Abstract:
A vehicle brake circuit including a brake actuator, a manually operable valve, an electrically operable valve and a fluid reservoir for the supply of fluid to the actuator. The manually operated valve may be connected to the fluid reservoir either directly or indirectly through the electrically operable valve, when its position blocks a direct fluid connection with the fluid reservoir. The electrically operated valve controls activation or deactivation of the actuator except when the mechanically operated valve is in an unblocked position and overrides the electrically operated valves control of the actuator.

Description:
BACKGROUND 
     Field of Invention 
       [0001]    This invention relates to a brake arrangement for a vehicle. In particular, the invention relates to a parking brake arrangement on an agricultural tractor. 
       SUMMARY 
       [0002]    Agricultural tractors are provided with at least one service brake circuit and a parking brake circuit which operates brakes on the front or rear wheels of the tractor. With the increasing number of controls a driver has to contend with inside the cab, it is easy to forget to activate and deactivate the parking brake when parking. If parking on a steep incline this can lead to a heavy load on the transmission which is undesirable. Moreover, upon starting travel in the tractor, the parking brake must be turned off. 
         [0003]    WO2009/156833 describes a hydraulic parking brake circuit in which the brake is operable by two manually operable means, or an electric means. The electric means is controlled by a tractor control unit and therefore takes the onus of operating the parking brake from the driver. However, because of the arrangement of the brake circuit, one of the manually operable means is connected to the control unit and the other manual means is a lever which cannot override the electric means whether operated by a control unit or the first manual means. Such an arrangement is uncomfortable. 
         [0004]    Accordingly, it is an object of the present invention to provide a vehicle park brake which is operable both by an electric means and a manually operable means and whereby the manually operable means overrides the control of the brake by the electric means. It is a further aim of the invention to provide a vehicle park brake which can easily be used for a hill start. 
         [0005]    In accordance with the invention, there is provided A vehicle brake circuit comprising a brake actuator, a first manually operable valve, a second electrically operable valve and a fluid reservoir for the supply of fluid to the actuator, said first valve connectable to the fluid reservoir both directly and via the second valve, said first valve biased in a position such that a direct fluid connection between the fluid reservoir and the first valve is blocked and a fluid connection between the fluid reservoir and the first valve is via the second valve and wherein movement of the second valve controls activation or deactivation of the actuator and wherein movement of the first valve out of the biased position overrides control of the actuator by the second valve. 
         [0006]    The vehicle operator can rely on the second valve controlling the brake which may be controlled by a vehicle control unit. Advantageously, the operator can override the control of the actuator by the second valve by activating the first valve. Movement of the first valve may be controlled by a handbrake lever. 
         [0007]    Preferably, the actuator is connected to the fluid reservoir via the first valve. 
         [0008]    More preferably, the actuator is connected to the fluid reservoir via a relay valve, said relay valve connected to the first valve. 
         [0009]    Preferably, the control unit detects different vehicle conditions and positions of the first and second valves. This means that the actuator may be applied or disengaged automatically depending on a sensed condition or parameter of the vehicle. 
         [0010]    The brake circuit is preferably a parking brake circuit. More preferably the circuit is a pneumatic circuit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The invention will now be described, by way of example only, with reference to the following drawings in which: 
           [0012]      FIG. 1  is a parking brake circuit in accordance with the invention, and 
           [0013]      FIG. 2  is an alternative parking brake circuit in accordance with the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    An agricultural tractor is provided with two or three independent brake circuits. Usually one or two brake circuits operate the service brakes which are applied to the front and rear wheels of the tractor and one brake circuit is provided for operating the parking brake which is usually applied to the front or the rear wheels of the tractor. Each brake circuit has its own fluid reservoir. 
         [0015]      FIG. 1  shows a pneumatic parking brake circuit  2  having an electrically operable valve  4  (a 3/2 way valve), a manually operable valve  6  (a 4/3 way valve), a relay valve  8 , a fluid reservoir  16  and brake actuation means  14 . Fluid reservoir  16  may be an accumulator. 
         [0016]      FIG. 2  is an alternative brake circuit which is similar to that in  FIG. 1  but without the relay valve  8 . 
         [0017]    The electrically operable valve  4  may be a solenoid valve which can be moved between open and closed positions  4   a  and  4   b  and is connected to and operated by a control unit  10  on the tractor. Electrically operable valve  4  is provided with snap-in locking devices  5  to hold the valve  4  in position when moved into position  4   a,  or  4   b . Tractor control unit  10  receives information concerning various parameters which are measured on the tractor, for example tractor speed, or the functioning of the engine. Valve  4  can thus be operated by control unit  10  to apply or disengage the brakes depending on a measured parameter. 
         [0018]    The manually operable valve  6  is actuated by a handbrake lever  12  which is moveable by the driver. Lever  12  and valve  6  have three different positions: a biased resting position in which the valve is in position  6   a  and the lever is in position  12   a,  an open position in which the valve is in position  6   b  and the lever is in position  12   b  and a closed position in which the valve is in position  6   c  and the lever is in position  12   c.  Lever  12  and the valve  6  are spring tensioned into the biased resting positions  6   a  and  12   a  respectively, so that when no forces are applied to the lever, the valve  6  will rest or move back to position  6   a.  Lever  12  and therefore valve  6  can be held in the closed positions  6   c  and  12   c  respectively by a ratchet  13  on the handbrake lever  12 . 
         [0019]    The brake actuator  14  comprises a piston chamber  14   a  inside which a piston  14   b  is arranged. Piston  14   b  divides the piston chamber  14   a  into two chambers. One chamber is provided with a spring element  14   c  which is in contact with the piston  14   b . The other chamber is fluidly connected to the fluid reservoir  16  via the relay valve  8 . An end of the piston  14   b  is in contact with a brake element (not shown). Movement of brake actuator  14  applies and disengages the parking brake or brakes accordingly. An increase in pressure in chamber  14   a  disengages the brake and causes spring element  14   c  to compress. A decrease in pressure in chamber  14   a  causes spring element  14   c  to expand and apply the brake. 
         [0020]    In  FIG. 1 , valve  6  is shown in the biased resting position  6   a . Valve  6  has four ports and in position  6   a,  two ports of valve  6  are aligned with connections to the reservoir  16  (in which one port is connected to the reservoir  16  via valve  4  by line A and a second port being connected directly to the reservoir  16  by line C). A third port is connected to relay valve  8  by line B and a fourth port is connected to a ventilation outlet. In position  4   a,  line C is blocked by a closed port and line A is connected to line B by an open port. In position  4   b  (as shown) one port is aligned with a connection directly to the reservoir, one is connected to ventilation and the other is connected to valve  6 . If valve  4  were to be moved from position  4   b  into the open position  4   a  by control unit  10 , fluid from the reservoir  16  would flow through lines A and B to the relay valve  8  causing it to move so that chamber  14   a  of the actuator  14  is fluidly connected to the reservoir  16  by line D. An increase in pressure in chamber  14   a  causes piston  14   b  to move against the spring  14   c  and disengage a brake. Normally the handbrake lever  12  and valve  6  are spring tensioned into the biased positions  6   a,    12   a.  When valve  6  is in position  6   a,  the release or activation of the parking brake occurs via valve  4  which is controlled by the control unit  10   
         [0021]    In an automatic mode, the brake may be activated or deactivated automatically depending on an operating condition, or conditions of the tractor, for example, a vehicle speed. 
         [0022]    If, for example in an automatic mode the vehicle speed falls below a predetermined value, the control unit senses the drop in speed and will switch the valve  4  to position  4   b  and relay valve  8  is not activated. As a result, the connection between chamber  14   a  and the fluid reservoir  16  is blocked and the brake is activated. 
         [0023]    If, the vehicle speed exceeds a predetermined value whilst in an automatic mode, the control unit  10  switches the valve  4  to open position  4   a  and the relay valve  8  is activated via lines A, B so that chamber  14   a  is connected with the fluid reservoir  16  which leads to the brake being deactivated. 
         [0024]    When the tractor is at a standstill when in an automatic mode, the control unit  10  senses that the tractor engine is not operating and will switch valve  4  to the closed position  4   b  and the brake will be applied. 
         [0025]    In a non automatic mode when lever  12  is moved to position  12   b,  valve  6  is moved to position  6   b.  In this position line A is blocked and line C is connected to line B so that relay valve  8  is activated and line D is connected to the reservoir  16 . Fluid from the reservoir flows into chamber  14   a  and works against the force of spring  14   c  to disengage the brake. 
         [0026]    If lever  12  is moved to position  12   c,  valve  6  is moved to position  6   c  and fluid from line B is vented which closes relay valve  8  causing the brake to be applied. 
         [0027]    It can be seen that whether valve  4  or valve  6  is moved to disengage the brake, fluid from the reservoir  16  must pass through valve  6  to enable appropriate actuation of the actuator, but does not have to pass through valve  4 . Moreover, when lever  12  is moved to position  12   c  and therefore valve  6  is moved to position  6   c  to apply the brake, the two ports between valve  6  and the reservoir, via valve  4  along line A and directly along line C are blocked and therefore movement of valve  4  has no effect. 
         [0028]    This means that at any time the driver can override the command of the control unit  10  by using the handbrake lever  12  and moving it out of its biased position  6   a  and into a desired position. In this way control of the tractor is maintained by the driver. 
         [0029]    To release the parking brake, the relay valve  8  is activated and chamber  14   a  is fluidly connected to the fluid reservoir  16 . 
         [0030]    Sensor means  18  can be used to detect the position of the handbrake lever  12  and valve unit  6  and inform the control unit  10 . 
         [0031]      FIG. 2  shows a similar circuit to  FIG. 1  but without relay valve  8 . In this figure, valve  6  is connectable to chamber  14   a  of actuator  14  by line E. It can be seen again that whether valve  4 , or valve  6  is moved to disengage the brake, fluid from the reservoir  16  must pass through valve  6  to actuator  14 . To activate the brake, movement of valve  6  to position  6   c  blocks the ports between the reservoir and valve  6 . As a result movement of the hand lever  12  out of biased position  12   a  will override any control of the brake by valve  4 . 
         [0032]    Different operating conditions of the tractor will result in the control unit  10  controlling the brake differently when the valve  6  is in position  6   a.    
         [0033]    If the tractor is in standstill with the engine off and the brake lever is moved to position  12   c  and therefore valve  6  moves to position  6   c,  the brake is applied. The control unit  10  cannot control the brake since the connection of line A to valve  6  is blocked. If the handbrake lever  12  is then moved to position  12   b  (and therefore valve  6  is moved to position  6   b ), the brake is released while handbrake lever  12  is held in position  12   b.  If handbrake lever  12  is moved either manually, or by letting it go to position  12   a,  valve  6  will move to position  6   a  The control unit  10  can only be switched off if the brake is applied. This way, if the tractor is not operating (that is the engine is off) and is at a standstill, it is guaranteed that the handbrake is applied. As a result before the control unit is switched off, or powered off, the electric valve  4  must be in position  4   b  to enable activation of the handbrake. 
         [0034]    If the tractor is in standstill with the engine on and handbrake lever  12  is moved to position  12   c  and therefore valve  6  moves to position  6   c,  the brake is applied. Control unit  10  cannot control the brake. If handbrake lever  12  is moved to position  12   b  and therefore valve  6  is moved to position  6   b,  the brake is released while handbrake  12  lever is held in this position. If handbrake lever is then moved to position  12   a  manually, or by letting it go, the brake will be applied by valve  4  being moved to position  4   b  if the control unit  10  detects that no forces are applied through the transmission (indicating the tractor is not moving) or if the driver seat is not seated. If the control unit senses that a driver is seated in the driver&#39;s seat, the handbrake is released by moving valve  4  to position  4   a.    
         [0035]    If the tractor is moving with the engine on, or off and handbrake lever  12  is moved to position  12   c  and therefore valve  6  is moved to position  6   c,  the brake is applied. Control unit  10  cannot control the brake. If handbrake lever  12  is moved to position  12   b  and therefore valve  6  is moved to position  6   b,  the brake is released while handbrake lever  12  is held in this position. If handbrake lever  12  is moved to position  12   a  manually, or by letting it go and therefore valve  6  is moved to position  6   a,  the control unit will move the valve  4  to position  4   b  so that the parking brake remains in the last braking condition. When the control unit  10  then detects another vehicle condition, it then controls either the release or activation of the parking brake. 
         [0036]    In the figures shown the braking circuits are intended for a pneumatic system but a hydraulic system with the same circuits in accordance with the invention is possible. 
         [0037]    The braking circuits shown in  FIGS. 1 and 2  are intended for a prompt braking action but a gradual braking action is possible if a brake proportioning valve is used.