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Claims  |
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We claim:
1. A contactless electromechanical actuator comprising:
a) an electric brushless motor with a rotatable motor shaft;
b) a rotable output shaft;
c) first and second magnets connected for rotation with the motor shaft and
output shaft, respectively;
d) a geartrain (i) coupled for torque amplification from the motor shaft to
the output shaft, and (ii) having a step-down ratio coupling the first
magnet to the second magnet;
e) magnetic field sensors (i) positioned for sensing the magnetic fields of
the magnets, and (ii) providing output signals indicative of the magnetic
fields sensed;
f) a processor module receiving said output signals, and operative to (i)
calculate the rotational angles of the magnets therefrom, and (ii) compute
the angular position of the output shaft as a function of said rotational
angles and the step-down ratio, wherein the rotational angle of the second
magnet provides a coarse indication of the angular position of the output
shaft, and the rotational angle of the first magnet divided by the gear
ratio provides a refined indication of the angular position of the output
shaft; and
g) a motor control module responsive to the computed angular position of
the output shaft to commutate the motor and control the angular position
of the output shaft.
2. The actuator as defined in claim 1 in which the magnets are each
provided with two poles that rotate about the magnet's center axis to
establish a periodic variation in magnetic field intensity as the magnets
rotate.
3. The actuator as defined in claim 2 in which (a) the magnets are annular
in shape, and are each provided with two poles spaced 180 degrees apart;
(b) the magnetic field sensors each comprise a pair of magnetic field
sensor elements associated with each of the magnets, the sensor elements
of each pair (i) being positioned for sensing different components of the
magnetic fields, and (ii) having output signals indicative of the
component strength of the magnetic field sensed; and (c) the processor
module is operative to calculate the rotational angles of the magnets
according to a relationship between the ratio of the output signals from
the sensor elements of each pair.
4. The actuator as defined in claim 3 in which the sensor elements of each
pair comprise ratiometric Hall-effect devices positioned for sensing
orthogonal non-saturating components of the magnetic fields and providing
periodic output signals phase shifted 90 degrees apart as the magnets
rotate.
5. The actuator as defined in claim 4 further comprising an additional pair
of ratiometric Hall-effect devices positioned for sensing orthogonal
non-saturating components of the magnetic fields and providing redundant
periodic output signals phase shifted 90 degrees apart as the magnets
rotate.
6. The actuator as defined in claim 3 in which the sensor elements of each
pair comprise magnetoresistive bridge sensor elements having magnetically
sensitive axes positioned for sensing orthogonal non-saturating components
of the magnetic fields and providing periodic output signals phase shifted
90 degrees apart as the magnets rotate.
7. The actuator as defined in claim 1 in which the step-down ratio is an
integer ratio.
8. The actuator as defined in claim 1 in which the geartrain comprises a
multiple-stage planetary geartrain.
9. The actuator as defined in claim 8 in which the geartrain includes first
and second sides, the motor and magnets are located on said first side and
the output shaft is located on said second side, and the actuator further
comprises a center shaft passing therethrough and connected between the
output shaft and the second magnet.
10. The actuator as defined in claim 1 in which the processor module
obtains a rotational angle of the first magnet from the calculated
rotational angle of the second magnet multiplied by the step-down ratio,
and uses the obtained rotational angle of the first magnet in computing
the angular position of the output shaft.
11. The actuator as defined in claim 10 in which the processor module
compares the obtained and calculated rotational angles of the first
magnet, and uses the comparison divided by a function of the step-down
ratio in computing the angular position of the output shaft.
12. The actuator as defined in claim 11 in which the processor module uses
said comparison divided by said step-down ratio function to obtain the
complete turns of the first magnet, and applies said complete turns
divided by the step-down ratio and the calculated rotational angle of the
first magnet divided the step-down ratio in computing the angular position
of the output shaft.
13. The actuator as defined in claim 11 in which the processor module uses
said comparison divided by said step-down ratio function to obtain a
second-magnet error correction angle, and applies the error correction
angle to the calculated rotational angle of the second magnet in computing
the angular position of the output shaft.
14. The actuator as defined in claim 1 further comprising a memory storage
module containing motor shaft angle error correction data, and in which
the processor module computes the angular position of the output shaft as
a further function of the stored error correction data.
15. The actuator as defined in claim 1 in which the processor module is
operative to compare an input command signal with the computed angular
position of the output shaft, and the control module is responsive to said
comparison to commutate the motor and control the angular position of the
output shaft.
16. The actuator as defined in claim 1 in which the processor module is
further operative to calculate time derivatives of the rotational angles
of the magnets from said output signals, and compute an associated time
derivative of the angular position of the output shaft as a function of
the time derivatives of said rotational angles and the step-down ratio,
wherein the time derivative of the rotational angle of the second magnet
provides a coarse indication of the time derivative of the output shaft,
and the time derivative of the rotational angle of the first magnet
divided by the gear ratio provides a refined indication of the time
derivative of the output shaft; and in which the motor control module is
further responsive to the computed time derivative of the angular position
of the output shaft to commute the motor and control said time derivative
of the output shaft.
17. A contactless electromechanical actuator comprising:
a) an electric brushless motor with a rotatable motor shaft;
b) a rotable output shaft;
c) first and second annular magnets connected for rotation with the motor
shaft and output shaft, respectively, the magnets each having two poles
180 degrees apart that establish a sinusoidal variation in magnetic field
intensity as the magnets rotate;
d) a geartrain (i) coupled for torque amplification from the motor shaft to
the output shaft, and (ii) having a step-down ratio coupling the first
magnet to the second magnet;
e) first and second pairs of magnetic field sensor elements positioned to
sense the magnetic fields of the first and second magnets, respectively,
--the magnetic field sensor elements of each pair
i) being positioned to sense orthogonal non-saturating components of the
magnetic fields, and
ii) providing output signals proportional to the strength of the magnetic
fields sensed and phase shifted 90 degrees;
f) a processor module receiving said output signals and an input command
signal, and operative to
i) calculate the rotational angles of the magnets from the 90 degree
out-of-phase signals from the sensor elements of each pair,
ii) obtain a rotational angle of the first magnet from the calculated
rotational angle of the second magnet multiplied by the step-down ratio,
iii) compute the angular position of the output shaft as a function of the
obtained rotational angle of the first magnet, and the calculated
rotational angle of the first magnet divided by the step-down ratio, and
iv) compare the computed angular position of the output shaft with the
input command signal; and
g) a motor control module responsive to said comparison to commutate the
motor and control the angular position of the output shaft.
18. The actuator as defined in claim 17 in which the processor is operative
to obtain a full-turn rotational angle of the first magnet from the
rotational angle of the second magnet multiplied by the step-down ratio.
19. The actuator as defined in claim 17 in which the processor is operative
to obtain a fractional-turn rotational angle of the first magnet from the
rotational angle of the second magnet multiplied by the step-down ratio.
20. A contactless electromechanical actuator comprising:
a) an electric brushless motor with
i) first and second sides, and
ii) a rotatable motor shaft extending between said first and second sides;
b) a rotatable output shaft extending coaxial with the motor shaft between
said first and second sides of the motor;
c) a geartrain
i) coupled between the motor shaft and the output shaft on the second side
of the motor, and
ii) having a step-down gear ratio for torque amplification from the motor
shaft to the output shaft;
d) first and second annular magnets
i) located on the first side of the motor;
ii) secured concentric around said motor shaft and said output shaft,
respectively, for rotation therewith, and
iii) each having two poles 180 degrees apart to establish a periodic
variation in magnetic field intensity as the magnet rotates with its
associated shaft;
e) first and second pairs of magnetic field sensor elements positioned to
sense the magnetic fields of the first and second magnets, respectively,
--the sensor elements of each pair.
i) being positioned to sense orthogonal non-saturating components of the
associated magnetic field, and
ii) providing 90 degree phase shifted output signals proportional to the
strength of the magnetic fields sensed;
f) a processor module receiving said output signals and operative to
i) calculate the rotational angles of each magnet according to the ratio of
the 90 degree phase-shifted signals from the sensor elements of each pair,
and
ii) compute the angular position of the output shaft as a function of the
rotational angles of the magnets, wherein the rotational angle of the
second magnet provides a coarse indication of the angular position of the
output shaft, and the rotational angle of the first magnet divided by the
gear ratio provides a refined indication of the angular position of the
output shaft, and
g) a motor control module responsive to the computed angular position of
the output shaft to commutate the motor and control the angular position
of the output shaft.
21. The actuator as defined in claim 20 in which the geartrain comprises a
multiple-stage planetary geartrain, and the output shaft passes through
the center of the geartrain.
22. A method for controlling a rotatable output shaft of an
electromechanical actuator, the method comprising the steps of
a) providing:
i) said actuator having:
a) said rotatable output shaft,
b) an electric brushless motor with motor windings and a rotatable motor
shaft, and
c) a geartrain for torque transmission from the motor shaft to the output
shaft, the geartrain having step-down gear ratio that divides the rotation
of the output shaft into rotational segments having an angular measure
equal to 360 degrees divided by the step-down ratio of the geartrain, each
angular segment being associated with a full rotation of the motor shaft
through said gear ratio,
d) first and second annular magnets connected to rotation with the motor
shaft and output shaft, respectfully, the magnets each having two poles
that establish a periodic magnet field intensity as the magnets rotate,
and
e) first and second magnetic field sensor sets positioned to sense the
magnetic fields of the first and second magnets, respectfully, the first
magnetic field sensor set providing a first sinusoidal signal indicative
of the rotational angle of the motor shaft between 0 and 360 degrees
rotation, and the second magnetic field sensor set providing a second
sinusoidal signal indicative of the rotational angle of the output shaft
between 0 and 360 degrees rotation, and
ii) an input command signal;
b) determining
i) in which one of the segments the output shaft is in from the second
signal, and
ii) the angular position of the output shaft in said one segment from the
first signal;
c) combining the information from said determining step as to said one
segment and the angular position of the output shaft therein to obtain a
corrected signal accurately indicative of the angular position of the
output shaft;
d) comparing the corrected signal with the input command signal; and
e) selecting and energizing selected motor windings to rotate the motor
shaft and output shaft in response to said comparison and rotate the
output shaft towards the input command signal. |
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Claims |
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Description |
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CROSS-REFERENCE TO RELATED APPLICATIONS
None
REFERENCE TO SEQUENCE LISTING, TABLE, OR COMPUTER PROGRAM LISTING APPENDIX
SUBMITTED ON A COMPACT DISC
N/A.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
N/A.
BACKGROUND OF THE INVENTION
1. Field of Invention
This invention relates generally to electromechanical actuators of the type
having an electric motor driving a rotary output shaft through a torque
amplifying gear train.
More specifically, the invention relates to an electromechanical actuator
with a brushless motor and contactless angular position sensors that
provide both motor commutation signals and output shaft angular position
signals to achieve a high-reliability, precision actuator.
2. Background Art
Electromechanical actuators have historically utilized AC or brush DC
motors with potentiometers for feedback. Brushes in the motors and wipers
in the potentiometers have led to limited life and low reliability for
these types of actuators. More recently, the trend in precision
electromechanical actuators is to utilize a brushless DC motor with a
resolver, optical encoder, or switching Hall-effect device for motor
commutation, a gear reducer for torque amplification, and a resolver,
optical encoder, or rotary-variable-differential-transformer (RVDT) for
sensing the angular position of the output shaft. These output shaft
position feedback sensors are typically self-contained units driven by
gears off the actuator output shaft. They are also substantially more
expensive than conventional potentiometers, often requiring AC excitation
and demodulation electronics to obtain useable output signals, and/or are
unreliable in low temperature, moist environments. Consequently, precision
actuators utilizing these types of sensors are generally more complicated
and more expensive than actuators with more conventional potentiometer
feedback.
Recent efforts to achieve lower-cost, yet reliable and accurate
electromechanical actuators have included use of integrated contactless
magnetic field sensor elements such as Hall-effect devices or
magnetoresistive (MR) sensors. These sensor elements are relatively low
cost, and are capable of generating electrical output signals when exposed
to a rotating magnetic field. Hall-effect sensors utilize a
current-carrying semi-conductor membrane to generate a low voltage
perpendicular to the direction of current flow when subjected to a
magnetic field normal to the surface of the membrane. Magnetoresistive
sensors utilize an element whose resistance changes in the presence of a
changing external magnetic field.
One group of prior electromechanical actuators utilize integrated
Hall-effect sensors to provide signals that are digital in nature,
generating pulses as a function of shaft rotation, or discrete signals for
incremental shaft angles. These digital signals are generally developed by
sensing the passage of notches, magnets, saturating magnet poles, or other
discrete signal generating arrangements on a rotating shaft, and are used
for motor commutation and/or actuator output shaft position sensing in the
actuator. For example, Takeda et al., U.S. Pat. No. 5,422,551 uses
Hall-effect sensors to generate pulse signals for motor control in a power
window drive mechanism. Collier-Hallman et al., U.S. Pat. No. 6,002,226
uses Hall-effect sensors to generate pulse signals for motor control in an
electric power steering system. Integrated Hall-effect sensors generating
digital control signals are also shown in the motor controls of Coles et
al., U.S. Pat. Nos. 6,104,152 and 6,124,688; Redelberger, U.S. Pat. No.
6,091,220; and Hans et al., U.S. Pat. No. 5,598,073. In Ritmanich et al.,
U.S. Pat. No. 6,198,243, integrated Hall-effect devices generate a pulsed
output from rotation of an actuator output shaft for stepper motor
control. As noted above, actuator and motor controls utilizing integrated
magnetic field sensors as digital signal generators often require
pulse-width modulation, or are otherwise relatively complicated to obtain,
process and utilize the digital output signals from the sensors. And the
accuracy of such devices is limited by the number of pulses per revolution
developed from the sensed rotating element.
Another group of prior electromechanical actuators utilize integrated
Hall-effect devices to produce analog signals indicative of the angular
position of the output shaft for closed-loop control of the actuator.
Electromechanical actuators of this type are shown in Peter et al., U.S.
Pat. No. 5,545,961, Weiss et al., U.S. Pat. No. 6,097,123, and Fukumoto et
al., U.S. Pat. No. 6,408,573. In general, these include annular magnets
provided with sets of alternating N-pole/S-pole combinations coupled to
the rotary output elements of the actuator, and Hall-effect sensors
arranged around the magnet to produce analog output signals that are
processed to obtain the angular position of the output element. Although
capable of sensing angular position through 360 degrees of rotation, the
accuracy of these types of actuators is limited to the accuracy of the
Hall-effect sensing elements, which is currently, typically in the
neighborhood of .+-.2 degrees, without provisions for special magnet
magnetization processes, special sensor configurations, temperature
compensation or reference calibration.
To advance the electromechanical arts, and to address the above-identified
drawbacks of prior actuators of the same general type, there is a need for
an improved electromechanical actuator that is capable of accurately
controlling the angular position of a rotary output shaft, with the high
reliability and long life available with the use of a brushless motor and
contactless sensors, but without the high cost and complexity associated
with use of resolver, encoders, or RVDTs. There is also a need for an
improved accurate, high-reliability actuator that can be economically
manufactured and compactly packaged.
For detailed discussion of position sensor configurations utilizing such
magnetic field sensor elements, reference is made to Frederick et al, U.S.
patent application Ser. No. 10/087,322, filed Feb. 28, 2002, and Seger et
al, U.S. patent application Ser. No. 10/367,459, filed Feb. 14, 2003, both
of which are assigned to the assignee of the present invention, and the
discussions of which are incorporated herein by reference.
SUMMARY OF THE INVENTION
An important objective of the present invention is to provide an improved
electromechanical actuator which can precisely control the angular
position of an output shaft, but which is economical to manufacture.
Another important objective of the invention is to provide an actuator
without motor and sensor contacts, brushes and wipers to improve actuator
life and reliability as compared with many prior economical actuators of
the same general type.
Another important objective of the invention is to provide an actuator that
accurately computes and controls the position of the output shaft with
enhanced accuracy without the high cost and complexity associated with use
of resolver, encoders, RVDTs and like sensor components of many prior
precision actuators.
Another important objective of the invention is to provide the foregoing
high-reliability, accurate actuator in a compact package utilizing
economical, standard components.
A detailed objective is to achieve the foregoing by providing an
electromechanical actuator with high-reliability contactless brushless
motor and contactless angular position sensing elements comprising simple
magnets and magnetic field sensing elements to produce both motor
commutation signals and shaft position signals.
Another detailed object is to achieve a compact actuator design by
integrating the functional motor and sensor components around a common
axis of rotation.
Another detailed objective is to use both the motor commutation signals and
output shaft position signals in a unique algorithm to achieve enhanced
precision control of the angular position of the output shaft.
These and other objectives and advantages of the invention will become
apparent from the following detailed description when taken in conjunction
with the accompanying drawings.
The objectives of the invention are accomplished in one preferred
embodiment actuator with a brushless electric motor, an integrated motor
commutation sensor comprising an annular two-pole magnet connected to
rotate with the motor shaft and a pair of ratiometric Hall-effect devices
for sensing the angular position of the magnet, a step-down geartrain
coupled between the motor shaft and an output shaft (i.e., a rotatable
output element), an integrated output shaft position sensor comprising a
second annular two-pole magnet connected to rotate with the output shaft
and a second pair of ratiometric Hall-effect devices for sensing the
angular position of the output shaft magnet, and a digital-signal
processor-based sensor computation and motor control circuit. The
Hall-effect devices sense the magnetic field of each magnet as it rotates
and provide output signals indicative of the angular position of the
magnet over a full 360 degrees of rotation. A controller module computes
the precise angle of the output shaft from the sensed positions of the
magnets, compares the computed output shaft angle with an input position
command, and provides logic signals to a motor power controller module to
energize the appropriate motor windings and turn the motor in the
direction necessary to drive the output shaft towards the commanded
position.
As in any closed-loop control system, the accuracy of the actuator is
primarily dependent upon the accuracy of the output shaft position sensing
system. In the present invention, a highly accurate position sensing
system is implemented economically and compactly by adding a pair of
magnets and associated magnetic field sensors, wherein one magnet is
connected to the motor shaft, the second magnet is connected to the output
shaft, and rotation of the two magnets is coupled by the step-down ratio
of the actuator geartrain such that the motor shaft rotates multiple
revolutions for one turn of the output shaft.
With this arrangement, the output shaft magnet is used to generate signals
to calculate a coarse indication of output shaft angle. In other words,
the sensed angle of the output shaft magnet, as calculated by the digital
signal processor, provides an indication of output shaft angle within the
sensing accuracy of the magnet and magnetic field sensors. Current
state-of-the-art in standard magnets and solid-state flux sensors can
typically provide an indication of shaft angle within .+-.2 degrees over
360 degrees of rotation and -54 to 125.degree. C. of temperature variation
without special magnetization or sensor configurations, electronic
temperature compensation, or reference calibration data. Since the angular
rotation of the motor shaft magnet can be sensed with the same degree of
accuracy, and its rotational angle is a fixed multiple of the angular
rotation of the output shaft, the sensed position of the motor shaft
magnet can be used to obtain a more accurate indication of the output
shaft angle with an improvement in accuracy approximately proportional to
the interconnecting gear ratio.
To compute the precise angular position of the output shaft, i.e., to
compute the angular position of the output shaft with the improved
accuracy, the sensed angular position of the output shaft magnet is used
to provide an absolute measure of the output shaft position at all motor
shaft angles, and to predict the angle of the motor shaft as calculated by
multiplying the sensed angle of the output shaft magnet by the gear ratio.
The difference in the calculated angles of the two magnets is then divided
by the gear ratio to obtain a correction factor that is applied to the
sensed angle of the output shaft to compute a more precise output shaft
position. Alternately, the angle of the output shaft magnet is utilized to
count the number of complete turns of the motor shaft magnet, the result
of which is added to the sensed angle of the motor shaft magnet. This
total motor shaft rotation is divided by the gear ratio to provide an
accurate measure of output shaft angle. Thus, the position of the output
shaft is accurately computed as a function of the sensed positions of both
the output shaft magnet, the motor shaft magnet, and the gear ratio
connecting the two magnets. In implementing this aspect of the invention,
the gear ratio between the magnets must be less than 360 degrees divided
by the maximum position sensing error of the output shaft magnet to
accurately predict the number of revolutions the motor shaft has
traversed. Alternately stated, the step-down ratio must be less than the
inverse of the accuracy in parts per hundred for which the rotational
angle of the output shaft magnet can be sensed.
In the preferred embodiment actuator, a circular or annular magnet is fixed
to or around the motor shaft. This configuration allows the shaft, or an
extension therefrom, to extend through the center of the magnet for ease
of attachment, and for compact packaging of the magnet in the actuator.
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