Method for calculating the stiffness of a motor vehicle air suspension

Abstract

The invention relates to a method for calculating the stiffness of an air suspension system for a motor vehicle seat. The air suspension system comprises a pocket, a reservoir, a piston against the pocket and coupled to the seat. The reservoir comprises an extensible wall with an extensible material, the extensible wall bulging by elongating its extensible material during an increase in pressure in the reservoir. The method comprises the steps: b) calculating (102) a maximum displacement of the outer wall; c) estimating (104) an elongation of the extensible material as a function of the maximum displacement; d) calculating (106) an adjusted modulus of elasticity as a function of the elongation; e) iterating (108) steps b) calculating a displacement to d) calculating the modulus of elasticity until convergence; h) calculating (114) a stiffness coefficient of the air suspension system as a function of volumes and a stroke. Figure to be published with the abstract: Figure 2

Description

Title of the invention: Method for calculating the stiffness of a motor vehicle pneumatic suspension

[0001] The invention relates to a seat with an air suspension system for a motor vehicle. The invention also relates to a method for calculating the stiffness of an air suspension system for a motor vehicle seat. The invention also relates to a motor vehicle. The invention also relates to a computer program.

[0002] A motor vehicle seat is commonly associated with an air suspension system. Such a system makes it possible to filter the vibrations generated by the movement of the vehicle over irregularities in the road. Such an air suspension system notably comprises an impactor and an air-filled pocket having deformable walls and delimiting a volume of the air pocket.

[0003] The impactor, also called a piston, and the air pocket are thus configured to move relative to each other in order to absorb the vertical movement of the seat. As is known, a volume of the air bladder can be modified by removing or adding air from an air reservoir fluidly communicating with the air bladder. Adding or removing air from the air bladder makes it possible to modify the filtering conditions of the air suspension system. In particular, when the impactor is brought into contact with the air bladder, it compresses the air housed inside: by varying the volume and pressure of the bladder. It is thus possible to modify a stiffness of the air suspension system.

[0004] The disadvantage of these air suspension systems is that it is difficult to predict their stiffness, both during design and in operation. Consequently, it is necessary to multiply the experimental tests according to various configurations in order to determine the operating conditions in which these air suspension systems will meet the requirements expected by manufacturers and users of motor vehicles. The multiplication of these experimental tests negatively impacts the time and cost of designing the motor vehicle.

[0005] Document FR3113129B1 presents a method for predicting the stiffness of a damping system comprising an impactor in relative motion with respect to an air pocket. The prediction method comprises a step of determining an effective surface area of ​​the air pocket compressed by the impactor. The stiffness of the damping system is a function of the effective surface area. The stiffness of the damping system is determined by the product of a pressure in the air pocket by the quotient between a derivative of the effective surface area and a derivative of a translational movement of the impactor towards the air pocket.

[0006] Such a damping system makes it possible to improve comfort, and the associated method makes it possible to accurately predict its stiffness. However, there is a need to further increase comfort, reduce size, and improve or preserve the accuracy of stiffness determination.

[0007] The invention aims to address at least one of the problems or drawbacks encountered in the prior art. In particular, the invention aims to improve the comfort of an air suspension system for a motor vehicle seat. The invention aims to improve the accuracy of determining the stiffness of an air suspension system for a motor vehicle seat. The invention also aims to optimize the comfort, compactness, and accuracy of calculating the stiffness of an air suspension system for a motor vehicle seat.

[0008] According to a first aspect, the invention provides a method for calculating the stiffness of an air suspension system for a motor vehicle seat, the air suspension system comprising a pocket, a reservoir communicating with the pocket, the pocket and the reservoir defining an internal volume filled with a compressible gas; a piston against the pocket and coupled to the seat, the air suspension system being configured so that a predefined force of the piston on the pocket causes a flow of gas from the pocket to the reservoir and an increase in pressure; remarkable in that the reservoir comprises an extensible wall with an extensible material, the extensible wall being intended to bulge by elongating its extensible material during the increase in pressure; and in that the method comprises the following steps: a) obtaining a theoretical modulus of elasticity of the extensible material;b) calculating a maximum displacement of the extensible wall as a function of an internal pressure and the modulus of elasticity of the extensible material; c) estimating an elongation of the extensible material as a function of the maximum displacement; d) calculating an adjusted modulus of elasticity as a function of the elongation calculated during step c) estimation of the elongation; e) iterating steps b) calculating a maximum displacement to d) calculating the adjusted modulus of elasticity until the elongation obtained in step c) estimation of the elongation meets a convergence criterion; f) calculating a first internal volume as a function of the maximum displacement corresponding to the elongation obtained in step e) iteration, and a second internal volume as a function of the first volume and the predefined force applied by the piston on the pocket;g) estimation of a piston stroke based on a comparison of the first internal volume, the second internal volume, an area of ​​the piston against the pocket; h) calculation of a stiffness coefficient of the suspension system; pneumatic according to the predefined effort divided by the stroke.

[0009] The invention equips the tank with an extensible wall, and estimates the resulting stiffness via an iterative calculation. This solution makes it possible to materialize a compact air suspension system and to increase its capacity to follow the travel of the seat. In addition, the calculation method optimizes the accuracy of stiffness calculation despite the complexity of the corresponding mathematical model.

[0010] Document EP0166671B1 discloses a vehicle having a spring forming a suspension component. The air spring has inner and outer chambers that are independently pressurized to provide variable spring rates. The chambers are formed by gas-impermeable membranes sealingly attached to an axially spaced retainer and piston. The effective diameter of the inner chamber is greater than that of the outer chamber, allowing the spring to exert a compressive or extended force. However, this document does not propose a solution allowing communication between the chambers in order to exchange gas.

[0011] Preferably, in step h) calculation of the stiffness coefficient, the stiffness coefficient is a function of a difference in forces divided by a difference in strokes.

[0012] Preferably, in step f) calculation of the first internal volume, the second total volume is a function of the atmospheric pressure and an internal pressure in the reservoir when the pocket is at rest.

[0013] Preferably, in step b) calculation of the maximum displacement, said calculation comprises a third-degree polynomial depending on the maximum displacement and a thickness of the extensible wall.

[0014] Preferably, in step c) estimation of the elongation; said elongation is calculated as a function of the maximum displacement, of said thickness, of a ratio specific to the extensible wall.

[0015] Preferably, the convergence criterion comprises a variation in the elongation less than or equal to 1%.

[0016] Preferably, the reservoir is cylindrical, has a central axis and two opposite faces along the central axis, the extensible wall forming one of the two opposite faces.

[0017] Preferably, the expandable wall forms a half-sphere and / or the pocket is spherical.

[0018] Preferably, the predefined force of the piston on the pocket causes a compression of said gas in order to generate a force resisting the movement of the piston.

[0019] Preferably, in step a) obtaining the theoretical modulus of elasticity, said theoretical modulus of elasticity is derived from a tensile test.

[0020] Preferably, in step b) calculation of the maximum displacement, the calculation comprises the equation A(w / h)A3+(w / h) = B*P / E*(a / h)A4.

[0021] Preferably, in step c) estimation of the elongation; said elongation is calculated with the equation St=((asin((2*a*w) / (a2+w2))*(a2+w2)) / (2*a*w)) -1.

[0022] Preferably, step d) calculation of the adjusted elasticity modulus is a step of updating the adjusted elasticity modulus as a function of the elongation calculated during step c) estimation.

[0023] According to another aspect, the invention provides a seat with a piston and an air suspension system which comprises a pocket cooperating with the piston, a reservoir communicating with the pocket, the pocket and the reservoir defining an internal volume filled with a compressible gas; a stiffness regulating device of the air suspension system; the air suspension system being configured so that a predefined force of the piston on the pocket causes a flow of gas from the pocket to the reservoir; remarkable in that the reservoir comprises an extensible wall with an extensible material, the extensible wall being intended to bulge by elongating its extensible material in the event of an increase in pressure in the reservoir; the stiffness regulating device being configured to regulate the stiffness of the air suspension system using the calculation method according to the invention.

[0024] According to another aspect, the invention provides a computer program comprising instructions which, when executed by a computer, cause the latter to execute the calculation method according to the invention.

[0025] According to another aspect, the invention provides a motor vehicle comprising a seat, an air suspension system comprising a pocket, a reservoir communicating with the pocket, the pocket and the reservoir defining an internal volume filled with a compressible gas; a piston against the pocket and coupled to the seat; a stiffness regulating device of the air suspension system; the air suspension system being configured so that a predefined force of the piston on the pocket causes a flow of gas from the pocket to the reservoir; remarkable in that the reservoir comprises an extensible wall with an extensible material, the extensible wall being intended to bulge by elongating its extensible material in the event of an increase in pressure in the reservoir; the stiffness regulating device being configured to regulate the stiffness of the air suspension system using the stiffness calculation method according to the invention.

[0026] Preferably, the expandable wall comprises an elastomeric material or rubber.

[0027] Each characteristic introduced by the expression “preferably” given in relation to one of the aspects of the invention applies to all the other aspects of the invention.

[0028] The invention will be well understood and other aspects and advantages will appear clearly on reading the description which follows, given with reference to the figures appended and listed below.

[0029] [Fig.l] is a side view of a motor vehicle according to the invention.

[0030] [Fig.2] is a diagram of a method for calculating the stiffness of a system of pneumatic suspension for a motor vehicle seat according to the invention.

[0031] In the following description, the term "comprise" is synonymous with "include" and is not limiting in that it allows the presence of other elements in the motor vehicle or other steps in the calculation method to which it relates. It is understood that the term "comprise" includes the terms "consist of".

[0032] The X axis represents the longitudinal direction, the Y axis represents the transverse direction, and the Z axis represents the vertical direction of the motor vehicle. These three axes define a direct trihedron whose orientation is preserved throughout the figures.

[0033] In the present description, the ranges of values ​​include the limits which delimit them.

[0034] In the present description, the equalities between the values ​​are not to be understood in the strict sense insofar as each equality authorizes a variation of at most 10%, preferably at most 5%, more preferably at most 2%, between these values.

[0035] In this description, the technical characteristics are defined in the air suspension system mounting configuration, unless explicitly stated otherwise.

[0036] Throughout the description, the different figures use the same reference signs to designate identical or similar entities.

[0037] [Fig.l] represents a motor vehicle 10 according to the invention. The motor vehicle 10 comprises energy storage means and at least one motor (not shown) adapted to drive said motor vehicle 10.

[0038] The motor vehicle 10 comprises a structure 12. The structure 12 forms an outer body, or even a main frame of the motor vehicle. The structure 12 delimits different compartments of the motor vehicle 10, including the passenger compartment 14, the cargo compartment, the engine compartment (not shown). The structure 12 forms a mounting support for the powertrain, the shock absorber systems, the steering system, the braking systems.

[0039] The motor vehicle 10 also comprises at least one seat 16, preferably several seats 16 (only one shown). The seats 16 are arranged in the passenger compartment 14. They are fixed to the structure 12. The seats 16 comprise front seats and rear seats. At least one seat 16, for example the seat conductor, is able to oscillate vertically. It is mobile.

[0040] The motor vehicle 10 comprises an air suspension system 18. The air suspension system 18 accommodates the at least one seat 16. The air suspension system 18 generally forms a spring. It forms a mechanism allowing vertical movements; such as oscillations. Its pneumatic nature allows it to accumulate mechanical energy and then restore it. It forms a base for fixing the seat 16 to the structure 12. It may comprise energy dissipation means in order to provide cushioning. The seat 16 comprises a vertical slide or arms for guiding its vertical movements.

[0041] The air suspension system 18 comprises a piston 20, also called an impactor. The piston 20 is secured to the seat 16. It follows its vertical oscillations. The air suspension system 18 comprises a pocket 22 communicating with a reservoir 24. The reservoir 24 and the pocket 22 are connected by a pipe. According to an alternative, they are joined at a common partition.

[0042] The pocket 22 is also called an air pocket. It forms a compressible bladder, which is compressed by the movements of the piston 20. The pocket 22 comprises a flexible outer casing, and deformed by the piston 20 when the seat 16 presses on it. The material of the pocket 22 is non-stretchable. It may comprise a waterproof fabric.

[0043] The pocket 22 and the reservoir 24 define an internal volume filled with a compressible gas. The internal volume varies depending on the vertical position of the piston 20. Different physical models can be used to calculate it. The compressible gas can be air. Alternatively, it can be nitrogen or helium.

[0044] At rest, that is to say when the seat 16 is free of passengers, preferably when the piston is not pressing against the pocket; the specific volume of the reservoir 24 is greater than the specific volume of the pocket 22.

[0045] The reservoir 24 has an expandable wall 26 with an expandable material. The expandable wall 26 forms a half-sphere and / or the pocket is spherical. Other shapes are contemplated. For example, the expandable wall 26 comprises an elastomeric material or rubber. When the piston 20 exerts a predefined force on the pocket 22, this causes a flow of gas from the pocket 22 to the reservoir 24, and an increase in pressure. Since the expandable wall 26 can elongate, it inflates during an increase in pressure. It bulges. The volume it delimits increases, however less than the pocket compresses. The material of the expandable wall 26 elongates in addition to changing curvature. In order to contain the pocket 22, the air suspension system 18 has a liner 28. The liner 28 forms a housing housing the pocket 22.

[0046] The reservoir 24 is cylindrical. It has a central axis 38 and two opposite faces. Along the central axis 38, the extensible wall 26 forms one of the two opposite faces. According to one option of the invention, the tank has an extensible wall on each of the opposite faces.

[0047] The air suspension system 18 also includes a stiffness control device 30. The stiffness control device 30 includes pressure adjustment means (not shown) for the reservoir 24 and the bag 22. The pressure adjustment means may include a pump and / or an exhaust communicating with the environment. The pressure adjustment means may include an inflatable balloon in the reservoir 24 to modulate the internal pressure. An auxiliary reservoir (not shown) may be coupled to said reservoir 24.

[0048] The stiffness regulating device 30 is configured to be able to regulate a stiffness of the air suspension system 18. In particular, the stiffness regulating device 30 is configured to maintain a stiffness of the air suspension system 18 constant. The stiffness can remain constant despite the mass of a user on the seat 16.

[0049] Generally, the air suspension system 18 includes a gas pressure sensor in the reservoir 24 and the bag 22.

[0050] The stiffness regulation device 30 comprises computer means 32. The computer means 32 comprise a processor 34 and a memory 36. The computer means 32 may correspond to the on-board computer of the motor vehicle.

[0051] The processor 34 may include one or more programmable electronic microprocessors or microcontrollers. In addition, the processor may include a central processing unit (CPU), a memory (in addition to or like the separate memory illustrated by reference numeral 36), and an input / output (I / O) interface through which the processor may receive a plurality of input signals. Such an I / O interface is also configured to generate a plurality of output signals including, but not limited to, those used to control and / or provide data.

[0052] The memory 36 is provided for storing data and instructions or code (i.e., software) for and readable and / or writable by the processor 34. The memory 36 may include various forms of non-volatile (i.e., non-transient) memory. The non-volatile memory includes flash or read-only memory (ROM), any type of programmable read-only memory (e.g., PROM, EPROM, EEPROM). The memory optionally includes volatile memory, including random access memory (RAM), static random access memory (SRAM), dynamic random access memory (DRAM), and synchronous dynamic random access memory (SDRAM). According to the invention, the memory may be internal to the processor, or alternatively tively form a separate component.

[0053] According to an alternative of the invention, the expandable wall is flat at rest. It bulges, forms a bump, when the pressure increases in the tank.

[0054] According to an alternative of the invention, the expandable wall forms the tubular part of the cylindrical tank. The tubular part surrounds the central axis. It connects the opposite faces. Its axial ends are fixed. When the pressure increases in the tank, the diameter of its central part elongates.

[0055] According to an alternative of the invention, the reservoir is spherical, the extensible wall forming all or part of the spherical surface of the spherical reservoir. When the pressure increases in the spherical reservoir, its radius lengthens.

[0056] The motor vehicle may, for example, be a private motor vehicle or a utility motor vehicle.

[0057] According to a preferred embodiment, the piston is made of a plastic or composite material. The plastic material may comprise a thermoplastic material or a thermosetting material. According to a preferred embodiment, the plastic material is selected from the group comprising polypropylene, polyamide, polyphthalamide, polyetheretherketone, polyphenylene sulfide, polyamide-imide, polyetherimide, polyarylamide, polyepoxide, unsaturated polyester, vinyl ester or polyester-vinyl ester resins. For example, the composite material comprises a matrix of a plastic material as described above with a reinforcement. For example, the reinforcement comprises glass or carbon fibers.

[0058] The piston comprises at least 10% by weight of recycled plastic material based on the total weight of the plastic material; preferably from 10 to 80% by weight; more preferably from 20 to 60% by weight or from 30 to 40% by weight. The use of recycled plastic material reduces the ecological footprint of the vehicle.

[0059] [Fig.2] shows a diagram of a method for calculating the stiffness of an air suspension system for a motor vehicle seat. The motor vehicle corresponds to that shown in relation to [Fig.l].

[0060] The method comprises the following steps, in particular executed in the following order.

[0061] a) obtaining 100 of a theoretical modulus of elasticity of the extensible material;

[0062] b) calculation 102 of a maximum displacement of the extensible wall as a function of a internal pressure and the modulus of elasticity of the stretchable material;

[0063] c) estimation 104 of an elongation of the extensible material as a function of the maximum displacement;

[0064] d) calculation 106 of an elastic modulus adjusted as a function of the elongation calculated during step c) estimation of the elongation;

[0065] e) iteration 108 of steps b) calculation of a maximum displacement to d) calculation of the modulus adjusted elasticity until the elongation obtained in step c) estimation meets a convergence criterion;

[0066] f) calculation 110 of a first internal volume as a function of the maximum displacement corresponding to the elongation obtained in step e) iteration 108, and of a second internal volume as a function of the first volume and the predefined force applied by the piston on the pocket;

[0067] g) estimation 112 of a stroke of the piston as a function of a comparison of the first internal volume, of the second internal volume, of an area of ​​the piston against the pocket;

[0068] h) calculation 114 of a stiffness coefficient of the air suspension system as a function of the predefined force divided by the stroke.

[0069] In step a) obtaining 100 of the theoretical modulus of elasticity, said theoretical modulus of elasticity can be determined beforehand via a tensile test. This makes it possible to obtain a first estimate of the behavior of the extensible material, the behavior of which can be non-linear. The modulus of elasticity can be a secant modulus of elasticity. Step a) obtaining 100 can be a step of reading a value of modulus of elasticity in a memory.

[0070] In step b) calculation 102 of the maximum displacement, the calculation comprises a third-degree polynomial which is a function of the divided maximum displacement and a thickness of the extensible wall. In step b) calculation 102 of a maximum displacement, the internal pressure, at the first iteration, may correspond to the initial internal pressure.

[0071] In step b) calculation 102 of the maximum displacement, the calculation includes the following equation. This equation is optionally a third-degree equation. Its variable is a quotient between the maximum displacement and the thickness of the extensible wall.

[0072] A*(f)3 + f V h J h E \ h /

[0073] In this equation A is an empirical constant equal to 1.852; B is an empirical constant equal to 0.696; the variable w is the maximum displacement of the extensible wall; h is the thickness of the extensible wall; P is the pressure in the pocket and in the reservoir; the variable E is the Young's modulus of the extensible material; a is a radius of the extensible wall. The maximum displacement w corresponds to the displacement of the point of the extensible wall presenting the longest displacement.

[0074] This equation is solved using Cardamo's formula:

[0075] / 3 / ¢,2 / 7V-, P3 \, 3 Fl 2 / / ?2, A T" i

[0076] In this formula, we set p=l / A; and q is obtained by the equation below.

[0077] 9 “ A *E kh )

[0078] Generally, in step c) estimation 104 of the elongation; said elongation is calculated as a function of the maximum displacement, of said thickness, of a ratio specific to the extensible wall. In step c) estimation 104 of the elongation; the elongation St is calculated with the following equation. Preferably, the ratio is the radius of the extensible wall.

[0079] asin(^)(M^2) >3l A 'L

[0080] Where "asin" corresponds to the inverse function of the sinusoidal function. It is also noted sin-1.

[0081] In step d) calculation 106 of an adjusted modulus of elasticity, the elongation is expressed as a percentage. It is equal to the difference in length divided by the total length of the test piece or portion of material taken into account. The modulus of elasticity may be a Young's modulus E or a secant modulus.

[0082] During step e) iteration 108, the convergence criterion comprises a variation in the elongation less than or equal to 5%, preferably less than or equal to 1%, more preferably less than or equal to 0.2%. As long as the convergence criterion is not reached, the calculation continues in order to refine the value of the elongation and the value of the adjusted modulus of elasticity. For example, as long as the following inequality is verified ((wi+l-wi) / wi) > 1%, the calculation continues in a loop. Step e) iteration 108 comprises at least two iterations, preferably at least five iterations. The convergence of the elongation implies the convergence of stress in the extensible material, as well as a convergence of the maximum displacement.

[0083] At iteration i, the maximum displacement is noted wi. At iteration i, the elongation is noted Sti. At iteration i, the adjusted modulus of elasticity is noted Ei.

[0084] In step f) calculation 110, the second total volume is a function of atmospheric pressure and the internal pressure in the pocket at rest. The first volume is denoted VTo. It is calculated using the following equation.

[0085] VTo = VO + VAT + Vmem

[0086] The parameter V0 corresponds to the volume of the pocket at rest; the parameter VAT corresponds to the volume of the reservoir at rest; the parameter Vmem corresponds to the variation in volume of the reservoir resulting from the movement of the extensible wall following pressure of the piston on the pocket. The parameter Vmem is calculated using the following formula.

[0087] t T * lvvo / j Vmem= ttAv^-—g--

[0088] Other equations are considered, in particular depending on the geometry of the extensible wall.

[0089] The second total volume is calculated using the following equation.

[0090] VT _ {Po+Patm) vt,T P+Patm

[0091] Other equations are envisaged, notably with weighting coefficients.

[0092] In this equation, the variable P is the pressure inside the reservoir and the pocket. It is calculated by the equation P= F / S; where F is the predefined force, and S is the area of ​​the piston in contact with the pocket. This is the extent of the lower surface of the piston. The second total volume corresponds to a weighted value of the first total volume as a function of the pressure without support Po, and the pressure with support P of the piston against the pocket.

[0093] In step g) estimation 112 of the piston stroke, the following equation is used.

[0094] 7_ VT-VAT-Vmem(F)-Vo

[0095] In other words, this equation compares two volumes VT and VTo calculated in two ways, and divides the difference by the piston.

[0096] According to one option, in step h) calculation 114, the stiffness coefficient is a function of a difference in force divided by a difference in travel. The stiffness coefficient K is calculated using the following equation. For example, for a given force F, the method takes into account this force F plus 5%; F+; and this force F minus 5%; F-. Similarly, the corresponding displacements Z+ and Z- are taken into account.

[0097] “z+-z.

[0098] Thus, the stiffness coefficient K corresponds to a resistance opposed to a stroke. It corresponds to a counter-force resisting the descent of the piston. The use of the above equation improves the relevance of the stiffness coefficient. It provides an average to smooth out the peak values.

[0099] The invention comprises the combination of all the embodiments illustrated by all the figures.

Claims

Claims

1. A method for calculating the stiffness of an air suspension system (18) for a seat (16) of a motor vehicle (10), the air suspension system (18) comprising a pocket (22), a reservoir (24) communicating with the pocket (22), the pocket (22) and the reservoir (24) defining an internal volume filled with a compressible gas; a piston (20) against the pocket (22) and coupled to the seat (16), the air suspension system (18) being configured such that a predefined force of the piston (20) on the pocket (22) causes a flow of gas from the pocket (22) to the reservoir (24) and an increase in pressure; characterized in that the reservoir (24) comprises an expandable wall (26) with an expandable material, the expandable wall (26) being intended to bulge by elongating its expandable material during the increase in pressure; and in that the method comprises the following steps: • a) obtaining (100) a theoretical modulus of elasticity of the extensible material; • b) calculation (102) of a maximum displacement of the extensible wall (26) as a function of an internal pressure and the modulus of elasticity of the extensible material; • c) estimation (104) of an elongation of the extensible material as a function of the maximum displacement; • d) calculation (106) of an elastic modulus adjusted as a function of the elongation calculated during step c) estimation (104) of the elongation; • e) iteration (108) of steps b) calculation (102) of a maximum displacement to d) calculation (106) of the adjusted modulus of elasticity until the elongation obtained in step c) estimation (104) of the elongation meets a convergence criterion; • f) calculation (110) of a first internal volume as a function of the maximum displacement corresponding to the elongation obtained in step e) iteration (108), and of a second internal volume as a function of the first volume and the predefined force applied by the piston (20) on the pocket (22); • g) estimation (112) of a stroke of the piston (20) as a function of a comparison of the first internal volume, the second internal volume, an area of ​​the piston (20) against the pocket (22); • h) calculation (114) of a stiffness coefficient of the air suspension system (18) as a function of the predefined force divided by the stroke.

2. Method for calculating stiffness according to claim 1, characterized in that in step h) calculation (114) of the stiffness coefficient, the stiffness coefficient is a function of a difference in forces divided by a difference in strokes.

3. Method for calculating stiffness according to one of claims 1 to 2, characterized in that in step f) calculation (110) of the first internal volume, the second total volume is a function of atmospheric pressure and an internal pressure in the reservoir (24) when the pocket (22) is at rest.

4. Method for calculating stiffness according to one of claims 1 to 3, characterized in that in step b) calculation (102) of the maximum displacement, said calculation comprises a third-degree polynomial which is a function of the maximum displacement and of a thickness of the extensible wall (26).

5. Method for calculating stiffness according to claim 4, characterized in that in step c) estimation (104) of the elongation; said elongation is calculated as a function of the maximum displacement, of said thickness, of a ratio specific to the extensible wall (26).

6. Method for calculating stiffness according to one of claims 1 to 5, characterized in that the convergence criterion comprises a variation in the elongation less than or equal to 1%.

7. Method for calculating stiffness according to one of claims 1 to 6, characterized in that the reservoir (24) is cylindrical, has a central axis (38) and two opposite faces along the central axis, the extensible wall (26) forming one of the two opposite faces.

8. Method for calculating stiffness according to one of claims 1 to 7, characterized in that the extensible wall (26) forms a half-sphere and / or the pocket (22) is spherical.

9. Computer program comprising instructions which when executed by a computer cause the latter to execute the calculation method according to one of claims 1 to 8.

10. A motor vehicle (10) comprising a seat (16), an air suspension system (18) comprising a pocket (22), a reservoir (24) communicating with the pocket (22), the pocket (22) and the reservoir (24) defining an internal volume filled with a compressible gas; a piston against the pocket (22) and coupled to the seat (16); a stiffness regulating device (30) of the air suspension system (18); the air suspension system (18) being configured such that a predefined force of the piston on the pocket (22) causes a flow of gas from the pocket (22) to the reservoir (24); characterized in that the reservoir (24) comprises an expandable wall (26) with an expandable material, the expandable wall (26) being intended to bulge by extending its expandable material in the event of an increase in pressure in the reservoir (24); the stiffness regulating device (30) being configured to regulate the stiffness of the air suspension system (18) using the stiffness calculation method according to one of claims 1 to 8; preferably, the expandable wall (26) comprises an elastomeric material or rubber.

Images