Hydraulic actuation system for hydraulically powered parking brakes

The hydraulic actuation system (10) utilizes efficiently stored hydraulic pressure to effect a higher final pressure in the system (10) for a parking brake application. The system includes a pump (12) which communicates separately with accumulators or pressure storage mechanisms (20, 30) isolated from one another. A first accumulator (20) communicates with a first three-position apply and release valve mechanism (40) that can communicate via an apply hydraulic line (26) with a second two-position apply valve mechanism (50). The second accumulator (30) communicates with the second apply valve mechanism (50). The second apply valve mechanism (50) communicates the apply hydraulic line (26) with apply lines (55, 63, 73) and at least one brake (60, 70) that is hydraulically actuated for a parking brake application. The brake (60, 70) includes a release hydraulic line (62, 72, 49) which communicates with the first apply and release valve mechanism (40). Activation of the first apply and release valve mechanism (40) to an apply position (41 ) communicates the first apply and release valve mechanism (40) with the second apply valve mechanism (50) and brake (60, 70) via the apply hydraulic lines (26, 55, 63, 73), while the release hydraulic line (62, 72, 49) is communicated with reservoir (18), to effect an initial phase of the hydraulically actuated parking application.

The present invention relates generally to an actuation system for 
hydraulically powered brakes, and in particular to a two-phase system for 
effecting a higher pressure within the system and increased parking load 
by the brakes. 
BACKGROUND OF THE INVENTION 
Powered parking brake systems may utilize stored energy to apply the brakes 
upon command from a vehicle operator. These parking brake systems may be 
electrically or mechanically actuated. A majority of powered parking brake 
systems utilize a compressed coil spring as the energy source. This spring 
is usually held in the energized state by a pressurized piston. Another 
energy source is a charged hydraulic accumulator wherein hydraulic fluid 
stored under pressure is utilized for a parking brake application. When 
applying the parking brake, the fluid is used to move a piston which 
applies the parking load, either directly or indirectly. This may be an 
integral component of the service brake system or a separate parking 
assembly. When a hydraulic accumulator is employed, the final pressure 
applied is dependent upon the initial accumulator pressure, the stiffness 
of the braking system, and the volumetric capacity of the accumulator. The 
operating clearances and lightly loaded segments of the brake system 
consume the initial delivered volume at the highest pressures of the 
accumulator. The accumulator pressure decreases as fluid displaces the 
piston until the hydraulic system is in balance with the mechanical load 
which is at a maximum at this point in time. If additional brake load is 
desired for the system, the general practice is to increase the volumetric 
capacity of the accumulator so that the discharged or delivered volume of 
the accumulator is a smaller percentage of the total, and thereby 
resulting in a higher final pressure and thus higher braking load when 
fluid flow ceases. 
It is desirable to provide a hydraulic actuation system for hydraulically 
powered parking brakes such as disclosed is copending U.S. Ser. No. 
07/704,586 now U.S. Pat. No. 5,161,650 entitled "Disc Brake with Powered 
Integral Parking Mechanism" and assigned to the same assignee as herein. 
It is desirable to provide a system which effects a higher resultant final 
system pressure and, therefore, a higher mechanical brake load than is 
typically achieved in a system. It is advantageous if the system can 
deliver a higher final pressure with the same initial volume of fluid 
displacement. Thus, the system would be more efficient. 
SUMMARY OF THE INVENTION 
The present invention provides solutions to the above problems by providing 
an actuation system for a hydraulically powered parking brake mechanism, 
the system comprising pump means having an inlet connected with reservoir 
means and an outlet for supplying fluid pressure to at least two pressure 
storage means, the two pressure storage means comprising first pressure 
storage means communicating with first apply and release valve means and 
second pressure storage means communicating with second apply valve means, 
the first apply and release valve means communicating with the second 
apply valve means, and at least one brake mechanism which is actuated 
hydraulically to effect a parking application, the brake mechanism 
connected with the apply and release valve means and with the apply valve 
means to effect said parking application, such that operation of the pump 
means effects the supply of fluid pressure to said first and second 
pressure storage means and, independently, an actuation of the first apply 
and release valve means communicates the first pressure storage means with 
the second apply valve means and brake mechanism to effect an initial 
phase of said parking application, and a deactivation of said first apply 
and release valve means and activation of the second apply valve means 
disconnects the first pressure storage means from communication with the 
brake mechanism and connects the second pressure storage means with the 
brake mechanism to effect a second phase of said parking application.

DESCRIPTION OF THE INVENTION 
Referring to FIG. 1, curve 3 represents the pressure output curve of a 
single accumulator hydraulic actuation system for a hydraulically powered 
parking brake, and curve 4 is the corresponding torque arm load effected 
by the system. Curve 5 is the hydraulic pressure output curve of a 
two-accumulator hydraulic actuation system for hydraulically powered 
parking brakes, and curve 6 is the corresponding torque arm load curve for 
the two-accumulator system. Referring to curve 3, during the application 
cycle hydraulic pressure curve 3 has a typical "S" geometry which settles 
at a final pressure of approximately 1240 psi. The accompanying load 
represented by curve 4 illustrates a static load achievement of about 1400 
pounds. For curves 5 and 6, the two-accumulator actuation system comprises 
two accumulators each of which contains half of the volume of the single 
accumulator of the first hydraulic actuation system. The accumulators act 
independently of each other although each is charged to the same pressure 
to which the single accumulator was charged. When the accumulators are 
independently and serially applied, a higher final pressure and 
consequently a higher final braking load is achieved. Referring to curve 
5, the pressure curve has the signature "S" shape with the curve peaking 
at approximately 970 psi ("P.sub.2 ") at the end of curved portion 
A.sub.1. This is the result of the application of the first, half-volume 
accumulator. At this time the second half-volume accumulator is actuated 
as evidenced by the vertical curved portion A.sub.2. The small "S" curved 
portion A.sub.2 illustrates fluid flow under pressure, and finally the 
pressure release is indicated by A.sub.R2. The final pressure P.sub.3 for 
curve 5 is approximately 1500 psi or about 20% higher than the 1240 psi 
with the nonsequential accumulator system. The braking load (curve 6) 
achieved as a result of utilizing a sequential system is about 1675 pounds 
torque arm force, again approximately 20% higher than the load or force 
achieved by the single accumulator system illustrated by curve 4. The 
initial or first half-volume accumulator supplies the fluid volume 
required to take-up piston motion in the unloaded and lightly loaded state 
of the braking system. Once this has been accomplished, the second 
half-volume accumulator, at the same initial pressure, is applied to the 
system. The sequencing of the accumulators may be made by either 
mechanical or electro-mechanical means. 
Referring to FIG. 2, the hydraulic actuation system is referenced generally 
by numeral 10. Motor 11 drives pump 12 which communicates separately by 
lines 13 and 14 with a first pressure storage means or accumulator 20 and 
a second pressure storage means or accumulator 30. Accumulators 20, 30 are 
isolated from one another by check valves 15 and 16. Pressure switch 19 is 
utilized in system 10 to effect an automatic recharge of accumulators 20 
and 30 by motor pump 12. The inlet side of pump 12 is connected with 
reservoir 18. Accumulator or pressure storage means 20 communicates via 
check valve 22 and line 23 with first apply and release valve 40. Apply 
and release valve 40 communicates via line 24 with reservoir 18, and via 
apply hydraulic line 26 with second apply valve 50. Apply and release 
valve 40 comprises a three-position electrically operated solenoid valve 
which may be, alternatively, a mechanically actuated three-position valve. 
However, valve 40 is illustrated as an electrically applied valve which 
includes apply position 41, release position 42, at-rest or neutral 
position 43, apply transition position 41A, and release transition 
position 42A. 
Second accumulator or pressure storage means 30 communicates via check 
valve 32 and line 33 with apply valve 50. Apply valve 50 comprises a 
two-position electrically operated apply valve which may, alternatively, 
be mechanically applied, but is illustrated as an electrically operated 
solenoid valve. Apply valve 50 communicates via apply position 51 with 
apply hydraulic line 55 that communicates with hydraulic 
application/release apparatus 61 of dual piston disc brake 60. Brake 60 
may comprise the disc brake with powered integral parking mechanism as 
disclosed in copending U.S. Ser. No. 07/704,586, now U.S. Pat. No. 
5,161,650 incorporated by reference herein. Hydraulic apparatus 61 
includes hydraulic release line 62 which communicates via line 49 with 
first apply and release valve 40. The at-rest or neutral position 51 of 
second apply valve 50 connects apply hydraulic lines 26, 55 and terminates 
line 33 at termination 59 so that second pressure storage means or 
accumulator 30 does not communicate with any other parts of the system. 
Apply position 52 of apply valve 50 connects line 26 with termination 59 
so that it does not communicate with any other part of the system while 
line 33, and thus accumulator 30, is connected directly to apply hydraulic 
lines 55, 63 and hydraulic application/release apparatus 61 for a parking 
operation of brake 60. System 10 includes similar or identical brake 70 
with hydraulic application/release apparatus 71, and apply line 73. 
During the operation of system 10, motor pump 12 charges accumulators 20 
and 30 simultaneously through lines 13 and 14 so that the accumulators are 
charged to an initial desired pressure controlled by pressure switch 19, 
while remaining separated from one another via check valves 15 and 16. 
During a parking brake application cycle, apply and release valve 40 is 
energized by the vehicle operator via a not shown switch or other 
mechanism, so that apply position 41 is displaced to an activated position 
and connects line 23 with line 26 so that accumulator 20 communicates with 
second apply valve 50. At the same time, release lines 62 and 72 are 
communicated via line 49 with line 24 and reservoir 18. The communication 
of first accumulator 20 by first apply and release valve 40 with second 
apply valve 50 permits, via the at-rest or neutral position 51, first 
accumulator 20 to communicate fluid pressure to apply hydraulic lines 26, 
55 and hydraulic apparatuses 61, 71 so that brakes 60, 70 experience a 
hydraulically actuated parking application. Fluid exiting hydraulic 
application/release apparatuses 61, 71 is transmitted through lines 62, 
72, 49 and 24 to reservoir 18. When brakes 60, 70 and first accumulator 20 
are in balance, fluid flow ceases at pressure P.sub.2, which may 
correspond to approximately 970 psi as shown by curve portion A.sub.1 of 
curve 5 in FIG. 1. At the appropriate time, second apply valve 50 is 
sequentially actuated and effects the apply position 52 whereby 
accumulator 20 is connected via lines 23 and 26 with termination 59 and 
second accumulator 30 communicated via lines 33 and 55 with the hydraulic 
application/release apparatuses 61, 71. This allows the high pressure 
fluid at the initial pressure in accumulator 30 to be applied to brakes 
60, 70. Because the brakes are already under the initial pressure P.sub.2, 
a further increase in braking load is effected until a hydraulic 
pressure/braking load balance is reached between the brakes and second 
accumulator 30. A final pressure P.sub.3 is achieved and may correspond to 
approximately 1500 psi as illustrated by the top part of curved portion 
A.sub.2 of curve 5. This corresponds to the braking load of approximately 
1675 pounds effected by curve 6 in FIG. 1. Before or after second apply 
valve 50 is actuated, apply and release valve 40 is deactivated so that 
neutral valve position 43 returns to the position illustrated in FIG. 2 
and lines 26 and 24 return to atmospheric pressure. Line 26 still remains 
isolated from the accumulators 20, 30 by means of its connection within 
termination 59 in the apply position 52 of apply valve 50. When the new 
balance at the second or additional (or final) phase of the hydraulic 
parking brake application is achieved, second apply valve 50 is 
deactivated and returns to at-rest or neutral position 51 which then 
effects a complete relief of pressure from the parking system. Neutral 
position 51 connects apply hydraulic lines 26, 55, first apply and release 
valve 40, line 24, and reservoir 18. The parking load within brakes 60, 70 
is maintained via mechanical means. Optionally, the operation of apply 
valve 50 may be controlled by an external sensor such as a height, load, 
or grade sensing mechanism which will call for the additional parking 
force only when the need exists. 
To effect a hydraulic release of hydraulically powered brakes 60, 70, apply 
and release valve 40 is activated to displace into position release 
position 42. This connects hydraulic apparatuses 61, 71, lines 63, 73, 55, 
apply valve 50, line 26, and line 24 with reservoir 18. Pressurized Fluid 
available in first accumulator 20 is transmitted via lines 23, 49, 62 and 
72 to hydraulic apparatuses 61, 71 to effect complete releases of the 
brakes 60, 70. Thus, second apply valve 50 does not require energization 
during the release application cycle. 
Apply and release valve 40 includes transitional positions 41A and 42A. 
Transitional position 41A is moved into place during an application of 
parking brake system 10 so that line 26 is disconnected briefly from 
communication with line 24 and reservoir 18, and before first accumulator 
20 is connected via line 23 with line 26. Release transitional position 
42A effects an intermittent disconnection of release line 49 with line 24 
that communicates with reservoir 18 and a connection of line 26 with line 
24 and reservoir 18 so that a return flow passage is in effect prior to 
moving into effect release position 42 which connects the first 
accumulator 20 and line 23 with release lines 49, 62 and 72. 
The hydraulic actuation system of the present invention provides 
substantial advantages over prior systems. The system is able to deliver a 
higher final delivered pressure to the brakes and achieve higher final 
braking loads with the displacement of the same amount of fluid as 
required for a single accumulator hydraulic actuation system. The 
half-volume accumulators 20 and 30 are reduced in size and yet are able to 
deliver the necessary braking loads. Because of the size reductions, there 
is a lower cost effected by the dual accumulator system. Additionally, the 
system has the inherent ability to provide a two level system which may be 
incorporated with height sensing, load sensing, and grade sensing systems. 
Finally, the brake components of the system may experience longer useful 
lives when height, load or grade sensing systems are employed.