Rapid preliminary helmet shell design based on three-dimensional anthropometric head data

Transcription

1 Journal of Engineering Design Vol. 19, No. 1, February 2008, Rapid preliminary helmet shell design based on three-dimensional anthropometric head data HONG LIU, ZHIZHONG LI* and LI ZHENG Tsinghua University, China The ergonomic design of helmets is very important for those who wear them for long periods on the job; for example, construction workers and security personnel. A helmet s weight, stability, and ability to protect are especially important. According to our case study, fitting design according to head shape can significantly reduce the weight and enhance the stability of a helmet. The traditional helmet design process takes a long time, and is thus unsuitable for individually customized shape design. In this paper, a rapid preliminary design method for the helmet shell and a corresponding toolkit are introduced, taking advantage of three-dimensional (3D) anthropometric head scans. A 3D head model is first generated from the 3D head scan of the intended user or representative user of an intended population group. Then a semi-parametric surface modelling tool is applied to quickly generate the helmet shell by simply inputting several parameters related to helmet protection, size, and shape requirements and adjusting several key curves. In a case study, the new design by the proposed method and the existing design by the traditional method were compared with regard to weight, centroid, and moments of inertia to demonstrate the effectiveness of the proposed method. Keywords: Helmet design; 3D anthropometry; Computer-aided design 1. Introduction Helmets are personal protective equipment widely used in construction, manufacturing, sports, security, riding/driving, and the military. In some occupations, such as construction work and bank security, helmets are worn for long periods of time, causing complaints of discomfort and pain. Hall and Campbell (1992) described the helmet of the future, showing the complexity of the design task and the importance of integration for lightweight design and comfort. Fitting design is essential to this integration. By fitting design, only the necessary amount of material is used, uneven compression on the body surface can be avoided, spaces or gaps for protection or ventilation purposes can be rationalized, and other comfort requirements can be carefully considered. Adjustability design, population grouping, and customization (individualization) are major strategies of fitting design. Although the hanging system of a helmet is usually designed to be adjustable, the helmet shell, which is the hard component that protects the head and normally accounts for most of the helmet s weight, is unfortunately difficult to make adjustable. Since population grouping based on shape is still a technical challenge, population grouping based on key dimensions is a realistic strategy for the fitting design of a helmet. In the production *Corresponding author. zzli@tsinghua.edu.cn Journal of Engineering Design ISSN print/issn online 2008 Taylor & Francis DOI: /

2 46 H. Liu et al. of some helmets, the forming process does not require high temperature and produces limited force on the mould, thus a cheap resin or silica gel mould (rather than an expensive metal mould) can be used to produce a small number of the helmets. In such a case, customization is feasible since the cost is limited while the wearing comfort can be significantly improved. Typically, the traditional development process for the helmet shell follows these steps. First, the basic dimensions of the helmet shell are determined according to anthropometric data tables. Then the shell is modelled by an industrial designer with plaster based on the basic dimensions. After that, an engineering drawing of the mould to produce the shell by a forming process is finished by measuring key section curves of the plaster model. Finally, the mould is machined according to the drawing and then the shell can be produced with the mould (Li et al. 1999). There are two major disadvantages associated with the traditional process: (1) the design does not always fit the human head because the traditional anthropometric information available to most designers is misleading and can lead to poor helmet sizing (Robinette and Whitestone 1994); and (2) all of these steps are expensive and time consuming, and it is difficult to modify a design. It is obvious that the traditional method cannot meet the requirements of individual helmet design. Computer-aided design (CAD) has revolutionized the method of helmet design. Several researchers have shown the potential of CAD in helmet design (Cadogan et al. 1993, Armstrong et al. 2001, Palmer et al. 2001). However, thus far, in most of the work the mannequins and helmets were treated separately. Computer-generated models of three-dimensional (3D) head data could only be used to visually analyse whether the helmet being designed fit the head. When the head model is changed, this involves a tedious and time-consuming task of helmet modification. The main objective of this study was to develop a rapid preliminary helmet shell design method to remedy this problem. A 3D anthropometric head scan is taken as the design reference so that satisfactory shape fitting can be obtained. A semi-parametric surface modelling method based on a 3D head model is proposed for the rapid generation of helmet shells by simply inputting several parameters related to helmet protection, size, and shape requirements and adjusting several key curves. A corresponding CAD tool was developed and has already been used by a helmet company. A case study is presented to demonstrate the effectiveness of the proposed method. 2. 3D head modelling Traditionally, anthropometric data have been collected as one-dimensional values with anthropometers, calipers, a tape, and so on. The fluctuation of posture, identification of landmarks and instrument position, and orientation result in measurement error. The accuracy and precision of anthropometric measurements are dependent on the measurers who take the measurements (Meunier and Shi 2000). The difficulty in controlling all potential sources of error is such that it has been said that true values are seldom measured in anthropometry (Jamison and Zegura 1974). Additionally, lack of shape information and spatial relationship information make it difficult to design a fitted helmet based on one-dimensional anthropometric data. Robinette and Whitestone s (1992) technical report describes approaches for characterizing the human to provide shape and contour information in the design process. With the development of 3D anthropometric measurement technology, it has become possible to obtain 3D information about the surface of the human body. After the influential project CAESAR (Robinette 2000), more and more mass 3D anthropometric measurement projects have been conducted. 3D head scanning has opened a new realm in helmet design (Meunier et al. 2000). Meunier et al. (2000) presented a method to determine the probable population accommodation of a helmet sizing system with 3D laser scanning.

3 Rapid preliminary helmet shell design 47 Based on 3D anthropometric data, it is possible to create more accurate 3D models of the human body surface with CAD or reverse engineering software. Actually, various modelling methods can be adopted to generate a 3D human body surface model from a raw scan. For example, Zhang and Molenbroek (2004) used a bi-cubic B-spline to fit 3D scanned heads. Application programming interfaces of high-end CAD systems can also be very helpful in the processing of raw data and modelling of human body surfaces. The data used in our case study (described in a later section) were collected by a helmet company. 3D head scans of more than 2000 subjects aged from 18 to 36 were collected with a Philips CT-scanner at a low dosage (10 15% of the normal dosage for a medical examination). The slice-to-slice distance was set at 5.0 mm. The original data format was Digital Imaging and Communications in Medicine ( The eight-connected border tracking algorithm (Sonka et al. 1993) was adopted to identify the pixels of the head surfaces in binary computed tomography images. Coordinates of head surfaces were then calculated. The helmet company selected a representative head scan (see figure 1) for our study. There are several strategies to select a representative head. Based on statistical analysis (such as principle component analysis) of the anthropometric data, key parameters that account for the most variation in the data-set can be identified and then used in the design of a sizing system. For helmet design, head circumference was selected as the one and only key dimension in earlier infantry helmet sizing (Claus et al. 1974). This sizing method is still valid for the design of soft head coverings. However, for the design of rigid helmets, generally two key parameters are selected for the design of the sizing system, such as head length and width (Robinette and Whitestone 1992). According to the sizing system, the population is segmented into several groups. In each group, an individual with key parameter values closest to the average (or boundary) values can be chosen as a representative person of his population group. Sometimes, a representative person can even be an artificial human model with average values in all dimensions. The representative head for helmet design is normally determined by the helmet company according to their own sizing system and design strategy. With the modelling application of Unigraphics, a widely used high-end CAD system, the non-uniform rational B-spline surface of the representative head was generated, as shown in figure 2. The later case study using our rapid preliminary helmet design tool is based on this head model. However, it should be pointed out that the rapid design tool does not depend on this head model. In fact, any individual head model can be used as the design reference and thus custom-fit helmet design can be achieved. Figure 1. A representative head scan.

4 48 H. Liu et al. Figure 2. The non-uniform rational B-spline surface of the head scan. 3. Rapid preliminary design of helmet shell based on a head model The main components of a safety helmet are the shell and the hanging system. Basically, the function of the hanging system is to help to ensure proper fit and comfort, while the function of the shell is to protect the wearer from the energy of falling or approaching objects, such as bricks, stones, sticks, bullets, flying fragments, and any other foreign objects. The hard shell is generally made of fibreglass, Kevlar, or other light-weight but high-strength materials. The latest engineering materials and manufacturing technology have been able to provide satisfactory protection, and thus wearing comfort has become the major concern of helmet shell design. The hanging system only provides adjustable and comfort interface with the head surface, but the wearing comfort still heavily depends on the weight, centre of gravity, moment of inertia, and other properties of the shell. In order to improve the wearing comfort, the shape of the helmet shell must conform to the major shape (ignoring micro shapes such as wrinkles, local knobs, or concavity) of the wearer s head so that less weight, a lower centre of gravity, and a smaller moment of inertia can be expected. The helmet company thus requires an effective design tool to ensure the conformation, to control the shell s shape and size, and to evaluate the comfort-critical properties during the design process. Considering the necessity of customized design, the design tool should be a quick tool. 3.1 Main shape of a safety helmet shell The surface of the safety helmet shell to be considered in this study is eudipleural along the mid-sagittal plane. It can be divided into four main sections (figure 3): the main body, two identical ear covers, and the visor. The main body protects most of the surface of the skull and it is expected to at least have a minimal distance from the skull for protection purposes. As discussed above, it is better to let the main body follow the basic shape of the head surface. Technically, we can simply generate the main body from the head surface by an equidistant offset operation using the parameter of minimal protective distance. The ear covers protect the ear areas and they should have space to accommodate earphones. The visor stands out along the front edge of the main body to shield the wearer from sunshine and rain. It is also an important aesthetic feature of a helmet. There are obvious boundaries between each section. In addition to these four sections, we added a bottom surface section for the generation of the irregular bottom edge of the helmet shell. By identifying the key curves that represent the major shape of the above portions, modelling of the shell can be simplified. It is highly

5 Rapid preliminary helmet shell design 49 Figure 3. Main portions of the safety helmet. desirable that the computer-aided preliminary helmet shell design tool be simple, easy to use, and very efficient. Based on the discussion of the main sections of the helmet shell, a semi-parametric model is proposed. With this model, a user can easily obtain a preliminary helmet shell model by simply inputting key parameters and editing some key curves. It is called a semi-parametric model because the user has to do the curve editing. 3.2 The semi-parametric model of the helmet shell In this section, the method of modelling each section will be discussed and the corresponding key curves and controlling parameters will be defined. Figure 4 demonstrates the key curves and parameters in our semi-parametric model of the helmet shell. The main body portion can be obtained by an equidistant offset of the head surface model, notated as Offset M. The offset parameter D represents the protection distance (which should be greater than the minimal requirement) and determines the space between the helmet and the head. It provides a buffer zone against the after-effects of impact and room for effective air circulation (Meunier et al. 2000). Figure 4. Key curves and controlling parameters of the helmet shell.

6 50 H. Liu et al. The extruded surface modelling method is used to generate the bottom surface. The extrusion modelling method allows the designer to create a surface by extruding a curve to a linear distance along a specified direction. The key curve of the bottom surface can be defined as a section curve to be extruded, notated as SecCrv B in figure 4. Three controlling parameters of the key curve are defined in our design tool: the distance (H1) between the lowest point of SecCrv B and the top point of the head, which determines the height of the shell; the distance (H2) between the front segment (nearly linear) of SecCrv B and the top point of the head, which determines range of vision; and the inclination angle (A) of the back part (nearly linear) of SecCrv B to the horizontal surface, which determines how far the head can tilt upwards. The whole SecCrv B curve decides the protection area of the head. The sweeping method is used to model the visor. It is a special version of the skinning method. The sweeping method allows the creation of a single surface by extruding an open or closed curve along a guide (a path) formed by one or a set of curves. Two key curves are selected for the visor section. One is the section curve notated as SecCrv V, and the other is a guide curve notated as CtrlCrv V. The guide curve can be acquired by projecting a straight line to the main body. The distance (H3) between CtrlCrv V and the top of the head is an adjustable parameter that indicates the vertical position of the visor. Actually, the horizontal position of the visor also depends on H3, because the position of the section curve SecCrv V is determined by the intersection of the main body surface and the plane on which the guide curve CtrlCrv V is located. The method for modelling each ear cover section is similar to that for the visor portion. The key curves are one section curve notated as SecCrv E and one guide curve notated as CtrlCrv E in figure 4. The guide curve can be acquired by projecting a straight line to the main body. The distance (H4) between CtrlCrv E and the top of the head indicates the vertical position of an ear cover. Since the ear covers extend to the bottom edge of the helmet shell, H1 H4 determines the size of the ear covers in the vertical direction. For the convenience of sweeping, CtrlCrv E is divided into two segments by SecCrv E. The lengths of the two segments (L1 and L2) together control the size of the ear covers in the horizontal direction. When designing a helmet, parameters H1, H4, L1, and L2 should be valued with consideration of position and size of the ear and ear equipment. H1 should also be valued based on the required protection area. H2 should be valued according to the position of the eye and the required range of vision. H3 is mainly valued from an aesthetic viewpoint. 3.3 Obtaining the initial key curves When using the semi-parametric model of the helmet shell, parameter inputting is very simple, but curve editing is more complex. Because of this, the initial key curves given for further editing should be carefully determined, so that less editing work is required for an individual design. In our case study, the initial section curves were extracted from the model of a representative existing safety helmet. The section curve of the bottom surface (SecCrv B) was obtained by projecting the bottom edge to the mid-sagittal plane. The section curve of the visor (SecCrv V) was the intersection of the visor and the mid-sagittal plane. The section curve of the ear covers (SecCrv E) was the intersection of the ear cover and a coronal plane through the widest point of the helmet. For the convenience of later surface modelling, these section curves should be extended to a certain additional length (see figure 5). CtrlCrv V and CtrlCrv E are two lines whose vertical positions are determined by parameters H3 and H4. The beginning and ending points of the two lines were initially determined from the existing model. These key curves constitute a minimal curve set representing the fundamental shape of

7 Rapid preliminary helmet shell design 51 Figure 5. Extending the initial section curves. the helmet shell surface. The shape of each section can be modified conveniently by changing the shape of its key curve(s). 3.4 The semi-parametric modelling tool To facilitate the application of the above semi-parametric model, a computer-aided modelling tool, Helmet Design, was developed to work under a popular 3D CAD system, Unigraphics. The new tool is compatible with Unigraphics 17.0 or higher versions, and is fully integrated with the Unigraphics system; the user can use it like other modelling functions of the Unigraphics system. The tool can be launched from a menu item Helmet Design under the Unigraphics Application menu. The interface is user-friendly and allows users to easily input parameters, generate the helmet shell, and analyse the properties of the shell. Figure 6 shows an example of the Helmet Design tool interface. The main dialogue box provides an easy method for user input. For example, when L1 is clicked on (activated), the tool performs the following two functions: it displays a dialogue box for the user to input the Figure 6. Interface of the helmet design tool.

8 52 H. Liu et al. value of L1, and it highlights SecCrv E and CtrlCrv E for user modification. Essentially, the user interface of the dialogue box is designed following the user interface style of Unigraphics to ensure faster learning for Unigraphics users. The whole program has been written in such a way that the user only has to input the minimum and essential data to design a new helmet shell. After the eight parameters have been specified and all key curves have been modified or accepted, the user can click the Generate button to create the new helmet shell. The tool performs a series of actions: it generates the main body surface, the visor surface, the ear cover surfaces, and the bottom surface according to the key curves; it trims off the excess areas of the surfaces using the intersection curves between each pair of neighbouring sections; it joins the main body surface, visor surface, and ear cover surfaces together, resulting in a single helmet shell surface; and it thickens the helmet shell surface to create a solid body. When the Analyze button is activated, the Helmet Analyze dialogue appears, and the results of a quantitative evaluation of the helmet shell are displayed. A total of eight properties of the helmet shell are included in the analysis: length, width, height, surface area, volume of the solid body, mass, centroid position, and moments of inertia. With this information, the designer can determine whether the current design conforms to the design specifications. 4. A case study In the case study, an existing safety helmet was redesigned with the computer-aided helmet design tool referring to a representative head model, to demonstrate how the proposed method improves the helmet s wearing comfort, especially with regard to weight reduction. In addition to its protective function, weight is a critical criterion for a helmet to be well accepted by its wearers. The original helmet shell outer surface was measured by a coordinate measuring machine, with an accuracy of mm. Then the surface was reconstructed carefully with the freeform surface modelling tools of Unigraphics. By thickening the surface with the shell thickness, the solid model of the original shell was obtained. Its relationship with the 3D head model was established according to normal wearing conditions. With reference to the same 3D head model, the helmet shell was redesigned using the developed tool. The width of the redesigned shell was kept close to the original design. The bottom surface of the original shell was reused in designing the new shell, and thus the protection area was kept identical. Moreover, the shape of the visor and ear cover surfaces of the new shell were basically borrowed from the original shell by directly accepting key curves extracted from the original shell. The parameter D (safety distance) was set at 23 mm, which was also the design specification for the original shell. The safety distance is defined as the shortest distance between the inside surface of the solid shell model and the head surface. Under the above design conditions, it took just a couple of minutes to develop the new helmet shell as shown in figure 7. It would take about a month to design a helmet shell using the traditional method. There is no doubt that the proposed method and developed design tool can greatly improve the efficiency of safety helmet design. For in-depth comparison, geometric and physical properties related to the wearing comfort of the original and redesigned shell were evaluated. Table 1 presents the results. It can be seen that the redesigned helmet shell is obviously smaller in height than the original shell. In the traditional method, without the reference of a 3D head model, it is hard for the designer to accurately control the protection distance, and thus the designer is inclined to design a conservative space between the shell and the head. This may explain why the original shell

9 Rapid preliminary helmet shell design 53 Figure 7. Solid model of the redesigned helmet shell. Table 1. Properties of the original and redesigned safety helmet shell. Helmet shell Length (mm) Width (mm) Height (mm) Mass (g) Original Redesigned has such an excessive height. As shown in table 1, the redesigned helmet weighs 12.8% less than the original. Figure 8 shows that, in addition to the height difference, there is extra space in the upper-front and upper-back areas of the original shell compared with the redesigned shell. The extra space further results in a heavier helmet and requires a greater amount of material. Table 2 shows a comparison of the centroid position relative to the 3D head model and the moments of inertia regarding the centroid of the helmet shell. The centroid of the redesigned helmet shell is 10.9 mm lower than the original one (z-axis direction), and the moments of Figure 8. Comparison between the redesigned and the original helmet shell outer surface. Table 2. Comparison of centroid position and moments of inertia. Centroid position (mm) Moments of inertia (g mm 2 ) Helmet shell Hx Hy Hz Ixx Iyy Izz Original Redesigned

10 54 H. Liu et al. inertia of the helmet shell are also obviously decreased. These results imply that the stability is also improved. 5. Conclusions This study has demonstrated that a computer-aided tool based on the proposed semi-parametric helmet shell model can rapidly generate a preliminary design of a safety helmet shell in reference to 3D anthropometric measurement of a human head. The tool is still limited to the preliminary design of helmet shells. The designer may need to modify some details about the shape of the shell to reach a final design. In spite of this, the proposed method obviously speeds up and improves the development of customized safety helmets. The Analyze function provides a quick evaluation to support the designer s decisions. The integrated helmet head model makes it easier to evaluate a design with ergonomics considerations. As an example, we redesigned a safety helmet, showing significant improvement of the preliminary design efficiency and fitting comfort. With this experience, we believe that there are many benefits to widely apply and extend the proposed method in industry. The proposed method strongly supports the customized design of safety helmets, offering improved wearing comfort and thus better protection for the wearers. Acknowledgement This study was supported by the National Natural Science Foundation of China (No ). References Armstrong, M.G., Palmer, R., Wakes, S.J. and Tiu, W., The design by computational methods of a safety helmet for racing car drivers: application of integrated software tools, in 2001 ASME Pressure Vessels and Piping Conference, 2001, pp Cadogan, D.P., George, A.E. and Winkler, E.R., Aircrew helmet design and manufacturing enhancements through the use of advanced technologies, in Proceedings of SPIE, 1993, pp Claus, W.D., McManus, L.R. and Durand, P.E., Development of headforms for sizing infantry helmets. Technical report AD , Mass Clothing Equipment And Materials Engineering Lab, US Army Natick Labs, Hall, P.S. and Campbell, B.L., Helmet-mounted systems technology planning for the future, in Proceedings of SPIE, 1992, pp Jamison, P. and Zegura, S., A univariate and multivariate examination of measurement error in anthropometry. Am. J. Phy. Anthropol., 1974, 40(2), Li Shaodong, Zhang Jianchun and Ling Zhiguang, Development and test of a Kevlar bulletproof helmet. J. Tianjin Inst. Textile Sci. Technol., 1999, 18(4), (in Chinese). Meunier, P. and Yin, S., Performance of a 2D image-based anthropometric measurement and clothing sizing system. Appl. Ergonom., 2000, 31(5), Meunier, P., Tack, D., Ricci, A., Bossi, L. and Angel, H., Helmet accommodation analysis using 3D laser scanning. Appl. Ergonom., 2000, 31(4), Palmer, R., Tiu, W. and Armstrong, M.G., Design by computational methods of a safety helmet for racing car drivers: geometry design and peripheral vision analysis, in 2001 ASME Pressure Vessels and Piping Conference, 2001, pp Robinette, K.M., CAESAR measures up. Ergonom. Design., 2000, 8(3), Robinette, K.M. and Whitestone, J.J., Methods for characterizing the human head for the design of helmets. Technical report ADA263875, Crew Systems Directorate, Human Engineering Division, Armstrong Laboratory, Robinette, K.M. and Whitestone, J.J., The need for improved anthropometric methods for the development of helmet systems. Aviation Space Environ. Med., 1994, 65(4), Sonka, M., Hlavac, V. and Boyle, R., editors, Image Processing, Analysis, and Machine Vision, 1993 (Chapman & Hall Computing: London). Zhang, B. and Molenbroek, J.F.M., Representation of a human head with bi-cubic B-splines technique based on the laser scanning technique in 3D surface anthropometry. Appl. Ergonom., 2004, 35(5),