U.S. patent application number 09/886874 was filed with the patent office on 2002-02-14 for power assist vehicle.
Invention is credited to Ulrich, Nathan, Yoerger, Dana R..
Application Number | 20020019686 09/886874 |
Document ID | / |
Family ID | 23532806 |
Filed Date | 2002-02-14 |
United States Patent
Application |
20020019686 |
Kind Code |
A1 |
Ulrich, Nathan ; et
al. |
February 14, 2002 |
Power assist vehicle
Abstract
A power-assist vehicle such as a wheelchair senses driving
torque/force applied by a user through a transmission. The detected
force/torque is utilized in a control system with a control map
defining a desired dynamic of the vehicle and programmed with the
desired mass and drag parameter.
Inventors: |
Ulrich, Nathan; (Lee,
NH) ; Yoerger, Dana R.; (North Falmouth, MA) |
Correspondence
Address: |
WIGGIN & DANA LLP
ATTENTION: PATENT DOCKETING
ONE CENTURY TOWER, P.O. BOX 1832
NEW HAVEN
CT
06508-1832
US
|
Family ID: |
23532806 |
Appl. No.: |
09/886874 |
Filed: |
June 21, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09886874 |
Jun 21, 2001 |
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09388124 |
Aug 31, 1999 |
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09886874 |
Jun 21, 2001 |
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PCT/US00/23815 |
Aug 30, 2000 |
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60272216 |
Feb 28, 2001 |
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Current U.S.
Class: |
701/1 ; 180/374;
701/55; 701/62 |
Current CPC
Class: |
B60L 15/20 20130101;
Y02T 10/64 20130101; B60L 2240/463 20130101; B60L 2240/423
20130101; B60Y 2200/84 20130101; Y02T 10/72 20130101; B60L 2250/12
20130101; B62D 51/04 20130101; B60L 50/51 20190201; A61G 5/048
20161101; A61G 2203/38 20130101; B60K 2007/0061 20130101; B60L
2200/34 20130101; B60L 2240/12 20130101; B60L 2220/14 20130101;
Y02T 10/70 20130101; Y10S 180/907 20130101; B60K 7/0007 20130101;
A61G 5/1054 20161101; B60L 2220/44 20130101; B60K 17/043 20130101;
F16H 1/20 20130101; B60L 2240/461 20130101; B60L 2220/16 20130101;
B60L 2240/421 20130101; B60K 2007/0046 20130101; B60L 2220/46
20130101; A61G 5/045 20130101; B60L 15/2054 20130101 |
Class at
Publication: |
701/1 ; 701/55;
701/62; 180/374 |
International
Class: |
G06F 017/00 |
Claims
What is claimed is:
1. A gearing system for use in a motor assisted vehicle including
at least a drive wheel rotatably connected to a frame comprising: a
power input device for receiving an input force from a user; a
first gear assembly coupling the power input device to the drive
wheel and operatively associated with a transducer, the transducer
being configured to output at least one signal indicative of the
input force received by the power input device; a second gear
assembly operatively associated with a motor and coupling the motor
to the drive wheel; a sensor configured to output at least one
signal indicative of a motion of either of the motor or the drive
wheel; and a controller electrically connected to the sensor and to
the transducer, the controller being configured to receive the
output signal from the sensor and the output signal from the
transducer, to compute a control signal based upon one of at least
one programmed function, and to output the control signal to the
motor.
2. The gearing system of claim 1 wherein the controller comprises:
a computing device for determining the control signal from the
transducer signal using one of the at least one programmed
function; and a motor driver electrically connected to the
computing device for transmitting the control signal to the motor,
the control signal including a voltage and a polarity for
controlling a speed and a direction of the motor.
3. The gearing system of claim 2 wherein the motor driver
comprises: a digital-to-analog converter for producing said voltage
for controlling the speed of the motor; and an H-bridge circuit for
determining the polarity for controlling the direction of the
motor.
4. The gearing system of claim 2 wherein the computing device is a
microprocessor.
5. The gearing system of claim 1 wherein the controller further
comprises a memory for storing the at least one programmed
function.
6. The gearing system of claim 5 wherein the memory is an
electrically erasable programmable read only memory.
7. The gearing system of claim 1 wherein sensor output signal is
operatively associated with an error correction program.
8. The gearing system of claim 1 wherein the controller is
operatively associated with an external computer, the external
computer having at least one input/output port and being configured
to receive a functional input, and to output a programmed function
to the controller.
9. The gearing system of claim 8 wherein the at least one
input/output port includes an infrared port.
10. The gearing system of claim 1 wherein the controller is
operatively associated with a user interface, the interface
comprising: a switch for selecting a programmed function; and a
display for identifying the selected function.
11. A gearing system for use with a motor assisted vehicle
including at least a drive wheel rotatably connected to a frame
comprising: a power input device for receiving a first input force
from a user to power the vehicle; a gear assembly operatively
connecting the input device to the drive wheel; a motor operatively
associated with the gear assembly for providing a torque to the
drive wheel and thereby provide a second force to power the
vehicle; a transducer configured to output at least one signal
indicative of the first input force; a sensor configured to output
at least one signal indicative of a motion of either of the motor
or the drive wheel; and a controller electrically connected to the
sensor and to the transducer, the controller being configured to
receive the output signal from the sensor and the output signal
from the transducer, to compute a control signal based upon one of
at least one programmed function, and to output the control signal
to the motor.
12. A gearing system for use with a wheelchair including at least a
drive wheel rotatably connected to a frame comprising: a first gear
assembly operatively associated with a handrim and a transducer,
the transducer being configured to output at least one signal
indicative of a force applied to the handrim by a user; a second
gear assembly operatively associated with a motor; a sensor
configured to output at least one signal indicative of a motion of
either of the motor or the drive wheel; and a controller
electrically connected to the sensor and to the transducer, the
controller being configured to receive the output signal from the
sensor and the output signal from the transducer, to compute a
control signal based upon one of at least one programmed function,
and to output the control signal to the motor.
13. The gearing system of claim 12 wherein the controller
comprises: a computing device for determining the control signal
from the transducer signal using one of the at least one programmed
function; and a motor driver electrically connected to the
computing device for transmitting the control signal to the motor,
the control signal including a voltage and a polarity for
controlling a speed and a direction of the motor.
14. The gearing system of claim 13 wherein the computing device is
a microprocessor.
15. The gearing system of claim 13 wherein the motor driver
comprises: a digital-to-analog converter for producing said voltage
for controlling the speed of the motor; and an H-bridge circuit for
determining the polarity for controlling the direction of the
motor.
16. The gearing system of claim 12 wherein the controller further
comprises a memory for storing the at least one programmed
function.
17. The gearing system of claim 16 wherein the memory is an
electrically erasable programmable read only memory.
18. The gearing system of claim 12 further comprising an external
computer having at least one input/output port, the external
computer configured to receive a functional input, and to output a
programmed function to the controller.
19. The gearing system of claim 18 wherein the at least one
input/output port includes an infrared port.
20. The gearing system of claim 12 wherein the controller is
operatively associated with a user interface, the interface
comprising: a switch for selecting a programmed function; and a
display for identifying the selected function.
21. A gearing system for use with a wheelchair including at least a
drive wheel rotatably connected to a frame comprising: a handrim
configured to receive an input force from a user for providing a
first torque to the drive wheel; a gear assembly operatively
connecting the handrim to the drive wheel for transmitting said
first torque; a motor operatively associated with the gear assembly
for providing a second torque to the drive wheel; a transducer
configured to output at least one signal indicative of the user
input; a sensor configured to output at least one signal indicative
of a motion of either of the motor or the drive wheel; and a
controller electrically connected to the sensor and to the
transducer, the controller being configured to receive the output
signal from the sensor and the output signal from the transducer,
to compute a control signal based upon one of at least one
programmed function, and to output the control signal to the
motor.
22. A drive assembly for driving a drive wheel of a wheelchair,
comprising: a handrim for the application of torque by a user; a
handrim shaft carrying the handrim and extending coaxially through
a hub of the drive wheel; a motor driving a driven element secured
coaxially to the hub; a gear assembly operatively connecting the
handrim shaft to the wheel hub to permit the handrim to drive the
drive wheel; and a sensor coupled to the gear assembly for
measuring a parameter indicative of the torque applied by the user
and providing output utilized to control the motor.
23. A wheelchair comprising: first and second drive wheels each
having an axis and a coaxial handrim for receiving a driving torque
from a user; for each of the first and second drive wheels: a
motor; and a transmission assembly coupling the motor to the drive
wheel to permit the motor to drive the drive wheel and coupling the
associated handrim to the drive wheel so as to permit the handrim
to simultaneously drive the drive wheel; and a control system
programmed with at least one control map defining a desired dynamic
of the wheelchair and programmed with a desired mass parameter and
a desired drag parameter, the control system utilizing said control
map and measured values of said driving torque from said user and
an actual velocity to determine a desired velocity and control the
motor to reduce an error component between said actual velocity and
said desired velocity.
24. The wheelchair of claim 23 wherein said drag parameter
comprises a coulomb damping parameter.
25. The wheelchair of claim 23 wherein said drag parameter further
includes a linear damping parameter.
26. The wheelchair of claim 23 wherein said desired velocity is
produced in response to an iterative integration of: measured
handrim torque multiplied by a first constant; the negative of the
desired velocity multiplied by a second constant; and the negative
of the sign of the desired velocity multiplied by a third
constant.
27. A power assist vehicle comprising: a drive wheel; input means
for receiving a driving torque from a user; a motor; a transmission
assembly coupling the motor to the drive wheel to permit the motor
to drive the drive wheel and coupling the input means to the drive
wheel so as to permit the input means to simultaneously drive the
drive wheel; and a control system programmed with at least one
control map defining a desired dynamic of the vehicle and
programmed with a desired mass parameter and a desired drag
parameter, the control system utilizing said control map and
measured values of said driving torque from said user and an actual
velocity to determine a desired velocity and employs a single-axis
velocity servo loop to control the motor to reduce an error
component between said actual velocity and said desired
velocity.
28. The wheelchair of claim 27 wherein said desired velocity is
produced in response to an iterative integration of: measured
user-applied torque multiplied by a first constant; and the
negative of the sign of the desired velocity multiplied by a second
constant.
29. The wheelchair of claim 27 wherein said desired velocity is
produced in response to an iterative integration of: measured
user-applied torque multiplied by a first constant; and a damping
effect provided by a function of the desired velocity.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This patent application claims priority of U.S. Provisional
Patent Application Ser. No. 60/272,216 entitled "POWER ASSIST
VEHICLE" that was filed on Feb. 28, 2001 and is a
continuation-in-part of U.S. patent application Ser. No. 09/388,124
entitled "POWER ASSIST VEHICLE" that was filed on Aug. 31, 1999 and
also a continuation-in-part of International Application
PCT/US00/23815 entitled "POWER ASSIST VEHICLE" that was filed on
Aug. 30, 2000. The disclosures of patent application Ser. Nos.
60/272,216 and 09/388,124 and International Application
PCT/US00/23815 are incorporated by reference in their entireties
herein as if set forth at length.
BACKGROUND OF THE INVENTION
[0002] (1) Field of the Invention
[0003] This invention relates to power-assist vehicles, and more
particularly to power-assist wheelchairs.
[0004] (2) Description of the Related Art
[0005] A wide variety of wheelchair configurations exist.
Wheelchairs generally allow a seated occupant who has little or no
use of his legs to navigate from one place to another. Wheelchairs
are commonly configured to be powered in one or more ways. Many
wheelchairs have handles at their back to allow an attendant to
push the wheelchair. Many wheelchairs may be occupant-powered,
typically having a large diameter drive wheel at each side, each
drive wheel having a concentric handrim which may be gripped by the
occupant to rotate the drive wheel to drive the wheelchair.
Motor-driven wheelchairs may be used where the occupant is unable
to power the wheelchair himself. In some motor-driven wheelchairs,
two electric motors are respectively coupled to the left and right
drive wheels. The motors may be controlled by a joystick which can
drive the wheels at different speeds to provide steering. One
example of a motor-driven wheelchair with a suspension mechanism is
shown in U.S. Pat. No. 4,339,013 of Gerald I. Weigt.
[0006] The '124 application discloses a power-assist wheelchair.
Another power-assist wheelchair is disclosed in U.S. Pat. No.
5,818,189 (the '189 patent), the disclosure of which is
incorporated by reference in its entirety herein. In a power-assist
wheelchair, motor power supplements power provided by the occupant.
The '124 application and '189 patent teach a motor associated with
each drive wheel applying power to the drive wheel responsive to
the torque applied by the occupant to the associated handrim.
Electricity to power the motors may be provided one or more
rechargeable batteries.
[0007] In addition to the drive wheels, wheelchairs commonly
include at least one additional wheel. In a common wheelchair
configuration such as illustrated in the '124 patent, there are two
relatively small caster-like front wheels which freely pivot about
generally vertical caster axes to permit the wheelchair to turn.
Some racing wheelchairs, however, feature a single central front
wheel while other wheelchairs locate the additional wheel(s) behind
the drive wheels.
BRIEF SUMMARY OF THE INVENTION
[0008] In one aspect the invention is directed to a power assist
vehicle. The vehicle has a drive wheel and input means for
receiving a driving torque from a user. A transmission assembly
couples a motor to the drive wheel to permit the motor to drive the
drive wheel and couples the input means to the drive wheel so as to
permit the input means to simultaneously drive the drive wheel. A
control system is programmed with at least one control map defining
a desired dynamic of the vehicle and programmed with a desired
mass-indicative parameter and a desired drag parameter.
[0009] The control system advantageously utilizes the control map
and measured values of driving torque from the user and an actual
velocity to determine a desired velocity and employs a single-axis
velocity servo loop to control the motor to reduce an error
component between the actual velocity and the desired velocity. The
desired velocity may be produced in response to an iterative
integration of: measured user-applied torque multiplied by a first
constant; and the negative of the sign of the desired velocity
multiplied by a second constant. The desired velocity may be
produced in response to an iterative integration of: measured
user-applied torque multiplied by a first constant; and a damping
effect provided by a function of the desired velocity.
[0010] Aspects of the invention may be implemented in a wheelchair
having a pair of drive wheels and handrims.
[0011] The details of one or more embodiments of the invention are
set forth in the accompanying drawings and the description below.
Other features, objects, and advantages of the invention will be
apparent from the description and drawings, and from the
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a view of a wheelchair with a gearing system of
the present invention.
[0013] FIGS. 2 and 3 are partially exploded and partially
longitudinal sectional views of a powertrain unit of the wheelchair
of FIG. 1.
[0014] FIG. 4 is a first partially exploded view of the powertrain
unit of FIGS. 2 and 3.
[0015] FIG. 5 is a second partially exploded view of the powertrain
unit of FIGS. 2 and 3.
[0016] FIG. 6 is a longitudinal sectional view of an alternate
powertrain unit.
[0017] FIG. 7 is a control system schematic for use with the
powertrain unit of FIGS. 2 and 3.
[0018] FIG. 8 is a flow chart of an overall control method for use
with the powertrain unit of FIGS. 2 and 3.
[0019] FIG. 9 is a flow diagram of a velocity servo loop portion of
the method of FIG. 8.
[0020] FIG. 10 is a flow diagram of desired dynamics of the method
of FIG. 8.
[0021] FIG. 11 is a semi-schematic illustration of a user interface
for controlling the powertrain units of FIGS. 2 and 3.
[0022] FIG. 12 is a block diagram of velocity and current
controller.
[0023] FIG. 13 is a block diagram of a control algorithm.
[0024] Like reference numbers and designations in the various
drawings indicate like elements.
DETAILED DESCRIPTION
[0025] FIG. 1 shows an exemplary wheelchair 10 which represents one
of many wheelchair configurations to which the present invention
may be applied. The exemplary wheelchair includes a structural
frame 12, a seat 14, a seat back 16, two ground-engaging drive
wheels 18 and two ground-engaging nondriven caster wheels 20. The
wheels 18 may comprise a rim and apneumatic tire and each are
associated with a coaxial handrim 22 for receiving a driving input
from a user seated on the chair. Each drivewheel rim is mounted
onto associated drivewheel hub 26 by drivewheel spokes 24. Each
handrim is connected to an associated handrim shaft 28 by handrim
spokes 30. The spokes 24 and 30 may, without limitation, be of
metal or composite material or may be replaced by discs as
appropriate for any particular use.
[0026] The exemplary wheelchair has a pair of generally
mirror-image left and right powertrain units 100 respectively
mounting the left and right drive wheels to the wheelchair frame.
Each powertrain unit 100 (FIG. 2) includes an outboard housing
(e.g., an aluminum casting) 102 mounted to the frame and an inboard
cover (e.g., 6061 T6 aluminum machining) 104 bolted or screwed
together. These units contain the motor (e.g., a brushless
servomotor) 106 and the associated geartrain and include
appropriate bores or compartments for accommodating a variety of
bearings 108 supporting various shafts. The motor provides
electronically controlled power assistance that augments the force
applied by the user. The torque provided by the motor is
transmitted through a two-stage gear reduction, preferably having
spur gears. The combination of a high-performance, low-friction
motor and a small reduction ratio allows for minimal drivetrain
drag when the motor is unpowered, as could happen when the
batteries run low or a system failure occurs.
[0027] A motor drive pinion 110 on the motor shaft 111 drives a
larger driven gear 112 on a second shaft 113 which provides a first
stage reduction. The second shaft carries a second pinion 114
driving a larger output gear 116 (providing a second stage
reduction) connected to the wheel hub 26 by a splined or otherwise
faceted hollow shaft 118. A coaxial shaft 120 (FIG. 3) with splines
or other features for connecting to the handrim shaft 122 is
located within the shaft 118 and carries a second large gear 124
(FIG. 2) which, in the illustrated embodiment, is identical in
diameter to the gear 116. The gears 124 and 116 are respectively
engaged to identical smaller pinion gears 130 and 132 rigidly
mounted on a common shaft 134. In the illustrated embodiment this
arrangement links the handrim to the drivewheel with a 1:1 gear
ratio.
[0028] One end of the shaft 134 is rigidly held by the housing. The
other end is held by a distal end 150 of a sensor arm 152 (e.g., of
6061 T6 aluminum). The proximal end 154 is rigidly secured to the
cover (e.g., by screws). Intermediate its two ends, the sensor arm
has a narrowed neck 160 (FIG. 4). The thin neck permits the sensor
arm to flex slightly under load as if about a central transverse
axis 510 (e.g., an axis parallel to the various shaft axes). The
two opposite faces of the neck 160 bear strain gauges 164 mounted
so that the strain gauges measure such flexing. The flexing is
constrained by the close accommodation of the distal end 150 within
a compartment 170 (FIG. 5) of the cover. During assembly, the
distal end is shimmed between opposite faces 172 of the compartment
and the mounting screws tightened to secure the proximal end. The
shims are removed, establishing a neutral centered position for the
distal end.
[0029] The transmission of torque between the gears 124 and 116
(and thus the handrim and drivewheel) via the pinion gears 130 and
132 applies a torque to the shaft 134 about a longitudinal axis
perpendicular to the shaft axis. This torque is transmitted through
the bearings supporting the shaft and thus to the housing at one
end and to the distal end of the sensor arm at the other end. This
causes the flexing of the neck 160. Accordingly, the direction and
magnitude of such flexing provides a measure of the direction and
magnitude of the torque between the handrim and drivewheel and thus
the strain gauge output may be used to measure such torque.
[0030] The geartrain transmission of the illustrated powertrain
unit 100 provides a 1:1 ratio between handrim and drivewheel
rotation. This provides an intuitive, familiar, feel for many
users. In various other implementations, the powertrain units may
be provided with a ratio other than 1:1 (e.g., within arange of
0.5:1 to 2:1). The 1:1 ratio between handrim and drivewheel
rotation provides an intuitive, familiar feel for many users. A
higher reduction may be useful for low speed applications whereas a
low ratio may be useful for higher speed applications such as sport
wheelchairs. FIG. 6 shows a powertrain transmission unit 200
configured to provide a numerical advantage for the user and is
drawn from U.S. Ser. No. 09/388,124. In the transmission unit 200,
illustrated gears 216, 224, 230, and 232 are analogous to gears
116, 124, 130, and 132 of the transmission unit 100. Shafts 218,
222, and 234 are analogous to shafts 118, 122, and 134. The motor
and portion of the geartrain coupling it to the gear 216 are not
seen in this view.
[0031] Turning to FIG. 7, the handrim torque transducer 47 (formed
by the bridging of strain gauges 164) measures a parameter
indicative of the handrim torque T.sub.h and transmits this value
to a central processing unit (CPU) 56. Specifically, FIG. 7 shows
components of a control system and the data passing between these
components. The exemplary control system comprises an external
personal computer-based component 70, a battery 72, a velocity
sensor 74, the torque transducer 47, a LED circuit 76, a control
map 78 and associated circuitry, a user interface 80, the CPU 56,
and the motor 106 having an associated motor driver 82. The control
map 78 may be either constant to the system or selectable by the
user through the interface 80 with the CPU 56.
[0032] The control system operates through the CPU 56, which is
preferably implemented as a programmable microprocessor. The
circuitry for the control system is housed in a control box (not
shown) that is, preferably, either integral with the drive
unit/gear box or encased in a separate enclosure mounted on the
frame. The control system operates so that the user supplies a
tangential force and associated torque to the handrims 22 that is
measured by the associated torque transducers 47. Each torque
transducer 47 transmits this value to the CPU 56, which utilizes a
desired dynamic or control map to transform the measured torque
value into a desired drive wheel velocity. The desired dynamic may
be programmed into the CPU and may be specifically configured to
meet the needs of the individual user. A velocity servo loop is
used as an error measure to ensure proper system output based upon
the selected control map. The sensor 74 measures the actual
drivewheel velocity and compares that value against the optimum or
desired value through the velocity servo loop. The motor output is
then increased or decreased to reduce the error component to the
optimum value of zero.
[0033] To put this concept in operation, the CPU 56 accepts torque
input from the torque transducer 47, command input from the
interface 80 (when used) and velocity input from the sensor 74. In
response, the CPU 56 outputs a control signal to the motor 24 via
the motor driver 82. The CPU 56 is preferably programmable through
the use of the PC-based computer 70 having associated memory
storage. Resident on the computer is a design tool for specifying
and downloading these control maps to the CPU 56. The infra-red
(IR) link 83 facilitates data transfer between the CPU 56 and the
external computer 70.
[0034] The CPU 56 also directs information downloaded from the data
link, such as control maps, to an electrically erasable
programmable read only memory (EEPROM). And, if the data link is
appropriately configured to output information, the processor can
upload data from a DRAM, or other volatile memory, via the data
link. Software for governing the operation of the CPU 56 may also
reside here. Furthermore, the CPU 56 may, upon request by the
PC-based system 70, upload information that it has stored.
Downloading and uploading are preferably performed by an infrared
data link, although cabling, wireless data links, modems and other
data exchange means may also be used.
[0035] The various control maps may be accessed by the user through
the use of the interface 80 between the user and the CPU. The
interface 80 is provided with a switch 90 that allows the user to
select between the various control maps pre-programmed into the
CPU. The interface 80 may also have a display comprising a series
of LEDs 76 used to indicate which control map has been selected by
the user. Alternate displays (not shown), such as liquid crystal
devices, displaying this information, along with other status data
may be used in place of, or in addition to, the LEDs. The (IR) port
83 facilitates communication with the PC-based component 70 to
upload such data, and also to download control maps and other
software. As stated above, other data links may be used in place of
the IR port.
[0036] Once the user selects the desired control map, the CPU is
ready to compute the desired system output. FIG. 8 shows a control
flow utilizing a velocity servo loop (FIG. 9) and a desired
dynamics bock (FIG. 10). Computing the desired wheel velocity
.omega..sub.d (FIG. 8) is based upon the following algorithms: 1 .
d = 1 m [ N 1 T h - B 1 d - B 2 sgn ( d ) ] d = min ( d , d max ) d
= max ( d - d max )
[0037] In the above formulas, N.sub.1 is the gear ratio between the
handrim 22 to the outer drivewheel 18, m is a constant proportional
to the desired mass of the system, B.sub.1 is the desired linear
damping, B.sub.2 is the desired coulomb damping, and {dot over
(.omega.)}.sub.d represents the first derivative with respect to
time (integrated by 1/s, FIG. 10). Due to the above formulas, the
present invention is structured and tuned to mimic a
wheelchair-like system with specific inertia and prescribed drag
(combination of linear and coulomb friction) on a smooth, level
surface.
[0038] FIG. 8 also shows use of a particularly optional feedforward
signal path from the measured handrim torque. The feedforward path
applies a fixed ratio of torque to the motor, where the ratio is
determined by the gain, K.sub.F. When the system utilizes the
feedforward path, the desired wheel velocity T.sub.d is computed
based upon the same algorithm described above. K.sub.F is the
feedforward gain, B.sub.F1 is the linear friction compensation term
and B.sub.F2 is the coulomb friction compensation term. Both the
linear and coulomb friction compensation terms are used to
eliminate natural friction in the system. These components
(B.sub.F1 & B.sub.F2) add torque based upon speed, either
linearly for B.sub.F1 or based on the sign of speed for B.sub.B2.
The wheelchair may utilize the feedforward term in both servo mode
and by itself in feedforward mode. In servo mode, it helps the
control system respond more quickly to operator input. Alone, it
provides torque augmentation.
[0039] The variables in the above formula can be altered over a
wide range to tailor the control map to the specific needs of the
user. For example, by specifying low inertia, the system will
accelerate and decelerate more strongly in response to a torque
input at the handrims. The net effect is that the operator's inputs
are amplified by the reciprocal of the system mass. This is
referred as the "sensitivity" of the system.
[0040] The exemplary embodiment includes two types of damping,
linear and coulomb. These damping terms are used both to tailor the
response of the system to the needs of the user and to provide
system stability. For example, the damping terms help the operator
bring the speed to zero when desired and also keep the commanded
speed at zero despite small offsets in the torque sensors. The
linear damping term provides a resistive torque proportional to the
desired speed that is similar to moving through a viscous fluid.
The coulomb damping term provides a resistive torque of fixed
magnitude that is similar to sliding an object across a smooth
surface.
[0041] Other forms of damping may also be incorporated into the
system, such as quadratic drag where the force increases with the
square of the magnitude of the velocity. Increasing any damping
term causes the desired velocity to return to zero more quickly in
the absence of an applied torque input. At steady velocity, the
drag terms set the amount of applied torque required to maintain
that speed. If the damping terms are decreased, the chair will
maintain its speed longer without additional torque input. The
velocity limit simply prevents the desired velocity from exceeding
a preset magnitude. From the user point of view, this feels much
like heavy damping that cuts the limiting speed.
[0042] Advantageously there are regenerative capabilities. For
example, when the motor is slowing down the wheelchair (as it does
when in servo mode going downhill) power is routed back into the
battery. Similarly, the chair brakes actively on level ground when
the terms B.sub.1 and B.sub.2 (FIG. 10) reduce the desired
velocity, and the velocity controller reduces the actual speed by
applying the appropriate opposing torque. Then the regenerative
action transfers energy from the kinetic energy of the moving mass
back into the battery. Whenever the applied torque and velocity
have opposite signs, the system puts much of the resulting
dissipated power back into the battery. This capability is unique
to the low-friction environment of the present system. This
environment allows the wheelchair to coast when no current is
applied.
[0043] The desired dynamic (or control map) is created by varying
these parameters in association with the combination of the
computer control system, the sensors, and the entire
electromagnetic system (motors, gearing, etc). Due to the linearity
of the torque motor, the low friction and low backlash of the
gears, and the quality of the sensors, the computer control system
can shape the overall dynamic system response over a wide range.
Although the present invention will operate with high-friction,
high-reduction gearing, this is not desirable because these
components may constrain the ability to specify a desired system
behavior. Referring to FIG. 9, once the desired wheel velocity
.omega..sub.d has been computed based upon the selected control
map, the control circuitry computes the desired motor output
through the use of a single-axis velocity servo loop. The velocity
servo loop alters the motor torque to maintain the desired wheel
velocity .omega..sub.d despite changes in friction (external and
internal) or gravity loads imposed by sloped terrains. The measured
motor velocity V.sub.m is obtained through the use of the sensor 74
associated with the motor 24. Preferably, the sensor is an optical
encoder mounted on the corresponding motor 24.
[0044] Referring to FIG. 7, the controller uses the desired motor
output to transmit an appropriate control signal to the motor 24.
This signal contains magnitude and polarity information which are
presented to the motor driver 82 to produce an appropriate motor
output. The motor driver 82 converts this signal into a voltage of
the appropriate magnitude and polarity to be applied to the motor
24. For this, the motor driver comprises a digital-to-analog
converter (DAC), and an H-bridge circuit. The DAC converts the
control signal into an analog signal to be applied to the H-bridge
circuit, and the H-bridge circuit uses this signal, along with
polarity information, to drive the motor 24.
[0045] The gearing and control systems, described above, are
duplicated for each wheel. Due to the independence of each wheel,
the control system parameters can be varied to accommodate the
user's particular needs. For example, if the user has less strength
in one arm, the associated side of the wheelchair can be made more
sensitive by reducing the mass and drag parameters. Alternatively,
both systems can be coupled to produce a uniform response (e.g.,
with full or partial integration of the control components).
[0046] An exemplary controller has two parts implemented in a
digital signal processor (DSP) and a programmable interrupt
controller (PIC). The PIC handles the interface to the strain gauge
bridge, implements the high-level algorithm, and allows for mode
changes. The DSP closes a velocity loop with programmable
proportional and integral gains and current limit, with some
additional features.
[0047] The PIC communicates with the DSP, sending desired speed and
an additive current term. The DSP reports back the actual speed and
total current. These communications can be implemented using analog
or high speed digital communications such as serial peripheral
interface (SPI). An additional serial interface between the PIC and
DSP is used to provide configuration data to the DSP on
startup.
[0048] FIG. 12 shows velocity and current controllers. The DSP
implements sinusoidal or 6-step commutation (software selectable).
It has an inner loop consisting of a current servo, and an outer
loop that implements a velocity servo. The current loop produces a
current equal to the current demanded by the velocity servo plus
the additional current term provided by the PIC. The DSP controller
also has a programmable current limit.
[0049] The velocity servo uses a PI algorithm to compute the
current needed to obtain the commanded speed. An additional
controller then servos the pulse-width-modulated signal to ensure
that the desired current is obtained.
[0050] FIG. 13 is a block diagram of the algorithm. The high-level
algorithm resides in the PIC. It computes the desired velocity and
additive current terms that will be implemented by the DSP.
[0051] In the first stage of processing, the raw AD signal from the
strain gauge amplifier is converted into a torque value. To convert
AD counts to a floating point torque value, the zero value is
removed, a threshold applied, then appropriate gains are applied
for either a positive or negative value. A digital first-order
filter is then applied.
[0052] The feedforward torque is a scaled version of the input
torque. After the input torque is scaled, it is converted into an
integer value for the DA or SPI.
[0053] The desired velocity computation is a bit more complicated.
An acceleration parameter, Kt is applied to the input torque to
produce an acceleration term. A deceleration term is computed from
the sign of the desired velocity and a parameter that emulates
coulomb friction. This is the function Fdamp. The sum of the
acceleration term and the deceleration term is then integrated to
produce a desired velocity. The desired velocity is always kept
within a specified value of the actual velocity to prevent
"surging", which can occur when the velocity controller saturates
(velocity limiter block in FIG. 13). If the desired velocity
exceeds the actual velocity by more than a preset amount (a maximum
velocity error), the desired velocity is reset to the actual
velocity plus the amount (or minus the amount for negative desired
velocities). The desired velocity is finally converted to an
appropriate value for either DA or SPI.
[0054] One or more embodiments of the present invention have been
described. Nevertheless, it will be understood that various
modifications may be made without departing from the spirit and
scope of the invention. Accordingly, other embodiments are within
the scope of the following claims.
* * * * *