst type of decoupling, because the perceived inertia value changes so drastically.
Marcus Schick is the industry business developer for the motion control business of Siemens Industry Inc.