Motor can quench its own vibrations

Three engineers working for the disc drive manufacturer Seagate, have been granted a US patent for a brushless DC motor which also serves an actuator to stabilise vibrations and resonance in the motor. Although one promising application is to reduce vibrations in disc drive spindle motors, and thus to boost the drives` storage capacity, the technology is said to have other applications.

Conventional techniques for reducing vibrations include using non-contact bearings, such as magnetic or hydrodynamic bearings, or electronic damping. But both of these add considerably to the cost and complexity of the drive system. Another suggestion has been to add mechanical or electromechanical parts to the motor to damp the vibrations, but the Seagate engineers say that this is not effective for detecting resonance modes which can exist in rotating motors, and is also costly.

In the new technique (described in US patent 6603225), resonant movements are simulated and used to derive a lagging damping signal which controls the application of a damping force. The 90 degree out-of-phase force is said to correct the tendency for resonant movements to occur. Because there is a velocity component in the force calculations, the technique is said to be much more effective at higher frequencies than simply adding stiffness to a system.

The damping force is created using extra windings in the motor, grouped in phases. But the inventors say that a similar effect could be achieved by using a motor`s existing windings. For example, the windings could be tapped, and the currents needed to generate the stabilising radial force could be added to the normal motor driving currents.