Magnetoresistive sensor with reduced output signal jitter...

Details Magnetoresistive sensor with reduced output signal jitter... Magnetoresistive sensor with reduced output signal jitter...

1997-05-27

2001-06-12

Patidar, Jay (Department: 2862)

Electricity: measuring and testing

Magnetic

Displacement

C324S207250, C324S207120

Reexamination Certificate

active

06246233

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a magnetoresistive sensor that finds particular application in rotating shaft encoders. In particular, the magnetoresistive sensor reduces jitter in the output signal created by asymmetries in sensed magnetic fields.
BACKGROUND AND SUMMARY OF THE INVENTION
Magnetoresistive sensors are based on the principle that the resistance of a ferromagnetic material changes when subjected to a magnetic flux. Magnetoresistive sensors have numerous applications including ascertaining shaft rotation parameters (position, acceleration, etc.) in the context of encoders, tachometers, etc. In this regard, U.S. Pat. No. 5,293,125 entitled “Self Aligning Tachometer With Interchangeable Elements For Different Resolution Outputs” assigned to the present assignee is incorporated herein by reference. In these applications, magnetoresistive sensors detect changes in magnetic fields to measure motion.
FIG. 1
shows one application of the present embodiment in which a magnetic drum
100
includes a peripheral surface
112
having two distinct tracks: an incremental or INC track
116
and an index or Z track
118
. The rotary drum
100
is mounted to a shaft
114
which may be connected for example to a motor (not shown). The INC track
116
provides pulses indicating incremental shaft rotation and comprises an alternating series of magnetic north and south poles magnetically imprinted on the drum periphery
112
at a predetermined pitch which may be on the order of hundreds of microns, (e.g., 747 microns in the preferred embodiment). Depending on the diameter of the drum
100
, the peripheral surface
112
may encode a large number of pulses per revolution, (e.g., 480, 512, 600, etc.) The Z track
118
is used to generate one output pulse per revolution of the drum and thus provides information concerning the number of shaft revolutions and the absolute shaft position. Accordingly, whenever a Z pulse is generated, the motor shaft is known to be at a particular absolute position relative to the magnetoresistive sensor module
120
.
Magnetoresistive sensor module
120
and
120
′ include a plurality of magnetoresistive elements positioned adjacent to and separated by a predetermined gap from the drum peripheral surface
112
as will be described in more detail below. The magnetoresistive INC track
116
has corresponding sensor module
120
, and the Z track
118
has corresponding sensor module
120
′. Both sensor modules
120
and
120
′ are connected to signal sensing and conditioning circuitry
122
.
Each of the magnetoresistive sensors
120
,
120
′ consists of a glass, ceramic, or silicon substrate covered with a thin film permaloy, e.g., a Ni—Fe film, which is photoetched into a pattern of individual elements which are connected to the sensing and signal conditioning circuitry
122
via one or more flexible leads. Reference is made to FIG.
2
(
a
) which is a perspective view of a portion of the magnetized INC track
116
showing the adjacent north and south poles (N, S) on its incremental track and plural magnetoresistive elements
124
including elements MR
1
and MR
2
with connecting nodes A, B, and C, a dc voltage being connected to nodes A and B. As can be seen from the drawing, the magnetoresistive elements are formed parallel to each other and to the north and south poles formed on the peripheral surface of the drum
112
. The magnetoresistive elements are typically spaced some fraction of the pitch distance separating each adjacent magnetic pole, e.g. &lgr;/2 in FIG.
2
(
a
).
FIG.
2
(
b
) shows a current i generated in a linear magnetoresistive strip in response to an orthogonal magnetic field H. As shown in FIG.
2
(
c
), the magnetoresistive strip experiences a drop in electrical resistance R (corresponding to an increase in current i) in the presence of the saturated magnetic field H. More specifically, the electrical resistance R of the thin film magnetoresistive pattern inversely varies in accordance with the strength of magnetic field H which intersects a perpendicular current i running through the magnetoresistive pattern as shown. In theory, the change in resistance R is independent of the polarity of the magnetic field H. However, as will be described in more detail below, this assumption is not reliable in practical magnetoresistive sensor applications.
Referring to FIG.
2
(
d
), the magnetoresistive sensor elements MR
1
and MR
2
are conventionally connected in a resistive bridge array so as to provide differential outputs, e.g., the output signal is taken from node C. Note as the drum
100
rotates the magnetic pole pattern on INC track
116
past the magnetoresistive sensor elements MR
1
and MR
2
, an AC output generated at bridge circuit node C corresponds to the movement of the magnetic pole pattern and therefore the rotation of the drum
100
.
Magnetoresistive sensors are designed to increase the output voltage level and to improve the temperature properties of the device by making bridge connections between several elements. Two phase outputs (i.e., A and B phases) are typically obtained from the sensor by offsetting the magnetoresistive sensor's pattern of elements from the north-south pole pattern on the INC track
116
of the magnetic drum
100
by one quarter of the pole pitch &lgr;.
FIG. 3
illustrates a simple configuration of magnetic resistive elements a
1
, b
1
, a
2
, and b
2
positioned parallel to and above the magnetic pole surface corresponding magnetic field lines between four adjacent poles. One phase or channel of a magnetoresistive sensor comprises two magnetoresistive strips displaced an odd multiple of a half pole pitch &lgr; from each other which in the layout in
FIG. 3
is 3&lgr;/2. As the drum
100
rotates one pole pitch &lgr;, the one channel sensor output (which can be assumed for simplicity to be an approximately sinusoidal output waveform) completes one cycle having a particular phase A. A quadrature signal (phase B) which is 90° out of phase from phase A is generated by the B channel magnetoresistive sensor elements B
1
and B
2
which are formed on the same substrate as elements A
1
and A
2
but displaced an odd multiple of a quarter pole pitch from the first pair A
1
, A
2
.
The phase A and phase B bridge outputs are typically amplified and converted into square waveforms using conventional comparators or other zero crossing detection methods. The square waveforms for phases A (&thgr;
A
) and B (&thgr;
B
) shown in
FIG. 4
are in a quadrature relationship, i.e. &thgr;
A
leads &thgr;
B
by 90°. By combining the two quadrature phases &thgr;
A
and &thgr;
B
in an exclusive-OR gate, a single channel output of twice the frequency of the quadrature signals is obtained. This means that the output resolution of the magnetoresistive sensor is “doubled” without any increase in the number of magnetized poles formed on the rotary drum peripheral surface. Such sensors are referred to as frequency doubling sensors and achieve higher resolution without having to increase the manufacturing accuracy that would otherwise be required to reduce the separation between the poles needed to achieve higher resolution. In theory, additional exclusive-OR outputs may be recombined using further exclusive OR-gates to produce even higher resolutions by frequency tripling, quadrupling, etc. Although the present invention may be applied to frequency tripling, quadrupling, etc. embodiments, the present invention is described in the context of a frequency doubling sensor for the sake of simplicity.
A frequency doubling magnetoresistive sensor is shown in FIG.
5
. The magnetoresistive sensor includes two sets of five magnetoresistive elements, the first set including elements
5
-
9
and second set including elements
10
-
14
. The first and second sets of magnetoresistive elements are separated by one magnetic pole pitch &lgr;. Each magnetoresistive element within a group is spaced by some fraction of the pitch, e.g., by 3&lgr;/8. Magnetoresistive elements
5
-
9
are connected to

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