A New Generation Of Inflatable Protection Device Engineering Essay

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1. Introduction:

Airbags are advanced and well established safety devices that were first adopted in the 1980s to provide additional protection to the existing seatbelts for vehicle passengers. Existing airbag systems, despite being successful in automotive applications, cannot be universally employed to protect from injury that may occur during activities at home, in the workplace, or during sports, mainly because they are single-use, bulky, and expensive. These drawbacks are primarily due to the airbag inflator, which relies on an irreversible chemical reaction. The goal of the proposed Discovery application is to establish a program that facilitates the development a new generation of inflatable protection device for many injury prevention applications where the use of conventional airbags cannot be practically or commercially justified. The applicant proposes an initial research theme to investigate a new design for foam inflated airbags that exploits the characteristics of flexible foam, e.g. Polyurethane foam (PU), including high (70-90%) compressibility, memory (shape recovery), and durability (tolerates many loading cycles). In general, foam airbag systems comprise a bulk foam block (any desired shape), which is modified by cutting out substantial but structured patterns of foam pockets (air cells), and a flexible material covering (thin foam sheet). The foam airbag originates as low-volume compressed foam, which expands to a cushion of sufficient bulk upon deployment. When the foam regains its original shape the pockets will be filled up by air and form the airbag. The airbag owes its high impact attenuation to air compression energy and subsequent discharge (high pressure air expulsion) in the cells. The cells, in addition to acting as mini-airbags, enhance the compressibility of the foam and enable the deployment of larger airbags. The compressed foam's rebound speed is also faster than the required deployment time needed to mitigate most accidents that can cause injury. The idea for this novel airbag was introduced by Dr. Arzanpour and his team in 2012 and patented by the SFU Innovation Office [P2]. So far, the team has successfully demonstrated overall practicality and feasibility. An in-depth analysis of foam airbags that targets performance enhancement in terms of injury criteria (design optimization) is now needed and the results will be used to develop novel foam airbag designs for imbedding in injury protection devices. The outcomes of this research program will include new knowledge and HQP training in injury protection and safety device analysis and development, and a potential major breakthrough in the injury protection devices field. The research is based on an original idea and novel patentable solutions are expected to arise.

2. Recent Progress:

During the applicant's previous Discovery Grant, he developed his expertise in the modeling, parameter optimization, design and control of active and semi-active vibration isolators and shock absorbers. Work involved developing a Root Mean Square (RMS) based optimization technique for suspension systems [C25] and two engine mount prototypes (active and a semi-active) in collaboration with General Motors (GM) and Cooper Standard Co. (Tier 1 engine mount manufacturer) for GM's Displacement on Demand (DOD) engines. The semi-active version [J4,J12, C22, C27] utilizes a novel retrofitable Magnetorheological fluid (MRF) chamber inside a hydraulic engine mount while the active version achieves the isolation requirement by a custom designed solenoid valve actuator [J5,J10,J11,C24,C26]. The applicant has also utilized his expertise in sound and vibration and smart materials in developing novel biomedical devices. He invented a sound recognition technique to design dental hand-pieces to help dentists avoid cutting healthy parts of teeth during fillings [P3]; this is very common due to the lack of tactile sensing in current instruments. He also invented a percussion based dental implant stability measurement system (employs modal testing concepts) [P1] and a MRF force feedback platform for rehabilitation and teleoperations [J3, J9]. The applicant will build on this expertise gained through this previous research in modeling and optimization, and combine it with his expertise in mechanical and mechatronic systems to focus on injury prevention devices.

Preliminary work has been carried out developing and prototyping an inflatable hip protector (IHP) to protect the elderly against fall related injuries. The motivation for the IHP stems from the lack of user compliance (bulky) and insufficient protection provided by commercially available hip protectors. Collaborating with Dr. Robinovitch's Injury Prevention group at SFU, a mathematical model of the inflatable hip protector was first developed and experimentally verified [C1]. This model can be employed to predict the effect of several factors including the initial pressure, airbag size and impact intensity in the force attenuation performance of an IHP. Several prototypes of various sizes were fabricated and tested in the hip protector simulator setup (see Form 100). The experimental results show the IHPs can improve the force attenuation three-fold. While the new design addresses the performance requirements, some other practical issues, including being single use and bulky (inflated by CO2), need to be addressed. These issues were the inspiration for the investigation into making airbags from foam. So far, the feasibility of the idea has been demonstrated in small scale samples made by cutting a matrix of identical squares in foam blocks and covering the cavity by two thin foam sheets. Experimental results demonstrate significantly more impact attenuation (~5times) compared to the same size solid foam block. The results would be even more impressive if the comparison were made to unpadded surfaces; however, those tests were avoided due to potential damages to sensors.

3. Objectives: Long and Short Term:

The long-term objective of this research program is the development of a new generation of inflatable injury protection devices for applications where conventional airbags cannot be practically or commercially justified. Initially, the program will focus on developing a foam airbag system that can be customized (design, configuration, and shape) and installed on current protective devices (helmet, life jackets, hip protectors) for more comprehensive protection, or in other equipment that currently has no protection, such as mobility assistive devices (wheelchairs and scooters falls) and beds (bed fall cushions). Although the performance, specifications and requirements of foam airbags are not identical to those used in passenger vehicles, they are multiple-use, light, inexpensive, and deploy fast enough for most cases and as such offer a viable solution for many other injury protection applications. Over the next five years, the research program will have two short-term objectives:

Objective A: Characterize foam airbag systems and establish an effective design, analysis and optimization tool based on various injury criteria.

(A1) Undertake a parameter study of air pockets (cells) as the basic component of the foam airbag and derive expressions that correlate the design variables with the performance of the foam airbag system;

(A2) Develop a numerical method that correlates the foam airbag performance to common injury criteria; and

(A3) Develop an optimization tool to generate design parameters based on injury protection requirements.

Objective B: Develop foam airbag prototypes that employ novel design approaches to address both the safety and reliability requirements of injury protection devices.

(B1) Design foam airbag samples that can regenerate the required performances and address reliability, deployment time and sufficient coverage area;

(B2) Fabricate a working prototype of an inflatable foam airbag hip protector; and

(B3) an inflatable foam airbag helmet as a demonstration of the approach.

4. Literature Review:

Airbags, first adopted in the 1980s, have now become the gold standard in automobiles as a safety device to reduce the morbidity and mortality associated with car collisions and accidents. Statistical analysis and many surveys providing estimations on the number of lives saved by their application in conjunction with seatbelts leave no doubt about their effectiveness [1]. However, airbags can also cause more injury than they prevent if the passenger is a child or diminutive adult [2], improperly seated, or is not wearing a seatbelt [3]. Such injuries include head and eye damage [4,5], chemical side effects [6], facial abrasions and lacerations [7], burns [8], and even death [9]. Some of these injuries are related to the high rate of expansion that can increase the relative head-airbag velocity if it is not fully deployed by the time of head impact. Reducing this relative speed is a practical approach that has been achieved by seatbelts. In fact, some studies suggest seatbelts are up to three times more effective than airbags [10]. To solve this problem, a few novel airbag designs [11] have been proposed; however, it is also recommended that a dynamic adjustment of internal pressure (high energy air discharge) during and after the deployment (smart airbags) [12] can result in significantly improved impact attenuation. Although the idea of smart airbags has been around for a while, no product has been released yet. In aerospace, however, the idea of active pressure regulation has already been considered for spacecraft landing and human impact attenuation systems. NASA explored the idea of dynamic airbag pressure control by incorporating a "bag within a bag" approach [13], which is basically an anti-bottoming bag, placed inside an external vented airbag triggered by employing pressure transducers. ExpoMars is a mission to Mars landing by the European Space Agency that is planning to adopt a controlled airbag venting idea for landing [14]. Airbag utilization has also been explored for other applications including seat ejection of fighter jets [15], helicopter landing systems [16], and wearable impact protection devices [17,18]. Dainese D-Air and Mugen Denko Hit-Air are two human airbag motorcycle racing suits to protect the neck and shoulders during motorcycle accidents [19]. These airbags are heavy and expensive [20] (~1 Kg, and ~$1400) and so far have had no significant traction in the market.

Polymeric foams are low-density solids that can dissipate high impact energy and are widely used in many protective devices such as helmets and hip protectors as mechanical energy absorbers. Polymeric foams absorb and dissipate energy by several mechanisms, including elastic and/or plastic bending, buckling or fracture. In particular, the air inside the cells has a dominant role in impact attenuation. Air is compressed in the initial stage and expelled at high foam deformation [21,22]. For large impacts, however, the time it takes for the air entrapped in the cell to leave the foam is considerably longer than the time of the compression (impact). Hence, the open cell foam reacts more like a closed cell foam (conventional airbag). Although foam has exceptional properties such as high compression rate and the ability to inherently dissipate energy through air compression and discharge, no one has yet used it as an inflatable injury protection device. This research program intends to enhance the compressibility, energy dissipation capability (effective compression/discharge) and deployment time of foam, and design airbags that can be applied to many injury prevention applications.

5. Methodology:

Obj. A) Characterization and parameter study of the foam airbag system and model development (MASc 1, PhD 1, Co-ops 1-2)

Increasing the impact attenuation efficiency (per airbag unit volume), improving compression/expansion ratio (to deploy larger airbags) and achieving fast airbag deployment are the main challenges to meet. Foam airbags owe its current enhanced high compression/expansion ratio, improved impact attenuation and relatively high response time to the hollow vertical columnar hexagonal pockets (cells) cut inside PU foam (honeycomb structure). This cell geometry is known for its low density, relative high out-of-plane compression and high strength-to-weight ratio [23]. As the basic structural unit of a foam airbag, gaining knowledge about the effect of hexagonal cell parameters, i.e. geometry, foam material characteristic, discharge pressure, on the impact attenuation is an important step for characterizing foam airbags. Therefore, the proposed Objective A1 is to establish an expression that correlates the design variables with the performance of the foam airbag system. Both mathematical and empirical approaches will be considered. A mathematical model to capture cell behavior can be expressed with two or more sub-functions. For example, the air compression and discharge can be modeled using a similar approach employed for vented airsprings [24], and for foam densification the best match can be found from the models that use high strain rate loading [25-27] and nonlinear viscoelastic models [28-30] to express the impact behavior of foam. Empirical relationships, as the alternative, can also be explored due to the low cost and simplicity of sample fabrication and flexibility in implementing parameter variations. The results of these analyses can be expanded to the whole foam airbag structure and potentially used to determine if parameter optimization can be correlated with injury criteria. Another approach for design optimization is employing professional human injury analysis software packages that can directly correlate foam airbag parameters with human injury criteria.

The proposed Objective A2 is the development of a numerical model of the foam airbag to facilitate the analysis of human injury analysis. Some widely used injury criteria includes head injury criterion (HIC), neck injury criterion (NIC), pelvic injury criterion, dynamic response index (DRI, for spine injury), maximum chest deflection, combined thorax index (CTI), and 3MS (highest acceleration with a duration of at least 3 ms), etc. Depending on the type of accident and the body parts involved, one or more injury criteria should be considered in each analysis. There are several software packages, e.g. SIMM and Life MOD (both used mainly for musculoskeletal analysis), for human injury analysis. MADYMO is a powerful simulation package with a variety of dummy models (male, female, and children) constructed of ellipsoids, which can replicate the contact surfaces during impact more accurately. The software is validated experimentally and has been used extensively for automotive crash simulations and other injury analysis applications such as manual and powered wheelchairs [31,32]. Another powerful feature of MADYMO is the Gasflow module which uses a coupled computational fluid dynamics (CFD) and finite element (FE) model. The Gasflow module is mainly developed to simulate air in conventional airbags. MADYMO's FE model will be used to simulate the foam airbag structure and the air inside the cavities will be modeled using CFD. The effects of foam porosity will be either modeled similar to the airbag fabric pores or with single or multiple vents (also used in MADYMO). The software can automatically calculate the air discharge from compressed cells from the average pressure inside the cells and the airbag structural movement using an efficient approximate geometric calculation. To run the simulations, the property of the foam will be obtained from our dynamic tests (using Instron material testing machine) and will be manually inserted into the model. MADYMO models will first be experimentally verified (using head form drop tests) and then employed in injury analysis and optimal foam airbag design parameters. The proposed Objective A3 is to develop an optimization tool based on injury protection requirements. MADYMO also has a toolbox called Optimizer which utilizes a mid-range multipoint approach for crashworthiness optimization. Unlike other crashworthiness optimization techniques, such as response surface approximate optimization [33], orthogonal arrays [34], and heuristic methods [35], the Optimizer deals with noisy responses caused by numerical inaccuracies of estimating nonlinear cost functions (injury criteria). Moreover, this optimization package can automatically change the input file, modify the design variables, and analyze the results. The Optimizer performs the optimization process using the results of several simulations; each uses a new combination of foam airbag design variables. These design variables or parameters will be incorporated as the input files to the optimizer and the results used as the starting point of a new cycle of optimization to reach the final optimum design.

Obj. B) Development of foam airbag prototypes (MASCs 2 and 3, PhD 2, Co-ops 3-5, Capstone 1-2)

In Objective B1 novel design solutions will be investigated to address practical issues of foam airbags as a viable injury protection solution. The goal is to ensure the performance of foam airbags will reliably remain almost the same independent of external conditions (impact direction, intensity, etc.). Replacing the sources of uncertainty, e.g. fixing the top and bottom covers to the structure (closing the cells), is one approach to be considered. A more deterministic discharge pressure can then be achieved with several options including (i) venting, (ii) foam based one-way valves, (iii) pressure distribution to adjacent cells through internal foam channels. A number of series and parallel inter-connected channels can also be implemented in the bottom cover (using a mold) to enhance air delivery to the cells for rapid deployment. Several network constructions can be investigated including fractal (seen in veins in leaves, human lungs and arterial vessels [36]) and linear arrangements. It is important to know that under low forces, foam deformation is local (only affects contacted surface). As a result, one strategy will be to place inhaling channels inside the bottom cover (to receive the impact first) and the pressure distribution channels inside the structure. In Objectives B2 and B3, a working prototype of an inflatable foam airbag will be developed for two applications, i.e. hip protectors and helmets. These two applications are primarily selected because of the existing knowledge, expertise and infrastructure at SFU. Dr. Robinovitch is an expert in injury prevention and hip protection is one of his active research subjects. Also, developing a more effective motorcycle helmet is an active area of research for Drs. Wang and Golnaraghi. These collaborations will ensure faster progress and lower risk of failure. In the beginning, foam airbag samples that comply with the optimal design parameters obtained from our MADYMO simulations will be fabricated and tested. An NSERC RTI grant has also been submitted for a Hybrid III 50th Percentile dummy which will be the most accurate test device for injury analysis. Another option will be using existing hip impact simulator and head-form test setups in the preliminary stages and paying outside laboratories for final comprehensive verifications. The final prototype will contain the ideal foam airbag design, packaged inside a casing that will have a fixed (sits on hip) and a detachable cover (fabricated with our 3D printer or CNC). The prototypes will also have a miniature electronic locking mechanism and a detection unit. It should be emphasized that accident detection (fall and impact) is not in the scope of this research and a suitable methodology to trigger the airbag's deployment for the two applications will be selected from the literature and implemented in the final design.

6. Impact:

This research program aims to develop the core knowledge and expertise needed to develop foam airbag systems as a new generation of inflatable impact attenuation devices and also investigate potential applications to provide a more comprehensive protection for injuries that happen during our daily activities. The new airbag system will have many advantages: it will be inexpensive, light, and multiple-use which reduces the aftermarket costs and permits the safety device to be ready for re-use shortly after deployment. It will not have any of the potentially harmful effects of conventional airbags and can be simply cut and formed to desired shapes. Thus, it will provide a high degree of flexibility to design customized safety solutions for a wide variety of applications. It could be even produced and marketed as a raw material which gives safety equipment manufacturers, privately owned small businesses, research labs, etc. the flexibility to develop customized safety equipment.

This airbag is intended to reduce/prevent injuries in different groups of people who are most vulnerable and require more attention, including children, parents, wheelchair users, and those who are involved in high risk activities such as construction, transportation and sports. These injuries reduce the quality of life, often leading to chronic pain, dependence on others for daily activities, disability, and even death. The medical and recovery expenses of these accidents impose significant economic and social burdens to the patient and the healthcare system. For example, according to the BC's Workers Compensation Board (WCB), falls and being struck by/against objects (all protectable) are responsible for 48% of injuries (~18,000 cases), about 1M days lost, and $120M spending for salary compensation[37]. Fall-related injuries in elderly people is another example of a common accident [38,39] that can pose serious health problems such as bone fracture, subdural hematoma, soft tissue injury, head injury and even death [40]. In Canada alone, nearly 1.4 million falls by seniors were reported in 2005, and it is predicted that the number will increase by 2.35-fold in 2036, [41] which will need nearly $4.4 billion dollars to cover the cost of injuries [42].

This research program is building the foundation to develop a new generation of human injury protection devices. HQP will be trained in the various areas of the design and development of the technology. We will find wider use in other application areas by participating and presenting our technology and results to audiences in the injury prevention community, and mobility assistive device and sport safety equipment industry, and by reinforcing existing collaborations with researchers, care givers and industrial partners. In this way, we hope to advance the state-of-the-art foam airbag technology further to advance the human safety devices to improve the quality of life and bring peace of mind for families.