Electricity: measuring and testing – Magnetic – Displacement
Reexamination Certificate
1999-03-10
2001-08-07
Oda, Christine (Department: 2862)
Electricity: measuring and testing
Magnetic
Displacement
C324S207250, C338S03200R
Reexamination Certificate
active
06271662
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method for sensing precise angular position of a rotating object and more particularly to a method for sensing crankshaft or camshaft rotational position wherein a sensor, preferably but not exclusively a dual element magnetoresistive (MR) sensor, is matched to a predetermined location of a target wheel mounted on the crankshaft or camshaft as part of the manufacturing process of the sensor.
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, 5,714,883, 5,731,702, and 5,754,042).
An example of a known sensor used with a target wheel is depicted at
FIGS. 1A and 1B
. The sensor
10
consists of a first magnetoresistor MR
1
, a second magnetoresistor MR
2
, a biasing magnet
16
, a first resistor R
1
, a second resistor R
2
, and terminals
22
,
24
,
26
, and
28
. The positive terminal
30
of voltage source +V is applied to terminal
22
and the negative terminal
32
, considered to be at ground, is applied to terminal
28
. As a result, a voltage V
MR
appears at terminal
24
which is produced by the voltage divider circuit of MR
1
and MR
2
, and a voltage V
REF
appears at terminal
26
which is produced by the voltage divider circuit of R
1
and R
2
. V
MR
is input to the non-inverting terminal of a comparator (with hysteresis)
34
and V
REF
is input to the inverting terminal of the comparator, wherein the comparator is supplied with power through voltage source +V which is applied to terminal
36
and ground applied to terminal
38
.
As shown at
FIG. 1B
, the sensor
10
′ may optionally include the comparator
34
, whereupon the only outputs therefrom are terminal
30
for the voltage source +V, terminal
40
for V
OUT
and terminal
32
for ground.
According to the prior art, the following method is used to match MR
1
with MR
2
. During the manufacturing process of the sensor
10
, R
1
and R
2
do not have the same value to ensure that V
OUT
40
of comparator
34
is at a high voltage level (or, if desired, a low voltage level). V
REF
is gradually changed by trimming of R
1
or R
2
until V
OUT
switches voltage levels. For example, trimming is performed by a laser beam
42
of a laser
44
, wherein a portion of the cross-section
46
,
46
′ of the selected resistor is ablated to thereby change its resistance.
Instead of the reference signal being adjusted, the DC offset of the input signal (V
MR
) can be adjusted to achieve the same goal.
FIG. 1A
depicts a sensor
10
having an optional known DC offset
52
therefor incorporated therein, wherein the active path of the two alternative paths shown by dashed lines in
FIG. 1A
depends on whether the DC offset is present.
FIG. 1C
depicts an example of a DC offset
52
in the form of a variable resistance R
3
, as for example a rheostat (potentiometer) or a resistor selector box having a plurality of selectable resistors. Alternatively, the DC offset can be effected by individually adjustable current sources.
According to this method of the prior art, the reference signal or the DC offset is adjusted to match the MRs in free space. Now, the sensor
10
undergoes final packaging. Thus, this manufacturing process eliminates MR and bias magnet mismatch.
However, accurate engine crank position information is needed for ignition timing and state and federally mandated misfire detection. 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. Preferably, the output signal of the sensor should indicate a tooth edge passing through the nominal centerline of the sensor, although, a small fixed offset is acceptable. Usually, the specified accuracy is plus or minus 0.5 degrees with respect to the actual edge, which provides a one degree tolerance band. Quite frequently, however, fundamentally good sensors with even tighter tolerance bands must be rejected because they do not fall into the plus or minus 0.5 degree range, e.g. a sensor having a tolerance band from 0.25 degrees to 0.75 degrees.
Accordingly, what is needed in the art is a method to adjust a sensor so that its output signal will indicate a tooth edge location coinciding with the nominal center line of the sensor, or coinciding with any other specified point on the face of the sensor.
SUMMARY OF THE INVENTION
The present invention is a method for adjusting a sensor so that its output signal will indicate a tooth edge location coinciding with the nominal center line of the sensor, or coinciding with any other specified point on the face of the sensor. The proposed method applies to all types of sensors (magnetic and otherwise) which rely on the comparison between input and reference signals to generate a binary output signal.
As a part of the manufacturing process, each sensor is oriented and located relative to a stationary segment of a target wheel identical to that which will be subsequently used in operation with the sensor, wherein the location provides a nominal air gap with respect to the segment. In the preferred form of the present method, this segment must consist of at least a portion of a tooth, a portion of a slot, and one tooth edge specified as either falling or rising. The tooth edge is precisely aligned with the centerline of the sensor at a desired offset angle with respect to the actual tooth edge measured from the center of the target wheel. Now the reference level is adjusted until the output signal switches state.
For example, in a paired MR sensor first voltage divider wired in parallel with a second voltage divider composed of first and second resistors, one of the resistors is trimmed, wherein a portion of the cross-section is laser ablated to thereby change its resistance, until the output signal switches state, as indicated on an indicator. Instead of the reference signal, the DC offset of the input signal can be adjusted to achieve the same goal. Depending on the design of the sensor and a variety of other factors, this match adjustment can take place either prior to or after the sensor assembly process, but most preferably after the sensor assembly process so that stacking of all tolerances is present when the match adjustment occurs.
There are three interesting cases of match adjusting involving the trimming methodology, wherein the thermal coefficient of resistance is the same for each of R
1
and R
2
.
In case 1, switching occurs on a rising tooth edge (from low to high output). If this is the only mode, then initially R
2
>R
1
sufficiently to have a low output within all tolerances of the o
Delphi Technologies Inc.
Dobrowitsky Margaret A.
Oda Christine
Zaveri Subhash A
LandOfFree
Method of matching sensors to crankshafts and camshafts does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method of matching sensors to crankshafts and camshafts, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method of matching sensors to crankshafts and camshafts will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2441469