Design and Comparison of Permanent Magnet Self-Bearing Linear-Rotary Actuators

Linear-rotary actuators (LiRAs) are electric machines that can perform linear and rotary movements. They are used in many different applications, for example, for pick-and-place robots, in packaging or sorting lines, or as gearbox actuators. A linear-rotary movement can be obtained with various combinations of linear and rotary machines, whereas depending on the specifications of the underlying application the most suitable actuator arrangement has to be identified. In order to simplify the selection of the appropriate actuator configuration, this paper first gives an overview of possible realization concepts of linear-rotary actuators, which are also suitable to implement magnetic bearings (MB). Afterwards, fundamental scaling laws concerning achievable axial forces and torques of linear and rotary machines with interior and exterior rotor arrangement are derived, enabling a qualitative comparison in order to figure out the most suitable actuator concept. In this context, it is important that the derivation also considers the machine-internal heat flow and the heat dissipation to the ambient, which finally leads to a maximum current density depending on the selected topology. All findings are verified by finite element method simulations. In order to show the applicability of the derived scaling laws, a design example is discussed.