Electrohydraulic parking brake control system for preventing brake engagement when vehicle is in motion
BACKGROUND OF THE INVENTION 
     The present invention relates to vehicle parking brakes and more specifically relates to a system for controlling spring-applied, hydraulically released parking brakes. 
     A known self-propelled agricultural vehicle has a pair of driven ground wheels and is equipped with a braking system including a pair of lever-actuated parking brakes respectively associated with the pair of driven ground wheels. While operators are instructed to set or engage the parking brakes when the vehicle is parked, such action is not always taken creating the hazard of the vehicle possibly rolling unattended and causing property damage and/or physical injury. Interlocks are commonly provided, however, for preventing the vehicle engine from being started unless the parking brake lever is first placed in its brake-engage position. 
     In order to eliminate the problem of operators forgetting or neglecting to set or engage the parking brakes when parking a vehicle, it is known to automate this function by providing vehicles with parking brakes that are spring-engaged and hydraulic pressure-released. The source of fluid pressure for disengaging these brakes is usually provided by an engine-driven pump so that any time the engine is shut down the brakes are automatically engaged. These systems also include an interlock with the vehicle transmission shift lever which results in the brakes being automatically engaged any time the shift lever is moved to its &#34;neutral&#34; position. Thus, the parking brakes are set any time the vehicle is parked with the transmission in neutral and the engine running. Because it is possible that the vehicle may be rolling when the transmission shift lever is placed in its &#34;neutral&#34; position, the known brake system includes an auxiliary ground wheel driven pump that provides sufficient fluid pressure to keep the brakes disengaged until the vehicle has slowed to a predetermined speed. Such a braking system is disclosed in U.S. Pat. No. 5,203,616 granted to Douglas Johnson on 20 April 1993. 
     A drawback of the patented system is that the components used for preventing brake engagement when the vehicle speed is excessive for braking add a significant cost to the system, and engagement of the brakes at even a slow speed has been found to result in jerking which is uncomfortable to the operator. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided an improved control system for spring-applied, pressure-released parking brakes. 
     A broad object of the invention is to provide a parking brake control system for automating the engagement of the parking brakes to ensure that the brakes will be engaged whenever the vehicle is parked with or without the engine running, while preventing the brakes to be engaged so long as the vehicle is in motion. 
     A more specific object of the invention is to provide an electrohydraulic control system which incorporates the vehicle tachometer, the tachometer acting to complete a circuit for actuating a solenoid-operated parking brake control valve so that pressure is supplied to keep the brakes released whenever the vehicle ground speed indicated by the tachometer is other than zero. 
     These and other objects will become apparent from a reading of the ensuing description together with the appended drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The sole FIGURE is a schematic diagram of an electrohydraulic control system for controlling the operation of spring-applied, pressure-released parking brakes. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the drawing, there is shown a parking brake system including identical right- and left-hand parking brakes 10 and 12, respectively, each including a piston 14 slidably mounted in a cylinder with a release-pressure chamber 16 located at one side, and with a brake-engaging spring 18 located at another side, of the piston. 
     An electrohydraulic control system 20 is provided for controlling the flow of brake-release pressure fluid to and from the release-pressure chambers 16. Specifically, the control system 20 includes a solenoid-operated, two-position brake control valve 22 movable between a brake-engage position, as shown, to which it is biased by a biasing spring 23 and in which it couples the release-pressure chambers 16 to a sump 24 while blocking them from pressure supplied by an by an engine-driven, release-pressure supply pump 26. An electrical circuit for selectively supplying power to a solenoid 28 of the brake control valve 22 includes a power source, here shown as a battery 30. A power supply lead 32 is connected between the battery 30 and a switch element 34 of a transmission neutral switch 36, the latter being positioned so that a transmission control lever 37 engages and moves the element 34 against an &#34;off&#34; or &#34;neutral&#34; contact 38, as shown, only when the lever is in its &#34;neutral&#34; position. The neutral switch 36 also includes an &#34;on&#34; or &#34;not neutral&#34; contact 40. A lead 42 is connected between the &#34;on&#34; contact 40 and a grounded relay coil 44 of a parking brake relay switch 46, the latter having a switching element 48 connected to the power supply lead 32 by a branch lead 50. When the relay coil 44, and hence the switch 46, is deactivated, the switch element 48 is connected to a first relay contact 51, as shown, which is connected by a lead 52 to a grounded brake light 54. The relay switch 46 includes a second relay contact 56 connected, by a lead 58, to the solenoid 28 of the brake control valve 22. 
     Thus, when the transmission control lever is not in its &#34;neutral&#34; position, the switch element 34 of the neutral switch 36 will be in contact with the &#34;on&#34; contact 40 so as to complete a power circuit to energize the relay coil 44 and cause it to pull the switching element 48 into engagement with the second relay contact 56 so as to complete a power circuit to the valve solenoid 28. The actuated solenoid 28 will cause the brake control valve 22 to be shifted, against the bias of the spring 23, to its brake-release position coupling the pump 26 in fluid communication with the release-pressure chambers 16 of the cylinders of the parking brakes 10 and 12. 
     To keep the parking brakes 10 and 12 from being set even when the transmission control lever is in its &#34;neutral&#34; position, if the vehicle ground speed is greater than a preselected minimum speed, a ground speed signal processing device in the form of a digital tachometer 60, a ground speed sensing device 62 and a tachometer output or ground speed relay switch 64 are coupled in the control circuit. Specifically, the tachometer 60 includes a port 66 coupled to the ground speed sensing device 62 which generates and sends the tachometer an electrical signal representative of the vehicle ground speed. The tachometer 60 includes a central processing unit (CPU) 70 having an output connected to a grounding switch 71, the CPU 70 being programmed to send a signal to the grounding switch 70 to cause the latter to connect a port 72 to ground in the event the ground speed is above a preselected minimum value (U.S, Pat. No. 4,281,388 granted to Friend et al on 28 July 1981 discloses a tachometer having a signal-shaping circuit and CPU). The preselected ground speed value is preferably zero but may be some small amount greater than zero. In any event, the second port 72 is coupled to a ground side of a coil 74 of the tachometer output relay switch 64, the power side of the coil being coupled to the power supply lead 32 by a branch lead 76. The relay switch 64 includes a switching element 78 that is normally coupled to an &#34;off&#34; contact 80 when the coil 74 is de-energized, as shown. When the vehicle is in motion, the coil 74 will be energized causing the switching element 78 to be pulled into engagement with an &#34;on&#34; contact 82 which is connected to the power supply lead 32 by a branch lead 84. 
     Accordingly, when the transmission neutral switch 36 is in its &#34;off&#34; or &#34;neutral&#34; position, as shown, engagement of the parking brakes 10 and 12 will be prevented if the vehicle is rolling since this movement will be sensed by the speed sensing device 62 which will generate and send a signal to the tachometer 60, the latter acting to process the signal and to effect actuation of the switch 71 to connect the coil 74 to ground resulting in the coil being energized causing the switching element 78 of tachometer output relay switch 64 to be moved to complete a circuit energizing the coil 44 of the parking brake relay switch 46 resulting in its switching element 48 being moved to establish a circuit through the brake control valve solenoid 28. The brake control valve 22 then shifts to its brake-release position coupling the pump 26 to the release-pressure chambers 16 so that the parking brakes remain disengaged. This assures that, in the case where the transmission is a dual-path hydrostatic transmission, the hydrostatic pumps are completely destroked so they do not push against the brakes, and also assures that the vehicle is completely stopped before the brakes are applied so as to prevent uncomfortable jerking and unnecessary loads to the brake components.