PARKING BRAKE CONTROL DEVICE

A parking brake control device controls a hydraulic pressure unit for braking wheels hydraulically and a parking brake device for braking the wheels by transmitting power of an electric motor to the wheels mechanically. The parking brake control device includes a hydraulic brake control unit capable of exercising a hydraulic brake control under which a brake is applied to the wheels by the hydraulic pressure unit, on condition that a signal is received from an actuation switch for actuating the parking brake device, while a vehicle is running, and a pressure decrease rate setting unit configured to set a pressure decrease rate according to a closing condition satisfied upon entry into the closing stage of the hydraulic brake control when a pressure decrease control is exercised in a closing stage of the hydraulic brake control.

TECHNICAL FIELD

This disclosure relates to a parking brake control device.

BACKGROUND ART

A parking brake control device that controls a hydraulic pressure unit for braking wheels hydraulically, and a parking brake device for braking the wheels by transmitting power of an electric motor to the wheels mechanically is known in the art (see JP H10-76931 A).

SUMMARY OF INVENTION

The parking brake control device capable of exercising the hydraulic and mechanical brake control as known in the conventional art can be configured such that while a vehicle is running, the hydraulic brake control can be triggered by a driver actuating a parking switch. However, thus-activated hydraulic brake control will be brought to an end in time, and the end will come under various situations; thus, decrease of pressure at a uniform rate in the closing stage of the hydraulic brake control would potentially entail various disadvantages. For example, if one who has actuated the parking switch in the moving vehicle now wants to accelerate the vehicle again by actuating the accelerator, the hydraulic brake control should be brought to an end; in this situation, however, if the rate of decrease of the pressure in the closing stage has been set low, the braking power decreases very slowly; therefore, the driver feels frustrated as if “dragging”, by failing to feel anticipated acceleration or good pickup in the vehicle velocity. On the other hand, when actuation of the parking switch effected while the vehicle is running makes the vehicle stop running, one may presume that the hydraulic brake control should be brought to an end and the brake control should be switched to the mechanical brake control; however, if the rate of decrease of the pressure in the closing stage has been set high, the fluid pressure would disadvantageously become zero before application of the braking power of the mechanical brake control becomes sufficiently effective. In that event, the vehicle once stopped would move again slightly, and make a passenger feel uncomfortable.

It would be desirable to minimize the passenger’s discomfort even when the brake is applied by actuation of the parking switch.

Against the backdrop, a parking brake control device is disclosed which controls a hydraulic pressure unit for braking wheels hydraulically and a parking brake device for braking the wheels by transmitting power of an electric motor to the wheels mechanically. In one aspect, the parking brake control device comprises: a hydraulic brake control unit capable of exercising a hydraulic brake control under which a brake is applied to the wheels by the hydraulic pressure unit, on condition that a signal is received from an actuation switch for actuating the parking brake device, while a vehicle is running; and a pressure decrease rate setting unit configured to set a rate of decrease of pressure in such a manner that when a pressure decrease control is exercised in a closing stage of the hydraulic brake control, the rate of decrease of pressure is set according to a closing condition satisfied upon entry into the closing stage of the hydraulic brake control.

With this configuration, in which the rate of decrease of pressure is set according to the closing condition of the hydraulic brake control, a passenger’s discomfort can be restrained even when the brake is applied by the use of the actuation switch for actuating the parking brake device

The pressure decrease rate setting unit may be configured to set the rate of decrease of pressure at a first pressure decrease rate having a value greater than a minimum value thereof, if the closing condition is receipt of a request for cancellation of the hydraulic brake control as submitted by a driver’s operation while the vehicle is running.

With this configuration, in which the pressure decrease control is exercised at the rate of decrease of pressure as set at the first pressure decrease rate having a value greater than the minimum value if the pressure decrease control is exercised on the basis of the request for cancellation of the hydraulic brake control, a driver can be relieved from feeling frustrated as if “dragging”, in comparison, for example, with an alternative configuration in which the pressure decrease control is exercised at the minimum rate of decrease of pressure.

The parking brake control device may comprise a switching unit configured to switch a brake control from the hydraulic brake control to a mechanical brake control under which a brake is applied to the wheels by the electric motor of the parking brake device, and the switching unit may be configured to switch the brake control from the hydraulic brake control to the mechanical brake control when the vehicle has been stopped, wherein if the closing condition is a stop of the vehicle, the pressure decrease rate setting unit sets the rate of decrease of pressure at a second pressure decrease rate lower than the first pressure decrease rate.

With this configuration, in which the pressure decrease control is exercised at the rate of decrease of pressure as set at the second pressure decrease rate lower than the first pressure decrease rate if the pressure decrease control is exercised on the basis of the stop of the vehicle, the braking power to be applied when the brake control is switched from the hydraulic brake control to the mechanical brake control can be maintained so that the vehicle can be kept in a stopped state properly, in comparison, for example with an alternative configuration in which the pressure decrease control is exercised at the first pressure decrease rate when the vehicle has been stopped.

The switching unit may be configured to switch the brake control from the hydraulic brake control to the mechanical brake control when an abnormal condition has been detected in the hydraulic pressure unit, wherein if the closing condition is an abnormal condition in the hydraulic pressure unit, the pressure decrease rate setting unit sets the rate of decrease of pressure at a third pressure decrease rate lower than the first pressure decrease rate and higher than the second pressure decrease rate.

With this configuration, in which the pressure decrease control is exercised at the rate of decrease of pressure as set at the third pressure decrease rate lower than the first pressure decrease rate and higher than the second pressure decrease rate if the pressure decrease control is exercised on the basis of the abnormal condition in the hydraulic pressure unit, the brake control can be switched from the hydraulic brake control to the mechanical brake control properly when the abnormal condition has been detected in the hydraulic pressure unit.

The pressure decrease rate setting unit may be configured to set the first pressure decrease rate at a first rate if a vehicle body velocity is a first velocity, and set the first pressure decrease rate at a second rate higher than the first rate if the vehicle body velocity is a second velocity higher than the first velocity.

With this configuration, the higher the vehicle body velocity, the less likely the driver can be to feel frustrated as if “dragging”, so that the driver’s discomfort can be restrained more effectively.

In a configuration of the parking brake control device comprising a switching unit configured to switch a brake control from the hydraulic brake control to a mechanical brake control under which a brake is applied to the wheels by the electric motor of the parking brake device, when an abnormal condition has been detected in the hydraulic pressure unit, the pressure decrease rate setting unit may be configured to set the rate of decrease of pressure at a third pressure decrease rate lower than the first pressure decrease rate, if the closing condition is an abnormal condition in the hydraulic pressure unit.

With this configuration, in which the pressure decrease control is exercised at the rate of decrease of pressure as set at the third pressure decrease rate lower than the first pressure decrease rate, if the pressure decrease control is exercised on the basis of the abnormal condition in the hydraulic pressure unit, the brake control can be switched from the hydraulic brake control to the mechanical brake control properly when the abnormal condition has been detected in the hydraulic pressure unit.

In a configuration of the parking brake control device comprising a switching unit configured to switch the brake control from the hydraulic brake control to a mechanical brake control under which a brake is applied to the wheels by the electric motor of the parking brake device, when the vehicle has been stopped or when an abnormal condition has been detected in the hydraulic pressure unit, the pressure decrease rate setting unit may be configured to set the pressure decrease rate at a second pressure decrease rate if the closing condition is a stop of the vehicle, and set the pressure decrease rate at a third pressure decrease rate higher than the second pressure decrease rate if the closing condition is an abnormal condition in the hydraulic pressure unit.

With this configuration, the pressure decrease control is exercised at the rate of decrease of pressure as set at the second pressure decrease rate lower than the third pressure decrease rate if the pressure decrease control is exercised on the basis of the stop of the vehicle; therefore, the braking power to be applied when the brake control is switched from the hydraulic brake control to the mechanical brake control can be maintained so that the vehicle can be kept in a stopped state properly, in comparison, for example, with an alternative configuration in which the pressure decrease control is exercised at the third pressure decrease rate when the vehicle has been stopped. In addition, since the pressure decrease control is exercised at the rate of decrease of pressure as set at the third pressure decrease rate higher than the second pressure decrease rate if the pressure decrease control is exercised on the basis of the abnormal condition in the hydraulic pressure unit, the brake control can be switched from the hydraulic brake control to the mechanical brake control properly when the abnormal condition has been detected in the hydraulic pressure unit.

DESCRIPTION OF EMBODIMENTS

A detailed description will be given of one embodiment of a parking brake control device with reference made to the drawings where appropriate.

As shown inFIG.1, a vehicle2comprises wheel brakes FR, FL, RR, RL provided at right front, left front, right rear, and left rear wheels3, and a vehicle brake fluid pressure control device1that brakes the wheels3by transmitting fluid pressure to the wheel brakes FR, FL, RR, RL.

Each of the front wheel brakes FR, FL comprises a brake rotor BR and a wheel cylinder4. Each of the rear wheel brakes RR, RL comprises a brake rotor BR, a wheel cylinder4, and a parking brake device200. The wheel cylinder4and the parking brake device200applies braking power to the wheel3by pressing a friction pad260(seeFIG.3) against a brake rotor BR that is a rotating member that integrally rotates together with the wheel3.

The vehicle brake fluid pressure control device1mainly includes a hydraulic pressure unit10for braking wheels3hydraulically, and a controller100. The hydraulic pressure unit10is provided with fluid paths and various parts. The controller100exercises control over the various parts in the hydraulic pressure unit10in a suitable way.

A master cylinder5as a fluid pressure source and the wheel cylinders4are connected to the hydraulic pressure unit10. The brake fluid pressure produced by the master cylinder5according to the depressing force of a brake pedal6(driver’s braking operation) is regulated under control of the controller100and the hydraulic pressure unit10, and provided to the wheel cylinders4.

Wheel velocity sensors91, a parking switch92, and an accelerator sensor93are connected to the controller100. Each of the wheel velocity sensors91detects a wheel velocity of a corresponding wheel3. The parking switch92is an example of an actuation switch. The parking switch92is a switch for actuating the parking brake device200, and is provided near a driver’s seat. The parking switch92may be configured to turn ON when a parking lever (not shown) operated by the driver is pulled by the driver, and turn OFF when the driver takes his/her hand off the parking lever. The accelerator sensor93detects a motion of an accelerator pedal7.

The controller100comprises a CPU (Central Processing Unit), a RAM (Random Access Memory), a ROM (Read Only Memory), and an input/output circuit, and is configured to exercise control by performing various operations based on inputs from the parking switch92and the sensors91,93, etc., and programs and data stored in the ROM. Details of the controller100will be explained later.

As shown inFIG.2, the hydraulic pressure unit10is located between the master cylinder5and the wheel brakes FR, FL, RR, RL.

The hydraulic pressure unit10is comprised of fluid paths and various solenoid valves arranged in a pump body11that is a base body having the fluid paths (hydraulic line) formed therein through which a brake fluid circulates. Output ports5a,5bof the master cylinder5are connected to input ports11aof the pump body11, and output ports11bof the pump body11are connected to the wheel brakes FL, RR, RL, FR. During normal operations, fluid paths from the input ports11ato the output ports11binside the pump body11are connected to each other and form a hydraulic line so that the depressing force of the brake pedal6is transmitted to the respective wheel brakes FL, RR, FL, FR. The hydraulic pressure system connected to the output port5aof the master cylinder5is connected to the wheel brakes FL, RR, and the hydraulic pressure system connected to the output port5bof the master cylinder5is connected to the wheel brakes RL, FR, and these hydraulic pressure systems have approximately the same configuration.

Each of the hydraulic pressure systems has a pressure-regulating valve12provided on the hydraulic line connecting the input port11aand the output port11b. The pressure-regulating valve12is a normally open proportional solenoid valve that can regulate a difference between fluid pressures upstream and downstream thereof according to an electric current supplied thereto. A check valve12athat allows a flow only to the output port11bis provided in parallel with the pressure-regulating valve12.

Hydraulic lines between the pressure-regulating valve12and the wheel brakes FL, RR, RL, FR are each bifurcated off at some midpoint and connected to the corresponding output port11b. An inlet valve13that is a normally open proportional solenoid valve is provided on each of the branch hydraulic lines corresponding to the output ports11b. A check valve13athat allows a flow only to the pressure-regulating valve12is provided in parallel with each inlet valve13.

A return hydraulic line19B that is branched off from a point between the output port11band the inlet valve13corresponding thereto, and connected to a point between the pressure-regulating valve12and the inlet valve13via an outlet valve14that is a normally closed solenoid valve is provided.

On the return hydraulic line19B, a reservoir16for temporarily accommodating an excessive brake fluid, a check valve16a, a pump17, and an orifice17aare arranged from a position closest to the outlet valve14in this order. The check valve16ais arranged to allow a flow only to the point between the pressure-regulating valve12and the inlet valve13. The pump17is driven by a motor21, and provided to generate pressure toward the point between the pressure-regulating valve12and the inlet valve13. The orifice17aattenuates pulsation of pressure of a brake fluid discharged from the pump17and pulsation caused by operation of the pressure-regulating valve12.

An influent hydraulic line19A connecting the input port11aand the pressure-regulating valve12is connected to a section between the check valve16aand the pump17on the return hydraulic line19B by a suction hydraulic line19C. A suction valve15that is a normally closed solenoid valve is provided on the suction hydraulic line19C.

The hydraulic pressure unit10configured as described above is operated in normal times in such a manner that each of the solenoid valves is not energized, and a brake fluid pressure introduced from the input ports11ais outputted through the pressure-regulating valves12and the inlet valves13to the output ports11b, and thus given as-is to the wheel brakes FL, RR, RL, FR. When an antilock braking control is exercised, or on any other occasions, to reduce an excessive brake fluid pressure in the wheel brakes FL, RR, RL, FR, the relevant inlet valve(s)13is closed and the relevant outlet valve(s)14is opened so that the brake fluid is let flow through the return hydraulic line19B into the reservoir16so that the brake fluid in the wheel cylinder4can be drained out. To exert pressure on the wheel brakes FL, RR, RL, FR without driver’s operation of the brake pedal6, the suction valve15is opened, and the motor21is activated, so that the brake fluid can be proactively supplied to the wheel brakes FL, RR, RL, FR by the action of the pump17. Furthermore, to adjust the levels of pressure exerted on the wheel brakes FL, RR, RL, FR, the electric current supplied to the pressure-regulating valve12can be regulated so that the levels of pressure can be adjusted as desired.

As shown inFIG.3, the wheel brakes RR, RL each comprise a wheel cylinder4, a parking brake device200, a pair of friction pads260, and a brake rotor BR. The friction pads260are located to hold the brake rotor BR.

The parking brake device200comprises an electric motor210, a speed reducer220, and a nut250. The wheel cylinder4comprises a housing230and a brake piston240.

The electric motor210is a reversible motor, and includes an output shaft (not shown) coupled to the speed reducer220.

The speed reducer220is a mechanism for reducing the speed with which power of the electric motor210is transmitted, and comprises a plurality of gears inside. An external thread222is formed on the output shaft221of the speed reducer220.

The housing230has a cylinder hole231in which the brake piston240is supported and allowed to move in the axial direction of the speed reducer220. The cylinder hole231is formed to have a cylindrical shape with one base closed and the other base opening toward the friction pads260.

The brake piston240is formed to have a hollow cylindrical shape, oriented with one base closed and the other base opening toward the closed base of the cylinder hole231, and located inside the cylinder hole231. One of the friction pads260is attached to the brake piston240. A fluid pressure chamber232formed by the brake piston240and the cylinder hole231is configured to be filled with a brake fluid supplied from the hydraulic pressure unit10described above through a fluid path233formed in the housing230or the like. Accordingly, the brake piston240advances with the friction pad260at the forefront thereof toward the other one of the friction pads260, by fluid pressure exerted by the hydraulic pressure unit10, so that the friction pads260can be pressed against the brake rotor BR. In short, the wheel brakes RR, RL have a function of pressing the friction pads260against the brake rotor BR by the fluid pressure derived from the hydraulic pressure unit10.

The nut250has an internal thread251which allows the external thread222of the output shaft221of the speed reducer220to be screwed therein. The nut250is located inside the brake piston240, and engaged with the brake piston240, thereby prevented from rotating relative to the brake piston240, and allowed to move in the axial direction. Accordingly, when the electric motor210runs in the normal direction, the nut250advances toward the friction pads260, and thus can cause the friction pads260to be pressed against the brake rotor BR. When the electric motor210runs in the reverse direction, the nut250moves backward away from the friction pads260, so that the pressing force of the friction pads260exerted on the brake rotor BR is removed. In other words, the parking brake device200has a function of transmitting a power generated by the electric motor210rotating in the normal direction, mechanically to the friction pads260without the use of fluid pressure, to thereby cause the friction pads260to be pressed against the brake rotor BR.

Next, details of the controller100will be described below.

As shown inFIG.4, the controller100comprises a wheel velocity acquisition unit110, a vehicle body velocity computation unit120, a slippage computation unit130, a wheel deceleration computation unit140, a hydraulic brake control unit150, a mechanical brake control unit160, and a storage unit170. The controller100functions as a parking brake control device for exercising control over the parking brake device200.

The wheel velocity acquisition unit110has a function of acquiring a wheel velocity Vw of each wheel3from a corresponding wheel velocity sensor91. The wheel velocity acquisition unit110which has acquired wheel velocities Vw of the wheels3outputs the acquired wheel velocities Vw to the vehicle body velocity computation unit120, the slippage computation unit130, and the wheel deceleration computation unit140.

The vehicle body velocity computation unit120has a function of computing (estimating) a vehicle body velocity Vc by a known method of computation, based on the wheel velocity Vw outputted from the wheel velocity acquisition unit110. The method of computation of the vehicle body velocity Vc may be selected from among various methods; to cite one example, a method may be such that, for example, a wheel velocity Vw of a front wheel is assumed as the vehicle body velocity Vc in principle, and if the acceleration or deceleration of the wheel velocity Vw of the front wheel exceeds a predetermined upper limit, the vehicle body velocity Vc is set at such a value as assumed if the acceleration or deceleration of the wheel velocity Vc is the upper limit. It is to be understood that if the vehicle is provided with an acceleration sensor for detecting an acceleration in a forward or backward direction, the vehicle body velocity Vc may be computed based on the acceleration in the forward or backward direction. The vehicle body velocity computation unit120which has computed the vehicle body velocity Vc outputs the computed vehicle body velocity Vc to the slippage computation unit130and the hydraulic brake control unit150.

The slippage computation unit130has a function of computing a slippage SL of each wheel3, based on the wheel velocity Vw outputted from the wheel velocity acquisition unit110and the vehicle body velocity Vc outputted from the vehicle body velocity computation unit120. To be more specific, the slippage SL can be determined as a difference between the vehicle body velocity Vc and the wheel velocity Vw. The slippage computation unit130which has computed the slippage SL outputs the slippage SL to the hydraulic brake control unit150and the mechanical brake control unit150.

In the present embodiment, a value determined by subtracting the wheel velocity Vw from the vehicle body velocity Vc is used as a slippage SL; however, a slip ratio expressed as (Vc-Vw)/Vc may be used as a slippage SL, instead.

The wheel deceleration computation unit140has a function of computing a wheel deceleration Dw of each wheel3, based on the wheel velocity Vw of each wheel3. Herein, the wheel deceleration Dw taking a positive value indicates that the wheel is decelerating, and the wheel deceleration Dw taking a negative value indicates that the wheel is accelerating. The wheel deceleration Dw can, for example, be computed by subtracting the present value from the last value of the wheel velocities Vw. The wheel deceleration computation unit140which has computed the wheel deceleration Dw of each wheel3outputs the computed wheel decelerations Dw to the mechanical brake control unit160.

The hydraulic brake control unit150has a function of exercising a hydraulic brake control under which the brakes are applied on the wheels3by the hydraulic pressure unit10, for each of the wheels3, based on an ON signal outputted from the parking switch92while the vehicle is running. To be more specific, the hydraulic brake control unit150executes the process of the hydraulic brake control when all of the following conditions have been satisfied: the vehicle body velocity Vc is equal to or higher than a predetermined value; and the parking switch92has been turned ON.

The hydraulic brake control includes an emergency brake control under which a brake fluid pressure is increased by the pump17, and an anti-lock control under which the wheels3are restrained from locking up. The hydraulic brake control unit150starts the emergency brake control upon receipt of an ON signal outputted from the parking switch92while the vehicle is running, and thereafter executes the process of the anti-lock control.

The hydraulic brake control unit150, in the process of the emergency brake control, activates the motor21of the hydraulic pressure unit10, based on the ON signal from the parking switch92.

The hydraulic brake control unit150, in the process of anti-lock control, makes a determination, for each of the wheels3, based on the wheel acceleration Aw and the slippage SL estimated from the wheel speed Vw, as to which state the brake fluid pressure to be exerted on each wheel3is set at, a decreasing state, a increasing state, or a unchanging state. To be more specific, if the slippage SL is equal to or greater than a predetermined threshold SLth and the wheel acceleration Aw is equal to or lower than0, the hydraulic brake control unit150determines that the wheel3is about to lock, and decides to bring the brake fluid pressure into the decreasing state. If the wheel acceleration Aw is higher than0, the hydraulic brake control unit150decides to bring the brake fluid pressure into the unchanging state, while if the slippage SL has become smaller than the predetermined threshold SLth and the wheel acceleration Aw is equal to or lower than0, the hydraulic brake control unit150decides to bring the brake fluid pressure into the increasing state.

When the hydraulic brake control unit150has decided to bring the brake fluid pressure into the decreasing state, the hydraulic brake control unit150proceeds to exercise a pressure decrease control under which electric currents supplied to the inlet valve13and the outlet valve14of the hydraulic pressure unit10are regulated to close the inlet valve13and open the outlet valve14. On the other hand, when the hydraulic brake control unit150has decided to bring the brake fluid pressure into the unchanging state, the hydraulic brake control unit150proceeds to exercise a pressure maintaining control under which electric currents supplied to the inlet valve13and the outlet valve14are regulated to close the both of the inlet valve13and the outlet valve14.

Furthermore, when the hydraulic brake control unit150has decided to bring the brake fluid pressure into the increasing state, the hydraulic brake control unit150proceeds to exercise a pressure increase control under which electric currents supplied to the inlet valve13and the outlet valve14are regulated to open the inlet valve13and close the outlet valve14.

The hydraulic brake control unit150comprises an abnormality determination unit151, a switching unit152, and a pressure decrease rate setting unit153. The abnormality determination unit151has a function of determining whether or not an abnormal condition has occurred in the hydraulic pressure unit10. If the abnormality determination unit151determines that an abnormal condition has occurred in the hydraulic pressure unit10, then the abnormality determination unit151outputs an abnormality signal indicative of the occurrence of an abnormal condition, to the switching unit152. To be specific, for example, if it is determined that an abnormal condition has occurred in the hydraulic pressure unit10, then the abnormality determination unit151sets an abnormality flag at1, while if it is determined that no abnormal condition has occurred, then the abnormality determination unit151sets the abnormality flag at0.

The switching unit152has a function of switching the brake control, upon receipt of an abnormality signal from the abnormality determination unit151while the vehicle is running, from the hydraulic brake control to a mechanical brake control under which a brake is applied to rear wheels32by the electric motor210of the parking brake device200. To be more specific, upon receipt of the abnormality signal from the abnormality determination unit151during the hydraulic brake control, the switching unit152brings the hydraulic brake control to an end, and outputs a signal indicative of an ON/OFF state of the parking switch92to the mechanical brake control unit160. If the switching unit152receives an abnormality signal from the abnormality determination unit151before the hydraulic brake control is started, the switching unit152outputs a signal indicative of an ON/OFF state of the parking switch92to the mechanical brake control unit160, while leaving the hydraulic brake control unactuated.

The switching unit152also has a function of switching the brake control from the hydraulic brake control to the mechanical brake control when the vehicle2has been stopped, specifically, when the vehicle body velocity Vc has become a value approaching 0 (e.g., 0). To be more specific, when the vehicle body velocity Vc has become a value approaching 0, the switching unit152brings the hydraulic brake control to an end, and outputs a signal indicative of an ON/OFF state of the parking switch92to the mechanical brake control unit160.

The hydraulic brake control unit150has a function of bringing the hydraulic brake control to an end, not only upon switching of the control done by the switching unit152but also upon receipt of a request for cancellation of the hydraulic brake control as submitted by a driver’s operation while the vehicle is running. To be more specific, a stop to receive an ON signal from the parking switch92or receipt of a signal from the accelerator sensor93, as the request for cancellation, triggers the cancellation and causes the hydraulic brake control unit150to bring the hydraulic brake control to an end.

In other words, three conditions for bringing the hydraulic brake control to an end are set up, as a closing condition in the present embodiment, that is: receipt of a request for cancellation of the hydraulic brake control as submitted by a driver’s operation while the vehicle is running; a stop of the vehicle2; and occurrence of an abnormal condition in the hydraulic pressure unit10. Thus, the hydraulic brake control unit150brings the hydraulic brake control to an end if at least one of these plurality of closing conditions is satisfied. Specifically, if at least one of the plurality of closing conditions is satisfied, the hydraulic brake control unit150proceeds to exercise a pressure decrease control and ends the hydraulic brake control. Note that the pressure decrease control exercised in the closing stage of a process of the hydraulic brake control will also be called “closing-stage pressure decrease control” in following description.

The pressure decrease rate setting unit153has a function of setting a rate of decrease of pressure (pressure decrease rate R) in such a manner that when the closing-stage pressure decrease control is exercised, the pressure decrease rate R is set according to a closing condition of hydraulic brake control, satisfied upon entry into the closing stage of the process of the hydraulic brake control (hereinafter referred to also as “closing-stage condition”). To be more specific, if the closing-stage condition is receipt of a request for cancellation of the hydraulic brake control as submitted by a driver’s operation while the vehicle is running, the pressure decrease rate setting unit153sets the pressure decrease rate R at a first pressure decrease rate R1having a value greater than a minimum value thereof.

If the closing-stage condition is a stop of the vehicle2, the pressure decrease rate setting unit153sets the pressure decrease rate R at a second pressure decrease rate R2lower than the first pressure decrease rate R1. If the closing-stage condition is an abnormal condition in the hydraulic pressure unit10, the pressure decrease rate setting unit153sets the pressure decrease rate R at a third pressure decrease rate R3lower than the first pressure decrease rate R1and higher than the second pressure decrease rate R2.

Moreover, the pressure decrease rate setting unit153is configured to set the first pressure decrease rate R1according to the vehicle body velocity Vc. To be more specific, the pressure decrease rate setting unit153sets the first pressure decrease rate R1, based on the vehicle body velocity Vc and a first pressure decrease rate setting map as prepared for associating the first pressure decrease rate R1with the vehicle body velocity Vc.

In the first pressure decrease rate setting map, the first pressure decrease rate R1is set in such a manner that the higher the vehicle body velocity Vc, the lower the first pressure decrease rate R1is. That is, the pressure decrease rate setting unit153sets the first pressure decrease rate R1at a first rate R11 if the vehicle body velocity Vc is a first velocity V1, and sets the first pressure decrease rate R1at a second rate R12 higher than the first rate R11 if the vehicle body velocity Vc is a second velocity V2higher than the first velocity V1.

In the first pressure decrease rate setting map, the first pressure decrease rate R1may be set, for example, in a range closer to a maximum value of the pressure decrease rate R. Specifically, the first pressure decrease rate R1may be set at a value equal to or lower than the maximum value of the pressure decrease rate R and higher than the third pressure decrease rate R3.

The second pressure decrease rate R2may be set, for example, at a minimum value of the pressure decrease rate R1. The second pressure decrease rate R2may be set at a value equal to or higher than the minimum value of the pressure decrease rate R and lower than the third pressure decrease rate R3. It is understood that the maximum and the minimum values of the pressure decrease rate R mentioned above are determined, for example, by the capacities of the electric motor210an the speed reducer220, etc.

The mechanical brake control unit160has a function of exercising a mechanical brake control upon receipt of a signal from the switching unit152. The mechanical brake control includes a dynamic actuation control exercised while the vehicle2is running, and a static actuation control exercised while the vehicle2is stopped. The mechanical brake control unit160exercises the dynamic actuation control while the vehicle2is running, and exercises the static actuation control while the vehicle2is stopped.

The mechanical brake control unit160comprises an applying control unit161capable of exercising an applying control under which braking power applied to the wheels3is increased, a stopping control unit162capable of exercising a stopping control under which the braking power applied to the wheels3is maintained, and a releasing control unit163capable of exercising a releasing control under which the braking power applied to the wheels3is decreased.

Herein, the applying control is a control implemented by causing the electric motor210to run in the normal direction, thereby causing the nut250to advance toward the brake rotor BR at a constant speed. To be more specific, in the applying control, a steady electric current is applied to the electric motor210to increase a clamping force of the pair of friction pads260.

The stopping control is a control implemented by causing the electric motor210to stop rotation to stop the nut250. The releasing control is a control implemented by causing the electric motor210to run in the reverse direction, thereby causing the nut250to move back away from the brake rotor BR at a constant speed.

The mechanical brake control unit160exercises the applying control only, for the static actuation control. The mechanical brake control unit160exercises the applying control, the stopping control or the releasing control, selectively on an as appropriate basis for the dynamic actuation control.

The applying control unit161has a function of exercising an applying control which is started if a predetermined starting condition (i.e., the condition for starting the mechanical brake control) is satisfied.

To be more specific, the applying control unit161exercises the applying control in the static actuation control (i.e., control implemented by causing the electric motor210to run in the normal direction for a predetermined period of time), if a starting condition, for example, of the vehicle body velocity Vc assuming a value approaching 0 (e.g., 0) and the parking switch92being in the ON state is satisfied.

The applying control unit161exercises the applying control in the dynamic actuation control, if a starting condition, for example, of the vehicle body velocity Vc assuming a value equal to or higher than a predetermined value and the parking switch92being in the ON state is satisfied. In short, the applying control unit161exercises an applying control if a starting condition for actuating the parking brake device200is satisfied while a vehicle is running.

When the stopping control or the releasing control which will be described below are started, the applying control unit161brings the applying control to an end. To be more specific, the applying control unit161exercises the applying control if the wheel deceleration Dw is lower than a predetermined threshold Dth, for a period of time when the releasing control as will be described below is not exercised.

The stopping control unit162has a function of exercising a stopping control based on a wheel deceleration Dw outputted from the wheel deceleration computation unit140. To be more specific, the stopping control unit162exercises the stopping control by stopping supply of an electric current to the electric motor210, at least if the wheel deceleration Dw during the applying control is equal to or higher than the predetermined threshold Dth. To be more specific, the stopping control unit162exercises the stopping control if the wheel deceleration Dw is equal to or higher than the predetermined threshold Dth for a period of time when the releasing control as will be described below is not exercised. It is to be understood that the threshold Dth is a fixed value in this embodiment.

The releasing control unit163has a function of exercising a releasing control based on a slippage SL outputted from the slippage computation unit130. To be more specific, the releasing control unit163exercises the releasing control if the slippage SL has become equal to or greater than the threshold SLth.

In short, the dynamic actuation control exercised by the mechanical brake control unit160is configured such that: if SL<SLth and Dw<Dth, then the applying control is exercised; if SL<SLth and Dw≥Dth, then the stopping control is exercised; and if SL≥SLth, then the releasing control is exercised.

The storage unit170stores the first pressure decrease rate setting map, the thresholds SLth, Dth, etc. as described above. The first pressure decrease rate setting map, and the thresholds SLth, Dth are set by experiment, simulation, etc. as appropriate.

Next, a detailed description will be given of an operation or process of closing-stage pressure decrease control of the controller100.

If one condition of the closing conditions of the hydraulic brake control is satisfied, the hydraulic brake control unit150starts the closing-stage pressure decrease control according to the flowchart shown inFIG.5(START).

In the closing-stage pressure decrease control, the hydraulic brake control unit150, first, sets a first pressure decrease rate R1according to the vehicle body velocity Vc (S1). After step S1, the hydraulic brake control unit150makes a determination as to whether or not the closing-stage condition is the receipt of a cancellation request (S2).

If it is determined in step S2that the closing-stage condition is the receipt of a cancellation request (Yes), then the hydraulic brake control unit150sets the pressure decrease rate R at the first pressure decrease rate R1set in step S1(S3). If it is determined in step S2that the closing-stage condition is not the receipt of a cancellation request (No), then the hydraulic brake control unit150makes a determination as to whether or not the closing-stage condition is the stop of the vehicle2(S4).

If it is determined in step S4that the closing-stage condition is the stop of the vehicle2(Yes), then the hydraulic brake control unit150sets the pressure decrease rate R at the second pressure decrease rate R2(S5). If it is determined in step S4that the closing-stage condition is not the stop of the vehicle2(No), which means that the closing-stage condition is the remaining one condition (i.e., occurrence of abnormal condition in the hydraulic pressure unit10), then the hydraulic brake control unit150sets the pressure decrease rate R at the third pressure decrease rate R3(S6).

After the pressure decrease rate R is set in step S3, S5or S6, the hydraulic brake control unit150exercises a closing-stage pressure decrease control based on the set pressure decrease rate R (S7). To be more specific, in step S7, the hydraulic brake control unit150regulates an electric current to be passed though the outlet valve14in such a way that the set pressure decrease rate R is achieved.

Next, a detailed description will be given of a closing-stage pressure decrease control depending on each of the closing-stage conditions, with reference toFIGS.6to8.

As shown inFIG.6, when a driver turns on the parking switch92while the vehicle is running (time t1), the hydraulic brake control unit150starts the hydraulic brake control. When the driver takes his/her hand off the parking lever or depresses the accelerator pedal7and causes a request for cancellation to be outputted to the hydraulic brake control unit150, the hydraulic brake control unit150sets the pressure decrease rate R at the higher first pressure decrease rate R1as represented with a sharply declining slope, to cause the brake fluid pressure to decrease rapidly at the set first pressure decrease rate R1. Accordingly, the braking power applied to the wheels3can be removed in a very short time (t2 to t3), so that a driver can be relieved from feeling frustrated as if “dragging”,

As shown inFIG.7, when the hydraulic brake control unit150determines, during the hydraulic brake control, that the vehicle body velocity Vc has become a value approaching 0 (e.g., 0), that is, the vehicle2has been stopped (time t11), the hydraulic brake control unit150starts a closing-stage pressure decrease control. On the other hand, when the closing-stage pressure decrease control is started (time t11), the mechanical brake control unit160starts a mechanical brake control by actuating the electric motor210of the parking brake device200.

In the closing-stage pressure decrease control, the hydraulic brake control unit150sets the pressure decrease rate R at the second pressure decrease rate R2lower than (as represented with a slope declining more gently than that of) the first pressure decrease rate R1, to cause the brake fluid pressure to decrease slowly at the set second pressure decrease rate R2. Accordingly, the brake fluid pressure can be applied to the wheels3for a relatively long time (t11 to t13) in comparison, for example, with an alternative configuration in which the brake fluid pressure is caused to decrease rapidly at the first pressure decrease rate R1as indicated by the chain double-dashed line in the chart, so that the braking power can be maintained when the brake control is switched from the hydraulic brake control to the mechanical brake control, and the vehicle2can be kept at a halt properly.

It is to be understood that the second pressure decrease rate R2may preferably but not necessarily be set to have a slope such that the brake fluid pressure becomes 0 after a time (time t12) when the mechanical braking power (the force of the nut250pushing the brake piston240), which increases under the mechanical brake control starting at approximately the same time as the time when the closing-stage pressure decrease control starts, reaches a target value (e.g., the maximum value) as shown in the chart. In other words, the second pressure decrease rate R2may preferably but not necessarily be set to have a slope such that the total braking power as the sum of the braking power derived from the brake fluid pressure and the mechanical braking power becomes equal to or higher than a predetermined value while the closing-stage pressure decrease control is exercised effectively.

As shown inFIG.8, if the hydraulic brake control unit150determines that an abnormal condition has occurred in the hydraulic pressure unit10during the hydraulic brake control (time t21), then the hydraulic brake control unit150starts the closing-stage pressure decrease control. On the other hand, at the time when the closing-stage pressure decrease control is started (time t21), the mechanical brake control unit160starts the mechanical brake control.

In the closing-stage pressure decrease control, the hydraulic brake control unit150sets the pressure decrease rate R at the third pressure decrease rate R3lower than (as represented with a slope declining more gently than that of) the first pressure decrease rate R1and higher than (as represented with a slope declining more sharply than that of) the second pressure decrease rate R2, to cause the brake fluid pressure to decrease at the set third pressure decrease rate R3. Accordingly, the brake control can be switched from the hydraulic brake control to the mechanical brake control properly when the abnormal condition has been detected in the hydraulic pressure unit10.

Herein, when the brake control is switched from the hydraulic brake control to the mechanical brake control based on the abnormal condition in the hydraulic pressure unit10, the rapid decrease of the brake fluid pressure at such a rate as the first pressure decrease rate R1with the sharply declining slope would potentially make the brake fluid pressure into 0 before the mechanical braking power is raised to a sufficiently high level, with the result that the brake control could not be exercised properly. When the brake control is switched from the hydraulic brake control to the mechanical brake control based on the abnormal condition in the hydraulic pressure unit10, the slow decrease of the brake fluid pressure at such a rate as the second pressure decrease rate R2with the gently declining slope would potentially make the total braking power as the sum of the brake-fluid-pressure-derived braking power and the mechanical braking power excessively high, with the result that the wheels3would disadvantageously become likely to lock up. To address these problems, the third pressure decrease rate R3is set, as in the present embodiment, at a rate (with a slope) between (the slope of) the first pressure decrease rate R1and (the slope of) the second pressure decrease rate R2, so that the brake control can be switched properly from the hydraulic brake control to the mechanical brake control.

According to the present embodiment as described above, the following advantageous effects, in addition to the aforementioned effects, can be achieved.

Since the pressure decrease rate R is set according to the closing-stage condition that is a closing condition satisfied upon entry into the closing stage of the process of the hydraulic brake control, a passenger’s discomfort can be restrained even when the brake is applied by the use of the parking switch92.

Since the higher the vehicle body velocity Vc, the higher the first pressure decrease rate R1is set, the higher the vehicle body velocity Vc, the less likely the driver can be to feel as if “dragging”, and thus the driver’s discomfort can be restrained more effectively.

The above-described embodiment may be modified and implemented in various forms as will be illustrated below. In the following description, substantially the same members and process steps as those in the above-described embodiment will be designated by the same reference characters, and a description thereof will be omitted.

In the above-described embodiment, three conditions are adopted as the closing condition of the hydraulic brake control; however, any number of the closing condition may be adopted, instead. For example, the closing condition may be two conditions consisting of receipt of a request for cancellation of the hydraulic brake control and a stop of the vehicle; in this setting, the pressure decrease rate R may be set at the first pressure decrease rate R1or the second pressure decrease rate R2. Another alternative example of such two closing conditions may consist of the receipt of a request for cancellation of the hydraulic brake control and an abnormal condition in the hydraulic pressure unit10; in this setting, the pressure decrease rate R may be set at the first pressure decrease rate R1or the third pressure decrease rate R3. Yet another alternative of the two closing conditions may be the stop of the vehicle2and an abnormal condition in the hydraulic pressure unit10; in this setting, the pressure decrease rate R may be set at the second pressure decrease rate R2or the third pressure decrease rate R3.

In the above-described embodiment, the wheel deceleration Dw is set at a value taking a positive value when the vehicle2is decelerating; however, the the wheel deceleration may be set at a value taking a positive value when the vehicle is accelerating. That is, the wheel deceleration Dw may be computed by subtracting the last value from the present value of the wheel velocities Vw. In this configuration, the stopping control may be exercised if the wheel deceleration, which takes a negative value when decelerating, passes a negative threshold to a decreasing side (with increasing difference from zero).

In the above-described embodiment, the controller100of the vehicle brake fluid control device1is taken as an example of the parking brake control device; however, the parking brake control device may be configured, for example, as an ECU (Electronic Control Unit) for controlling an engine, or the like.

In the above-described embodiment, the parking brake device200is illustrated as comprising the electric motor210, the speed reducer220, the nut250, and the brake piston240; however, any type of the parking brake device may be feasible as long as it transmits power of an electric motor to the friction member(s) mechanically. For example, the parking brake device may be configured to transmit power of an electric motor to the friction member(s) by a wire.

In the above-described embodiment, the so-called disc brake which comprises the brake rotor BR is illustrated as an example of the wheel brakes FR, FL, RR, RL; however, each of the wheel brakes may be configured, for example, as a drum brake.. In this configuration, a rotating member is configured to be a drum rotating together with a wheel, and a friction member may be a brake shoe that is slidably contactable with the inner peripheral surface of the drum.

The actuation switch may not be limited to the parking switch92which detects motion of the parking lever as in the above-described embodiment; for example, a switch which turns ON by a driver’s pressing operation and turns OFF by his/her subsequent pressing operation may also be feasible.

The elements explained in the above-described embodiments and modified examples may be implemented in combination where appropriate.