1. Field of the Invention
The invention relates to variable reluctance speed sensing devices and is particularly useful in such devices used with vehicle engines and transmissions.
Variable reluctance speed sensing devices are widely used for sensing the speed of a rotating or moving part. For example, they are used to sense the speed of a rotating shaft to provide a tachometric signal which may be used to control an engine or vehicle speed display. The same sensor may also provide synchronization information for injection of fuel or triggering an ignition spark.
In vehicles, a common speedometer drive includes a worm gear coupling to a flexible shaft. Typically, the coupling is inserted through a threaded hole whose axis is generally tangential with a rotating gear; or the sensor is inserted through a smooth bore until a shoulder and an axial locating surface abut, and an internally threaded cap is turned down over external threads on the transmission to hold the speedometer cable in place. Known variable reluctance speed sensors cannot readily be used as replacement items with these arrangements, because the known sensors sense fields which are parallel to the sensor axis.
2. Description of the Prior Art
A typical variable reluctance sensor has an elongated pole piece made of a soft magnetic material (i.e., one with low remanence), which extends through a coil and has a permanent magnet disposed at one axial extremity of the pole piece. The other axial extremity of the elongated pole piece is axially spaced by a relatively small gap from the teeth of a flywheel or other gear whose rotational speed is to be sensed. Thus, at any specific time either one of a plurality of teeth, or a space between adjacent teeth, of a ferrous toothed wheel is opposite the pole piece. The passage of each tooth by the pole piece produces an increase of the flux coupling the coil, thereby generating an alternating voltage in the coil. The frequency of this voltage is utilized to measure the speed of rotation of the ferrous wheel.
Well-known types of such sensors include the constructions shown in FIGS. 1 and 2. These constructions differ in that, in the embodiment of FIG. 2, the magnet end of the pole piece is provided with an enlarged head. This head corresponds generally to the shape of a nail. Because each of these known designs utilize the axial extremity of the elongated pole piece to cooperate with the teeth of the ferrous wheel or gear, they must be oriented generally along a radius line of the wheel or gear. This design cannot be substituted readily in a machine or vehicle which had used a worm gear mechanical sensor.