Low cost single magnetoresistor position and speed sensor

Details Low cost single magnetoresistor position and speed sensor Low cost single magnetoresistor position and speed sensor

1999-07-01

2001-05-15

Patidar, Jay (Department: 2862)

Electricity: measuring and testing

Magnetic

Displacement

C324S207120, C324S207250

Reexamination Certificate

active

06232770

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a method of sensing precise angular position and speed of rotation of a rotating object and more particularly to a method for sensing crankshaft or camshaft rotational position and speed of rotation wherein a sensor, preferably but not exclusively a single element magnetoresistive (MR) sensor, is utilized to sense crankshaft position and rotational speed from the passage of single tooth edges of an encoder or target wheel.
BACKGROUND OF THE INVENTION
It is well known in the art that the resistance modulation of Hall elements or 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 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. The use of a constant current excitation source provides an output voltage from the MR that varies as the resistance of the MR varies.
Accurate engine crank position information is needed for ignition timing and OBDII mandated misfire detection. Increasingly more sophisticated spark timing and emission controls introduced the need for crankshaft sensors capable of providing precise position information during cranking. Various combinations of magnetoresistors and single and 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. 5,570,016, 5,714,883, 5,731,702, and 5,754,042).
The crank position information is encoded on a rotating target wheel in the form of teeth and slots. The edges of the teeth define predetermined crank positions. The sensor is required to detect these edges accurately and repeatably over a range of air gaps and temperatures. Virtually all such sensors are of the magnetic type, either variable reluctance or galvanomagnetic (e.g. Hall generators or magnetoresistors). Galvanomagnetic sensors are becoming progressively most preferred due to their capability of operation down to a zero speed, greater encoding flexibility and standardized output signals.
Furthermore, temperature and the size of the air gap affect the output signal of a magnetic sensing element. Consequently, operation over wide temperature and air gap size ranges requires some form of compensation for the resultant signal drift, both in amplitude and offset. The most common approach is the use of two matched sensing elements operating in a differential mode thereby providing a common mode rejection.
An example of such a sensor is the sequential crankshaft sensor used on several of General Motors Corporation trucks. This sensor employs two matched InSb magnetoresistor elements located radially proximate to the target wheel, one being slightly displaced with respect to the other in the direction of target wheel rotation.
FIG. 1
is a schematic representation of an exemplar automotive environment of use according to this prior art scheme, wherein a target wheel
10
is rotating, such as for example in unison with a crankshaft, a drive shaft or a cam shaft, and the rotative position thereof is to be sensed. Rotative position of the target wheel
10
is determined by sensing the passage of a tooth edge
12
, either a rising tooth edge
12
a
or a falling tooth edge
12
b,
using a single dual MR differential sequential sensor
14
. A tooth edge
12
is considered rising or falling depending upon the direction of rotation of the target wheel
10
with respect to the magnetoresistive sensors MR
1
and MR
2
. MR
1
is considered leading and MR
2
is considered lagging if the target wheel
10
is rotating in a clockwise (CW) direction whereas if the target wheel is rotating in a counterclockwise (CCW) direction then MR
1
is considered lagging whereas MR
2
is considered leading. For purposes of example, the target wheel
10
will be assumed to be rotating in a CW direction in the views.
The single dual MR differential sequential sensor
14
employs two matched magnetoresistor elements, MR
1
and MR
2
, which are biased by a permanent magnet
16
, wherein the magnetic flux
18
and
20
emanating therefrom is represented by the dashed arrows. The magnetic flux
18
and
20
passes from the permanent magnet
16
through the magnetoresistors MR
1
and MR
2
and through the air gaps
22
and
24
to the target wheel
10
. The target wheel
10
is made of a magnetic material having teeth
26
and spacings
28
therebetween. The spacing L between MR
1
and MR
2
is generally such that the trigger points for the rising and falling edges of the output signal V
OUT
as shown in
FIG. 2C
are dependent on the leading MR only as depicted in
FIGS. 2A
,
2
B and
2
C, as will be later described.
Power is supplied to CURRENT SOURCE
1
30
and CURRENT SOURCE
2
32
through voltage source
34
. Power is also supplied to a comparator
36
(with hysteresis) through voltage source
34
but is not shown. CURRENT SOURCE
1
30
supplies current to MR
1
thereby providing for an output voltage V
MR1
from MR
1
. CURRENT SOURCE
2
32
supplies current to MR
2
thereby providing for an output voltage V
MR1
from MR
2
. Output voltages V
MR1
and V
MR2
are input into the comparator
36
whose output voltage V
OUT
, as shown in
FIG. 2C
, is an indication of the position of rotation of the target wheel
10
. It is to be understood that all voltages are measured with respect to ground unless otherwise indicated herein, and that CURRENT SOURCE
1
is matched to CURRENT SOURCE
2
.
As shown in
FIG. 2A
, the lagging MR element, in this case MR
2
, provides a delayed signal in every respect identical to the signal from the leading MR, in this case MR
1
. The differential signal V
D
=V
MR1
−V
MR2
, shown in
FIG. 2B
is electronically generated within the comparator
36
and is then used by the comparator to reconstruct the signal V
OUT
(shown in
FIG. 2C
) emulating the profile of the target wheel
10
. Upon a closer inspection of
FIGS. 2A
,
2
B and
2
C, it becomes evident that the rising edges
42
and the falling edges
44
of the sensor output signal V
OUT
are determined only by first points
46
corresponding to the rising edges and second points
48
corresponding to the falling edges where the signal from the leading MR, in this example MR
1
, crosses a first threshold voltage
50
corresponding to the first points and a second threshold voltage
52
corresponding to the second points wherein the first and second threshold voltages are determined by the hysteresis applied to the comparator
36
. The lagging MR, in this example MR
2
, has no part in the generation of the rising edges
42
or the falling edges
44
of the output signal V
OUT
. The lagging MR simply determines the offset voltage
54
of the leading MR.
What is needed is a method and apparatus wherein a single element sensor, preferably, but not exclusively, a single element magnetoresistive sensor, is utilized to sense crankshaft position and rotational speed from the passage of single tooth edges of an encoder or target wheel.
SUMMARY OF THE INVENTION
The present invention provides a method and apparatus wherein a single element galvanomagnetic sensor, herein exemplified by a single element magnetoresistive (MR) sensor, is utilized to sense crankshaft position and rotational speed from the passage of single tooth edges of an encoder or target wheel and determines its offset voltage by measuring the MR voltage over a slot in the target wheel. This is accomplished by inclusion of a valley detector

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