Eccentricity, Wobble, and How a Servo System Can Help

By definition, Eccentricity is a measure of how much a roll deviates from being perfectly circular. Ideally, eccentricity should be zero, but in reality, it is never zero. Practically not a single roll is a perfect circle because it is produced using a machine which itself is prone to some machining errors (since it is product of some other machine and so on).

Eccentricity in the roll gives rise to wobble when rotated using a motor. Wobble is the angular deviation of the axis of rotation over one revolution.

Now let's understand how eccentricity can affect the machine performance.

Now consider two rolls in contact, one having half the diameter of the other.

With an induction motor, it expects to deliver a fixed torque at a fixed/constant speed. But as the load varies (due to eccentricity), the torque starts varying. The result is higher acceleration of the motor.

T = J x α (T - Torque, α - Angular Acceleration, J – Moment of Inertia)
Hence, α = T / J

So change in the torque gives rise to change in acceleration & hence change in speed.
If the motor is to deliver a fixed torque, then due to the load variation, the speed profile will be as shown below:

To maintain constant speed, the motor has to deliver torque as per the load variation.

Typically, the motor has to respond to changes in torque every 100 ms (e.g. for roll running at 600 rpm).Typically for induction motor, torque rise time is in the tune of 350-400 ms. So despite having superior drive control, a normal induction motor cannot respond in the required time frame. The more connected rolls, the greater the load variation. In production machines, there are many rolls rotating together at very high speeds.

How can Servo system help here?

1. Conventional systems driven by induction motors are not dynamic enough to respond to such quick changes (to the tune of 100 ms).
2. A permanent magnet synchronous servo motor (e.g. AKM from Kollmorgen) has torque rise times of 80-100 ms. Therefore, the response is faster.
3. A servo drive (e.g. AKD from Kollmorgen) has current controller cycle time of 670 ns (fastest in the industry) which helps the servo motor achieve the desired response.

So in order to take care of varying torque demands due to eccentricity issues in the mechanical systems, servo based motion control systems can perform much better as compared to conventional systems. What kind of eccentricity issues have you had to deal with recently?

Application Team India

This was written by one of our Application Engineers in Mumbai. You can reach the Indian team by email: [email protected]
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