Vehicle control device and vehicle control method

A control device for a vehicle includes: a charge control portion that adjusts an upper limit of charging power to a battery to prevent a negative electrode potential of the battery from dropping to a lithium reference potential, based on a charge/discharge history of the battery; a braking control portion that detects a sharing ratio between hydraulic braking force by a braking device and regenerative braking force for desired braking force according to a brake pedal depression amount so that a motor generator generates a regenerative braking force within a range of the adjusted upper limit of charging power; and a setting portion that variably sets, according to the hydraulic response rate detected by the detection portion, a degree of limitation of the upper limit when restricting charging current to the battery by restricting the upper limit.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to Japanese Patent Application No. 2011-45080 filed on Mar. 2, 2011, which is incorporated herein by reference in its entirety including the specification, drawings and abstract.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a vehicle control device and control method, and more specifically to break cooperative control by regenerative braking force and hydraulic braking force.

2. Description of Related Art

A motor vehicle having a vehicle driving electric motor mounted thereon, such as a hybrid vehicle or an electric vehicle, performs braking force control when braked, to ensure desired braking force for the overall vehicle by cooperation between regenerative braking force provided by a vehicle driving electric motor and braking force provided by a hydraulic braking device (hereafter, also referred to as hydraulic braking force) (see, for example, Japanese Patent Application Publication No. 2004-196064 (JP-A-2004-196064), Japanese Patent Application Publication No. 2004-155403 (JP-A-2004-155403), and Japanese Patent Application Publication No. 2010-141997 (JP-A-2010-141997)). In the description below, this braking force control shall be also referred to as “cooperative brake control”. Electric power generated by regenerative braking is recovered to charge an on-vehicle electric storage device, whereby the energy efficiency, that is, the fuel economy of the vehicle is improved.

JP-A-2004-196064 describes cooperative brake control of regenerative braking and friction braking in order to prevent occurrence of insufficient vehicle deceleration due to delayed response to friction braking command value when braking is switched from regenerative braking to friction braking. More specifically, it describes a technique in which when the sharing ratio is changed by reducing the sharing ratio of regenerative braking torque while increasing the sharing ratio of friction braking torque, the rate of reduction of the regenerative braking torque is suppressed according to a delay in response of the friction braking torque.

JP-A-2004-155403 describes a cooperative control device of a composite brake which calculates a braking torque due to a hydraulic control error based on a wheel cylinder hydraulic reference model and corrects the command value of regenerative braking torque in consideration of this control error.

JP-A-2010-141997 describes cooperative brake control for a motor vehicle in which a reference value of the upper limit of charging power of an electric storage device, which limits regenerative braking force, is set variably according to a change rate of the upper limit of charging power. This makes it possible to ensure a period of time required for rise of hydraulic pressure during transition from a state in which both regenerative braking and hydraulic braking are used to a state in which only hydraulic control is used.

On the other hand, the application of lithium ion secondary batteries as on-vehicle electric storage devices has been increased. The lithium ion secondary batteries have high energy density and high output voltage, and thus can be used as on-vehicle electric storage devices requiring large battery capacity and high voltage.

However, a lithium ion secondary battery has a problem that lithium metal may deposit on a surface of the negative electrode depending on a status of use, resulting in heat generation in the battery or deterioration in performance thereof. In order to solve this problem, WO 2010/005079 discloses a control technique in which deposition of lithium metal on the negative electrode of a lithium ion secondary battery is suppressed by adjusting the power that is allowed to input to the battery based on its charge and discharge history. More specifically, it describes a technique in which, based on the history of battery current, a maximum current value that will not cause deposition of the lithium metal is successively calculated, while the power that is allowed to input to the battery is adjusted so as not to exceed the maximum current value.

In a vehicle performing cooperative brake control as disclosed in JP-A-2004-196064, JP-A-2004-155403, and JP-A-2010-141997, the fuel economy is improved further as the ratio of regenerative braking force is increased during deceleration. Therefore, in a vehicle using a lithium ion secondary battery as the on-vehicle electric storage device, the sharing ratio of regenerative braking must be increased as much as possible while suppressing deposition of lithium metal.

However, as described in WO 2010/005079, the charging power to the battery must be restricted and thus the regenerative braking force must also be restricted when the charging state becomes such that the risk of deposition of lithium metal is expected. Once charging limitation like this is started, the cooperative brake control must be performed to reduce the regenerative braking force in order to suppress the deposition of lithium metal, while substituting the shortfall caused thereby with hydraulic braking force.

If the regenerative braking force is changed at a high rate, as described in JP-A-2004-196064, JP-A-2004-155403, and JP-A-2010-141997, instantaneous fluctuations may occur in the vehicle braking force due to delayed response of the hydraulic braking force. Such fluctuations in the braking force may possibly give uncomfortable feeling to a passenger in the vehicle even if the braking performance of the vehicles is not affected thereby.

SUMMARY OF THE INVENTION

The invention provides a vehicle control device and control method for use in a vehicle having a lithium ion secondary battery mounted thereon, wherein control is performed to restrict charging to the battery in order to suppress deposition of lithium metal, and cooperative brake control is performed such that no uncomfortable feeling is given to a passenger in the vehicle due to instantaneous fluctuations in the vehicle braking force possibly caused by the control to restrict charging, while ensuring recovered energy obtained by regenerative power generation.

A first aspect of the invention is related to a control device for a vehicle having: a battery formed of a lithium ion secondary battery; a braking device configured to exert braking force on a drive wheel according to hydraulic pressure supplied from a hydraulic pressure generation circuit; a motor generator configured to transmit rotation force reciprocally with the drive wheel; and a power controller that performs bidirectional power conversion between the battery and the motor generator to control output torque of the motor generator. The vehicle control device includes: a charge control portion that adjusts an upper limit of charging power to the battery so as to prevent a negative electrode potential of the battery from dropping to a lithium reference potential, based on a charge and discharge history of the battery; a braking control portion that determines a sharing ratio between hydraulic braking force by the braking device and regenerative braking force for desired braking force according to a brake pedal depression amount so that the motor generator generates the regenerative braking force within a range of the adjusted upper limit of charging power; a detection portion that detects an actual value of hydraulic response rate in the hydraulic pressure generation circuit; and a setting portion that variably sets, according to the hydraulic response rate detected by the detection portion, a degree of limitation of the upper limit of charging power when restricting charging current to the battery by restricting the upper limit of charging power.

A second aspect of the invention is related to a control method for a vehicle having: a battery formed of a lithium ion secondary battery; a braking device configured to exert braking force on a drive wheel according to hydraulic pressure supplied from a hydraulic pressure generation circuit; a motor generator configured to transmit rotation force reciprocally with the drive wheel; and a power controller that performs bidirectional power conversion between the battery and the motor generator to control output torque of the motor generator. The vehicle control method includes: adjusting an upper limit of charging power to the battery so as to prevent a negative electrode potential of the battery from dropping to a lithium reference potential, based on a charge and discharge history of the battery; determining a sharing ratio between hydraulic braking force by the braking device and regenerative braking force for desired braking force according to a brake pedal depression amount so that the motor generator generates the regenerative braking force within a range of the adjusted upper limit of charging power; variably setting, according to the detected hydraulic response rate, a degree of limitation of the upper limit of charging power when restricting charging current to the battery by restricting the upper limit of charging power.

According to the above aspects, in a vehicle having a lithium ion secondary battery mounted thereon, after performing control to restrict charging to the battery in order to suppress deposition of lithium metal, cooperative brake control can be performed such that no uncomfortable feeling is given to a vehicle user due to instantaneous fluctuations in the vehicle braking force possibly caused by the control to restrict charging, while ensuring recovered energy obtained by regenerative power generation.

DETAILED DESCRIPTION OF EMBODIMENTS

An exemplary embodiment of the invention will be described with reference to the accompanying drawings. The same or equivalent components in the drawings are assigned with the same reference numerals and description thereof will be omitted in principle.

FIG. 1is a block diagram illustrating a schematic configuration of a hybrid vehicle as a representative example of a vehicle having mounted thereon a vehicle control device according to an embodiment of the invention.

Referring toFIG. 1, a hybrid vehicle5includes a braking device10, a drive wheel12, a reduction gear14, an engine20, a first motor generator (hereafter, abbreviated as first MG)40for power generation and starting the engine, a second motor generator (hereafter, abbreviated as second MG)60for driving the vehicle, a brake hydraulic circuit80, a power dividing mechanism100, and a transmission200.

The hybrid vehicle5further includes a power control unit (PCU)16, a battery18formed by a lithium ion secondary battery, a brake pedal22, a brake electronic control unit (brake ECU)300, a hybrid vehicle electronic control unit (HV-ECU)302, an engine electronic control unit (engine ECU)304, and a power-supply electronic control unit (power-supply ECU)306. Each ECU is typically formed by a micro computer (not shown) including a central processing unit (CPU), a memory, an input/output port, and a communication port. At least some of the ECUs may be configured to perform predetermined numerical and logical data processing by means of hardware such as an electronic circuit or the like.

The brake ECU300, the HV-ECU302, the engine ECU304, and the power-supply ECU306are communicably connected to each other via a communication bus310.

The power-supply ECU306is connected to an ignition (IG) switch36. When a driver operates the IG switch36to start up the system of the hybrid vehicle5, the power-supply ECU306turns on an IG relay (or an IG relay and an accessory (ACC) relay) (not shown) on the condition that the brake pedal22is depressed. Thus, a group of electric devices composing the hybrid vehicle5are supplied with power, whereby the hybrid vehicle5is made ready to drive.

Although the description of the embodiment has been made on the assumption that the brake ECU300, the HV-ECU302, the engine ECU304, and the power-supply ECU306are separate ECUs, some or all of these ECUs may be integrated into one ECU.

The engine20is an internal combustion engine, such as a gasoline engine or a diesel engine, which outputs power by burning a fuel. The engine20is designed to be able to electrically control the operating conditions such as throttle (intake amount), fuel supply amount, and ignition timing. The engine20is provided with an engine speed sensor34. The engine speed sensor34detects a rotational speed of the engine20and transmits a signal indicating the detected rotational speed of the engine20to the engine ECU304.

The engine ECU304controls the amount of fuel injection, the ignition timing, and the air intake amount of the engine20based on signals from various sensors including the engine speed sensor34, so that the engine20operates at a target speed and target torque determined by the HV-ECU302.

The battery18is formed by a lithium ion secondary battery. A lithium ion secondary battery has high energy density, and exhibits higher initial circuit voltage and higher average operating voltage than other types of secondary batteries. This is a reason why a lithium ion secondary battery is used as an on-vehicle electric storage device of a vehicle requiring large battery capacity and high voltage. In addition, a lithium ion secondary battery has a coulombic efficiency of approximately 100%. This means that a lithium ion secondary battery has high charge and discharge efficiency and, therefore, is capable of providing more effective utilization of energy in comparison with other types of secondary batteries.

However, as described also in WO 2010/005079, a lithium ion secondary battery has a drawback that lithium metal may be deposited on the surface of the negative electrode depending on charging conditions. In the embodiment, therefore, a control is performed to restrict the charge to the battery18in order to suppress deposition of lithium metal in the lithium ion secondary battery (hereafter, also referred to as “Li deposition suppression control”).

The battery18is provided with a battery sensor for detecting state values of the battery18. The battery sensor is configured, for example, to detect battery current Ib, battery voltage Vb, and battery temperature Tb as the state values. The state values detected by the battery sensor are transmitted to the HV-ECU302.

In the description below, the battery current Ib shall be represented by a positive value (Ib>0) when the battery18is discharged, whereas the battery current Ib shall be represented by a negative value (Ib<0) when the battery18is charged. The HV-ECU302is thus enabled to acknowledge the charge/discharge history of the battery18based on the values of battery current Ib successively transmitted thereto.

The first MG40and the second MG60are both a three-phase dynamo-electric machine, for example, and have a function as a motor and a function as a generator. The second MG60can correspond to the motor generator according to the invention.

The first MG40and the second MG60are provided with rotational position sensors41and61, respectively for each detecting a rotational position (angle) of a rotor (not shown).

The first MG40and the second MG60are connected to the battery18via the PCU16. The PCU16has an inverter and/or a converter (not shown) including a plurality of semiconductor power switching elements. The PCU16performs bidirectional power conversion between the first MG40and second MG60and the battery18according to a control instruction from the HV-ECU302. The HV-ECU302controls the power conversion in the PCU16so that the output torque of the first MG40and the output torque of the second MG60match their respective torque command values. The PCU16can correspond to a power controller according to the invention. The HV-ECU302controlling the PCU16can correspond to a charge control portion according to the invention.

The power dividing mechanism100is a planetary gear set provided between the engine20and the first MG40. The power dividing mechanism100divides the power input from the engine20into power to the first MG40and power to the reduction gear14which is coupled to the drive wheel12via a drive shaft164. The drive shaft164is provided with a vehicle speed sensor165. A vehicle speed V of the hybrid vehicle5is detected based on a rotational speed of the drive shaft164detected by the vehicle speed sensor165.

The power dividing mechanism100includes a first ring gear102, a first pinion gear104, a first carrier106, and a first sun gear108. The first sun gear108is an external gear coupled to an output shaft of the first MG40. The first ring gear102is an internal gear arranged concentrically with the first sun gear108. The first ring gear102is coupled to the reduction gear14via a ring gear shaft102arotating together with the first ring gear102. The first pinion gear104engages with the first ring gear102and the first sun gear108. The first carrier106holds the first pinion gear104such that the first pinion gear104can rotate and revolve, and is coupled to the output shaft of the engine20.

In other words, the first carrier106is an input element, the first sun gear108is a reaction element, and the first ring gear102is an output element. Driving force (torque) output to the ring gear shaft102ais transmitted to the drive wheel12via the reduction gear14and the drive shaft164.

When the engine20is in operation, a reaction torque generated by the first MG40is input to the first sun gear108while an output torque of the engine20is input to the first carrier106, whereby a torque of a magnitude obtained by adding/subtracting these torques appears in the first ring gear102functioning as the output element. In this case, the first MG40functions as a generator since the rotor of the first MG40is rotated by that torque. When the rotational speed (output rotational speed) of the first ring gear102is assumed to be fixed, the rotational speed of the engine20can be changed continuously (not stepwise) by changing the rotational speed of the first MG40. This means that a control for setting the engine rotational speed, for example, to a rotational speed at which the best fuel economy can be achieved is performed by the HV-ECU302controlling the first MG40.

When the engine20is in stop while the hybrid vehicle5is traveling, the second MG60is rotating positively while the first MG40is rotating reversely. When the first MG40in this condition is caused to function as a motor and to output a torque in a positive rotational direction, this torque in the positive rotational direction can be made to act on the engine20connected to the first carrier106. Accordingly, the engine20can be started (motored or cranked) by the first MG40. In that case, the torque acts on the reduction gear14in a direction to stop the rotation. Therefore, the driving force for driving the vehicle can be maintained by controlling the output torque of the second MG60and, at the same time, the engine20can be started up smoothly. The hybrid system of the hybrid vehicle5shown inFIG. 1is referred to as a mechanical distributing type or split type.

When the hybrid vehicle5is in a regenerative braking mode, the second MG60is driven by the drive wheel12via the reduction gear14and the transmission200, and hence the second MG60is activated as a generator. Thus, the second MG60operates as a regenerative brake which converts braking energy to electric power. The electric power generated by the second MG60is stored in the battery18via the PCU16. The electric power generated by the second MG60is determined by a product of the torque and rotational speed of the second MG60. Therefore, the electric power generated by the regenerative braking can be adjusted by controlling the torque of the second MG60.

The transmission200is a planetary gear set provided between the reduction gear14and the second MG60. The transmission200changes the rotational speed of the second MG60and transmits the changed rotational speed to the reduction gear14. An alternative configuration is possible in which the transmission200is omitted and the output shaft of the second MG60is directly connected to the reduction gear14.

The transmission200includes a second ring gear202, a second pinion gear204, a second carrier206, and a second sun gear208. The second sun gear208is an external gear connected to the output shaft of the second MG60. The second ring gear202is an internal gear arranged concentrically with the second sun gear208. The second ring gear202is connected to the reduction gear14. The second pinion gear204engages with the second ring gear202and the second sun gear208. The second carrier206holds the second pinion gear204such that the second pinion gear204can rotate and revolve. The second carrier206is fixed to a case or the like (not shown) so as not to rotate.

The transmission200may be configured to use a frictional engagement element so that rotation of the components in the planetary gear is restricted or synchronized based on a control signal from the HV-ECU302, and so that the transmission200thereby changes the rotational speed of the second MG60in one or more steps and transmits the same to the reduction gear14.

The HV-ECU302performs travel control for causing the vehicle to travel in a suitable manner for the vehicle state. For example, when starting or traveling at a low speed, the hybrid vehicle5is driven by the output of the second MG60with the engine20being stopped. During normal traveling, the engine20is activated and the hybrid vehicle5is driven by the output from the engine20and the second MG60. The fuel economy of the hybrid vehicle5can be particularly improved by causing the engine20to operate at a highly efficient operating point. Specifically, the HV-ECU302sets a driving force required for the entire vehicle by reflecting the depression amount of an accelerator pedal (not shown), while the HV-ECU302also sets operation command values (typically, rotational speed command value and/or torque command value) for the engine20, the first MG40, and the second MG60so that the aforementioned travel can be realized.

The HV-ECU302estimates the state of charge (SOC) of the battery18based on the state values (battery current Ib, battery voltage Vb, battery temperature Tb) detected by the battery sensor19. The SOC is represented by a percentage obtained by the current charge amount by the full charge amount. Since a conventionally available method can be applied to the estimation of the SOC, description thereof will be omitted here.

The HV-ECU302sets an allowable input power value (hereafter, also referred to as Win) indicating a limit value of electric power to be charged to the battery18and an allowable output power value (hereafter, also referred to as Wout) indicating a limit value of electric power to be discharged from the battery18, at least based on the SOC. The input/output power to/from the battery18(hereafter, also referred to simply as the battery power) shall also be indicated by a positive value when the battery18is discharged, and by a negative value when charged. Therefore, Wout assumes zero or a positive value (Wout≧0), while Win assumes zero or a negative value (Win≦0). The HV-ECU302sets operation command values for the first MG40and the second MG60such that the battery power is within the range of Win to Wout.

Description will be made of a brake system of the hybrid vehicle5. The braking device10includes a brake caliper160and a circular-disk-shaped brake disk162. The brake disk162is fixed such that its axis of rotation is coincident with that of the drive shaft164. The brake caliper160includes a wheel cylinder (not shown inFIG. 1) and a brake pad. The wheel cylinder is activated by supplying a hydraulic pressure to the brake caliper160from the brake hydraulic circuit80. Rotation of the brake disk162is restricted by the activated wheel cylinder pressing the brake pad against the brake disk162. The braking device10thus generates hydraulic braking force according to the hydraulic pressure Pwc supplied from the brake hydraulic circuit80. This means that braking device10can correspond to the “braking device” of the invention.

The brake hydraulic circuit80is controlled according to an operation command from the brake ECU300and controls the hydraulic pressure Pwc supplied to the braking device10. This means that the brake hydraulic circuit80can correspond to the “hydraulic pressure generation circuit” according to the invention”.

FIG. 2is a block diagram illustrating a specific configuration example of the brake hydraulic circuit80shown inFIG. 1.

Referring toFIG. 2, the brake hydraulic circuit80includes a hydraulic pressure booster81, a pump motor82, an accumulator83, a reservoir84, a brake actuator85, and a stroke simulator89.

The brake actuator85includes switching solenoid valves SSC, SMC, SRC and SCC, linear solenoid valves SLA and SLR for controlling the wheel cylinder hydraulic pressure during normal braking, control solenoid valves FLH, FLR, FRH, FRR, RLH, RLR, RRH and RRR, and hydraulic pressure sensors86to88. Operation of each of the solenoid valves is controlled in response to a control signal Sv1from the brake ECU300.

The hydraulic booster81is configured to generate a hydraulic pressure (regulator pressure Prg) according to the depressing force of the brake pedal22in order to amplify the depressing force. The regulator pressure Prg generated by the hydraulic booster81can be detected by a hydraulic pressure sensor87. The detection value by the hydraulic pressure sensor87is transmitted to the brake ECU300.

The brake ECU300is able to detect the depression amount (depressing force) of the brake pedal22based on the detected regulator pressure Prg. Alternatively, the depression amount (depressing force) of the brake pedal22may be detected by providing a stroke sensor for directly detecting the depression amount (depressing force) of the brake pedal22.

The stroke simulator89is configured such that a reaction force according to the operation of the brake pedal22acts on the brake pedal22to give the driver an optimum brake feeling.

The pump motor82increases the pressure of working fluid stored in the reservoir84. The working fluid output from the pump motor82is supplied to the brake actuator85via the accumulator83. The hydraulic supply pressure (accumulator pressure Pac) can be detected by the hydraulic pressure sensor88. The detection value of the accumulator pressure Pac by the hydraulic pressure sensor88is transmitted to the brake ECU300. The brake ECU controls the pump motor82such that the accumulator pressure is coincident with the command value.

The switching solenoid valve SSC opens and closes the passage to the stroke simulator89. The switching solenoid valve SCC opens and closes the passage between a fluid passage90on the rear wheel side and a fluid passage91on the front wheel side. The switching solenoid valve SMC opens and closes the passage from the hydraulic booster81to the fluid passage90. The switching solenoid valve SRC opens and closes the passage from the hydraulic pressure booster81to the fluid passage91. The switching solenoid valves SMC and SRC are normally closed, whereas the switching solenoid valve SCC is normally open.

The linear solenoid valves SLA and SLR control the hydraulic pressure (wheel cylinder pressure Pwc) in the fluid passages90and91detected by the hydraulic pressure sensor86. The detection value of the wheel cylinder pressure Pwc by the hydraulic pressure sensor86is transmitted to the brake ECU300.

The brake ECU300controls the aperture of the linear solenoid valves SLA and SLR so that the wheel cylinder pressure Pwc matches with the target hydraulic pressure. When the wheel cylinder pressure is to be increased, the aperture of the linear solenoid valve SLA is set to be greater than zero, while the linear solenoid valve SLR is closed (aperture=0). In contrast, when the wheel cylinder pressure is to be decreased, the aperture of the linear solenoid valve SLR is set to be greater than zero, while the linear solenoid valve SLA is closed (aperture=0).

The control solenoid valves FLH, FLR, FRH, FRR, RLH, RLR, RRH and RRR are provided to independently control the hydraulic pressure supplied to the wheel cylinders161of the wheels when an anti-lock brake system (ABS) or traction control system is activated. The control solenoid valves include holding valves FLH, FRH, RLH and RRH for holding the hydraulic pressure supplied to their corresponding wheel cylinders161, and pressure-reducing valves FLR, FRR, RLR and RRR for reducing the hydraulic pressure supplied to their corresponding wheel cylinders161. When the ABS or the like is not activated, the control solenoid valves are kept closed.

During normal braking, a wheel cylinder pressure Pwc controlled by the linear solenoid valves SLA and SLR is supplied to the wheel cylinders161. This means that the wheel cylinder pressure Pwc detected by the hydraulic pressure sensor86corresponds to the hydraulic pressure Pwc supplied to the braking device10shown inFIG. 1.

When the brake actuator85is in an abnormal state, the switching solenoid valves SMC and SRC are opened, whereby a hydraulic pressure (regulator pressure) according the brake depressing force from the hydraulic booster81is supplied to the wheel cylinders161. The switching solenoid valve SCC can be closed in accordance with the location of the abnormality, so that the passage on the front wheel side can be separated from the passage on the rear wheel side.

Referring toFIG. 1again, the hybrid vehicle5performs cooperative brake control so that a desired braking force (total braking force) for the entire vehicle according to the driver's operation of the brake pedal22is output while it is shared between regenerative braking force by the second MG60and hydraulic braking force by the braking device10.FIG. 3illustrates an example of the cooperative brake control by the hydraulic braking and the regenerative braking.

Referring toFIG. 3, the reference numeral W10indicates a total braking force based on the driver's operation of the brake pedal. The reference numeral W20indicates a regenerative braking force generated by the second MG60. It can be understood that the total braking force is ensured by the sum of the regenerative braking force and the hydraulic braking force. Although not shown, in a hybrid vehicle having an engine, an engine braking force is also generated by a so-called engine brake in addition to the hydraulic braking force and the regenerative braking force. Therefore, strictly speaking, the regenerative braking force and the hydraulic braking force should be determined in consideration of the engine braking force if necessary. However, for the simplicity of description, the following description will be made on the assumption that the engine braking force=0.

The regenerative braking force, that is, the braking torque output by the second MG60is limited within such a range that electric power input to the battery18will not exceed the Win (that is, within such a range that Pb>Win). Therefore, if the Win is limited, it may cause a problem that the original regenerative braking force shown inFIG. 3cannot be obtained. When Li deposition suppression control described also in WO 2010/005079 is applied, in particular, the Win may possibly vary in the positive direction during generation of the regenerative braking force. In this case, the regenerative braking force must be immediately reduced in order to protect the battery18.

On this occasion, the hydraulic braking force must be increased in accordance with the reduction of the regenerative braking force. However, as described in JP-A-2004-196064, if the regenerative braking force is reduced rapidly, the increase of the hydraulic braking force (the increase of the hydraulic pressure) cannot follow the reduction of the regenerative braking force, and it may momentarily become impossible to ensure the target total braking force. If this happens, uncomfortable feeling may be given to the vehicle user even if the braking performance of the vehicle is not affected thereby.

In order to avoid such problems, the vehicle according to the embodiment of the invention sets the limitation to the regenerative electric power in the cooperative brake control as described below, in consideration of application of the Li deposition suppression control.

FIG. 4is a flowchart illustrating control processing procedures of the cooperative brake control performed in the hybrid vehicle5shown inFIG. 1.

The control processing illustrated in the flowchart ofFIG. 4is performed by the HV-ECU302at constant control intervals. Processing steps shown inFIG. 4are implemented by software and/or hardware processing by the HV-ECU302.

Referring toFIG. 4, the HV-ECU302inputs vehicle state values of the hybrid vehicle5in step S100. The vehicle state values include a brake pedal depression amount BP that is a depression amount of the brake pedal22, vehicle speed V, rotational speed Nm1of the first MG40, and rotational speed Nm2of the second MG60.

The brake pedal depression amount BP is detected for example based on a regulator pressure Prg detected by the hydraulic pressure sensor87shown inFIG. 2. The vehicle speed V is detected based on an output from the vehicle speed sensor165. The rotational speeds Nm1and Nm2can be calculated based on outputs from the rotational position sensors41and61attached to the first MG40and the second MG60, respectively.

In step S110, the HV-ECU302sets a required braking torque Tr* for the entire vehicle based on the vehicle state of the hybrid vehicle5. The required braking torque Tr* corresponds to the total braking force illustrated inFIG. 3.

Typically, the required braking torque Tr* is calculated as a braking torque to be output to the ring gear shaft102a, based on the brake pedal depression amount BP and vehicle speed V input in step S100. For example, a map may be preliminarily generated, defining a relationship between the brake pedal depression amount BP and vehicle speed V and the required braking torque Tr*, and the map may be stored in a memory (not shown) in the HV-ECU302. In step S110, the HV-ECU302is enabled to set the required braking torque Tr* by referring to the map based on the brake pedal depression amount BP and the vehicle speed V input in step S100.

Subsequently, the HV-ECU302retrieves the Win to the battery18in step S120. Control processing for setting the Win will be described later in detail. |Win|(Win≦0) denotes a maximum value of magnitude of the charging power, or “the upper limit of charging power” to the battery18in the current control cycle. The magnitude of the battery current Ib (|Ib|) during charging (Ib<0) will be represented also as “charging current” in the description below.

In step S130, the HV-ECU302determines a share amount of the required braking torque Tr* that is assigned to the regenerative braking torque according to the cooperative brake control shown inFIG. 3. A torque command value of the second MG60generating regenerative braking force (second MG torque Tm2*) is set based on this share amount.

During regenerative braking, the second MG60generates electric power according to a product of torque and rotational speed. Therefore, the battery power (Pb=Vb·Ib) must not exceed the Win retrieved in step S120. Specifically, the relation of |Pb|<|Win| must be established. Accordingly, in step S130, a second MG torque Tm2* for the cooperative brake control is set while being limited within such a range that the relation of |Pb|<|Win| is established.

Further, in step S140, the HV-ECU302sets a hydraulic brake torque Tbk* according to the following equation (1). In the equation (1), Gr denotes a reduction ratio of the transmission200.
Tbk*=Tr*−Tm2*·Gr(1)

In this manner, the cooperative brake control is realized, in which the required braking torque Tr* is shared by the regenerative braking torque (Tm2*) and the hydraulic brake torque (Tbk*). The HV-ECU302performing the processing of steps S100to S140can correspond to the “braking control portion” of the invention.

Further, in step S150, the HV-ECU302outputs the hydraulic brake torque Tbk* set in step S140to the brake ECU300(FIG. 1).

The brake ECU300calculates a target hydraulic pressure supplied to the braking device10, based on the hydraulic brake torque Tbk*. The brake actuator85shown inFIG. 2is then controlled so that the wheel cylinder pressure (Pwc) detected by the hydraulic pressure sensor86matches this target hydraulic pressure.

Description will be made of the Li deposition suppression control on the battery18formed by a lithium ion secondary battery.

FIGS. 5 and 6are waveform diagrams for explaining the Li deposition suppression control performed by the hybrid vehicle5according to the embodiment of the invention.

Referring toFIG. 5, the battery current Ib changes to a negative direction from time t0, and charging to the battery18is started.

An allowable input current value Ilim for the battery18is set according to the charge and discharge history of the battery18. As described in WO 2010/005079, the allowable input current value Ilim is obtained as a maximum current value at which lithium metal will not deposit when the potential of the negative electrode of the battery drops to lithium reference potential within a unit time. The allowable input current value Ilim may be set in the same manner as in WO 2010/005079. Specifically, an allowable input current value Ilim[t] at time t is successively obtained, at each control interval, by adding or subtracting a reduction amount due to continued charging and a recovery amount due to continued discharging or a recovery amount due to the battery being left to stand, based on an initial value Ilim[0] of the allowable input current value in the state where no charge and discharge history exists.

A margin current ΔImr is set for the allowable input current value Ilim, and an input current limit target value Itag is set for preventing deposition of lithium metal. As described in WO 2010/005079, the input current limit target value Itag can be set by offsetting the allowable input current value Ilim in a positive direction. In this case, the offset current value constitutes the margin current ΔImr.

As shown inFIG. 5, the allowable input current value Ilim and the input current limit target value Itag gradually vary in a positive direction as a result of continuous charging. It will be understood that the allowable charging current (|Ib|) is decreased by this. When the battery current Ib becomes lower than the input current limit target value Itag at the time t1(Ib<Itag), limitation of the charging current becomes necessary to suppress deposition of the lithium metal.

Therefore, as shown inFIG. 6, the charging power (i.e. regenerative electric power) is limited by changing the Win to the battery18in a positive direction from the time t1. For example, the Win is changed in a positive direction at a constant rate (time-change rate). This causes |Win|, namely “upper limit of charging power” to drop down. The change rate of the Win shall be hereafter referred to also as the “regeneration limitation rate”. The regeneration limitation rate corresponds to an example of a degree of limitation in charging power limitation by the Li deposition suppression control (hereafter, referred to also as the “regeneration limitation”).

Referring toFIG. 5again, the charging current is reduced (that is, Ib varies in a positive direction) by limiting the Win from the time t1, and the battery current Ib becomes greater than the input current limit target value Itag again at the time t2. Thus, as shown in FIG.6, the regeneration limitation is terminated from the time t2. As a result, the Win to the battery18gradually returns to its normal value.

As described above, during the regenerative braking in which large charging current is generated, regeneration limitation is started to change the Win at a constant regeneration limitation rate in order to suppress deposition of lithium metal, once the charging current reaches the input current limit target value Itag. As seen fromFIGS. 5 and 6, the conditions for starting regeneration limitation become stricter as the margin current ΔImr becomes greater. On the other hand, if the margin current ΔImr is reduced, the conditions for starting regeneration limitation will be alleviated, and thus the energy recovered by regenerative power generation can be increased.

When the Win is changed by regeneration limitation during regenerative braking, the share of the regenerative braking torque (absolute value of the MG2torque) is decreased by the processing in steps S120and S130ofFIG. 4, while the share of the hydraulic brake torque Tbk* is increased. Accordingly, the brake ECU300controls the brake hydraulic circuit80to increase the hydraulic supply pressure Pwc to the braking device10. As described above, the increase in the hydraulic supply pressure Pwc is somewhat delayed with respect to the increase in the command value. This causes a momentary shortage in the hydraulic brake torque, leading to momentary fluctuations of the vehicle braking force, which may give uncomfortable feeling to the user.

In order to prevent completely such uncomfortable feeling given to the user, it is conceivable to set the regeneration limitation rate to be uniformly low to ensure a sufficient margin for the control responsiveness of the hydraulic pressure. However, if the regeneration limitation rate is reduced, the limitation to the charging current started from when Ib<Itag becomes moderate. Therefore, in order to prevent deposition of lithium metal, the margin current ΔImr shown inFIG. 5needs to be increased from the viewpoint of reliably preventing the battery current Ib from reaching the allowable input current value Ilim. This means that, when the degree of limitation of the charging power for the regeneration limitation is reduced, the conditions for starting the regeneration limitation must be made stricter. As a result, the recovered energy by the regenerative power generation is decreased by the excessive limitation of the Win.

FIG. 7is a conceptual waveform diagram for explaining delay in response of hydraulic control in the brake hydraulic circuit.

Referring toFIG. 7, when the hydraulic supply pressure to the braking device10is to be raised, the command value of the wheel cylinder pressure is increased. The brake ECU300controls the linear solenoid valve SLA shown inFIG. 2so that the wheel cylinder pressure Pwc matches the command value. This raises the actual value of the wheel cylinder pressure Pwc (i.e. the hydraulic supply pressure to the braking device10), following the command value.

A certain period of time (delay time) ΔT is required until the wheel cylinder pressure Pwc reaches the command value. The actual value of hydraulic response rate is obtained by dividing the hydraulic pressure variation ΔP by the delay time ΔT.

The delay time ΔT varies according to the circuit state of the brake actuator85or the state of the working fluid. The delay time ΔT generally tends to be greater when the temperature is low, whereas the delay time ΔT may vary depending on other factors than temperature. Therefore, it is difficult to accurately estimate the hydraulic response rate based on temperature or other indirect conditions in response to the command to raise the hydraulic pressure.

On the other hand, the brake ECU300is able to successively obtain an actual value of the hydraulic response rate (ΔP/ΔT) Prt based on the command value and a detection value of the hydraulic pressure sensor86when a control command is output to the brake actuator85(linear solenoid valves SLA and SLR) in accordance with variation in the command value of the wheel cylinder pressure Pwc.

Therefore, in the embodiment of the invention, as described below, the regeneration limitation rate of the Li deposition suppression control is set variably based on an actual value of the hydraulic response rate Prt in the brake hydraulic circuit80.

FIG. 8is a functional block diagram illustrating cooperative brake control combined with Li deposition suppression control in a vehicle according to the embodiment of the invention. The functional blocks shown inFIG. 8can be embodied by software processing and/or hardware processing by the brake ECU300or the HV-ECU302.

Referring toFIG. 8, the brake ECU300includes a hydraulic control unit315and a response rate detection unit320. The HV-ECU302has a regeneration limitation rate setting unit330, a SOC calculation unit340, a current determination unit350, and a Win setting unit360. The functional blocks shown inFIG. 8are those relating to the “charge control portion” of the Li deposition suppression control among the functions of the HV-ECU302.

The hydraulic control unit315generates a command value Pwc* of the wheel cylinder pressure based on a hydraulic brake torque Tbk* set by the HV-ECU302according to the flowchart ofFIG. 4. The hydraulic control unit315also generates a control signal Sv1for controlling the brake actuator85shown inFIG. 2(in particular, the linear solenoid valves SLA and SLR), based on the command value Pwc* and an actual value of the wheel cylinder pressure Pwc detected by the hydraulic pressure sensor86. The control signal Sv1is transmitted to the brake hydraulic circuit80.

The response rate detection unit320calculates a hydraulic response rate Prt described inFIG. 7for the wheel cylinder pressure in the brake hydraulic circuit80, based on the command value Pwc* set by the hydraulic control unit315and the actual value (Pwc) detected by the hydraulic pressure sensor86. The response rate detection unit320can correspond to the detection portion of the invention.

The response rate detection unit320detects a hydraulic response rate Prt, for example, every time the command value Pwc* is varied to exceed a certain amount. The detected hydraulic response rate Prt is successively transmitted to the HV-ECU302. This makes it possible to obtain the latest hydraulic response rate Prt based on the current state of the brake hydraulic circuit80. The hydraulic response rate Prt is updated to the latest value at the first timing of generation of the hydraulic braking force according to the cooperative brake control shown inFIG. 3, at least every time the driver operates the brake.

The SOC calculation unit340calculates a SOC estimated value of the battery18based on the state values (battery current Ib, battery voltage Vb, and battery temperature Tb) detected by the battery sensor. In the description hereafter, the SOC estimated value shall be also referred to simply as the “SOC”.

The current determination unit350determines whether or not regeneration limitation by the Li deposition suppression control is necessary, based on the state values and SOC of the battery18. As is described later in more detail, the history of the battery current Ib is reflected in this determination. The current determination unit350turns a flag FLG on when the regeneration limitation by the Li deposition suppression control is necessary, and turns the flag FLG off when not necessary. A determination value of the flag FLG is transmitted to the Win setting unit360.

The regeneration limitation rate setting unit330sets a regeneration limitation rate Pr based on the hydraulic response rate Prt (Pa/sec) calculated by the response rate detection unit320and the rotational speed Nm2of the second MG60. The regeneration limitation rate setting unit330can correspond to a “setting portion” of the invention. The regeneration limitation rate Pr is calculated based on the rotational speed Nm2and a hydraulic braking force change rate Tbrt according to the following equation (2).
Pr(W/sec)=2π×Nm2(rpm)×Tbrt(N·m/sec)  (2)

The rotational speed Nm2(rpm) of the second MG60in the equation (2) can be calculated based on a vehicle speed V (km/h) detected by the vehicle speed sensor165according to the following equation (3). Alternatively, the rotational speed Nm2can be calculated based on an output of the rotational position sensor61of the second MG60.
V(km/h)=2π×Nm2(rpm)×r(m)×60/Gm/1000  (3)

In the equation (3) above, Gm denotes an overall reduction ratio represented by a product of a reduction ratio between the ring gear shaft102aand the drive shaft164and a reduction ratio by the transmission200, and r denotes a radius of a tire.

The hydraulic braking force change rate Tbrt (N·m/sec) in the equation (2) can be calculated based on the detected hydraulic response rate Prt (Pa/sec) according to the following equation (4).
Tbrt(N·m/sec)=Prt(Pa/sec)×C1×C2  (4)

In the equation (4) above, C1is a conversion factor (N·m/Pa) for converting a wheel cylinder pressure into a hydraulic brake torque. The conversion factor C1is a constant determined according to a cylinder area of the wheel cylinder161, a diameter of the braking action point and a coefficient of friction of the brake disk162, and so on. The conversion factor C2corresponds to an inverse of the overall reduction ratio Gm in the equation (3).

The equation (4) makes it possible to calculate a change rate of the hydraulic brake torque when the wheel cylinder pressure is varied according to the detected hydraulic response rate Prt. Accordingly, it is understood that no delay occurs in the hydraulic braking force even if the regenerative braking force is reduced while the hydraulic braking force is increased according to the regeneration limitation rate Pr calculated by the equation (2). It is also possible that the processing until the calculation of the hydraulic braking force change rate Tbrt (N·m/sec) based on the equation (4) is performed by the brake ECU300(response rate detection unit320), and then the hydraulic braking force change rate Tbrt is transmitted from the brake ECU300to the HV-ECU302.

The Win setting unit360sets a Win for the battery18, based on the current battery state (SOC, state value), the flag FLG determination value from the determination unit350, and the regeneration limitation rate Pr set by the regeneration limitation rate setting unit330.

FIG. 9is a flowchart illustrating control processing for implementing the Li deposition suppression control shown inFIG. 8. The control processing according to the flowchart ofFIG. 9is performed by the HV-ECU302at regular control intervals. The processing steps shown inFIG. 9can be embodied by software processing and/or hardware processing by the HV-ECU302.

Referring toFIG. 9, in step S200, the HV-ECU302inputs the actual value Prt of the hydraulic response rate detected by the brake ECU300(response rate detection unit320). When the processing until the calculation of the hydraulic braking force change rate Tbrt (N·m/sec) is performed by the brake ECU300(response rate detection unit320), as described above, the hydraulic braking force change rate Tbrt is input to the HV-ECU302in step S200.

In step S210, the HV-ECU302inputs state values of the battery18based on the output from the battery sensor. As described before, the state values input in step S210include values of voltage Vb, current Ib, and temperature Tb of the battery18. The HV-ECU302calculates SOC of the battery18in step S220. This means that the processing in210corresponds to the function of the SOC calculation unit340shown inFIG. 8.

In step S230, the HV-ECU302sets Win0for the battery18. The Win0means the Win before performing the regeneration limitation rate processing, and is set based on the current battery state (SOC, state values, etc.) without taking into consideration of the Li deposition suppression control. In other words, this Win0set here corresponds to the Win to the battery18that is normally set according to a conventional technique. The Win0is set based on SOC and battery temperature Tb, for example.

The HV-ECU302sets, in step S240, a regeneration limitation rate by the Li deposition suppression control based on the hydraulic response rate Prt (actual value). The regeneration limitation rate corresponds to the change rate in a positive direction (time-change rate) of the Win shown inFIG. 5.

The processing in step S240corresponds to the function of the regeneration limitation rate setting unit330shown inFIG. 8. Therefore, in step S240, a regeneration limitation rate Pr responsive to the hydraulic response rate Prt (actual value) is calculated according the equations (2) to (4). When the hydraulic braking force change rate Tbrt is input in step S200, the processing in step S240is performed by the brake ECU300.

The HV-ECU302sets a margin current ΔImr in step S245according to the regeneration limitation rate Pr.

When the regeneration limitation rate is increased in order to increase the degree of limitation of the charging power, the charging current can be reduced rapidly when Ib becomes smaller than Itag. This makes it possible to alleviate the conditions for starting the regeneration limitation, and thus the margin current ΔImr can be reduced. As a result, the energy amount recovered by regenerative braking can be increased.

As described above, the margin current ΔImr in the Li deposition suppression control must be changed in an opposite sense to the regeneration limitation rate. Specifically, the margin current ΔImr must be set smaller as the regeneration limitation rate Pr during regeneration limitation becomes greater. On the contrary, the margin current ΔImr must be set greater as the regeneration limitation rate Pr during regeneration limitation becomes lower. This makes it possible to realize both prevention of deposition of lithium metal and securement of a sufficient energy amount recovered by regenerative braking.

In view of this, a map can be preliminarily generated, defining a relation between regeneration limitation rate Pr and margin current ΔImr. This map is prestored in a memory (not shown) in the HV-ECU302. Thus, in step S245, the margin current ΔImr can be set based on the regeneration limitation rate Pr set in step S240, by referring to the map.

Further, in step S250, the HV-ECU302performs current control calculation for suppression control of Li deposition. Specifically, as described inFIG. 5, an allowable input current value Ilim[t] in the current control cycle is calculated based on the charge and discharge history of the battery18by means of the method disclosed in WO 2010/005079. The input current limit target value Itag can be calculated by providing the margin current ΔImr set in step S245to the allowable input current value Ilim.

In step S260, the HV-ECU302compares the battery current Ib input in step S210with the input current limit target value Itag calculated in step S250. The processing steps in steps S245to S260correspond to the functions of the current determination unit350shown inFIG. 8.

When Ib>Itag (determined NO in S260), the HV-ECU302turns off the flag FLG shown inFIG. 8since the charging current has not reached the Itag. In this case, the HV-ECU302does not perform regeneration limitation in step S270. Instead, the HV-ECU302sets, in step S275, the Win0set in step S230directly as the Win to the battery18(Win=Win0).

In contrast, when Ib<Itag (determined YES in S260), the HV-ECU302turns on the flag FLG shown inFIG. 8since the charging current has reached the Itag. In this case, the HV-ECU302performs regeneration limitation in step S280. This is because if the charging current is not reduced from the current state, Ib may reach the allowable input current value Ilim.

When the regeneration limitation is performed, the HV-ECU302sets, in step S285, the Win according to the regeneration limitation rate Pr set in step S240. More specifically, the Win is set so as to be changed in a positive direction from the Win in the previous control cycle. The upper limit of charging power (|Win|) to the battery18is reduced according to the regeneration limitation rate Pr, whereby the charging current to the battery18is reduced. As a result, the potential of the negative electrode is prevented from being reduced and thus the deposition of lithium metal can be prevented. The processing steps in steps S230and S270to S285correspond to the functions of the Win setting unit360shown inFIG. 8.

According to the embodiment as described above, in a vehicle having a lithium ion secondary battery mounted thereon, limitation of charging power (regeneration limitation) is performed by Li deposition suppression control based on an actual value of the hydraulic response rate Prt in the brake hydraulic circuit80, when the charging current restricted for Li deposition suppression control during regenerative power generation.

Accordingly, the limitation of charging power (regeneration limitation) by the Li deposition suppression control can be limited rapidly according to such a rate that no delay in hydraulic braking force will occur. As a result, it is made possible to prevent that uncomfortable feeling is given to the user due to instantaneous fluctuations of the braking force caused by the limitation of charging power (regeneration limitation). In addition, the conditions for stating the limitation of charging power can be alleviated (that is, the margin current ΔImr can be reduced) in such a state in which the regenerative braking force can be reduced rapidly reduced. This makes it possible to improve the energy efficiency (that is, the fuel economy) of the vehicle by ensuring maximum regenerative energy.

The vehicle to which the cooperative brake control according to the embodiment of the invention is applicable is not limited to the hybrid vehicle5shown inFIG. 1. The invention is commonly applicable to motor vehicles in general including hybrid vehicles, electric vehicles and fuel cell powered vehicles not having an engine, as long as they are configured to ensure the braking force by combination of regenerative braking force by the motor and hydraulic braking force according to supply of hydraulic pressure, regardless of the number of mounted motors (motor generators) or configuration of driving systems. The configuration of the hybrid vehicle is also not limited to the example shown inFIG. 1, and the invention is applicable to any type of hybrid vehicles including those of a parallel type. The invention is also applicable to vehicles having no traveling motor as long as they has a motor generating regenerative braking force.