Bearing-less four-coil oscillatory linear motor

Abstract

Linear direct oscillatory actuation has the prime advantage to eliminate all components connected with the transformation of rotatory to translational motion. All negative side effects of these components, which are mainly losses and the need of a lubrication system, belong to the past. Moreover, the absence of a crankshaft reduces the constraints on the linear motion. On the other side, the target power density known from conventional drives pushes the requirements to higher operation frequencies and lower mover masses. This favors slotted motor geometries with small air gaps. Thus, unfortunately friction issues again require a closer look. The bearing forces now result to a considerable amount from the unbalanced magnetic pull. Though they ideally cancel in double-sided systems, manufacturing tolerances result in inevitable bearing forces and increase the requirements on dry running bearings. The linear oscillatory system analyzed in this paper follows the approach of keeping the moving mass small, the motor compact and eliminating any mechanical bearing issues at the same time. A minimal system is found in a four-coil arrangement capable of simultaneously generating axial thrust and bearing forces. The plate structure of the mover stabilizes the remaining degrees of freedom of the rigid body motion. The system is modeled and linearized to allow a decoupling of the thrust and bearing forces. The proposed control decouples the forces and is implemented on an evaluation model to validate the operation of the oscillatory bearing-less (self bearing) linear motor.