Brake-by-wire system having conditioned brake boost termination at key off

An electric powered brake system (10) for braking road-engaging wheels of a motor vehicle. An ignition switch for turning a motor of the vehicle on and off also controls the application of electric power to the electric powered brake system. A control and method (FIGS. 2 and 3) for maintaining application of full electric power to the electric powered brake system when the ignition switch is operated from ON to OFF so long as at least one parameter (60, 62, 64, 66) related to a respective component of the vehicle and indicative of a need to maintain full electric power application to the brake system continues to exist and for discontinuing application of full electric power to the brake system when all of selected ones of several such parameters are indicative of lack of need to maintain full electric power application to the brake system.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 This invention relates generally to brake systems for wheeled motor
 vehicles, and more specifically it relates to a brake-by-wire system.
 2. Background Information
 A brake-by-wire system may comprise a pedal that depresses a master
 cylinder plunger to perform the service brake function for a motor
 vehicle. The service brake function is performed by selectively applying
 electrically boosted hydraulic brakes or regenerative brakes. Hydraulic
 brakes generally apply friction forces to the wheels to dissipate energy
 as heat. Regenerative brakes may take any of various forms, but are often
 considered to comprise a rotary electric machine load that is applied to
 one or more driven wheels. When a regenerative brake is applied, energy
 flows from the wheels to operate the electric machine as a generator that
 converts the energy into electricity. The electricity is often used to
 charge an energy storage device, such as a battery. Whether hydraulic or
 regenerative brakes are applied is determined by an electronic brake
 controller.
 The ability to operate a motor vehicle is typically controlled by a
 key-operated switch, sometimes referred to as an ignition switch. Such
 switches may assume various forms, but perhaps the most common in present
 motor vehicles is a mechanical switch that is turned by a metal key. When
 a proper key is used, the switch may operated from an OFF position to
 additional positions, such as START, RUN (i.e. ON), and ACCESSORY, to
 deliver electric power to electric circuits and devices that perform
 certain functions related to vehicle starting and operation.
 Certain electric circuits need to be energized for a brake-by-wire system
 to properly operate. Placing the energization of such circuits under the
 control of the ignition switch is desirable so that they do not
 needlessly, drain the battery when the ignition switch is OFF. However,
 turning the ignition switch off to de-energize those circuits may remove
 power from devices that when powered, aid in reducing the pedal effort
 required to apply the brakes via the master cylinder. Hence, brake pedal
 effort may increase significantly when the ignition switch is turned off.
 Governmental regulation applicable to certain motor vehicles proposes to
 mandate certain consistency in brake pedal effort that is independent of
 whether the ignition switch is OFF or ON. In the case of a brake system
 that comprises only boosted hydraulic brakes, a vacuum reservoir is used
 to provide a vacuum reserve that allows the booster to aid the pedal
 effort for several brake applications after the ignition switch is turned
 off to stop the engine. In the case of certain brake-by-wire systems, the
 de-energization of associated electric circuits that occurs when the
 ignition switch is turned off, precludes continuance of the ability to aid
 the pedal effort that was present when the ignition switch was on.
 A preliminary novelty search developed the following U.S. Pat. Nos.:
 5,139,121; 5,752,748; 5,769,509; 5,800,025; 5,834,854; and 5,923,096.
 SUMMARY OF THE INVENTION
 The present invention relates to a strategy that is believe suitable for
 accomplishing the intent of the aforementioned regulatory proposal in a
 brake-by-wire system.
 One general aspect of the invention relates to a motor vehicle comprising
 an electric powered brake system for braking road-engaging wheels of the
 motor vehicle. The vehicle has an ignition switch for turning the motor on
 and off, including controlling the application of electric power to the
 electric powered brake system. A control maintains application of full
 electric power to the electric powered brake system when the ignition
 switch is operated from ON to OFF so long as at least one parameter
 related to a respective component of the vehicle and indicative of a need
 to maintain full electric power application to the brake system continues
 to exist. The control discontinues application of full electric power to
 the brake system when all of selected ones of several such parameters are
 indicative of lack of need to maintain full electric power application to
 the brake system.
 Another general aspect of the invention relates to a method for use in a
 motor vehicle that has an electric powered brake system for braking
 road-engaging wheels of the vehicle. The vehicle has an ignition switch
 for turning the motor on and off, including controlling the application of
 electric power to the electric powered brake system. A control controls
 the application of electric power to the brake system and functions to
 maintain application of full electric power to the electric powered brake
 system when the ignition switch is operated from ON to OFF so long as at
 least one parameter related to a respective component of the vehicle and
 indicative of a need to maintain full electric power application to the
 brake system continues to exist. The control also discontinues application
 of full electric power to the brake system when all of selected ones of
 several such parameters are indicative of lack of need to maintain full
 electric power application to the brake system.
 Further aspects will be seen in various features of two presently preferred
 embodiments of the invention that will be described in detail.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
 FIG. 1 shows a representative brake-by-wire system comprising a hydraulic
 brake master cylinder 12 having a plunger 14 which is adapted to be
 depressed by operation of a brake pedal 16 to brake road-engaging wheels
 of a motor vehicle. Braking may be accomplished either by hydraulic brakes
 at individual wheels, such as 18, 20, 22, 24, or by regenerative braking,
 as determined by an electronic brake controller 26. Hydraulic braking is
 applied through an ABS unit 28. Boost for aiding hydraulic braking so as
 to reduce pedal effort that would otherwise be required is derived from a
 hydraulic power source 30 whose pressure is modulated by a pressure
 modulation unit 32 under the control of brake controller 26. For
 developing the proper modulation, controller 26 senses brake pedal input
 with the use of one or more of the following: stroke position sensor;
 master cylinder pressure sensor; or any other sensor that may be used to
 indicate pedal input.
 FIG. 1 includes a back-up hydraulic circuit path from master cylinder 12 to
 ABS unit 28 through a switching valve unit 34 that allows hydraulic brakes
 to alternatively be applied directly by hydraulic pressure from master
 cylinder 12. The regenerative braking system does not expressly appear in
 FIG. 1.
 The delivery of electric power for operating brake-by-wire system 10 is
 controlled by the vehicle ignition switch, not specifically shown in FIG.
 1. When that switch is OFF, power is not delivered; when the switch is ON,
 power is delivered. When the power is off, hydraulic power source 30 is
 unable to deliver the brake boost that it does when the power is on.
 Hence, when the ignition switch is turned from ON to OFF to shut down the
 vehicle powertrain (engine and/or motor), brake boost is also lost.
 The present invention provides continued power to system 10, and hence
 continued brake boost, after the ignition switch is turned off, based on
 the non-occurrence of one or more particular events involving the vehicle.
 When all such events have occurred, power is removed from system 10 to
 avoid battery drain. The events are monitored by monitoring certain
 parameters related to positions and/or conditions of certain components of
 the vehicle.
 FIG. 2 shows an exemplary strategy that implements the inventive principles
 in a processor of the vehicle. The strategy involves monitoring for the
 ignition switch to be operated to ON (step 50). When the ignition switch
 is ON electric power is applied to system 10 (step 52).
 Thereafter the strategy monitors for the ignition switch to be operated to
 OFF (step 54). When the ignition switch is OFF, a timer starts (step 56).
 Power continues to be applied to system 10 based on the non-occurrence of
 the following events: all vehicle wheels not turning; brake pedal not
 applied; vehicle transmission not in K position; parking brake applied.
 Hence step 60 monitors for all wheels not turning; step 62, for pedal 16
 not being applied; step 64, for transmission in K; and
 step 66 for parking brake applied. It can be seen that only after all these
 events have occurred is the elapsed time on the timer checked (step 68).
 If the elapsed time does not exceed a defined minimum (X minutes, for
 example), steps 60, 62, 64, and 66 continue to be executed.
 When the events monitored by steps 60, 62, 64, and 66 have all occurred,
 and the elapsed time exceeds the defined minimum, power is removed from
 system 10. That concludes the strategy. The monitored events are
 considered to be indicative that there is no need for the service brakes
 to be applied, and hence that it is appropriate to de-energize system 10.
 Individual wheel speeds may determined by respective wheel speed sensors;
 non-application of brake pedal 12, from an associated sensor or switch;
 transmission in K from an associated transmission, or gear selector,
 switch; and parking brake applied from an associated sensor or switch. A
 representative defined time for X is 10 minutes.
 FIG. 3 shows a second embodiment of strategy that utilizes the steps of the
 first embodiment, but further includes a series of steps that provide for
 a reduced power sleep mode before power is completely removed from system
 10. The timing step 56 does not commence until after the events monitored
 by steps 60, 62, 64, and 66 have all occurred. Power, but at a reduced
 level, continues to be applied to system 10 after the monitored events
 have all occurred and the sleep mode commences. The monitored events, and
 the state of the ignition switch, continue to be monitored during the
 sleep mode (steps 72, 74, 76, 78, 80), and if there is a change in any one
 of those events or the ignition switch before the defined time X has
 elapsed, full power is promptly re-applied to system 10, and the strategy
 starts anew at step 54. It is only after there is no change in any
 monitored event (steps 72, 74, 76, 78) or in the ignition switch (step 80)
 after an elapsed time X, measured in hours rather than minutes, 10 hours
 for example, that power is completely disconnected from system 10. The
 sleep mode may provide for more quickly powering up system 10.
 From the foregoing, one can understand that the present invention
 conditions the termination of brake boost at key off. It is believed that
 the present invention can enhance the effectiveness and acceptability of
 electrically-powered brake-by-wire systems.
 While a presently preferred embodiment has been illustrated and described,
 it is to be appreciated that the invention may be practiced in various
 forms within the scope of the following claims. Examples of such
 additional forms include the non-use of certain events, or parameters,
 already specifically mentioned above and/or the use of certain events, or
 parameters, not previously mentioned above, including occupant restraint
 engagement (i.e. seat belt engagement), driver position, steering input,
 accelerator pedal input, and clutch input. Furthermore, monitoring of
 various events, or parameters, may occur serially, as described above, or
 in parallel, or in various serial and parallel combinations.