Patent Description:
Many vehicles are often provided with attached trailers for the transportation of goods and materials. For large-scale use, and in particular in the area of agricultural tractors, such trailers may be provided with trailer-side braking systems to allow for safe control of the trailer, and to prevent jack-knifing or skidding of the trailer when braking.

In the EU, the vehicle braking requirements (RVBR) of the so-called "Mother Regulation" introduces increased safety requirements for the operation of trailer brake systems in the agricultural sector. In particular, agricultural trailers are required to have trailer braking systems responsive to user actuation, where such trailer braking systems must provide for improved response times, and have failsafe systems which act to brake the towed trailer in the event of a systems or communications fault with the towing vehicle.

The present applicants International patent application <CIT> describes a method of controlling a hydraulic trailer brake system using a hydraulic fluid supply provided on a vehicle, wherein a control line (CL) and a supplementary line (SL) are arranged between the vehicle and a vehicle trailer for supply of hydraulic fluid to and from the trailer. The method comprises monitoring the hydraulic fluid pressure in the CL and in the SL, receiving a user braking input; and controlling the pressure of hydraulic fluid in the CL and in the SL to control the operation of the trailer brake system. The controlling is based on a combination of the received user braking input and the detected pressure levels in the CL and SL.

Such an arrangement is not applicable where engine braking is used to slow the vehicle, relying on internal resistance in the vehicle (from the transmission and other components) rather than the application of a service or parking brake of the vehicle.

The detection of a critical jack-knife situation is a major issue here. Jack-knifing occurs when the trailer pushes the towing vehicle such as a tractor (known as a PUSH condition) rather than the tractor pulling the trailer (known as a PULL condition). In general terms, a PUSH condition exists if the torque input to the vehicle wheels (which are driven by the trailer inertia) is greater than the nominal torque supplied by the engine (in certain conditions). It is also known to refer to a PUSH condition as a PUSH mode and to refer to a PULL condition as a PULL mode.

A known parameter used to detect a PUSH condition in trucks is the difference of the set engine speed and the current engine speed. When going downhill, the trailer pushes the truck tractor and thereby wheels begin to rotate faster so the setting of the engine speed (depending on the desired speed) and the speed of the engine (transmitted via wheels) would show a deviation. In agricultural vehicles such as farm tractors the engine speed difference may not be considered as various secondary drives (PTO, hydraulic supply pump) may have an influence. This may have the result that PUSH/PULL conditions cannot be detected by engine speed.

Alternative approaches, e.g. comparing torque supply by engine with the torque transmitted via wheels (indicating a PUSH condition) fail due to complex physical relationships and/or the fact that direct torque measuring requires a high degree of effort.

Further alternative approaches, requiring the provision of two or more pressure sensors in a continuously-variable transmission to determine PULUPUSH conditions, are described in <CIT> and <CIT>.

In accordance with the present invention there is provided a trailer braking system as defined in claim <NUM>.

The trailer braking system defined in claim <NUM> is able to make determine whether a PUSH condition exists based on two direct measurements from the transmission and so is less prone to problems than e.g. torque measurement. The system can operate with only a single pressure sensor in the transmission in an arrangement which is simpler than known systems requiring two pressure sensors to identify a PUSH condition. However, a trailer braking system according to the invention can make use of a second pressure sensor in a secondary or back-up detection to improve overall safety.

Further optional features of the trailer braking system according to this aspect of the invention are set out in the claims dependent on claim <NUM>.

In accordance with a further aspect of the invention, there is provided a method of operating a trailer braking system according to the first aspect of the invention as defined in claim <NUM>.

Further optional features of the method according to the further aspect of the invention are set out in the claims dependent on claim <NUM>.

The invention will now be described, by way of example only, in which.

With reference to <FIG> a vehicle in the form of an agricultural tractor is indicated at <NUM>. The tractor <NUM> comprises front wheels <NUM> and rear wheels <NUM>, a forward engine section <NUM> and a cab section <NUM>. The tractor <NUM> includes a hydrostatic transmission <NUM> driven by the engine and providing motive power to the wheels <NUM>, <NUM> via a driveline <NUM> including a speed sensor <NUM>. The transmission <NUM> includes at least one pressure sensor <NUM>, <NUM> and a rotation sensor <NUM> connected with an electronic control unit (ECU) <NUM>. The operation of the sensors <NUM>, <NUM>, <NUM>, <NUM> and ECU <NUM> in the control of trailer braking is described further below. It will be recognised that ECU <NUM> is suitably a programmable device that may control a number of features and functions of the tractor <NUM>.

The tractor <NUM> is coupled with a towed trailer <NUM>. The trailer <NUM> comprises a trailer body <NUM>, and at least one pair of wheels <NUM>. A trailer brake system <NUM> is provided on the trailer <NUM>, which is connected (as schematically represented by control line <NUM>) with a hydraulic or pneumatic trailer brake valve <NUM> system provided on the tractor <NUM>. The trailer brake valve <NUM> is operated under control of the ECU <NUM>. It will be understood that hydraulic fluid lines (not shown) may extend between the tractor <NUM> and the trailer <NUM>, for powering the braking system <NUM> of the trailer <NUM>.

<FIG> shows a schematic representation of an adjustment unit ADU and a hydraulic power unit of the transmission <NUM>. In this situation, the components belonging to the adjustment unit ADU are located inside the rectangle defined by the broken line <NUM>, and the components belonging to the transmission <NUM> are located outside the rectangle defined by the broken line <NUM>.

The hydraulic power unit of the transmission <NUM> has a pair of hydrostats <NUM>, <NUM>, wherein hereafter the hydrostat <NUM> is designated as the hydraulic pump <NUM> and the hydrostat <NUM> as the hydraulic motor <NUM>. The hydrostats <NUM>, <NUM> illustrated in <FIG> are axial piston machines of oblique-axle design, of which the delivery/intake volume is changed by the pivoting of the axis of rotation of the pistons to an axle drive shaft, not shown.

By means of a first valve unit <NUM> allocated to a hydraulic pump <NUM>, and by means of a second valve unit <NUM> allocated to the hydraulic motor <NUM>, the individual pivot angle of the hydraulic pump <NUM> and/or of the hydraulic motor <NUM> can be adjusted.

In this situation, depending on the specified revolution speed transmission ratio, an actuator element <NUM> is rotated by means of an actuator motor <NUM>. The actuator motor <NUM> is in this case controlled by a control device, such as the ECU <NUM> shown in <FIG>. Because the valve units <NUM>, <NUM> are coupled to the actuator element <NUM>, these valve units <NUM>, <NUM> are displaced corresponding to the actuator element <NUM>. As a result, oil present in a line <NUM> can flow into a cylinder <NUM>, <NUM>', <NUM>, <NUM>' allocated to the valve unit <NUM>, <NUM>.

Due to the displacement of the actuator element <NUM>, the oil flow is accordingly directed out of the line <NUM> into the cylinders <NUM>, <NUM>', <NUM>, <NUM>' and with it the pivot angle of the hydraulic pump <NUM> and of the hydraulic motor <NUM> is adjusted. The pivot angle, and therefore the delivery volume of the hydraulic pump <NUM> and the intake volume of the hydraulic motor <NUM> can accordingly be changed. This makes it possible for the revolution speed of the axle drive shaft, not shown in <FIG>, to be adjusted, and with it the revolution speed transmission ratio of the transmission <NUM>.

In addition, the hydraulic pump <NUM> is connected by fluid means to the hydraulic motor <NUM> by means of a fluid circuit HC. The fluid circuit HC in this situation has an upper circuit UHC and a lower circuit LHC. In this situation, the direction of the arrow F represents a flow direction of the fluid located inside the hydraulic circuit HC during forwards travel of the tractor <NUM> and the direction of the arrow R represents a flow direction of the fluid during reverse travel of the tractor <NUM>.

By means of a first pressure sensor <NUM>, the pressure value pUHC prevailing in the upper circuit UHC can be measured. This pressure value pUHC is then sent to the ECU <NUM> represented in <FIG>. Moreover, both the pressure in the upper circuit UHC as well as the lower circuit LHC are conducted by means of a shuttle valve <NUM> to a second pressure sensor <NUM> in order to measure the pressure value pHCmax.

In this situation, the shuttle valve <NUM> in the transmission <NUM> is designed in such a way so as to communicate to the second pressure sensor <NUM> the greater of the two pressures present in the upper circuit UHC or of the lower circuit LHC as a pressure value pHCmax. When the tractor <NUM> is stationary, the second pressure sensor <NUM> issues a system pressure arising in the upper circuit UHC or the lower circuit LHC as pressure value pHCmax. The second pressure sensor <NUM> serves the purpose to detect if the transmission is facing any load, which is explained in further detail below.

The rotation sensor <NUM> is arranged at the output shaft of the hydraulic motor <NUM>, which is drivingly connectable to the driveline <NUM>. The direction of the rotation of the hydraulic motor <NUM> is determined by rotation sensor <NUM> and the direction of travel of the tractor <NUM> can be concluded. Alternately, the sensor <NUM> may detect speed and direction of rotation for other components of the transmission <NUM>, as will be described further below.

Preferably, when the vehicle is stationary a system pressure of about <NUM> MPa (<NUM> bar) is set in the fluid circuit HC. This system pressure of <NUM> MPa (<NUM> bar) results from the fact that, by means of a supply line SL, the fluid circuit HC is supplied with a constant system pressure by means of a constant hydraulic pump, not shown, driven by the engine of the tractor <NUM>. As soon as the tractor <NUM> moves or the transmission is no longer stationary, the pressure inside the fluid circuit rises, depending on the drive torque, to a high-pressure value of over <NUM> MPa (<NUM> bar). With an average loading of the transmission <NUM>, a high-pressure value of <NUM>-<NUM> MPa (<NUM>-<NUM> bar) is provided. A limit value of <NUM> MPa (<NUM> bar) must not be exceeded, however, since otherwise overstressing of the transmission and its components is to be expected. Due to the pressure prevailing in the fluid circuit HC, the torque of the drive shaft leaving the hydraulic motor <NUM> is determined and therefore the traction force of the transmission unit <NUM>.

In this situation, the first pressure sensor <NUM> serves to identify the presence of a PUSH condition or a PULL condition of the utility vehicle and therefore of the transmission unit <NUM>. The term "PULL condition" is understood hereinafter to mean an operational state in which the tractor <NUM> is driven by the transmission unit <NUM>. The term "PUSH condition" designates the operational state in which, in an uninterrupted positive or adhesion engagement, the tractor <NUM>, including the transmission <NUM>, is kept in a rotational movement by the tractor <NUM> itself, or the trailer <NUM>. This can be the case, for example, with downhill travel or the onset of jack-knifing.

The detection of forward travel or reverse travel of the utility vehicle, and therefore the direction of rotation of the transmission, can be carried out by the rotation sensor <NUM>. In this situation, the direction of rotation of the hydraulic motor <NUM> is detected by the rotation sensor <NUM>. The individual directions of rotation of the hydraulic motor <NUM> during forward or reverse travel of the utility vehicle are in each case opposed to one another. In this situation, a PULL condition of the transmission <NUM>, and therefore also of the tractor <NUM> during forward travel, creates a high pressure PH in the upper circuit UHC which is measured by the first pressure sensor <NUM>. The high pressure PH measured is greater than a system pressure PSYS with the tractor <NUM> at rest.

By means of the hydraulic motor <NUM>, the hydraulic output created by the hydraulic pump <NUM> is converted into a mechanical output. Consequently, in the lower circuit LHC, which with forward movement of the vehicle corresponds to the hydraulic range which forms in the direction of flow between the hydraulic motor <NUM> and hydraulic pump <NUM>, the system pressure PSYS of around <NUM> MPa (<NUM> bar) pertains. At reverse movement of the utility vehicle in pulling mode, by contrast, the system pressure PSYS pertains in the upper circuit UHC and the high pressure PH in the lower circuit LHC.

A PUSH condition on the transmission <NUM>, and therefore also on the tractor <NUM> in forward movement, can be detected by the fact that in this case the hydraulic motor <NUM> of the transmission <NUM> is driven by the tractor <NUM> itself. Consequently, the pressure in the lower circuit LHC rises to the high pressure PH and is measured by the second pressure sensor <NUM>. Moreover, with the tractor <NUM> in PUSH condition, no hydraulic output is created by the hydraulic pump <NUM>. Rather, the hydraulic output produced by the hydraulic motor <NUM> is converted by the hydraulic pump <NUM> into a mechanical output. Consequently, the system pressure PSYS now pertains in the upper circuit UHC, which is measured by the first pressure sensor <NUM>. With the tractor <NUM> in reverse travel in PUSH condition, by contrast, the high pressure PH pertains in the upper circuit UHC and the system pressure PSYS in the lower circuit LHC.

As mentioned above, the second pressure sensor <NUM> serves the purpose to detect if the transmission is facing any load in circuit HC. In standstill, the second pressure sensor <NUM> would therefore detect PSYS. If load in the circuit HC is present, the second pressure sensor <NUM> would detect PH. If e.g. the tractor rolls down a hill without any PUSH/PULL condition (vehicle only overcomes friction resistance of the road contact) the second pressure sensor <NUM> would detect PSYS. Therefore, second pressure sensor <NUM> is relevant for detecting whether an indicated PUSH/PULL condition is correctly identified.

Thereby, the relationship between the detected direction (via rotational speed) and the value of first pressure sensor <NUM> and second pressure sensor <NUM> enables the control system of ECU <NUM> to determine PUSH/PULL conditions. This is summarised in the table of <FIG> which also shows (in the right-hand column) that the first pressure sensor <NUM> may alternatively be positioned in the lower circuit LHC to detect PUSH/PULL conditions whereby the assignment of the pressures values to PUSH/PULL conditions would be opposite to that of the upper circuit UHC.

So to summarize: The combination of rotational speed sensor <NUM> plus pressure sensor <NUM>, <NUM> are indicative of PUSH/PULL conditions:.

Detection of a PUSH condition by the ECU <NUM> is then used to trigger the actuation of the trailer brakes <NUM> - independently of the service or parking brake of the tractor <NUM>. The actuation of the trailer brakes (following previous detection of a PUSH condition) may be aborted or interrupted when the combination of rotational speed plus pressure sensor <NUM> indicates that the vehicle returns to PULL condition.

According to a further aspect of the invention, the system offers a redundant system for the PUSH/PULL condition determination as explained below.

In a first check, the value determined with rotation sensor <NUM> (arranged at the output shaft of the hydraulic motor <NUM>) is compared with the value of the speed sensor <NUM> in driveline <NUM>. If these values do not correlate in terms of the driving direction (taking account of where in the driveline the speed sensor is located), a failure may have occurred (i.e. the identification of a PUSH condition based on the first pressure sensor <NUM> and rotation sensor <NUM> may be erroneous). In the event of failure, alternative measures are suitably triggered.

In a second (or alternative) check, the values of first pressure sensor <NUM> and second pressure sensor <NUM> are compared based on the table shown in <FIG>. For example, if in FORWARD-PULL condition first pressure sensor <NUM> would measure PH while second pressure sensor <NUM> would deviate from PH, a failure may have occurred.

The comparison of the values in the first and second checks may be executed in a time-controlled manner (e.g. in certain time intervals) or it may be event controlled (e.g. if a change in PULUPUSH condition or FORWARD/REVERSE travel is determined).

Providing this redundancy for the PUSH/PULL condition determination is very important in case of operating any brake system. Other triggers for the comparison may also be applicable without leaving the scope of the invention.

Claim 1:
A trailer braking system, comprising:
a vehicle (<NUM>) having a continuously-variable hydrostatic transmission (<NUM>);
a trailer (<NUM>) coupled for towing by the vehicle, the trailer having two or more wheels (<NUM>) with trailer brakes (<NUM>) operable from the vehicle;
the system including a first pressure sensor (<NUM>) arranged to measure a fluid pressure (PH, PSYS) at a predetermined point (UHC) within the transmission, and a rotation sensor (<NUM>) arranged to determine a rotation direction of a predetermined component (<NUM>) in a driveline of the vehicle;
the system further comprising an electronic control unit (ECU <NUM>) coupled to the first pressure sensor (<NUM>) and to the rotation sensor (<NUM>), characterized in the ECU (<NUM>) being configured to determine, based on respective output signals from the first pressure sensor (<NUM>) and the rotation sensor (<NUM>), whether a PUSH condition exists in dependence a particular combination of fluid pressure at the predetermined point within the transmission and rotational direction of the predetermined component within the driveline of the vehicle, and further configured to operate the trailer brakes (<NUM>) in response to a determination that a PUSH condition exists.