DE102009033953B4 - Vehicle cruise control using vehicle brakes in a cruise control mode - Google Patents

Abstract

A cruise control system comprising:a speed difference module (76) that compares an actual vehicle speed (Vs) to a target cruise speed (Vt); anda brake control module (72) that selectively actuates a brake (34) when the actual vehicle speed (Vs) is greater than the target travel speed (Vt) by a predetermined speed difference (Vdiff); characterized in thatthe brake control module (72) applies the brake (34) only when a transmission (14) can no longer be shifted into a lower gear without overspeeding an engine (20) and when no other control modules can be used, to reduce vehicle speed.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to US Provisional Application Serial No. 61/082,859 filed July 23, 2008. The entire disclosure of the above application is incorporated herein by reference.

AREA

The present disclosure relates to motor vehicles and, more particularly, to control systems and methods for controlling vehicle speed.

BACKGROUND

The background description provided herein is for the purpose of generally presenting the context of the disclosure. The work of the presently named inventors, to the extent that it is described in this background section, and aspects of the specification not otherwise identified as prior art at the time of filing are not admitted, either explicitly or implicitly, as prior art to the present disclosure.

Motor vehicles may include a powertrain consisting of a prime mover (e.g., an engine, an electric motor, and/or a combination thereof), a multi-speed transmission, and a differential or final drive. The prime mover produces drive torque that is transmitted to the driveline through one of a plurality of gear ratios of the transmission to drive wheels of the vehicle.

Operation of the powertrain may be regulated by one or more control modules, such as a powertrain control module, based on driver inputs. In particular, the powertrain control module may receive one or more signals from driver interface devices, such as an accelerator pedal, that are manipulated by the driver. The powertrain control module may regulate operation of the powertrain to produce a desired drive torque based on the signals it receives. The powertrain control module may also regulate operation of the powertrain to maintain a target ground speed, such as when the vehicle is equipped with a cruise control system.

Vehicles equipped with a cruise control system typically include a cruise control switch that allows the driver to specify the target vehicle speed. With such systems, the driver can specify the target running speed by operating the switch while driving at the target running speed. In turn, the cruise control switch may generate a signal that may be used in conjunction with a wheel speed signal by the powertrain control module to determine the target cruise speed. The wheel speed signal may be generated using one or more wheel speed sensors that may be included in the vehicle.

In cruise control mode, the powertrain control module may regulate the powertrain to maintain an actual vehicle speed within a target range of the target cruise speed. Actual vehicle speed depends on several factors such as engine drive torque and road grade. The powertrain control module may typically control the powertrain to maintain the actual vehicle speed within the target speed range by increasing or decreasing engine-produced drive torque. However, on a downhill grade, the vehicle's actual speed can continuously increase without the aid of drive torque produced by the powertrain. In addition, the actual vehicle speed may exceed an upper threshold speed of the target speed range.

In this case, the powertrain control module may control the powertrain to produce negative drive torque that may be used to decelerate the vehicle and thereby bring the actual vehicle speed within the target speed range. For example, the powertrain control module may control an engine included in the powerplant in a deceleration enleanment mode. In deceleration lean mode, the powertrain control module may limit fueling to the engine, thereby causing the engine to produce negative drive torque.

In a hybrid power plant, the control module may similarly regulate an electric motor included in the hybrid power plant into a regeneration mode. In the regeneration mode, the final drive's rotational energy is used by the moving Vehicle is generated, used to drive the electric motor. The powertrain control module may also control the powertrain into a powertrain braking mode. In the powertrain braking mode, the powertrain control module may control the transmission to shift to a lower gear, thereby increasing the speed of the powerplant, thereby increasing the negative torque produced by the powerplant.

However, even when the powertrain control module is operating in one or more of the above modes, it may not be able to prevent the actual vehicle speed from exceeding the upper speed threshold of the target speed range.

the DE 695 08 778 T2 discloses a brake control system for controlling the descent of a vehicle on slopes which, when activated, applies the brakes of the vehicle when an actual vehicle speed exceeds a target ground speed. Depressing the accelerator pedal increases the target vehicle speed, while depressing the brake pedal overrides the brake control system.

WO 2007/141631 A2 discloses a system and method for controlling a vehicle in which a driving force is maintained at a predetermined value in a cruise control mode even when an actual vehicle speed exceeds a target vehicle speed during downhill travel by which To be able to maintain the target driving speed during a subsequent upward drive. The brakes are applied to maintain the target driving speed when driving downhill.

the DE 101 18 708 A1 discloses a method for controlling the speed of a motor vehicle in which an acceleration request signal is generated which represents a positive or negative target acceleration of the vehicle. Depending on this signal, either a control command is sent to the engine or a control command is sent to a brake system of the vehicle.

In the DE 60 2004 007 805 T2 discloses a method for optimizing a braking sequence in vehicle adaptive cruise control (ACC) by simulating various combinations of gear shift engine speed, gear shift increments, and braking forces and selecting a braking sequence that minimizes service brake wear.

SUMMARY

Accordingly, the present disclosure provides an example control system and method for maintaining a vehicle speed within a target speed range while operating in a cruise control mode. In one aspect, the present disclosure provides a cruise control system that includes a speed difference module that compares an actual vehicle speed to a target speed, and a brake module that selectively applies a brake when the actual vehicle speed is greater than the target speed by a predetermined speed difference . In one feature, the brake module applies the brake while the actual vehicle speed is greater than the target travel speed. In another feature, the brake module selectively applies the brake based on negative input torque from a power plant.

In another aspect, the present disclosure provides a cruise control system that includes a speed difference module that determines a target braking force based on a comparison of an actual vehicle speed to a target vehicle speed, and a braking module that selectively actuates a brake to generate an actual braking force. which is less than or equal to the target braking force when the actual vehicle speed is greater than the target vehicle speed by a predetermined speed difference. In one feature, the speed difference module determines the target braking force based on a negative input torque of a power-plant. In another feature, the speed difference module determines the desired braking effort using a closed loop feedback routine.

In yet another aspect, the present disclosure provides a method for controlling vehicle speed in a cruise control mode. The method includes comparing an actual vehicle speed to a target travel speed and selectively applying a brake when the actual vehicle speed is greater than the target travel speed by a predetermined speed difference. In one feature, selectively applying the brake includes applying the brake while the actual vehicle speed is greater than the target travel speed. In another feature, selectively actuating the brake further includes selectively actuating the brake based on negative drive torque of a power-plant.

In yet another feature, the method further includes determining a target braking force based on the comparison of the actual vehicle speed and the target travel speed, and applying the brake to produce an actual braking force that is less than or equal to the target braking force. In a related feature, determining the target braking force may further include determining the target braking force based on a negative drive torque of a power-plant. In another related feature, determining the desired braking force includes determining the desired braking force using a closed-loop feedback routine.

Further areas of applicability of the present disclosure will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the disclosure.

character list

• 1 ein Funktionsblockdiagramm ist, das ein beispielhaftes Fahrzeugsystem gemäß den Prinzipien der vorliegenden Offenbarung veranschaulicht;
• 2 ein Funktionsblockdiagramm ist, das ein beispielhaftes Fahrgeschwindigkeitsregelungssystem gemäß den Prinzipien der vorliegenden Offenbarung veranschaulicht; und
• 3 ein Flussdiagramm ist, das beispielhafte Schritte für ein Fahrgeschwindigkeitsregelungsverfahren gemäß den Prinzipien der vorliegenden Offenbarung veranschaulicht.

The present disclosure will be better understood from the detailed description and the accompanying drawings, in which:

• 1 Figure 12 is a functional block diagram illustrating an exemplary vehicle system according to the principles of the present disclosure;
• 2 Figure 12 is a functional block diagram illustrating an exemplary cruise control system according to the principles of the present disclosure; and
• 3 14 is a flow chart illustrating exemplary steps for a cruise control method according to the principles of the present disclosure.

PRECISE DESCRIPTION

The following description is merely exemplary in nature and is in no way intended to limit the disclosure, its application, or uses. For purposes of clarity, the same reference numbers will be used throughout the drawings to identify similar elements. As used herein, the phrase at least one of A, B, and C should be construed to mean a logical (A or B or C) using a non-exclusive logical or. It is understood that steps in a method may be performed in a different order without altering the principles of the present disclosure.

As used herein, the term module means an application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or cluster) and memory executing one or more software or firmware programs, a combinational logic circuit and/or or other suitable components that provide the described functionality.

A vehicle traveling on surfaces such as a road may hit on a down slope. The downward incline may cause the vehicle to gain speed. In a cruise control mode, the downward incline may cause the vehicle speed to exceed a target speed range. Accordingly, the present disclosure provides an example control system and method for controlling vehicle brakes in cruise control mode to maintain vehicle speed within the target speed range. In this way, the control system and method can be used to limit vehicle speed in cruise control mode while traveling on downhill grades.

With special reference to 1 1, a functional block diagram illustrating an exemplary vehicle system 10 according to the principles of the present disclosure is shown. The vehicle system 10 includes a power plant 12 that produces drive torque that can be used to accelerate the vehicle and maintain a desired vehicle speed. Drive torque may be transmitted through a transmission 14 at variable gear ratios to a driveline 16 to drive at least one or more wheels 18 of the vehicle. The powerplant 12 may also be used to decelerate the vehicle by allowing the wheels 18 to drive the powerplant 12 in reverse via the transmission 14 and driveline 16 .

The powerplant 12 may be a hybrid powerplant including an internal combustion engine 20 and a hybrid propulsion system 22 as shown. The hybrid propulsion system 22 may be one of several types and may be a belt driven alternator/starter (BAS) system. Accordingly, the hybrid powertrain system 22 may include a machine accessory drive 24 , an electric motor/generator 26 , and a power pack assembly 28 . The engine accessory drive 24 serves to transmit torque between the engine 20 and the electric motor/generator 26. In a drive mode, the engine/ Generator 26 for providing drive torque to engine 20 while receiving electrical power from power pack assembly 28 . In a regeneration mode, the motor/generator 26 is operative to provide electrical charge to power the power pack assembly 28 while being driven by the engine 20 . The power pack assembly 28 includes a battery (not shown) for storing energy provided by the motor/generator 26 and for supplying the energy to various components of the vehicle system 10 including the motor/generator 26.

The vehicle system 10 further includes a braking system 30 that can be used to generate a braking force to decelerate and thereby stop the vehicle. The braking system 30 is connected to the wheels 18 and is operable to selectively apply a desired braking torque to the wheels 18 to slow the vehicle. The braking system 30 may include a brake modulator 32 connected to brakes 34 located at each of the wheels 18 . The brake modulator 32 can be used to selectively apply the brakes 34 and thereby distribute desired braking torque to the wheels 18, if desired. The brake modulator 32 may distribute the desired braking torque to front and rear wheels of the vehicle, for example. The brake modulator can also distribute the target braking torque to the individual wheels 18 of the vehicle.

Operation of the vehicle system 10 including the powerplant 12, transmission 14, and braking system 30 may be controlled by one or more control modules, such as a powertrain control module (PCM) 40 and a chassis control module 42, as shown. In this manner, the PCM 40 may be used to regulate drive torque produced by the powerplant 12 and transmitted through the transmission 14 . Similarly, the chassis control module 42 can be used to regulate the braking force generated by the braking system 30 . Further, as will be described in more detail below, the PCM 40 and chassis control module 42 may work together to maintain vehicle speed within a desired speed range of a target vehicle speed during a cruise control mode of the vehicle system 10 . In particular, the PCM 40 and the chassis control module 42 may work together to limit vehicle speed while the vehicle is traveling on a downgrade.

With continued reference to 1 For example, the PCM 40 and chassis control module 42 may control the components of the vehicle system 10 based on signals they receive from various devices and sensors that may be included in the vehicle system 10 . For example, the PCM 40 and chassis control module 42 may receive signals from driver interface devices 44 operated by a driver of the vehicle.

Driver interface devices 44 may include, but are not limited to, devices such as an accelerator pedal 50 , a brake pedal 52 , and a cruise control switch 54 . Accelerator pedal 50 may generate an accelerator pedal signal (not shown) that may be used to determine desired drive torque. The brake pedal 52 may generate a signal (BRAKEsw) that may be used to determine the desired braking torque. The cruise control switch 54 may generate a cruise control switch signal (CRUISE _Sw ) that indicates whether the driver has requested that the vehicle system 10 operate in the cruise control mode.

The PCM 40 and chassis control module 42 may also receive signals from various sensors included in the powerplant 12 and braking system 30 . Sensors 56 included in the power-plant 12 may be used to sense various operating parameters of the power-plant 12 . Sensors 56 may include, but are not limited to, a mass air flow sensor (not shown) of engine 20 and a battery voltage sensor (not shown) of power pack assembly 28 .

Sensors included in the braking system 30 can be used to sense various operating parameters of the braking system 30 . The sensors may include, but are not limited to, a brake fluid pressure sensor 58 and/or a wheel speed sensor 60 . The pressure sensor 58 can detect a working pressure of the brake fluid in the brake system 30 . The pressure sensor 58 may generate a pressure signal (BRAKE _Ps ) that may be used to determine the working pressure of the brake fluid. The BRAKE _Ps pressure signal may be output to the chassis control module 42 . The wheel speed sensor 60 can measure the speed of the wheels 18 . The wheel speed sensor 60 may generate a signal that is transmitted to the chassis control module 42 and may be used by the chassis control module 42 to determine an actual speed (V _s ) of the vehicle.

Although the sensors 56 and the sensors 58, 60 in 1 are shown in such a way that they are at of the engine 20 and braking system 30, respectively, it should be appreciated that the various sensors 56-60 may be located throughout the vehicle system 10 if desired. The signals generated by the driver interface devices 44 and the sensors 56-60 may be transmitted to one or more of the modules of the vehicle system 10, including the PCM 40 and the chassis control module 42, as shown. In addition, the PCM 40 and chassis control module 42 may transmit the various signals they receive to each other and to other modules of the vehicle system 10 .

Based on signals generated by driver interface devices 44 and sensors 56 - 60 , PCM 40 may regulate drive torque produced by powerplant 12 by controlling actuators 62 included within powerplant 12 . For example, the PCM 40 may control a throttle valve (not shown) used to vary the amount of air provided to the engine 20 for combustion. Similarly, the chassis control module 42 may control braking torque generated by the braking system 30 by controlling actuators included in the braking system 30, such as the brake modulator 32 and the brakes 34. The chassis control module 42 may generate a brake control signal (BRAKE _Control ). , which is output to the brake modulator 32 to regulate braking torque generated by the brakes 34 and distribute braking torque to the wheels 18 .

From the foregoing, it can be appreciated that the PCM 40 and chassis control module 42 can control the vehicle system 10 to vary the speed of the vehicle. The PCM 40 and chassis control module 42 may also control the vehicle system 10 to maintain an actual vehicle speed at or near the target vehicle speed in the cruise control mode, as will now be described.

Still referring to 1 For example, the PCM 40 may include a cruise control module 70 that cooperates with a brake control module 72 of the chassis control module 42 to maintain the vehicle speed within the target speed range. In particular, the cruise control module 70 may cooperate with the brake control module 72 to reduce the vehicle speed into the target speed range when the vehicle speed becomes excessive. For example, vehicle speed may become excessive when the vehicle is operating on a downhill slope.

The cruise control module 70 and the brake control module 72 may also cooperate with other modules of the vehicle system 10 that regulate operation of the vehicle system 10 to slow the vehicle. As an example, the vehicle system 10 may include one or more modules (not shown) for controlling the power-plant 12 in a power-plant braking mode. In the power-plant braking mode, the modules may control the transmission 14 to shift to a lower gear, thereby increasing the speed of the power-plant 12 and thereby increasing the negative drive torque produced by the power-plant 12 .

With specific reference to 2 For example, the vehicle speed control module 70 may include a vehicle speed module 74 and a speed difference module 76 . The vehicle speed module 74 may receive CRUISE _Sw from the cruise control switch 54 and the actual vehicle speed (Vs). Based on CRUISE _Sw and V _s , the cruising speed module 74 may determine the target cruising speed (Vt) and generate a cruise control signal (CRUISE _Control ) that indicates whether the vehicle system 10 is currently operating in the cruise control mode. The vehicle speed module 74 may output Vt and CRUISE _Control to one or more modules of the PCM 40 including the speed difference module 76 .

The speed difference module 76 may receive Vt, Vs, and _{CRUISEControl} and generate a cruise control mode brake signal (BRAKE _Cruise ) that may be output to the brake control module 72 as shown. In addition, the speed difference module 76 may also generate BRAKE _Cruise based on signals generated by other modules included in the vehicle system 10 . For example, the other signals may include a PTBRAKE signal that indicates whether the vehicle system 10 is operating in a powertrain braking mode. The PTBRAKE signal may also be indicative of an actual negative propulsion torque being produced by the powerplant 12 . The PTBRAKE signal can also indicate a future available negative drive torque of the power plant.

The BRAKE _Cruise signal may indicate whether cruise control mode braking is desired to keep the vehicle speed at or near the target cruise speed Vt to keep. Accordingly, BRAKE _Cruise may be a binary signal that indicates whether vehicle braking is desired in a cruise control braking mode. Cruise control mode braking may be desired when the actual vehicle speed V _s exceeds the target vehicle speed Vt by a predetermined amount (V _diff ). Thus, the speed difference module 76 may generate BRAKE _Cruise based on a comparison of V _s , V _t , and V _diff . For example, cruise control mode braking may be desired when the value of V _s minus Vt is greater than the value of V _diff . Stated another way, cruise control _{mode braking may be desired when the value of Vs is greater than an upper threshold equal to the value of Vt plus Vdiff .}

The speed difference module 76 may also generate BRAKE _Cruise to indicate a desired magnitude of a cruise control mode braking force in the cruise control braking mode. Consequently, BRAKEcruise may indicate a value of zero while braking is not desired in cruise control mode. In addition, BRAKEcruise may indicate a special non-zero value when cruise control mode braking is desired, which corresponds to the cruise control mode braking force setpoint.

The setpoint variable can represent an absolute braking amount, such as a specific amount of setpoint braking force or setpoint braking torque. Alternatively, the magnitude may be a relative amount, such as a percentage of a predetermined threshold braking force or torque. The threshold braking force may be predetermined in a number of ways. For example, the threshold braking force or torque may be predetermined to minimize the driver's perception of vehicle braking during cruise control braking mode. Alternatively, the threshold braking force may be predetermined to produce a desired vehicle deceleration rate.

The braking force target value in the vehicle speed control mode can depend on the actual vehicle speed V _s . For example, the magnitude can be proportional to _Vs. The magnitude may also be proportional to the difference between the actual vehicle speed _Vs and the target vehicle speed Vt. In addition, the variable can be dependent on the negative actual drive torque generated by the drive unit 12 . The setpoint variable can also be based on the negative drive torque of the drive unit 12 that will be available in the future.

The speed difference module 76 may determine the cruise control mode braking force setpoint in a number of ways. From the foregoing, it can be appreciated that the speed difference module 76 may determine the cruise control mode braking force setpoint based on the value of Vs minus _Vt . In addition, the speed difference module 76 may implement a closed-loop feedback routine to determine the desired braking force magnitude in cruise control mode, such as a proportional-integral-derivative (PID) control routine. A PID controller can be used to determine size based on _Vt and Vs. Alternatively, the PID control can be based on the values of V _diff and the value of V _s minus Vt. Closed-loop feedback may be implemented to improve vehicle speed control and ensure smooth response during cruise control mode braking.

The brake control module 72 may receive BRAKE _Cruise and other signals generated by the vehicle system 10 . Based on the signals it receives, the brake control module 72 may generate the BRAKE _Control signal to generate a control amount of vehicle braking force. The brake control module 72 may receive signals such as BRAKE _Sw and BRAKE _Ps generated by the braking system 30 . Additionally, the brake control module 72 may receive signals such as, but not limited to, a brake antilock brake (ABS) signal, a vehicle stability control (VSC) signal, and a vehicle rollover control (VRC) signal. Based on the above signals, the brake control module 72 may generate BRAKE _Control to generate the control amount of vehicle braking force.

The amount of control of vehicle braking may be equal to a target braking torque indicated via the brake pedal 52 by the driver of the vehicle. Alternatively, in the cruise control braking mode, the control amount may be equal to or less than the target magnitude of the braking force in the cruise control mode in the cruise control braking mode. The control amount of vehicle braking may be less than the desired magnitude of braking force in cruise control mode if it is desirable to limit vehicle braking for oversteer reasons. Slippery or uneven roads can cause oversteer provide reason to limit the amount of regulation of the vehicle braking force generated by the brakes 34. The negative drive torque available in the future may provide another override reason to limit the amount of regulation of the vehicle braking force.

The brake control module 72 may generate the BRAKE _Control signal to apportion the control amount of vehicle braking force to the wheels 18 via the brakes 34 . In other words, the brake control module 72 may generate the BRAKEcontrol signal to apportion braking torque generated by each of the wheels 18 . For example, the brake control module 72 may limit braking torque generated by one or more of the wheels 18 to avoid locking a wheel in slippery or rough road conditions to prevent loss of vehicle control. It is therefore appreciated that the brake control module 72 may generate the BRAKE _Control signal to generate a proportion of the desired amount of vehicle braking effort, which may be indicated by the signals it receives, including BRAKE _Cruise .

From the foregoing, it can be appreciated that in the cruise control mode, the vehicle system 10 can control the brakes 34 to maintain the vehicle speed within the target speed range of the target vehicle speed. The vehicle system 10 may regulate the brakes 34 based on the difference between the actual vehicle speed and the target travel speed. In this manner, the vehicle system 10 may be used to limit vehicle speed while the vehicle is traveling on a down grade. It is also appreciated that the vehicle system 10 can regulate the magnitude of a braking force generated by the brakes 34 to limit vehicle speed.

With specific reference to 3 An example control method 200 for implementing braking in cruise control mode is shown. For simplicity, the control method 200 is described with reference to the vehicle system 10 described hereinabove. However, it should be appreciated that the control method 200 can be implemented with other vehicle systems. The control method 200 may be implemented using the various modules previously described herein, such as the PCM 40 and the chassis control module 42. The control method 200 may be implemented to complement other control methods for controlling vehicle speed in various modes of operation, such as the cruise control mode and the Powertrain Braking Mode discussed hereinabove.

The control method 200 begins with step 202, where control determines whether the vehicle system 10 is operating in the cruise control mode. If the vehicle system 10 is operating in the cruise control mode, control proceeds to step 204; otherwise control loops back as shown.

In step 204, control determines whether the current vehicle speed is excessive. In general, the controller will determine that the actual vehicle speed is excessive when the actual vehicle speed is above an upper threshold of a target speed range (ie, not within the target speed range) in the cruise control mode. Thus, the controller can determine that the actual vehicle speed is excessive by comparing V _s , V t and V _diff . For example, the controller may determine that the current vehicle speed is excessive when the value of Vs minus Vt is greater than _Vdiff , as discussed _hereinabove . If the current vehicle speed is excessive, control continues with step 206; otherwise control loops back and continues to step 202 as previously described.

In step 206, control determines whether cruise control mode braking entry conditions are met in a cruise control braking mode. In general, the cruise control braking mode entry conditions will be met when there are no override reasons to prevent cruise control mode braking. Override reasons for preventing cruise control mode braking may exist when cruise control mode braking may be undesirable for reasons such as vehicle stability and/or vehicle control. In addition, override reasons may exist when other control modules operating the vehicle system 10 to produce negative drive torque may continue to be used to reduce vehicle speed. In other words, an oversteer cause may exist when additional negative drive torque is available to reduce vehicle speed. For example, an oversteer cause may exist when the vehicle system 10 is operating in a powertrain braking mode and the transmission 14 can be shifted to a lower gear to generate additional negative drive torque and thereby reducing vehicle speed. The entry conditions may be met when the transmission 14 can no longer be shifted into a lower gear without overspeeding the engine 20 .

If the entry conditions are met, control continues to step 208, where control generates a request for cruise control mode braking in a cruise control braking mode. The request for cruise control mode braking may be a general request indicating that cruise control mode braking is desired. Alternatively, the cruise control mode braking request may indicate a target amount of cruise control mode braking force. As discussed hereinabove, the magnitude may be based on an actual vehicle speed and a target vehicle speed. The magnitude may also be based on a threshold braking force, negative drive torque, and/or a desired vehicle deceleration rate. At step 208, the controller may generate a signal, such as BRAKEcruise, indicative of the request for braking in cruise control mode. From step 208 control proceeds to step 210 .

At step 210, control generates a brake control signal requesting a control amount of braking force. In particular, the controller generates the brake control signal to control the brakes (e.g., brakes 34) and thereby generate the control amount of braking force. In general, the control amount of braking force will correspond to the braking force target value in the vehicle speed control mode. Thus, the brake control request may be based on the cruise control mode braking force setpoint indicated by the cruise control mode braking request generated in step 208 .

Alternatively, the vehicle braking force control amount may be smaller than the braking force target magnitude in the vehicle speed control mode. For example, the amount of regulation may be limited when slippery or rough roads exist. The amount of regulation may also be limited based on vehicle system 10 operating conditions. For example, the amount of control may be limited based on the target vehicle speed. In addition, the amount of control can be limited while the vehicle is turning.

From step 210 control proceeds to step 212 where control selectively applies the brakes to generate the control amount of vehicle braking force. Next, in step 214, control determines whether the actual vehicle speed exceeds the target travel speed. Therefore, at step 214, control may compare V _s to V _t . If the actual vehicle speed exceeds the target travel speed (eg, V _s >V t ), then control proceeds to step 216; otherwise control proceeds to step 218.

In step 216, control determines whether there is an override reason to not continue braking in cruise control mode. An override reason not to continue may exist if the driver has indicated that the vehicle system 10 should no longer operate in the cruise control mode. In addition, an override condition may exist when one or more of the entry conditions are no longer met. An override reason for not proceeding may also exist where failure to proceed may result in overheating of the brakes or other components of the braking system. If there are no override reasons not to continue cruise control mode braking, control loops back and continues to step 206 as previously described. Control proceeds to step 218 if there is an override reason to exit.

At step 218, control aborts the request for braking in cruise control mode, thereby not continuing to regulate the brakes in cruise control braking mode. Control loops back from step 218 and continues at step 202 as previously described.

From the foregoing, it can be appreciated that the control method 200 may control the brakes in the cruise control mode based on a comparison of the actual vehicle speed and the target cruise speed. The control method 200 may control the brakes in the cruise control mode to limit the vehicle speed and thereby maintain the vehicle speed within the target speed range of the target travel speed. Accordingly, it is also appreciated that the control method 200 may limit vehicle speed in cruise control mode while operating on a downhill slope by controlling the brakes.

Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the disclosure can be implemented in a variety of forms. Therefore, while this disclosure includes particular examples, the true scope of the disclosure should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification, and the following claims.

Claims (12)

A cruise control system comprising: a speed difference module (76) that compares an actual vehicle speed (Vs) to a target cruise speed (Vt); and a brake control module (72) that selectively actuates a brake (34) when the actual vehicle speed (Vs) is greater than the target travel speed (Vt) by a predetermined speed difference (V _diff ); characterized in that the brake control module (72) applies the brake (34) only when a transmission (14) can no longer be downshifted without overspeeding an engine (20) and when no other control modules can be used to decrease vehicle speed. control system claim 1 , wherein the brake control module (72) applies the brake (34) while the actual vehicle speed (V _s ) is greater than the target travel speed (Vt). control system claim 1 wherein the brake control module (72) selectively applies the brake (34) based on negative drive torque of a power plant (12). A vehicle speed control system comprising: a speed difference module (76) that determines a target braking force based on a comparison of an actual vehicle speed (Vs) and a target vehicle speed (Vt); and a brake control module (72) that selectively actuates a brake (34) to generate an actual braking force that is less than or equal to the target braking force when the actual vehicle speed (Vs) is greater than the target vehicle speed by a predetermined speed difference (V _diff ). (Vt) is; characterized in that the brake control module (72) applies the brake (34) only when a transmission (14) can no longer be downshifted without overspeeding an engine (20) and when no other control modules can be used to decrease vehicle speed. control system claim 4 , wherein the speed difference module (76) determines the target braking force on the basis of a negative drive torque of a drive unit (12). control system claim 4 wherein the speed difference module (76) determines the desired braking effort using a closed loop feedback routine. A vehicle speed control method (200) comprising: comparing an actual vehicle speed (Vs) to a target vehicle speed (Vt); and a brake (34) is selectively applied when the actual vehicle speed (V _s ) is greater than the target vehicle speed (Vt) by a predetermined speed difference (V _diff ); characterized in that the brake (34) is only actuated when a transmission (14) of the vehicle can no longer be shifted into a lower gear without overspeeding an engine (20) of the vehicle and when no other control modules can be used to decrease vehicle speed. Method (200) according to claim 7 , wherein selectively applying the brake (34) comprises applying the brake (34) while the actual vehicle speed (Vs) is greater than the target travel speed (V _t ). Method (200) according to claim 7 wherein selectively actuating the brake (34) further comprises selectively actuating the brake (34) based on negative input torque of a power-plant (12). Method (200) according to claim 7 , further comprising determining a target braking force based on the comparison of the actual vehicle speed (V _s ) to the target ground speed (Vt), and wherein selectively applying the brake (34) further comprises applying the brake (34). is to generate an actual braking force that is less than or equal to the target braking force. Method (200) according to claim 10 , wherein the determination of the target braking force further comprises that the target braking force is determined on the basis of a negative drive torque of a drive unit (12). Method (200) according to claim 10 , wherein determining the desired braking force comprises determining the desired braking force using a closed-loop feedback routine.

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