Brake force retaining control apparatus

A brake force retaining control apparatus operates on the principle that an amount of depression of an accelerator pedal 20 varies with an angle of inclination of a road surface upon which a motor vehicle starts moving. An appropriate clutch position CLTi for releasing the motor vehicle brake is thereby established according to the amount of accelerator pedal depression. The brake force applied to the motor vehicle is released when a clutch 11 reaches the clutch position CLTi while the driver is depressing the accelerator pedal 20 and the clutch 11 is being automatically shifted from a disengaged position to an engaged position.

Cross-referenced to related application, the present invention is an 
improvement of the invention described in Japanese Patent Application No. 
58-117483 (Japanese Laid-Open Patent Publication No. 60-11719). 
BACKGROUND OF THE INVENTION 
The present invention relates to a brake force retaining control apparatus 
for a motor vehicle equipped with an automatic transmission having a 
friction clutch controlled by an electronic control unit. 
DESCRIPTION OF RELATED ART 
When a motor vehicle equipped with a standard transmission is stopped on a 
hill and starts to move up hill, the driver pulls the emergency or parking 
brake with one hand to keep the motor vehicle braked, then depressing the 
accelerator pedal while simultaneously releasing the clutch, the driver 
releases the emergency brake. Since such a process is complex and requires 
coordinated operation of the emergency brake, accelerator pedal, and 
clutch, the driver is required to be highly skilled in order to start the 
vehicle moving smoothly up hill. If the procedure is not properly 
executed, the engine may stall and the motor vehicle may roll backwards. 
In an effort to solve the above problem, the applicant has proposed a brake 
force retaining control apparatus as disclosed in Japanese Laid Open 
Patent Publication No. 60-11719 referred to above. 
The disclosed brake force retaining apparatus is used on a motor vehicle 
having a synchromesh-type automatic transmission with a friction clutch 
controlled by an electronic control unit. The brake force retaining 
apparatus receives signals from various detectors on the motor vehicle to 
retain a braking force until the clutch is engaged by a hydraulic clutch 
actuator. By retaining the braking force, the motor vehicle is allowed to 
restart upon an uphill road smoothly without requiring skill on the part 
of the driver. 
In such a brake force retaining apparatus, a check valve for the brake is 
always released at a constant clutch engaging position. Therefore, 
depending on the angle at which the road is inclined, the brake may be 
released too early allowing the motor vehicle to roll backward, or too 
late causing the motor vehicle to surge forward. 
SUMMARY OF THE INVENTION 
The present invention has been made in view of the aforesaid drawbacks. 
It is an object of the present invention to provide a brake force retaining 
control apparatus capable of controlling a motor vehicle so as to start 
smoothly upon an incline by releasing the braking force at a suitable time 
dependent upon the amount of accelerator pedal depression. The depression 
of the accelerator pedal is varied by the operator as a function of the 
angle of inclination upon which the motor vehicle is to start. 
To achieve the above object, there is provided in accordance with the 
present invention, a brake force retaining control apparatus for releasing 
a braking force in timed relation to the engaged condition or stroke of a 
clutch controlled by a signal from an electronic control unit. The brake 
force retaining control apparatus comprises sensor means for detecting the 
amount of accelerator pedal depression and engagement position determining 
means for determining an engaging position of the clutch in which the 
retained braking force is to be released. 
With the present invention, irrespective of the road gradient on which the 
motor vehicle is stopped, an optimum braking force release timing can be 
determined according to the inclination of the road as judged by the 
driver of the motor vehicle, and a time taking into account a gear 
position for restarting the motor vehicle can be established. 
The above and other objects, features and advantages of the present 
invention will become more apparent from the following description when 
considered with the accompanying drawings in which the preferred 
embodiment of the present invention is shown by way of illustrative 
example.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 is a block diagram of a brake force retaining control apparatus for 
a motor vehicle having and electronically controlled automatic 
transmission, according to the present invention. As shown in FIG. 1, 
braking system A comprises a brake pedal 1, a brake pedal switch 1a, a 
master cylinder 2, a braking valve assembly 3, a wheel cylinder 4, and a 
solenoid coil 5. The braking valve assembly 3 comprises a check valve 3a 
for allowing a brake fluid to flow in a direction from the master cylinder 
2 to the wheel cylinder 4 and prevents the brake fluid from flowing in the 
opposite direction when solenoid operated cutoff valve 3b connected in 
parallel with the check valve 3a is actuated. The solenoid operated valve 
3b is switchable by the solenoid coil 5 to control the flow of brake fluid 
therethrough. While the solenoid coil 5 is energized after the brake pedal 
1 has been depressed, the braking force is retained by the braking valve 
assembly 3 irrespective of the position of the brake pedal 1. If the brake 
pedal 1 is released and the solenoid coil 5 is de-energized, the braking 
force is no longer retained, i.e., the motor vehicle is no longer braked. 
An electronic control unit 6 comprises a microcomputer for receiving 
signals from various detectors on the motor vehicle such as the brake 
pedal sensor 1a and energizes the solenoid coil 5 in order to retain the 
braking force or de-energizes the solenoid coil 5 in order to release the 
braking force. A clutch system B comprises a fluid pressure source 7, a 
fluid tank 8, a solenoid operated valve 9, a clutch actuator 10 operated 
by the solenoid operated valve 9, a clutch stroke sensor 10a, and a 
friction clutch 11. 
Reference numeral 12 is a transmission, and reference numeral 13 identifies 
an actuator for actuating the transmission 12. The transmission 12 has 
output shaft 12a coupled to a differential gear 14 for transmitting engine 
power to wheels 15. The output shaft 12a is associated with the vehicle 
speed sensor 16 which applies its output signal to the electronic control 
unit 6. A select lever 17 is connected via a select position switch 18 to 
the electronic control unit 6 for determining a gear range of the 
transmission 12. 
An engine 19 is coupled to the friction clutch 11. The power output from 
the engine 19 is controlled by depression of accelerator pedal 20 to 
control the opening of a throttle valve 19b. The engine 19 is associated 
with an engine rotation sensor 19a which supplies an output signal to the 
electronic control unit 6. The electronic control signal 6 is also 
supplied with an output signal from an accelerator pedal sensor 20a which 
is associated with the accelerator pedal 20 and detects the amount of 
depression thereof. The signal from the accelerator pedal sensor 20a and 
the vehicle speed signal from the vehicle speed sensor 16 are processed by 
the electronic control unit 6 for determining an optimum transmission gear 
position and producing a gear change signal to control the clutch actuator 
10. The clutch stroke sensor 10a supplies the electronic control unit 6 
with a signal indicative of the engaging condition (stroke) of the clutch 
11 between its engaged and disengaged positions. 
Reference numeral 21 is an emergency or parking brake, and reference 
numeral 21a identifies a parking brake switch for supplying the electronic 
control unit 6 with a signal representative of the parking brake 21 being 
depressed or released. 
FIG. 2 illustrates a graph of the clutch position (CLTi) when the brake is 
released, with respect to the amount of accelerator pedal depression 
detected by the accelerator pedal sensor 20a. As is apparent from FIG. 2, 
when the motor vehicle starts to move upon a gentle incline similar to a 
flat road, the clutch position CLTi is shallow at the time of releasing 
the brake of the braking system A. When the motor vehicle starts upon a 
steep sloping road, the clutch position CLTi at the time of releasing the 
brake, is deep so that after the friction clutch 11 has been engaged to 
transmit sufficient torque, the braking system A is released. A graph of 
data similar to that shown in FIG. 2 is stored in a memory within 
electronic control unit 6 for each of the gear positions which can be 
selected by the transmission 12. 
Operation of the brake force retaining control apparatus will briefly be 
described below. Only when; (1) the selected gear position is other than a 
neutral gear position, (2) the vehicle speed is zero, (3) the gear 
position is in the lowest gear position (normally first gear position), 
(4) the engine is rotating, (5) the accelerator pedal sensor is off, (6) 
the brake pedal switch is on, and (7) the clutch stroke sensor is off, 
does the braking valve assembly 3 operate to prevent the break fluid from 
flowing from the wheel cylinder 4 to the master cylinder 2. Therefore, the 
motor vehicle continues to be braked even if the brake pedal 9 is 
released. The braking valve assembly 3 is released to allow the motor 
vehicle to move unbraked uphill only when; (1) the selected gear position 
is other than a neutral gear position, (2) the vehicle speed is not zero 
(the motor vehicle is not at rest), (3) the gear position is the lowest 
position (normally first gear position), (4) the engine is rotating, (5 ) 
the accelerator pedal sensor is on, and (6) the clutch stroke sensor is 
on. 
Such operation of the braking force retaining control apparatus will 
hereinafter be described with reference to the flowchart in FIG. 3. 
First, a vehicle speed signal, a select lever signal, a parking brake 
signal, a clutch stroke signal, and an amount of accelerator pedal 
depression are read from the vehicle speed sensor 16, the select position 
switch 18, the parking brake switch 21a, the clutch stroke sensor 10a, and 
the accelerator pedal sensor 20a, respectively, in steps a through e. 
Then, the present gear position is recognized by a signal from the 
transmission actuator 13 in step f. 
Then, step g ascertains if the select lever 17 is in a neutral position 
(equal to zero), step h ascertains if the parking lever 21 is operated, 
and step i ascertains if the vehicle speed is at least a preset value 
SPD1. 
If the select lever 17 is not in the neutral position, the parking lever 21 
is released, and the vehicle speed is smaller than the preset value SPD1, 
then a graph of clutch positions for releasing the braking system A with 
respect to the present gear position is selected in step j. The clutch 
position CLTi for releasing the brake is determined from this graph in 
step k by referring to the amount of accelerator pedal depression read in 
step e. 
The accelerator pedal 20 is depressed, and the clutch which has been 
disengaged starts being automatically and gradually engaged under a 
command from the electronic control unit 6. Then step 1 ascertains whether 
the present position CLT of the friction clutch 11 is closer to the 
engaged position than the clutch position CLTi for releasing the brake. If 
the clutch position CLT is not closer to the engaged position than the 
clutch position CLTi, the braking system A retains its braking force in 
step m. 
If the clutch engaging position CLT is shifted closer to the engaged 
position than the clutch position CLTi, then the braking force is released 
in step n. 
If the select lever 17 is in the neutral position in step g, or if the 
parking lever 21 is operated in step h, or if the vehicle speed is equal 
to or higher than the preset value SPD1 in step i, then the braking force 
is released in step n. 
If the engaging position CLT of the friction clutch 11 does not reach the 
position CLTi, the braking force retention is not released and the cycle 
of steps a through m is repeated periodically after a preset period such 
as 100 msec. At this time, the select lever 17 is in the forward or 
reverse gear position, and the friction clutch 11 is engaged automatically 
based on a signal from the electronic control unit 6. 
In the above embodiment, there has been described a brake force retaining 
control apparatus for a motor vehicle having an automatic transmission 
with a friction clutch which is controlled by an electronic control unit. 
However, the present invention is not limited to the illustrated 
embodiment, and various brake force retaining control modes may be 
constructed within the scope of the invention dependent upon the type of 
motor vehicle, the electronic control unit, etc. 
According to the present invention, when a motor vehicle begins to move 
uphill, the motor vehicle is prevented from rolling backward due to the 
brake being released to early and from surging forward due to the brake 
being released to late. The clutch position for releasing the brake is 
varied according to the amount of accelerator pedal depression. By 
releasing the braking force at a suitable time according to the condition 
in which the motor vehicle runs, the motor vehicle can be smoothly started 
upon an inclined road or hill.