The present invention relates to a linear magnetic sensor for indicating the position of two relatively movable parts and more particularly to a sensing device comprising a bar magnet and a Hall effect device mounted a fixed distance apart but relatively movable with respect to each other.
As disclosed in U.S. prior art Pat. No. 4,107,604, a position-indicating device utilizes a Hall effect device and provides a predictable linear output. A magnet and Hall effect device are movably mounted in a fixed orientation such that they cannot shift or rotate relatively from the original pre-set position except for relative movement translationally in a given direction. It is stated that in this way variations in magnetic characteristics and hence variations in output from one application to another are eliminated. The magnet is in the form of an elongated bar which is magnetized from end-to-end, meaning that the North Pole is on one end and the South Pole on the other. Therefore, as the Hall effect device is moved parallel to the rod, it encounters strong fields of opposite polarities at the ends but a reduced field as the Hall effect device approaches the rod center.
It must be remembered that a Hall effect element is essentially a thin, planar structure which produces an output voltage proportional to magnetic flux normal to the plane thereof. Thus, as flux lines tend to approach a plane parallel to the Hall element, the output voltage substantially diminishes even to the point of zero volts relative to the offset voltage.
In this prior art device, since the field impinging the Hall element is not changed linearly as the element and magnet are moved relatively, the voltage output from the Hall element cannot be truly linear for any significant portion of the magnet's length.