Tailoring force-stroke characteristics in medium-stroke linear reluctance actuators

Summary form only given. Linear reluctance actuators based on a pair of C-cores offer a robust and high efficiency solution for controlled linear motion when strokes of the order several mm are required, e.g. electro-hydraulic servo valves. Although such actuators are capable of producing a relatively constant force over a significant proportion of their stroke, there is inevitably a roll-off in force as the armature moves towards full alignment. In some cases that require a near constant-force, restricting the working stroke can accommodate this roll-off in force. However, there are some applications in which the space envelope for the actuator is very constrained, e.g. electromagnetic valve actuation systems for internal combustion engines, as a consequence of which it may be necessary to employ the entire stroke. One approach that is investigated in this paper is to incorporate a degree of overhang in order to generate a component of normal reluctance force, which increases in magnitude as the actuator comes into alignment. The ability to optimise the dimensions and the shape of the overhanging pole and the upper edge of the armature affords considerable scope to tailor the force-stroke characteristic to the specific demands of the end application. This feature is illustrated by means of two detailed design studies. In the first case, the aim is to generate a near constant force over the entire stroke, while in the second case, the aim is to supplement a near constant force with a significant holding force near alignment, as might be required for example in electromagnetic valve actuation systems. The design studies include extensive finite element analysis of various actuators and are supported by experimental measurements on prototype devices.