Electricity: measuring and testing – Magnetic – Displacement
Reexamination Certificate
2001-01-09
2002-10-22
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Magnetic
Displacement
C324S207210, C324S207240
Reexamination Certificate
active
06469497
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method of sensing position using magnetoresistors.
BACKGROUND OF THE INVENTION
It is well known in the art that the resistance modulation of magnetoresistors can be employed in position and speed sensors with respect to moving magnetic materials or objects (see for example U.S. Pat. Nos. 4,835,467, 4,926,122, and 4,939,456). In such applications, the magnetoresistor (MR) is biased with a magnetic field and electrically excited, typically, with a constant current source or a constant voltage source. A magnetic (i.e., ferromagnetic) object moving relative and in close proximity to the MR, such as a toothed wheel, produces a varying magnetic flux density through the MR, which, in turn, varies the resistance of the MR. The MR will have a higher magnetic flux density and a higher resistance when a tooth of the moving target wheel is adjacent to the MR than when a slot of the moving target wheel is adjacent to the MR.
Increasingly more sophisticated spark timing and emission controls introduced the need for crankshaft sensors capable of providing precise position formation during cranking. Various combinations of magnetoresistors and single dual track toothed or slotted wheels (also known as encoder wheels and target wheels) have been used to obtain this information (see for example U.S. Pat. Nos. 570,016, 5,731,702, and 5,754,042).
The shortcoming of MR devices is their temperature sensitivity. They have a negative temperature coefficient of resistance and their resistance can drop as much as 50% when heated to 180 degrees Celsius. Generally, this led to the use of MR devices in matched pairs for temperature compensation. Additionally, it is preferable to drive MR devices with current sources since, with the same available power supply, the output signal is nearly doubled in comparison with a constant voltage source.
To compensate for the MR resistance drop at higher temperatures, and thus, the magnitude decrease of the output signal resulting in decreased sensitivity of the MR device, it is also desirable to make the current of the current source automatically increase with the MR temperature increase. This is shown in U.S. Pat. No. 5,404,102 in which an active feedback circuit automatically adjusts the current of the current source in response to temperature variations of the MR device. It is also known that air gap variations between the MR device and ferromagnetic materials or objects will affect the resistance of MR devices with larger air gaps producing less resistance and decreased output signals.
What is needed is a position sensor capable of self compensation over wide ranges of temperature and air gaps, including tilts.
SUMMARY OF THE INVENTION
The present invention is a position sensor system capable of self compensation over wide temperature ranges and air gaps, including tilts. It employs three matched MRs (also commonly referred to as MR elements) with either one common bias magnet or separate bias magnets. The MRs are aligned in the direction of movement of a magnetic target. The middle MR is the actual position sensor. The two outer MRs serve as reference sensors which sense the magnetic field at the limits of the position sensing range. The cooperating magnetic target assures that one of the two outer MR elements is always exposed to some maximum magnetic field, B
MAX
, corresponding to a position X
MAX
, and the other MR element is always exposed to some minimum magnetic field, B
MIN
, corresponding to a position X
MIN
, such that B
MIN
≦B
X
≦B
MAX
, corresponding to X
MIN
≦X≦X
MAX
, where B
X
is the magnetic field measured by the middle MR and varies with the position, X, of the target.
The actual position, X, is computed assuming a linear relation between MR resistance in the magnetic field range from B
MIN
to B
MAX
and the position, X, of the target. This is accomplished by making the magnetic field a linear function of position in the range X
MIN
to X
MAX
and using MRs whose resistance is a linear function of magnetic field in the range B
MIN
to B
MAX
.
Alternatively, the shape of the magnetic field profile can be tailored to the MR characteristics in the operating magnetic field range B
MIN
to B
MAX
to yield the desired linear relation between MR resistance and position X. That is, the sensor always operates on the linear part of the MR resistance versus magnetic field characteristic curve in the magnetic field range B
MIN
to B
MAX
.
Accordingly, it is an object of the present invention to provide a magnetic position sensor system capable of self compensation over wide ranges of temperatures, air gaps and tilts.
REFERENCES:
patent: 4835467 (1989-05-01), Gokhale
patent: 4926122 (1990-05-01), Schroeder et al.
patent: 4939456 (1990-07-01), Morelli et al.
patent: 5327077 (1994-07-01), Honda
patent: 5351003 (1994-09-01), Bauer et al.
patent: 5570016 (1996-10-01), Schroeder et al.
patent: 5585719 (1996-12-01), Endo et al.
patent: 5731702 (1998-03-01), Schroeder et al.
patent: 5754042 (1998-05-01), Schroeder et al.
Delphi Technologies Inc.
Dobrowitsky Margaret A.
Lefkowitz Edward
Zaveri Subhash
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