SE541596C2

SE541596C2 - A method for controlling a hydraulic retarder system, a control unit, a hydraulic retarder system, a vehicle, a computer program and a computer-readable medium

Info

Publication number: SE541596C2
Application number: SE1651421A
Authority: SE
Inventors: Erik Gustaf Lilljebjörn
Original assignee: Scania Cv Ab
Priority date: 2016-10-28
Filing date: 2016-10-28
Publication date: 2019-11-12
Also published as: DE102017009225A1; SE1651421A1

Abstract

The invention relates to a method for controlling a hydraulic retarder system (10), comprising: a first valve unit (14) controlling a fluid pressure in a retarder (12); and a second and a third valve unit (16; 18), wherein the first valve unit (14) is controllable by an air pressure in an air conduit (20), the second and the third valve units (16, 18) are on/off valves, the second valve unit (16) being in communication with a supply pressure (P) and the third valve unit (18) being in communication with atmospheric pressure (P), the method comprising: determining a current air pressure in the air conduit (20); determining a requested air pressure in the air conduit (20); and controlling the second and/or third valve unit (16; 18) based on the requested air pressure in relation to; the current air pressure; the atmosphere pressure (P); the supply pressure (P); and an average pressure (P).

Description

A method for controlling a hydraulic retarder system, a control unit, a hydraulic retarder system, a vehicle, a computer program and a computer-readable medium.

TECHNICAL FIELD The present invention relates to a method for controlling a hydraulic retarder system, a control unit, a hydraulic retarder system comprising such a control unit, a vehicle, a computer program and a computer-readable medium.

BACKGROUND Vehicle retarders serve to provide retarding torque for braking the vehicle. There are various types of retarders, such as engine brakes, hydraulic retarders or electric retarders. A hydraulic retarder typically comprises dynamic and static vanes in a chamber filled with a fluid, such as oil or water. The vanes are connected to the output shaft of a transmission, for example via gears. The fluid in the chamber induces viscous drag forces between the vanes and a braking torque is thereby applied on the output shaft. Braking of the vehicle is thus achieved. The braking torque provided by the retarder can be controlled by controlling the amount of fluid in the chamber. A larger amount of fluid in the chamber will increase the retardation. The retarder is suitably part of a retarder system further comprising for example control means for controlling the amount of fluid in the retarder chamber and a heat exchanger for cooling the retarder fluid.

There are many different ways of controlling the amount of fluid in the retarder chamber. Typically a hydraulic mechanical valve unit is used, where the valve unit is controlled to control the amount of fluid in the retarder chamber. The mechanical valve unit is typically controlled by an air pressure acting on the mechanical valve unit, thereby determining the position of the valve unit. Thus, a certain air pressure acting on the mechanical valve unit corresponds to a certain amount of fluid in the retarder chamber and thereby a certain braking torque. The mechanical valve unit can be controlled in various ways. One commonly known way is to control the air pressure acting on the mechanical valve unit by means of a proportional valve, which is controlled by an electric current. The proportional valve unit is arranged in fluid communication with a source of pressurized air and ambient air and can thereby regulate the air pressure affecting the mechanical valve unit. Using a proportional valve may however cause a stick slip phenomenon due to friction in the proportional valve unit. This means that the proportional valve unit may remain closed until enough current is provided to overcome the friction, whereby the proportional valve unit opens too much. It is thus difficult to control the air pressure acting on the mechanical valve unit by means of a proportional valve unit.

Another way of controlling the air pressure acting on the mechanical valve unit is to use on/off valves, which only can be controlled to be either completely open or completely closed. Typically two on/off valves are used, a first valve connected to pressurized air and a second valve connected to ambient air. Thus, by opening the first valve the amount of pressurized air is increased and the pressure acting on the mechanical valve increases and by opening the second valve air is drained and the pressure acting on the mechanical valve decreases. The problem with such on/off valves is that large opening areas of the valves are required in order to be able to rapidly activate or inactivate the retarder. Large opening areas, however, result in rapid changes of pressure which makes it difficult to fine tune the pressure acting on the mechanical valve unit and which thereby affects the accuracy of the braking torque. One way of trying to enable a better control of the air pressure by means of on/off valves, is to control how fast/slow the on/off valves are opened/closed. This can be done by controlling the pulse-width of the signal controlling the on/off valves by means of pulse-width modulation (PWM).

Document W02005/120918 A1 discloses two on/off valves with different opening areas connected to pressurized air and two on/off valves with different areas connected to the ambient air. This way, the on/off valves with larger opening areas can be controlled to rapidly increase/decrease the pressure and the valve units with smaller opening areas can be controlled for fine adjustments of the pressure.

Another problem with controlling on/off valves is that a certain delay typically occurs in the system. A control signal may be sent to the on/off valve to achieve a desired air pressure in association with the mechanical valve unit and a pressure sensor determines the actual air pressure. Once the air pressure has been determined to comply with a desired air pressure a new control signal is sent to close both on/off valves. Due to the delay in the system the on/off valves may be closed too late, causing a too high/low air pressure compared to the desired air pressure.

SUMMARY OF THE INVENTION Despite known solutions in the field, it would be desirable to develop a method for controlling a hydraulic vehicle retarder arrangement, which overcomes or at least alleviates the drawbacks mentioned above.

An object of the present invention is therefore to achieve a new and advantageous method for controlling a hydraulic retarder system, which increases the braking torque accuracy. Another object of the invention is to achieve a new and advantageous control unit, a hydraulic retarder system, a vehicle, a computer program and a computer-readable medium which increase the braking torque accuracy.

The herein mentioned objects are achieved by a method for controlling a hydraulic retarder system, a control unit, a computer program and a computerreadable medium according to the independent claims.

Hence, according to an aspect of the present invention a method for controlling a hydraulic retarder system is provided. The hydraulic retarder system comprising: a retarder unit; a first valve unit arranged to control a fluid pressure in the retarder unit for influencing a retarding torque of the retarder unit; and a second valve unit and a third valve unit each arranged in fluid communication with the first valve unit via an air conduit. The first valve unit is controllable by an air pressure in the air conduit, wherein the second and the third valve units are on/off valves which are controllable to an open state or a closed state, the second valve unit, in the open state, being in fluid communication with pressurized air having a supply pressure and the third valve unit, in the open state, being in fluid communication with ambient air having atmosphere pressure. The method comprises: - determining a current air pressure in the air conduit; - determining a requested air pressure in the air conduit; and - for providing the requested air pressure in the air conduit, controlling the second valve unit and/or the third valve unit based on the requested air pressure in relation to: the current air pressure; the atmosphere pressure; the supply pressure; and an average pressure, wherein the average pressure is the air pressure in the air conduit achievable when the second valve unit and the third valve unit are in their open states at the same time.

The second and the third valve units may e.g. be solenoid on/off valves. The second valve unit may be referred to as a filling valve. The second valve unit may e.g. be arranged in fluid communication with a supply unit of pressurized air, where the pressurized air has a certain supply pressure. With only the second valve unit open, the air pressure in the air conduit will eventually become similar to the supply pressure. Hence, the maximum air pressure that can be achieved in the air conduit is the same as the supply pressure. The third valve unit is arranged in fluid communication with ambient air and may thus be referred to as a drain valve. With only the third valve unit open, the air pressure in the air conduit will eventually become similar to atmospheric pressure. The minimum air pressure that can be achieved in the air conduit is thus the same as the atmospheric pressure. By controlling the second valve unit to the open state the amount of pressurized air in the air conduit is increased and the air pressure acting on the first valve unit is increased. By controlling the third valve unit to the open state pressurized air is exhausted from the air conduit and the air pressure acting on the first valve unit is thus decreased. The average pressure is the air pressure which eventually will prevail in the air conduit when the second valve unit and the third valve unit are in their open states at the same time. With both valve units open pressurized air will be entering the air conduit via the second valve unit and pressurized air will at the same time be exhausted via the third valve unit. The average pressure will thereby be lower than the supply pressure and higher than the atmospheric pressure. The average pressure depends on the opening areas of the respective on/off valve. If the opening area of the second valve unit is larger than the opening area of the third valve unit the average pressure is higher than 50% of the supply pressure. If the opening area of the second valve unit is the same as the opening area of the third valve unit the average pressure is similar to 50% of the supply pressure. If the opening area of the second valve unit is smaller than the opening area of the third valve unit the average pressure is lower than 50% of the supply pressure.

A common problem with using on/off valves to control the air pressure for controlling a retarder unit is that the opening areas of the on/off valves affect the possibility of fine adjustments of the air pressure. Typically, large opening areas are desired, such that the retarder unit can be rapidly activated or inactivated. However, large opening areas may cause rapid changes of air pressure in the air conduit which makes it difficult to fine tune the air pressure. With smaller opening areas it is easier to control the air pressure but too small areas may result in too long time to active or inactivate the retarder unit. It is thus difficult to decide which opening areas to use and it often leads to a compromise. For safety reasons such a compromise typically results in somewhat larger areas than desired and thereby difficulties to fine adjust the air pressure. By determining the current air pressure; determining a requested air pressure and; for providing the requested air pressure, controlling the second and/or the third valve unit based on the requested air pressure in relation to; the current air pressure; the supply pressure; the atmosphere pressure; and the average pressure, several factors are taken into consideration whereby the second valve unit and the third valve unit can be controlled in a better way. This way, the first valve unit can be more accurately controlled and the braking torque accuracy of the retarder unit is thereby improved.

The method may be performed by a control unit of a hydraulic retarder system. The second valve unit and the third valve unit may be controlled by control signals from the control unit. For example, a high control signal (current) sent to the second respective the third valve unit may mean that the valve unit is controlled to the open state. A low (zero) control signal may mean that the valve unit is in the closed state.

For example, the current air pressure may be determined by means of a pressure sensor arranged in fluid communication with the air conduit. The pressor sensor may be arranged in communication with the control unit whereby the control unit determines the current air pressure based on signals from the pressor sensor. The requested air pressure is the pressure required to provide a retarding torque requested by the operator of the vehicle or requested by a vehicle system. The requested air pressure is thus the pressure that is required to control the first valve unit such that the fluid pressure in the retarder unit corresponds to the requested retarding torque. The requested air pressure may be determined by the control unit, based on signals from other vehicle systems or components.

According to an embodiment of the invention the second and/or the third valve unit are controlled, such that an air pressure rate of change slows down as the air pressure in the air conduit approaches the requested air pressure. With a low air pressure derivative closest to the requested air pressure (as compared to the air pressure derivate more far away from the requested air pressure) it is easier to more accurately control the air pressure and it is thus easier to achieve the requested air pressure.

To achieve a slower air pressure rate of change around (and at) the requested air pressure the second valve unit and the third valve unit may be controlled in various ways, depending on the relationship between the requested air pressure and the current air pressure, the supply pressure, the atmosphere pressure and the average pressure. According to the law of physics, the air pressure rate of change is higher the greater the difference is between the air pressure in the air conduit and the maximum/minimum air pressure achievable in the air conduit. Hence, the air pressure rate of change is lower when the air pressure in the air conduit is similar to the supply pressure, the atmosphere pressure or the average pressure. Embodiments of the invention utilize knowledge of these air pressure change rate phenomena in order to achieve an air pressure change rate slowing down as it approaches the requested pressure. How this may be accomplished will be described in more detail in the following.

According to an embodiment of the invention the second and/or the third valve unit are controlled, such that the air pressure in the air conduit first is increased past the requested air pressure, whereafter the second and/or the third valve unit are controlled, such that the air pressure in the air conduit is decreased with a slower rate of change as the air pressure approaches the requested air pressure. The second and/or the third valve units may further be controlled, such that the air pressure in the air conduit first is decreased past the requested air pressure, whereafter the second and/or the third valve units are controlled, such that the air pressure in the air conduit is increased with a slower rate of change as the air pressure approaches the requested air pressure. Depending on the requested air pressure in relation to the current air pressure, it might be difficult to achieve the requested air pressure by increasing or decreasing the air pressure, due to a rapid air pressure rate of change. It may therefore be advantageous to control the valve units and the air pressure in the air conduit, such that the requested air pressure first is deliberately missed (while the air pressure rate of change is relatively high), and thereafter control the valve units such that the air pressure rate of change becomes slower close to the requested air pressure and thereby more easily achieve the requested air pressure.

According to an embodiment of the invention, if the requested air pressure is higher than the current air pressure and within a predetermined range from the atmospheric pressure, the second valve unit is controlled to the open state until the air pressure in the air conduit is higher than the requested air pressure; thereafter the second valve unit is controlled to the closed state and the third valve unit is controlled to the open state. This way, the requested air pressure is first deliberately missed while increasing the air pressure, and instead the requested air pressure is achieved while decreasing the air pressure in the air conduit. If the requested air pressure is higher than the current air pressure and within a predetermined range from the atmospheric pressure, the second valve unit may be controlled to the open state until the air pressure in the air conduit is similar to the supply pressure; thereafter the second valve unit is controlled to the closed state and the third valve unit is controlled to the open state. The requested air pressure may be an air pressure close to the atmospheric pressure. The predetermined range may be 0-2,5 bar from the atmospheric pressure. By controlling the second valve unit to the open state the air pressure in the air conduit is increased. When the air pressure is higher than the requested air pressure or similar to the supply pressure the second valve unit may be controlled to the closed state. By controlling the third valve unit to the open state the air pressure starts decreasing. The longer the third valve unit is open the more decreases the air pressure. According to the laws of physics the air pressure will increase/decrease more slowly when the air pressure approaches a maximum/minimum pressure. Thus, when the air pressure in the air conduit approaches atmospheric pressure and thus the requested air pressure, a slower air pressure rate of change is achieved. The requested air pressure can thereby be achieved more easily.

To maintain the requested air pressure both the second valve unit and the third valve unit are in the closed state. Thus, each time a new retarding torque is requested the second valve unit and the third valve unit are in their closed states.

According to an embodiment of the invention, if the requested air pressure is higher than the current air pressure and within a predetermined range from the supply pressure, the second valve unit is controlled to the open state. This way, the requested air pressure is achieved while increasing the air pressure in the air conduit. The requested air pressure may be an air pressure close to the supply pressure. The predetermined range may be 0-2,5 bar from the supply pressure. Since the air pressure in the air conduit will increase more slowly when the air pressure approaches the supply pressure and thus the requested air pressure, it is easier to hit the target and achieve the requested air pressure.

In an embodiment, if the requested air pressure is lower than the current air pressure and within a predetermined range from the supply pressure, the requested air pressure may be provided by controlling the third valve unit to the open state until the air pressure in the air conduit is lower than the requested air pressure. Thereafter the third valve unit may be controlled to the closed state and the second valve unit may be controlled to the open state, such that the requested air pressure is achieved by increasing the air pressure in the air conduit. The requested air pressure could thus be deliberately missed while decreasing the air pressure in the air conduit, so that a slower air pressure rate of change can be obtained when increasing the air pressure.

If the difference between the current air pressure in the air conduit and the requested air pressure is within a predetermined tolerance, the current air pressure may be considered to comply with the requested air pressure and the second and the third valve units may be maintained closed.

According to an embodiment of the invention, if the requested air pressure is higher than the current air pressure and within a predetermined range higher than the average pressure, the second valve unit is controlled to the open state, such that the air pressure in the air conduit is higher than the requested air pressure, and thereafter the third valve unit is controlled to the open state. This way, the requested air pressure is deliberately missed while increasing the air pressure. If the requested air pressure is higher than the current air pressure and within a predetermined range higher than the average pressure, the second valve unit may be controlled to the open state, such that the air pressure in the air conduit is similar to the supply pressure, and thereafter the third valve unit may be controlled to the open state. By having both on/off valves open the air pressure in the air conduit will eventually be similar to the average pressure. The minimum air pressure in the air conduit is thereby similar to the average pressure. The air pressure in the air conduit will thus decrease with a slower rate of change as the air pressure approaches the average pressure. The requested air pressure can thereby be achieved more easily. The requested air pressure may be an air pressure close to the average pressure. The predetermined range may be 0-2,5 bar higher than the average pressure. Suitably, the second valve unit is open during a time period enough to reach the supply pressure whereafter it is closed. After some time the second valve unit and the third valve units are both opened to decrease the air pressure in a slow and controlled way.

According to an embodiment of the invention, if the requested air pressure is lower than the current air pressure and within a predetermined range higher than the average pressure, the requested air pressure is provided by controlling the second valve unit and the third valve unit to the open state. The requested air pressure may be an air pressure close to the average pressure. The predetermined range may be 0-2,5 bar higher than the average pressure.

By having both on/off valves open the air pressure in the air conduit will eventually be similar to the average pressure. The air pressure in the air conduit will thus decrease with a slower rate of change as the air pressure approaches the average pressure. The requested air pressure can thereby be achieved more easily.

According to an embodiment of the invention, if the requested air pressure is higher than the current air pressure and within a predetermined range lower than the average pressure, the second valve unit and the third valve unit are controlled to the open state. The requested air pressure may be an air pressure close to the average pressure. The predetermined range may be 0-2,5 bar lower than the average pressure. With the second valve unit and the third valve unit open, starting from an air pressure lower than the average pressure, the maximum air pressure that can be achieved in the air conduit is the average pressure. Thus, a slower air pressure rate of change will be achieved as the air pressure in the air conduit approaches the average pressure. This way, the air pressure can more easily be controlled to achieve the requested air pressure while increasing the air pressure towards the average pressure.

If the requested air pressure is lower than the current air pressure and within a predetermined range lower than the average pressure, the requested air pressure may be provided by controlling the third valve unit to the open state until the air pressure in the air conduit is lower than the requested air pressure, and thereafter controlling the second valve unit to the open state as well, such that the air pressure is increased towards to average pressure. The method may comprise to control the third valve unit to the open state until the air pressure in the air conduit is similar to atmospheric pressure, and thereafter controlling the second valve unit to the open state as well, such that the air pressure is increased towards the average pressure. The air pressure rate of change will thereby be slower as the air pressure in the air conduit approaches the requested air pressure. This way, the requested air pressure can more easily be achieved.

According to an embodiment of the invention, to achieve the requested air pressure, the second valve unit and/or the third valve unit are maintained in the open state for a time period determined based on an air pressure rate of change in the air conduit, wherein after said time period it is ensured that the second valve unit and the third valve units are in the closed state. Irrespective of whether the requested air pressure is achieved by increasing the air pressure or by decreasing the air pressure the control signal to close the second valve unit and the third valve unit should be sent prior to reaching the requested air pressure, to make sure that the air pressure in the air conduit doesn’t exceed or fall below the requested air pressure more than the predetermined tolerance value due to delays in the system. The time period during which the second and/or the third valve unit should be open in order to reach the requested air pressure may be determined based on a known model over the change of air pressure over time. The model may be stored in the control unit as a non-linear curve. The air pressure rate of change may be determined from the model over the air pressure change over time. The model will vary depending on the control strategy on thus depending on the state of the second and the third valve unit. The model over the change of air pressure over time will thus be different depending on the requested air pressure in relation to the current air pressure; the supply pressure; atmospheric pressure and the average pressure. Thus, based on the known models, the current air pressure and the requested air pressure, the control unit may determine how long the second valve unit and/or the third valve unit should be open in order to achieve the requested air pressure. The non-linear curve may be approximated with at least one linear curve in order to determine the time period during which the second and/or third valve unit should be open.

According to an embodiment of the invention the second valve unit and the third valve unit are controlled to the open state by a control signal (current) corresponding to an (essentially) 100 percent duty cycle. The control unit thus controls the second and the third valve unit to the open state with a consistent current. This means that according to the invention the second and the third valve units are controlled to be opened as fast as possible.

The herein mentioned predetermined range from the supply pressure, the atmospheric pressure respective the average pressure may depend on the supply pressure and the average pressure. In the case where the supply pressure is around 10 bar and the average pressure is around 5 bar, the predetermined range from the supply pressure, the atmospheric pressure respective the average pressure may be 0-2,5 bar. With different values on the supply pressure and the average pressure, the predetermined range may be different.

According to an aspect of the invention a control unit for controlling a second valve unit and a third valve unit is provided. The second valve unit and the third valve unit each being arranged in fluid communication with a first valve unit via an air conduit, wherein the first valve unit is controllable by an air pressure in the air conduit, wherein the second and the third valve units are on/off valves which are controllable to an open state and a closed state, the second valve unit, in the open state, being in fluid communication with pressurized air having a supply pressure and the third valve unit, in the open state, being in fluid communication with ambient air having atmospheric pressure. The control unit comprising: - means for determining a current air pressure in the air conduit; - means for determining a requested air pressure in the air conduit; and - means for, for providing the requested air pressure in the air conduit, controlling the second valve unit and/or the third valve unit based on the requested air pressure in relation to; the current air pressure; the atmosphere pressure; the supply pressure and an average pressure, wherein the average pressure is the air pressure in the air conduit achievable when the second valve unit and the third valve unit are in their open states at the same time.

It will be appreciated that all the embodiments described for the method aspect of the invention are also applicable to the control unit aspect of the invention. That is, the control unit may be configured to perform any one of the steps of the method according to various embodiments described herein.

The means for determining the current air pressure in the air conduit; the means for determining the requested air pressure in the air conduit; and the means for, for providing the requested air pressure in the air conduit, controlling the second valve unit and/or the third valve unit, may e.g. be different software modules/portions in the control unit, program code or similar.

According to an aspect of the invention a hydraulic retarder system is provided. The hydraulic retarder system comprising: a retarder unit; a first valve unit arranged to control a fluid pressure in the retarder unit for influencing a retarding torque of the retarder unit; and a second valve unit and a third valve unit each arranged in fluid communication with the first valve unit via an air conduit, wherein the first valve unit is controllable by an air pressure in the air conduit, wherein the second and the third valve units are on/off valves which are controllable to an open state and a closed state, the second valve unit, in the open state, being in fluid communication with pressurized air having a supply pressure and the third valve unit being, in the open state, in fluid communication with ambient air having atmosphere pressure. The system further comprises a control unit comprising: - means for determining a current air pressure in the air conduit; - means for determining a requested air pressure in the air conduit; and - means for, for providing the requested air pressure in the air conduit, controlling the second valve unit and/or the third valve unit based on the requested air pressure in relation to; the current air pressure; the atmosphere pressure; the supply pressure and an average pressure, wherein the average pressure is the air pressure in the air conduit achievable when the second valve unit and the third valve unit are in their open states at the same time.

Further objects, advantages and novel features of the present invention will become apparent to one skilled in the art from the following details, and also by putting the invention into practice. Whereas embodiments of the invention are described below, it should be noted that it is not restricted to the specific details described. Specialists having access to the teachings herein will recognise further applications, modifications and incorporations within other fields, which are within the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS For fuller understanding of the present invention and further objects and advantages of it, the detailed description set out below should be read together with the accompanying drawings, in which the same reference notations denote similar items in the various diagrams, and in which: Figure 1 schematically illustrates a vehicle according to an embodiment of the invention; Figure 2 schematically illustrates a hydraulic retarder system according to an embodiment of the invention; Figure 3 illustrates a flow chart for a method for controlling a hydraulic retarder system according to an embodiment of the invention; Figure 4a-d illustrates diagrams of the change of air pressure over time pertaining to a hydraulic retarder system according to an embodiment of the invention; Figure 5 illustrates a diagram of air pressure over time pertaining to a hydraulic retarder system according to an embodiment of the invention; and Figure 6 schematically illustrates a control unit or computer according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS Figure 1 schematically shows a side view of a vehicle 1 according to an embodiment of the invention. The vehicle 1 comprises an engine 2 and a transmission 4 connected to the engine 2. The transmission 4 is connected to the driving wheels 6 of the vehicle 1 via an output shaft of the transmission (not shown). The vehicle 1 may further comprise a hydraulic retarder system 10 arranged in association with the transmission 4. The vehicle 1 may be a heavy vehicle, e.g. a truck or a bus. The vehicle 1 may alternatively be a passenger car.

Figure 2 schematically shows a hydraulic retarder system 10 according to an embodiment of the invention. The hydraulic retarder system 10 may comprise a retarder unit 12; a first valve unit 14 arranged to control a fluid pressure in the retarder unit 12 for influencing a retarding torque of the retarder unit 12; and a second valve unit 16 and a third valve unit 18 each arranged in fluid communication with the first valve unit 14 via an air conduit 20. The first valve unit 14 is controllable by an air pressure in the air conduit 20. The second and the third valve units 16, 18 are on/off valves which are controllable to an open state and a closed state. The second valve unit 16 is in the open state in fluid communication with pressurized air having a supply pressure Psupand the third valve unit 18 is in the open state in fluid communication with ambient air having atmospheric pressure Patm. The retarder unit 12 suitably comprises dynamic and static vanes (not shown) in a chamber (not shown) filled with a retarder fluid, such as oil or water. The retarder fluid in the chamber induces viscous drag forces between the vanes and a retarding torque is thereby provided. The retarder unit 12 may be connected to the output shaft of a vehicle transmission, whereby the retarding torque provided by the retarder unit 12 is applied on the output shaft and thereby brakes the vehicle. The retarder fluid may be circulated in a retarder circuit 22 arranged in fluid communication with the retarder unit chamber. The retarder circuit 22 may be connected to a heat exchanger 24, such that the retarder fluid is cooled by a cooling fluid circulated through the heat exchanger 24 via a cooling circuit 26. The first valve unit 14 is arranged to control the amount of retarder fluid in the retarder unit 12, which thereby affects the provided retarding torque. The air pressure in the air conduit 20 affects the first valve unit 14, such that the first valve unit 14 controls how much of the retarder fluid flow in the retarder circuit 22 that is led to the retarder unit 12. The retarder fluid that is not led to the retarder unit 12 is suitably diverted to a reservoir 28.

The hydraulic retarder system 10 may comprise a pressure sensor P arranged in fluid communication with the air conduit 20. The pressure sensor P is adapted to determine the current pressure in the air conduit 20.

The air pressure in the air conduit 20 may be controlled by means of the second valve unit 16 and the third valve unit 18. The second valve unit 16 may be referred to as a filling valve. By controlling the second valve 16 unit to the open state the amount of pressurized air in the air conduit 20 is increased and the air pressure acting on the first valve unit 14 is increased. With only the second valve unit 16 open, the air pressure in the air conduit 20 will eventually become similar to the supply pressure Psup. The third valve unit 18 may be referred to as a drain valve. By controlling the third valve unit 18 to the open state air is exhausted from the air conduit 20 and the air pressure acting on the first valve unit 14 is thus decreased. With only the third valve unit 18 open, the air pressure in the air conduit 20 will eventually become similar to atmospheric pressure Patm. With both on/off valve units 16, 18 open pressurized air will enter the air conduit 20 via the second valve unit 16 and air will at the same time be exhausted via the third valve unit 18. With the second and the third valve unit 16, 18 open at the same time the air pressure in the air conduit 20 will eventually reach an average pressure. The average pressure is lower than the supply pressure Psupand higher than the atmospheric pressure Patm. The average pressure depends on the opening areas of the respective on/off valve The second valve unit 16 and the third valve unit 18 are controlled by a control unit 30. The control unit 30 may be a transmission control unit, engine control unit or any other control unit of a vehicle. The control unit 30 may comprise a plurality of different control units. A computer 32 may be arranged in communication with the control unit 30. The control unit 30 may comprise means for determining the current air pressure in the air conduit 20; means for determining a requested air pressure in the air conduit 20; and means for, for providing the requested air pressure in the air conduit 20, controlling the second valve unit 16 and/or the third valve unit 18 based on the requested air pressure in relation to; the current air pressure; the atmospheric pressure Patm; the supply pressure Psupand the average pressure. The requested air pressure is the air pressure required to provide a requested retarding torque by the retarder unit 12. The requested air pressure is thus the pressure that is required to control the first valve unit 14, such that the fluid pressure in the retarder unit 12 corresponds to a requested retarding torque. The requested air pressure may be determined by the control unit 30. The control unit 30 may be arranged in communication with the pressure sensor P. The control unit 30 may thus be adapted to determine the current air pressure by means of the pressure sensor P.

The control unit 30 may comprise means for controlling the second valve unit 16 and/or the third valve unit 18 such that a slower air pressure rate of change is achieved when the air pressure in the air conduit 20 approaches the requested air pressure.

The control unit 30 may comprise means for controlling the second valve unit 16 and/or the third valve unit 18 such that the air pressure in the air conduit 20 first is increased past the requested air pressure, whereafter the second and/or the third valve units 16, 18 are controlled such that the air pressure in the air conduit 20 is decreased with a slower rate of change as the air pressure approaches the requested air pressure.

The control unit 30 may be adapted to control the second valve unit 16 and the third valve unit 18 to the open state with a control signal (current) corresponding to an essentially 100% duty cycle. This means that the control unit 30 may be adapted to control the second and the third valve units 16, 18 with a consistent current.

Figure 3 illustrates a flow chart for a method for controlling a hydraulic retarder system according to an embodiment of the invention. The hydraulic retarder system may be configured as the hydraulic retarder system 10 as disclosed in Figure 2. The method comprises: determining s101 a current air pressure in the air conduit 20; determining s102 a requested air pressure in the air conduit 20; and for providing the requested air pressure in the air conduit 20, controlling s103 the second valve unit 16 and/or the third valve unit 18 based on the requested air pressure in relation to; the current air pressure; the atmosphere pressure Patm; the supply pressure PsuP; and an average pressure, wherein the average pressure is the air pressure in the air conduit 20 achievable when the second valve unit 16 and the third valve unit 18 are in their open states at the same time.

The method may be performed by means of a control unit 30 of the hydraulic retarder system 10.

The current air pressure may be determined by means of the pressure sensor P arranged in fluid communication with the air conduit 20 and the control unit 30. The current air pressure may be determined by the control unit, 30 based on signals from the pressure sensor P. The requested air pressure may be determined by the control unit 30, based on signals from other vehicle systems or components.

The second and the third valve unit 16, 18 may be controlled, such that the air pressure rate of change is slower when the air pressure in the air conduit 20 approaches the requested air pressure. With a low air pressure derivative close to the requested air pressure it is easier to control the air pressure and it is thus easier to achieve the requested air pressure.

The second and/or the third valve units 16, 18 may be controlled such that the air pressure in the air conduit 20 first is increased past the requested air pressure, whereafter the second and/or the third valve units 16, 18 are controlled such that the air pressure in the air conduit 20 is decreased with a slower rate of change as the air pressure approaches the requested air pressure. The second and/or the third valve units 16, 18 may further be controlled such that the air pressure in the air conduit 20 first is decreased past the requested air pressure, whereafter the second and/or the third valve units 16, 18 are controlled, such that the air pressure in the air conduit 20 is increased with a slower rate of change as the air pressure approaches the requested air pressure. Depending on the requested air pressure in relation to the current air pressure, it might be difficult to achieve the requested air pressure by increasing or decreasing the air pressure, due to a rapid air pressure rate of change. It may therefore be advantageous to control the air pressure in the air conduit 20 such that the requested air pressure first is deliberately missed, and thereafter control the on/off valves 16, 18 such that the air pressure rate of change is slower close to the requested air pressure and thereby more easily achieve the requested air pressure.

If the requested air pressure is higher than the current air pressure and within a predetermined range from the atmosphere pressure Patm, the second valve unit 16 may be controlled to the open state until the air pressure in the air conduit 20 is higher than the requested air pressure; thereafter the second valve unit 16 is controlled to the closed state and the third valve unit 18 is controlled to the open state. This way, the requested air pressure is first deliberately missed while increasing the air pressure, and instead the requested air pressure is achieved while decreasing the air pressure in the air conduit 20. The requested air pressure is suitably an air pressure close to the atmospheric pressure Patm.

If the requested air pressure is higher than the current air pressure and within a predetermined range from the supply pressure Psup, the second valve unit 16 may be controlled to the open state. This way, the requested air pressure is achieved while increasing the air pressure in the air conduit 20. The requested air pressure is suitably an air pressure close to the supply pressure Psup. Since the air pressure in the air conduit 20 will increase more slowly when the air pressure approaches the supply pressure Psup, it is easier to achieve the requested air pressure.

If the requested air pressure is higher than the current air pressure and within a predetermined range higher than the average pressure, the second valve unit 16 may be controlled to the open state, such that the air pressure in the air conduit 20 is higher than the requested air pressure, and thereafter the third valve unit 18 may be controlled to the open state. This way, the requested air pressure is deliberately missed while increasing the air pressure to the supply pressure Psup. By having both on/off valves 16, 18 open the air pressure in the air conduit 20 will eventually be similar to the average pressure. The air pressure in the air conduit 20 will thus decrease with a slower rate of change as the air pressure approaches the average pressure. The requested air pressure can thereby be achieved more easily. The requested air pressure is suitably an air pressure close to the average pressure. Suitably, the second valve unit 16 is open during a time period enough to reach an air pressure higher than the requested air pressure, whereafter it is closed. After some time the second valve unit 16 and the third valve units 18 are both controlled to the open state to decrease the air pressure in a slow and controlled way.

If the requested air pressure is lower than the current air pressure and within a predetermined range higher than the average pressure, the second valve unit 16 and the third valve unit 18 may be controlled to the open state. The requested air pressure is suitably an air pressure close to the average pressure.

If the requested air pressure is higher than the current air pressure and within a predetermined range lower than the average pressure, the second valve unit 16 and the third valve unit 18 may be controlled to the open state. The requested air pressure is suitably an air pressure close to the average pressure. Whit the second valve unit 16 and the third valve unit open 18, starting from an air pressure lower than the average pressure, the maximum air pressure that can be achieved in the air conduit is the average pressure. Thus, the air pressure rate of change will be slower as the air pressure in the air conduit 20 approaches the average pressure. This way, the air pressure can more easily be controlled to achieve the requested air pressure.

If the requested air pressure is lower than the current air pressure and within a predetermined range lower than the average pressure, the third valve unit 18 may be controlled to the open state until the air pressure in the air conduit 20 is similar to atmospheric pressure Patm, and thereafter the second valve unit 16 may be controlled to the open state as well, such that the air pressure is increased towards to average pressure. The air pressure rate of change will thereby be slower as the air pressure in the air conduit 20 approaches the requested air pressure. This way, the requested air pressure can more easily be achieved.

The second valve unit 16 and the third valve unit 18 may be controlled to the open state by a control signal (current) from the control unit 30. How long time the respective valve unit 16, 18 is open depends on the control signal. When no current is supplied to the valve unit 16, 18, the valve unit may be in the closed state. Alternatively, the second valve unit 16 may be controlled to the open state by a control signal (current) from the control unit 30 and when no current is supplied to the second valve unit 16, the second valve unit 16 may be in the closed state. The third valve unit 18 may be controlled to the closed state by a control signal (current) from the control unit 30, and when no current is supplied to the third valve unit 18, the third valve unit 18 may be in the open state. The second valve unit 16 and the third valve unit 18 may be controlled by control signals corresponding to a 100 percent duty cycle. This means that according to the invention the second and the third valve units 16, 18 are controlled to be opened/closed as fast as possible.

To achieve the requested air pressure, the second valve unit 16 and/or the third valve unit 18 may be maintained in the open state for a time period determined based on an air pressure rate of change in the air conduit 20, wherein after said time period it is ensured that the second valve unit 16 and the third valve units 18 are in the closed state. Thus, the method may comprise to ensure that the second valve unit 16 and the third valve unit 18 receive control signals for moving to the closed state prior to reaching the requested air pressure in the air conduit 20. This way, it is ensured that the air pressure in the air conduit 20 doesn’t exceed or fall below the requested air pressure due to delays in the system 10. In order to determine the time period during which the second and the third valve units 16, 18 should be open to achieve the requested air pressure, a model over the change of air pressure over time may be used. This is further described in Figure 5.

Figures 4a-4d illustrate diagrams of the change of air pressure over time pertaining to a hydraulic retarder system according to an embodiment of the invention. The figures illustrates diagrams of how the air pressure in an air conduit changes depending on how a second valve unit and a third valve unit of a hydraulic retarder system are controlled according to an embodiment of the invention. The figures thus show details of the method for controlling a hydraulic retarder system as disclosed in Figure 3. The hydraulic retarder system which is controlled may be the hydraulic retarder system 10 as disclosed in Figure 2. The figures comprises a first diagram [1] showing control signals for controlling the second valve unit 16, a second diagram [2] showing the control signals for controlling the third valve unit 18, and a third diagram [3] showing how the air pressure in the air conduit 20 changes depending on the control signals to the second and the third valve units 16, 18. In these diagrams a control signal (current) to the respective on/off valve unit 16, 18 means that the valve unit 16, 18 is controlled to the open state. No control signal means that the valve unit 16, 18 is in the closed state.

Figure 4a shows diagrams relating to the air pressure in the air conduit 20 according to an embodiment of the invention. In the third diagram [3] the supply pressure Psupis the upper dashed line and the requested air pressure Preqis the lower dashed line. Atmosphere pressure Patmis the lowest air pressure shown in the diagram [3].

Thus, in this embodiment the requested air pressure Preqis within a predetermined range from the atmosphere pressure Patm. The predetermined range may be 0-2,5 bar from the atmospheric pressure Patm. Depending on the current air pressure in the air conduit 20, the second valve unit 16 and the third valve unit 18 are suitably controlled in different ways. In the case where the requested air pressure Preqis higher than the current air pressure a control signal is suitably sent to the second valve unit 16, such that it is controlled to the open state. This is illustrated in the first diagram [1], When the second valve unit 16 is open the air pressure in the air conduit 20 increases towards the supply pressure Psup. The control signal to the second valve unit 16 is suitably such that the second valve unit 16 is open for a certain time period, enough to achieve an air pressure in the air conduit which higher than the requested air pressure Preqand similar to the supply pressure Psup. This way, the requested air pressure Preqis deliberately missed while increasing the air pressure. Thereafter, the second valve unit 16 is controlled to the closed state and with both valve units 16, 18 closed the air pressure will remain similar to the supply pressure Psup. After a certain time a control signal is sent to the third valve unit 18 such that it is controlled to the open state. This is illustrated in the second diagram [2], By controlling the third valve unit 18 to the open state the air pressure in the air conduit 20 starts decreasing. According to the laws of physics the air pressure will increase/decrease more slowly when the air pressure approaches a maximum/minimum pressure. Thus, when the air pressure in the air conduit 20 approaches the requested air pressure Preq, which is close to atmospheric pressure Patm, a slower air pressure rate of change is achieved. The requested air pressure Preqcan thereby be achieved more easily. The control signal to the third valve unit 18 may be such that the third valve unit 18 is open for a certain time period, enough to achieve the requested air pressure Preq. Due to a delay in the system, it may be predetermined how long the third valve unit 18 should be open to reach the requested air pressure Preq. The third valve unit 18 is thus not controlled based on feedback from the pressure sensor P. When the second valve unit 16 and the third valve unit 18 are closed, the air pressure in the air conduit 20 may be determined by means of the pressure sensor P. If the air pressure is not similar to the requested air pressure Preqthe procedure of controlling the air pressure in the air conduit 20 may be performed once again. If the air pressure is within a predetermined tolerance from the requested air pressure Preqthe requested air pressure Preqis considered to have been reached (achieved) and the second valve unit 16 and the third valve unit 18 are maintained closed.

Should the requested air pressure Preqbe lower than the current air pressure, for example if the current air pressure is similar to the supply pressure PSUp, then a control signal is sent to the third valve unit 18 such that it is controlled to the open state as described above.

Figure 4b shows diagrams relating to the air pressure in the air conduit 20 according to an embodiment of the invention. In the third diagram [3] the supply pressure Psupis the upper dashed line and the requested air pressure Preqis the lower dashed line. Atmosphere pressure Patmis the lowest air pressure shown in the diagram [3].

Thus, in this embodiment the requested air pressure Preqis higher than the current air pressure and within a predetermined range from the supply pressure Psup. The predetermined range may be 0-2,5 bar from the supply pressure Psup. A control signal may thus be sent to the second valve unit 16, such that it is controlled to the open state. This is illustrated in the first diagram [1], When the second valve unit 16 is open the air pressure in the air conduit 20 increases towards the supply pressure Psup. According to the laws of physics the air pressure will increase/decrease more slowly when the air pressure approaches a maximum/minimum pressure. Thus, when the air pressure in the air conduit 20 approaches the requested air pressure Preq, which is close to supply pressure Psup, the air pressure rate of change is slower. The requested air pressure Preqcan thereby be achieved more easily. The control signal to the second valve unit 16 may be such that the second valve unit 16 is open for a certain time period, enough to achieve the requested air pressure Preq. Due to a delay in the system, it may be predetermined how long the second valve unit 16 should be open to reach the requested air pressure Preq. The second valve unit 16 is thus not controlled based on feedback from the pressure sensor P. When the second valve unit 16 and the third valve unit 18 are closed, the air pressure in the air conduit 20 may be determined by means of the pressure sensor P. If the air pressure is not similar to the requested air pressure Preqthe procedure of controlling the air pressure in the air conduit 20 may be performed once again. If the air pressure is within a predetermined tolerance from the requested air pressure Preqthe requested air pressure Preqis considered to have been reached and the second valve unit 16 and the third valve unit 18 are maintained closed.

Figure 4c shows diagrams relating to the air pressure in the air conduit 20 according to an embodiment of the invention. In the third diagram [3] the supply pressure Psupis the upper dashed line, the requested air pressure Preqis the middle dashed line, and the average pressure Pavis the lower dashed line. The average pressure Pav is the pressure that eventually will prevail in the air conduit 20 when the second valve unit 16 and the third valve unit 18 are in the open state at the same time. Atmosphere pressure Patmis the lowest air pressure shown in the diagram [3].

Thus, in this embodiment the requested air pressure Preqis within a predetermined range higher than the average pressure Pav. The predetermined range may be 0-2,5 bar higher than the average pressure Pav. In the case where the requested air pressure Preqis higher than the current air pressure a control signal may be sent to the second valve unit 16 such that it is controlled to the open state. This is illustrated in diagram [1], When the second valve unit 16 is open the air pressure in the air conduit 20 increases towards the supply pressure Psup. The control signal to the second valve unit 16 may be such that the second valve unit 16 is open for a certain time period, enough to achieve an air pressure in the air conduit 20 which is similar to the supply pressure Psup. This way, the requested air pressure Preqis deliberately missed while increasing the air pressure. Thereafter, the second valve unit 16 is controlled to the closed state and with both valve units 16, 18 closed the air pressure will remain similar to the supply pressure Psup. After a certain time a control signal is sent to the third valve unit 18 such that it is controlled to the open state. A control signal is also sent to the second valve unit 16 such that it is controlled to the open state. This is illustrated in the first diagram [1] and the second diagram [2], By controlling the third valve unit 18 to the open state the air pressure in the air conduit 20 starts decreasing. However, the air pressure in the air conduit 20 can only decrease to the average pressure Pav since the second valve unit 16 is open and thereby supplies pressurized air to the air conduit 20. According to the laws of physics the air pressure will increase/decrease more slowly when the air pressure approaches a maximum/minimum pressure. Thus, when the air pressure in the air conduit 20 approaches the requested air pressure Preq, which is close to the average pressure Pav, a slower air pressure rate of change is achieved. The requested air pressure Preqcan thereby be achieved more easily. The control signal to the second valve unit 16 respective the third valve unit 18 may be such that the respective valve unit 16, 18 is open for a certain time period, enough to achieve the requested air pressure Preq. Due to a delay in the system, it may be predetermined how long the valve units 16, 18 should be open to reach the requested air pressure Preq. The second valve unit 16 and the third valve unit 18 are thus not controlled based on feedback from the pressure sensor P. When the second valve unit 16 and the third valve unit 18 are closed, the air pressure in the air conduit 20 may be determined by means of the pressure sensor P. If the air pressure is not similar to the requested air pressure Preqthe procedure of controlling the air pressure in the air conduit 20 may be performed once again. If the air pressure is within a predetermined tolerance from the requested air pressure Preqthe requested air pressure Preqis considered to have been reached and the second valve unit 16 and the third valve unit 18 are maintained closed.

Should the requested air pressure Preqbe lower than the current air pressure, for example if the current air pressure is similar to the supply pressure PSUp, then a control signal is sent to the second valve unit 16 and the third valve unit 18 such that they are controlled to the open state as described above.

Figure 4d shows diagrams relating to the air pressure in the air conduit 20 according to an embodiment of the invention. In the third diagram [3] the supply pressure Psupis the upper dashed line, the average pressure Pavis the middle dashed line, and the requested air pressure Preqis the lower dashed line. The average pressure Pav is the pressure that eventually will prevail in the air conduit 20 when the second valve unit 16 and the third valve unit 18 are in the open state at the same time. Atmosphere pressure Patmis the lowest air pressure shown in the diagram [3].

Thus, in this embodiment the requested air pressure Preqis higher than the current air pressure and within a predetermined range lower than the average pressure Pav. The predetermined range may be 0-2,5 bar higher than the average pressure Pav. Control signals may be sent to the second valve unit 16 and the third valve unit 18 such that they are controlled to the open state. This is illustrated in diagram [1] and diagram [2], With both the second valve unit 16 and the third valve unit 18 open the average pressure Pav is the highest possible air pressure that can be achieved in the air conduit. According to the laws of physics the air pressure will increase/decrease more slowly when the air pressure approaches a maximum/minimum pressure. Thus, when the air pressure in the air conduit 20 approaches the requested air pressure Preq, which is close to the average pressure Pav, a slower air pressure rate of change is achieved. The requested air pressure Preqcan thereby be achieved more easily. The control signal to the second valve unit 16 respective the third valve unit 18 may be such that the respective valve unit 16, 18 is open for a certain time period, enough to achieve the requested air pressure Preq. Due to a delay in the system, it may be predetermined how long the valve units 16, 18 should be open to reach the requested air pressure Preq. The second valve unit 16 and the third valve unit 18 are thus not controlled based on feedback from the pressure sensor P. When the second valve unit 16 and the third valve unit 18 are closed, the air pressure in the air conduit 20 may be determined by means of the pressure sensor P. If the air pressure is not similar to the requested air pressure Preqthe procedure of controlling the air pressure in the air conduit 20 may be performed once again. If the air pressure is within a predetermined tolerance from the requested air pressure Preqthe requested air pressure Preqis considered to have been reached and the second valve unit 16 and the third valve unit 18 are maintained closed.

Figure 5 illustrates a diagram of air pressure over time pertaining to a hydraulic retarder system according to an embodiment of the invention. The figure shows an example of the method for controlling a hydraulic retarder system as disclosed in Figure 3. The hydraulic retarder system which is controlled may be the hydraulic retarder system 10 as disclosed in Figure 2. As was previously described with regard to Figures 4a-4d it may be determined how long the valve units 16, 18 should be open to reach the requested air pressure Preqin order to avoid the problem with delays in the system 10. The time period during which the second valve unit 16 and/or the third valve unit 18 should be open in order to achieve the requested air pressure Preqmay be determined based on an air pressure rate of change in the air conduit 20. The air pressure rate of change will vary depending on the control strategy and thus depending on the state of the second and the third valve unit 16, 18. The time period may be determined based on a model of the change of air pressure in the air conduit 20 over time. The air pressure rate of change may be determined from the model of the change of air pressure over time. The model may be illustrated as a non-linear curve as shown in Figure 5. The curve in Figure 5 shows the air pressure changes over time according to an embodiment of the invention where the requested air pressure Preqis within a predetermined range higher than the average pressure Pav. The current air pressure Pcur is shown in Figure 5 and is higher than the requested air pressure Preq. The second valve unit 16 and the third valve unit 18 would then suitably be controlled to the open state, such that the air pressure rate of change would be slower as the air pressure approaches the average pressure Pavand thus the requested air pressure Preq. The non-linear curve may be approximated with at least one linear curve in order to determine how long the second and the third valve unit 16, 18 should be open to reach the requested air pressure 16, 18. In this example, the non-linear curve is approximated with two linear curves, one between the current air pressure Pcurand an intermediate air pressure Pint, and one between the intermediate air pressure Pintand the requested air pressure Preq. It may then be determined how long time t1 the valve units 16, 18 should be open to reach the intermediate air pressure Pintand how long time t2 they should be open to go from the intermediate air pressure Pintto the requested air pressure Preq. The total time that the second valve unit 16 and the third valve unit 18 should be open to reach the requested air pressure Preqis thus t1 12.

Figure 6 is a diagram of a version of a device 500. The control unit 30 and/or computer 32 described with reference to Figure 2 may in a version comprise the device 500. The term “link” refers herein to a communication link which may be a physical connection such as an optoelectronic communication line, or a non-physical connection such as a wireless connection, e.g. a radio link or microwave link. The device 500 comprises a non-volatile memory 520, a data processing unit 510 and a read/write memory 550. The non-volatile memory 520 has a first memory element 530 in which a computer programme, e.g. an operating system, is stored for controlling the function of the device 500. The device 500 further comprises a bus controller, a serial communication port, I/O means, an A/D converter, a time and date input and transfer unit, an event counter and an interruption controller (not depicted). The non-volatile memory 520 has also a second memory element 540.

There is provided a computer programme Pr which comprises routines for controlling a hydraulic retarder system. The computer programme Pr comprises routines for determining the current air pressure. The computer programme Pr comprises routines for determining a requested air pressure. The computer programme Pr comprises routines for controlling a second valve unit 16 and/or a third valve unit 18 based on the requested air pressure in relation to; the current air pressure; the atmosphere pressure; the supply pressure; and an average pressure, wherein the average pressure is the air pressure, which eventually will prevail in the air conduit when the second valve unit and the third valve unit are in their open states at the same time. The computer programme Pr comprises routines for controlling the second valve unit and the third valve unit, such that the air pressure rate of change is decreased when the air pressure in the air conduit approaches the requested air pressure. The computer programme Pr comprises routines for controlling the second valve unit and the third valve unit, such that the requested air pressure is deliberately missed.

The programme Pr may be stored in an executable form or in a compressed form in a memory 560 and/or in a read/write memory 550.

Where the data processing unit 510 is described as performing a certain function, it means that the data processing unit 510 effects a certain part of the programme stored in the memory 560 or a certain part of the programme stored in the read/write memory 550.

The data processing device 510 can communicate with a data port 599 via a data bus 515. The non-volatile memory 520 is intended for communication with the data processing unit 510 via a data bus 512. The separate memory 560 is intended to communicate with the data processing unit 510 via a data bus 511. The read/write memory 550 is adapted to communicating with the data processing unit 510 via a data bus 514.

When data are received on the data port 599, they are stored temporarily in the second memory element 540. When input data received have been temporarily stored, the data processing unit 510 is prepared to effect code execution as described above.

Parts of the methods herein described may be effected by the device 500 by means of the data processing unit 510 which runs the programme stored in the memory 560 or the read/write memory 550. When the device 500 runs the programme, methods herein described are executed.

The foregoing description of the preferred embodiments of the present invention is provided for illustrative and descriptive purposes. It is not intended to be exhaustive or to restrict the invention to the variants described. Many modifications and variations will obviously be apparent to one skilled in the art. The embodiments have been chosen and described in order best to explain the principles of the invention and its practical applications and hence make it possible for specialists to understand the invention for various embodiments and with the various modifications appropriate to the intended use.

Claims

1. A method for controlling a hydraulic retarder system (10), the hydraulic retarder system (10) comprising: a retarder unit (12); a first valve unit (14) arranged to control a fluid pressure in the retarder unit (12) for influencing a retarding torque of the retarder unit (12); and a second valve unit (16) and a third valve unit (18) each arranged in fluid communication with the first valve unit (14) via an air conduit (20), wherein the first valve unit (14) is controllable by an air pressure in the air conduit (20), wherein the second and the third valve units (16, 18) are on/off valves which are controllable to an open state or a closed state, the second valve unit (16), in the open state, being in fluid communication with pressurized air having a supply pressure (Psup) and the third valve unit (18), in the open state, being in fluid communication with ambient air having atmosphere pressure (Patm), the method comprising: - determining (s101 ) a current air pressure in the air conduit (20); - determining (s102) a requested air pressure in the air conduit (20); and - for providing the requested air pressure in the air conduit, controlling (s103) the second valve unit (16) and/or the third valve unit (18) based on the requested air pressure in relation to: the current air pressure in the air conduit (20); the atmosphere pressure (Patm); the supply pressure (Psup); and an average pressure (Pav), wherein the average pressure (Pav) is the air pressure in the air conduit (20) achievable when the second valve unit (16) and the third valve unit (18) are in their open states at the same time.

2. The method according to claim 1 , wherein the second and/or the third valve units (16, 18) are controlled such that an air pressure rate of change slows down as the air pressure in the air conduit (20) approaches the requested air pressure (Preq).

3. The method according to claim 1 or 2, wherein the second and/or the third valve units (16, 18) are controlled, such that the air pressure in the air conduit (20) first is increased past the requested air pressure (Preq), whereafter the second and/or the third valve units (16, 18) are controlled such that the air pressure in the air conduit (20) is decreased with an air pressure rate of change slowing down as the air pressure approaches the requested air pressure (Preq).

4. The method according to any one of the preceding claims, wherein, if the requested air pressure (Preq) is higher than the current air pressure and within a predetermined range from the atmosphere pressure (Patm), the second valve unit (16) is controlled to the open state until the air pressure in the air conduit (20) is higher than the requested air pressure (Preq); thereafter the second valve unit (16) is controlled to the closed state and the third valve unit (18) is controlled to the open state.

5. The method according to any one of the preceding claims, wherein, if the requested air pressure (Preq) is higher than the current air pressure and within a predetermined range from the supply pressure (Psup), the second valve unit (16) is controlled to the open state.

6. The method according to any one of the preceding claims, wherein, if the requested air pressure (Preq) is higher than the current air pressure and within a predetermined range higher than the average pressure (Pav), the second valve unit (16) is controlled to the open state such that the air pressure in the air conduit (20) is higher than the requested air pressure (Preq), and thereafter the third valve unit (18) is controlled to the open state.

7. The method according to any one of the preceding claims, wherein, if the requested air pressure (Preq) is lower than the current air pressure and within a predetermined range higher than the average pressure (Pav), the second valve unit (16) and the third valve unit (18) are controlled to the open state.

8. The method according to any one of the preceding claims, wherein, if the requested air pressure (Preq) is higher than the current air pressure and within a predetermined range lower than the average pressure (Pav), the second valve unit (16) and the third valve unit (18) are controlled to the open state.

9. The method according to any one of the preceding claims, wherein, to achieve the requested air pressure (Preq), the second valve unit (16) and/or the third valve unit (18) are maintained in the open state for a time period determined based on an air pressure rate of change in the air conduit (20), wherein after said time period it is ensured that the second valve unit (16) and the third valve units (18) are in the closed state.

10. The method according to any one of the preceding claims, wherein the second valve unit (16) and the third valve unit (18) are controlled with a 100 percent duty cycle.

11. A computer program (Pr), wherein said computer program comprises programme code for causing a control unit (30; 500) or a computer (32; 500) connected to the control unit (30; 500) to perform the method according to any one of the preceding claims.

12. A computer-readable medium comprising instructions, which when executed by a control unit (30; 500) or a computer (32; 500) connected to the control unit (30; 500), cause the control unit (30; 500) or the computer (32; 500) to perform the method according to any one of claims 1-10.

13. A control unit (30) for controlling a second valve unit (16) and a third valve unit (18) each arranged in fluid communication with a first valve unit (14) via an air conduit (20), wherein the first valve unit (14) is controllable by an air pressure in the air conduit (20), wherein the second and the third valve units (16, 18) are on/off valves which are controllable to an open state and a closed state, the second valve unit (16), in the open state, being in fluid communication with pressurized air having a supply pressure (Psup) and the third valve unit (18), in the open state, being in fluid communication with ambient air having atmosphere pressure (Patm), the control unit (30) comprising: - means for determining a current air pressure in the air conduit (20); - means for determining a requested air pressure (Preq) in the air conduit (20); and - means for, for providing the requested air pressure (Preq) in the air conduit (20), controlling the second valve unit (16) and/or the third valve unit (18) based on the requested air pressure (Preq) in relation to; the current air pressure in the air conduit (20); the atmosphere pressure (Patm); the supply pressure (Psup) and an average pressure (Pav), wherein the average pressure (Pav) is the air pressure in the air conduit (20) achievable when the second valve unit (16) and the third valve unit (18) are in their open states at the same time.

14. A hydraulic retarder system (10), comprising: a retarder unit (12); a first valve unit (14) arranged to control a fluid pressure in the retarder unit (12) for influencing a retarding torque of the retarder unit (12); and a second valve unit (16) and a third valve unit (18) each arranged in fluid communication with the first valve unit (14) via an air conduit (20), wherein the first valve unit (14) is controllable by an air pressure in the air conduit (20), wherein the second and the third valve units (16, 18) are on/off valves which are controllable to an open state and a closed state, the second valve unit (16), in the open state, being in fluid communication with pressurized air having a supply pressure (Psup) and the third valve unit (18), in the open state, being in fluid communication with ambient air having atmosphere pressure (Patm), wherein the system (10) further comprises a control unit (30) according to claim 13.

15. A vehicle (1) comprising a hydraulic retarder system (10) according to claim 14.