Mecos Traxler (Switzerland)

Unbalance of magnetically levitated rotors causes undesirable synchronous vibrations which may lead to saturation of the magnetic actuator. To avoid this problem we propose a generalized narrow-band notch filter which is inserted into the multivariable feedback without destabilizing the closed loop. The parameters of the generalized notch filter strongly depend on the inverse sensitivity matrix evaluated at rotational speed. This matrix is easily measured a priori and stored in a look-up table. It is shown that a decentralized notch is feasible for weakly gyroscopic rotors. The proposed notch filter approach has advantages in terms of run-time complexity and analytical verification of closed-loop stability. Results from the implementation of the proposed unbalance compensation in industrial magnetic bearing systems are included.

The problem of achieving high mechanical stiffness in an active controlled flexible structure is investigated. A wideband disturbance force is assumed to act at the effector (e.g., a cutting tool of an electromagnetically borne spindle). Explicit and quite general limits on the best possible achievable performance are found. Mechanical (or any other) systems can be divided in two basic categories. Either arbitrarily low compliance can be achieved at the cost of high controller gain or there exists a nontrivial bound on the disturbance rejection. The tradeoffs are computed explicitly for some simple systems using H/sub infinity / techniques. They enable one to assess a given simple controller, as it can be estimated how close such a controller is to the theoretical optimum. >