Speed sensor encoder wheel and method of making

A method of making an encoder for use in a speed sensor includes providing a metal tube. A material containing ferrite is then extruded over the metal tube to produce an encoder tube. The encoder tube is cut to a given length to form an encoder. The encoder is then magnetized within a magnetic field.

FIELD OF THE INVENTION

The present invention relates to speed sensors and more particularly to a speed sensor encoder wheel and a method of making same.

BACKGROUND OF THE INVENTION

Encoder wheels are used in speed sensor devices to determine the rotational speed of a component. For example, in motor vehicle applications, an encoder wheel often forms part of an anti-lock braking system for determining the rotational speed of the wheels. Other applications include coupling to engine crankshafts and within transmissions.

The encoder wheel typically includes an outer surface having magnetized ferrite therein. The ferrite is magnetized to form alternating north and south poles around the circumference of the encoder wheel. As the encoder wheel rotates, a magnetic sensor measures the change in the magnetic field of the alternating poles. From this alternating magnetic field, the rotational speed of the encoder wheel may then be calculated, and in turn the rotational speed of the component to which the encoder wheel is coupled is known. Alternatively, hall effect sensors, which do not use magnets, can also be utilized for detecting the rotational speed.

A conventional encoder wheel is produced from a tube of stamped steel with a ferrite loaded rubber compression molded thereon. However, in order to retain the ferrite loaded rubber, the stamped steel must have flanges or features to engage the rubber. This in turn enlarges the size of the encoder wheel. Moreover, such features increase the cost of manufacturing the encoder wheel and increase the costs of packaging. Finally, each encoder wheel is made separately and individually. This can lead to non-uniformity between encoders. Accordingly, it is an object of the present invention to provide an encoder wheel and method of making that addresses these limitations.

SUMMARY OF THE INVENTION

A method of making an encoder for use in a speed sensor includes providing a metal tube. A material containing ferrite is then extruded over the metal tube to produce an encoder tube. The encoder tube is cut to a given length to form an encoder wheel. The encoder wheel is then magnetized within a magnetic field.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference toFIG. 1of the drawings, an encoder wheel10constructed according to the principles of the present invention is shown in operative association with an exemplary motor vehicle12. In the particular example provided, the encoder wheel10forms part of an anti-lock braking system14. The anti-lock braking system14further includes a magnetic field sensor16in communication with a vehicle control module18. The encoder wheel10is preferably coupled to an axle shaft20that rotatingly drives a pair of wheels22. With brief reference toFIG. 1A, the encoder wheel10is magnetized such that it includes a plurality of alternating north and south magnetic poles around its circumference. As the wheel22and axle shaft20rotate, the encoder wheel10in turn rotates. The magnetic sensor16senses the magnetic field emitted from the encoder wheel10as the magnetic field alternates between positive and negative magnetic fields as the encoder wheel10rotates. The vehicle control module18may then use this information to determine the rotational speed and/or rotational position of the encoder wheel10, and therefore of the axle shaft20and the wheel22. It should be appreciated that the encoder wheel10may be part of any magnetic speed sensing system, for example, as part of an engine speed sensing system where the encoder wheel10is mounted on a crankshaft or camshaft (not shown) so that the rotational speed and/or rotational position of the crankshaft or camshaft can be accurately determined for use in an engine control system.

Turning toFIG. 2, a plurality of encoder wheels10are illustrated, each being identical to another. The encoder wheels10are cut from a continuous length of encoder tubing28, the manufacturing of which will be described in greater detail below. The encoder10includes an inner tube24and an outer tube26extruded overtop the inner tube24. The inner tube24is preferably steel formed by an extrusion method. However, various other metals may be employed for the inner tube24and various other methods of making, such as stamping, may be employed.

As noted above, the outer tube26is an extruded material that encases the inner tube24. The outer tube26is preferably rubber loaded with ferrite, although any extrudable material that can be loaded with ferrite may be employed, such as, for example, a plastic or thermoplastic material. In the preferred embodiment, the encoder wheel10includes an inner diameter of 40.45 mm and an outer diameter of 44.45 mm with a width of 20 mm. However, it should be appreciated that the encoder10may be of virtually any size without departing from the scope of the invention.

With reference toFIG. 3, the continuous length of encoder tubing28is formed using an extrusion process on the inner tube24. For example, the inner tube24is pre-fabricated and fed into a mold30. The mold30encases the inner tube24and defines a mold cavity32. The mold cavity32in turn will define the outer surface of the outer tube26(FIG. 2). The mold30is coupled to an extrusion apparatus34.

The extrusion apparatus34may take many forms without departing from the scope of the invention. Moreover, more than one extrusion apparatus34may be used with the mold30(e.g., an extrusion apparatus34at 120 degree intervals around the mold30). In the particular example provided, a mixture of plastic granules and ferrite36are fed into a hopper38. The hopper38feeds the plastic granules and ferrite36into a cylinder40. The cylinder40is heated at an elevated temperature by heating elements42surrounding the cylinder40. A screw mechanism44is located within the cylinder40and is rotatingly driven by a motor (not shown).

As the plastic granules36enter the cylinder40, they are heated by the heating elements42and become semi-molten/molten plastic, generally indicated by reference numeral46. The screw mechanism44as it rotates forces the mixed semi-molten plastic and ferrite46into the mold30, thereby filling the mold cavity32surrounding the inner tube24. The semi-molten/molten plastic and ferrite46then cools and forms the outer tube26(FIG. 2). The mold30may then be opened and the continuous encoder tube28removed therefrom. Alternatively, the inner tube24may be fed through the mold30in a continuous process without opening the mold30. Once the encoder tube28has been formed, any number of encoders10may be cut from the encoder tube28at any desired width. Magnetization of the ferrite within the outer tube26to form alternating poles (seeFIG. 1A) may be accomplished either before or after cutting of the encoder tube28to form the encoder wheels10. The ferrite within the outer tube26is magnetized to preferably include 32 pole pairs all of equal widths, though any number of pole pairs may be used and may include asymmetrical widths.

By extruding the outer tube26onto the inner tube24, the encoder wheel10finishes with a low profile (i.e., no flanges or bumps) that allows the encoder10to fit within small areas, including driveshafts. Moreover, extrusion of the outer tube26onto the inner tube24allows a large number of encoders10to be cut from a single encoder tube28, thereby saving on manufacturing costs and increasing uniformity among encoders10.