A new bi-directional inductive force sensor

This paper presents a new kind of inductive force sensor that gives the amplitude and the direction of the force in a plane, and that is different from the lately developed technologies utilizing the magnetostrictive or magnetoresistive effect. It is constituted of a magnetic circuit with airgaps that vary in length with the applied force. When a force is exerted, a bar is bent and one of the airgaps becomes wider, the reluctance of the corresponding magnetic circuit increases and the inductance of the coil located on this circuit decreases; the other airgap becomes narrower. The bending of the bar is proportional to the applied force, so, the inductance variations are an image of this force. The use of two coils leads to a differential structure. The same construction is done to measure the force in the orthogonal direction. This structure gives the two components of a force in a plane. The sensor is studied first to optimize its geometry with regard to the dimensional parameters. An optimum is found that gives the maximum airgap variations for a given force, and so the maximum signal variations. A magnetic study is done with finite elements calculations. A prototype was built to measure lift and drag of a rudder on a ship.