Design of a linear transverse flux actuator for fast positioning

Transverse flux motors feature high force densities and hence compact designs. Unfortunately, their complex three-dimensional geometries hamper industrial series production. This is especially true when the machine's dimensions are very small. The numerical computation of a linear transverse flux actuator, which is simultaneously compact and manufacturable, is presented here. The use of powdered soft magnetic composites is appropriate for this application. The finite element method is used to calculate the forces, the inductance and the back-emf. The actuator is designed for high dynamic and accurate positioning. LINEAR TRANSVERSE FLUX ACTUATOR When compared to other machine types, transverse flux machines feature high achievable force densities up to 4 N/cm and even more, because the coil flux is linked to all the mover poles [1]. Figure 1 shows a linear transverse flux actuator. The mover comprises three high energy NdFeB-magnets magnetised in the direction of motion and two ferromagnetic connections representing the mover poles (Fig. 2). Powdered soft magnetic composites are particularly appropriate for this application, because the use of permanent magnets enlarges the airgap magnetically and hence reduces the relative importance of the material's permeability [2]. Furthermore, the stator comprises two components of the same geometry, which are easily formed in a die followed by a sintering process. This permits industrial series production, even when the dimensions are very small. FUNDAMENTAL BEHAVIOUR The analysis of this actuator requires a 3D-approach, due to the complexity of the flux patterns. The total force Ftot is the sum of a reluctance force Frel and a force originating from the current in the coil, called the net force Fnet. The y-component of the force is not considered here, as it remains small relative to the others. The fundamental behaviour of this device is studied by the finite element method. The 3D mesh, obtained by applying solid modelling, is strongly refined close to the teeth and the mover in order to obtain an accurate field solution around the mover. The forces are determined from this solution by further improving the accuracy of the local field solution around the mover, using a local analytical approximation of the Laplace equation [3]. The position-dependency of the surface force density components, for a constant current in the coil, is plotted in Fig. 3. The position is expressed in terms of the number of pole pitches (Fig. 2). DOUBLE MOTOR REQUIREMENT It is impossible to generate a constant driving force in the direction of motion because zero-force positions appear along the trajectory. Moreover, holding the mover in a fixed position requires a constant current in the coil to compensate for the reluctance force. This yields very low efficiencies. These problems are solved by joining a second, identical actuator to the construction to form a double arrangement. Both movers are connected by a non-ferromagnetic material. The second actuator is shifted relatively to the first motor over a distance of half a pole pitch, in order to obtain an approximately constant resulting net force in the direction of motion when both coils are driven by the same current. Mover Stator