Electro-hydraulic brake apply system displacement augmentation

A brake apply system includes a modulator that has a first bore carrying a power driven dual-effect piston. The piston has a base piston surface and an augmentation piston surface. A wheel brake is hydraulically connected to the modulator through an open fluid communication route that is provided between the wheel brake and the base piston surface. A selectively open fluid communication route is provided between the wheel brake and the augmentation piston surface with a check valve mechanism positioned in the selectively open fluid communication route. An augmentation chamber is defined adjacent the augmentation surface of the dual-effect piston. The modulator includes a second bore carrying a valve mechanism that provides an open flow path between the augmentation chamber and a reservoir when pressure at the wheel brake is relatively high and that significantly restricts the flow path when the wheel brake pressure is relatively low. Through this mechanism the augmentation surface is added to the base piston surface for a dual-effect pressure generation area that increases the initial pressurization rate of the brake apply system. Advantageously, the faster the apply rate of the braking system, the greater the augmentation pressure and displacement that is available.

TECHNICAL FIELD 
The present invention relates to an electro-hydraulic brake apply system 
with displacement augmentation and more particularly, to an 
electro-hydraulic brake apply system with a modulator that effects 
hydraulic fluid flow augmentation under certain brake application 
conditions. 
BACKGROUND OF THE INVENTION 
A typical vehicle braking system includes a master cylinder with a power 
booster that intensifies a manual input force and applies it to the master 
cylinder to effect pressurization of an associated braking system. Within 
the master cylinder selective movement of primary and secondary pistons 
develops elevated fluid pressure which is transmitted to the braking 
system. During base brake operation, the primary and secondary pistons 
generate operating fluid pressure which is used to actuate brake calipers 
or wheel cylinders at each vehicle wheel brake. 
Electro-hydraulic brake apply systems are also known wherein the pressure 
applied to a vehicle's wheel brakes is controlled by a electronic unit 
that evaluates several parameters and delivers a control signal to a 
hydraulic modulator that sets the wheel brake pressure. A key parameter 
used to determine the appropriate braking pressure at the wheel brake is 
the driver's command, delivered as an input on the brake pedal. Generally, 
a pressure sensor or brake pedal force sensor monitors apply action 
providing feedback to the control system for use in setting the braking 
pressure. 
A typical hydraulic modulator includes a pressure generation mechanism and 
a means of controlling delivery of the generated pressure to the wheel 
brakes. This may take the form of a pump and proportional hydraulic valve, 
a pump with a pair of two way valves or a movable piston variable pressure 
chamber device. The number and arrangement of these elements included in a 
braking system is determined by the system layout and selected control 
scheme. With a movable piston variable chamber device, a piston is driven 
linearly in a bore to vary the size of a pressure chamber. The pressure 
chamber is connected to a brake line leading to the wheel brake. For an 
application of braking pressure, the size of the variable chamber is 
reduced to take up compliance in the system, and to increase braking 
pressure. The piston applies an increased force to the contained 
non-compressible fluid to apply the brake. To decrease braking pressure, 
the force on the piston is reduced and when appropriate, the size of the 
variable chamber is increased to release the brake. 
An automotive braking system may operate without a booster under electrical 
or electro-hydraulic control in the traditional base brake mode wherein 
manual actuation of the master cylinder effects a desired application of 
the wheel brakes with assistance from a remote pressure modulator. In 
addition to the base brake mode of operation, braking systems are often 
capable of controlling vehicle deceleration through anti-lock operation, 
controlling vehicle acceleration through traction control operation, and 
improving lateral and longitudinal vehicle stability through stability 
enhancement systems which provide a level of dynamic handling 
augmentation. During operation of a braking system in a brake-by-wire type 
of control, the typical master cylinder is isolated from the remainder of 
the braking system and power is effected through an ancillary pressure 
generation mechanism such as a motor driven pump or pressure 
chamber/piston arrangement. In order to effect a fast response time, a 
relatively large motor and pump combination or a large piston is typically 
required. Another known method of providing a fast cycle response time is 
to utilize a separate high pressure accumulator to store a fluid 
pre-charge, which can be applied to the vehicle wheel brakes when 
required. These approaches are somewhat undesirable since they tend to 
increase the overall costs of the system. 
SUMMARY OF THE INVENTION 
A brake apply system according to an aspect of the present invention 
includes a modulator that has a first bore carrying a power driven 
dual-effect piston. The piston has a base piston surface and an 
augmentation piston surface. A wheel brake is hydraulically connected to 
the modulator through an open fluid communication route that is provided 
between the wheel brake and the base piston surface. A selectively open 
fluid communication route is provided between the wheel brake and the 
augmentation piston surface with a check valve mechanism positioned in the 
selectively open fluid communication route. Preferably, a fluid reservoir 
is hydraulically connected to the modulator. An augmentation chamber is 
defined adjacent the augmentation surface of the dual effect piston. The 
modulator includes a second bore carrying a valve mechanism that provides 
an open flow path between the augmentation chamber and the reservoir when 
pressure at the wheel brake is relatively high, and that significantly 
reduces the flow path through a fixed orifice when the wheel brake 
pressure is relatively low. Through this mechanism the augmentation 
surface is added to the base piston surface for a dual-effect pressure 
generation area that increases the initial pressurization rate of the 
brake apply system. For example, greater fluid displacement is provided 
during the initial 100-150 psi pressure build during brake apply, which 
corresponds with the highest compliance portion of the pressure versus 
displacement performance curve of a caliper based braking system. Since 
there is a fixed orifice flow through the augmentation valve mechanism, 
the faster the rate of apply of the power driven piston, the greater the 
pressure rise in the augmentation chamber. Therefore, a greater 
augmentation gain is available for rapid apply situations.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to the drawings, a brake apply system is illustrated in FIG. 1 
and is designated generally at 10. Brake apply system 10 is of the 
electro-hydraulic apply type and operates without a power booster between 
the system's brake pedal and master cylinder assembly. This type of system 
is disclosed in U.S. Pat. No. 5,246,283 to Shaw, which issued Sep. 21, 
1993, and is specifically incorporated herein by reference. The brake 
apply system 10 includes a modulator 11 that has a body 12 made of an 
acceptably rigid material such as metal. The modulator 11 is hydraulically 
connected to a wheel brake 14 and a fluid reservoir 15. In addition, the 
modulator 11 interfaces with a bi-directional drive system (not 
illustrated), through the input shaft 16. Through rotation of the input 
shaft 16, the modulator 11 is functionally capable of applying the wheel 
brake 14 through the use of a pressure generation mechanism and is capable 
of releasing the wheel brake 14 by means of effecting fluid flow back to 
the modulator 11. Fluid compensation (make-up) and discharges from the 
modulator 11 are communicated from and to the reservoir 15. 
Body 12 of modulator 11 includes a main bore 18 that receives the pressure 
generation mechanism in the form of a ball screw driven piston assembly. 
The input shaft 16 is supported on body 12 within bore 18 by a bearing 
assembly 19, and includes a threaded section 20. The threaded section 20 
is drivingly engaged with a ball nut assembly 21 that translates within 
dry chamber 22 of bore 18 through selective rotation of the input shaft 
16. A dual-effect piston assembly 24 is engaged with the ball nut 21 and 
translates within the bore 18 in coordinated movement therewith. Piston 
assembly 24 includes a tubular section 25 that engages the ball nut 
assembly 21 and receives the threaded section 20 of input shaft 16. The 
tubular section 25 is formed as one piece with a body section 26 that 
includes an annular protruding waistline section 27 and a head section 28. 
The waistline section 27 includes a groove that carries an annular seal 29 
that bears against the wall of bore 18 separating the dry chamber 22 from 
an augmentation chamber 30. The waistline section 27 also forms annular 
augmentation piston surface 31, which faces into augmentation chamber 30 
and operates as a moving boundary of augmentation pressure chamber 30. The 
head section 28 similarly includes a groove that carries an annular seal 
32 that bears against the wall of bore 18 separating the augmentation 
pressure chamber 30 from a main pressure chamber 33. The head section 28 
also forms a base piston surface 34 facing into main pressure chamber 33 
and operating as a moving boundary of main pressure chamber 33. With the 
modulator 11 in the fully released position shown, a release opening 35 
allows fluid communication between the main pressure chamber 33 and the 
augmentation pressure chamber 30. This allows full release of the pressure 
at wheel brake 14 to the reservoir 15 when the piston assembly 24 
approaches the full release position. 
The main pressure chamber 33 is open to a bleed valve port 37 and is 
continuously open to the wheel brake 14 through bore 38. The augmentation 
pressure chamber is also open to the main pressure chamber through bore 
39, conduit 40.and bore 38. The conduit 40 includes a check valve 41 that 
allows fluid to flow from augmentation pressure chamber 30 to main 
pressure chamber 33 alongside piston head section 28 of dual effect piston 
assembly 24, while preventing the flow of fluid from main pressure chamber 
33 to augmentation pressure chamber 30 through conduit 40. Optionally, as 
shown in FIG. 3, the conduit 40 and check valve 41 can be replaced by an 
annular lip seal 42 used on the piston head 28 in place of annular seal 
32. The lip seal 42 flexes to allow fluid to flow into main pressure 
chamber 33 from augmentation pressure chamber 30 while preventing fluid 
flow in the opposite direction. 
With the modulator 11 as thus far described, rotation of the input shaft 16 
causes linear translation of the ball nut 21 and the piston assembly 24 
within bore 18. As this occurs, the release opening 35 is closed, and the 
fluid within main pressure chamber 33 and augmentation pressure chamber 30 
is displaced. The fluid from main pressure chamber 33 moves through bore 
38 and conduit 44 to wheel brake 14. The fluid from augmentation pressure 
chamber 30 moves through bore 39, conduit 40, check valve 41, bore 38 and 
conduit 44 to wheel brake 14. Through this mechanism the maximum-effective 
augmentation piston area is equal to the sum of base piston surface 34 and 
augmentation piston surface 31, (or an equivalent piston area resulting 
from the diameter of the waistline section 27), which is significantly 
larger than the head section 28 by itself. However, with this enlarged 
area, a significantly higher torque is required to drive the system 
through the input shaft 16 to generate a given pressure than would be 
required if only the area of base piston surface 34 is used to generate 
pressure. Accordingly, the apply system 10 includes a cut-off mechanism to 
drop the augmentation piston area 31 out of the pressure generation 
mechanism when pressures begin to significantly increase. This permits the 
generation of maximum braking pressures approaching 2000 psi with an 
acceptable level of input torque, while providing higher fluid 
displacements rates when most need to overcome system compliance at lower 
pressures. 
The cut-off mechanism includes a bore 48 that is formed in body 12 
alongside bore 18. The bore 48 intersects bore 38 through reduced diameter 
section 52, bore 39 and a bore 49 that opens to the reservoir 15 through 
conduit 50. A valve obturator in the form of shuttle 51 is carried in bore 
48 and is biased toward step 53 by a spring 54 that is grounded against a 
stop 55 that is fixed in the bore 48. Referring to FIG. 2, the shuttle 51 
is generally provided as a spool shaped member with a pair of lands 
separated by an undercut 56. Each land carries a seal that bears against 
the wall of bore 48 providing fluid separation at the undercut 56 as the 
shuttle moves within the bore 48. A cross bore 58 extends through the 
shuttle 51 at the undercut 56 and a longitudinal bore 59 extends into the 
shuttle 51 and registers with the cross bore 58. The longitudinal bore 59 
threadedly receives a tubular insert 60 that includes a valve seat 61 
formed at the end of central opening 62. A ball 63 is trapped in the 
longitudinal bore 59 by insert 60 and engages the valve seat 61 
substantially closing off flow from the bore 49. A slot 64 is formed in 
the insert 60 at the valve seat 61 providing a continuously open orifice 
bypass around the ball 63. In addition to providing a surface 65 to 
support the spring 54, the stop 55 includes a pin 67 that extends into the 
central opening 62 with clearance 68 for fluid flow. Stop 55 has a groove 
70 that provides an opening between the chamber 71 and the central opening 
62 when the shuttle 51 is forced against the face 72 of stop 55. Stop 55 
also includes a groove carrying an annular seal that closes the chamber 
71. 
Fluid flow control provided through the shuttle 51 and spring 54 
establishes the augmentation cut-off point based on the pressure level 
provided to the wheel brake 14. Referring again to FIG. 1 in conjunction 
with FIG. 2, when the input shaft 16 is rotated to compress the main 
pressure chamber 33 and the augmentation pressure chamber 30, fluid 
pressure increases at wheel brake 14. The pressure increase is initially 
due to fluid being displaced by both the base piston surface 34 and the 
augmentation piston surface 31. Some of the fluid displaced by 
augmentation piston surface 31 travels through bore 39 to bore 48. At bore 
48, with the shuttle 51 in an augmentation position, fluid enters the area 
of undercut 56 and moves into cross bore 58. From cross bore 58, fluid 
moves through longitudinal bore 59, slot 64, central opening 62 and 
chamber 71 to bore 49 for transmission to the reservoir 15. This 
controlled flow through the slot 64 acts to make augmentation pressure 
build up dependent upon flow rate. The result is that significant 
augmentation is provided at fast apply rates and little, or no 
augmentation is provided during slow apply rates. During the apply action, 
the pressure building in the bore 38 acts against the face 73 of shuttle 
51 through the section 52 of bore 48. The force resulting from the 
pressure acting on face 73 is opposed by the spring 54, which has a rate 
selected for the application. When a predetermined pressure level is 
reached, the shuttle 51 moves to compress the spring 54 and the pin 67 
moves further into central opening 62 until the ball 63 is unseated by the 
pin 67. This results in free-flow from the bore 39 to the bore 49 through 
the valve seat 61, cutting-off augmentation and discharging the fluid 
displaced from augmentation chamber 30 to the reservoir 15. The check 
valve 41 prevents the escape of the fluid displaced from main pressure 
chamber 33. The at-rest gap 79 between the face 72 and the base of shuttle 
51 is tightly controlled so that minimal displacement loss occurs when the 
shuttle 51 moves to unseat the ball 63. In the present embodiment the gap 
79 is approximately 0.35 millimeters. 
During a rapid release, when the input shaft 16 rotates in the opposite 
direction to expand the main pressure chamber 33 and the augmentation 
pressure chamber 30, a partial vacuum is developed in the augmentation 
pressure chamber 30. As a result, even though the shuttle 51 is moved so 
that the ball 63 is disengaged from the pin 67, the ball 63 lifts off the 
valve seat 61. This allows rapid back flow from the reservoir 15 and into 
the augmentation pressure chamber 30. Optionally, a small spring force may 
be applied to the ball 63 to ensure its return onto the valve seat 61 when 
the modulator 11 returns to an at-rest condition. 
Referring to FIG. 4, an alternative embodiment of the cut-off mechanism is 
illustrated. In this arrangement, the shuttle does not include an internal 
valve to control flow through the bore 48 between the bores 39 and 49. 
Instead, the shuttle 81 itself operates to obstruct flow. As shown, in an 
augmentation state, the shuttle 81 is positioned to close the bore 39 off 
from the undercut 82. A small amount of fluid can move through the slight 
clearance space between the shuttle 81 and the wall of bore 48. During an 
apply, when the fluid pressure in bore section 52 overcomes the force of 
spring 83, the shuttle will move to open the bore 39 to the bore 49 
through the cross bore 84, cutting-off augmentation. The cross bore 
registers with a longitudinal bore 85 that opens to the blind chamber 86 
to enable the shuttle 81 to move. 
Thus, an augmented brake apply system provides increased fluid displacement 
for initially taking-up compliance in a rapid manner and, by cutting-off 
augmentation, shifts into a main pressure generation mode for applying 
maximum braking pressures. The peak augmentation pressure level is 
tailored to the application and can be modified up to relatively high 
levels by changing the shuttle face area and the spring load applied to 
the shuttle.