U.S. patent application number 14/529385 was filed with the patent office on 2016-05-05 for active brake retraction during regeneration.
The applicant listed for this patent is GM GLOBAL TECHNOLOGY OPERATIONS LLC. Invention is credited to Paul A. Stanowski, Goro Tamai.
Application Number | 20160121726 14/529385 |
Document ID | / |
Family ID | 55643090 |
Filed Date | 2016-05-05 |
United States Patent
Application |
20160121726 |
Kind Code |
A1 |
Tamai; Goro ; et
al. |
May 5, 2016 |
ACTIVE BRAKE RETRACTION DURING REGENERATION
Abstract
A method of operating a brake-retract system during a
regenerative braking event begins with monitoring an amount of
regenerative braking achieved. A set point threshold is linearly
ramped from a first threshold to a max regeneration capacity if the
amount of regenerative braking achieved exceeds a second threshold.
The second threshold is less than the first threshold, and the
first threshold is less than the max regeneration capacity. The max
regeneration capacity is representative of a maximum amount of
regenerative braking capable of being produced. A friction element
is transitioned from a first, retracted-state to a second,
ready-state if the amount of regenerative braking achieved exceeds
the set point threshold.
Inventors: |
Tamai; Goro; (West
Bloomfield, MI) ; Stanowski; Paul A.; (Trenton,
MI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
GM GLOBAL TECHNOLOGY OPERATIONS LLC |
Detroit |
MI |
US |
|
|
Family ID: |
55643090 |
Appl. No.: |
14/529385 |
Filed: |
October 31, 2014 |
Current U.S.
Class: |
701/22 ;
701/70 |
Current CPC
Class: |
B60L 7/26 20130101; B60T
2250/00 20130101; B60L 7/18 20130101 |
International
Class: |
B60L 7/18 20060101
B60L007/18; F16D 65/52 20060101 F16D065/52 |
Claims
1. A method of operating a brake-retract system during a
regenerative braking event comprising: monitoring an amount of
regenerative braking achieved; linearly ramping a set point
threshold from a first threshold to a max regeneration capacity if
the amount of regenerative braking achieved exceeds a second
threshold; wherein the second threshold is less than the first
threshold, wherein the first threshold is less than the max
regeneration capacity, and wherein the max regeneration capacity is
representative of a maximum amount of regenerative braking capable
of being produced; transitioning a friction element of a braking
mechanism from a first, retracted-state to a second, ready-state if
the amount of regenerative braking achieved exceeds the set point
threshold; and wherein the friction elements are closer to a brake
rotor in the second, ready-state than in the first,
retracted-state.
2. The method of claim 1, further comprising transitioning the
friction element from the first, retracted-state to the second,
ready-state if a braking request is received from a vehicle
system.
3. The method of claim 1, further comprising transitioning the
friction element from the first, retracted-state to the second,
ready-state if a vehicle speed is less than a predetermined
threshold.
4. The method of claim 1, further comprising linearly ramping the
set point threshold toward the first threshold if the amount of
regenerative braking achieved falls below the first threshold and
the friction element is in the second, ready-state.
5. The method of claim 4, further comprising transitioning the
friction element from the second, ready-state to the first,
retracted-state if the amount of regenerative braking achieved
falls below the first threshold and the friction element is in the
second, ready-state.
6. The method of claim 1, further comprising transitioning the
friction element from the first, retracted-state to the second,
ready-state if a state of charge of a vehicle traction battery is
within a predefined tolerance of a maximum state of charge.
7. The method of claim 1, wherein the set point threshold, the
first threshold, the second threshold, and the max regeneration
capacity are all torque values measured from the point of view of a
motor/generator disposed in power flow communication with a
plurality of wheels of a vehicle.
8. A vehicle comprising: a vehicle traction battery; a
motor/generator in power-flow communication with a plurality of
vehicle wheels and in electrical communication with the vehicle
traction battery, wherein the motor/generator is configured to
perform regenerative braking such that a torque is received from
the plurality of vehicle wheels and converted into an electrical
energy that is provided to the vehicle traction battery; a friction
braking mechanism in communication with each of the plurality of
vehicle wheels, each respective friction braking mechanism
including a movable friction element and a rotor, wherein the rotor
is configured to rotate with the wheel, and wherein the movable
friction element is configured to selectively apply a contact
pressure to the rotor; wherein each friction element is configured
to translate between a first, retracted-state and a second,
ready-state, and wherein the friction elements are closer to a
brake rotor in the second, ready-state than in the first,
retracted-state; and a controller configured to: monitor an amount
of regenerative braking achieved by the motor/generator; linearly
ramp a set point threshold from a first threshold to a max
regeneration capacity if the amount of regenerative braking
achieved exceeds a second threshold; wherein the second threshold
is less than the first threshold, wherein the first threshold is
less than the max regeneration capacity, and wherein the max
regeneration capacity is representative of a maximum amount of
regenerative braking capable of being produced; transition each
respective friction element from the first, retracted-state to the
second, ready-state if the amount of regenerative braking achieved
exceeds the set point threshold.
9. The vehicle of claim 8, wherein the controller is further
configured to transition each respective friction element from the
first, retracted-state to the second, ready-state if a braking
request is received from a vehicle system.
10. The vehicle of claim 8, wherein the controller is further
configured to transition each respective friction element from the
first, retracted-state to the second, ready-state if a vehicle
speed is less than a predetermined threshold.
11. The vehicle of claim 8, wherein the controller is further
configured to linearly ramp the set point threshold toward the
first threshold if the amount of regenerative braking achieved
falls below the first threshold and the friction element is in the
second, ready-state.
12. The vehicle of claim 8, wherein the controller is further
configured to transition each respective friction element from the
second, ready-state to the first, retracted-state if the amount of
regenerative braking achieved falls below the first threshold and
each respective friction element is in the second, ready-state.
13. The vehicle of claim 8, wherein the controller is further
configured to transition each respective friction element from the
first, retracted-state to the second, ready-state if a state of
charge of the vehicle traction battery is within a predefined
tolerance of a maximum state of charge.
14. The vehicle of claim 8, wherein the set point threshold, the
first threshold, the second threshold, and the max regeneration
capacity are all torque values measured from the point of view of a
motor/generator.
15. A method of operating a brake-retract system during a
regenerative braking event comprising: monitoring an amount of
regenerative braking achieved; linearly ramping a set point
threshold from a first threshold to a max regeneration capacity if
the amount of regenerative braking achieved exceeds a second
threshold; wherein the second threshold is less than the first
threshold, wherein the first threshold is less than the max
regeneration capacity, and wherein the max regeneration capacity is
representative of a maximum amount of regenerative braking capable
of being produced; transitioning a friction element of a braking
mechanism from a first, retracted-state to a second, ready-state if
the amount of regenerative braking achieved exceeds the set point
threshold; and transitioning the friction element from the first,
retracted-state to the second, ready-state if a braking request is
received from a vehicle system transitioning the friction element
from the first, retracted-state to the second, ready-state if a
vehicle speed is less than a predetermined threshold wherein the
friction elements are closer to a brake rotor in the second,
ready-state than in the first, retracted-state.
16. The method of claim 15, further comprising linearly ramping the
set point threshold toward the first threshold if the amount of
regenerative braking achieved falls below the first threshold and
the friction element is in the second, ready-state.
17. The method of claim 16, further comprising transitioning the
friction element from the second, ready-state to the first,
retracted-state if the amount of regenerative braking achieved
falls below the first threshold and the friction element is in the
second, ready-state.
18. The method of claim 15, further comprising transitioning the
friction element from the first, retracted-state to the second,
ready-state if a state of charge of a vehicle traction battery is
within a predefined tolerance of a maximum state of charge.
19. The method of claim 15, wherein the set point threshold, the
first threshold, the second threshold, and the max regeneration
capacity are all torque values measured from the point of view of a
motor/generator disposed in power flow communication with a
plurality of wheels of a vehicle.
Description
TECHNICAL FIELD
[0001] The present disclosure relates generally to a manner of
controlling active brake retraction in a hybrid electric
automobile.
BACKGROUND
[0002] A brake is a device that is included in automotive vehicles
to inhibit motion. Brakes commonly use friction to convert kinetic
energy into heat, though other methods of energy conversion may be
employed. For example, regenerative braking converts much of the
kinetic energy to electric energy, which may be stored for later
use.
[0003] On vehicles, braking systems are employed to apply a
retarding force, typically via frictional elements at the vehicle's
rotating axles or wheels, to inhibit vehicle motion. Friction
brakes often include stationary shoes or pads that are lined with
friction material and configured to be applied against a rotating
wear surface, such as a rotor or a drum. Common configurations
include shoes that contact to rub on the outside of a rotating
drum, commonly called a "band brake," a rotating drum with shoes
that expand to rub the inside of a drum, commonly called a "drum
brake," and pads that pinch a rotating disc, commonly called a
"disc brake."
[0004] Another form of braking involves applying a torque that is
counter to the direction of rotation of the wheel using an electric
motor. In effect, the inertia of the vehicle drives the electric
motor in reverse. Doing so then causes the inertial torque to drive
the motor as a generator, which can regenerate the vehicle
batteries, while simultaneously slowing the vehicle. As such this
form of braking is often referred to as regenerative braking.
SUMMARY
[0005] A method of operating a brake-retract system during a
regenerative braking event begins with monitoring an amount of
regenerative braking achieved. A set point threshold is linearly
ramped from a first threshold to a max regeneration capacity if the
amount of regenerative braking achieved exceeds a second threshold.
The second threshold is less than the first threshold, and the
first threshold is less than the max regeneration capacity. The max
regeneration capacity is representative of a maximum amount of
regenerative braking capable of being produced.
[0006] A friction element is transitioned from a first,
retracted-state to a second, ready-state if the amount of
regenerative braking achieved exceeds the set point threshold. The
friction elements are closer to a brake rotor in the second,
ready-state than in the first, retracted-state.
[0007] In further embodiments, the method may additionally include
transitioning the friction element from the first, retracted-state
to the second, ready-state if a braking request is received from a
separate vehicle system. Likewise, the method may include
transitioning the friction element from the first, retracted-state
to the second, ready-state if a vehicle speed is less than a
predetermined threshold.
[0008] If the amount of regenerative braking achieved falls below
the first threshold and the friction element is in the second,
ready-state the set point threshold may be linearly ramped toward
the first threshold. Additionally, the friction element may then be
transitioned from the second, ready-state to the first,
retracted-state.
[0009] The present method may be performed by a controller
associated with a vehicle. The vehicle can include a vehicle
traction battery, a motor/generator, a plurality of wheels, and a
friction braking mechanism in communication with each of the
plurality of vehicle wheels. The motor/generator is in power-flow
communication with the plurality of vehicle wheels and is in
electrical communication with the vehicle traction battery. The
motor/generator is configured to perform regenerative braking such
that a torque is received from the plurality of vehicle wheels and
converted into an electrical energy that is provided to the vehicle
traction battery.
[0010] Each friction braking mechanism includes a movable friction
element and a rotor. The rotor is configured to rotate with the
wheel, and the movable friction element is configured to
selectively apply a contact pressure to the rotor. Each friction
element is configured to translate between a first, retracted-state
and a second, ready-state, and wherein the friction elements are
closer to a brake rotor in the second, ready-state than in the
first, retracted-state.
[0011] The above features and advantages and other features and
advantages of the present invention are readily apparent from the
following detailed description of the best modes for carrying out
the invention when taken in connection with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a schematic diagram of a hybrid electric
vehicle.
[0013] FIG. 2 is a schematic isometric view of a vehicle wheel and
friction braking mechanism.
[0014] FIG. 3 is a schematic flow diagram of a regenerative braking
brake-retract algorithm.
[0015] FIG. 4 is a schematic illustration of a regenerative braking
graph including a brake-retract state.
DETAILED DESCRIPTION
[0016] Referring to the drawings, wherein like reference numerals
are used to identify like or identical components in the various
views, FIG. 1 schematically illustrates a vehicle 10, such as an
automobile, that includes a motor/generator 12 and an energy
storage system, such as a traction battery 14. While only one
motor/generator 12 is shown for simplicity, multiple
motor/generators may be used depending on the design. The vehicle
10 may be configured as a hybrid electric vehicle (HEV), a battery
electric vehicle (BEV), an extended-range electric vehicle (EREV),
or the like. Such vehicles can generate torque using the
motor/generator 12 at levels suitable for propelling the vehicle in
an electric-only (EV) mode. Alternatively, or in addition, the
motor/generator 12 may operate to varying degrees with an internal
combustion engine 16 for the purpose of propelling the vehicle 10.
As may be appreciated, the traction battery 14 (or simply "battery
14") may include one or more battery cells of any suitable
construction and/or composition. The battery 14 may be capable of
discharging high voltage electrical energy as a motive power source
for the vehicle 10, and storing high voltage electrical energy
provided from an outside source.
[0017] In one configuration, an internal combustion engine 16,
shown in phantom in FIG. 1, may be used to generate torque via an
engine output shaft 18. Torque from the engine output shaft 18 can
be used to either directly propel the vehicle 10, i.e., in an HEV
design, or to power a generator 20, i.e., in an EREV design. The
generator 20 can deliver electricity (arrow 22) to the battery 14
to recharge the battery 14. A clutch and/or damping assembly 24 may
be used to selectively connect/disconnect the engine 16 from a
transmission 26. Torque is ultimately transmitted from the
motor/generator 12 and/or the engine 16 to a set of drive wheels 28
via an output member 30 of the transmission 26. While FIG. 1 only
illustrates two drive wheels 28, it should be understood that the
vehicle may have, for example, two or more drive wheels, and/or up
to two or more passive, non-drive wheels. The specific power flow
configuration illustrated in FIG. 1 is intended to be generally
illustrative of one embodiment of a vehicle that employs electric
propulsion, and should not be viewed as limiting.
[0018] The motor/generator 12 may be embodied as a multi-phase
permanent magnet/AC induction machine rated for approximately 60
volts to approximately 300 volts or more depending on the vehicle
design. The motor/generator 12 is electrically connected to the
battery 14 via a power inverter module (PIM) 32 and a high-voltage
bus bar 34. The PIM 32 is configured to convert high voltage DC
power into three phase AC power and vice versa as needed. The
battery 14 may be selectively recharged via the motor/generator 12
when the motor/generator is actively operating as generator 20,
e.g., by capturing energy during a regenerative braking ("regen")
event.
[0019] An engine control unit (ECU) 36 may be in electrical
communication with each of the battery 14, PIM 32, and
motor/generator 12 and may be configured to monitor and control
their respective performance. The ECU 36 may be embodied as one or
multiple digital computers or data processing devices, having one
or more microcontrollers or central processing units (CPU), read
only memory (ROM), random access memory (RAM),
electrically-erasable programmable read only memory (EEPROM), a
high-speed clock, analog-to-digital (A/D) circuitry,
digital-to-analog (D/A) circuitry, input/output (I/O) circuitry,
and/or signal conditioning and buffering electronics. In practice,
the ECU 36 illustrated in FIG. 1 may be a generalized controller
that may include various sub-modules such as an engine control
module (ECM) hybrid control module (HCM), and/or transmission
control module (TCM). The ECU 36 may be configured to automatically
perform one or more control/processing routines that may be
embodied as software or firmware, and may either be stored locally
on the ECU 36, or may be stored in a device that is readily
accessible by the ECU 36.
[0020] As shown in FIG. 2, the vehicle 10 may further include a
vehicle braking system 40 that is in mechanical communication with
two or more wheels of the vehicle and is configured to selectively
decelerate the vehicle. The vehicle braking system 40 includes one
or more braking mechanisms 42 disposed at each respective wheel.
Each braking mechanism 42 may be configured as either a disc brake
(shown in FIG. 2) or a drum brake (not shown, but understood by
those skilled in the art). Each friction braking mechanism 42
includes a rotor 44 configured for synchronous rotation with the
respective wheel 28, and an actuator 46 configured to translate a
friction element 48, such as a brake pad, into contact with the
rotor 44. In one configuration, the actuator 46 may be
hydraulically actuated, such that a volume of a positively or
negatively pressurized fluid acts as the basis for translating the
friction element 48 and/or creating a contact pressure between the
friction element 48 and the rotor 44. The actuator force is
generally controlled by an operator of the vehicle 10 via an
application of the brake pedal, though it may also be controlled by
an electronic brake controller.
[0021] During operation, the braking system 40 may be configured to
operate in one of three states: brake-apply; brake-un-apply; and
brake-retract. Brake-apply is the situation where the friction
elements 48 are increasing pressure against the rotor 44 to
increase the friction force, and decelerate the vehicle.
Conversely, brake-un-apply is the situation where the friction
elements 48 are decreasing pressure against the rotor 44, thus
resulting in a decreasing friction force. Brake-apply is most
commonly experienced when a user is applying pressure to a brake
pedal, and brake-un-apply results when the user releases pressure
from the brake pedal.
[0022] To decrease the braking response time while waiting for a
brake-apply condition, even when the brake pedal is not pressed by
the user, the friction elements 48 (i.e., "brake pads 48") are
typically positioned in close proximity and/or in light contact
with the brake rotor 44. While this positioning decreases brake
response time, it may also result in an incidental frictional force
being created against the rotor 44 (often referred to as driveline
drag). During the brake-retract state, the brake pads 48 are
configured to lift off the rotor 44 to a more distant position.
Said another way, during the brake retract state, the brake pads 48
translate from a first position in contact with the brake rotor 44
to a second position that is more distant from the brake rotor 44.
In doing so, the braking system 40 reduces the rolling resistance
of the vehicle that is attributable to the brake pads 48. In a
hydraulically actuated system, the brake pads 48 are generally
maintained in the first position (i.e., including brake-apply and
brake-un-apply states) by supplying positively pressurized fluid to
the actuator 50. Conversely, in such a system, the brake pads 48
are maintained in the second position by supplying a negatively
pressurized fluid to the actuator 46.
[0023] In one configuration, the brake-retract state may be
initiated under the direction of the ECU 36 during regenerative
braking in an effort to maximize the potential energy capture
during the regenerative braking ("regen") event (i.e., where the
motor/generator 12 is driven as a generator to both slow the
vehicle and recharge the battery 14). As such, during regenerative
braking, the ECU 36 is configured to execute a corresponding regen
brake-retract algorithm 80, such as shown in FIG. 3.
[0024] Referring to FIG. 3, the algorithm 80 begins at 82 with the
brake pads 48 in a retracted state (B.sub.r=1). The algorithm 80
assumes that brake retraction (B.sub.r) is desirable under most
driving situations to reduce driveline drag, unless it determines
otherwise and disables brake retraction at 84 (B.sub.r=0).
"Disabling" the brake retraction at 84 involves transitioning the
brake pads 48 from the retracted, more distant position from the
rotor 44, back to the position that is more proximate to and/or in
contact with the rotor 44.
[0025] The algorithm 80 may begin by determining at 86 if brake
retraction should be disabled at the request of a separate vehicle
system. Said another way, if another vehicle system indicates that
it may require the use of friction brakes, the ECU 36 may prevent
the brakes from being retracted. Examples of vehicle system
requests (VSR) that may disable the brake retraction include
requests from antilock braking systems, vehicle stability control
systems, and communication systems (i.e., in the event of a fault
code). While this determination is shown as a discrete step, it may
also operate as an interrupt, where an appropriate interrupt
handler can override other aspects of the algorithm to disable
brake retraction.
[0026] Brake retraction may also be disabled at 88 if the vehicle
speed (VS) falls below a threshold vehicle speed where regeneration
becomes unavailable (VS.sub.min). In one example, this threshold
vehicle speed may be about 2.0 mph. Furthermore, at 90, brake
retraction may be disabled at 86 if the state of charge (SOC) of
the battery 14 is over a particular threshold (SOC.sub.T) where
further charging would negatively affect battery performance. Said
another way, brake retraction may be disabled at 84 if the SOC of
the battery 14 is within a particular tolerance of a maximum
SOC.
[0027] The algorithm 80 may be further configured to determine if
brake retraction should be disabled according to the amount of
regenerative braking that is actually being achieved at 92. The
disablement scheme shown at 92 may be best illustrated with
reference to a regenerative braking graph 120, shown in FIG. 4.
This graph 120 generally illustrates instantaneous regen with
respect to time (t) 122, as well as the brake retraction state
(B.sub.r) 124 with respect to time 122. As shown, instantaneous
regen can be measured as a regen-torque 126 (.tau.) that is
received by the motor/generator 12. For the purpose of this
description, the sign of the regen-torque 126 (.tau.) is viewed
from the point of view of the motor/generator 12, where a positive
regen-torque 126 is torque received by the motor/generator 12 and
converted into stored energy. Correspondingly, a negative
regen-torque would be a torque that is generated by the
motor/generator 12 (i.e. acting as a motor) from the stored energy.
In practice, this torque may be an instantaneous measurement (or
computed value) at the motor output shaft, at an output shaft of a
mechanically linked transmission, or at other such places along the
driveline.
[0028] As mentioned above, the brake retraction state 124 has two
states: a first state 128 (B.sub.r=0), where the brake pads 48 are
positioned proximate to/in contact with the brake rotor 44; and a
second state 130 (B.sub.r=1), where the brake pads 48 are
retracted/more distant from the brake rotor 44 (i.e., B.sub.r=1
corresponds to brake-retract-enabled, and B.sub.r=0 corresponds to
brake-retract-disabled).
[0029] On the regen graph 120, regen-achieved (R.sub.a) 132
illustrates the instantaneous amount of regen-torque 126 converted
by the motor/generator 12, and max-regen (R.sub.max) 134 is the
maximum torque-absorbing capacity of the regenerative braking
system. R.sub.max 134 may vary according to various operating
parameters of the vehicle, but ultimately represents the maximum
amount of regen-torque 126 that is capable of being
absorbed/converted by the motor/generator 12 at a given
instant.
[0030] The graph 120 further illustrates a first threshold
(T.sub.1) 136 and a second threshold (T.sub.2) 138, where (T.sub.1)
136 is less than R.sub.max 134 by a first offset 140, and (T.sub.2)
138 is less than (T.sub.1) 136 by a second offset 142.
Additionally, the graph 120 illustrates a Controller Set Point
(CSP) 144 that may vary between (T.sub.1) 136 and (R.sub.max)
134.
[0031] The graph 120 generally illustrates a regenerative braking
event, where all, or nearly all of the vehicle braking torque is
being supplied by the motor/generator 12. The graph 120 illustrates
a first period of time 150 where a regenerative vehicle braking
force is increasing (i.e., regen-achieved (R.sub.a) 132 is
increasing), and a second period of time 152 where a regenerative
vehicle braking force is decreasing (i.e., regen-achieved (R.sub.a)
132 is decreasing). These periods of time 150, 152 may correspond
to, for example, a user applying an increasing amount of pressure
to a brake pedal (in period 150), and subsequently decreasing the
amount of pressure to the brake pedal (in period 152). In this
example, because regen-achieved (R.sub.a) 132 is still less than
R.sub.max 134, the motor/generator 12 is fully capable of supplying
the entirety of the requested vehicle braking torque, and even if
readied, the friction brakes may not be actively engaged.
[0032] With reference to FIG. 3, the disablement scheme 92 may
begin by determining at 94 if the regen-achieved (R.sub.a) 132
torque exceeds T.sub.2 138. This second threshold T.sub.2 138 may
serve as an advanced warning/warning-track of a braking event that
is nearing the capacity of the regenerative braking system
(R.sub.max) 134. If R.sub.a<T.sub.2, the brake pads 48 may
remain in a retracted state at 82. If R.sub.a>T.sub.2, such as
at 154 in FIG. 4, the Controller Set Point (CSP) 144 may begin
linearly ramping at 96 from T.sub.1 136 toward R.sub.max 134 (shown
graphically at 156 in FIG. 4).
[0033] At 98, the algorithm 80 may determine if either R.sub.a 132
is greater than CSP 144, or if the brake retract is disabled
(B.sub.r=0) (i.e., the brake pads are positioned adjacent to the
rotors) and R.sub.a 132 is greater than T.sub.1 136. If either
condition answers in the affirmative then brake retraction should
be disabled at 84. Referring to FIG. 4, the first condition
(R.sub.a 132 is greater than CSP 144) occurs in the time period
shown at 158, and the second condition (brakes are in the first
state 128 (B.sub.r=0) and Ra 132 is greater than T.sub.1 136)
occurs in the time period shown at 160 (i.e., until T.sub.1 136 is
crossed).
[0034] If neither condition at 98 answers in the affirmative, then
the algorithm 80 then inquires at 100 whether brake retraction is
disabled (B.sub.r=0) and R.sub.a 132 is less than T.sub.1 136, such
as at 162. If either condition is not true, then the algorithm 80
continues monitoring at 94 to determine if R.sub.a 132 is still
greater than T.sub.2 138. If it is, CSP 144 continues to ramp at
96, such as at 164. If, however, brake retraction is disabled and
R.sub.a 132 is less than T.sub.1 136 (such as at 166), then the
algorithm 80 causes CSP 144 to begin linearly ramping (at 102)
toward T.sub.1 136 (such as at 168), and causes the brakes to
retract away from the rotors 44 (B.sub.r=1) at 84.
[0035] In operation, CSP 144 serves as the trigger to cause the
brakes to transition into a ready-state against the rotors 44.
Because it takes a certain amount of time to effectuate the
transition, the brakes should begin to transition earlier under an
aggressive braking condition than under a smoother braking
condition. Said another way, CSP 144 may be spaced from R.sub.max
134 by a given regen-capacity buffer. This buffer may remain at
certain size until R.sub.a 132 exceeds the second "warning track"
threshold T.sub.2 138, at which point it may begin decreasing
toward zero. In an aggressive braking condition, where R.sub.a 132
is increasing rapidly, it is more likely that R.sub.a 132 will
exceed CSP 144 when the buffer is at or near its largest size. This
will provide the brakes with advanced notice to move toward the
rotors 44 prior to maxing out the regen braking torque at R.sub.max
134. In a comparatively less aggressive braking condition (i.e.,
where R.sub.a 132 is increasing with a smaller slope), it is more
likely that R.sub.a 132 will exceed CSP 144 when the buffer is at a
comparatively smaller size. This, however, would likely still
provide sufficient time for the brakes to transition, as the slope
of R.sub.a 132 is smaller.
[0036] While the best modes for carrying out the invention have
been described in detail, those familiar with the art to which this
invention relates will recognize various alternative designs and
embodiments for practicing the invention within the scope of the
appended claims. It is intended that all matter contained in the
above description or shown in the accompanying drawings shall be
interpreted as illustrative only and not as limiting.
* * * * *