Quasi-zero stiffness vibration isolator based on pneumatic-mechanical adjustable combination

Abstract

The invention discloses a quasi-zero stiffness vibration isolator based on pneumatic-mechanical adjustable combination, and belongs to the technical field of vibration isolation. According to the technical scheme, the vibration isolator comprises an upper connecting plate, a lower connecting plate, a pneumatic positive stiffness module and at least three mechanical negative stiffness modules, the pneumatic positive stiffness module forms an adjustable air pressure closed air chamber through an air cylinder, a piston and an air pressure adjusting unit, and non-linear positive stiffness is provided. The mechanical negative stiffness module adopts a symmetrical rhombic connecting rod-spring mechanism and generates negative stiffness near a balance position; and a wide and stable quasi-zero stiffness region is formed near the balance position of the system by accurately matching the pneumatic positive stiffness curve and the mechanical negative stiffness curve. The device has the beneficial effects that the device has the characteristics of online stepless adjustment of rigidity, compact structure, stable bearing and excellent low-frequency vibration isolation performance, and is particularly suitable for ultralow-frequency vibration isolation of precise instruments and optical platforms.

Description

Technical Field

[0001] This invention belongs to the field of vibration isolation technology, specifically a quasi-zero stiffness vibration isolator based on a pneumatic-mechanical adjustable composite. Background Technology

[0002] Quasi-zero stiffness vibration isolators achieve effective isolation of low-frequency and even ultra-low-frequency vibrations by connecting a negative-stiffness element in parallel with a positive-stiffness element, making the equivalent stiffness of the system near the equilibrium position approach zero. Existing technologies for achieving quasi-zero stiffness mainly fall into the following categories: Mechanical springs offer a simple structure, but typically exhibit linear stiffness, making it difficult to independently achieve a wide-band quasi-zero region, and their stiffness cannot be adjusted in real time. Pre-bent beam structures utilize buckling characteristics to generate negative stiffness, but their negative stiffness range is narrow, making them extremely sensitive to machining and installation precision and exhibiting poor robustness. Magnetic structures utilize magnetic force to generate negative stiffness, achieving non-contact and frictionless operation, but their magnetic models are complex, costly, and susceptible to external magnetic field interference, and the negative stiffness is difficult to adjust linearly over a wide range. Bionic structural designs offer novel inspiration, but their mechanical models are often inaccurate, resulting in poor performance repeatability and difficulty in engineering. Composite material structures can integrate the properties of multiple materials, but they are typically used to improve damping or strength, making it difficult to achieve precise and adjustable stiffness characteristics as the main structure.

[0003] In summary, existing quasi-zero stiffness vibration isolation technologies generally suffer from one or more of the following problems: complex structure, fixed and unadjustable stiffness characteristics, narrow negative stiffness range, poor adaptability to load changes, high friction, or difficulty in achieving precise engineering manufacturing. Therefore, there is an urgent need for a quasi-zero stiffness vibration isolator with a relatively simple structure, independently designable positive and negative stiffness with online stepless adjustment of positive stiffness, low friction, strong adaptability, and ease of engineering implementation. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide a pneumatic-mechanical composite quasi-zero stiffness vibration isolator with a compact structure, independently designable positive and negative stiffness characteristics, online stepless adjustment of positive stiffness, the ability to form a wide and stable quasi-zero stiffness range, and particularly suitable for light-load precision vibration isolation.

[0005] To achieve the above objectives, the present invention provides the following technical solution: A quasi-zero stiffness vibration isolator based on a pneumatic-mechanical adjustable composite system, comprising: Upper connecting plate and lower connecting plate; A pneumatic positive stiffness module is disposed in the central region between the upper connecting plate and the lower connecting plate, and is used to provide nonlinear positive stiffness; Several mechanical negative stiffness modules are uniformly and symmetrically distributed along the circumference and arranged in parallel between the upper connecting plate and the lower connecting plate; The nonlinear positive stiffness provided by the pneumatic positive stiffness module and the negative stiffness provided by the mechanical negative stiffness module compensate each other within a preset displacement range near the system equilibrium position, so that the total dynamic stiffness of the vibration isolator within the displacement range approaches zero, forming a quasi-zero stiffness region.

[0006] Furthermore, the aerodynamic positive stiffness module includes: A cylinder, the cylinder body of which is fixedly mounted on the lower connecting plate; The piston is located inside the cylinder and fits precisely with the inner wall of the cylinder. The piston rod has one end connected to the piston and the other end rigidly connected to the upper connecting plate; the piston, the inner wall of the cylinder, and the piston rod form a variable-volume sealed air chamber. A pressure regulating unit is connected to the sealed air chamber via a pipeline and is used to set and regulate the initial absolute pressure inside the sealed air chamber. A pressure relief port is located on the upper side wall of the cylinder to prevent the pressure in the sealed chamber from exceeding a safety threshold.

[0007] Furthermore, the cylinder is fixed at the center position of the lower connecting plate.

[0008] Furthermore, the mechanical negative stiffness module is a symmetrical rhomboid linkage-spring mechanism, which is uniformly and symmetrically distributed along the circumference.

[0009] Furthermore, each of the aforementioned mechanical negative stiffness modules includes: Four connecting rods of equal length are connected in the middle by a connecting shaft to form an X-shaped structure that can deform in a vertical plane; The connecting components located at the upper and lower ends of the rhomboid structure are used to connect to the brackets connected to the upper connecting plate and the lower connecting plate, respectively. A tension spring is connected between a set of diagonal points of the rhomboid structure in a pre-stretched state; when the vibration isolator undergoes vertical displacement, the geometric deformation of the rhomboid structure causes the tension force of the tension spring to exhibit negative stiffness characteristics in the vertical direction.

[0010] Furthermore, the connecting assembly includes a sleeve and a fastening bolt, which are connected together.

[0011] Furthermore, the pressure regulating unit includes a miniature vacuum / air pump, a high-precision electro-proportional valve, and a pressure sensor.

[0012] Furthermore, by adjusting the initial absolute pressure through the air pressure regulating unit, the positive stiffness of the pneumatic positive stiffness module can be adjusted online steplessly, thereby reconstructing the quasi-zero stiffness region under different load conditions.

[0013] Furthermore, it also includes a control system, which includes: A pressure sensor is used to monitor the pressure of the sealed air chamber in real time. A displacement sensor is used to monitor the vertical displacement of the upper connecting plate relative to the lower connecting plate; a controller is configured to calculate the required target pressure based on the offset of the equilibrium position caused by load changes, and control the miniature vacuum / air pump and high-precision electro-proportional valve for adjustment.

[0014] Furthermore, the pressure relief port is connected to a safety valve, which automatically opens to relieve pressure when the pressure in the sealed air chamber exceeds a preset safety threshold.

[0015] The beneficial effects of this invention are: Compared with the prior art, the quasi-zero stiffness vibration isolator based on a pneumatic-mechanical adjustable composite described in this invention has the following technical features and beneficial effects: This invention achieves near-zero stiffness near the equilibrium position, resulting in extremely low dynamic stiffness of the system and significantly reducing the equivalent natural frequency, thus providing a basis for effectively isolating ultra-low frequency vibrations.

[0016] This invention allows for real-time and continuous adjustment of the aerodynamic stiffness by changing the initial air pressure through an air pressure regulating unit. This enables the system to quickly adapt to different load conditions and exhibits extremely high robustness.

[0017] This invention allows for offline design matching of negative stiffness characteristics by selecting tension springs of different stiffness or adjusting the initial installation angle of connecting rods, providing a matching basis for online adjustment of positive stiffness.

[0018] The pneumatic module of this invention provides high static support force, and multiple symmetrically distributed rhomboid mechanisms ensure uniform and stable load bearing. The system has high static stiffness at the equilibrium position, meeting the static load bearing requirements of precision equipment.

[0019] The central pneumatic module and the circumferentially distributed mechanical modules of the present invention are integrated between the upper and lower plates, resulting in a symmetrical and compact structure that is easy to install and integrate into various precision equipment.

[0020] This invention features a clear parametric design model. By rationally setting the key parameters of the pneumatic and mechanical modules, a quasi-zero stiffness zone of a certain width can be formed near the equilibrium position, achieving excellent low-frequency vibration isolation. By scaling or adjusting the key parameters proportionally, this design concept can be easily applied to vibration isolation scenarios with different load ranges. Attached Figure Description

[0021] To more clearly illustrate the technical solutions of the embodiments of the present invention, the present invention will be described in detail below with reference to the accompanying drawings and detailed embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. Other features, objects, and advantages of the present invention will become more apparent by reading the following detailed description of non-limiting embodiments with reference to the accompanying drawings: wherein: Figure 1 This is a three-dimensional structural diagram of the overall structure of the present invention, showing the arrangement relationship and structural symmetry between the upper connecting plate, the lower connecting plate, the aerodynamic positive stiffness module and the circumferentially distributed mechanical negative stiffness module; Figure 2 This is a longitudinal sectional view of the aerodynamic positive stiffness module in an embodiment of the present invention; Figure 3 This is a schematic diagram of the working principle of the mechanical negative stiffness module in the near-zero stiffness state in an embodiment of the present invention; the pneumatic positive stiffness module is simplified by being equivalent to a spring model in the figure, and the focus is on explaining the geometric deformation of the symmetrical rhomboid link-spring mechanism near the equilibrium position, and its superposition relationship with the pneumatic positive stiffness; Figure 4 This is a three-dimensional rendering of the present invention; The attached diagram is labeled as follows: 1-Upper connecting plate, 2-Piston rod, 3-Bracket, 4-Tension spring, 5-Lower connecting plate, 6-Sleeve, 7-Bolt, 8-Connecting shaft, 9-Connecting rod, 10-Cylinder, 11-Pressure relief port, 12-Piston. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. The following description, in conjunction with the accompanying drawings... Figure 1-4 Further explanation is given regarding the quasi-zero stiffness vibration isolator based on a pneumatic-mechanical adjustable composite.

[0023] Example 1 A quasi-zero stiffness vibration isolator based on a pneumatic-mechanical adjustable composite includes an upper connecting plate 1, a lower connecting plate 5, and a pneumatic positive stiffness module and at least three mechanical negative stiffness modules arranged in parallel between the two. The overall structure is symmetrically distributed to ensure balanced force distribution.

[0024] The pneumatic stiffness module includes a vertically mounted cylinder 10, piston 12, piston rod 2, and a pressure regulating unit. The cylinder body of cylinder 10 is fixed to the center of the lower connecting plate 5 by a high-strength bracket. The upper end of piston rod 12 is rigidly connected to the upper connecting plate 1, ensuring effective force transmission. Piston 12 and the inner wall of cylinder 10 are precisely sealed together, forming a variable-volume sealed air chamber with excellent sealing performance, ensuring air pressure stability. The pressure regulating unit is connected to the sealed air chamber via a sealed pipeline for precisely setting and real-time adjustment of the initial absolute pressure within the air chamber. This unit has a fast response speed and high adjustment accuracy. When the upper connecting plate 1 and the lower connecting plate 5 undergo relative displacement, piston 12 moves up and down accordingly, changing the air chamber volume. Based on the compressibility of gas, a nonlinear restoring force is generated, and its stiffness characteristics dynamically change with the displacement and initial air pressure, enabling wide-range adjustment.

[0025] The mechanical negative stiffness module is a symmetrical rhomboid link-spring mechanism, uniformly and symmetrically distributed circumferentially to maintain stability. Each module includes multiple links 9 of equal length, a connecting shaft 8, and a tension spring 4. The links 9 are connected at the middle by the connecting shaft 8, forming a symmetrical rhomboid structure that can deform in a vertical plane. Each end of the rhomboid structure has a connection point, which is connected to corresponding mounting seats on the upper connecting plate 1 and the lower connecting plate 5 via connecting components. The tension spring 4 is connected in a pre-stretched state between a set of diagonal points of the rhomboid structure. When the system experiences vertical displacement, the geometry of the rhomboid structure changes non-linearly, causing the vertical component of the tension force of the tension spring 4 to exhibit non-linear characteristics, thereby providing a negative stiffness effect near the equilibrium position.

[0026] By precisely matching the positive stiffness curve of the pneumatic positive stiffness module with the negative stiffness curve of the mechanical negative stiffness module, the total stiffness is made close to zero near the system's equilibrium position, thereby forming a quasi-zero stiffness region within a certain displacement range.

[0027] Preferably, the pressure regulating unit includes a miniature vacuum / air pump, a high-precision electro-proportional valve, and a real-time pressure sensor, which together form a closed-loop control circuit, enabling precise setting, real-time monitoring, and dynamic adjustment of the initial absolute pressure.

[0028] This invention achieves ultra-low frequency vibration isolation with extremely low dynamic stiffness: the nonlinear positive stiffness provided by the pneumatic positive stiffness module (cylinder 10, piston 12, and sealed air chamber) is precisely matched and mutually compensated with the negative stiffness provided by at least three mechanical negative stiffness modules (symmetrical rhomboid connecting rod-spring mechanism) uniformly and symmetrically distributed along the circumference near the system's equilibrium position. This results in the total dynamic stiffness of the vibration isolator approaching zero within a preset displacement range, forming a quasi-zero stiffness region. Within this region, the system's equivalent natural frequency is significantly reduced, providing a fundamental guarantee for effectively isolating low-frequency and even ultra-low-frequency vibrations.

[0029] This invention uses a pneumatic pressure regulating unit (including a miniature vacuum / air pump, a high-precision electro-proportional valve, and a pressure sensor) to set and adjust the initial absolute pressure within the sealed air chamber in real time, allowing for online and continuous modification of the positive stiffness provided by the pneumatic positive stiffness module. This feature enables rapid adjustment and rematching of positive and negative stiffness when load changes cause a shift in the system's equilibrium position, reconstructing a near-zero stiffness region near the new equilibrium position. This significantly enhances the vibration isolator's adaptability and robustness to different operating conditions and loads.

[0030] The mechanical negative stiffness module adopts a parametrically designed symmetrical rhomboid link-spring mechanism. By selecting tension springs 4 with different stiffnesses or adjusting the initial installation angle of the link 9, its negative stiffness characteristics can be designed and precisely matched offline, providing a stable and predictable compensation basis for online adjustment of positive stiffness, which is convenient for optimization for different applications.

[0031] The pneumatic positive stiffness module relies on the static air pressure of the sealed air chamber to provide the main static support force, and has high static stiffness. At the same time, multiple (at least three) circumferentially uniformly and symmetrically distributed mechanical negative stiffness modules ensure the uniform distribution and stable support of the load on the structure, so that the system has high static stiffness at the equilibrium position, which can meet the static load requirements of precision instruments or optical platforms.

[0032] The aerodynamic positive stiffness module located at the center and multiple mechanical negative stiffness modules evenly distributed along the circumference are integrated in parallel between the upper connecting plate 1 and the lower connecting plate 5, resulting in a symmetrical and compact overall layout. This modular and symmetrical design not only ensures stable mechanical performance but also facilitates installation, debugging, and integration into the foundations or frames of various precision equipment.

[0033] A pressure relief port 11 is provided on the upper side wall of the cylinder 10, and a safety valve can be optionally installed. When the pressure in the sealed air chamber exceeds a preset safety threshold, the pressure can be automatically released, effectively preventing component damage or safety accidents caused by overpressure, and ensuring the reliability and safety of the vibration isolator in long-term operation.

[0034] By integrating a closed-loop control system consisting of a pressure sensor, a displacement sensor, and a controller, the system status (pressure and displacement) can be monitored in real time, and the target air pressure can be automatically calculated and adjusted according to load changes. This achieves automatic maintenance and dynamic optimization of the near-zero stiffness state, improving the intelligence and practicality of the vibration isolator.

[0035] Example 2 refer to Figure 1 This embodiment provides a quasi-zero stiffness vibration isolator suitable for precision vibration isolation.

[0036] The structure of the pneumatic stiffness module is as follows: Figure 2As shown. This module includes a vertically mounted cylinder 10, whose cylinder body is fixedly mounted at the center of the lower connecting plate 5 via a bracket 3. A piston 12 is located inside the cylinder, and the upper end of the piston rod 2 is rigidly connected to the upper connecting plate 1. The piston 12, the inner wall of the cylinder, and the piston rod 2 together form a variable-volume sealed air chamber. The cylinder 10 is equipped with an air passage interface, which is connected to an external air pressure regulating unit via a pipeline for precise setting and real-time adjustment of the initial absolute pressure within the sealed air chamber. When the upper and lower connecting plates undergo relative displacement, the piston moves within the cylinder, changing the air chamber volume, thereby generating a nonlinear positive stiffness restoring force based on the compressibility of the gas. Figure 2 As shown, the upper side wall of the cylinder 10 is also provided with a pressure relief port 11, which is connected to the sealed air chamber. When the pressure in the air chamber exceeds a preset safety threshold, the safety valve automatically opens and releases gas through the pressure relief port to prevent overpressure in the air chamber from causing structural damage or safety accidents.

[0037] The mechanical negative stiffness module is a symmetrical rhomboid linkage-spring mechanism. In this embodiment, three such mechanisms are provided and evenly and symmetrically distributed circumferentially to ensure the balance and stability of the system's forces. Linkage 9, via a support shaft in the middle, forms a symmetrical rhomboid structure that can deform in a vertical plane. The upper and lower ends of the rhomboid structure are connected to corresponding mounting seats on the upper and lower connecting plates via sleeves 6 and fastening bolts 7, respectively. Tension springs 4 are connected in a pre-stretched state between a set of diagonal points of the rhomboid structure. The working principle of this mechanism is as follows: Figure 3 As shown (the pneumatic positive stiffness module in the figure has been simplified to an equivalent spring model): each of the mechanical negative stiffness modules contains an adjustable rhomboid structure composed of points A, B, C, and D, wherein a tension spring connects points B and C; when the load causes the top point A' to be vertically displaced downwards from the original state shown by the dashed line to the state A shown by the solid line, the rhomboid structure undergoes geometric nonlinear deformation, driving points B and C to move outwards synchronously, resulting in the extension of the tension spring BC and an increase in tension; the vertical component of this tension increases upwards with the increase of displacement, forming a negative stiffness characteristic.

[0038] The working principle of a quasi-zero stiffness vibration isolator based on a pneumatic-mechanical adjustable composite isolator of the present invention is explained below: The working principle of this invention is as follows: The total stiffness of the vibration isolator is formed by the superposition of the nonlinear positive stiffness provided by the aerodynamic positive stiffness module and the negative stiffness provided by the mechanical negative stiffness module. Through optimized design of structural parameters, the slopes of the two modules can be made equal in magnitude and opposite in direction within a certain displacement range near the system's equilibrium position, thereby making the total stiffness approach zero and forming a wide and stable quasi-zero stiffness region. Within this region, the dynamic stiffness of the system is extremely low, and the equivalent natural frequency decreases significantly, thus achieving effective isolation of low-frequency and even ultra-low-frequency vibrations.

[0039] One of the key advantages of this invention lies in its excellent adjustability and adaptability. When changes in load conditions cause a shift in the system's equilibrium position, the initial absolute pressure within the sealed air chamber can be adjusted in real time via the air pressure regulating unit. This adjustment not only re-matches the system's static support force but also simultaneously alters the magnitude of the aerodynamic positive stiffness, enabling it to precisely match the mechanical negative stiffness characteristics once again, thereby reconstructing the quasi-zero stiffness region near the new equilibrium position. This online, stepless stiffness adjustment capability greatly enhances the vibration isolator's adaptability and practicality under different operating conditions.

[0040] The principles and specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various changes or modifications within the scope of the claims, which do not affect the substantive content of the present invention. Unless otherwise specified, the embodiments and features described in the principles of this application can be arbitrarily combined with each other.

[0041] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A quasi-zero stiffness vibration isolator based on a pneumatic-mechanical adjustable composite system, characterized in that, include: Upper connecting plate (1) and lower connecting plate (5); A pneumatic positive stiffness module is disposed in the central region between the upper connecting plate (1) and the lower connecting plate (5) to provide nonlinear positive stiffness; Several mechanical negative stiffness modules are evenly and symmetrically distributed along the circumference and arranged in parallel between the upper connecting plate (1) and the lower connecting plate (5); The nonlinear positive stiffness provided by the pneumatic positive stiffness module and the negative stiffness provided by the mechanical negative stiffness module compensate each other within a preset displacement range near the system equilibrium position, so that the total dynamic stiffness of the vibration isolator within the displacement range approaches zero, forming a quasi-zero stiffness region.

2. The quasi-zero stiffness vibration isolator based on a pneumatic-mechanical adjustable composite as described in claim 1, characterized in that, The aerodynamic stiffness module includes: The cylinder (10) has its cylinder body fixedly mounted on the lower connecting plate (5); The piston (12) is disposed inside the cylinder (10) and is precisely fitted with the inner wall of the cylinder; The piston rod (2) has one end connected to the piston and the other end rigidly connected to the upper connecting plate (1); the piston (12), the inner wall of the cylinder (10) and the piston rod (2) form a variable volume sealed air chamber; A pressure regulating unit is connected to the sealed air chamber via a pipeline and is used to set and regulate the initial absolute pressure inside the sealed air chamber. A pressure relief port (11) is provided on the upper side wall of the cylinder (10) to prevent the pressure in the sealed chamber from exceeding the safety threshold.

3. The quasi-zero stiffness vibration isolator based on a pneumatic-mechanical adjustable composite as described in claim 2, characterized in that, The cylinder (10) is fixed at the center of the lower connecting plate (5).

4. The quasi-zero stiffness vibration isolator based on a pneumatic-mechanical adjustable composite as described in claim 1, characterized in that, The mechanical negative stiffness module is a symmetrical rhomboid link-spring mechanism, which is uniformly and symmetrically distributed along the circumference.

5. The quasi-zero stiffness vibration isolator based on a pneumatic-mechanical adjustable composite as described in claim 1 or 4, characterized in that, Each of the aforementioned mechanical negative stiffness modules includes: Four connecting rods (9) of equal length are connected in the middle by a connecting shaft (8) to form an X-shaped structure that can deform in a vertical plane; The connecting components provided at the upper and lower ends of the rhomboid structure are used to connect to the brackets connected to the upper connecting plate (1) and the lower connecting plate (5), respectively. A tension spring (4) is connected between a set of diagonal points of the rhomboid structure in a pre-stretched state; when the vibration isolator generates a vertical displacement, the geometric deformation of the rhomboid structure causes the tension of the tension spring (4) to exhibit negative stiffness characteristics in the vertical direction.

6. The quasi-zero stiffness vibration isolator based on a pneumatic-mechanical adjustable composite according to claim 5, characterized in that, The connecting assembly includes a sleeve (6) and a fastening bolt (7), which are connected together.

7. The quasi-zero stiffness vibration isolator based on a pneumatic-mechanical adjustable composite according to claim 2, characterized in that, The pressure regulating unit includes a miniature vacuum / air pump, a high-precision electro-proportional valve, and a pressure sensor.

8. The quasi-zero stiffness vibration isolator based on a pneumatic-mechanical adjustable composite according to claim 2 or 7, characterized in that, By adjusting the initial absolute pressure through the air pressure regulating unit, the positive stiffness of the pneumatic positive stiffness module can be adjusted online steplessly, thereby reconstructing the quasi-zero stiffness region under different load conditions.

9. The quasi-zero stiffness vibration isolator based on a pneumatic-mechanical adjustable composite according to claim 8, characterized in that, It also includes a control system, which includes: A pressure sensor is used to monitor the pressure of the sealed air chamber in real time. A displacement sensor is used to monitor the vertical displacement of the upper connecting plate (1) relative to the lower connecting plate (5); a controller is configured to calculate the required target pressure based on the offset of the equilibrium position caused by load changes, and control the micro vacuum / air pump and high-precision electro-proportional valve for adjustment.

10. The quasi-zero stiffness vibration isolator based on a pneumatic-mechanical adjustable composite according to claim 2, characterized in that, The pressure relief port (11) is connected to a safety valve. When the pressure in the sealed air chamber exceeds a preset safety threshold, the safety valve will automatically open to relieve pressure.

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