Electricity: measuring and testing – Magnetic – Displacement
Reexamination Certificate
2001-02-09
2002-11-26
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Magnetic
Displacement
C324S207250
Reexamination Certificate
active
06486657
ABSTRACT:
TECHNICAL FIELD
The present invention relates to semiconductor magnetoresistive devices, also known in the art as magnetoresistors, employed in position and speed sensors, and more particularly to a malfunction detector which uses the output signal to monitor the functionality of speed and position sensors.
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 ferromagnetic 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 rotating 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 rotating target wheel is adjacent to the MR than when a slot of the rotating target wheel is adjacent to the MR. Angular position information is contained in the location of target wheel tooth edges (i.e., tooth/slot transitions), and at these locations the output signals of the MRs are by design unequal so that their differential signal is nonzero.
High accuracy and repeatability magnetic position sensors employ two matched sensing elements such as magnetoresistors or Hall generators. They are spaced a few millimeters apart from each other, either in the axial direction (dual track target wheels) or along the target periphery (sequential sensors). The primary purpose of using two matched sensing elements is common mode signal rejection, since the sensing elements are equally affected by temperature and air gap. Presently, selection of matched MR pairs, a tight process control during all phases of sensor manufacture with a final testing of each sensor, is employed to build sensors meeting the required specifications. Unfortunately, this approach increases the final cost of the sensor.
Currently, variable reluctance (VR) sensors are the most common types of anti-lock braking system (ABS) wheel speed sensors. They are rugged and inexpensive, but are incapable of sensing zero wheel speed. A feature demanded by an increasing number of sophisticated ABS implementations. Also, they do not lend themselves to easy monitoring and automated fault detection. In contrast, semiconductor magnetoresistors manufactured from InSb, InAs, and other compound semiconductors provide large signal outputs down to zero wheel speed and, being resistors, they allow for continuous monitoring and fault detection by simple means without interfering with the wheel speed sensing process.
FIG. 1
shows a present wheel speed sensor utilizing a single MR sensor
100
driven by a constant current source
120
powered by a supply voltage V
B
with output voltage V
S
wherein the passage of a tooth
140
of the rotating target wheel
180
produces a high output voltage and the passage of a slot
160
produces a low output voltage. A constant current source
100
is the preferred drive method for single MR sensors. The use of a constant voltage drive, however, would not affect a malfunction detection system.
FIG. 2
depicts the output voltage V
S
corresponding to the two extreme operating conditions within the specified tolerance range of the sensor
100
as V
S1
and V
S2
. The MR sensor
100
will produce the highest output voltage signal V
S
=V
S1
when the MR sensor is simultaneously operating at the lowest temperature, smallest air gap, and largest MR drive current, all within the specified tolerance range, however. The MR sensor
100
will produce the lowest output voltage signal V
S
=V
S2
when the MR sensor is simultaneously operating at the highest temperature, largest air gap, and smallest MR drive current, all within the specified tolerance range, however. The voltage span between the largest value of V
S
=V
MAX
and the smallest value of V
S2
=V
MIN
defines the correct operating range of the sensor
100
with a corresponding output signal voltage V
S
. That is, V
MIN
<V
S
<V
MAX
. Monitoring a failure of the MR sensor
100
requires that the maximum output voltage V
S
not exceed V
MAX
and that the minimum output voltage V
S
will not fall below V
MIN
.
The output signal V
S
exceeding V
MAX
may, for example, be indicative of such potential problems as too large a MR drive current
120
, defective MR die, bad wiring, bad connector, insecure sensor, or loose target wheel mount. The output signal V
S
falling below the value of V
MIN
may, for example, be indicative of a partial short circuit, total short circuit, insufficient MR drive current
120
, defective MR die, insecure sensor, or loose target wheel mount.
FIG. 3
shows a present wheel speed sensor utilizing a dual MR sensor
200
driven by a constant supply voltage V′
B
with output voltage V′
S
wherein the passage of a tooth
240
of the rotating target wheel
280
produces a high output voltage and the passage of a slot
260
produces a low output voltage. A constant voltage source V′
B
is the preferred drive method for dual MR sensors. The use of constant current drives, however, would not affect a malfunction detection system.
FIG. 4
depicts the output voltage V′
S
corresponding to the two extreme operating conditions within the specified tolerance range of the sensor
200
as V′
S1
and V′
S2
. The MR sensor
200
will produce the highest output voltage signal V′
S
=V
S1
when the MR sensor is simultaneously operating at the lowest temperature, smallest air gap, and largest MR drive voltage V′
B
, all within the specified tolerance range, however. The MR sensor
200
will produce the lowest output voltage signal V′
S
=V′
S2
when the MR sensor is simultaneously operating at the highest temperature, largest air gap, and smallest MR drive voltage V′
B
, all within the specified tolerance range, however. The voltage span between the largest value of V′
S1
=V
MAX
and the smallest value of V′
S1
=V′
MIN
defines the correct operating range of the sensor
200
with a corresponding output signal voltage V′
S
.
That is, V′
MIN
<V′
S
<V′
MAX
. Monitoring a failure of the MR sensor
200
requires that the maximum output voltage V′
S
not exceed V′
MAX
and that the minimum output voltage V′
S
will not fall below V′
MIN
.
The output signal V′
S
exceeding V′
MAX
may, for example, be indicative of such potential problems as too large a MR drive voltage V′
B
, one or two defective MR dies, bad wiring, bad connector, insecure sensor, or loose target wheel mount. The output signal V′
S
falling below the value of V′
MIN
may, for example, be indicative of a partial short circuit, total short circuit, insufficient MR drive voltage V′
B
, one or two defective MR dies, insecure sensor, or loose target wheel mount.
Accordingly, it is necessary when monitoring either single or dual MR speed and position sensors for malfunction to observe the value of the output signal within a maximum and minimum voltage envelope.
SUMMARY OF THE INVENTION
The present invention is a sensor malfunction detection method and system applicable to MR speed and position sensors. The sensor malfunction detection method and system is used in conjunction with passive MR sensor configurations such as depicted in
FIGS. 1 and 2
wherein the sensor contains only the MR or MRs and no processing electronics. The raw MR signal V
S
or V′
S
of
FIGS. 1 and 2
could be transmitted to an off-site processor containing the malfunction detection circuitry. If the processing electronics is integrated with the sensor, the malfunction detection circuitry could also be integrated with the se
Delphi Technologies Inc.
Dobrowitsky Margaret A.
Lefkowitz Edward
Zaveri Subhash
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