A method for simulating the dynamics of an arrayed wedge airbag care mattress

Abstract

The application is a kind of array wedge-shaped air bag nursing mattress dynamics simulation method, first, the minimum feature unit is extracted from the mattress configuration, and the minimum feature unit, weight and boundary model are established;Define material properties, and carry out mesh division;Then, model assembly and contact setting;Set the boundary conditions of each simulation step;Finally, set the parameters, run the simulation;Simulation step one is used to simulate the air bag inflation process, simulation step two is used to simulate the process of lying down, and simulation step three is used to simulate the body turning movement.The ratio of the load received by the air bag to the pressure in the air bag is defined as the effective contact area between the human body and the air bag, if the effective contact area change rate is greater than or equal to the threshold value, it indicates that the contact area between the human body and the air bag changes greatly, which increases the risk of pressure ulcer, so when the air bag nursing mattress makes the human body turn over, the human body position should be avoided to stay in the position with large effective contact area change rate.Through simulation, the body position and part prone to pressure ulcer are obtained, which provides guidance for the control and optimization design of air bag nursing mattress.

Description

Technical Field

[0001] This invention relates to the field of airbag mattress simulation technology, and in particular to a dynamic simulation method for an array-type wedge-shaped airbag nursing mattress. Background Technology

[0002] Critically ill patients, often in a completely or semi-conscious state, develop pressure sores due to prolonged bed rest caused by pressure, hypoxia, and ischemia in localized tissues. Airbag nursing mattresses, composed of an array of multiple inflatable airbags, allow for controlled inflation and deflation of the airbags, enabling patients to turn over and improving pressure distribution. This prevents prolonged pressure on localized tissues and thus prevents pressure sores, achieving the desired nursing effect. The pressure distribution between the patient and the airbag mattress varies significantly depending on the patient's lying position. To improve this pressure distribution, a finite element model of the airbag nursing mattress is established during its design and development. This model simulates and analyzes the mechanical properties of the airbag nursing mattress under external forces and internal gas pressure, a crucial prerequisite for precise control and optimized design.

[0003] Existing technologies rarely involve numerical simulation of airbags. Patent application number 201410063266.0 discloses a finite element simulation analysis method for airbags. By creating a finite element Lagrange network model of the airbag before folding, an Eulerian fluid network model of the airbag's exterior after folding, and an Eulerian fluid network model of the gas generator's interior, this method can realistically and effectively simulate the pressure values ​​at various points inside the airbag during deployment in a collision. This allows for the assessment of injuries to passengers in abnormal sitting positions during airbag deployment, providing some guidance for the design and development of airbag systems. However, this method only analyzes the mechanical properties of individual airbags. Since airbags in airbag-supported mattresses are interconnected and compress each other, this method cannot accurately reflect the state changes during airbag coupling and therefore cannot provide a reference for pressure ulcer control in airbag-supported mattresses. Summary of the Invention

[0004] To address the shortcomings of existing technologies, the technical problem this invention aims to solve is to provide a dynamic simulation method for an array-type wedge-shaped airbag nursing mattress.

[0005] The technical solution adopted by the present invention to solve the aforementioned technical problem is as follows:

[0006] A dynamic simulation method for an array-type wedge-shaped airbag care mattress, the mattress comprising two rows of airbags along its length, each row consisting of multiple double-layered wedge-shaped airbags closely arranged, with the wedge-shaped surfaces of two double-layered wedge-shaped airbags in the same row in contact; the double-layered wedge-shaped airbags are formed by stacking two wedge-shaped airbags together, with the inner cavities of the two wedge-shaped airbags communicating with each other; characterized in that the method includes the following steps:

[0007] Step 1: Extract the minimum feature unit for simulation from the mattress configuration. This involves treating the two double-layered wedge-shaped airbags with wedge-shaped surfaces as a whole and dividing them into two equal parts along the plane of symmetry. Each part is a minimum feature unit. Therefore, the minimum feature unit is formed by coupling half of one double-layered wedge-shaped airbag with half of the other double-layered wedge-shaped airbag. The half of one double-layered wedge-shaped airbag and the half of the other double-layered wedge-shaped airbag are denoted as airbag unit one and airbag unit two, respectively. Establish a solid model in the finite element simulation software, including the minimum feature unit, the weight, and the boundary. The weight is used to simulate the human body.

[0008] Step 2: Define material properties and mesh the material. The material property parameters for the weight and boundary include density, Young's modulus, and Poisson's ratio. The property parameters of the airbag material are obtained from the hyperelastic material constitutive model. The mesh of the smallest feature element uses shell elements, while the mesh of the weight and boundary uses solid elements.

[0009] Step 3: Model assembly and contact setup; Contact is divided into general contact and surface contact. General contact is applied to the entire model, while surface contact is used between two airbag units, between the weight and an airbag unit, and between an airbag unit and the boundary.

[0010] Step 4: Set the boundary conditions for each simulation step;

[0011] Set the pressure boundary conditions for the airbag unit, that is, select the inner surface of the airbag unit as the surface on which the gas pressure acts, set the magnitude of the gas pressure, and set the direction as the normal direction of the surface on which the gas pressure acts.

[0012] The entire simulation process consists of three steps: step one simulates the airbag inflation process, step two simulates the process of a person lying down, and step three simulates the process of a person turning over. In step one, the positions of the weight, airbag unit one, solid boundary one, and solid boundary two are fixed. Airbag unit two and solid boundary three are moved the same distance along the negative Z-axis so that the distance between the two airbag unit's bisecting surfaces equals the actual distance in the nursing mattress. Simultaneously, the gas pressure in both airbag units is increased until the internal pressure of the airbag unit reaches a set value. In simulation step two, the pressure boundary conditions of the airbag unit and the fixed position constraint of the weight are deleted. The positions of solid boundary one, solid boundary two, and solid boundary three are fixed, and the displacement of the boundary points on the two airbag unit split surfaces along the Z-axis is set to zero. The weight falls onto the airbag unit under the action of gravity and causes the airbag unit to deform until the airbag unit reaches a steady state. In simulation step three, the gas pressure of one of the airbag units is increased or decreased to a set value. The height direction of the airbag is taken as the X-axis, the length direction as the Y-axis, and the width direction as the Z-axis.

[0013] Step 5: Set the parameters, run the simulation, and obtain the simulation data;

[0014] The effective contact area between the human body and the airbag is defined as the ratio of the load on the airbag to the internal pressure of the airbag. The effective contact area between the human body and the airbag in step three of the simulation is obtained, and the effective contact area curve is obtained. The slope of the curve is the effective contact area change rate. If the effective contact area change rate is greater than or equal to the threshold, it indicates that the contact area between the human body and the airbag changes a lot, which increases the risk of pressure sores. Therefore, when controlling the airbag nursing mattress to turn the human body, the human body position should be avoided in the position with a large effective contact area change rate.

[0015] After the turning movement is completed, randomly select multiple grid nodes from the contact area between the airbag unit and the weight. If the stress of these grid nodes exceeds the limit value, it indicates that the risk of pressure sores in the corresponding parts of the human body has increased, and the human body position should be adjusted.

[0016] Furthermore, in step 2, a uniaxial tensile test is performed on the specimen in a quasi-static environment to obtain the deformation of the specimen under different loads; the deformation of the specimen under different loads is substituted into the constitutive model of the hyperelastic material for curve fitting to obtain the material property parameters; the constitutive model of the hyperelastic material adopts the Yeoh model.

[0017] Furthermore, in step 2, the shell element is a quadrilateral shell element with a mesh size of 2-4 mm; the solid element is an 8-node hexahedral solid element with a mesh size of 5-10 mm.

[0018] Compared with the prior art, the beneficial effects of the present invention are:

[0019] Based on the structural characteristics of air-cushioned nursing mattresses, the smallest feature units are extracted from the overall configuration. Simulation of only these smallest feature units reflects the overall changes, eliminating the need to build a finite element model of the entire mattress. This reduces simulation complexity, computational load, and convergence speed. The entire simulation process includes three steps: air-cushion inflation, the patient lying down, and turning over. This visually reflects the state changes of the nursing mattress during each process and obtains parameters of the coupling region. The rate of change of the effective contact area between the patient and the air-cushion during turning over is used as a pressure ulcer risk indicator, providing a basis for the design and control of nursing mattresses. When controlling the air-cushioned nursing mattress to turn the patient, the patient's position should be avoided in areas with a large rate of change of effective contact area to reduce the risk of pressure ulcers. The stress of the mesh nodes in the contact area between the patient and the air-cushion also reflects the risk of pressure ulcers. Therefore, by obtaining parameters in the contact area through simulation, the areas where patients are prone to pressure ulcers can be identified. By controlling the inflation and deflation of the air-cushion to adjust the patient's position, prolonged pressure on localized body tissues can be avoided, reducing the risk of pressure ulcers and providing guidance for the design and development of pressure ulcer-preventing nursing mattresses. Attached Figure Description

[0020] Figure 1This is a schematic diagram of the structure of the array-type wedge-shaped airbag nursing mattress of the present invention;

[0021] Figure 2 This is an overall flowchart of the present invention;

[0022] Figure 3 This is a structural diagram of the smallest feature unit of the present invention;

[0023] Figure 4 This is a structural diagram of the solid model established in this invention;

[0024] Figure 5 This is a schematic diagram of the boundary points of the airbag unit's split surface in this invention;

[0025] Figure 6 This is a graph showing the change in internal pressure of the airbag unit obtained from simulation step two;

[0026] Figure 7 This is a graph showing the displacement changes of the mesh nodes obtained in step two of the simulation.

[0027] In the diagram, 1-weight; 2-airbag unit one; 3-entity boundary one; 4-entity boundary two; 5-entity boundary three; 6-airbag unit two. Detailed Implementation

[0028] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments, but this does not limit the scope of protection of this application.

[0029] like Figure 1 As shown, the airbag nursing mattress of the present invention includes two rows of airbags along the length of the mattress. Each row consists of multiple double-layered wedge-shaped airbags arranged closely together. The two rows of airbags are interlocked and coupled together, meaning the wedge-shaped surfaces of two double-layered wedge-shaped airbags in the same row are in contact. A double-layered wedge-shaped airbag refers to two wedge-shaped airbags stacked together, with their internal cavities communicating with each other. By inflating and deflating the two rows of airbags, the patient's turning over is controlled, preventing prolonged pressure on local tissues and thus preventing pressure sores. The interlocking arrangement of the double-layered wedge-shaped airbags makes the spatial layout of the airbag nursing mattress more rational, the mattress surface flatter, and the coupling area between the airbags larger, providing more effective support.

[0030] The present invention provides a dynamic simulation method for an array-type wedge-shaped airbag nursing mattress, comprising the following steps:

[0031] Step 1: Create a solid model in the finite element simulation software, including the smallest feature element, the weight, and the boundary model;

[0032] The minimum feature unit for simulation is extracted from the airbag nursing mattress configuration, and a minimum feature unit model is established based on the geometric parameters of the minimum feature unit. Due to the symmetry of the double-layer wedge-shaped airbag structure, the two double-layer wedge-shaped airbags in contact with their wedge surfaces are considered as a whole and divided into two equal parts along the plane of symmetry. Each part is a minimum feature unit. Therefore, the minimum feature unit is formed by coupling one half of one double-layer wedge-shaped airbag with the other half. The half of one double-layer wedge-shaped airbag and the half of the other double-layer wedge-shaped airbag are denoted as airbag unit 1-2 and airbag unit 2-6, respectively. (See [reference]). Figure 2 The weight is used to simulate the human body. When a person lies on the airbag nursing mattress, it is equivalent to applying a load to the airbag. Assuming that the weight applies a uniform load to the airbag, the weight model is simplified to a cuboid. The boundary refers to the contact boundary between the airbag and the other structures, including the contact surface between the airbag and the bed and the split surface of the airbag unit. All of them are surface contacts. Therefore, solid boundary 2 4 is set on the contact surface between the airbag and the bed, solid boundary 3 is set on the split surface of airbag unit 1 2, and solid boundary 3 5 is set on the split surface of airbag unit 2 6. All solid boundaries are cuboids.

[0033] Step 2: Define material properties and mesh the model;

[0034] The material properties of the weight and the boundary include density, Young's modulus and Poisson's ratio; the airbag is made of rubber-like hyperelastic material, and the material properties are obtained by the Yeoh model, namely the coefficients C10, C20 and C30 of the three polynomials in the Yeoh model; under small deformation, C10 represents the initial shear modulus; since the coefficient C20 of the second polynomial is negative, it can reflect the softening phenomenon of the material under medium deformation; since the coefficient C30 of the third polynomial is positive, it can describe the hardening phenomenon of the material under large deformation, see (Yanshan, Wang Wei. Determination of material parameters in rubber-like hyperelastic constitutive model [J]. Rubber Industry, 2014, 61(08):453-457.);

[0035] To determine the material properties of the airbag, tensile tests are required to obtain both load and deformation data. According to the national standard GB / T1040.3-2006, uniaxial tensile tests were conducted on the specimens in a quasi-static environment to obtain the deformation under different loads. The specimens had a length greater than or equal to 150 mm, a width of 10 mm to 25 mm, and a height less than 1 mm. The experimental data were then substituted into the hyperelastic material constitutive model (Yeoh model) in the finite element simulation software for curve fitting to obtain three material property parameters: C10, C20, and C30. To verify the accuracy of the airbag material property parameters, the C10, C20, and C30 material property parameters were input into the finite element simulation software, and tensile simulations were performed on the specimens to obtain the deformation under different loads. The error between the simulation data and the experimental data was no more than 5%.

[0036] The smallest feature element uses a quadrilateral shell element with a mesh size of 2–4 mm; the weight and boundary elements use an 8-node hexahedral solid element with a mesh size of 5–10 mm.

[0037] Step 3: Model assembly and setup contact;

[0038] Assemble the smallest feature unit, weight, and boundary model after mesh generation, and adjust the position of each model so that airbag unit 12 and airbag unit 26 are on the same horizontal plane and separated from each other to simulate the initial state before airbag inflation; the weight does not contact either airbag unit; the contact attribute is represented by friction, with the direction of friction along the tangent of the contact surface and the coefficient of friction being 0.3; there are two types of contact between models: one is general contact, which is applied to the entire model; the other is surface contact, which is used for contact between the two airbag units, between the weight and the airbag unit, and between the airbag unit and the boundary.

[0039] Step 4: Set boundary conditions;

[0040] Pressure boundary conditions are set for the airbag unit, i.e., gas pressure is applied inside the airbag unit to simulate the aerodynamic behavior of the airbag during inflation and deflation. The inner surface of the airbag unit is selected as the surface on which the gas pressure acts, and the magnitude and direction of the gas pressure are set to the normal direction of the surface on which the gas pressure acts. During the inflation and deflation of the airbag, the airbag is in an open state, and gas is considered to be able to flow in or out. Therefore, the inflation and deflation process of the airbag is reflected by changing the magnitude of the gas pressure. The inflation process of the airbag is characterized by a linear increase in gas pressure, and the deflation process is characterized by a linear decrease in gas pressure. When no pressure boundary conditions are set for the airbag unit, it is considered that the airbag is in a closed state, and gas is considered to be unable to flow in or out. That is, the gas pressure inside the airbag unit will not change actively until it is passively changed by external force. By randomly setting a cavity point on the segmented surface of the airbag unit and obtaining the pressure data of the cavity point during the simulation, the gas pressure change curve inside the airbag unit can be obtained.

[0041] The entire simulation process consists of three steps: step one simulates the airbag inflation process, step two simulates the process of a person lying down, and step three simulates the process of a person turning over. In step one, the positions of weight 1, airbag unit 1 2, solid boundary 1 3, and solid boundary 2 4 are fixed, meaning their displacement along the three coordinate axes and their rotation around the three coordinate axes are all zero. Airbag unit 2 6 and solid boundary 3 5 are moved the same distance along the negative Z-axis. The distance moved must ensure that the distance between the two airbag unit's bisecting surfaces is equal to the actual distance in the nursing mattress. Simultaneously, the gas pressure of the two airbag units is linearly increased until the internal pressure of the airbag unit reaches the set value (the internal pressure after the airbag is fully inflated). Step two continues the simulation based on step one. Okay, delete the pressure boundary conditions of the airbag unit and the position fixed constraint of the weight 1. At the same time, fix the positions of solid boundary 3, solid boundary 4 and solid boundary 5. Set the displacement of the boundary points on the two airbag unit split surfaces along the Z-axis to zero. Then these boundary points can only move in the XY plane, so that the airbag unit still has the performance of a complete double-layer wedge airbag when inflated and under pressure. The weight is released based on inertia and falls onto the airbag unit under the action of gravity, causing the airbag unit to deform until the airbag unit reaches a steady state. Simulation step three is carried out based on simulation step two. Set the gas pressure of one of the airbag units to increase or decrease linearly to a set value, that is, the internal pressure of the airbag after the human body turns over, to simulate the state change of the airbag nursing mattress during the human body turning over.

[0042] The height direction of the airbag is taken as the X-axis, the length direction as the Y-axis, and the width direction as the Z-axis; the boundary conditions are all applied to the mesh nodes on the two airbag unit split surfaces.

[0043] Step 5: Set the parameters and run the simulation to obtain simulation data;

[0044] The parameters include simulation time, simulation step size, and weight mass. The simulation time for step one is 2 seconds, and the simulation times for steps two and three are both 4 seconds or longer, until the airbag unit reaches a steady state. The simulation step size is 0.01 seconds, and the step size can be appropriately increased while ensuring convergence. The target internal pressure of the airbag unit refers to the internal pressure of the airbag unit after step one, with a value ranging from 106 kPa to 131 kPa. The weight mass ranges from 2.5 kg to 9 kg. Given a fixed weight volume, changing the density can be considered as changing the weight mass.

[0045] The simulation data includes the internal pressure of the airbag unit, the displacement of each grid node, stress, etc.

[0046] The effective contact area between the human body and the airbag is defined as the ratio of the load on the airbag to the internal pressure of the airbag, i.e.:

[0047]

[0048] Among them, A eff F represents the effective contact area between the human body and the airbag; F represents the load on the airbag, i.e., the load exerted by the weight on the airbag unit; P represents the internal pressure of the airbag.

[0049] Read the effective contact area between the human body and the airbag in step three of the simulation to obtain the effective contact area curve. The slope of the curve is the rate of change of the effective contact area. If the rate of change of the effective contact area is greater than or equal to the threshold, it is considered that the rate of change of the effective contact area is large, indicating that the contact area between the human body and the airbag changes a large abruptly, which increases the risk of pressure sores. Therefore, when controlling the airbag care mattress to turn the human body, the human body position should be avoided in positions with a large rate of change of the effective contact area. For example, if the position with a large rate of change of the effective contact area is when the human body is tilted to the right at 10°, the airbag care mattress should be controlled to avoid staying in a position tilted to the right at 10°. During the turning process, when passing through positions with a large rate of change of the effective contact area, the inflation and deflation speed should be slowed down.

[0050] After the turning movement is completed, multiple grid nodes are randomly selected from the contact area between the airbag unit and the weight, and the stress of these grid nodes is obtained. If the stress of all grid nodes exceeds the limit value, it indicates that the risk of pressure sores in the corresponding parts of the human body has increased, and the human body position should be adjusted.

[0051] Any aspects not covered in this invention are applicable to existing technologies.

Claims

1. A dynamic simulation method for an array-type wedge-shaped airbag nursing mattress, the mattress comprising two rows of airbags along the length of the mattress, each row consisting of multiple double-layered wedge-shaped airbags tightly arranged, with the wedge-shaped surfaces of two double-layered wedge-shaped airbags in the same row in contact; the double-layered wedge-shaped airbags are formed by stacking two wedge-shaped airbags together, with the inner cavities of the two wedge-shaped airbags communicating with each other; characterized in that, The method includes the following steps: Step 1: Extract the minimum feature unit for simulation from the mattress configuration. Treat the two double-layer wedge-shaped airbags with wedge-shaped surfaces as a whole and divide them into two parts along the plane of symmetry. Each part is a minimum feature unit. Therefore, the minimum feature unit is formed by coupling half of one double-layer wedge-shaped airbag with half of the other double-layer wedge-shaped airbag. Denote half of one double-layer wedge-shaped airbag and half of the other double-layer wedge-shaped airbag as airbag unit one and airbag unit two, respectively. Build a solid model in the finite element simulation software, including the minimum feature unit, weight, and boundaries. The weight is used to simulate the human body. Set solid boundary one on the bisecting surface of airbag unit one, solid boundary two on the contact surface between the airbag and the bed, and solid boundary three on the bisecting surface of airbag unit two. Step 2: Define material properties and mesh the material. The material property parameters for the weight and boundary include density, Young's modulus, and Poisson's ratio. The property parameters of the airbag material are obtained from the hyperelastic material constitutive model. The mesh of the smallest feature element uses shell elements, while the mesh of the weight and boundary uses solid elements. Step 3: Model assembly and contact setup; Contact is divided into general contact and surface contact. General contact is applied to the entire model, while surface contact is used between two airbag units, between the weight and an airbag unit, and between an airbag unit and the boundary. Step 4: Set the boundary conditions for each simulation step; Set the pressure boundary conditions for the airbag unit, that is, select the inner surface of the airbag unit as the surface on which the gas pressure acts, set the magnitude of the gas pressure, and set the direction as the normal direction of the surface on which the gas pressure acts. The entire simulation process consists of three steps: step one simulates the airbag inflation process, step two simulates the process of a person lying down, and step three simulates the process of a person turning over. In step one, the positions of the weight, airbag unit one, solid boundary one, and solid boundary two are fixed. Airbag unit two and solid boundary three are moved the same distance along the negative Z-axis so that the distance between the two airbag unit's bisecting surfaces equals the actual distance in the nursing mattress. Simultaneously, the gas pressure in both airbag units is increased until the internal pressure of the airbag unit reaches a set value. In simulation step two, the pressure boundary conditions of the airbag unit and the fixed position constraint of the weight are deleted. The positions of solid boundary one, solid boundary two, and solid boundary three are fixed, and the displacement of the boundary points on the two airbag unit split surfaces along the Z-axis is set to zero. The weight falls onto the airbag unit under the action of gravity and causes the airbag unit to deform until the airbag unit reaches a steady state. In simulation step three, the gas pressure of one of the airbag units is increased or decreased to a set value. The height direction of the airbag is taken as the X-axis, the length direction as the Y-axis, and the width direction as the Z-axis. Step 5: Set the parameters, run the simulation, and obtain the simulation data; The effective contact area between the human body and the airbag is defined as the ratio of the load on the airbag to the internal pressure of the airbag. The effective contact area between the human body and the airbag in step three of the simulation is obtained, and the effective contact area curve is obtained. The slope of the curve is the effective contact area change rate. If the effective contact area change rate is greater than or equal to the threshold, it indicates that the contact area between the human body and the airbag changes a lot, which increases the risk of pressure sores. Therefore, when controlling the airbag nursing mattress to turn the human body, the human body position should be avoided in the position with a large effective contact area change rate. After the turning movement is completed, randomly select multiple grid nodes from the contact area between the airbag unit and the weight. If the stress of these grid nodes exceeds the limit value, it indicates that the risk of pressure sores in the corresponding parts of the human body has increased, and the human body position should be adjusted.

2. The dynamic simulation method for the array-type wedge-shaped airbag nursing mattress according to claim 1, characterized in that, In step 2, a uniaxial tensile test was performed on the specimen in a quasi-static environment to obtain the deformation of the specimen under different loads; the deformation of the specimen under different loads was substituted into the constitutive model of the hyperelastic material for curve fitting to obtain the material property parameters; the constitutive model of the hyperelastic material adopted was the Yeoh model.

3. The dynamic simulation method for the array-type wedge-shaped airbag nursing mattress according to claim 1 or 2, characterized in that, In step 2, the shell element is a quadrilateral shell element with a mesh size of 2~4mm; the solid element is an 8-node hexahedral solid element with a mesh size of 5~10mm.

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