Title: Non-contacting compliant torque sensor
Abstract: A variable reluctance rotational displacement sensor with: an annular sleeve; a coil coaxially aligned within the sleeve; a first ring in magnetic communication with the sleeve, coaxially aligned and configured to rotate relative to the sleeve. The first ring including a first plurality of axially directed teeth arranged about a circumference of the ring on a front portion thereof. The sensor also includes a second ring in magnetic communication with the first ring and the sleeve, the second ring coaxially aligned and configured to rotate relative to the first ring and the sleeve and including a second plurality of axially directed teeth configured substantially the same as the first plurality of axially directed teeth and oriented adjacent to the first plurality of axially directed teeth on a rear portion of the second ring. The coil generates a signal responsive to a differential displacement between the first and second rings.
Patent Number: 6,851,324 Issued on 02/08/2005 to Islam, et al.
| Inventors:
|
Islam; Mohammad S. (Saginaw, MI);
Mir; Sayeed A. (Saginaw, MI);
Sebastian; Tomy (Saginaw, MI);
Ross; Christian E. (Hemlock, MI)
|
| Assignee:
|
Delphi Technologies, Inc. (Troy, MI)
|
| Appl. No.:
|
319952 |
| Filed:
|
December 16, 2002 |
| Current U.S. Class: |
73/862.328; 73/862.336 |
| Intern'l Class: |
G01L 003/02 |
| Field of Search: |
73/862.328-862.336
|
References Cited [Referenced By]
U.S. Patent Documents
| 4724710 | Feb., 1988 | Murty | 73/862.
|
| 5497667 | Mar., 1996 | Nakaura | 73/862.
|
| 5739616 | Apr., 1998 | Chikaraishi et al. | 73/862.
|
| 5811695 | Sep., 1998 | Satoh et al. | 73/862.
|
| 6035960 | Mar., 2000 | Wakao et al. | 180/446.
|
| 6362586 | Mar., 2002 | Naidu.
| |
| 6370967 | Apr., 2002 | Kouketsu et al. | 73/862.
|
| 6400142 | Jun., 2002 | Schroeder.
| |
| 6424896 | Jul., 2002 | Lin et al.
| |
| 6481297 | Nov., 2002 | Kim et al. | 73/862.
|
| 6581479 | Jun., 2003 | Goto et al. | 73/862.
|
| 6622576 | Sep., 2003 | Nakano et al. | 73/862.
|
Other References
U.S. Appl. No. 10/320,328, filed Dec. 16, 2002, Mir et al.
KOYO Engineering Journal English Edition No. 160E; "Development Of Custom
IC for EPS Torque Sensor, Author: K. Yoshida, pp. 48-51, 2002.
|
Primary Examiner: Lefkowitz; Edward
Assistant Examiner: Ellington; Alandra
Attorney, Agent or Firm: Smith; Michael D.
Claims
What is claimed is:
1. A variable reluctance rotational displacement sensor comprising:
an annular sleeve;
a coil coaxially aligned within said sleeve;
a first ring shaped toothed structure m magnetic communication with said
sleeve, coaxially aligned and configured to rotate relative to said
sleeve, said first ring including a first plurality of axially directed
teeth arranged substantially equidistant about a circumference of said
ring on a front portion thereof;
a second ring shaped toothed structure in magnetic communication with said
first ring and said sleeve, said second ring coaxially aligned and
configured to rotate relative to said first ring and said sleeve and
including a second plurality of axially directed teeth configured
substantially the same as said first plurality of axially directed teeth
and oriented adjacent to said first plurality of axially directed teeth on
a rear portion of said second ring;
wherein said coil generates a signal responsive to a differential
rotational displacement between said first ring and said second ring; and
wherein said annular sleeve includes an internal flange configured to
maintain a selected magnetic air gap with a rear portion of said first
ring shaped toothed structure.
2. The sensor of claim 1 wherein said coil substantially surrounds said
first plurality of axially directed teeth and said second plurality of
axially directed teeth.
3. The sensor of claim 1 wherein said second ring includes a flange about
its circumference configured to maintain a selected magnetic air gap with
an internal surface of said sleeve.
4. The sensor of claim 1 wherein said sleeve, first ring, and second ring
are fabricated from ferrite.
5. The sensor of claim 1 wherein said differential rotation displacement is
responsive to the twist of a torsion bar.
6. The sensor of claim 1 wherein said sensor is responsive to a torque.
7. The sensor of claim 1 wherein said sleeve, said first ring, and said
second ring are configured to reduce temperature sensitivity.
8. The sensor of claim 1 further including a temperature sensor for
providing temperature compensation.
9. The sensor of claim 1 further including an oscillator circuit
operatively connected with said coil, said oscillator circuit generating a
frequency of oscillation responsive to said differential rotational
displacement.
10. The sensor of claim 9 wherein said oscillator circuit includes
temperature compensation.
11. A system for detecting a sensed parameter for a motor control system
comprising:
a means for receiving a sensor signal, said sensor signal responsive to an
inductance of a non-contacting variable reluctance rotational displacement
sensor, wherein said inductance is indicative of a displacement of said
sensor and responsive to said sensed parameter;
a means for applying said sensor signal to an oscillator circuit a
frequency of which is indicative of said inductance;
a means for determining an oscillation period for said oscillator circuit;
and a means for computing a value for said sensed parameter.
Description
BACKGROUND
This invention relates to non-contacting torque sensor and an algorithm for
processing signals therefrom. Currently, many of the non-contacting torque
sensors employ a permanent magnet in the structure. Others utilize eddy
current principles to determine the torque.
It is well known in the torque sensing art that the deformation, or twist,
of a rotary shaft under load can be sensed as a measure of the torque
being transmitted through the shaft. In relatively low torque applications
where the amount of twist may be too small for accurate measurement, such
as in automotive power steering systems, the twist is typically augmented
by inserting a torsion bar between two relatively rigid sections of the
shaft. The relative rotation of the more rigid sections of the shaft may
be mechanically or electrically detected using a variety of techniques.
Ideally, a torque sensing arrangement should have the following
characteristics. The sensor, if electrical or electromagnetic, should have
a stationary transducer element, avoiding the need for slip rings or other
rotating or sliding contact arrangements. This is true even in limited
rotation applications, such as in automotive steering, since movement of
the electrical cables increases the likelihood of failures due to
mechanical fatigue or interference. The sensor should be non-contacting,
meaning that the sensor elements do not physically contact each other in
normal operation. Contacting operation invariably introduces hysteresis
error and the possibility of failure due to mechanical bind-up. The sensor
should be amenable to mass production, with liberal tolerances on the
parts. The sensor should be reasonably easy to calibrate in mass
production. And finally, the sensor should be capable of redundant torque
measurement to permit continued operation in the event of a transducer
failure.
Therefore, it would be beneficial to provide a non-contacting torque sensor
that inductance variation of a coil due to the change in reluctance in a
magnetic circuit when the torsion bar is under torque.
BRIEF SUMMARY
Disclosed herein is a variable reluctance rotational displacement sensor
comprising: an annular sleeve; a coil coaxially aligned within the sleeve;
a first ring in magnetic communication with the sleeve, coaxially aligned
and configured to rotate relative to the sleeve. The first ring including
a first plurality of axially directed teeth arranged substantially
equidistant about a circumference of the ring on a front portion thereof.
The sensor also includes a second ring in magnetic communication with the
first ring and the sleeve, the second ring coaxially aligned and
configured to rotate relative to the first ring and the sleeve and
including a second plurality of axially directed teeth configured
substantially the same as the first plurality of axially directed teeth
and oriented adjacent to the first plurality of axially directed teeth on
a rear portion of the second ring. The coil generates a signal responsive
to a differential rotational displacement between the first ring and the
second ring.
Also disclosed herein is a method of detecting a sensed parameter for a
motor control system comprising: receiving a sensor signal, the sensor
signal responsive to an inductance of a non-contacting variable reluctance
rotational displacement sensor, wherein the inductance is indicative of a
displacement of the sensor and responsive to the sensed parameter. The
method also includes: applying the sensor signal to an oscillator circuit
a frequency of which is indicative of the inductance; determining an
oscillation period for the oscillator circuit; and computing a value for
said sensed parameter.
Further disclosed herein is a system for detecting a sensed parameter for a
motor control system comprising: a means for receiving a sensor signal,
said sensor signal responsive to an inductance of a non-contacting
variable reluctance rotational displacement sensor, wherein the inductance
is indicative of a displacement of the sensor and responsive to the sensed
parameter. The system further includes: a means for applying the sensor
signal to an oscillator circuit a frequency of which is indicative of the
inductance; a means for determining an oscillation period for the
oscillator circuit; and a means for computing a value for the sensed
parameter.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will now be described, by way of an example, with
references to the accompanying drawings, wherein like elements are
numbered alike in the several figures in which:
FIG. 1 depicts an electric power steering system employing an exemplary
embodiment;
FIG. 2 is diagram depicting torque sensor in accordance with an exemplary
embodiment;
FIG. 3 is diagram depicting an upper toothed structure aligned with a lower
toothed structure of the torque sensor in accordance with an exemplary
embodiment;
FIG. 4 is diagram depicting an upper toothed structure not aligned with a
lower toothed structure of the torque sensor in accordance with an
exemplary embodiment;
FIG. 5 is diagram depicting an equivalent circuit for the torque sensor in
accordance with an exemplary embodiment;
FIG. 6 depicts a schematic of an exemplary circuit for the oscillator
circuit;
FIG. 7 depicts a schematic of an exemplary circuit for the oscillator
circuit with temperature compensation;
FIG. 8 depicts a flow chart of an exemplary algorithm for the capture of an
oscillation period and determination of a torque; and
FIG. 9 depicts an illustration of the period capture, and the parameters
associated with determination of an oscillation frequency.
DESCRIPTION OF AN EXEMPLARY EMBODIMENT
Referring to FIG. 1, reference numeral 40 generally designates a motor
vehicle electric power steering system suitable for implementation of the
disclosed embodiments. The steering mechanism 36 is a rack-and-pinion type
system and includes a toothed rack (not shown) within housing 50 and a
pinion gear (also not shown) located under gear housing 52. As the
operator input, hereinafter denoted as a steering wheel 26 (e.g. a hand
wheel and the like) is turned, the upper steering shaft 29 turns and the
lower steering shaft 51, connected to the upper steering shaft 29 through
universal joint 34, turns the pinion gear. Rotation of the pinion gear
moves the rack, which moves tie rods 38 (only one shown) in turn moving
the steering knuckles 39 (only one shown), which turn a steerable wheel(s)
44 (only one shown).
Electric power steering assist is provided through the control apparatus
generally designated by reference numeral 24 and includes the controller
16 and the electric motor 46. The controller 16 is powered by the vehicle
power supply 10 through line 12. The controller 16 receives a vehicle
speed signal 14 representative of the vehicle velocity. Steering pinion
gear angle is measured through position sensor 32, which may be an optical
encoding type sensor, variable resistance type sensor, or any other
suitable type of position sensor, and supplies to the controller 16 a
position signal 20. Motor velocity may be measured with a tachometer and
transmitted to controller 16 as a motor velocity signal 21. A motor
velocity denoted .omega..sub.m may be measured, calculated or a
combination thereof For example, the motor velocity .omega..sub.m may be
calculated as the change of the motor position .theta. as measured by a
position sensor 32 over a prescribed time interval. For example, motor
speed .omega..sub.m may be determined as the derivative of the motor
position .theta. from the equation .omega..sub.m =.DELTA..theta./.DELTA.t
where .DELTA.t is the sampling time and .DELTA..theta. is the change in
position during the sampling interval. Alternatively, motor velocity may
be derived from motor position as the time rate of change of position. It
will be appreciated that there are numerous well-known methodologies for
performing the function of a derivative.
As the steering wheel 26 is turned, torque sensor 28 senses the torque
applied to the steering wheel 26 by the vehicle operator. The torque
sensor 28 may include a torsion bar (not shown) and a variable
resistive-type sensor (also not shown), which outputs a variable torque
signal 18 to controller 16 in relation to the amount of twist on the
torsion bar. Although this is the preferable torque sensor, any other
suitable torque-sensing device used with known signal processing
techniques will suffice. In response to the various inputs, the controller
sends a command 22 to the electric motor 46, which supplies torque assist
to the steering system through worm 47 and worm gear 48, providing torque
assist to the vehicle steering.
It should be noted that although the disclosed embodiments are described by
way of reference to motor control for electric steering applications, it
will be appreciated that such references are illustrative only and the
disclosed embodiments may be applied to any instance where rotational
displacement, e.g., torque sensing is desired. Moreover, the references
and descriptions herein may apply to many forms of parameter sensors,
including, but not limited to torque, position, speed and the like. It
should also be noted that reference herein to electric machines including,
but not limited to, motors, or more specifically sinusoidally excited
brushless DC motors, hereafter, for brevity and simplicity, reference will
be made to motors only without limitation.
In the control system 24 as depicted, the controller 16 utilizes the
torque, position, and speed, and like, to compute a command(s) to deliver
the required output power. Controller 16 is disposed in communication with
the various systems and sensors of the motor control system. Controller 16
receives signals from each of the system sensors, quantifies the received
information, and provides an output command signal(s) in response thereto,
in this instance, for example, to the motor 46. Controller 16 is
configured to develop the necessary voltage(s) out of inverter (not shown)
such that, when applied to the motor 46, the desired torque or position is
generated. Because these voltages are related to the position and speed of
the motor 46 and the desired torque, the position and/or speed of the
rotor and the torque applied by an operator are determined. A position
encoder is connected to the steering shaft 51 to detect the angular
position .theta.. The encoder may sense the rotary position based on
optical detection, magnetic field variations, or other methodologies.
Typical position sensors include potentiometers, resolvers, synchros,
encoders, and the like, as well as combinations comprising at least one of
the forgoing. The position encoder outputs a position signal 20 indicating
the angular position of the steering shaft 51 and thereby, that of the
motor 46.
Desired torque may be determined by one or more torque sensors 28
transmitting torque signals 18 indicative of an applied torque. An
exemplary embodiment includes such a torque sensor 28 and the torque
signal(s) 18 therefrom, as may be responsive to a compliant torsion bar,
T-bar, spring, or similar apparatus (not shown) configured to provide a
response indicative of the torque applied. In an exemplary embodiment, a
non-contacting torque sensor is disclosed employs the inductance variation
of a coil due to a change in reluctance in a magnetic circuit when the
torsion bar is experiencing torque.
Referring now to FIG. 2 as well, the torque sensor 28 transmits torque
signal(s) 18 to controller 16 for processing in accordance with another
exemplary embodiment. Controller 16 executes a digital signal processing
(DSP) algorithm 200 is employed for processing the torque signal(s) for
the non-contacting torque sensor 28. In an exemplary embodiment, the
variable inductance information of a non-contacting torque sensor is
encoded into a time-period of square wave oscillation. Preferably, the
sensing electronics is configured integral with the torque sensor 28 or
controller 16 of the EPS system 40. Advantageously, this approach further
reduces cost and simplifies the interface electronics. The algorithm 200
depends on the frequency signal output from an oscillator circuit 120. The
torque sensor 28 forms an element of the oscillator circuit 120. In an
exemplary embodiment, the period of the oscillation is proportional to the
inductance of the torque sensor 28. Hence, as the torque varies, the
inductance of the torque sensor 28 shifts resulting in a variation of the
inductance. Thus, the torque information is embedded in the frequency or
period of the oscillation, which may readily be observed and measured.
Optionally, a temperature sensor(s) 23 located at the torque sensor 28.
Preferably the temperature sensor 23 is configured to directly measure the
temperature of the sensing portion of the torque sensor 28. The
temperature sensor 23 transmits a temperature signal 25 to the controller
16 to facilitate the processing prescribed herein and compensation.
Typical temperature sensors include thermocouples, thermistors,
thermostats, and the like, as well as combinations comprising at least one
of the foregoing sensors, which when appropriately placed provide a
calibratable signal proportional to the particular temperature.
The position signal 20, velocity signal 21, and a torque signal(s) 18 among
others, are applied to the controller 16. The controller 16 processes all
input signals to generate values corresponding to each of the signals
resulting in a rotor position value, a motor speed value, and a torque
value being available for the processing in the algorithms as prescribed
herein. Measurement signals, such as the abovementioned are also commonly
linearized, compensated, and filtered as desired or necessary to enhance
the characteristics or eliminate undesirable characteristics of the
acquired signal. For example, the signals may be linearized to improve
processing speed, or to address a large dynamic range of the signal. In
addition, frequency or time based compensation and filtering may be
employed to eliminate noise or avoid undesirable spectral characteristics.
In order to perform the prescribed functions and desired processing, as
well as the computations therefore (e.g., the torque sensor signal
processing, control algorithm(s), and the like), controller 16 may
include, but not be limited to, a processor(s), computer(s), DSP(s),
memory, storage, register(s), timing, interrupt(s), communication
interface(s), and input/output signal interfaces, and the like, as well as
combinations comprising at least one of the foregoing. For example,
controller 16 may include input signal processing and filtering to enable
accurate sampling and conversion or acquisitions of such signals from
communications interfaces. Additional features of controller 16 and
certain processes therein are thoroughly discussed at a later point
herein.
As exemplified herein and disclosed above one such process may be
determining a torque value from torque signal(s) 18. Controller 16
receives various input signals including, but not limited to, those
identified above, to facilitate such processing and may provide one or
more output signals in response. Once again, it will be appreciated that
while the embodiment disclosed herein is explained by way of discussion
concerning torque signals and torque sensors, other sensors and sensed
parameters may be equally applicable.
In an exemplary embodiment, the controller 16 obtains as input signals or
receives signals to facilitate computing, among others, a torque signal 18
from a torque sensor 28. The torque signal 18 is representative of the
torque felt/applied by an operator of the vehicle or they may be combined
in series to achieve a larger magnitude signal. Also received by the
controller 16 are a variety of implementation specific parameters, signals
and values for initialization and characterization of the prescribed
processes and to identify various states of the processes herein.
Continuing now with FIG. 2, there is depicted an expanded view of an
exemplary embodiment of a non-contacting torque sensor 28. The
electromagnetic structure of the torque sensor 28 comprises a ring shaped
upper toothed-structure (UTS) hereinafter denoted UTS 102, a lower ring
shaped toothed-structure (LTS) hereinafter denoted LTS 104, a sleeve 106
and coil/bobbin assembly hereinafter denoted coil 108. The UTS 102, LTS
104, sleeve 106 and coil 108 are assembled and installed in the steering
system 40 by means of some mechanical members configured to position the
UTS 102 and LTS 104 as rotating members while the sleeve 106 and coil 108
are stationary.
The operation of the torque sensor 28 is based on the reluctance variation
of the magnetic circuit when there is a relative rotation between UTS 102
and LTS 104. The reluctance is an embedded element in the inductance of
the coil 108, which can be encoded into a measurable electrical quantity
such as a voltage, time-period, and the like. The UTS 102 and LTS 104 are
attached to the upper steering shaft 29 and lower steering shaft 51,
respectively, of the steering system 40. The two assemblies of the
steering system 40 are linked by the torsion bar or similar torque
responsive apparatus (not shown). When the torque is applied (for example,
at the steering wheel 26, the compliance of the torsion bar provides a
relative circumferential shift between the UTS 102 and LTS 104, which
varies the overlapping area between the UTS 102 and LTS 104, hence the
reluctance of the magnetic circuit or the inductance of the coil 108
varies. Thus, it will be appreciated that the inductance of the coil 108
facilitates an encoding circumferential displacement of the UTS 102 and
LTS 104, and thereby an encoding of the torque information as applied on
the upper steering shaft 29 relative to the lower steering shaft 51.
Similarly, the applied torque (e.g., at steering wheel 26) is converted to
an angle by means of the same torsion bar.
Referring now to FIGS. 3 and 4 as well, the UTS 102 and LTS 104 are each
comprised of substantially similar annular rings of magnetic material
(e.g., soft magnetic material, ferrite, and the like). The UTS 102 and LTS
104 each include an equal number of teeth shown generally as cut into the
magnetic material about their circumference and projecting in an opposing
radial direction and configured so that a selected finite air gap is
maintained between the opposing teeth. In an exemplary embodiment, an air
gap of one millimeter is employed between the teeth 110 of the UTS 102 and
LTS 104.
The number of teeth 110 on both the UTS 102 and LTS 104 are same and also
same in terms of geometry except at the back iron portion 112 of both,
which is used to pass flux to the sleeve 106. In an exemplary embodiment
nine teeth 110 are utilized for both the UTS 102 and the LTS 104. It will
be apparent and made further evident from discussion herein that other
numbers of teeth may readily be employed.
Continuing with FIGS. 2, 3, and 4, the LTS 104 also includes a flange 114
or lip projecting radially on the back iron portion 112 configured to
maintain a selected magnetic air gap with an internal surface of the
sleeve 106. In an exemplary embodiment, an air gap of 0.28 millimeter is
employed.
However, it will be readily appreciated that other air gaps may be employed
based upon the particular configuration and arrangement of the torque
sensor 28.
In an exemplary embodiment, the coil 108 is an assembly of two conductors
wound around a bobbin, which surrounds only the toothed structure
(projected portion not the back iron 112) of UTS 102 and LTS 104. The coil
108 is surrounded by a soft magnetic sleeve 106, which provides closed
path to the magnetic flux produced by the current flowing through the coil
108. The sleeve 106 also includes and internal lip 107 configured to
maintain a selected magnetic air gap with a rear portion of UTS 102. In an
exemplary embodiment, an air gap of 0.28 millimeter is employed. The coil
108 in conjunction with the UTS 102, LTS 104, and sleeve 106 forms the
electromagnetic part of the torque sensor 28. In an exemplary embodiment,
the sleeve 106, UTS 102, and LTS 104 are made of soft iron material.
Referring now to FIG. 5 as well, the inductance of the coil 108 varies due
to the variation in the reluctance of the magnetic circuit of the torque
sensor 28. FIG. 5 depicts an equivalent magnetic circuit 150 for the
torque sensor 28. The equivalent magnetic circuit 150 includes the
magnetomotive force denoted as NI and 152 with the core reluctance 154 and
air gap reluctances 156, 158, and 160. The air gaps in the physical sensor
and the reluctances associated therewith are:
(1). The air gap between the sleeve 106 and the back iron of the UTS 102
denoted ({character pullout}.sub.ag.sub..sub.-- .sub.UTS.sub..sub.--
.sub.sleeve) and 158 in the figure.
(2). The air gap between the sleeve 106 and the back iron of the LTS 104
denoted ({character pullout}.sub.ag.sub..sub.-- .sub.LTS.sub..sub.--
.sub.sleeve). and 160 in the figure.
(3). The air gap between the UTS 102 and the LTS 104 denoted ({character
pullout}.sub.ag.sub..sub.-- .sub.UTS.sub..sub.-- .sub.LTS) and 156 in the
figures.
Ideally, it will be readily understood that only the reluctance {character
pullout}.sub.ag.sub..sub.-- .sub.UTS.sub..sub.-- .sub.LTS 156 should vary
as the UTS 102 and LTS 104 are rotated relative to one another as a torque
is applied. In practice, because of tolerances, the parallel and angular
offsets between the UTS 102 and LTS 104, and a temperature dependence of
the properties of the soft magnetic material, the other reluctances may
also vary. Therefore, an ideal design objective would be to make the
reluctance variation, and thereby the inductance variation proportional to
the reluctance variation in {character pullout}.sub.ag.sub..sub.--
.sub.UTS.sub..sub.-- .sub.LTS 156 irrespective of any other variation.
To minimize the sensitivity of and effects from other reluctance sources,
optimally, either the air gaps should be minimal or the areas should be
maximal. A finite air gap is dictated by a non-contacting constraint,
therefore, the area should be maximized to minimize the effects from
{character pullout}.sub.ag.sub..sub.-- .sub.UTS.sub..sub.-- .sub.sleeve
158 and {character pullout}.sub.ag.sub..sub.-- .sub.LTS.sub..sub.--
.sub.sleeve 160. Increasing the length (axially) of the back iron 112 and
sleeve 106 increases the area, making the inductance and thereby the
magnetic circuit less sensitive to the variation in those particular
reluctances. However, increases in the length of the back iron 112 and
sleeve 106 increase the overall size and weight of the torque sensor 28.
Therefore, a balance between the size of the sensor and the desired
insensitivity to the reluctances may be appropriate.
In an exemplary embodiment, the air gap between UTS 102 and LTS 104 should
be such that enough and reasonable variation in inductance between aligned
and unaligned positions of the teeth 110. The aligned and unaligned
positions of UTS 102 and LTS 104 are shown in FIGS. 3 and 4. Assuming the
aligned position for the teeth 110 to be 0 degrees, the unaligned position
becomes 20 degrees for a UTS 102 and LTS 104 exhibiting 9 teeth 110 each.
The center or neutral position, therefore, occurs at 10 degrees, which is
in between the aligned and unaligned positions. In an exemplary
embodiment, the torque sensor 28 is limited by the travel of the torsion
bar such that the relative movement between UTS 102 and LTS 104 may be
.+-.8 degrees from the center/neutral position. Hence, it should be
apparent, that the various air gaps, the geometry of the structure, and
the material properties may all be selected to achieve a non-contacting
torque sensor that achieves desirable characteristics.
In an exemplary embodiment, a torque sensor 28 with a one millimeter (mm)
air gap between the teeth 110 of the UTS 102 and LTS 104 respectively,
exhibited substantially linear response over the desired range. It will be
further appreciated that the inductance and response is frequency
dependent. As the frequency increases, the core loss influences the
inductance to the extent that beyond 10 kHz, the inductance values may
become saturated. Different configurations will, of course exhibit various
frequency dependent characteristics. In an exemplary embodiment the
inductance level is maintained to be sufficient such that the frequency of
the oscillation of the oscillator circuit 120 (of which the inductance of
the coil 108 is an integral part) is not too high (avoids core losses).
Recall that the time period/frequency of the oscillation for the
oscillator circuit 120 is proportional to the inductance of the coil 108
assuming the other circuit parameters remain unchanged. In an exemplary
embodiment, the UTS 102 and LTS 104 and sleeve 106 are made of soft
magnetic material having the properties of minimum core loss and high
initial permeability (at low magnetomotive force). The frequency of the
oscillation is selected such that the effect of core losses are minimal.
In an exemplary embodiment a frequency of about 10 kHz is employed. Of
course, other frequencies are possible, depending upon the selected values
for R, L, and the acceptable core losses.
Torque Measurement and Determination
Referring now to FIG. 6, a schematic of an exemplary circuit for the
oscillator circuit 120 is depicted. The oscillator circuit 120 is
configured to translate the inductance (L) information into
time-period/frequency of the oscillation. More specifically, the
oscillator circuit 120 converts the variation in inductance into a
variation in time-period/frequency from which the torque may be readily
ascertained. Ideally, the time-period (T.sub.p) of oscillation is given by
assuming the output of the oscillator circuit 120 varies between V.sub.L
=0, V.sub.H =V.sub.cc, and the ration R.sub.2 /(R.sub.1 +R.sub.2)=1/2
yielding:
##EQU1##
Equation (1) show that as the inductance L varies with applied torque, the
period T.sub.p also varies. Hence, by measuring the period T.sub.p, the
torque information from the torque sensor 28 may be extracted. A DSP
algorithm may be employed for this purpose as will be discussed at a later
point herein.
In an exemplary embodiment, the oscillator circuit 120 is configured to
accommodate operation from a single voltage supply operation as shown in
FIG. 6. Turning to the figure, in an exemplary embodiment, the oscillator
circuit 120 comprises an operational amplifier 124 with the inductance L
and resistance R.sub.L of the coil 108 in the feedback. Resistance R also
denoted 130 provides input from a reference voltage source denoted
V.sub.REF, selected in this instance to be half of the supply voltage
denoted V.sub.CC. Moreover, resistors R.sub.1 and R.sub.2 also denoted 126
and 128 respectively, operate to facilitate providing hysteresis. One
skilled in the art would appreciate and understand the operation of the
oscillator circuit 120, therefore, further description of the operation is
omitted.
Effect of Temperature and Its Compensation
Theoretically, the inductance (L) of the coil 108 varies with temperature
though it is minimized from magnetic design of the core. The permeability
and resistivity of the magnetic core material change with temperature,
which in turn influences the inductance variation. Finally, the winding
resistance (R.sub.L) of the coil 108 varies with temperature. Therefore,
the effect of the variation of R.sub.L and L with temperature on T.sub.p
is preferably compensated. It will be appreciated that Equation (1) may be
simplified in by ensuring that the value for R is much larger than R.sub.L
yielding:
##EQU2##
T.sub.P.apprxeq.kL (2a)
Therefore, if the proportionality constant k of Equation (2a) may be kept
constant over the whole operating temperature range, temperature
insensitivity of the oscillator circuit 120 may readily be achieved if the
inductance variation is maintained within acceptable limits. Therefore, by
using a precision resistor for resistor R 130, where R>>R.sub.L.
Advantageously, this approach also makes the oscillator circuit 120
substantially independent of the temperature as the resistors R.sub.1 126
and R.sub.2 128 are used as voltage divider, and thus it can be assumed
that the resistance ratio of the voltage divider remains substantially the
same with temperature variation. Therefore, only component tolerance and
variation have effect, which may be readily addressed with precision
resistors and components. Assuming a change in the resistance of the coil
108 R.sub.L as .DELTA.R.sub.L, the change in time period .DELTA.T.sub.p
can be expressed as:
##EQU3##
Equation (3) may be therefore, be employed to design resistance R 130 for a
given bound on .DELTA.T.sub.p. The value of R 130 for a given maximum
inductance L.sub.MAX of the coil 108 for given bound on .DELTA.T is the
appropriate design. In other words, based upon an expected possible range
of inductance L for the coil 108, a selected value for R will result in a
bounded range for T.sub.p. Therefore, resistor R may readily be selected
to achieve a desired range of frequencies/period T.sub.p. In an exemplary
embodiment for a inductance L of with a range from about 1.6 milliHenries
to about 1.8 milliHenries, resistance R 130 may be selected to be about 40
ohms to achieve a frequency in desired range. Advantageously, the drift in
supply voltage to the operation amplifier 124 does not change the period
T.sub.p, but the amplitude of the oscillation.
Turning to FIG. 7, in yet another exemplary embodiment an oscillator
circuit 120a is depicted where a temperature-compensating resistor
R.sub.c, also denoted 132 is incorporated to nullify the effect of
variation in L and/or R.sub.L with temperature on time-period (T.sub.p) of
the oscillation.
It will be appreciated that in an exemplary embodiment the resistors e.g.,
126, 128, 130, 132 may be the precision resistors, which do not vary more
than 0.1% over the whole temperature range. The inductance to time-period
(L-T.sub.p) expression from Equation (1) may be rewritten as a function of
temperature as:
##EQU4##
where
R.sub.L (T)=R.sub.La (1+.alpha..sub.R.sub..sub.L (T-T.sub.a)) (4a)
and: R.sub.La denotes the winding resistance at ambient temperature T.sub.a
.alpha..sub.RL is the temperature coefficient.
The nature of variation of L and R.sub.L with temperature is similar.
Careful investigation shows that if R can be made to exhibit similar
properties to L and R.sub.L with temperature, then, in the oscillator
circuit 120 (120a) operation it has the opposite effect on Tp. Therefore,
using R 130, it will be appreciated that the variations in Tp with
temperature may be balanced. The temperature dependence of L and R.sub.L
may be determined experimentally for initial design of the compensating
resistor R.sub.c. 132 or analytically.
Considering that the sum of R.sub.o 130 and R.sub.c 132 establishes the
operating frequency of oscillation, while at ambient temperature, the
total resistance should be equal to the desired value for R 130 (from
above).
It will be evident that R.sub.L would ideally be zero to make T.sub.p
insensitive to temperature assuming R and L do not vary with temperature.
As is well known, in practice, both L and R.sub.L exhibit variation with
temperature. As disclosed hereinbefore, the temperature variation in L
(the inductance of coil 108) may be accounted for and compensated to
within selected constraints by magnetic design specification. Therefore,
the compensation disclosed herein may address only the variation with
temperature in R.sub.L, the variation in L or both to achieve temperature
insensitivity for the torque sensor 28.
Continuing with FIG. 7, the compensating resistor R.sub.c 132, may be
placed in series with R.sub.o, also denoted 130a, which exhibits similar
temperature properties as the winding resistance, in this instance copper
wire used in the coil 108. The relation in Equation (4) may be rewritten
illustrating the temperature dependency and considering resistor R.sub.c
132 as:
##EQU5##
where R.sub.ca 132 and R.sub.La represent the nominal values for these
resistances respectively at ambient temperature T.sub.a and .alpha..sub.RL
and .alpha..sub.RC are the temperature coefficients of resistance for
winding and compensating resistance respectively.
Observation of Equation (5) indicates the following parametric
relationships:
T.uparw.R.sub.L.uparw.L.uparw.{character pullout}T.sub.p.uparw.
T.dwnarw.R.sub.L.dwnarw.L.dwnarw.{character pullout}T.sub.p.dwnarw.
T.uparw.(R.sub.o +R.sub.c (T)).uparw.{character pullout}T.sub.p.dwnarw.
T.dwnarw.(R.sub.o +R.sub.c (T)).dwnarw.{character pullout}T.sub.p.uparw.
where T is temperature,
R.sub.L is the resistance of the coil,
L is the inductance,
R.sub.o is a resistor in the oscillator circuit 120a,
R.sub.c is a temperature compensation resistor in the oscillator circuit
120a,
T.sub.p is the period of oscillation.
It will now be appreciated from Equation (5) that one may readily design
R.sub.c 132 to make T.sub.p independent of temperature T. Mathematically,
taking the derivative of T.sub.p with respect to T and equating it to
zero, the value of R.sub.ca (R.sub.c 132 at ambient) is readily computed.
Hence, the final expression to solved becomes:
##EQU6##
To extract the value of R.sub.ca, a numerical solution to Equation (6) is
desired. Depending on temperature, it will be appreciated that a range of
values for R.sub.c 132, which satisfy Equation (6) may be ascertained. It
will also be appreciated, that the lower the nominal value of R.sub.L, the
smaller the potential range of values for R.sub.c 132 over the temperature
range. Therefore, because the resistance R.sub.L should be kept as small
as possible, a value for resistance R.sub.c 132 may readily be implemented
employing track resistance of a printed circuit board so that the
compensation resistance R.sub.c 132 exhibits similar properties to the
copper winding. In general, the ratio R/R.sub.L greater than 10 has
negligible effect on period (T.sub.p) due to the change in R.sub.L over
the operating temperature range.
Implementing the compensation resistance R.sub.c 132 and incorporating the
magnetic design for steady inductance L over the temperature range, the
torque sensor 28 may readily provide superior characteristics including
but not limited to, simple construction, very low-cost compared to the
existing technologies and art, and a minimum of interface electronics.
In yet another embodiment, it will readily be appreciated that direct
temperature compensation is also possible. Temperature can be measured
with temperature sensor 23, and the torque signal directly compensated
with appropriate scaling.
DSP Methodology for Time Period Measurement
FIG. 8 depicts a flow chart of an exemplary algorithm 200 for the capture
of the period and determination of the torque. FIG. 9 depicts an
illustration of period capture, and the parameters associated with
determination of the oscillation frequency and ultimately the torque
sensed. In an exemplary embodiment, the output of the oscillator circuit
120 (120a) (FIGS. 6 and 7 respectively) is connected to an input capture
circuit or function of a DSP (or other like processor). Moreover, in an
exemplary embodiment, the algorithm 200 may by implemented as a recursive
loop for processing the torque signal(s) 18 and determination of the
oscillation period as may be implemented in software, firmware, dedicated
field programmable gate array, and the like, as well as combinations
including at least one of the foregoing.
A capture clock is employed to facilitate determination of the oscillation
period T.sub.p. The maximum time of the capture clock hereinafter denoted
(TCP) is set to a selected maximum period. In an exemplary embodiment, the
capture clock (TCP) is set to be more than the maximum expected period
T.sub.p of the input capture signal from the oscillator circuit 120 (120a)
(FIGS. 6 and 7 respectively). The selected frequency band is generated
from the inductance profile of the torque sensor 28 as discussed above.
Advantageously, this also means that a failure of the torque sensor 28, or
inoperative torque sensor 28 will result in generation of a frequency,
which is outside the selected band indicating a fault. Similarly, improper
operation of the oscillator circuit 120, 120a will also result in
generation of a frequency, which is outside the selected band. If the
computed period T.sub.p is outside the selected band including a selected
tolerance, an invalid sensed torque is flagged. This flag may be
transmitted to the diagnostics portion of the algorithm 200 and/or overall
processing for the steering system 40 (FIG. 1).
Continuing with FIGS. 8 and 9, the input capture is configured to capture a
transitioning edge of the input frequency signal (V.sub.o of the
oscillator circuit 120, 120a) as depicted at process block 202. The data
from the input capture is processed after two consecutive captures as
indicated by decision block 204 and an associated loop. If two consecutive
captures are acquired, at process block 206 the times associated with a
first capture denoted in the figure as CTE1 and second capture denoted in
the figure CTE2 respectively, are determined. The time difference between
the two respective captures is computed at process blocks 210 (or 212) and
yields the period T.sub.p for the input frequency from the oscillator
circuit 120, (120a). Decision block 208 selects an appropriate computation
at process blocks 210 and 212 respectively to address numerical
computation issues. It will be further appreciated that while an exemplary
embodiment discusses two consecutive captures for measurement of timing it
should be evident that other configurations of the process are possible.
For example, any two successive transitions would facilitate computation
of half of the period, and the like. Finally, at process block 214 the
period T.sub.p scaled to calculate the torque as measured. As discussed
above, the period is proportional to the time constant of the coil 108 and
hence the displacement of the UTS 102 and LTS 104 (FIG. 2). Once again,
the torque may be directly obtained from the period T.sub.p.
The disclosed invention can be embodied in the form of computer or
controller implemented processes and apparatuses for practicing those
processes. The present invention can also be embodied in the form of
computer program code containing instructions embodied in tangible media
13, such as floppy diskettes, CD-ROMs, hard drives, or any other
computer-readable storage medium, wherein, when the computer program code
is loaded into and executed by a computer or controller, the computer
becomes an apparatus for practicing the invention. The present invention
may also be embodied in the form of computer program code as a data signal
15, for example, whether stored in a storage medium, loaded into and/or
executed by a computer or controller, or transmitted over some
transmission medium, such as over electrical wiring or cabling, through
fiber optics, or via electromagnetic radiation, wherein, when the computer
program code is loaded into and executed by a computer, the computer
becomes an apparatus for practicing the invention. When implemented on a
general-purpose microprocessor, the computer program code segments
configure the microprocessor to create specific logic circuits.
It will be appreciated that the use of first and second or other similar
nomenclature for denoting similar items is not intended to specify or
imply any particular order unless otherwise stated.
While the invention has been described with reference to an exemplary
embodiment, it will be understood by those skilled in the art that various
changes may be made and equivalents may be substituted for elements
thereof without departing from the scope of the invention. In addition,
many modifications may be made to adapt a particular situation or material
to the teachings of the invention without departing from the essential
scope thereof. Therefore, it is intended that the invention not be limited
to the particular embodiment disclosed as the best mode contemplated for
carrying out this invention, but that the invention will include all
embodiments falling within the scope of the appended claims.
*
