Electricity: measuring and testing – Magnetic – Displacement
Reexamination Certificate
2000-02-28
2002-02-12
Metjahic, Safet (Department: 2862)
Electricity: measuring and testing
Magnetic
Displacement
C324S207250
Reexamination Certificate
active
06346808
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to a method of sensing crankshaft rotational position.
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 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.
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, 5,731,702, and 5,754,042).
The electronic control module (ECM) of an engine specifies the required format of the crankshaft position signal. Invariably, the target wheel (i.e., encoder) is designed to generate a magnetic signal conforming to the format of the required signal. That is, preferably, the target wheel will have teeth at crank angles where the position signal should have a high value and slots at crank angles where the position signal should have a low value. The position sensor should convert the mechanical pattern of the target wheel, as closely as possible, into a corresponding electrical signal.
FIG. 1A
is a schematic representation of an exemplar automotive environment of use according to this prior art scheme, wherein a target wheel
410
is rotating about an axis
410
′, 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
410
is determined by sensing the passage of a tooth edge
412
, either a rising tooth edge
412
a
or a falling tooth edge
412
b,
using a differential MR sequential sensor
50
. A tooth edge
412
is considered rising or falling depending upon the direction of rotation of the target wheel
410
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
410
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
410
will be assumed to be rotating in a CW direction in the views.
The differential MR sequential sensor
50
employs two magnetoresistor elements, MR
1
and MR
2
, which are biased by a permanent magnet
56
, wherein the magnetic flux
418
and
420
emanating therefrom are represented by the dashed arrows. The magnetic flux
418
and
420
pass from the permanent magnet
56
through the magnetoresistors MR
1
and MR
2
and through the air gaps
422
and
424
to the target wheel
410
. The target wheel
410
is made of a magnetic material having teeth
426
and spacings
428
therebetween and the sensor signal V
S
is available between terminals
430
and
432
.
The example of the target wheel
410
in
FIG. 1A
is a 3X target wheel. This target wheel
410
and the associated sensor
50
utilize analog signals, available between terminals
430
and
432
, which are converted into a 3 bit digital signal that is repeated every 360 degrees of rotation of the wheel. The ideal, error free, situation is depicted by the digital signal in
FIG. 1B
wherein each bit
426
′ represents a particular angular position of the target wheel
410
and adjacent bits are angularly separated by
120
degrees representing the tooth pattern
426
of the target wheel and the desired signal pattern whereby the rising edges
412
a
of the teeth occur at the rising edges of the signal
412
′
a
and the falling edges
412
b
of the teeth occur at the falling edges of the signal
412
′
b.
However, the actual digital signal is depicted in
FIG. 1C
wherein each bit
426
″ represents a particular angular position of the target wheel
410
and adjacent bits are not angularly separated by 120 degrees due to an angular position error E whereby the rising edges
412
a
of the teeth
426
do not occur at the rising edges of the signal
412
″
a
and an angular position error E′ whereby the falling edges
412
b
of the teeth do not occur at the falling edges of the signal
412
″
b.
The angular position errors E and E′ are caused by graduality of change of magnetic field at approach and recession of the teeth, which is sometimes compensated by making the teeth narrower. Another component of the error is caused by variations in the air gaps
422
and
424
as well as variations in temperature.
Another target wheel of interest is the 24X target wheel (see for example U.S. Pat. No. 5,570,016). This wheel and its associated sensor utilize analog signals which are converted into a 24 bit digital signal that is repeated every 360 degrees of rotation of the wheel. Each bit represents a particular position of the wheel and adjacent bits are angularly separated by 15 degrees. In general, target wheels of interest may be specified as nX target wheels where n is an integer number of teeth or slots. These wheels and their associated sensors utilize analog signals which are converted into an n bit digital signal that is repeated every 360 degrees of rotation of the wheel. Each bit represents a particular position of the wheel and adjacent bits are angularly separated by (360
) degrees. Prior art uses of these wheels have utilized sensors incorporating two matched MRs with a more costly dual track wheel when high accuracy was required or with less expensive single track wheels when less accuracy was acceptable.
What is needed is a method and apparatus to accurately locate the rising and falling edges of the teeth of a single track target wheel whereby the position of the crankshaft can be obtained very accurately and inexpensively.
SUMMARY OF THE INVENTION
The present invention provides a method of emulating any desired tooth/slot format of a desired target wheel from a predetermined tooth/slot arrangement of a rotating actual target wheel used in conjunction with an MR position sensor.
According to the method of the present invention, the passage of two sequential slots of differing widths or two sequential teeth of differing widths of the actual target wheel determine the rising and falling edge of one tooth of the desired target wheel and define one tooth and one slot of the desired target wheel. The actual target wheel, has, preferably, 2n teeth and 2n slots of two distinct sequential widths whereby the desired target wheel is emulated to have n teeth and n slots.
For example, an actual target wheel having 6 teeth and 6 slots of two distinct sequential widths, can be used to emulate a 3X desired target wheel with an accuracy attainable previously with a two track target wheel; or, for another example, a actual target wheel having 24 teeth and 24 slots of two distinct sequential widths can be used to emulate a 12X desired target wheel.
The two MRs of the position sensor, are matched, having matched magnetic biasing and powered by matched current sources, and are aligned in the circumferential direction of the actual target wheel so as to generate
Delphi Technologies Inc.
Dobrowitsky Margaret A.
Metjahic Safet
Zaveri Subhash
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