Device for controlling hydraulic braking of a trailer hitched to a tractor

A hydraulic circuit having a brake pipe suitable for being connected to a pressurized fluid source or to a pressure-free reservoir, i.e. to a reservoir without excess pressure, via a proportional pressure reducer controlled by an electronic control unit (ECU). The circuit includes link solenoid-valve means suitable for connecting the proportional pressure reducer to the reservoir or, via a calibrated pressure reducer, to the pressurized fluid source.

BACKGROUND OF THE INVENTION

The present invention relates to a device for controlling hydraulic braking of a trailer hitched to a tractor, which device comprises a hydraulic circuit having a brake pipe suitable for being connected to a pressurized fluid source or to a pressure-free reservoir, i.e. a reservoir without excess pressure, via a proportional pressure reducer controlled by control means comprising an electronic control unit.

Such a device can serve for braking trailers of various types, in particular farm trailers. Trailers of the “Italian” type exist in which the brake can be activated in a parking situation (i.e. in a rest situation) through lack of pressure in a hydraulic control chamber, whereas it is deactivated in order to enable the trailer to move when a medium pressure (e.g. in the range 8 bars to 15 bars) is maintained in that chamber, and it is reactivated for service braking (i.e. braking while the trailer is moving) when the pressure reaches a much higher value (in particular in the range 15 bars to 140 bars). The parking braking also performs emergency braking which is activated naturally when an incident occurs.

Trailers of the “French” type also exist that have service braking only, that braking being controlled by increasing the pressure in a hydraulic control chamber. In a variant, certain French-type trailers are each equipped with an energy accumulation device making emergency braking possible by feeding the control chamber.

Braking control devices of this type are subjected to contradictory requirements. Firstly, they must make it possible to increase the pressure rapidly in the brake pipe, so as to obtain intense braking whenever necessary. That requires the proportional pressure reducer to be dimensioned in such a manner as to enable a large amount of fluid to flow through in a short time. In addition, it is necessary to adjust the pressure finely in the brake pipe so that, when the trailer is of the Italian type, it is possible to cause a pressure in the range 8 bars to 15 bars to prevail in said brake pipe, since such a pressure is necessary to enable the trailer to move unbraked, and make it possible to avoid the brakes being engaged, even partially, since such engagement might damage them.

An object of the present invention is to propose an improved and simple device that makes it possible to satisfy those two contradictory requirements.

This object is achieved by means of the fact that the circuit includes link solenoid-valve means suitable for connecting the proportional pressure reducer to the reservoir or, via a calibrated pressure reducer, to the pressurized fluid source.

In particular, the calibrated pressure reducer is set for a pressure lying in the range 8 bars to 15 bars, e.g. in the range 10 bars to 13 bars, making it possible to guarantee release of the brake system of a trailer of the Italian type. In which case, in order to achieve such brake release, the proportional pressure reducer is connected to the calibrated pressure reducer rather than being connected directly to the reservoir. It is then by operating said calibrated pressure reducer, i.e. by connecting it to the pressurized fluid source or to the reservoir, that it is possible to obtain the desired pressure in the brake pipe.

When hard braking is necessary, the link solenoid-valve means connect the proportional pressure reducer to the reservoir, and it is by operating the proportional pressure reducer, between the pressurized fluid source to which it is also connected and this link to the reservoir, that it is possible to have the desired pressure prevail in the brake pipe.

Advantageously, a pressure limiter is disposed on a link between the calibrated pressure reducer and the link solenoid-valve means.

This pressure limiter makes it possible to limit the peak pressure at the outlet of the calibrated pressure reducer, so as to make obtaining the desired pressure (e.g. in the range 8 bars to 15 bars) in the brake pipe more reliable.

In an advantageous embodiment, the proportional pressure reducer is a solenoid valve, controlled by the electronic control unit.

In another advantageous embodiment, the proportional pressure reducer is a hydraulically controlled valve including a control chamber suitable for being connected to a control fluid feed via a proportional pilot solenoid valve, itself controlled by the electronic control unit.

In which case, the electronic control unit controls the proportional pressure reducer indirectly, said electronic unit controlling the pilot solenoid valve which, itself, controls the proportional pressure reducer by suitably feeding the control chamber thereof.

Depending on the type of trailer, emergency braking is obtained by emptying the hydraulic control chamber of the brake (Italian-type trailer), or by feeding said control chamber to a sufficient pressure (French-type trailer).

With existing devices, specific adaptation is necessary when it is desired to hitch the tractor to a trailer of one of the above-indicated types, and when emergency braking is to be obtained. Such adaptation operations are tedious and costly, and they are detrimental to the flexibility of use of the tractor.

In a particularly advantageous variant embodiment, the invention makes it possible to obtain such emergency braking, without any specific adaptation. In particular, no adaptation of the trailer is necessary, since all of the elements of the device are provided on the tractor.

Advantageously, the circuit includes a hydraulic brake selector valve that, when in a link position in which the inlet and the outlet of the brake selector valve are interconnected, is suitable for causing a link pipe to be connected to a fluid feed or to a pressure-free enclosure, i.e. to an enclosure without excess pressure, and control solenoid-valve means for controlling the brake selector valve, which means are controlled by the electronic control unit, the control solenoid-valve means for controlling the brake selector valve being suitable for taking up a first working configuration in which they make it possible to feed a first control chamber of the brake selector valve with fluid so as to cause it to go into an isolation position in which the inlet and the outlet of the brake selector valve are isolated from each other by the brake selector valve, a second working configuration in which they make it possible for a second control chamber of the brake selector valve to be fed with fluid so as to cause it to go into said link position, and a neutral configuration in which neither of the two control chambers of the brake selector valve is fed with fluid, and in which the inlet of the brake selector valve is connected to the fluid feed; the device includes means for holding the brake selector valve in position when the control solenoid-valve means go from their first working configuration or from their second working configuration to their neutral configuration; the link pipe is connected to link valve means themselves connected to the brake pipe so as to enable said brake pipe to be fed with fluid or to be connected to the reservoir depending on whether the link pipe is connected to the fluid feed or to the pressure-free enclosure; and the device includes means for delivering to the electronic control unit information indicating whether the brake of the trailer is of a first type requiring brake release by fluid feed, or of a second type, the control unit taking that information into account in controlling the control solenoid-valve means for controlling the brake selector valve.

Advantageously, the link valve means include the hydraulically controlled proportional pressure reducer, and the link pipe is connected to a pilot pipe for piloting said pressure reducer.

In which case, for applying emergency braking to a trailer of the French type, the fluid coming from the link pipe urges the proportional pressure reducer to allow the brake pipe to be connected to the fluid source. For applying emergency braking to a trailer of the Italian type, the link pipe is not fed with fluid so that the proportional pressure reducer allows the brake pipe to be connected to the reservoir.

It is also possible to imagine having the link pipe connected to the brake pipe via selection means for selecting the higher of the pressures at the outlet of the proportional reducer and in the link pipe, so as to feed the brake pipe directly when applying emergency braking to a trailer of the French type, or so as to enable said brake pipe to be connected to the reservoir when applying emergency braking to a trailer of the Italian type.

For example, the control solenoid-valve means comprise two two-position solenoid valves or one solenoid valve suitable for taking up three positions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Firstly, the right portion ofFIG. 1is described. That portion shows the possible examples of trailer braking systems S1, S2, S3and shows a situation S0in which no trailer is hitched to the tractor.

System S1is of the Italian type. It comprises mechanical brake members, e.g. brake linings10that must be moved apart against the inside periphery of a hub in order to perform braking. They are controlled by a hydraulic actuator system comprising a piston12mounted to move in a cylinder13, and hydraulic control means which, as can be seen more clearly in the enlargement, comprise a main enclosure14and a secondary enclosure16. The secondary enclosure16is defined, inside the main enclosure, by a moving cup18that is urged continuously in the braking direction by a main return spring20. The rod of the piston12is urged continuously towards the end wall of the cup by a secondary spring22. A hydraulic pipe24is connected to the main enclosure14. It can be understand that, when said hydraulic pipe is not fed with fluid, the actuator is in the position shown inFIG. 1, in which position the spring20pushes the piston away, so that braking is active, in the parking brake situation. In order to release the brake, i.e. in order to deactivate the parking brake so as to enable the trailer to move, the main enclosure must be fed with pressurized fluid via the pipe24. For a given pressure corresponding to the calibration of the spring20(e.g. approximately in the range 8 bars to 15 bars), the cup18is pushed back towards the end wall14A of the main enclosure against the return force of the spring20, thereby enabling the piston12to move to the extent necessary for releasing the brake. When, starting from this situation, it is necessary to apply service braking, the feed pressure in the pipe24is increased further, and the increased pressure makes it possible to move the piston rod12away from the end wall of the cup18against the return force of the secondary spring22, thereby urging the mechanical brake members towards their braking position. If the brake fluid feed ceases, emergency braking is automatically applied because the situation returns to the parking brake situation. It can thus be understood that this type of trailer requires:absence of pressure in the pipe24in order to enable the parking brake or emergency brake to be activated;medium pressure in that pipe in order to enable the brakes to be released; andincreased pressure in that pipe in order to activate the service brake.

System S2comprises mechanical brake members10and12, e.g. of the same type as the mechanical brake members of system S1. The piston12is mounted to move in a cylinder13, and braking is caused by feeding a brake chamber26with fluid, which brake chamber is situated on one side of the piston, against the return force exerted by a return spring28that is, for example, situated on the other side. Braking system S2is of the French type, with accumulation. It includes an accumulator30that enables emergency braking to be activated. The braking system equipping the trailer of the type S2also includes a solenoid valve32interposed on the brake pipe34connected to the hydraulic chamber26, and a solenoid valve36disposed on the branch38that connects the accumulator30to the pipe34, downstream from the solenoid valve32. These solenoid valves are controlled by an electrical unit UE, which unit also controls the brake lights40of the trailer.

When the trailer is to be enabled to move, the solenoid valves32are36are caused to go to their controlled position II, and the hydraulic fluid can flow in the pipe34so as to empty the brakes. In order to apply braking for the first time, the hydraulic fluid flows in the direction for filling the brake chamber26which, by being filled, causes braking to take place. While the chamber26is being filled, since the solenoid valve36is in its controlled position II, the fluid can flow from the pipe34towards the accumulator30via the solenoid valve36, without being able to return in the opposite direction towards the chamber26. The accumulator is thus filled with fluid the first time braking is applied.

For emergency braking, the solenoid valves32and36cease to be fed, so that they return to their position I. Thus, the fluid contained in the accumulator30can flow towards the chamber26so as to cause braking to occur, and the fluid is not removed from said chamber because the valve32is in its position I.

The system S3is the system of a French-type trailer, without accumulation, the brake members10,12being of the same type as above, and braking being achieved by feeding a hydraulic chamber26with fluid via the brake pipe34so as to cause the brake piston to move against the return force of a spring28.

Conventionally, a trailer is hitched to the tractor via a coupler CT that is secured to the tractor and via a coupler CR that is secured to the trailer. These couplers make it possible to establish both an electrical connection and a hydraulic connection.

In the situation S0, no trailer is hitched and the coupler CT is not connected.

A description follows of the left portion ofFIG. 1that shows the device of the invention in a first embodiment.

The hydraulic circuit of this device includes a hydraulic pump50that could, in particular, be a load-sensing pump whose flow rate is regulated so as to avoid variations in flow rate under the effect of variations in load on the pump. This pump feeds a main pipe52in which the pressure is limited by a pressure limiter54.

The pipe52makes it possible to feed the brake circuit. However, it can be desirable for the pump50also to be used for controlling auxiliary functions of the tractor, e.g. the suspension thereof, the actuators for raising its tow bar, the hydraulic drive for a tool, or indeed various options. For this reason, a priority valve56is disposed on the pipe52. This valve serves to make it possible to perform the auxiliary functions while also making it possible to give priority to performing braking which is a safety function. Thus, the valve56is suitable for taking up two positions, one of which is its position I in which its inlet56A is connected only to its first outlet56B to which an inlet segment58of the brake circuit is connected. If the pressure in the pipe52is sufficient, the valve56can take up its position II, in which its inlet56A is also connected to its second outlet56C, making it possible, via a pipe FA, to feed the hydraulic devices necessary for performing auxiliary functions.

The inlet segment58can be the brake pipe directly or else, as in the example shown, it can be connected to a pipe FT that serves to provide the necessary feed for braking the tractor (the system for braking the tractor not being shown), and to a pipe FR that serves for braking the trailer, the pipes FT and FR branching off from the pipe58.

For braking the trailer, the circuit includes a brake pipe60which, via a valve system, can be connected either to the pipe FR, or to a pressure-free reservoir R, i.e. to a reservoir without excess pressure. It can be understood that connecting the pipe60to the pipe FR via the valve system actually makes it possible to connect it to the pump50. As can be seen inFIG. 1, the brake pipe60is connected to the coupler of the tractor CT so as to enable the brake of the trailer to be hydraulically controlled.

In accordance with the invention, the above-mentioned valve system comprises a proportional pressure reducer62, link solenoid-valve means64, and a calibrated pressure reducer66.

More precisely, the proportional pressure reducer presents a first port62A that is connected to the link solenoid-valve means64, a second port62B that is connected to the pump50via the pipe FR and a third port62C that is connected to the brake pipe60.

Depending on the position of the link solenoid-valve means, the first port62A of the reducer62can be connected either to the reservoir R, via an emptying pipe61, or to the calibrated pressure reducer66.

The link solenoid-valve means comprise at least one solenoid valve. In this example, it is a solenoid valve64having three ports and two positions. The first port64A of this solenoid valve is connected to the first port62A of the proportional pressure reducer62, its second port64B is connected to the reservoir R via the pipe61, and its third port64C is connected to an auxiliary pipe65.

It is this auxiliary pipe65that, depending on the position of the calibrated pressure reducer66, can be connected either to the pump50or to the reservoir R. The calibrated pressure reducer66has a first port66A connected to the reservoir R via the pipe61, a second port66B connected to the pump50via the pipe FR, and a third port66C connected to the auxiliary pipe65.

In conventional manner, the proportional pressure reducer62is controlled to go between a first end position I, in which its ports62B and62C are interconnected so as to connect the brake pipe60to the pump50, and a second end position II, in which its ports62A and62C are interconnected so as to connect the brake pipe60to the first port64A of the solenoid valve64. The rest position of the reducer62is its position II.

The solenoid valve64can take up a position I that is its rest position, in which its ports64A and64B are interconnected, and a second position II in which its ports64A and64C are interconnected. It can thus be understood that, when the solenoid valve64is in the position I, the first port62A of the reducer62is connected to the reservoir. Therefore, when the solenoid valve64is in this position, controlling said reducer62serves to connect the brake pipe60either to the pump50or to the reservoir R, via said reducer. Conversely, when the solenoid valve64is in its position II, then the first port62A of the reducer62is connected to the auxiliary pipe65. Therefore, in order to adjust finely the pressure in the brake pipe60, as is necessary for releasing the brakes of a trailer of the Italian type, the proportional pressure reducer62and the solenoid valve64are placed in their positions II, and the pressure is finely adjusted by the calibrated reducer66, to the value of the calibration pressure thereof.

A pressure limiter68is disposed on a branch67of the auxiliary pipe65. It is connected at its inlet to the auxiliary pipe while its outlet is connected to the reservoir R. This limiter makes it possible to limit the peak pressure in the auxiliary pipe65and thus in the pipe60when the reducer62and the solenoid valve64are in their positions II for fine adjustment of the pressure.

The pressure reducer66is a conventional pressure reducer that maintains a pressure that is substantially constant in the auxiliary pipe65. The reducer62and the solenoid valve64are controlled electrically, by an electronic control unit ECU, via respective control lines l62and l64.

In the example shown, the unit ECU also receives information on the pressure in the brake pipe60, via a line IP connected to a pressure sensor CP.

The unit ECU also receives information IS relating to the type of trailer hitched to the tractor, and braking information IF. This braking information is a function of the desired braking level, it being possible for the braking-level information to be related to the extent to which the brake control pedal or lever is actuated or to the force exerted on said pedal or lever, to the pressure of the fluid in the brake circuit of the tractor, or to any other suitable means. The control of the reducer62by the unit ECU takes account of the desired braking level and the control of the solenoid valve64takes account of the type of trailer.

In normal operation, when the information IS reveals the presence of a trailer of the Italian type, the unit ECU causes the solenoid valve64to go into its position I, and causes the reducer62to go into its rest position II, so as to connect the outlet of said reducer to the reservoir R. The brake pipe60is then connected to the reservoir and the parking brake is thus applied.

In order to release the parking brake and in order to enable the trailer to move, the unit ECU causes the solenoid valve64to go into its position II, and, for a short time, it causes the reducer62to go into its position I, thereby connecting the port62B of the reducer to the pump50, and thereby enabling the brake pipe to be fed rapidly with fluid until a pressure of in the range 10 bars to 15 bars is obtained therein. The control signal for operating the reducer62is rapidly brought to zero so that said reducer returns to its rest position II. Since the solenoid valve64is in its position II, the brake pipe60is then connected to the port66C of the reducer66which adjusts more finely the pressure in the brake pipe so as to maintain therein a pressure of approximately in the range 10 to 13 bars, so as to guarantee that the brakes of the trailer are released.

The duration of the control signal for operating the reducer62to achieve a pressure of in the range 10 bars to 15 bars in the brake pipe, before performing fine adjustment by the reducer66, can be determined by trials. By way of a variant, the unit ECU can be parameterized to bring this signal to zero once the pressure in the pipe60, as measured by the sensor CP, reaches the desired value.

In order to enable service braking to be applied to the trailer while it is moving, it is the reducer62that is caused to go between its positions II and I in such a manner as to obtain the desired pressure level (in the range 18 bars and 140 bars) in the brake pipe.

When the information IS reveals the presence of a trailer of the French type, the unit ECU places the solenoid valve64and the reducer62in their rest positions, so that the brake pipe60is connected to the reservoir. The French-type trailer can then move. In order to brake such a trailer, the reducer62is caused by the control unit ECU to deliver a desired pressure that can reach a maximum value of 140 bars or 150 bars, while the solenoid valve64remains in its rest position I.

A description follows ofFIG. 2, in which the elements that are unchanged relative toFIG. 1are designated by like references.

InFIG. 2, the proportional pressure reducer is a hydraulically controlled valve82comprising a control chamber84that can be connected to a fluid feed via a proportional pilot solenoid valve86, itself controlled by the electronic control unit ECU, via a control line l86.

More precisely, the pilot solenoid valve86has a first port86A that, via a control pipe87, is connected to a pilot pipe85itself connected to the control chamber84, a second port86B that is connected to a pump50′ for feeding that pipe with fluid, and a third port86C that is connected to a pressure-free enclosure R′. The pumps50and50′ could be constituted by a single pump, or else by two different pumps. In particular, the fluid used for piloting the proportional pressure reducer82can have properties different from the properties of the fluid that is used for braking via the brake pipe60, and, for example, can have different viscosity. In which case, the pump50′ is an auxiliary fluid source, different from the pump50, generally having a lower flow rate and a lower pressure, and thus being smaller in size and less expensive, and the pressure-free enclosure R′ is different from the reservoir R.

The pilot solenoid valve86takes up, at rest, an end position I, in which its ports86A and86C are interconnected, while it can be caused, by a signal issued into the control line l86by the unit ECU, to go into its end position II, in which its ports86A and86D are interconnected.

The proportional pressure reducer82has ports82A,82B, and82C that are respectively analogous to the ports62A,62B, and62C of the reducer62ofFIG. 1. In its rest position II, its ports82A and82C are interconnected while, in its controlled position I, its ports82B and82C are interconnected. The links between the ports82A,82B and82C and the various pipes of the circuit are the same as the links between the ports62A,62B, and62C of the reducer ofFIG. 1and the same pipes.

When the pilot solenoid valve86takes up its position I, the link between its ports86A,86C connects the control chamber84of the reducer82to the pressure-free enclosure R′, thereby enabling the reducer82to take up its position II. Thus, the same operation is obtained as when the reducer62ofFIG. 1is in its position II.

When the pilot solenoid valve86is in the position II, the link between its ports86A and86B makes it possible to connect the pilot pipe85to the pump50′. Thus, when said solenoid valve86is piloted by an electrical signal in the line l86to its position II, said solenoid valve makes it possible to establish in the control pipe87a pressure that is proportional to the signal transmitted by the unit ECU, and to feed the control chamber84of the reducer82with fluid so as to urge it into its position I in which it feeds the brake pipe60to the pressure that corresponds to the braking intensity needed for braking the trailer. Operation is then the same as operation of the reducer62ofFIG. 1.

The advantage of the hydraulically controlled pressure reducer82with a pressure multiplication stage is that it accommodates high flow rates while requiring only one electrical control stage, implemented by the pilot solenoid valve86, which can then be of small size in order to generate a control pressure and a flow-rate that are low.

The circuit shown inFIG. 2also includes a hydraulic brake selector valve140controlled hydraulically by control solenoid-valve means which, in this example, comprise two solenoid valves, respectively142and144. This selector valve140can thus take up an link position I in which the inlet140A and the outlet140B of the selector valve140are interconnected and an isolation position II in which said inlet and said outlet are isolated from each other. In addition to its inlet and to its outlet, the brake selector valve140has an auxiliary port140C that is connected to the pressure-free enclosure R′. When this selector valve is in the link position I, the inlet and the outlet140A and140B are interconnected while being isolated from the auxiliary port140C, whereas, when the selector valve is in the isolation position II, its outlet140B is connected to said auxiliary port140C while being isolated from the inlet140A.

The first solenoid valve142for controlling the selector valve140has a first port142A connected to the pump501, a second port142B connected to the reservoir R′ and a third port142C connected to a first control chamber141A of the selector valve140. The second solenoid valve144has a first port144A connected to the pump501, a second port144B connected to the reservoir, a third port144C connected to the second control chamber141B of the selector valve140, and a fourth port144D connected to the first port140A of the selector valve140.

The solenoid valves142and144are controlled by the unit ECU via respective ones of the control lines l142and l144. In the absence of a control signal, the solenoid valve142takes up its position I, in which its second and third ports142B and142C are interconnected while being isolated from its first port142A whereas, when it is controlled, the solenoid valve takes up its second position II in which its first and third ports142A and142C are interconnected while being isolated from its second port142B. It can be understood that when it takes up its first position I shown inFIG. 2, the solenoid valve142connects the hydraulic control chamber141A of the selector valve140to the reservoir R′ whereas, when it takes up its position II, it connects said chamber to the pump50′.

Similarly, the second solenoid valve144takes up its position I without any electrical signal in its control line l144connected to the unit ECU, in which position its second and third ports144B and144C are interconnected (so that the hydraulic control chamber141B of the selector valve140is connected to the reservoir R′) whereas its first and fourth ports144A and144D are interconnected (so that the inlet140A of the selector valve is connected to the pump50′). When it is controlled by a signal in the line l144, the solenoid valve144takes up its position II, in which its first and third ports are interconnected and its second and fourth ports are also interconnected, so that the hydraulic control chamber141B of the selector valve is connected to the pump50′.

The inlet140A of the selector valve140is connected to the fourth port144D of the solenoid valve144, whereas its outlet140B is connected to a link pipe145and its auxiliary port140C is connected to the reservoir R′. It can be understood that, when the selector valve140is in its position I shown inFIG. 2, and when the solenoid valve144is also in its position I, the link pipe145is fed with fluid coming from the pump50. Conversely, when the selector valve140is in its position II, its outlet140B is isolated from its inlet140A, and is thus no longer fed by the pump50′. Conversely, when the auxiliary port140C is present, the outlet140B is then connected to the reservoir R′.

The solenoid valves142and144can take up a first working configuration, in which the solenoid valve142is in the position II whereas the solenoid valve144is in the position I, so that the control chamber141A is fed with fluid and thus urges the selector valve140into its isolation position II in which its inlet and its outlet are isolated from each other. In which case, the link pipe145is no longer fed with fluid. In this example, since the two solenoid valves142and144are urged naturally back into their positions I, this first working configuration is obtained by issuing a control signal into the line l144but not into the line l144.

The solenoid valves142and144can take up a second working configuration, in which the solenoid valve142is in its position I while the solenoid valve144is in its position II, so that the control chamber141B is fed with fluid and urges the selector valve140to be moved into its link position I in which its inlet and its outlet140A and140B are interconnected. In this second working configuration, the inlet140A of the selector valve140is, in addition, connected to the reservoir R′ via the link between the ports144B and144D of the second solenoid valve144, so that the pipe145can be connected to said reservoir R′. Thus, the inlet140A of the selector valve140is connected to the reservoir R′ at least when the solenoid valves142and144are in their second working configuration.

The solenoid valves142and144can, in addition, take up a neutral configuration, in which they are both in position I, so that neither of the two control chambers141A and141B of the brake selector valve140is fed with fluid; they are then substantially at the same pressure. This neutral configuration is obtained in the absence of any control signal in the lines l142and l144.

When the solenoid valves142and144go from one of their working configurations to their neutral configuration, the selector valve140remains in the position it has taken up, through the existence of holding means that are described below. Thus, if the solenoid valves142and144were in their second working configuration, the selector valve140remains in its link position I. However, since the solenoid valve144is in its position I in said neutral configuration, the link pipe145is then connected to the fluid feed via the link between the first and second ports144A and144D of the solenoid valve144. Conversely, if the solenoid valves go from the first working configuration to their neutral configuration, the selector valve140remains in its isolation position II.

The link pipe145is connected to the pilot pipe85of the reducer82. More precisely, the pilot pipe85is connected to said link pipe and to the control pipe87of the pilot solenoid valve86via selection means for selecting the higher of the pressures in the pipes87and145. These means are constituted, in this example, by a shuttle valve122.

Operation is as follows. When the information IS transmitted to the unit ECU reveals that a trailer of the Italian type is hitched to the tractor, the unit ECU issues a signal into the control line l142so as to cause the solenoid valve142to go into its position II, while the solenoid valve144remains in its position I. The solenoid valves142and144are thus in their first working configuration. In which case, the selector valve140is placed in its isolation position II and the link pipe145is not fed with fluid (it is even connected to the reservoir R′ when, as in the example, the auxiliary port140C is present). It is thus by means of the pressure in the control pipe87of the pilot solenoid valve86that the reducer82can be caused to go between its positions I and II. Thus, in order to release the parking brake of the trailer, the solenoid valve64is caused to go into its position II by a signal in the line l64, and it is by piloting the calibrated pressure reducer66that it is possible to deliver, in the brake pipe60, the pressure of in the range 10 to 15 bars that is necessary for this brake release. However, the pilot solenoid valve86can receive, for a short time, a signal different from zero via its control line l86in order to feed the control chamber84of the reducer with a pressure that takes account of the multiplication factor of the hydraulic control means of said reducer82so as to deliver, at the outlet of said reducer82, an outlet pressure of approximately in the range 10 bars to 15 bars with the advantage of rapidly filling the brake cylinders. This signal of line l86can be brought rapidly to zero so that the fine adjustment of the pressure in the brake pipe then takes place by adjusting the reducer66only. As inFIG. 1, the pressure limiter68makes it possible to limit the brake-release peak pressure to within the desired limits.

In order to apply service braking for braking the trailer while it is moving, the reducer82is piloted into its position I by controlling the pilot solenoid valve86as results from a signal issued into the line l86, so as to obtain the desired pressure in the brake pipe60.

The hydraulic brake selector valve140and its control solenoid valves142and144make it possible to obtain emergency braking in the event of electrical failure. In such an event, the solenoid valves return to their rest positions and, in particular, the solenoid valve142is brought to its position I, so that the solenoid valves142and144are in their neutral configuration. The selector valve140remains however in its isolation position II by being held in position by its holding means that are described below, so that its inlet140A is isolated from its outlet140B. The pilot solenoid valve86is then in its rest position I so that the control pipe87is connected to the reservoir R′. Thus, the control chamber84of the reducer82is not fed with pressurized fluid, and said reducer takes up its rest position II, while the link solenoid valve64is in its rest position I, so that the port82C of the reducer82is connected to the reservoir R. Thus, the brake pipe is not fed with fluid, and the parking brake is applied.

When the information IS given to the unit ECU reveals that a French-type trailer is hitched, said unit ECU issues a control signal into the line l144so as to cause the solenoid valve144to go into its position II, whereas no signal is issued into the line l142so that the solenoid valve142remains in its position I. These control solenoid valves142and144are then in their second working configuration. In this situation, the second control chamber141B of the selector valve140is connected to the pump501, so that the selector valve140is piloted into its link position I. Insofar as, in this situation, the solenoid valve144is in its position II, the link pipe145is then connected to the reservoir R′. It is thus by the pressure I in the pilot pipe87that the reducer82is piloted. In order to enable the trailer to move, the reducer82remains in its rest position II and the link solenoid valve64also remains in its rest position I. The brake pipe60is then connected to the reservoir R via the reducer62and via the solenoid valve64, thereby enabling the brake cylinders to be emptied. The trailer is then in the unbraked configuration.

For service braking, the reducer82is piloted in such a manner as to reach its position I. For this purpose, a control signal in the line l86corresponding to the desired braking level places the pilot solenoid valve86in its position II, so that the control pipe87is fed with fluid by the pump50′ at a pressure that enables the control chamber84of the reducer82to be fed with fluid so as to position said reducer in its position I, in which the brake pipe60is connected to the pump50and fed at the pressure corresponding to the desired braking level.

In the event of electrical failure, the solenoid valves take up their rest positions. Thus, the link solenoid valve64is in its position I, as is the pilot solenoid valve86. The control solenoid valves142and144are in their respective rest positions, corresponding to their neutral configurations. Therefore, the control chambers141A and141B of the selector valve140are not fed with fluid, but said selector valve remains in its link position I by means of its holding means for holding it in position. Insofar as the solenoid valve144is then in its rest position I, the link pipe145is fed with fluid coming from the pump50′, thereby making it possible to feed the control chamber84of the reducer82via said link pipe (the control pipe87is connected to the reservoir R′, so that the pressure in the pipe145prevails over the pressure in the pipe87and thus feeds the pilot pipe85via the shuttle valve122).

In the above-described example, the control solenoid-valve means for controlling the hydraulic brake selector valve140comprise two solenoid valves142and144, each of which has two positions.

A description follows ofFIG. 3, in which elements unchanged relative toFIG. 2are designated by like references. InFIG. 3, the control means of said selector valve140comprise a single three-position solenoid valve150. This solenoid valve has a first port150A connected to the pump50′, a second port150B connected to the reservoir R′, a third port150C connected to the first port140A of the selector valve140, a fourth port150D connected to the first control chamber141A of the selector valve140, and a fifth port150E connected to the second control chamber141B of the selector valve140. In this example, the solenoid valve150can be a six-port solenoid valve which is a type more common than a five-port solenoid valve, the sixth port150F being deactivated, by being, for example, connected continuously to the first port150A.

Issuing a signal into the control line l150B connected to the control means151B makes it possible to place the control solenoid valve150in its first position I, which is its first working configuration, in which the first control chamber141A of the selector valve140is fed with fluid. Issuing a signal into the line l150A connected to the control means151A makes it possible to place the control solenoid valve150in its second position II, which is the second working configuration, in which it is the control chamber141B that is connected to the fluid feed. When the solenoid valve150is in the position I, the ports150A and150D are interconnected, and the ports150B and150E are interconnected. In the second working configuration, the ports150B and150D are interconnected are the ports150A and150E are interconnected.

When no control signal is issued, the solenoid valve150takes up its neutral position0, in which the ports150A and150C are interconnected so as to enable the first port140A of the selector valve140to be connected to the pump50′, whereas the other ports of the solenoid valve are isolated. The device operates in the same way as the device described above with reference toFIG. 2.

With reference toFIGS. 4 and 5, a description follows of the means that make it possible to hold the selector valve140in position when it ceases to be urged by its control chambers141A and141B being fed. It can be seen that this selector valve comprises a slide152that is mounted to move in a bore154. Various ports of the selector valve open out into this bore, namely its inlet140A, its outlet140B, and its auxiliary port140C. InFIG. 4, the selector valve140is in its isolation position II, its outlet140B being connected to the auxiliary port140C while the inlet140A is isolated. InFIG. 5, the selector valve is in its link position I, the inlet140A being connected to the outlet140B while the auxiliary port140C is isolated.

The means for holding the selector valve in position comprise resilient means secured to the bore154and that are urged continuously back towards a holding position in which, when they are facing a groove in the slide, they project into said groove. In this example, the resilient means comprise a ball156urged back continuously against the wall of the slide152by a spring157disposed in a bore perpendicular to the bore154and closed off by a stopper158at its end opposite from the bore154. The slide is provided with two grooves, respectively G1and G2. InFIG. 4, the ball156penetrates into the groove G1, thereby tending to hold the slide152in its position II even when the pressure in the chamber141A decreases when the control solenoid-valve means return to their neutral position. InFIG. 5, the ball156is in contact with the bottom of the groove G2, thereby tending to hold the slide in its position I even if the pressure in the chamber141B decreases when the control solenoid-valve means return to their neutral position.

Naturally, it is possible to invert the holding means for holding the selector valve in position, by making provision for the resilient means to be secured to the slide, and, when they are facing a groove in the bore, for them to project into said groove. For example, in this situation, the resilient means could be of the spring clip type tending naturally to take up a radially expanded configuration.

The control signals for controlling the solenoid valves and the information received by the unit ECU can be transmitted by wire or wireless links.