U.S. patent application number 10/051333 was filed with the patent office on 2002-07-18 for methods for stair climbing in a cluster-wheel vehicle.
Invention is credited to Kerwin, John M., Morrell, John B..
Application Number | 20020092686 10/051333 |
Document ID | / |
Family ID | 26822549 |
Filed Date | 2002-07-18 |
United States Patent
Application |
20020092686 |
Kind Code |
A1 |
Morrell, John B. ; et
al. |
July 18, 2002 |
Methods for stair climbing in a cluster-wheel vehicle
Abstract
A method for operating a device ascending or descending stairs.
The device has a plurality of wheels rotatable about axes that are
fixed with respect to a cluster arm, where the cluster arm itself
is rotated about an axis so that wheels rest on successive stairs.
The wheels and cluster arms are controlled according to separate
control laws by a controller. Whether the device ascends or
descends the stairs is governed by the pitch of the device relative
to specified front and rear angles.
Inventors: |
Morrell, John B.;
(Manchester, NH) ; Kerwin, John M.; (Manchester,
NH) |
Correspondence
Address: |
BROMBERG & SUNSTEIN LLP
125 SUMMER STREET
BOSTON
MA
02110-1618
US
|
Family ID: |
26822549 |
Appl. No.: |
10/051333 |
Filed: |
January 18, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10051333 |
Jan 18, 2002 |
|
|
|
09757230 |
Jan 9, 2001 |
|
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|
09757230 |
Jan 9, 2001 |
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09428007 |
Oct 27, 1999 |
|
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|
60124403 |
Mar 15, 1999 |
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Current U.S.
Class: |
180/8.2 ;
180/65.8 |
Current CPC
Class: |
A61G 5/04 20130101; G05B
13/042 20130101; B60K 7/00 20130101; A61G 2203/46 20130101; B62D
57/00 20130101; A61G 5/061 20130101; A61G 2203/14 20130101 |
Class at
Publication: |
180/8.2 ;
180/65.8 |
International
Class: |
B62D 051/06 |
Claims
We claim:
1. A method for regaining static stability of a transport device
for climbing or descending stairs, the transport device being
characterized by a pitch and having: i. a cluster arm; ii. a
cluster arm actuator characterized by a temperature for driving the
cluster arm in rotation; iii. a brake capable of braking the
cluster arm; and iv. a sensor that monitors the temperature and
provides a sensor signal; the method comprising the steps of: a.
monitoring the sensor signal; and b. disengaging the brake when the
sensor signal exceeds a preset threshold so as to allow the cluster
arm to rotate freely in such a manner that the device assumes a
statically stable configuration on the stairs.
2. A method for limiting a load driven by an actuator for driving a
transport device in ascent and descent on stairs, the actuator
characterized by a temperature, the method comprising: a.
monitoring the temperature of the actuator; b. detecting an
over-temperature condition; and c. limiting the cluster actuator
load by allowing cluster rotation only in the direction of descent
for the duration of the over-temperature condition.
Description
[0001] The present application is a divisional application of Ser.
No. 09/757,230, now allowed, filed Jan. 9, 2001, which is a
divisional application of Ser. No. 09/428,007, U.S. Pat. No.
6,311,794, filed Oct. 27, 1999. This application claims priority
from U.S. Provisional Application, Ser. No. 60/124,403, filed March
15, 1999. These applications are hereby incorporated herein by
reference.
TECHNICAL FIELD
[0002] The present invention pertains to methods for controlling
the configuration and motion of a personal vehicle equipped with
one or more wheels or other ground-contacting members.
BACKGROUND OF THE INVENTION
[0003] Personal vehicles (those used by handicapped persons, for
example), may benefit from actively stabilization in one or more of
the fore-aft or left-right planes. Operation of a balancing vehicle
is described in U.S. Pat. No. 5,701,965 (incorporated herein by
reference, and referred to herein as the "'965 patent"). Personal
vehicles may advantageously be stabilized whether or not the
vehicles are self-propelled, whether or not the vehicles may be
guided by the occupant of the vehicle or by an assistant, and
whether or not the vehicles may operate in a balancing mode.
Vehicles of this sort may be more efficiently and safely operated
employing control modes supplementary to those described in the
prior art. A personal vehicle may be referred to in this
description, interchangeably, as a "transporter."
SUMMARY OF THE INVENTION
[0004] In accordance with a preferred embodiment of the invention,
terms are provided in both the wheel and cluster control laws of a
vehicle so that the amount of effort required by a rider to control
the vehicle by virtue of the location of the center of gravity is
significantly reduced. Thus, most riders are able to climb or
descend stairs unaided by an assistant. The addition of terms
referenced to a front and rear angle allows the rider to lean the
vehicle between the front and rear angle with relatively low
exertions because the pitch gain can be set to a small value while
a large gain can be used with the front and rear angles to maintain
a fast rotation of the cluster during stair climbing. The front and
rear angles are updated throughout the stair climbing process
thereby allowing the rider to vary the position of the vehicle CG
(center of gravity) with very little effort.
[0005] In accordance with embodiments of the invention, a wheel
control law is provided that mirrors the cluster control law in
that front and rear angles are added to the wheel control law. The
wheel control law differs slightly from the cluster control law in
order to assure that during stair climbing the wheels can only move
in the rearward direction. In addition, a damping term is included
in the wheel control law which changes depending on the wheel
direction and has the effect of accelerating the wheel into the
riser while decelerating the wheels in the forward direction. This
change alters the behavior of the vehicle at the top or bottom step
and provides for a safer transition at the beginning and end of
stair climbing. In accordance with further embodiments of the
invention, there is provided a brake pitch control algorithm. This
is a safety feature that monitors the cluster and wheel motor
temperature during stair climbing. If the cluster or wheel motor
overheats and fails during portions of the stair climb, the vehicle
may fall. If the temperature exceeds a preset value, the brake
pitch controller turns off the motor amplifiers and controls the
motion by modulating the cluster brakes. The controller places the
vehicle in a configuration where all four wheels are on the stairs
thus placing the vehicle in a statically stable configuration with
respect to gravity and preventing the rider from continuing in
stair mode. The algorithm continues to monitor the motor
temperature and if the motors cool sufficiently, will resume stair
mode in order to allow the rider to exit from the stairs.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] The invention will be more readily understood by reference
to the following description, taken with the accompanying drawings,
in which:
[0007] FIG. 1 is a side view of a prior art personal vehicle of the
type in which an embodiment of the invention may be advantageously
employed;
[0008] FIGS. 2a-2d illustrate the phases of a stair ascend/descend
cycle;
[0009] FIG. 3 shows a schematic block diagram of the sub-modes of
the present invention;
[0010] FIG. 4 illustrates the relation between the front and rear
angles of the present invention;
[0011] FIGS. 5a-5h illustrate the front and rear angle over one
stair climb cycle;
[0012] FIG. 6 provides a plot of functions used in the
specification of front and rear angles in accordance with
embodiments of the present invention; and
[0013] FIG. 7 shows a schematic block diagram of the brake pitch
controller.
DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS
[0014] In accordance with preferred embodiments of the invention,
personal vehicles designed for enhanced maneuverability and safety
include one or more clusters of wheels, with the cluster and the
wheels in each cluster capable of being motor-driven independently
of each other.
[0015] Referring to FIG. 1, a side view is shown of a balancing
vehicle, such as otherwise described in U.S. Pat. No. 5,701,965,
and as designated generally by numeral 10. Preferred embodiments of
the present invention may advantageously be practiced in
conjunction with balancing vehicle 10. FIG. 1 does not represent a
prior art device when capable of operation in modes herein
described and claimed in the appended claims.
[0016] Vehicle 10 has a support 12 that supports the rider 14 on
the vehicle. Support 12 is attached to a ground-contacting module
16. Ground-contacting module 16 contains the power source, drive
amplifiers, drive motors, and controller that are used to drive a
cluster 36 of wheels 18. Cluster 36 includes ground-contacting
elements 18 which are shown as wheels in FIG. 1. The wheels 18 are
mounted on a cluster arm 40 and each wheel 18 is capable of being
driven by controller 8. Cluster arm 40 rotates about a cluster axis
22, the rotation of the arm governed by the controller. Controller
8 comprises a processor in communication with a memory storage
device. The processor executes a control program stored in the
memory storage device. The processor is in communication with a
plurality of sensors 38 capable of sensing the state of the vehicle
and receiving commands from the rider 14. The processor is also in
communication with the drive amplifiers and sends commands to the
drive amplifiers. The drive amplifiers provide current to the drive
motors.
[0017] As stated, controller 8 is capable of independently
controlling cluster arm 40 and the rotating wheels 18. In addition,
sensors 38 on the ground-contacting module 36 are capable of
sensing the pitch, roll, and yaw of the vehicle 10 as well as such
variables as the angular position and/or rate of rotation of wheels
18 and cluster arm 40. The signals from the sensors are
communicated to the controller and are used by the controller to
drive the wheel and the cluster motors. The independent control of
the clusters and wheels allows the vehicle to operate in several
modes thereby allowing the rider or the controller to switch
between modes in response to the local terrain.
[0018] For example, in `standard mode,` cluster 36 is rotated to
lift the front wheel 14 of the cluster off the ground and is locked
in place and the controller is placed in a simple PID loop to allow
the vehicle to operate as a powered wheelchair. In `balance mode,`
cluster 36 is rotated 90.degree., locked in place, and the
controller is allowed to balance and drive the vehicle by commands
to the wheel motors. In `enhanced mode,` described in U.S.
application Ser. No. 09/321,401, filed on May 28,1999 and herein
incorporated by reference, controller 8 allows both the wheels 18
and clusters 36 to operate simultaneously.
[0019] In stair mode, vehicle 10 climbs and descends stairs by
rotating the cluster 36 in such a manner as to place the second
wheel 42 (i.e., the wheel not currently in ground contact) of each
cluster onto the appropriate stair. During cluster rotation, the
vehicle is supported by only one wheel 18 from each cluster and is
unstable with respect to falling. In a manner unlike its operation
in either `balance` or `enhanced` mode, controller 8 does not
attempt to balance the vehicle. Instead, the rider 14 maintains
balance by holding on to stair handrail (not shown). The handrail
also provides the rider with a leverage point to lean the vehicle
in the fore or aft direction in order to direct the vehicle to
either climb or descend the stairs. The cluster control law used by
controller 8 to operate the cluster motor has the same form as the
control law for the wheel motors described in the '965 patent and
may be represented as:
T.sub.c=K.sub.1.theta.+K.sub.2{dot over
(.theta.)}.sub.c+K.sub.3{dot over
(.phi.)}.sub.c+K.sub.4.phi..sub.c+K.sub.5.intg.{dot over
(.phi.)}.sub.cdt.
[0020] In the equation above, T.sub.c represents the cluster
torque, .theta. represents the vehicle pitch error, .phi..sub.c
represents the cluster angle error with respect to vertical, and
the K.sub.1 represent the gains for each dynamic variable. Pitch is
defined with respect to an earth-fixed vertical direction, while
error designates the difference between a currently measured value
and a fiducial value that may be set by the controller, either by
presetting or in real time. The cluster angle (otherwise referred
to as the `cluster position`) with respect to vertical is a
calculated value based in part on readings of sensors mounted to
the mechanical structure (referred to as an `E-box`) which is fixed
to the vehicle and contains controller 8. Alternatively, the
cluster angle may be referred to the E-box itself, in which case
the cluster angle with respect to the E-box is designated by
.delta., and gains K.sub.3 and K.sub.4 are adjusted accordingly.
The superscript dot indicates time differentiation of the dynamic
variable. The first four terms have the same form as the control
law for the wheels although gains are different for the wheel and
cluster control laws. For example, the wheel control law sets
K.sub.3, corresponding to the wheel velocity to zero while the
cluster control law sets K.sub.3, corresponding to the cluster
rotation rate to a non-zero value.
[0021] The fifth term in the cluster control law is used to ensure
that the cluster goes to a specified angle with substantially no
steady state error. For example, in balance mode, the cluster is
rotated to 0.degree. and locked in place. As the cluster is
rotated, the angle error decreases and the rotation to the vertical
orientation may stop before the cluster reaches 0.degree.. In order
to ensure that the cluster reaches 0.degree., the fifth term in the
cluster controller law adds an additional component to the cluster
motor command so that the cluster is able to achieve the specified
orientation of 0.degree..
[0022] The control law as heretofore described will serve to
operate the vehicle for ascending and descending stairs. In order
to climb or descend stairs, rider 14 leans vehicle 10 in the
forward direction to travel down the stairs or in the rearward
direction to travel up the stairs. The rider accomplishes this by
pushing off of or by pulling on the stair handrail. In cases where
riders lack sufficient upper body strength to exert the requisite
force on the handrail to cause the clusters to rotate the wheels to
the next step, an assistant may help the rider lean the vehicle in
the forward or rearward direction. Alternatively, the pitch gain,
K.sub.1, may be decreased. Decreasing K.sub.1 has the effect of
making the vehicle easier to lean into the stair thereby allowing
the rider to lean the vehicle with a lower force on the handrail.
However, a small K.sub.1 also results in a slower rotation of the
cluster during stair climbing thereby increasing the stair climbing
time.
[0023] In accordance with preferred embodiments of the present
invention, additional terms may be added to both the wheel and
cluster control laws, as described below, providing the advantage
that the effort required of the rider to lean the vehicle is
significantly reduced. Thus, most riders are enabled to climb or
descend stairs unaided by an assistant.
[0024] FIGS. 2a-2d illustrates the phases of stair assent and
descent, collectively known as stair climbing. The present
invention uses the same control modes for both assent and descent
and does not require the rider to select the direction of stair
travel thereby providing an additional safety factor by eliminating
the possibility of an incorrect selection by the rider. For
purposes of simplicity in the figure, only two wheels 210 and 212
attached to cluster arm 220 are shown. The rider and vehicle,
denoted generally as 20, are represented by the center of gravity
(CG) 230 of the rider and vehicle together. A line 240 is defined
by the CG 230 and the cluster axis 250. The angle defined by the
line 240 and a vertical line 252 with respect to gravity passing
through the cluster axis 250 defines the vehicle pitch (frame
pitch), .theta.. The angle defined by cluster arm 220 and a
vertical line with respect to gravity passing through the cluster
axis 250 defines the cluster position (or `cluster angle`),
.phi..sub.c.
[0025] FIG. 2a shows vehicle 20 on the bottom landing 260 of a
flight of stairs 270. In typical stair mode operation, the rider
faces away from the stairs. Thus, the direction facing away from
the stairs will be referred to in the following description as the
`front` direction, while the direction facing into the stairs will
be referred to as the `rear` direction. The wheel 212 closest to
the stair riser 280 is considered the rear wheel and the wheel 210
farthest from the stair riser 280 is considered to be the front
wheel.
[0026] After the rider positions the vehicle on the bottom landing
260 as shown in FIG. 2a, the rider enters `stair mode` by so
instructing controller 8. The rider moves CG 230 to a position over
the center 290 of the rear wheel 212 as shown in FIG. 2b. The rider
may move the CG 230 by grasping and pulling on a handrail.
Alternatively, CG 230 may be moved by an assistant tilting the
rider and vehicle 20. In a preferred embodiment, the controller 8
(shown in FIG. 1) may alter the frame pitch for the rider. As the
CG 230 moves behind the rear wheel center 290, the cluster arm
rotates to lift the front wheel off of the landing 260 and onto the
next step 295 as shown in FIG. 2c.
[0027] Referring now to FIG. 2d, the configuration of vehicle 20 is
shown after the cluster motor (not shown) has rotated the front
wheel onto the next step 295. If the rider decides to continue
ascending the stairs 270, the rider moves the CG 230 beyond a
specified line indicated by numeral 234. The angle defined by line
234 and a vertical with respect to gravity passing through the
cluster axis 250 is defined as the rear angle, .theta..sub.rear.
The angle defined by line 232 and a vertical 252 with respect to
gravity passing through the cluster axis 250 is defined as the
front angle, .theta..sub.front. The front and rear angles are more
particularly described below with reference to FIG. 4 where they
are shown explicitly.
[0028] If the rider decides to descend the stairs 270, the rider
moves the CG 230 in front of the front angle, .theta..sub.front.
The controller automatically decides the direction of cluster
rotation by comparing the measured pitch, .theta., to the front and
rear angles. If .theta.>.theta..sub.front, the controller
rotates the cluster in a counter-clockwise direction (with
reference to FIG. 2d) and vehicle 20 will descend the stairs 270.
If .theta.<.theta..sub.rear, the controller rotates the cluster
in a clockwise direction (with reference to FIG. 2d) and the
vehicle ascends the stairs 270.
[0029] Using the same controller for both upward and downward stair
travel provides an important safety feature to the rider by
preventing the rider from accidentally selecting the wrong stair
travel direction.
[0030] FIG. 3 shows a schematic block diagram of the sub-modes for
the stair mode. From enhanced mode 300 (which is described in U.S.
Ser. No. 09/321,401), the rider selects stair mode 310. Each
sub-mode is characterized by a set of gain coefficients that are
combined with the system dynamic variables in a generalized control
law. The generalized control law for the wheel motor (the wheel
control law) and the cluster motor (the cluster control law) are
shown in equations (1) and (2), respectively below.
V.sub.w=k.sub.w.theta.(.theta.'.sub.front-.theta.'.sub.rear)-k.sub.w.theta-
.r.vertline..theta..vertline.+k.sub.wd{dot over
(.phi.)}'.sub.w+k.sub.wp.p- hi.'.sub.w (1)
V.sub.c=k.sub..theta.(.theta.'.sub.front+.theta.'.sub.rear)+k.sub..theta.r-
{dot over (.theta.)}+k.sub.d.delta.'+k.sub.i.intg..delta.'dt
(2)
[0031] In the equations above, V.sub.w and V.sub.c are the
commanded voltages for the wheel and cluster motors, respectively.
The subscript w refers to the wheel variables, the subscript c
refers to the cluster variables, the subscript r indicates the time
derivative of the variable, as does a superscripted dot above the
variable. The prime notation indicates an error variable, namely,
the difference between the actual value of the variable and desired
value of the variable. For example, the wheel position error,
.phi.'.sub.w, is defined by the equation, .phi.'.sub.w=.phi..sub.w
desired-.phi..sub.w where .phi..sub.w represents the wheel position
as determined by on-board sensors and .phi..sub.w desired
represents the desired wheel position as calculated by the
controller. The k's represent the gains for each variable. The
pitch variable is represented by .theta., the wheel position by
.phi..sub.w, and the cluster position by .delta.. Each sub-mode is
characterized by the set of gains indicated in equations (1) and
(2). The selection of the specific value for each gain will depend
on factors such as the sub-mode objectives and rider comfort and
can be determined by one of ordinary skill in the control arts
without undue experimentation given the description of the
sub-modes below.
[0032] Referring further to FIG. 3, stair mode 310 is entered from
enhanced mode 300 by rider selection. The control laws for stair
mode are very different than the control laws for `enhanced mode`
and does not provide the same disturbance rejection. In order to
prevent such a situation, `stair mode` 310 is entered through the
sub-mode referred to as `Position Servo Cluster` or `PSC` 320.
Sub-mode PSC 320 provides a stable platform when the vehicle is on
level ground such as a stair landing. Stability on a stair landing
is provided in the cluster control law by setting the pitch gains,
k.sub..theta. and k.sub..theta.r, to small values relative to the
position gains, k.sub.d, k.sub.p, and k.sub.1. The large position
gains result in a stiff position control of the cluster position
relative to the CG. A similar strategy is also implemented in the
wheel control law by setting the wheel position gain, k.sub.wp, to
a large value relative to the other wheel gains. This has the
effect of keeping the vehicle in place as the rider prepares to
start stair climbing. The stiff position control of the cluster
enables the rider to shift the CG over the rear wheel as shown in
FIG. 2(b) by changing the desired cluster position,
.delta..sub.desired.
[0033] The desired cluster position is modified by the controller
which processes the rider commands input via an input device such
as a button or joystick 44 (shown in FIG. 1). As
.delta..sub.desired is modified by the controller, .delta.' becomes
non-zero and is multiplied by k.sub.p producing a non-zero voltage
command to the cluster motor resulting in the rotation of the
cluster relative to the CG and producing a configuration shown in
FIG. 2(b). An equivalent view of FIG. 2(b) is that vehicle pitch,
.function., has been decreased from 0.degree. to q. In this
description, a convention is employed wherein a positive pitch
angle corresponds to the vehicle leaning in the forward direction.
The controller will detect the non-zero pitch state of the vehicle
and automatically transition to the sub-mode `OnLanding` 330. The
transition between sub-modes is accomplished by replacing the gains
associated with the exiting sub-mode with the gains associated with
the entering sub-mode into the cluster and wheel control laws, as
apparent to one of ordinary skill in the control arts. In a
preferred embodiment of the present invention, the transition is
accomplished by control scheduling as described in U.S. application
Ser. No. 09/322,431 which is herein incorporated by reference.
[0034] In the OnLanding sub-mode 330, the controller allows the
rider to control the cluster motor by varying the vehicle pitch,
.theta.. Additionally, the OnLanding sub-mode 330 allows the
vehicle to roll freely in either the forward or rearward direction.
OnLanding sub-mode 330 permits rider pitch control by setting the
pitch gain, k.sub..theta., and the rate gains, k.sub..theta.r and
k.sub.d, to large values relative to the other cluster gains in the
cluster control law. The large k.sub..theta. has the effect of
making the cluster motor sensitive to small changes in the pitch of
the vehicle. The rate gains act to provide a damping effect on the
cluster motor and tends to smooth the motion of the vehicle while
stair climbing.
[0035] The pitch error state variable associated with
k.sub..theta., (.theta.'.sub.front+.theta.'.sub.rear), represents
the angle the CG is in front of the front angle, .theta..sub.front,
or the angle the CG is behind the rear angle, .theta..sub.rear. The
pitch error state variable is a slightly more complicated function
than the error variables defined in equations (1) and (2) and is
now described in conjunction with the description of the front and
rear angle.
[0036] The error state variables .theta.'.sub.front and
.theta.'.sub.rear are two sided functions show below. 1 front ' = {
front - for front 0 otherwise ( 3 ) rear ' = { 0 for rear rear -
otherwise . ( 4 )
[0037] The front and rear angles can more easily be understood with
reference to FIG. 4 which shows the cluster and wheel configuration
and identifies the angles and lengths used in the front and rear
angle calculations. FIG. 4 shows the configuration of wheels 410
and 411 of the vehicle as they would be on the staircase with the
front wheel 410 on the lower step 450 of the staircase and the rear
wheel 411 on the upper step 452 of the staircase. The front and
rear wheel centers, 412 and 413, respectively, are rotatably
mounted to the cluster arm 420 which rotates, in turn, about the
cluster axis 421. A reference vertical 430 passes through the
cluster axis 421. The position of the CG 440 of the occupied
vehicle is described by the length of a line, L.sub.1, 422 from the
CG 440 to the cluster axis 421 and by the pitch angle, 0, which is
the angle defined by the reference vertical 430 and L.sub.1 422.
The cluster position, .theta., is angle defined by the reference
vertical 430 and the cluster arm 420. The sign convention for
angles with respect to the reference vertical is positive for
counter-clockwise rotation. A specified front vertical 441
intersects a line defined by the cluster arm 420 and defines
X.sub.front as the horizontal distance between the cluster axis 421
and the intersection point of the front vertical 441 with the
cluster arm 420. Similarly, X.sub.rear is the horizontal distance
between the cluster axis 421 and the intersection of a specified
rear vertical 442 and the cluster arm 420. The front angle,
.theta..sub.front, is defined as the pitch angle when the CG 440 is
on the front vertical 441. The rear angle, .theta..sub.rear, is
defined as the pitch angle when the CG 440 is on the rear vertical
442.
[0038] The OnLanding sub-mode 330 also allows the vehicle to roll
freely in either direction. This is implemented by setting the
wheel position errors to zero. This allows the rider to position
the vehicle against the bottom riser before ascending the stairs or
to drive the vehicle off the top landing to begin stair
descent.
[0039] The rider can vary the vehicle pitch, .theta., by pushing or
pulling on a nearby object such as a stair railing. As the rider
varies .theta., the relatively large value of k.sub..theta.
produces a non-zero command to the cluster amplifier to drive the
cluster, and the cluster begins to rotate as shown in FIG. 2(c). As
the cluster rotates, the cluster position, .delta., also changes.
The controller monitors the cluster position and, when the
controller determines that the cluster is no longer in a horizontal
orientation, the controller automatically transitions to the
sub-mode `Climbing` 340 (shown in FIG. 3).
[0040] Sub-mode Climbing 340 allows the rider to control the
cluster motor by varying the vehicle pitch, .theta.. The cluster
control law used in Climbing 340 is similar or identical to the
cluster control law used in OnLanding 330.
[0041] While the Climbing 340 sub-mode allows the vehicle to roll
in the rearward direction, it does not allow rolling in the forward
direction. The wheels are prohibited from rolling in the forward
direction as a safety measure to prevent the vehicle from
accidentally rolling off a step. This function of the Climbing 340
sub-mode is implemented by setting the wheel position gain,
k.sub.wp, to a large value relative to the values of the other
wheel gains. The controller also resets the desired wheel position,
.phi..sub.w desired to the sensed wheel position, .phi..sub.w,
whenever .phi..sub.w desired>.phi..sub.w. This allows the wheel
to freely move in the rearward direction but creates a restoring
force if the wheel travels in front of .phi..sub.w desired.
[0042] The forward motion of the wheel during Climbing 340 is
further damped by setting the wheel velocity gain, k.sub.wd, to a
large positive value when wheel velocity, {dot over (.phi.)}.sub.w
is positive (resulting in a negative {dot over (.phi.)}.sub.w. This
combination of a large positive gain multiplied by a negative wheel
velocity error produces a large damping effect in the wheel control
law that results in the deceleration of the wheel in the forward
direction. A negative wheel velocity corresponds to motion that
would drive the wheel into the stair riser 280. In such a situation
k.sub.wd is set to a large negative value. This causes the wheel
control law to accelerate the wheel into the stair riser 280. The
controller monitors the state of .phi..sub.w and loads the
appropriate k.sub.wd into the control law when .phi..sub.w=0
resulting in no discontinuity in the motion of the vehicle when the
new gains are loaded into the wheel control law. The rearward
acceleration of the wheel into the stair riser has an additional
benefit to the rider during stair ascent when the rear wheel is on
the top landing. Without rearward acceleration, the rider may have
difficulty rolling the wheels backward onto the landing. With
rearward acceleration, the controller accelerates the rear wheel
backward to find the next stair riser. Since the rear wheel is on
the top landing which has no stair riser, the controller continues
to accelerate the rear wheel backward. The controller is
simultaneously commanding the cluster motor to place the CG in a
stable configuration directly over the cluster axis 421. The
simultaneous and independent operation of wheel and cluster control
laws appears to the rider as the vehicle moving rearward while the
cluster rotates just enough to bring the front wheel level with the
top landing.
[0043] In order to prevent the vehicle from accelerating backward
into a object on the top landing, the controller monitors the wheel
position, .phi..sub.w, When the rear wheel has moved no more than
one wheelbase length (the distance between centers 412 and 413)
rearward on the landing, the controller can safely assume that the
front wheel is over the top landing and automatically transitions
to sub-mode OnLanding 330.
[0044] The wheel control law for the Climbing 340 sub-mode also
uses the pitch state information to accomplish the second objective
of the sub-mode by using relatively large values for the wheel
pitch gains, k.sub.w.theta. and k.sub.w.theta.r. The signs of the
pitch terms in equation (1) are changed to ensure that the wheel
rotation is always in the rearward direction. In the case of the
pitch rate term, the absolute value of the pitch rate is taken and
the sign is negative to ensure that regardless of the pitch
rotation direction, the command to the wheel will always be in a
rearward direction. Similarly, the rear pitch error is subtracted
from the front pitch error.
[0045] As the cluster rotates the front wheel to the next step
during ascent or rotates the rear wheel to the next step during
descent, the front and rear angles are recalculated by the
controller. As the rotating wheel (the front wheel during ascent or
the rear wheel during descent) approaches the next step, the CG
will move in front of the rear angle during ascent or behind the
front angle during descent and the controller will automatically
transition to sub-mode Transfer 350 shown in FIG. 3.
[0046] The Transfer 350 sub-mode, allows the rider to pause on each
step and transfer the CG to the rear of .theta..sub.rear if the
rider desires to continue ascending or in front of
.theta..sub.front if the rider desires to descend the stairs.
Transfer 350 sub-mode allows the rider to shift the CG by pushing
or pulling on the stair railing. Additionally, Transfer 350
sub-mode allows the vehicle to roll in the rearward direction but
not in the forward direction, as implemented by the same wheel
control law as used in the Climbing 340 sub-mode.
[0047] In order to allow the rider to pause on each step and
transfer the CG to the rear of .theta..sub.rear if the rider
desires to continue ascending or in front of .theta..sub.front if
the rider desires to descend the stairs, the cluster is driven
according to the control law
V.sub.c=k.theta.'tm (5)
[0048] whenever .theta..sub.front>.theta.>.theta..sub.rear.
The gain, k, for this sub-mode is set sufficiently low to allow the
rider to shift the vehicle CG between the front and rear angles
using moderate force on the stair handrails. The low value for k
appears to the rider as a soft region where the rider can easily
shift the CG between the front and rear angles.
[0049] While in the Transfer 350 sub-mode, the controller checks
the cluster position. If the vehicle is on the stair case, the
cluster position will not be horizontal because the front wheel
will be on a different step from the rear wheel. If the controller
detects that the cluster position is in a horizontal orientation,
the controller assumes that the vehicle is on either the top or
bottom landing and automatically transitions to the OnLanding 330
sub-mode.
[0050] In order to make the vehicle responsive to rider leaning,
the front and rear angles are recalculated by the controller during
stair climbing. This is more easily understood with reference to
FIG. 5. In FIG. 5, wheels 501 and 502 of the vehicle are shown
during the ascent of a single stair. FIG. 5a shows a representation
of the vehicle with the front wheel 501 on the lower step and the
rear wheel 502 on the upper step. The wheels are attached to the
cluster arm 503 which rotates about cluster axis 504. The front
angle, .theta..sub.front is defined by a first line 511 and a
vertical reference, not shown, passing through the cluster axis
504. The rear angle, .theta..sub.rear, is defined by a second line
512 and a vertical reference. In FIG. 5a, the vehicle is in the
Transfer 350 sub-mode with the CG 520 of the occupied vehicle
between .theta..sub.front and .theta..sub.rear and the cluster arm
503 in a non-horizontal position. While in Transfer 350 sub-mode,
the rider can easily shift the CG 520 of the vehicle to the
configuration shown in FIG. 5b where the pitch, .theta., is equal
to .theta..sub.rear. The rider can easily shift the CG 520 because
the cluster control law for the Transfer 350 sub-mode shown in
equation (5) above has a relatively small value for the pitch error
gain, k.
[0051] In FIG. 5(b), .theta.=.theta..sub.rear and the controller
automatically transitions to the Climbing 340 sub-mode. As the
rider shifts the CG 520 behind .theta..sub.rear as shown in FIG.
5c, the controller rotates the cluster arm 503 and lifts the front
wheel 501 off the lower step. As shown in FIGS. 5(a)-(h), the
.theta..sub.front and .theta..sub.rear change as the cluster arm
503 rotates. In particular, during the rotation phase where the
vehicle is supported only by the rear wheels as shown in FIGS. 5(c)
through 5(g), the difference between .theta..sub.front and
.theta..sub.rear is small to minimize the time spent in Transfer
350 sub-mode should the rider decide to reverse the direction of
stair climbing during cluster arm 503 rotation. FIG. 5(f) shows
that cluster arm 503 has rotated to a vertical position placing the
front wheel 501 over the rear wheel 502. Since the CG 520 is still
behind .theta..sub.rear, the controller continues to rotate the
cluster arm 503. As the cluster arm 503 rotates, the controller
calculates new values of the front and rear angles and a
configuration shown in FIG. 5(g) is reached where the constantly
updated front and rear angles expand to a point where
.theta.=.theta..sub.rear. When the pitch equals the rear angle, the
controller automatically transitions to the Transfer 350
sub-mode.
[0052] FIG. 5(g) shows the configuration of the vehicle at the end
of one stair climb cycle where the front wheel has been rotated to
the next step to become the rear wheel for the next cycle.
[0053] The selection of the front and rear wheel functions used to
calculate the front and rear wheel angles are chosen to optimize
rider comfort and safety during stair climbing. In a preferred
embodiment, the front and rear angles are customized to the rider
and depend on the cluster position, .phi..sub.c. Using a small
angle approximation, the front and rear angles may be calculated by
the equations below.
.theta..sub.front=(1/L.sub.1)X.sub.front(.phi..sub.c)+.theta..sub.ref
(6)
.theta..sub.rear=(1/L.sub.1)X.sub.rear(.phi..sub.c)+.theta.ref
(7)
[0054] In the equations above, the quantities (1/L.sub.1) and
.theta..sub.ref are constants determined during rider customization
as described in U.S. pending application Ser. No. 09/321,401. The
functions X.sub.front and X.sub.rear both are functions of
.phi..sub.c and may be determined heuristically. In particular, the
functional relationships between X.sub.front and X.sub.rear, each,
respectively, as functions of .phi..sub.c, are plotted in FIG. 6 in
accordance with an embodiment of the invention, each normalized in
terms of L.sub.2, the distance between the wheel axes of each
cluster.
[0055] During stair climbing, the vehicle is supported by only the
rear set of wheels during cluster arm 503 rotation as shown in
FIGS. 5(c) through (g). Stability is maintained by the rider
holding onto a stair handrail. A safety system for automatically
placing the vehicle in a statically stable configuration, as shown
in FIG. 5(a), in the event of a system fault is now described. The
brake pitch controller may be more easily understood with reference
to FIG. 7 showing a block schematic of the brake pitch controller.
During stair climbing, the controller monitors the cluster motor
temperature and battery temperature. If the controller detects
overheating in the cluster motor or battery, the controller
automatically invokes a brake pitch controller, entered in the step
shown as 610 in FIG. 7. The brake pitch controller brings the
vehicle into a statically stable configuration. In step 620, the
controller turns off the cluster motor amplifier that drives the
cluster motor. The amplifier is turned off to allow the cluster
motor to cool down. The controller then checks for a falling
condition in 630. If a falling condition is not declared, the
controller disengages the cluster brakes in 640 and rechecks for a
falling condition. Disengaging the cluster brakes allows the
cluster arm 503 (shown in FIG. 5) to rotate freely and to place
front wheel 501 onto a step. If a falling condition is declared,
the controller engages the cluster brakes in 650. After engaging
the brakes in 650, the controller checks if the over-temperature
condition still exists in 660. If the over-temperature condition
has passed, the controller exits the brake pitch controller in 670.
If the over-temperature condition still exists, the controller
rechecks for a falling condition in 630. A falling condition may be
declared using state-based criteria alone or in combination with
each other. In one embodiment, the controller compares the absolute
value of the cluster rotation rate, {dot over (.phi.)}.sub.c, to a
preset maximum cluster rotation rate, {dot over (.phi.)}.sub.c max.
If the absolute value of {dot over (.phi.)}.sub.c is greater than
{dot over (.phi.)}.sub.c max, the controller assumes the vehicle is
falling and declares a falling condition. In another embodiment,
the pitch rate .theta. is compared to a preset falling pitch rate,
{dot over (.phi.)}.sub.fall. If .theta. is less than
.theta..sub.fall, the controller declares a falling condition
corresponding to a situation where the vehicle is falling backwards
onto the stairs.
[0056] In another embodiment, the controller declares a falling
condition when .theta.'.sub.rear is less than zero corresponding to
the situation where the CG of the occupied vehicle is behind the
rear angle, .theta..sub.rear. The choice of values of {dot over
(.phi.)}.sub.c max and .theta..sub.fall is within ordinary skill in
the control arts.
[0057] In an alternate embodiment of the brake pitch controller,
the cluster motor amplifier may continue to drive the cluster motor
but the load placed on the cluster motor is limited. The cluster
motor experiences the largest load when rotating the cluster to
ascend the stairs. During stair descent, the motor experiences a
much smaller load because the direction of travel of the vehicle is
in the same direction as the force of gravity. Therefore, the load
on the cluster motor can be limited by prohibiting stair ascent
when a motor or battery over-temperature is detected. In an
embodiment of the brake pitch controller, the cluster brake is
engaged if .theta.<0.degree. when a motor or battery
over-temperature is detected. In a further embodiment of the brake
pitch controller, if a motor or battery over-temperature is
detected and .theta.<0.degree., the cluster position and rate
error variables, .delta.' and .delta.' are reset to zero,
.delta..sub.desired is set to zero, and .delta..sub.desired is set
to .delta..sub.0, where .delta..sub.0 is the value of .delta. at
the time that the over-temperature and the .theta.<0.degree.
condition occurred. The effect of the reset is effectively to lock
the cluster if the rider tries to continue ascending stairs when an
over-temperature is detected while still allowing the rider to
descend the stairs. Additionally, the reset effectively locks the
cluster without the delay incurred when physically locking the
cluster brake.
[0058] Having thus described various illustrative embodiments of
the present invention, some of its advantages and optional
features, it will be apparent that such embodiments are presented
by way of example only and are not by way of limitation. Those
skilled in the art could readily devise alternations and
improvements on these embodiments, as well as additional
embodiments, without departing from the spirit and scope of the
invention. For example, though the wheels are connected to a single
linear cluster arm, the cluster arm may be angled to accommodate
more than two wheels in a cluster. In addition, the cluster arm may
be separated into two or more cluster arms, each with only a single
wheel wherein the cluster axes may or may not be co-incident.
Accordingly, the invention is limited only as defined in the
following claims and equivalents thereto.
* * * * *