An energy-based design criterion for magnetic microactuators

Magnetic actuators can be divided into two types: those in which motion changes the gap separation (type I) and those in which motion changes the gap overlap area but not the gap separation (type II). In conventional magnetic actuators of both types, it is assumed that most of the magnetic energy is stored in the gap due to the large reluctance of the gap compared with the negligibly small reluctance of the magnetic core. However, in magnetic microactuators, the fabrication limitations on the achievable cross-sectional area of the magnetic core as well as the finite core permeability increase the core reluctance to the point that this assumption may no longer be valid. In this case, the magnetic energy is distributed in both the gap and the magnetic core, in which the energy distribution is in proportion to the reluctance of the gap and the reluctance of the core respectively. Using an elementary structure of a magnetic actuator, it is shown that for type I microactuators, when the initial gap of the actuator is fixed (e.g., determining the stroke of the actuator), the generated magnetic force has maximum value when the gap overlap area is designed such that the reluctance of the gap is equal to the reluctance of the magnetic core (i.e., ). For type II actuators, the initial overlap area of the actuator is fixed (determining the stroke); therefore the generated magnetic force has a maximum value when the gap separation is designed such that the above equality holds. This paper details both analytical and finite element method (FEM) analysis confirmation for type I actuators. Extension to type II actuators is straightforward.