Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – With electromagnetic field
Reexamination Certificate
2000-10-23
2002-05-07
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
With electromagnetic field
C324S207120
Reexamination Certificate
active
06384592
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to air core gauges, and more specifically to techniques for determining the position of a magnetic rotor in such an apparatus including at least three coils in proximity to the rotor.
BACKGROUND OF THE INVENTION
Typical analog displays, such as those used in vehicle instrument panels, utilize air core gauges or stepper motors to position pointers in relation to sensor values. Conventional air core gauge mechanisms typically include a rotor formed of a substantially circular disk of magnetized material that is fixed to a spindle, wherein the rotor is surrounded by at least two coils of wire, with at least one coil typically perpendicular to another of the coils. When electric current passes through the coils, a magnetic field is produced that exerts a force on the rotor. The angular direction of the magnetic field produced by the coils primarily depends on the number of ampere-turns in each of the coils, wherein the resultant magnetic field can be represented by the vector addition of the fields produced by each of the coils.
Stepper motors are inherently more accurate than air core gauges, although to achieve the higher accuracy, stepper motors typically incorporate a high stepdown gear ratio between the magnetic rotor and the pointer shaft or multi-pole rotors in combination with a geartrain. The additional parts required in typical stepper motors as compared with air core gauges undesirably increases the mechanism cost and often times necessitates, at least from a cost standpoint, the use of air core gauges.
A conventional two-coil air core gauge is typically driven by one of two known techniques. According to a first known technique, as shown in
FIG. 1
, the two coils are designated by reference numerals
36
and
38
. Coil
38
is biased to a fixed voltage V
IGN
through resistor
34
. The resistance of resistor
32
typically varies in relation to a physical parameter such as fuel level. Resistor
30
supplies current to coil
36
from V
IGN
. The voltage across coil
36
is determined by a voltage divider comprised of resistor
30
and the parallel combination of resistor
32
and the resistance of coil
36
, and the current flowing through coil
36
varies in proportion to the voltage thereacross. Coil
36
and coil
38
are arranged to generate orthogonal magnetic vectors that sum to form a resultant magnetic vector. As the current flowing through coil
36
varies in response to the changing resistance of resistor
32
, the vector component of the magnetic field generated by coil
36
similarly varies. The direction and magnitude of the magnetic field resulting from vector addition of the field components generated by coils
36
and
38
thus varies in relation to the changing resistance of resistor
32
. The magnetic rotor aligns with the resultant magnetic field direction, and its rotational position is thus determined by the direction of the resultant magnetic field which is determined by resistor
32
.
According to a second known technique for driving a two-coil air gauge, as shown in
FIG. 2
, a signal on line
50
from a sensor (not shown), typically a signal with a frequency varying with a vehicle parameter, is converted to a corresponding analog voltage through a frequency-to-analog converting circuit
52
. The resultant analog signal is provided as an input to a sine/cosine drive circuit
56
, whereby the sine/cosine drive circuit
56
generates a current flowing through signal path
58
proportional to the cosine of the desired angle of deflection of the rotor, and a current flowing through signal path
60
proportional to the sine of the desired angle of deflection of the rotor. Coils
67
and
64
, in response to the currents flowing through signal paths
58
and
60
respectively, develop magnetic fields with sine and cosine component magnetic vectors correlating to the desired pointer rotation. Various other techniques which are not set forth here are also known and are used to drive air core gauges.
Air core gauge error sources include hysteresis, pointer staking errors and linearity errors. Pointer staking and linearity errors can be minimized with a calibration process, although calibration of the mechanism typically adds investment and cycle time to the system cost. Hysteresis errors, on the other hand, typically cannot be compensated for in an open-loop system, wherein most conventional drive techniques for air core gauges, including those set forth above, are typically open-loop systems in which actuation currents are applied to the coils without the use of any feedback information as to the actual pointer position to allow for corrections to the values of the currents. If the center of mass of the pointer does not lie on the axis of the pointer spindle, the weight of the pointer will generally cause the pointer to sag from the angular position in which the magnetic field of the rotor aligns with the resultant magnetic field of the two coils.
One approach to addressing such hysteresis and other errors to thereby improve pointer position accuracy is disclosed in U.S. Pat. No. 5,489,842 (hereinafter '842 patent), owned by the assignee of the present invention, and the disclosure of which is incorporated herein by reference. The '842 patent discloses an air core gauge
411
, as illustrated in
FIG. 3
, which includes a generally circular or cylindrical magnetic rotor
410
driven by two coils
412
and
414
about an axis
409
(shown in phantom), which are wound around perpendicular axes, B-F and O-D, respectively, and mounted within the proximity of the rotor
410
. In addition to the normal rotation drive signal (not shown), coil
412
is coupled to a high frequency AC signal source (not shown) such that a high frequency current is superimposed onto the portion of the drive signal applied to coil
412
. Since coils
412
and
414
are perfectly perpendicular, there is no magnetic coupling of the AC input signal from coil
412
to
414
. However, rotor
410
provides a magnetic flux linkage between coils
412
and
414
, thereby inducing a coupled AC output signal on coil
414
in response to the AC input signal on coil
412
. Since the rotation drive signal is substantially DC, the rotation drive signal does not cause signal coupling between coils
412
and
414
. Thus, because the frequency of the injected AC current is much higher than the frequency content of the nominally DC currents used to drive the rotor
410
to cause torque in the mechanism, the technique disclosed in the '842 reference makes it possible to simultaneously drive the rotor
410
to a desired position while determining the position of rotor
410
using filters (not shown) to separate the two activities.
The magnetic flux linkage between coil
412
and coil
414
is proportional to sin(i)*sin(j), where i is the angle between the north pole
416
(or south pole
418
) and a line drawn through points B and F, and j is the angle between the north pole
416
(or south pole
418
) and a line drawn through points D and O. The magnetic flux linkage between coils
412
and
414
is further dependent upon the rotational position of rotor
410
, so that the magnitude and phase of the AC output signal in coil
414
is accordingly dependent upon the rotational position of rotor
410
. Thus, a measurement of the AC output signal in coil
414
, or the ratio between the input and output AC signals, can be used to make a determination of the rotational position of rotor
410
, and therefore the position of a pointer or other mechanism attached to the rotor
410
.
While various causes of pointer position error, including hysteresis, can be compensated for with a closed-loop system of the type illustrated in
FIG. 3
, such systems have a number of drawbacks associated therewith. For example, the apparatus
411
illustrated in FIG.
3
and disclosed in the '842 reference has inherent accuracy limitations. More specifically, the feedback signal (output AC signal) is a substantially sinusoidal signal and, with two orthogo
Lippmann Raymond
Nelson James E.
Sylvester Gail M.
Aurora Reena
Chmielewski Stefan V.
Lefkowitz Edward
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