Method for estimating an amount of compressed air supplied to an air bellows in a vehicle

FIELD OF THE INVENTION AND PRIOR ART

The present invention relates to a method for estimating an amount of compressed air supplied to an air bellows which forms part of an air suspension system of a vehicle. The invention relates also to a computer programme product comprising data programme codes for implementing a method according to the invention, and an electronic control unit.

Good fuel economy is currently an important competition factor in the field of heavy motor vehicles, and manufacturers of heavy motor vehicles endeavour constantly to minimise the fuel consumption of motor vehicles of this kind. The compressor which is powered by the vehicle engine to deliver compressed air inter alia to the brake system and the air suspension system of a heavy motor vehicle is one of many factors which contribute to fuel consumption. Reducing the consumption of compressed air of a motor vehicle makes it possible to reduce the amount of fuel needed for powering the compressor, resulting in a reduction in the fuel consumption of the vehicle. For this reason, development work is currently being conducted to make it possible to minimise the compressed air consumption of heavy vehicles. As the air suspension system is a large consumer of compressed air of a heavy motor vehicle, the compressed air consumption of the air suspension system is of particular relevance in this context. Gaining a comprehensive understanding of the factors which affect the compressed air consumption of a heavy motor vehicle and hence making measures directed towards minimising the compressed air consumption possible would involve measuring and gathering operating data concerning the compressed air consumption of a large number of motor vehicles. However, the flowmeters usable for measuring the compressed air consumption in a motor vehicle are at present very expensive, so for cost reasons it is not possible for a vehicle manufacturer to equip the motor vehicles it releases to the market with such flowmeters. The work of minimising the compressed

air consumption of heavy motor vehicles, which is important with regard to the environment and fuel economy, is thus hampered.

OBJECT OF THE INVENTION

The object of the present invention is to provide a simple and cost-effective way of estimating the magnitude of an amount of compressed air supplied to an air bellows in an air suspension system.

SUMMARY OF THE INVENTION

According to the present invention, said object is achieved with a method having the features indicated in claim 1.

The method according to the invention calculates the magnitude of an amount of compressed air supplied to an air bellows by means of a computing model on the basis of the measured value representing the bellows height of the air bellows and the measured value representing the bellows pressure of the air bellows, whereby, when an increase in the bellows height of the air bellows is detected while the bellows pressure of the air bellows is kept substantially constant, the magnitude of an amount of compressed air supplied to the air bellows is calculated on the basis of information about the increase in the bellows height and a given relationship between the bellows height and the air mass accommodated in the air bellows at the prevailing bellows pressure. The solution according to the invention thus makes it possible to estimate the magnitude of an amount of compressed air supplied to an air bellows and hence the compressed air consumption of the air bellows without using an expensive flowmeter. Information about the bellows height and the bellows pressure of the air bellows which form part of an air suspension system is at present already recorded in many modern types of heavy motor vehicles and is thus often already available in most newly produced heavy motor vehicles such as trucks, tractor units and buses, which means that the method according to the invention can be implemented in such motor vehicles easily and at low cost with use of already existing equipment.

Advantageous embodiments of the method according to the invention are indicated by the dependent claims and the description set out below.

The invention relates also to a computer programme product having the features defined in claim 4.

Advantageous embodiments of the computer programme product according to the invention are indicated by the dependent claims and the description set out below.

The invention relates further to an electronic control unit having the features defined in claim 8.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in more detail below on the basis of embodiment examples with reference to the attached drawings, in which:

Fig. 1 is a skeleton diagram of a system for implementing a method according to the present invention,

Figs. 2a-b are schematic illustrations of an air bellows which forms part of an air suspension system of a vehicle during a change in the bellows height,

Fig. 3 is a diagram illustrating the relationship between bellows height and the air mass accommodated in said air bellows at a given bellows pressure and a given temperature,

Figs. 4a-c are schematic illustrations of said air bellows at the time of a change in the bellows pressure,

Fig. 5 is a diagram illustrating the relationship between bellows pressure and the air mass accommodated in said air bellows at a given bellows height and a given temperature,

Fig. 6 is a skeleton diagram of an electronic control unit for implementing the method according to the invention, and

Fig. 7 is a flowchart illustrating a method according to an embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Fig. 1 illustrates schematically components which form part of or cooperate with an air suspension system of a motor vehicle. A compressor 1 is connected to the vehicle's combustion engine 2 to generate compressed air during operation of the combustion engine. The compressor 1 generates compressed air for the vehicle's air suspension system and for other systems and functions of the vehicle which require compressed air. The compressor 1 is connected via an air dryer 3 to one or more compressed air containers 4 to generate compressed air in the respective compressed air container(s) 4. A valve (not depicted in Fig. 1) is usually arranged in or immediately after the air dryer 3 to distribute the compressed air generated to the various compressed air circuits of the vehicle. The vehicle's air suspension system comprises a number of air bellows 10, only one of which is illustrated in Fig. 1. Each air bellows 10 comprises a combined inlet/outlet 11 for feeding compressed air into the air bellows and feeding compressed air out from the air bellows. The inlet/outlet 11 of the air bellows is connectable via a regulating valve 5 to a compressed container 4 to make it possible to receive compressed air from the latter. The inlet/outlet 11 of the air bellows is also connectable via the regulating valve 5 to the surroundings. Alternatively, the air bellows 10 might be provided with separate inlet and outlet connectable respectively to the compressed air container and the surroundings via their respective regulating valves. The regulating valve 5 is controlled by an electronic control unit 6 in a

conventional manner on the basis of a number of variables including the bellows height h and the bellows pressure/? of the air bellows 10.

The air bellows 10 illustrated in the drawings is of conventional configuration and comprises a support 13, a top plate 14 and a bellows 15 which is fastened to, and extends between, the support 13 and the top plate 14 and is made of flexible material. In the example illustrated, the inlet/outlet 11 of the air bellows is arranged in the top plate 14. The bellows height h may for example be defined as the distance between the top plate 14 and the base 16 of the support, as illustrated in, for example, Fig. 2a. The bellows 15 can be caused to rise relative to the support 13 by feeding compressed air into the bellows 15 via the inlet/outlet 11 , thereby increasing the distance between the top plate 14 and the base 16 of the support and hence the bellows height h. The bellows 15 can be caused to sink relative to the support 13 by allowing air to flow out from the bellows 15 via the inlet/outlet 11 , thereby reducing the distance between the top plate 14 and the base 16 of the support and hence the bellows height h.

In this description and the claims set out below, the "bellows pressure" of an air bellows means the relative air pressure which prevails within the bellows 15 of the air bellows, i.e. the air pressure in the air bellows relative to the prevailing atmospheric pressure.

Each air bellows 10 is arranged between a wheelshaft of the vehicle and the vehicle's chassis frame to support the chassis frame relative to the wheelshaft. When the vehicle is in motion, the air bellows 10 serve as shock absorbers. The air bellows 10 may also be used for regulating the height of the vehicle's chassis frame, e.g. during loading or unloading or when a semitrailer is to be coupled to or uncoupled from a tractor unit.

The bellows height h of the air bellows 10 may for example be measured by means of a level sensor 7 adapted to detecting variations in the distance between the vehicle's chassis frame and the wheelshaft pertaining to the air bellows. The bellows pressure/? of the air bellows 10 is measured by means of a pressure sensor 8. The control unit 6 is connected to said sensors 7, 8, either directly as illustrated in Fig. 1 or via another

control unit of the vehicle, in order to receive measured values from them for the bellows height h and the bellows pressure/? of the air bellows 10.

According to the general gas law,

p - V = m - R - T

in which p is the bellows pressure, V the bellows volume, m the mass of the air accommodated in the air bellows, R the general gas constant for air (R _air = 287 Nm /(kg ^■ K)) and T the absolute temperature in kelvin.

The bellows volume V varies with the bellows height h, and the relationship between the bellows volume V and the bellows height A of a specific air bellows can easily be determined by calculations and/or empirical tests. The bellows pressure;? is proportional to the load F supported by the air bellows 10 and therefore varies with the axle load on the wheelshaft pertaining to the air bellows. With unchanged axle load, the bellows pressure/? is therefore constant.

It follows from the general gas law that p - V m =

R T which therefore means: - that the variation Am in the air mass m is proportional to the variation δFin the bellows volume V and therefore to the variation Ah in the bellows height h at a given bellows pressure p and a given temperature T, i.e. when there is no change in the axle load and the temperature T, the air mass variation Am is proportional to the bellows height variation Ah; and - that the variation Am of the air mass m is proportional to the variation Ap in the bellows pressure p at a given bellows volume F and a given temperature T, i.e.. when there is no change in the bellows height h and the temperature T, the air mass variation Am is proportional to the bellows pressure variation Ap.

At the very highest position and the very lowest position of the bellows 15, i.e. in the regions nearest to the maximum value and the minimum value of the bellows height h, the cross-sectional area of the bellows 15 changes in response to variation in the bellows height h, which means that the relationship between the bellows height h and the bellows volume V and hence the relationships indicated above between the air mass m and the bellows height h and between the air mass m and the bellows pressure p look somewhat different in these extreme states than in the region between these extreme states.

Fig. 3 is a diagram illustrating the relationship between bellows height h and the air mass m accommodated in the air bellows 10 according to Figs. 2a and 2b at a given bellows pressure p and a given temperature T, and Fig. 5 is a diagram illustrating the relationship between bellows pressure/* of and the air mass m accommodated in this air bellows at a given bellows height h and a given temperature T.

The air bellows 10 consumes compressed air whenever an increase in the bellows height h ordered via the control unit 6 is effected, since every increase in the bellows height which is not due to the load F on the air bellows 10 having decreased requires a supply of compressed air to the air bellows.

An increase in the bellows height ordered by the control unit 6 may be initiated manually, e.g. by the vehicle's driver, via an operating unit 20 connected to the control unit 6 for increasing the height of the vehicle's chassis frame, e.g. to position a load space of a truck at a suitable height for loading or unloading or to position the rear end of a tractor unit at a suitable height relative to a semitrailer when the latter is to be coupled to or uncoupled from the tractor unit. Figs. 2a and 2b illustrate a change in the bellows height h of the air bellows 10 from a first bellows height hi (see Fig. 2a) to a second bellows height /? ₂ (see Fig. 2b) at a time when there is no change in the load F and hence in the bellows pressure p. This height increase is effected by a certain amount of compressed air being supplied to the air bellows 10 via the inlet/outlet 11. The amount of compressed air supplied to the air bellows 10 to effect the increase in the bellows height from hi to h. ₂ can in this case be calculated by means of a computing

model on the basis of the relationship illustrated in Fig. 3 between bellows height h and air mass m. The relationship means that a first air mass mi is accommodated in the air bellows at the first bellows height hi and that a second air mass ni ₂ which is greater than said first air mass mj is accommodated in the air bellows at the second bellows height /^. The amount of compressed air supplied Am therefore corresponds to the difference between said second air mass m. ₂ and said first air mass mi, i.e. Am = m,2 - mi.

The control unit 6 may also be set to automatically control the supply of compressed air to the air bellows 10 and the feeding-out of compressed air from the air bellows on the basis of recorded values of the bellows height h and the bellows pressure/? in such a way that the bellows height h and hence the height position of the vehicle's chassis frame are kept substantially constant. An increase in the bellows height h ordered via the control unit 6 may in this case be initiated automatically by the control unit 6, e.g. by resetting the bellows height h to a given value when the air bellows 10 has been pressed downwards by an increase in the load F on the air bellows 10 or as a result of air having leaked out from the air bellows. Figs. 4a and 4b illustrate an increase in the load F on the air bellows 10 from a first value Fj to a second value F 2. The load increase leads to the air bellows 10 being pressed downwards so that the bellows height h decreases from a given bellows height hi (see Fig. 4a) to another bellows height hf (see Fig. 4b) while the bellows pressure/? of the air bellows 10 increases from a first bellows pressure/); (see Fig. 4a) to a second bellows pressure/^ (see Fig. 4b). As the control unit 6 is set to keep the bellows height h at the given value hi', the control unit orders a supply of compressed air to the air bellows 10 via the inlet/outlet 11 so that the bellows height increases from the reduced bellows height hj (see Fig. 4b) to the given bellows height hi (see Fig. 4c) while said second bellows pressure /?^ is maintained. The amount of compressed air supplied to the air bellows 10 to effect the increase in the bellows height from λ/ to hi can in this case be calculated by means of a computing model on the basis of the relationship illustrated in Fig. 5 between bellows pressure/? and air mass m. The relationship means that a first air mass mi ' is accommodated in the air bellows at the first bellows pressure/?/ and that a second air mass m^ ' which is larger than said first air mass air mass mi ' is

accommodated in the air bellows at the second bellows pressure p ₂ when the bellows height corresponds to the given bellows height hi. The amount of compressed air supplied Am therefore corresponds to the difference between said second air mass m,2 ' and said first air mass mi ', i.e. Am = m. ₂ '- mi '.

The amount of compressed air supplied to the air bellows 10 to effect the increase in the bellows height from hi to hi might alternatively, in the case illustrated in Figs. 4a-4c, be calculated by means of a computing model on the basis of the relationship illustrated in Fig. 3 between bellows height h and air mass m, since the bellows pressure/? remains unchanged (p = P ₂ ) during the increase in the bellows height from hi to hi.

The calculated value of the amount of compressed air supplied to the air bellows 10 may be expressed either by weight, e.g. in kilograms, or by volume. In cases where the value is expressed by volume, it is advantageous for the volume value to be converted to standard cubic metres, which indicates the volume to which the calculated amount of compressed air should correspond at a given pressure and a given temperature.

In a simplified form of the method according to the invention, the temperature T is taken as a constant which corresponds to an assumed ambient temperature value. In this case, the actual ambient temperature is therefore not taken into account in the computing model used for calculating the magnitude of the amount of compressed air supplied Am to the air bellows 10. In a more refined variant, however, measured values of the prevailing ambient temperature Tare taken into account in the computing model used for calculating the magnitude of the amount of compressed air supplied Am to the air bellows 10, in which case the relationship illustrated in Figs. 3 and 5 will vary with the prevailing ambient temperature T. The temperature T is measured by a temperature sensor 9. The control unit 6 is connected to this sensor 9, either directly as illustrated in Fig. 1 or via another control unit of the vehicle, in order to receive from it measured values of the ambient temperature T of the air bellows.

To generate a measure of the compressed air consumption of the air bellows 10 during a certain period of time, each new value of an amount of compressed air supplied Am to the air bellows is accumulated with the values for the amount of compressed air supplied to the air bellows 10 previously estimated during the respective period of time.

Fig. 7 is a flowchart illustrating an embodiment of a method according to the invention for estimating an amount of compressed air supplied to an air bellows forming part of an air suspension system of a vehicle. At a first step S 1 , information about the bellows height h and the bellows pressure/? of the air bellows is received. At a second step S2, there is checking to see whether an increase in the bellows height h or the bellows pressure/? and hence a supply of compressed air to the air bellows have taken place. If the check at step S2 shows that no increase in the bellows height h or the bellows pressure/? has taken place, steps Sl and S2 are repeated after a certain time delay based on new information about the bellows height h and the bellows pressure/? of the air bellows. If the check at step S2 shows that an increase in the bellows height h or the bellows pressure/? has taken place, the magnitude of the amount of compressed air supplied δm to the air bellows is calculated at step S3 in the manner described above by means of a computing model on the basis of the information received about the bellows height h and the bellows pressure/? of the air bellows.

Computer programme codes for implementing a method according to the invention are with advantage included in a computer programme which can be read into the internal memory of a computer, such as the internal memory of an electronic control unit of a vehicle, e.g. the abovementioned control unit 6 or a control unit connected to it. Such a computer programme is with advantage provided via a computer programme product 21 which comprises a data storage medium which is readable by computer and has the computer programme stored on it. Said data storage medium is for example an optical data storage medium in the form of a CD ROM disc, a DVD disc, etc., a magnetic date storage medium in the form of a hard disc, a diskette, a cassette tape etc. or a memory of the ROM, PROM, EPROM or EEPROM type or a Flash memory.

A computer programme product according to an embodiment of the invention comprises computer programme codes for causing a computer in a vehicle provided with an air suspension system which comprises air bellows:

- to generate or receive the height value which represents the bellows height h of an air bellows which forms part of the air suspension system;

- to generate or receive the pressure value which represents the bellows pressure/) of said air bellows 10; and

- to calculate the magnitude of an amount of compressed air supplied Am to said air bellows 10 by means of a computing model on the basis of said height value and pressure value.

Fig. 6 illustrates very schematically an electronic control unit 30 comprising an execution means 31, e.g. a central processor unit (CPU) for executing computer software. The execution means 31 communicates with a memory 33, e.g. of the ROM type, via a databus 32. The control unit 30 comprises also a data storage medium 34, e.g. in the form of a memory of the ROM, PROM, EPROM or EEPROM type or a flash memory. The execution means 31 communicates with the data storage medium 34 via the databus 32. A computer programme comprising computer programme codes for implementing a method according to the invention is stored on the data storage medium 34.

The invention is of course in no way limited to the embodiments described above, since many possibilities for modifications thereof are likely to be obvious to a specialist in the field without thereby having to deviate from the basic concept of the invention as defined in the attached claims.