U.S. patent application number 12/745424 was filed with the patent office on 2011-02-10 for three wheel vehicle electronic stability system and control strategy therefor.
This patent application is currently assigned to BOMBARDIER RECREATIONAL PRODUCTS INC.. Invention is credited to Mario Dagenais, Daniel Mercier.
Application Number | 20110035111 12/745424 |
Document ID | / |
Family ID | 40377553 |
Filed Date | 2011-02-10 |
United States Patent
Application |
20110035111 |
Kind Code |
A1 |
Dagenais; Mario ; et
al. |
February 10, 2011 |
THREE WHEEL VEHICLE ELECTRONIC STABILITY SYSTEM AND CONTROL
STRATEGY THEREFOR
Abstract
A method for enhancing stability of a three wheel vehicle having
a pair of front wheels and a single rear wheel, each of the wheels
having a tire with a tire grip threshold. The method including
deploying an electronic stability system (ESS) on the vehicle,
providing the ESS with input from various vehicle sensors related
to the longitudinal and lateral acceleration of the vehicle,
causing the ESS to determine whether (i) a precursory condition
indicative of a wheel lift exists and (ii) the tire grip threshold
of any of the tires has been exceeded; and when a precursory
condition indicative of a wheel lift exists and the tire grip
threshold of none of the tires has been exceeded, causing the ESS
to reduce the longitudinal acceleration of the vehicle by a first
amount less than that which would cause the tire grip threshold of
any of the tires to be exceeded.
Inventors: |
Dagenais; Mario; (Lac Brome,
CA) ; Mercier; Daniel; (Magog, CA) |
Correspondence
Address: |
OSLER, HOSKIN & HARCOURT LLP (BRP)
2100 - 1000 DE LA GAUCHETIERE ST. WEST
MONTREAL
QC
H3B4W5
CA
|
Assignee: |
BOMBARDIER RECREATIONAL PRODUCTS
INC.
Valcourt
QC
|
Family ID: |
40377553 |
Appl. No.: |
12/745424 |
Filed: |
December 1, 2008 |
PCT Filed: |
December 1, 2008 |
PCT NO: |
PCT/US08/85206 |
371 Date: |
May 28, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60991641 |
Nov 30, 2007 |
|
|
|
Current U.S.
Class: |
701/42 ;
701/36 |
Current CPC
Class: |
B62K 5/027 20130101;
B60T 8/1706 20130101; B60T 8/1755 20130101; B60G 2400/1042
20130101; B60G 2800/94 20130101; B62K 5/05 20130101; B60T 2240/06
20130101; B60G 17/0157 20130101; B62K 5/08 20130101 |
Class at
Publication: |
701/42 ;
701/36 |
International
Class: |
G06F 19/00 20060101
G06F019/00; B62D 6/00 20060101 B62D006/00 |
Claims
1. A three wheel vehicle having: a frame, a pair of front wheels,
the front wheels being connected to the frame via a front
suspension, each of the front wheels having a tire, the tire having
a tire grip threshold, a single rear wheel, the rear wheel being
connected to the frame via a rear suspension, the rear wheel having
a tire, each of the tires having a tire grip threshold, an engine
supported by the frame and operatively connected to at least one of
the wheels to provide power to propel the vehicle, a braking system
including brakes associated with each of the wheels to brake the
vehicle, a steering system including a handlebar operatively
connected to the front wheels to steer the vehicle, a straddle seat
disposed on the frame, the seat being suitable for accommodating at
least a driver of the vehicle sitting in straddle fashion, the tire
grip thresholds of tires being, for a set of combinations of
lateral and longitudinal accelerations that the vehicle may
undergo, greater than a wheel lift threshold of a vehicle, such
that the vehicle experiences wheel lift before the tires lose grip,
a plurality of sensors arranged on the vehicle so as to provide
electronic signals related to vehicle information including at
least engine speed, engine throttle position, lateral acceleration,
and longitudinal acceleration, and an electronic stability system
(ES S) including a processor and memory, the ESS being
electronically connected to at least the engine, the sensors, and
the braking system, the memory including instructions that when
executed by the processor: cause a determination, using information
from the sensors including information related to the longitudinal
acceleration of the vehicle, information related to the lateral
acceleration of the vehicle, and data from the memory, of whether
(i) a precursory condition indicative of a wheel lift before the
tires lose grip exists and (ii) the tire grip threshold of any of
the tires has been exceeded; and cause a reduction of the
longitudinal acceleration of the vehicle by a first amount less
than that which would cause the tire grip threshold of any of the
tires to be exceeded when a precursory condition indicative of a
wheel lift before the tires lose grip exists and the tire grip
threshold of none of the tires has been exceeded, thereby
increasing an amount of lateral acceleration that the vehicle can
undergo before experiencing wheel lift other than by reducing the
lateral acceleration of the vehicle.
2. A three wheel vehicle as recited in claim 1, wherein the memory
further includes instructions that when executed by the processor
will cause a reduction of the longitudinal acceleration by a second
amount that would exceed the tire grip threshold of at least one of
the tires, after having caused the reduction of the longitudinal
acceleration of the vehicle by the first amount less than that
which would exceed the tire grip threshold of any of the tires.
3. A three wheel vehicle as recited in claim 1, wherein causing the
reduction of the longitudinal acceleration of the vehicle is
carried out solely by causing reduction of engine torque.
4. A three wheeled vehicle as recited in claim 1, wherein causing
the reduction of the longitudinal acceleration of the vehicle is
carried out solely by causing actuation of the braking system of
the vehicle.
5. A three wheeled vehicle as recited in claim 1, wherein causing
the reduction of the longitudinal acceleration of the vehicle is
carried out by causing a reduction of engine torque and by causing
actuation of the braking system of the vehicle.
6. A three wheel vehicle as recited in claim 5, wherein the braking
system is actuated so as not to generate a specific yaw moment on
the vehicle.
7. A three wheel vehicle as recited in claim 1, further comprising,
when a precursory condition indicative of a wheel lift exists and
the tire grip threshold of none of the tires has been exceeded,
taking no action to directly reduce the lateral acceleration of the
vehicle.
8. A three wheel vehicle as recited in claim 1, wherein the memory
further includes instructions that when executed by the processor
will cause a reduction of the lateral acceleration of the vehicle,
after having caused the reduction of the longitudinal acceleration
of the vehicle by the first amount less than that which would
exceed the tire grip threshold of any of the tires.
9. A three wheeled vehicle as recited in claim 8, wherein the
plurality of sensors are arranged on the vehicle so as to provide
electronic signals related to further vehicle information including
a steering angle of the steering system, and wherein the ESS is
further electronically connected to the steering system, and
wherein causing the reduction of the lateral acceleration of the
vehicle is carried out by causing actuation of the steering
system.
10. A three wheel vehicle as recited in claim 1, wherein the memory
further includes instructions that when executed by the processor
cause a reduction of at least one of the lateral acceleration and
the longitudinal acceleration of the vehicle when a precursory
condition indicative of exceeding the tire grip threshold of at
least one of the tires exists but a precursory condition indicative
of a wheel lift does not exist.
11. A method for enhancing the stability of a three wheel vehicle,
the vehicle having: a frame, a pair of front wheels, the front
wheels being connected to the frame via a front suspension, each of
the front wheels having a tire, the tire having a tire grip
threshold, a single rear wheel, the rear wheel being connected to
the frame via a rear suspension, the rear wheel having a tire, the
tires having a tire grip threshold, an engine supported by the
frame and operatively connected to at least one of the wheels to
provide power to propel the vehicle, a braking system including
brakes associated with each of the wheels to brake the vehicle, a
steering system including a handlebar operatively connected to the
front wheels to steer the vehicle, a straddle seat disposed on the
frame, the seat being suitable for accommodating at least a driver
of the vehicle sitting in straddle fashion, a plurality of sensors
arranged on the vehicle so as to provide electronic signals related
to vehicle information including at least engine speed, throttle
position, lateral acceleration, and longitudinal acceleration, and
an electronic stability system (ESS) including a processor and
memory, the ESS being electronically connected to at least the
engine, the sensors, the braking system, the tire grip thresholds
of tires being, for a set of combinations of lateral and
longitudinal accelerations that the vehicle may undergo, greater
than a wheel lift threshold of a vehicle, such that the vehicle
experiences wheel lift before the tires lose grip, the method
comprising: providing the ESS with information from the sensors
related to at least the longitudinal acceleration of the vehicle
and the lateral acceleration of the vehicle; causing the ESS to
determine, using information from the sensors and data from the
memory, whether (i) a precursory condition indicative of a wheel
lift exists and (ii) the tire grip threshold of any of the tires
has been exceeded; and when a precursory condition indicative of a
wheel lift exists and the tire grip threshold of none of the tires
has been exceeded, causing the ESS to reduce the longitudinal
acceleration of the vehicle by a first amount less than that which
would cause the tire grip threshold of any of the tires to be
exceeded.
12. A method for enhancing the stability of a three wheel vehicle
as recited in claim 11, further comprising, after causing the ESS
to reduce the longitudinal acceleration of the vehicle by the first
amount less than that which would exceed the tire grip threshold of
any of the tires, causing the ESS to reduce the longitudinal
acceleration by a second amount that would exceed the tire grip
threshold of at least one of the tires.
13. A method for enhancing the stability of a three wheel vehicle
as recited in claim 11, wherein causing the ESS to reduce the
longitudinal acceleration of the vehicle is carried out solely by
causing the ESS to reduce the torque of the engine.
14. A method for enhancing the stability of a three wheeled vehicle
as recited in claim 11, wherein causing the ESS to reduce the
longitudinal acceleration of the vehicle is carried out solely by
causing the ESS to actuate the braking system of the vehicle.
15. A method for enhancing the stability of a three wheeled vehicle
as recited in claim 11, wherein causing the ESS to reduce the
longitudinal acceleration of the vehicle is carried out by causing
the ESS to reduce the torque of the engine and by causing the ESS
to actuate the braking system of the vehicle.
16. A method for enhancing the stability of a three wheel vehicle
as recited in claim 15, wherein the ESS actuates the braking system
so as not to generate a specific yaw moment on the vehicle.
17. A method for enhancing the stability of a three wheel vehicle
as recited in claim 11, further comprising, when a precursory
condition indicative of a wheel lift exists and the tire grip
threshold of none of the tires has been exceeded, having the ESS
take no action to directly reduce the lateral acceleration of the
vehicle.
18. A method for enhancing the stability of a three wheel vehicle
as recited in claim 11, further comprising, after causing the ESS
to reduce the longitudinal acceleration of the vehicle by the first
amount less than that which would exceed the tire grip threshold of
any of the tires, causing the ESS to reduce the lateral
acceleration of the vehicle.
19. A method for enhancing the stability of a three wheeled vehicle
as recited in claim 18, wherein the plurality of sensors are
arranged on the vehicle so as to provide electronic signals related
to further vehicle information including a steering angle of the
steering system, and wherein the ESS is further electronically
connected to the steering system, and wherein causing the ESS to
reduce the lateral acceleration of the vehicle is carried out by
causing the ESS to actuate the steering system.
20. A method for enhancing the stability of a three wheel vehicle
as recited in claim 11, further comprising, when a precursory
condition indicative of exceeding the tire grip threshold of at
least one of the tires exists but a precursory condition indicative
of a wheel lift does not exist, causing the ESS to reduce the
lateral acceleration of the vehicle.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application claims the benefit of priority to
U.S. Provisional Patent Application Ser. No. 60/991,641 (filed 30
Nov. 2007), which is incorporated by reference herein.
INCORPORATION BY REFERENCE
[0002] The entirety of each of the following documents is
incorporated in herein by reference: U.S. Provisional Patent
Applications Ser. Nos. 60/547,092 (filed 25 Feb. 2004), 60/547,089
(filed 25 Feb. 2004), 60/496,905 (filed 22 Aug. 2004); U.S. Patent
Application Ser. No. 10/920,226 (filed 18 Aug. 2004, published as
US 2006/0180372); International Application Nos. PCT/US2006/017477
(filed 5 May 2006, published as WO 2007/130043) and
PCT/US2006/016352 (filed 1 May 2006, published as WO 2007/130015);
and U.S. Pat. Nos. 6,263,261 (issued 17 Jul. 2001); 6,324,446
(issued 27 Nov. 2001); 6,086,168 (issued 11 Jul. 2000); 6,409,286
(issued 25 Jun. 2002); 6,338,012 (issued 1 Jan. 2002); 6,643,573
(issued 4 Nov. 2003); and 6,745,112 (issued 1 Jun. 2004).
FIELD OF THE INVENTION
[0003] The present invention relates to vehicle electronic
stability systems for vehicles, particularly such systems for three
wheel vehicles having two wheels in the front and one wheel in the
rear.
BACKGROUND OF THE INVENTION
[0004] Recently, there has come to be known a new class of road
vehicle, namely, the three wheeled road vehicle having two wheels
in the front and one wheel in the rear. Because of its novelty,
there is as of yet no generic name for this class of vehicle. One
example of a vehicle of this type may be found in U.S. Pat. No.
6,948,581 assigned to Bombardier Recreational Products Inc. (BRP
Inc.), the assignee of the present application. A commercial
example of such a vehicle is the CAN-AM.TM. SPYDER.TM. vehicle sold
by BRP Inc., details of which may be found at the internet web
address: spyder.brp.com/en-US/.
[0005] As would be recognized by one skilled in the art, and as has
been described in some of the patent documents incorporated by
reference into this application, the stability of these three wheel
road vehicles is inherently less than that of four wheel
automobiles. Although the stability of such three wheel vehicles is
both safe and adequate for the vehicles' intended purpose, i.e.
road use, it is nonetheless desirable for manufacturers of such
vehicles to further control their stability as much as possible.
This is true particularly in view of the fact that these vehicles
are new on the market and operating them is somewhat different than
operating an automobile or a motorcycle, vehicles with which riders
will be more familiar.
[0006] One means for increasing a vehicle's stability is through
the use of an Electronic Stability System (ESS). In basic terms an
ESS uses an on-board computer processor and associated memory that
have programming to manage various vehicle systems (e.g. engine,
braking, steering, etc.) to a degree to which the human operator of
the vehicle cannot. ESS's for four-wheel automotive vehicles and
the benefits thereof have been known for some time. Given their
benefits, such systems are now found, in one form or another, on
many automobiles currently on the market.
[0007] In view of the desirability of enhancing the stability of a
three wheel vehicle and in view of the benefits of an ESS on a
four-wheeler, one of the first attempts (if not the first attempt)
was made to incorporate a then existing ESS for an automobile into
such a three wheel vehicle. As a result of that attempt, as is
described in U.S. Patent Publication No. 2006/0180372 (incorporated
by reference hereinabove), it was realized that the lack of a
fourth wheel and the geometry of the vehicle (and particularly the
geometry of the remaining wheels) prohibited the direct usage of
such an automotive ESS system on a three wheeled vehicle.
Modifications (also as described in that patent publication) were
necessary.
[0008] While the system described in the '372 publication
functioned as intended, it was merely a first attempt. The efforts
described in the '372 publication were mainly focused on modifying
the then existing automotive ESS to cause it to simulate its
behaviour on a four-wheel vehicle. In words, the inventors of those
inventions focused their attention making the three wheel ESS
perform (to the extent possible) as if it were a four-wheel
ESS.
[0009] After experimentation with a vehicle equipped with the
system described in the '372 publication and theoretical
calculations, the present inventors realized that while the first
generation system adapted an ESS to a three wheel vehicle to
overcome the disadvantages of a three wheel vehicle with respect to
an ESS and four wheel vehicles, it did not take into account all of
the characteristics of a three wheeled vehicle. Specifically, while
it was known that it was easier to roll three wheel vehicles over,
previous efforts were not focused on why this was the case, they
were simply focused on stabilizing the vehicle when a situation
indicative of an imminent rollovered occurred.
[0010] Therefore, while the first generation ESS for three wheeled
vehicles was adequate for its intended purpose, improvement was
still possible and further enhancing the stability of the vehicle
was desirable.
SUMMARY OF THE INVENTION
[0011] It is therefore an object of the present invention to
provide an ESS for a three wheel vehicle having two wheels in the
front and one wheel in the rear being an improvement over the prior
art.
[0012] Depending on the circumstances, a wheeled vehicle may be
undergoing various types of change in its acceleration. For
example, the vehicle may be subjected to increasing lateral
acceleration, such as when the driver enters a curve or attempts to
turn the vehicle. It may be subjected to increasing positive
longitudinal acceleration, such as when the driver requests more
torque from the engine. It may be subjected to increasing negative
longitudinal acceleration, such as when the driver actuates the
braking system of the vehicle. (For ease of understanding, what is
commonly referred to as "deceleration" is referred to in the
present application by the more technical term "negative
acceleration".) The vehicle may also be subjected to various
combinations of these types of change in acceleration.
[0013] Acceleration to which the vehicle is subjected results from
the wheel(s) being acted upon by force(s) created by actuation of
one or more of the vehicle's systems, e.g. the engine, the brakes,
and/or the steering system, depending on the situation. As a result
of these forces(s), a friction force is generated at the wheel's
tire's contact with the ground, with varying effect. As an example,
in the case of a wheel driven by power from the vehicle's engine
subjected to increasing torque from the engine, the friction force
is responsible for maintaining the traction of the tire of the
wheel on the road surface causing the power from the engine to
propel the vehicle as opposed to causing the tire to slip against
the road service.
[0014] Each tire is, however, limited as to the amount of friction
force that can be generated. If the friction force that would be
required to be generated exceeds the maximum friction force that
the tire can generate, the tire will lose traction on the ground.
As a result, the tire will slip against the road surface as opposed
to gripping the road surface.
[0015] The maximum friction force of a tire can be expressed as the
"maximum coefficient of friction" or .mu..sub.max. .mu..sub.max is
generally not a constant. For a given tire in a given situation,
.mu..sub.max will vary according to many variables, including the
chemical composition of the tire, the ambient temperature, the tire
temperature, the road surface conditions (smooth, rough, cracked,
dry, wet, oily, etc.), the size of tire's contact patch with the
road surface, and the tire's age (to name only but a few).
[0016] In any given situation, the friction force can be
represented by the coefficient of friction (.mu.), which is the
force causing the generation of the friction force (F) over the
vertical load on the tire ground contact patch (N), i.e. .mu.=F/N.
As would be understood by a person skilled in the art, as long as
.mu. is not greater than .mu..sub.max, i.e. as long as the friction
required to maintain traction of the tire on the road surface is
not greater than the friction of which the tire is capable under
the circumstances, the tire will maintain traction on the road
surface. If not, the tire will lose traction.
[0017] The vehicle's acceleration also can effect wheel lift and
rollover of the vehicle. Rollover of a vehicle is a situation where
the body of the vehicle has contacted the ground. As is described
more fully in some of the patent documents incorporated by
reference herein, each vehicle has a center of gravity. Each
vehicle also has a number of rollover axes, each rollover axis
being defined by a line connecting the contact patches of adjacent
tires on each side of the vehicle. Rollover of the vehicle is
likely to occur when the center of gravity of the vehicle passes
over the rollover axis on that side of the vehicle. This can occur,
for instance, if the vehicle's lateral acceleration is so great
that the wheels on one side of the vehicle lift from the ground and
the vehicle begins to tilt about the rollover axis on the opposite
side of the vehicle. If not corrected, this situation may become
such that vehicle rolls over.
[0018] The present inventors have realized that in the prior art
the exact relationship for a three wheel vehicle between the tire
grip threshold and vehicle wheel lift was not understood, and
therefore it previously had not been understood how this
relationship should be taken into account when designing an ESS for
a three wheel vehicle. The present invention arose from an attempt
at understanding these relationships and how to exploit them.
[0019] In this respect, FIG. 1 shows a theoretical graph of the
relationship of tire grip and vehicle rollover/wheel lift of a
typical prior art four wheel automotive vehicle as a function of
longitudinal and lateral acceleration of the vehicle when, prior to
the acceleration, the vehicle was heading straight at a constant
velocity on a flat level horizontal road surface. The curved line
1010 represents the limit of the tire grip of the vehicle (i.e. the
limit of the tire grip of the first of the vehicles tires to lose
grip--although usually loss of grip will occur in pairs--the front
pair of tires, the rear pair of tires, or both) and can be referred
to as the tire grip threshold. If, when plotted on the graph, a
given set of longitudinal and lateral accelerations that the
vehicle undergoes falls at a point below or on the tire grip
threshold 1010 (i.e. in the space marked 1012), then the tires will
all grip the road. Whereas, if when plotted on the graph a given
set of "theoretical" longitudinal and lateral accelerations would
fall at a point above the tire grip threshold 1010 (i.e. in the
space marked 1014), at least one and usually at least two of the
tires will lose their grip on the road and the vehicle or a part
thereof will skid. As a person skilled in the art would understand,
these are "theoretical" accelerations, because once a tire has
exceeded its tire grip threshold, friction is no longer maintaining
traction of the tire on the road surface and no further increase in
acceleration is possible as long as the vehicle remains on flat
horizontal ground and does not encounter any obstacles. It would
thus not ordinarily be possible to have such accelerations, and
acceleration points on the graph in space 1014 other than those
bordering the tire grip threshold 1010 would not ordinarily exist
under such circumstances; increases in acceleration beyond the
threshold not being possible. The graph has simply been discussed
in this way to illustrate the principles being explained (such that
the "theoretical" acceleration may be thought of the acceleration
that would have been if the tires had not lost traction).
Similarly, in the context of the present application "exceeding"
the tire grip threshold simply means that the acceleration has
reached the point where all of the tire no longer grips the road
surface (i.e. the tire has completely lost traction)--which would
be the points on the graph bordering the tire grip threshold.
[0020] The straight line 1016 represents the wheel lift threshold
of the vehicle. Therefore, if when plotted on the graph, a given
set of longitudinal and lateral accelerations (or theoretical
accelerations--see above) would fall at a point immediately above
the wheel lift threshold (i.e. in the space marked 1018) the (then
already skidding) vehicle will experience wheel lift (i.e. one or
more--usually a pair--of wheels will lift off the ground entirely),
and the vehicle will almost certainly roll over immediately
thereafter. This situation will occur for instance when the
skidding vehicle hits an object.
[0021] As would be understood by a person skilled in the art, it is
important to note that for any given vehicle at any particular
point in time (e.g. given its load factor, load distribution, tire
conditions, the road conditions, the outside temperature and a
whole variety of other factors), the position (and shape) of the
tire grip threshold 1010 and the position of the wheel lift
threshold 1016 may vary, but the relationship between them will not
(i.e. the wheel lift threshold 1016 will not cross the tire grip
threshold 1010, and will always remain above it--the vehicle will
always require a greater acceleration to cross the wheel lift
threshold 1016 than to cross the tire grip threshold 1010). That is
to say that for a standard factory-equipped automotive four wheel
vehicle (previously heading straight at a constant velocity on flat
horizontal terrain) undergoing an increase (positive or negative)
longitudinal acceleration or increasing lateral acceleration, the
vehicle will always reach its tire grip threshold (and begin to
skid) before reaching its wheel lift threshold.
[0022] An important point that can be seen on the graph in FIG. 1
is that the wheel lift threshold 1016 is a straight line having no
slope. Therefore, a four-wheel automotive vehicle will reach its
wheel lift threshold 1016 only as a result of increasing lateral
acceleration. Increasing longitudinal acceleration (in either a
positive or negative direction) will not cause the vehicle to reach
the wheel lift threshold 1016 (unless lateral acceleration is also
increased--although it will cause the vehicle to reach its tire
grip threshold). As a result, only by decreasing the lateral
acceleration of the vehicle can an imminent wheel lift be avoided;
a change (solely) in longitudinal acceleration will not prevent the
vehicle from crossing the threshold 1016. Further, under these
conditions, a four wheel vehicle will only reach its wheel lift
threshold 1016 after it has crossed its tire grip threshold 1010,
indicating that the vehicle will be skidding before wheel lift and
rollover occur. (Which will likely occur when the vehicle "trips"
by having contacted an object or having had its tires dig into the
ground.) For this reason, as is described in the patent documents
incorporated by reference into this application, prior art four
wheel automotive ESS's were focused taking corrective action by
creating yaw moments to reduce the lateral acceleration of the
vehicle, before it would lose tire grip (i.e. cross its tire-grip
threshold) and certainly after it had to prevent wheel lift and
thus roll over. As is further described in those patent documents,
these yaw moments were created by selective braking of the one or
more of the wheels of the vehicle.
[0023] FIG. 2, however, shows the same graph (i.e. the relationship
of the tire grip threshold and the wheel lift threshold) but for a
single person (i.e. the operator) three wheel rear wheel drive
vehicle having two wheels in the front and a single wheel in the
rear. (In this case the curve 210 still represents the limit of the
tire grip of the vehicle (i.e. the limit of the tire grip of the
first of the vehicle's tires to lose grip)--although loss of grip
may occur in the front pair of tires, in the rear tire alone, or in
both.) This graph was obtained through experimentation rather than
theoretical calculation. There are major differences from that of
the graph of FIG. 1. Firstly, the wheel lift threshold 216 crosses
and is below the tire grip threshold 210 for a significant number
of combinations of lateral and longitudinal accelerations.
Secondly, the wheel lift threshold 216, while still being straight
line, no longer has a slope of zero; its slope is significantly
negative.
[0024] These differences are important in that they indicate that
the vehicle can have wheel lift and rollover without first having
lost tire grip. This is situation with which an operator is
unlikely to be familiar with given its general non-occurrence on
four-wheel automobiles. For obvious reasons (given that it makes
rollover likely), wheel lift of the vehicle should be avoided if at
all possible. Further, depending on the then current acceleration
of the vehicle, an increase in the longitudinal acceleration of the
vehicle alone (i.e. not accompanied by an increase in the lateral
acceleration of the vehicle) can cause the vehicle the wheels to
lift and the vehicle to roll over. This is again a situation with
which an operator is unlikely to be familiar with given its general
non-occurrence on four-wheel automobiles. Conversely, the
differences indicate that wheel lift of the vehicle may be avoided
(depending on the circumstances) solely by decreasing the
longitudinal acceleration of the vehicle. Further, if the
longitudinal acceleration of the vehicle is decreased not only may
wheel lift be avoided (depending on the circumstances), in certain
circumstances an increase in lateral acceleration can be tolerated
before the vehicle wheels lift. This is in contrast to an
automobile wherein as previously mentioned decreasing the
longitudinal acceleration has no effect on the amount of increase
in lateral acceleration that can be tolerated before the wheels
lift and the vehicle rolls over.
[0025] The present inventors have realized then, that as a result
of the foregoing, the control strategy implemented by an ESS on a
three wheel vehicle can (and should) differ from that on four wheel
vehicle.
[0026] As a result, in one aspect, the invention provides a method
for enhancing the stability of a three wheel vehicle, the vehicle
having: a frame, a pair of front wheels, the front wheels being
connected to the frame via a front suspension, each of the front
wheels having a tire, the tire having a tire grip threshold, a
single rear wheel, the rear wheel being connected to the frame via
a rear suspension, the rear wheel having a tire, the tires having a
tire grip threshold, an engine supported by the frame and
operatively connected to at least one of the wheels to provide
power to propel the vehicle, a braking system including brakes
associated with each of the wheels to brake the vehicle, a steering
system including handlebars operatively connected to the front
wheels to steer the vehicle, a straddle seat disposed on the frame,
the seat being suitable for accommodating at least a driver of the
vehicle sitting in straddle fashion, the tire grip thresholds of
tires being, for a set of combinations of lateral and longitudinal
accelerations that the vehicle may undergo, greater than a wheel
lift threshold of a vehicle, such that the vehicle experiences
wheel lift before the tires lose grip, a plurality of sensors
arranged on the vehicle so as to provide electronic signals
respecting vehicle information including at least engine speed,
throttle position, lateral acceleration, and longitudinal
acceleration, and an electronic stability system (ESS) including a
processor and memory, the ESS being electronically connected to at
least the engine, the sensors, the braking system, the method
comprising: providing the ESS with information from the sensors
related to at least longitudinal acceleration of the vehicle and
the lateral acceleration of the vehicle; causing the ESS to
determine, using information from the sensors and data from the
memory, whether (i) a precursory condition indicative of a wheel
lift exists and (ii) the tire grip threshold of any of the tires
has been exceeded; and when a precursory condition indicative of a
wheel lift exists and the tire grip threshold of none of the tires
has been exceeded, causing the ESS to reduce the longitudinal
acceleration of the vehicle by a first amount less than that which
would cause the tire grip threshold of any of the tires to be
exceeded.
[0027] In another as aspect, the invention provides a three wheel
vehicle having: a frame, a pair of front wheels, the front wheels
being connected to the frame via a front suspension, each of the
front wheels having a tire, the tire having a tire grip threshold,
a single rear wheel, the rear wheel being connected to the frame
via a rear suspension, the rear wheel having a tire, each of the
tires having a tire grip threshold, an engine supported by the
frame and operatively connected to at least one of the wheels to
provide power to propel the vehicle, a braking system including
brakes associated with each of the wheels to brake the vehicle, a
steering system including a handlebar operatively connected to the
front wheels to steer the vehicle, a straddle seat disposed on the
frame, the seat being suitable for accommodating at least a driver
of the vehicle sitting in straddle fashion, the tire grip
thresholds of tires being, for a set of combinations of lateral and
longitudinal accelerations that the vehicle may undergo, greater
than a wheel lift threshold of a vehicle, such that the vehicle
experiences wheel lift before the tires lose grip, a plurality of
sensors arranged on the vehicle so as to provide electronic signals
respecting vehicle information including at least engine speed,
throttle position, lateral acceleration, and longitudinal
acceleration, and an electronic stability system (ESS) including a
processor and memory, the ESS being electronically connected to at
least the engine, the sensors, and the braking system, the memory
including instructions that when executed by the processor: cause a
determination, using information from the sensors including
information related to the longitudinal acceleration of the
vehicle, information related to the lateral acceleration of the
vehicle, and data from the memory, of whether (i) a precursory
condition indicative of a wheel lift exists and (ii) the tire grip
threshold of any of the tires has been exceeded; and cause a
reduction of the longitudinal acceleration of the vehicle by a
first amount less than that which would cause the tire grip
threshold of any of the tires to be exceeded when a precursory
condition indicative of a wheel lift exists and the tire grip
threshold of none of the tires has been exceeded.
[0028] One of the basic functions then of an ESS of the present
invention is to determine whether wheel lift of the vehicle is
likely (as a result of proximity of the acceleration of the vehicle
to the wheel lift threshold) and whether the vehicle's tires have
lost grip (as a result of the acceleration of the vehicle having
crossed the wheel lift threshold), and to take corrective action
accordingly. As is described below, different corrective actions
may be (and preferably will be) taken depending on whether it has
crossed its tire grip threshold or not.
[0029] In this respect reference is had to FIG. 3 which is similar
to FIG. 2.
[0030] Shaded area 220 represents a range of combinations of
lateral and longitudinal accelerations in which there is believed
(by designers of the vehicle) to be an increased risk of wheel lift
(depending on how the acceleration changes over time). Thus, this
is an area where "a precursory condition indicative of a wheel lift
exists" as that expression is used in the context of the present
invention. This expression should not be interpreted as meaning
that a wheel lift will occur, only that the chances are greater
that one might occur (depending on driver input and other factors).
Further, this expression may not (and most likely will not)
encompass all situations in which a wheel lift might occur (all of
the situations that could occur in real-life are far to complicated
for a simple graph). A wheel lift might occur in other situations
as well. This expression is simply intended to cover those
situations that the designers of this vehicle have identified as
such. The shaded area 220 need not (and likely will not) be
constant. Depending on other factors (such as, for instance, the
rate and direction of change of the acceleration, whether such
changes are being monitored, whether the vehicle is on an inclined
road surface, etc.), the ESS may be programmed to recognise
different "precursory conditions indicative of a wheel lift" under
different circumstances such that different shaded areas would be
represented on the graph if such conditions were plotted on the
graph.
[0031] Data representing the shaded area 220 is stored within the
memory of the ESS. This data may be stored in a number of ways, for
example as discrete points or mathematic equations or some
combination thereof. Information respecting the longitudinal
acceleration of the vehicle and the lateral acceleration of the
vehicle is received from the sensors by the ESS. Depending on the
type of sensor used and/or the programming of the ESS, the
information received by the ESS may be the actual acceleration of
the vehicle; or it may simply be information sufficient to allow
the ESS processor to perform whatever calculations are necessary to
make a meaningful comparison between the input received from the
sensors and the data stored in memory, so as to determine whether
or not the aforementioned precursory condition exists. Thus, while
it is preferred that the sensors directly provide the ESS with the
acceleration of the vehicle, it is envisaged that the ESS could be
provided with "rawer data" that it could use to make its own
calculations to arrive at values representative of the acceleration
of the vehicle, and then could use those values in subsequent
calculations.
[0032] If the aforementioned precursory condition does exist, and
the tire grip threshold of none of the tires has been exceeded,
corrective action will be taken to avoid a wheel lift while, if at
all possible, at the same time preventing the vehicle from
skidding. Thus, the first corrective action that will be taken will
be a reduction in the longitudinal acceleration of the vehicle from
the vehicle's current longitudinal acceleration to one which is
likely outside of the "precursory condition" zone yet that does not
exceed the tire grip threshold of any of the tires.
[0033] There are two principal ways in which to reduce the
longitudinal acceleration of the vehicle: Either a reduction in the
torque produced by the engine can be effected or the braking system
of the vehicle may be actuated. These may each be used alone or in
combination, although ideally when actuating the braking system the
engine torque will at least not be permitted to increase. Depending
on the circumstances one way may be preferred over the other. For
instance, in a situation where the vehicle accelerates too rapidly
in a curve, it is likely that this situation will be dealt with by
cutting engine torque (which is generally a simpler and faster way
to effect a reduction in longitudinal acceleration and thus is the
preferred manner).
[0034] Whereas, if a rapid reduction in the longitudinal
acceleration is desired or required, such as during obstacle
avoidance, or if cutting the engine torque was without a sufficient
effect, the braking system will likely be actuated.
[0035] The amount by which the longitudinal acceleration is reduced
depends on the circumstances and the means by which the reduction
is carried out. For example, the torque of the engine may be
reduced by retarding or cutting off the ignition in one or more of
the cylinders of engine. Typically, retarding the ignition will
produce a lower reduction in the engine torque than cutting off the
ignition completely. For instance in a 4-stroke V-twin engine,
intermittent cutting of the ignition in one of the cylinders can
produce a 25% reduction in engine torque, whereas retarding the
ignition can produce a 12.5% reduction in engine torque.
Combinations of both techniques may also be used to produce, for
example, a 37.5% reduction in engine torque. The ESS will typically
carry out increasingly more severe reductions in engine torque if
previous reduction(s) were ineffective at changing the acceleration
of the vehicle significantly enough such that the precursory
condition of a wheel lift no longer exists. If the longitudinal
acceleration is being reduced by actuating the braking system, the
forces applied by the brakes on the various wheels may be a
calculated force or may be based on predetermined amounts.
[0036] The memory however does further include instructions to
cause the reduction of the longitudinal acceleration by a second
amount (to be understood as including a further amount if more than
one previous reduction in longitudinal acceleration not resulting
in the tire grip threshold of any of the tires being exceeded have
occurred) that would exceed the tire grip threshold (i.e. to a
point such that the tires no longer have traction), after having
caused the reduction of the longitudinal acceleration of the
vehicle by a first amount (or amounts) less than that which would
exceed the tire grip threshold of any of the tires. The
acceleration at this point, however, being beyond the tire grip
threshold of at least one of the tires, would mean that the vehicle
or a part thereof is skidding (e.g. the vehicle is being
understeered). While undesirable, this situation is nonetheless
tolerable under the extreme circumstances of the vehicle's
operation, whereas having done nothing would have resulted in a
wheel lift.
[0037] In the aforementioned examples, the braking system was
actuated so as to reduce the longitudinal acceleration of the
vehicle while not directly effecting the lateral acceleration of
the vehicle (i.e. not taking an action aimed at directly reducing
the lateral acceleration or even preventing such acceleration from
increasing). Such would be the case, for example, when all of the
brakes of the vehicle are actuated simultaneously or when solely
the engine torque is reduced. In such a case no (or no substantial)
yaw movement is generated about the vehicle and the lateral
acceleration is left unchanged (other than any secondary effect on
the lateral acceleration owing to the reduction in the longitudinal
acceleration, if any).
[0038] It is possible however, to reduce both the longitudinal
acceleration and the lateral acceleration simultaneously by braking
only one wheel (usually the outer front wheel) or by differentially
braking the wheels such that one wheel (again usually the outer
front wheel) is braked to a greater extent. In this way, in
addition to slowing down the vehicle and reducing the longitudinal
acceleration, a yaw moment will be induced about the vehicle which
will reduce the lateral acceleration of the vehicle as well. This
described in International Patent Application No. PCT/US2006/017477
incorporated by reference hereinabove.
[0039] In another aspect, the plurality of sensors may be arranged
on the vehicle so as to provide electronic signals respecting the
steering angle, and the ESS is further electronically connected to
at least the steering system (e.g. a power steering actuator), and
reducing the lateral acceleration of the vehicle may also be
carried out by causing the ESS to actuate the steering system
(whether alone or in combination with inducing a yaw moment via the
braking system and/or reducing the engine torque). In such cases
the steering system (usually via a power steering unit) may be
actuated so as to increase the effort required to turn the wheels,
thus, impeding or hindering the driver so doing, thereby reducing
or preventing (at the case may be) further increases in lateral
accelerations. It would be theoretically possible that in extremely
rare situations the steering system (again usually via the power
steering unit) could even be actuated so as to turn the wheels in a
direction that would reduce the lateral acceleration on the
vehicle.
[0040] It will be understood by persons skilled in the art that the
various methods of acceleration reduction described above may not
all take effect with the same speed. For example, cutting the
engine torque will typically take effect faster (relatively) than
actuating the braking system (as it takes time to actually actuate
the braking cylinders). This lag time may be taken into
consideration when selecting the action to be carried out (e.g. by
choosing the faster acting method in certain situations). Of
course, the ESS could be intentionally programmed with a delayed
implementation strategy (i.e. to allow a period of time to elapse
when the precursory condition exists before taking corrective
action) if so desired.
[0041] In yet another aspect, the memory further includes
instructions to cause a reduction of at least one of the lateral
acceleration and the longitudinal acceleration of the vehicle when
a precursory condition indicative of exceeding the tire grip
threshold exists but a precursory condition indicative of a wheel
lift does not exist. In this aspect, the ESS may combine the
aforementioned novel features with the operation of prior art
system.
[0042] Embodiments of the present invention each have at least one
of the above-mentioned objects and/or aspects, but do not
necessarily have all of them. It should be understood that some
aspects of the present invention that have resulted from attempting
to attain the above-mentioned objects may not satisfy these objects
and/or may satisfy other objects not specifically recited
herein.
[0043] Additional and/or alternative features, aspects, and
advantages of embodiments of the present invention will become
apparent from the following description, the accompanying drawings,
and the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0044] For a better understanding of the present invention, as well
as other aspects and further features thereof, reference is made to
the following description which is to be used in conjunction with
the accompanying drawings, where:
[0045] FIG. 1 is graph of the tire grip threshold and vehicle wheel
lift threshold of a typical prior art four wheel vehicle;
[0046] FIG. 2 is a graph of the tire grip threshold and vehicle
wheel lift threshold of a typical rear wheel drive three wheel
vehicle having two wheels in the front and one wheel in the
rear;
[0047] FIG. 3 is a graph of the tire grip threshold and vehicle
wheel lift threshold of a typical rear wheel drive three wheel
vehicle having two wheels in the front and one wheel in the rear,
the vehicle included an ESS of the present invention and the graph
showing typical actions of the ESS of the present invention;
[0048] FIG. 4 is a left side rear perspective view of a three wheel
vehicle having an ESS of the present invention;
[0049] FIG. 5 is a left side elevation view of the three wheel
vehicle of FIG. 4;
[0050] FIG. 6 is a top plan view of the three wheel vehicle of FIG.
4;
[0051] FIG. 7 is a left side elevation cut-away view of the three
wheel vehicle of FIG. 7, showing interior components of the
vehicle;
[0052] FIG. 8 is a right side elevation view of the frame of the
vehicle in FIG. 4;
[0053] FIG. 9 is a schematic view of the braking system of the
vehicle of FIG. 4;
[0054] FIG. 10 is a block diagram of the ESS of the present
invention showing its interconnections with other vehicle
components and systems; and
[0055] FIG. 11 is a flow diagram of a control strategy employed by
an ESS of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0056] FIGS. 4, 5 and 6, illustrate a three wheel vehicle 10 in
accordance with a specific embodiment of the invention. The
particular aesthetic design details of the three wheel vehicle 10
are not critical to this invention, and these figures merely
illustrate one possible configuration.
Vehicle Components and Systems
[0057] Vehicle 10 includes a frame 12 that supports and houses a
internal combustion engine 14, but which could be any type of
suitable power source such as an electric motor or hybrid engine if
so desired. The engine includes a conventional Engine Management
System (EMS) 107 (FIG. 8) that controls and regulates all engine
functions such as RPM, torque, ignition, throttle, fuel injection,
and emissions using a variety of conventional sensors and
controllers (e.g. those described in U.S. patent application Ser.
No. 11/627,780 and U.S. Pat. No. 6,626,140, both incorporated by
reference herein). The EMS is electronically connected with the
vehicle's Electronic Control Unit (ECU) 110 (in FIG. 7), described
below.
[0058] A straddle seat 16 is mounted on the frame 12 and has a
driver seat 17 and a passenger seat 19 disposed behind the driver
seat 17.
[0059] A single rear wheel 18 with a tire 20 suitable for road use
is suspended via a rear suspension 15 at the rear of the frame 12
and is operatively connected to the engine 14 through a
transmission including a gearbox and belt drive, although any
suitable power transmission mechanism (e.g. continuously-variable
transmission, chain drive, driveshaft assembly, etc.) could be
used. A pair of front wheels 22 and 24 are suspended from the front
of the frame 12 through suitable front suspension 21 including
upper and lower A-arms. Dampening mechanisms including shock
absorber and coil spring assemblies are associated with the front
suspension 21 to increase ride comfort and vehicle stability. Front
wheels 22 and 24 have tires 26 and 28 suitable for road use mounted
thereon. A vehicle speed sensor in the form of Hall-effect wheel
speed sensors 86, 88, and 90, located at each wheel, generates
signals representative of each individual wheel rotation rate.
Sensors 86, 88, and 90 are electronically interconnected with the
ECU 110.
[0060] Suitable tires 20, 26, 28 are those sold by Kenda USA of
Reynoldsburg, Ohio under model no. 79100. Front tires 26, 28 are
size 165/65 R14 and the rear tire 20 is size 225/50 R15. The tires
are made of Styrene-Butadiene (SBR) copolymer and an approximate
maximum coefficient of friction (.mu..sub.max) of 1.0 laterally and
1.1 longitudinally.
[0061] A steering system 30 is coupled to the front wheels 22 and
24 and is supported by the frame 12 for transmitting steering
commands to the front wheels 22 and 24. The steering system 30
includes a steering column 32 and a handlebar 34, although other
suitable steering control mechanisms such as a steering wheel could
also be used. A steering sensor 98 (in the form of a Hall effect
sensor, potentiometer, or anisotropic magnetoresistence sensor
(AMR)), is mounted to the steering system 30 and generates signals
representative of steering angle, a steering angle variation rate,
and steering torque applied to the vehicle. The steering sensor 98
is electronically connected to the ECU 110. The steering system
also includes a power steering apparatus 29 of the type commonly
used in recreational vehicles such as all-terrain vehicles (best
shown in FIG. 7) including an electric motor and a reduction gear
(see U.S. Pat. No. 7,216,733, incorporated herein by reference as
an example). The power steering apparatus 29 is electronically
connected to the ECU 110 to provide status information thereto and
receive control information therefrom.
[0062] As illustrated in FIG. 8, the frame 12 is a supporting
structure to which are connected the rear suspension 15 and the
front suspension system 21. The vehicle 10 is equipped with a yaw
sensor 100 having integrated lateral acceleration sensor and
longitudinal acceleration sensor, which is mounted onto the upper
longitudinal member 45 of the frame 12. The yaw sensor 100 is
positioned in proximity to the vertical axis Z of the vehicle and
center of gravity CG of the vehicle to improve the accuracy of the
readings of the sensor and thus the information provided thereby.
The yaw sensor 100 measures the rotational speed of the vehicle
about the vertical axis Z and is a gyrometer that uses secondary
Coriolis forces developed within non-stationary systems. The
integrated lateral and longitudinal acceleration sensors measure
the acceleration of the vehicle along the transverse axis x and the
longitudinal axis y. They are Hall-type sensors. Other sensors such
as a roll rate sensor (or, alternatively, a roll angle sensor), and
a pitch rate sensor may be added to provide more vehicle status
information. All of the sensors are interconnected with the ECU
110.
[0063] FIG. 9 schematically illustrates the braking system of the
three wheel vehicle 10. The braking system comprises individual
brakes 80, 82, and 84, at each wheel 18, 22, and 24 respectively, a
master cylinder 92 hydraulically connected to each brake 80, 82,
and 84, a hand brake lever 93 and a foot brake lever 95 either
hydraulically or mechanically connected to the master cylinder 92.
The braking system also includes an hydraulic modulator 96 with
integrated primer pump hydraulically positioned between the
individual brakes 80, 82, and 84 and the master cylinder 92. The
hydraulic modulator 96 is a basic component of an antilock braking
system (ABS) which comprises at least two inlets channels 61, 62
and three outlet channels 63, 64, 65 (one for each individual
brake). The master cylinder 92 typically comprises two outlet
hydraulic lines 66, 67, one for the front brake circuit (66) and
one for the rear brake circuit (67), which are hydraulically
connected to the two inlet channels 61, 62 of the hydraulic
modulator 96. The inlet channel 62 receiving the front brake
hydraulic line 66 splits into two outlet channels 64, 65, each
hydraulically connected to one of the front brakes 82 and 84. The
inlet channel 61 receiving the rear brake hydraulic line 67 is
connected to a single outlet channel 63 which is hydraulically
connected to the rear brake 80. The hydraulic modulator 96 is
adapted to regulate the pressure in the individual brakes 80, 82,
and 84 independently of braking pressure applied by the driver. The
braking system is therefore an integrated Anti-lock Braking System
(ABS) that prevents wheel lock and improve braking efficiency. The
braking system is electronically interconnected with the ECU.
[0064] The Electronic Control Unit (ECU) 110, comprising both a
computer processor and memory, is responsible for vehicle
electrical, electronic and closed loop control functions, including
power supply to system sensors, recording operating conditions,
converting, manipulating, and transmitting data, and network
linkage to other controllers such as the EMS. The ECU 110 receives
inputs from the various sensors and other vehicle operating systems
(e.g. braking, power steering), processes the input data, and
outputs signals to actuate certain operating parameters of the
vehicle.
Electronic Stability System
[0065] The three wheel vehicle 10 is equipped with a specifically
designed Electronic Stability System (ESS). In general, an ESS
includes a computer processor and processor readable memory
containing both programming information (software) and data
respecting the ESS's functions. In the case of vehicle 10 the ESS
is incorporated into the ECU 110 as part of the ECU's functions.
(The ESS is not separately physically distinguishable from the ECU
in this embodiment, but in other embodiments it would be possible
that it were.) The ECU determines the actual vehicle dynamic status
based on theses inputs, evaluates whether the vehicle dynamic
status falls within or outside the limits of the specific stability
envelope of the three wheel vehicle stored in memory and below or
above specific maximum rate of changes of the vehicle dynamic
status stored in memory. Thereafter, if required, the ECU outputs
specific signals to various vehicle systems of the three wheel
vehicle 10 to restore stability or in specific circumstances, to
prevent (if possible) the vehicle from reaching the limits of the
stability envelope of the three wheel vehicle.
[0066] FIG. 10 shows a basic block diagram of the ECU (ESS) 110 in
accordance with one embodiment of the invention. In operation, the
ECU 110 receives inputs relating to at least some of the following
factors: the yaw rate from the yaw sensor 100, wheel speed from the
each wheel speed sensors 86, 88, and 90, lateral acceleration also
from the integrated lateral acceleration sensor 100, longitudinal
acceleration also from the integrated longitudinal acceleration
sensor 100 and steering angle from the steering angle sensor 98.
This information is processed by the ECU 110 to evaluate the
dynamic status of the three wheel vehicle and compare it with data
stored in memory defining the stability envelope of the three wheel
vehicle 10 and specifically the wheel lift limits of the stability
envelope to determine whether an intervention to stabilize the
vehicle is required. Various intervention schemes corresponding to
specific dynamic status are stored in memory and are described
hereinbelow. If the dynamic status evaluated by the ECU requires an
intervention, the ECU generates output signals (according to an
intervention scheme) to cause the braking system or the Engine
Management System or the power steering system, or some combination
thereof, to take action to attempt to correct the situation.
[0067] FIG. 11 shows a flow diagram of a control strategy 500
employed by an ESS of the present invention. Initially, and
continuously, as a first step 502 the ECU receives input from the
various sensors related to vehicle information including the
longitudinal acceleration and the lateral acceleration and engine
information from the EMS (the ECU may or may not additionally
process this information--as the case may require). The ECU then
504 compares data with values stored in memory or values calculated
from information stored in memory depending on the circumstances.
The ECU then 506 determines whether a precursory condition
indicative of a wheel lift exists. If such a condition does exist,
the ECU then 508 determines whether the vehicle is over its tire
grip threshold (i.e. has the tire grip threshold of any of the
tires been exceeded). If the vehicle is not over its tire grip
threshold, the ECU will cause 510 the reduction of the longitudinal
acceleration of the vehicle by an amount less than that which would
cause the vehicle to exceed its tire grip threshold (i.e. the grip
threshold of at least one of the tires to be exceeded). If,
however, the vehicle is over its tire grip threshold, the ECU will
determine whether a reduction in lateral acceleration is necessary
512. If no reduction in lateral acceleration is necessary, the ECU
will cause 514 the longitudinal acceleration of the vehicle to be
reduced, which will cause the vehicle to exceed its tire grip
threshold if it has not already been exceeded (the vehicle or a
part thereof will be skidding or begin to skid, depending on the
circumstances). If a reduction in lateral acceleration is
necessary, the ECU will cause the longitudinal acceleration and
lateral acceleration to be reduced 516. Returning back to step 506,
if a precursory condition of a wheel lift does not exist, the ECU
will then 518 determine whether a precursory condition of exceeding
the tire grip exists. If this is true, then 520 the ECU will cause
a reduction in the acceleration of the vehicle (this, depending on
the circumstance could be the lateral acceleration, the
longitudinal acceleration, or a combination of both) or maintain
the vehicle acceleration at its current value (as the case may be)
to prevent the tire grip threshold from being exceeded. If not,
then no action will be caused to be taken by the ECU. In all cases,
the ECU returns back to 500 and receives new input from the sensors
and begins the process again.
[0068] Referring now to FIG. 3, as an example, if at a particular
point in time the acceleration of the vehicle is at point A when
plotted on the graph, point A being within the shaded area 220
(i.e. is at a point wherein a precursory condition indicative of
wheel lift exists) and the tire grip threshold not having been
exceeded, corrective action will be taken to reduce the
longitudinal acceleration to point B. It will be noted that point B
is outside of and lower on the graph than the shaded area 220 and
therefore (all other things being equal) is generally a more
acceptable acceleration in terms of likelihood of wheel lift. It
will also be noted that the lateral acceleration of the vehicle at
point B is the same as that as at point A. With acceleration being
as it is at point B, the vehicle can accommodate a greater increase
in lateral acceleration than at point A before reaching its wheel
lift threshold 216 (compare distance 224 with distance 222).
Further when the acceleration is at point B, the vehicle has not
crossed the tire grip threshold and therefore the tires still have
traction.
[0069] The ESS is continually operative and thus assuming that
after a short period of time the operating conditions of the
vehicle have changed such that acceleration of the vehicle when
plotted on the graph would now be at point C. Under such
circumstances the ECU could again reduce the longitudinal
acceleration to point D in a similar manner as described above.
[0070] Continuing with the above example, if after another short
period of time the operating conditions of the vehicle have again
changed such that the acceleration of the vehicle is at point E
when plotted on the graph. (The previous reductions from A to B and
from C to D collectively being the "first amount" of reduction
within the context of the present invention.) At this point, as a
precursory condition indicative of wheel lift exists, the ESS may
act in two different manners (depending on programming). In a first
instance the ESS may reduce both the lateral acceleration and the
longitudinal acceleration of the vehicle to point F. (Reduction by
unequally braking the wheels so as to generate a yaw moment would
be the preferred method of so doing.) In this manner, the ESS has
enhanced the stability of the vehicle such that the vehicle remains
within the tire grip threshold. Alternatively, the ESS may reduce
solely the longitudinal acceleration of the vehicle to a point G.
While wheel lift has been prevented, the acceleration at point G
is, however, beyond the tire grip threshold, so the vehicle has
begun to skid. This is situation which the driver of the vehicle
may correct by reducing the lateral acceleration of the
vehicle.
[0071] An ESS of the present invention will also act like a
conventional ESS (assuming it is so programmed) in situations where
there is no precursory condition indicative of a wheel lift yet the
tire grip threshold is likely to be exceeded (i.e. a precursory
condition indicative of exceeding tire grip threshold exists). Such
a situation would be at point H in FIG. 3. Point H is not within
shaded area 220 and thus a precursory condition indicative of a
wheel lift does not exist. Nonetheless, it can be seen that if the
longitudinal acceleration of the vehicle increases the tire grip
threshold will be crossed. In such a situation, the ESS will either
reduce the longitudinal acceleration or maintain the longitudinal
acceleration (i.e. prevent its further increase), depending on its
programming, in order to prevent the vehicle from crossing the tire
grip threshold. This "precursory condition indicative of exceeding
tire grip threshold" has been graphically shown in FIG. 3 as dotted
areas 226 and 228.
[0072] Modifications and improvements to the above-described
embodiments of the present invention may become apparent to those
skilled in the art. The foregoing description is intended to be
exemplary rather than limiting. The scope of the present invention
is therefore intended to be limited solely by the scope of the
appended claims.
* * * * *