Magnetic modeling of a mutually-coupled two-finger variable reluctance gripper

Variable reluctance actuators have been used widely as low-cost, on-off type mechanical actuators, due to their inherit advantages of rugged-construction and ease-of-manufacture. This paper reports on the investigation work of applying variable-reluctance principle to the design of a novel two-finger gripper. Contrary to previous on-off type variable reluctance devices, the proposed actuator can be operated in position-control, velocity-control, and force-control modes. To enable effective control of the two-finger variable-reluctance gripper, its electromagnetic and mechanical properties have to be fully investigated, and an efficient model has to be constructed. Since the forces on the two fingers of the gripper are produced by the variation of flux, and flux is a nonlinear quantity, the electromagnetic characteristics of the two-finger gripper are much more complex than other types of devices (e.g. permanent magnet voice coil). Moreover, the existences of coupling effects make the characterization of the gripper even more complicated. In the past, most mutual coupling effects in variable reluctance machines are neglected for the sake of simplicity. However, due to the unique structure of the two-finger variable-reluctance gripper, it was found that mutual coupling can constitute significant modeling discrepancy. The work described in this paper is the first of its kind to carry out a detail study on the electromagnetic properties of a tightly-coupled two-finger variable-reluctance gripper, including its mutual-coupling effects. In the study, the flux modeling of a mutually coupled variable reluctance gripper is carried out. Magnetic equations of the actuator structure are derived and the flux model is constructed. To verify the study results, a similar two-finger variable-reluctance gripper was fabricated and its electro-magnetic characteristics were measured. When the measured results were compared with the model simulation, it was found that the two sets of data matched quite well with each other and confirmed that the model was an accurate representation of the two-finger variable-reluctance gripper. The investigation work describes in this paper enables a better understanding of mutually-coupled limited-stroke variable-reluctance actuators. Furthermore, it produces an effective simulation model for the future design and control of this class of device.