TRAILER BREAKING CIRCUIT The present invention relates to a trailer braking circuit and relates particularly, though not exclusively to an hydraulic trailer braking circuit. Trailers for domestic vehicles are legally divided into the categories of below 2.0 Tons GTM (gross trailer mass) and above that limit. For trailers in the above 2.0 Tons GTM bracket the trailers must have four wheel disc brakes which are operable from the driver's normal seated position and incorporate a breakaway facility which must immediately apply the trailer brakes for a minimum of 1 5 minutes should the trailer become detached from the vehicle. The typical system used for such trailers is a vacuum system which supplies vacuum to a pair of vacuum hydraulic master cylinders. Such a system requires an expensive setup on the trailer and a further expensive modification to the vehicle. The modification to the vehicle includes "cutting in" to the vehicle's hydraulics.

This cut is a simple matter for old braking systems but is not applicable to modern braking systems with anti-lock features. Such a cut would invalidate the vehicle's warranty and affect the performance of the vehicle's braking system.

It is an object of the present invention to provide a trailer braking circuit which requires no modifications to the vehicle's hydraulics.

A further object of the invention is to provide a trailer braking circuit which is less expensive than prior art methods and easy to fit.

With these objects in view the present invention provides a trailer braking circuit, said circuit including a source of pressurised

fluid, wheel brake actuator means, a sensor for providing a first signal indicative of the braking force of a prime mover and a proportional valve coupled to said source of pressurised fluid for providing a predetermined pressure to said wheel brake actuator means dependent on said first signal.

In a first embodiment said proportional valve is coupled to said wheel brake actuator means through a control valve, said control valve adapted to be opened by a second signal from a stop light switch on said prime mover. In a second embodiment said circuit includes an emergency valve coupled to said source of pressurised fluid, said emergency valve diverting said pressurised fluid to said wheel brake actuator means if said trailer should be decoupled from said prime mover whilst in a driving mode. In a third embodiment said proportional valve includes a manual override and said circuit includes an emergency actuator coupled to said manual override, whereby, in use, said emergency actuator moves said proportional valve to a maximum pressure position to allow said pressurised fluid to actuate said wheel brake actuator means if said trailer should be decoupled from said prime mover whilst in a driving mode.

In order that the invention may be fully understood, there shall now be described by way of non-limitative examples preferred trailer braking circuits incorporating the principal features of the invention, the description being with reference to the accompanying illustrative drawings, in which:

Fig. 1 is a schematic drawing of a first embodiment of a braking circuit of a trailer coupled to a vehicle with the circuit in the driving mode with no brake pedal depression;

Fig. 2 is a similar view to that of Fig. 1 with the brake pedal depressed; and

Fig. 3 is a similar view to that of Fig. 2 showing the trailer decoupled from the vehicle in the driving mode.

Fig. 4 is a schematic drawing of a second embodiment of a braking circuit of a trailer coupled to a vehicle with the circuit in the driving mode with no brake pedal depression.

Fig. 5 is a similar view to that of Fig. 4 with the brake pedal depressed; and

Fig. 6 is a similar view to Fig. 5 showing the trailer decoupled from the vehicle in the driving mode. In Figs. 1 to 3 there is shown a braking circuit 10 for a trailer (not shown). Circuit 10 is to be coupled to a vehicle's electric circuits to obtain electrical power and various signals from the vehicle (not shown) . A vehicle battery 12 provides the electric power and a vehicle brake pedal switch 14 is activated by driver depression of brake pedal 16. Switch 14 will cause the vehicle brake lights (not shown) to be turned on through lines 18,20 and to provide a brake signal on line 22. A brake sensor 24 is located inside the vehicle's cabin and provides a deceleration signal on line 26. This signal varies from 0 to 12 volts depending on the braking force detected by sensor 24. Such sensors 24 are readily available stock items and the unit made by Hayman Reese has been found to be suitable.

In this preferred embodiment an hydraulic breaking circuit 10 is provided but it is evident that a pneumatic system could readily

be substituted. The trailer has at least a pair of disc brakes 28 but the number of such pairs 28 can be varied depending on the construction of the trailer. Disc brakes 28 receive hydraulic pressure from a master cylinder 30 having a coupling knuckle 32. An hydraulic cylinder 33 is connected to coupling knuckle 32 to actuate the piston in master cylinder 30.

The trailer has an hydraulic pressure supply unit 34 which comprises a pump 36 and electric motor 38. Pump 36 will charge an accumulator 40 until a pressure of 350 psi is reached on pressure switch 42. A solenoid 44 coupled to pressure switch 42 will de¬ activate motor 38 when the pressure of 350 psi is reached. If the pressure falls below 350 psi solenoid 44 will be re-activated to operate motor 38 again. A pressure relief valve 46 is also provided and is set at 500 psi as an emergency bypass if pressure switch 42 fails to operate. The pressures are indicative of a practical environment and are exemplary only. A check valve 48 will prevent flow back to pump 36. Hydraulic pressure from pump 36 is provided along line 50 and a branch line 52.

Line 50 is coupled to a proportional control valve 54. Valve 54 provides a variable regulated pressure which is dependent on the voltage supplied to a solenoid 56 of valve 54. The voltage applied to solenoid 56 is from sensor 24 via line 26. The hydraulic pressure from valve 54 is coupled to a directional control valve 58 via line 60. A solenoid 62 of valve 58 is energised when brake pedal 16 is depressed by the signal along line 22. The hydraulic pressure from valve 58 is passed to hydraulic cylinder 33 along line 64 to operate disc brakes 28.

If required, a breakaway valve 66 may be inserted between directional control valve 58 and hydraulic cylinder 33. Valve 66 allows disc brakes 28 to be automatically applied if the trailer is decoupled from the vehicle. In the preferred embodiment valve 66 has a handle 68 which has a lanyard (not shown) attached thereto for connection to the vehicle towbar. On an unscheduled decoupling of the trailer from the vehicle the lanyard will rotate handle 68 to cause valve 66 to move from its normal operating position.

In order to provide a compact unit, most of the connections are internally provided in manifold 70. Such a construction will provide little external exposure for components and assist in reduction of maintenance.

The operation of braking circuit 10 will now be described. Fig. 1 illustrates the normal driving mode without brake pedal 16 being depressed. Motor 38 will operate pump 36 to provide hydraulic pressure to line 50 and branch line 52.

There are no electric signals on lines 22 and 26 from brake pedal switch 14 and brake sensor 24 respectively. Accordingly, solenoids 56,62 will not be activated and valves 54,58 will be closed. There will be no hydraulic pressure from line 66 and valves

54,58 together with hydraulic cylinder 33 will exhaust to tank 37 of pump 36.

Fig. 2 shows the normal braking mode. Depression of brake pedal 16 causes activation of brake light switch 14 to provide a signal on line 22. This signal will energise solenoid 62 and open directional control valve 58. Valve 58 receives hydraulic pressure from proportional control valve 54 and hydraulic pressure is thus applied to hydraulic cylinder 33 to actuate disc brakes 28. Valve 54

provides a predetermined hydraulic pressure dependent on the voltage from brake sensor 24. Accordingly, the higher the voltage, i.e. greater deceleration, the higher the hydraulic pressure from valve 54 and the greater the braking force provided by disc brakes 28. Fig. 3 shows the breakaway mode where breakaway valve

66 has had handle 68 rotated. Handle 68 is rotated when the trailer is decoupled from the vehicle. Under such circumstances the trailer must have disc brakes 28 applied automatically. Hydraulic pressure will pass along branch line 52 through valve 66 to hydraulic cylinder 33. The hydraulic pressure will not be regulated by valve 54 and the full pressure will be applied.

If motor 38 fails, accumulator 40 will provide hydraulic pressure for operation of braking circuit 10.

Figs. 4 to 6 illustrate a second embodiment of the invention. In this embodiment the operation of the braking circuit has been simplified over that shown in Fig. 4 to 6. Changes made include omission of directional control valve 58 and breakaway valve 66 and a revision of the components for producing hydraulic pressure. In order to avoid duplication of description the same reference numerals are used in Figs. 4 to 6 where identical integers occur in Figs. 1 to 3.

The embodiment of Figs. 4 to 6 does not require a brake signal on line 22 (Figs. 1 to 3) and this has been omitted. Pump 36 will charge accumulator 40 until a pressure of 1700 psi is reached on pressure switch 42. Solenoid 44 coupled to pressure switch 42 will de-activate motor 38 when a pressure of 1700 psi is reached. If the pressure falls below 400 psi solenoid 44 will be re-activated via a second pressure switch 72 to operate motor 38 again. Pressure relief valve 46 is set at 2000 psi as an emergency bypass if pressure

switch 42 fails to operate. The pressure for switch 42 is substantially higher than that used in the embodiment of Figs. 1 to 3. The higher pressures allow, less operation of pump 36 when brake pedal 16 is depressed. Proportional control valve 54 of Figs. 1 to 3 is replaced by a proportional control valve 74. Again valve 74 provides a variable regulated pressure which is dependent on the voltage supplied to solenoid 76 of valve 74. The hydraulic pressure from valve 74 is passed to hydraulic cylinder 33 along line 64 to operate disc brakes 28. Directional control valve 58 (Figs. 1 to 3) is no longer necessary.

Valve 74 includes a manual override 78 which opens valve 74. In this embodiment a pin 80 is coupled to a spring loaded actuator 82 for co-operation with manual override 78. Pin 80 is held under spring tension from actuator 82 by release pin 84. Pin 84 is coupled to a lanyard 86 attached to the vehicle towbar. On an unscheduled decoupling of the trailer from the vehicle lanyard 86 will withdraw release pin 84 and pin 80 will depress manual override 78 through spring tension from actuator 82.

The operation of the braking circuit shown in Figs. 4 to 6 will now be described. Fig. 4 illustrates the normal driving mode without brake pedal 16 being depressed. Motor 38 will operate pump 36 to provide hydraulic pressure to line 50.

There are no electric signals on line 26 from brake pedal switch 14 and brake sensor 24 respectively. Accordingly, solenoid 76 will not be activated and valve 74 will be closed. There will be no hydraulic pressure from line 64 and valve 74, and thus hydraulic cylinder 33 will exhaust to tank 37 of pump 36.

Fig. 5 shows normal braking mode. Depression of brake pedal 16 causes activation of brake light switch 14 to provide a signal on line 26 via sensor 24. The signal on line 26 activates solenoid 76 of proportional control valve 74 and hydraulic pressure is thus applied to hydraulic cylinder 33 to actuate disc brakes 28.

Valve 74 provides a predetermined hydraulic pressure dependent on the voltage from brake sensor 24. Accordingly, the higher the voltage, i.e. greater deceleration, the higher the hydraulic pressure from valve 74 and the greater the braking force provided by disc brakes 28.

Fig. 6 shows the breakaway mode where spring loaded actuator 82 has been released. Pin 84 has been removed and spring, tension from actuator 82 causes pin 80 to operate manual override 78 on valve 74 when the trailer is decoupled from the vehicle. Under such circumstances the trailer must have disc brakes 28 applied automatically. Hydraulic pressure will pass along branch line. 64 to hydraulic cylinder 33. The hydraulic pressure will now be regulated by pressure from valve 74 at its maximum setting and will hold for about 1 5 minutes, using stored energy from accumulator 40. If motor 38 fails, accumulator 40 will provide hydraulic pressure for operation of the braking circuit.

The invention can be readily retrofitted to existing trailers or be provided as a complete braking circuit solution for new trailers. In its preferred form the invention has many failsafe aspects to provide a simple and effective braking circuit.

It is believed that the invention and many of its attendant advantages will be understood from the foregoing description and it will be apparent that various changes may be made in the form,

construction and arrangement of the parts and that changes may be made in the form, construction and arrangement of a braking circuit described without departing from the scope and spirit of the invention or sacrificing all of its material advantages, forms hereinbefore described being merely preferred embodiments hereof.