Method for calculating the stiffness of an automotive 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 bladder, a reservoir, and a piston against the bladder and coupled to the seat. The reservoir comprises an expandable wall made of an expandable material. The expandable wall bulges, elongating the expandable material, when the pressure in the reservoir increases. The method comprises the steps: b) calculation (102) of a maximum displacement of the outer wall; c) estimation (104) of an elongation of the expandable material as a function of the maximum displacement; d) calculation (106) of a modulus of elasticity adjusted as a function of the elongation; e) iteration (108) of steps b) calculation of a displacement to d) calculation of the modulus of elasticity until convergence; h) calculation (114) of 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 an air suspension in a motor vehicle

[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 filters out vibrations generated by the vehicle's movement over road irregularities. Such an air suspension system includes, in particular, an impactor and an air-filled chamber having deformable walls and defining a volume of the air chamber.

[0003] The impactor, also called the piston, and the air bag are thus configured to They move relative to each other to absorb the vertical movement of the seat. As is known, the volume of the airbag can be modified by removing or adding air from an air reservoir that is fluidly connected to the airbag. Adding or removing air from the airbag allows for modification of the air suspension system's filtering conditions. Specifically, when the impactor comes into contact with the airbag, it compresses the air inside, thus varying the volume and pressure of the airbag. This makes it possible to modify the stiffness of the air suspension system.

[0004] The drawback of these air suspension systems is that their stiffness is difficult to predict, both during the design phase and in operation. Therefore, it is necessary to conduct numerous experimental tests under various configurations to determine the operating conditions under which these air suspension systems will meet the requirements expected by vehicle manufacturers and users. The increased number of these experimental tests negatively impacts the time and cost of vehicle design.

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

[0006] Such a damping system improves comfort, and the associated method allows for the precise prediction of its stiffness. However, there is a need to further increase comfort, reduce size, and improve or maintain 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 stiffness calculation 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 bladder, a reservoir communicating with the bladder, the bladder and the reservoir defining an internal volume filled with a compressible gas; a piston against the bladder and coupled to the seat, the air suspension system being configured such that a predefined force of the piston on the bladder causes a flow of gas from the bladder to the reservoir and an increase in pressure; remarkable in that the reservoir comprises an expandable wall with an expandable material, the expandable wall 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 a theoretical modulus of elasticity of the expandable material;b) calculation of a maximum displacement of the expandable wall as a function of an internal pressure and the modulus of elasticity of the expandable material; c) estimation of an elongation of the expandable material as a function of the maximum displacement; d) calculation of an adjusted modulus of elasticity as a function of the elongation calculated in step c) estimation of the elongation; e) iteration of steps b) calculation of a maximum displacement to d) calculation of the adjusted modulus of elasticity until the elongation obtained in step c) estimation of the elongation meets a convergence criterion; f) calculation of a first internal volume as a function of the maximum displacement corresponding to the elongation obtained in step e) iteration, and of 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, and the area of ​​the piston against the pocket; h) calculation of a stiffness coefficient of the suspension system; pneumatic force based on the predefined force divided by the stroke.

[0009] The invention equips the reservoir with an expandable wall and estimates the resulting stiffness through an iterative calculation. This solution makes it possible to implement a compact air suspension system and increase its ability to follow the seat's movement. Furthermore, the calculation method optimizes the accuracy of the stiffness calculation despite the complexity of the corresponding mathematical model.

[0010] Document EP0166671B1 describes 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 hermetically sealed to a retaining device and to the axially spaced piston. The effective diameter of the inner chamber is larger than that of the outer chamber, allowing the spring to exert a compressive or extended force. However, this document does not propose a solution for communication between the chambers 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 atmospheric pressure and an internal pressure in the reservoir when the bag is at rest.

[0013] Preferably, in step b) calculation of the maximum displacement, said calculation includes a third-degree polynomial as a function of 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 includes a variation in 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 extensible wall forms a hemisphere 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 obtained from a tensile test.

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

[0021] Preferably, at 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 modulus of elasticity is a step of updating the adjusted modulus of elasticity 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 includes 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 device for regulating the stiffness 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; notable in that the reservoir includes an expandable wall with an expandable material, the expandable wall being intended to bulge by elongating its expandable material in the event of an increase in pressure in the reservoir; the stiffness regulation 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 proposes a computer program comprising instructions which, when executed by a computer, lead the computer to execute the calculation process according to the invention.

[0025] According to another aspect, the invention provides a motor vehicle comprising a seat, an air suspension system including 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 device for regulating the stiffness 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; notable in that the reservoir includes an expandable wall with an expandable material, the expandable wall being intended to bulge by elongating its expandable material in the event of an increase in pressure in the reservoir; the stiffness regulation device being configured to regulate the stiffness of the air suspension system using the stiffness calculation method according to the invention.

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

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

[0028] The invention will be well understood and other aspects and advantages will become clear upon reading the following description, given with reference to the attached figures listed below.

[0029] Fig. 1 is a side view of a motor vehicle according to the invention.

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

[0031] In the following description, the term "include" is synonymous with "include" and is not limiting in that it permits the presence of other elements in the motor vehicle or other steps in the calculation process to which it relates. It is understood that the term "include" 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 right-handed trihedron whose orientation is preserved throughout the figures.

[0033] In this description, the ranges of values ​​include the bounds that delimit them.

[0034] In the present description, the equality between the values ​​is not to be understood in the strict sense insofar as each equality allows 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 otherwise explicitly stated.

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

[0037] Figure 1 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 the main frame of the motor vehicle. The structure 12 delineates various compartments of the motor vehicle 10, including the passenger compartment 14, the cargo area, and the engine compartment (not shown). The structure 12 provides a mounting support for the powertrain, shock absorber systems, steering system, and braking systems.

[0039] The motor vehicle 10 also includes 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 include front seats and rear seats. At least one seat 16, for example the seat The conductor is capable of vertical oscillation. It is mobile.

[0040] The motor vehicle 10 includes an air suspension system 18. The air suspension system 18 accommodates 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 release it. It forms a base for attaching the seat 16 to the structure 12. It may include energy dissipation means to provide damping. The seat 16 includes a vertical slide or arms to guide its vertical movements.

[0041] The air suspension system 18 includes a piston 20, also called an impactor. The piston 20 is integral with the seat 16. It follows the vertical oscillations of the seat. The air suspension system 18 includes a pocket 22 communicating with a reservoir 24. The reservoir 24 and the pocket 22 are connected by a pipe. Alternatively, 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, which is deformed by the piston 20 when the seat 16 presses on it. The material of the pocket 22 is non-stretchable. It may include a waterproof fabric.

[0043] The pouch 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 passenger, preferably when the piston is not in contact with the pocket; the own volume of the reservoir 24 is greater than the own volume of the pocket 22.

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

[0046] The reservoir 24 is cylindrical. It has a central axis 38 and two opposite faces. Along the central axis 38, the expandable wall 26 forms one of the two opposite faces. According to an option of the invention, the tank has an expandable 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 means for adjusting the pressure (not shown) of the reservoir 24 and the pocket 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 its internal pressure. An auxiliary reservoir (not shown) may be coupled to said reservoir 24.

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

[0049] In general, the air suspension system 18 includes a gas pressure sensor in the tank 24 and the pocket 22.

[0050] The stiffness control device 30 includes computer means 32. The computer means 32 include 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. Furthermore, the processor may include a central processing unit (CPU), memory (in addition to or as separate memory as illustrated by reference number 36), and an input / output (I / O) interface through which the processor can 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 intended for storing data and instructions or code (i.e., software) for and readable and / or writable by the processor 34. The memory 36 may comprise various forms of non-volatile (i.e., non-transient) memory. Non-volatile memory includes flash memory or read-only memory (ROM), any type of programmable read-only memory (e.g., PROM, EPROM, EEPROM). The memory optionally comprises 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, forming a bump, when the pressure increases in the reservoir.

[0054] According to an alternative embodiment of the invention, the expandable wall forms the tubular portion of the cylindrical tank. The tubular portion 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 portion lengthens.

[0055] According to an alternative embodiment 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 can, for example, be a private motor vehicle or a commercial motor vehicle.

[0057] According to a preferred embodiment, the piston is made of a plastic or composite material. The plastic material may include a thermoplastic or 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-vinylester resins. For example, the composite material comprises a matrix of a plastic material as described above with 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 vehicle's environmental footprint.

[0059] Figure 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 the one shown in relation to Figure 1.

[0060] The process includes the following steps, in particular carried out in the following order.

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

[0062] b) calculation 102 of a maximum displacement of the expandable 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 elasticity 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 at d) calculation of the modulus elasticity adjusted 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 piston stroke as a function of a comparison of the first internal volume, the second internal volume, and 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 the theoretical modulus of elasticity, said theoretical modulus of elasticity can be determined beforehand via a tensile test. This allows obtaining a first estimate of the behavior of the extensible material, which may be nonlinear. The modulus of elasticity can be a secant modulus of elasticity. Step a) obtaining the theoretical modulus of elasticity can be a step of reading a value of the modulus of elasticity from a memory.

[0070] In step b) calculation 102 of the maximum displacement, the calculation includes a third-degree polynomial as 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, can 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 stretchable wall; h is the thickness of the stretchable wall; P is the pressure in the pocket and in the reservoir; the variable E is the Young's modulus of the stretchable material; a is a radius of the stretchable wall. The maximum displacement w corresponds to the displacement of the point on the stretchable wall with the greatest 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 based on the maximum displacement, the thickness, and a ratio specific to the extensible wall. In step c) estimation 104 of the elongation; the elongation St is calculated using 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 sinusoid function. It is also denoted 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 specimen or portion of material considered. The modulus of elasticity can be a Young's modulus E or a secant modulus.

[0082] During step e), iteration 108, the convergence criterion includes a variation in elongation less than or equal to 5%, preferably less than or equal to 1%, and more preferably less than or equal to 0.2%. As long as the convergence criterion is not met, 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 satisfied ((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 denoted wi. At iteration i, the elongation is denoted Sti. At iteration i, the adjusted modulus of elasticity is denoted 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 bag at rest; the parameter VAT corresponds to the volume of the reservoir at rest; the parameter Vmem corresponds to the change in volume of the reservoir resulting from the displacement of the expandable wall following pressure of the piston on the bag. 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, in particular with weighting coefficients.

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

[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 stroke.

[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 stroke. The stiffness coefficient K is calculated using the following equation. For example, for a given force F, the process 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 opposing a stroke. It corresponds to a counterforce resisting the downward movement of the piston. The use of the above equation improves the relevance of the stiffness coefficient. It provides an average to smooth out peak values.

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

Claims

Demands

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 bladder (22), a reservoir (24) communicating with the bladder (22), the bladder (22) and the reservoir (24) defining an internal volume filled with a compressible gas; a piston (20) against the bladder (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 bladder (22) causes a flow of gas from the bladder (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 designed to bulge by elongating its expandable material during the pressure increase; 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 adjusted modulus of elasticity based on the elongation calculated in step c) estimation (104) of the elongation; • e) iteration (108) of steps b) calculation (102) of a maximum displacement at 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. A method for calculating stiffness according to claim 1, characterized in that at 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. A method for calculating stiffness according to any one of claims 1 to 2, characterized in that at step f) calculation (110) of the first internal volume, the second total volume is a function of atmospheric pressure and of an internal pressure in the reservoir (24) when the pocket (22) is at rest.

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

5. Stiffness calculation method 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. A method for calculating stiffness according to any one of claims 1 to 5, characterized in that the convergence criterion includes a variation in elongation less than or equal to 1%.

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

8. A method for calculating stiffness according to any one of claims 1 to 7, characterized in that the extensible wall (26) forms a hemisphere and / or the pocket (22) is spherical.

9. A computer program comprising instructions which, when executed by a computer, cause the computer to perform the calculation method according to any one of claims 1 to 8.

10. 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 control 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 elongating its expandable material in the event of an increase in pressure in the reservoir (24); the stiffness control device (30) being configured to regulate the stiffness of the air suspension system (18) using the stiffness calculation method according to any one of claims 1 to 8; preferably, the expandable wall (26) comprises an elastomeric material or rubber.

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