Page 1 of 7 Year 28 Nº 112 fourth quarter 2008 HADA Assisted Design and Analysis Tool ERGONOMICS This article describes the HADA Assisted Design and Analysis Tool, so called from its Spanish initials (Herramienta de Análisis y Diseño Asistido). Fruit of a 3- year research-and-development project by Grupo ID ERGO of the Engineering Research Institute (I3A) of Zaragoza University and Instituto de Ergonomía MAPFRE, S.A. It enables all human movements to be captured outside a laboratory environment. This system has been designed to help in the analysis of work-related musculoskeletal risks under real conditions and in the design of workstations. The technology is based on inertial motion sensors fitted inside a jacket worn by the worker under study. The system includes software for displaying the captured motion on a male or female biomechanical model, adjusting its anthropometry to the subject under observation. A biomechanical analysis can be conducted and ergonomic evaluation methods can be applied to ascertain whether musculoskeletal injuries might occur during work performance. The system makes it possible to recreate scenarios, both real ones and those proposed as alternatives to improve working conditions. General system objective The aim of the HADA system is the capture and 3D analysis of human motion in workstations. It is based on inertial motion sensors, with the information they pick up being passed onto 3D biomechanical models. It is a portable system comprising a set of motion sensors fitted in an instrumented jacket worn by the worker, with minimum interference to his or her work, combined with motion-capture and analysis software. The information picked up by the sensors in the field is used in conjunction with 3D animation software to reproduce the worker s motion on a biomechanical model. This information can then be used for a subsequent ergonomic evaluation of the musculoskeletal risks of the activity under study. Figure 1. Field components of the HADA System. There are currently various very advanced motion-capture systems [2]. Most of them, however are restricted to use under laboratory conditions, are costly and call for considerable user instruction. This article presents a portable, low-cost, high-performance, user-friendly system. Motion in real working conditions can be captured merely by issuing the worker to be observed with a sensor-fitted jacket and then using a PDA to pool the information picked up by the sensors and a video camera, which can be mounted on a tripod or be hand held by the technician during filming.
Page 2 of 7 In a more advanced version the camera could be fitted with calibrated lenses. This option would make it possible, by means of photogrammetry, to reconstruct the workstation in 3D and accurately ascertain its dimensions without having to make any measurements of the real workstation; this is another great advantage of this system. The motion information captured in the field is processed by software to display the resulting movement on a biomechanical model (male or female), whose anthropometrical characteristics can be automatically varied to simulate and analyse the risks for different population percentiles. A 3D motion study can also be made together with a biomechanical analysis; softwareincorporated ergonomic evaluation methods can also be applied. Figure 2. VICON System. Fitting the markers. Camera detail. In sum, the HADA System has been designed to capture motion in the actual workstation and is geared towards occupationalrisk-prevention officers who carry out field studies, helping them to make the corresponding ergonomic studies and assess the risk involved. The system has been successfully applied for designing and redesigning workstations. Motion capture in real scenarios The virtual models" that mimic the movements and gestures of human beings have been developed on the basis of a key tool: Motion Capture, hereinafter shortened to MoCap [1]. MoCap systems are being widely used by many companies in the fields of 3D modelling, virtual animation and cinema applications. They are also being used in the fields of sports medicine and medical rehabilitation. The HADA equipment applies MoCap systems to study and assess possible ergonomic risks on the basis of motion analysis and also to design and redesign workstations. There are currently various different MoCap systems and methodologies [2,3], but the most widely used ones are probably those based on optical methods using spherical reflective markers and infrared cameras capable of picking up marker reflection (fig. 2). These are very advanced systems that can even capture facial movements. Figure 3. Inertial sensors and how they might be fitted to the body. Systems of this type are generally restricted to use in laboratory conditions, with a high number of cameras (typically from 4 to 8) suitably arranged and with an appropriate calibration system; this makes them little apt for real working conditions. The capture times are usually high and the systems need highly skilled and trained operators, especially to eliminate any errors deriving from hidden markers during the filming process. The HADA system, based on inertial motion sensors, has largely overcome the shortfalls of vision-based systems, allowing information to be picked up in real field conditions rather than based on task simulation. This design, furthermore, is totally portable and easy to use. The subsequent work of processing and working up the field information is also simple and user friendly. Inertial motion sensor technology The sensors used in this equipment are fitted in an instrumented jacket worn by the subject under study. Each sensor is placed in pre-defined positions to allow recording of the spatial position of each joint in real time (fig 3). Figure 4. Detail of the hub that communicates by bluetooth with a PDA.
Page 3 of 7 The HADA system uses motion sensors built up from both inertial and magnetic sensors, working from three rotation angles with respect to an overall coordinates system, together with the linear acceleration of the three axes and their corresponding angular acceleration rates. The selected sensors meet the requirements of portability and basically supply three degrees of freedom, specifically the 3 rotations in space. This choice facilitates management of the stored data, since there is no need for a desktop-sized processing unit. The information generated by each sensor is sent by wireless bluetooth connection to a PDA that stores captured data (fig. 4). Description of the system hardware The HADA system includes the following elements: Inertial motion sensor kit. The minimum configuration is 5 sensors, but the recommended number is 7 sensors and the maximum is 15, the last case covering all segments of bodily movement (fig. 5). A hub or communication unit that is connected to the sensors by cable and sends on the data by bluetooth to the PDA (fig. 5). Instrumented jacket specifically designed to hold the sensors with certain additional fixing elements (fig. 6). PDA including software for starting and stopping the motion capturing operation, recording the sensor information in a file (fig. 7). Video camera, sold as a standard product in the mass consumption market, for filming the worker s activity, then to be synchronised with the motion capture information (fig. 7). Other necessary elements for field tasks, such as tripod or transporting bags and additional batteries for the video camera or communication unit. Workstation evaluation usually involves fitting sensors to the upper limbs using the instrumented jacket, but if the lower limbs also need to be analysed certain additional fixtures can be supplied for fitting sensors to the legs. Figure 5. Possible sensor arrangement on the body. Detail of the communication unit. A working set-up with one or two calibrated cameras can be chosen. Specific photogrammetry software can then be used to accurately reconstruct the workstation s detailed dimensions. The result is a set of elements that make up a complete motion capture system applicable in real workstations (fig. 8). Figure 6. Instrumented jacket and sensor fixing elements Description of the HADA system software The HADA system software includes a set of motion capturing functions that have been implemented in a general-purpose 3D animation programme called Poser4 (fig. 9) [4]. The functions for translating the sensor-captured information to a biomechanical model are the following: Motion importation and sensor acceleration. In this process different methods can be chosen for importing the sensor motion, in turn depending on how these sensors have been set up: If there are 7 or more sensors it will be possible to fit sensors on the upper and lower body. For example 4 Figure 7. PDA for recording field data. Camera and tripod (optional).
Page 4 of 7 sensors could be fitted to the legs, 1 on the pelvis and the rest on the trunk, head and/or arms. If the choice is made to fit sensors to the arms, the software will automatically calculate the leg position by inverse kinematics and by applying certain parametrisable motion rules. Synchronising motion with the video. Once the motion has been imported and translated to the virtual model, the motion can then be synchronised with the video of the background images. This task is highly automated and has hence been greatly simplified in relation to the earlier version. Figure 8. MH-Sensors System working in the field. Motion analysis Once the worker s motion has been reconstructed and suitably adjusted to the anthropometry of the virtual model, access can then be gained to the motion analysis module, thereby obtaining the kinematic properties of the subject s motion: body segment angles at each instant and also positions, speeds and acceleration rates, both of translation and rotation (fig. 10). For each body segment of the virtual model a display can be made of the variation of certain parameters throughout the different filming frames; this information can then be printed out (fig. 10). Within the desired range of images or frames, the variation in the following parameters can be observed: Joint angles in relation to biomechanical planes and the rotation angles of each segment. Acceleration rates, speeds and positions. Displacements of the body s centre of gravity and arms and legs plus the speeds and acceleration rates of these displacements. Figure 9. Field components of the MH- Sensors System. Analysis of acceleration rates If the anthropometry of the virtual model is adjusted to the dimensions of the observed worker then the measurable displacements will correspond to real values. The motion analysis module can then be used to estimate the acceleration rates of certain parts of the body, such as hands or head. A more accurate measurement of acceleration rates, however, can be obtained from the sensor information, since the sensors are fitted with accelerometers. The precision measurement of acceleration rates, together with inclinometer information, enables the sensor orientation to be calculated at each moment. For measuring acceleration rates a set of functions has been developed, implemented on a spreadsheet for displaying the variation in the different acceleration parameters synchronised with the motion of the virtual actor and the video filmed in the field (fig. 11). Ergonomic evaluation. REBA analysis, NIOSH and OCRA. Figure 10. Graphic display of the variation in angular parameters and report. The 3D recreation of the motion gives us all information pertaining to heights, reaches, position of the different body segments, etc. Feeding in some parameters such as applied force, weights handled or motion frequency, therefore, enables us to apply various evaluation methods.
Page 5 of 7 The NIOSH lifting equation [5,6] can now be applied for evaluating manual weight handling tasks contained in the standard UNE-EN 1005-2:2004 (fig. 12) as well as the REBA method [7,8] for analysing the musculoskeletal risks deriving from the working posture (fig. 13) or the OCRA method for evaluating repeated movements of the upper limbs as detailed in the standard ISO 11228-3:2007 [9]. When redesigning a workstation, with the object of improving its conditions, a new risk assessment can be quickly obtained. Improvements can therefore be proposed after the corresponding ergonomic evaluation. The reports corresponding to each chosen evaluation method are generated in MS Word format. Users can select the postures and data (results obtained, graphs, statistical summary, etc.) that they wish to record in each report. This simplifies the technicians tasks, since they only have to fill in the standard report with the conclusions and recommendations they deem fitting, while the rest of the study data are filled in automatically. Figure 11. Analysis of the acceleration rates supplied by the sensors. Photogrammtry The HADA System can be rounded out with photogrammetry software called PhotoModeler [10]. An account is given below of the system s possibility-expanding functions, especially in terms of redesigning workstations. Figure 12. Application of the NIOSH Method. Angular parameters considered. Scene Photogrammetry The photogrammetry software builds up a 3D reconstruction of the scene from a set of workstation photos. The virtual or real actor, as desired, can then be introduced into the scene as well as displaying the various angles of the scene (Fig. 15 and 16). Photogrammetry of the Worker Anthropometry The photogrammetry module can be used to measure certain anthropometric dimensions of the worker. To do so it suffices to take two photos of the worker, preferably in cross position (fig. 17). Figure 13. Application of the REBA Method. Resulting Statistics. A simple procedure can then be followed to measure accurately certain references such as those indicated in the figure (fig. 18). These references can then be used to establish the worker s height and the length of his arms. Simulation of improvement proposals The HADA system is especially useful for analysing improvement proposals. To do so the abovementioned set of functions can be used and activated after modifying the arrangement of elements making up the workstation. Figure 14. Imported scene in Poser4 with two types of rendering. Analysis of the resulting movement will establish the soundness of the various proposals. An analysis can therefore be made of the impact of the modifications of the working conditions before physically going ahead with them (fig. 19). In sum, the modules of motion analysis, ergonomic evaluation and photogrammetry can be combined to simulate and propose workstation improvements, accompanied by the corresponding motion analysis and the resulting new ergonomic evaluation. Figure 15. Display of the real and virtual actors superimposed, varying the rendering. Conclusions The system presented herein, HADA in its current version,
Page 6 of 7 enables motion to be captured by means of an instrumented jacket with inertial motion-capturing sensors, combined with a set of functions implemented on simulation and 3D animation software, thereby recreating the movements of the observed subject on a biomechanical model. Its portability and ease of use make it ideal for use in real settings, and it has been successfully put through its paces in industrial workstations and sporting activities. Optical system usually work fine under laboratory conditions but they are in general unsuitable for real settings, in particular industrial environments where the problem of markers being hidden by unavoidable obstacles often makes them unviable. Their portability is also low and the worker often has to wear special clothes calling for very precise fitting of markers. The HADA system, on the contrary, even enables field objects that the observer does not see to be seen and measured. The overriding concerns in the design of the system were portability and ease of use in the field and also simple processing and working up of the data afterwards in the office. The result is a really portable system that quickly obtains results for analysis and gathers its data much more accurately than a direct-vision or video system. The system s set of functions, as described above, greatly simplifies the work of occupational risk prevention officers, furnishing them with important information on subject postures and movements and dependable measurements. It enables recognised evaluation methods to be applied and lends itself to simple and rapid application in subjects with different anthropometric dimensions, while enabling different improvement proposals to be compared. It can be integrated with other applications, such as photogrammetry, to reconstruct scenes in three dimensions from workstation photos and thereby facilitate the design and redesign of workstations and evaluation of risks before they are put into practice. Figure 16. Display of the animation from different and selectable points of view. Figure 17. Photogrammetry Software. Template with worker photos. Figure 18. Anthropometric worker references. Figure 19. Simulation of the current situation and improvement proposals REFERENCES 1. http://en.wikipedia.org/wiki/motion_capture 2. www.peakperform.com; http://www.simi.com; http://en.wikipedia.org/wiki/motion_capture; http://www.vicon.com 3. INITION. http://inition.co.uk/inition/products.php 4. POSER. Curious Labs. http://graphics.smithmicro.com/ 5. NIOSH. Work practices guide for manual handling. Technical report nº 81122. US Department of Health and Human Services. National Institute for Occupational Health, Cincinnati, Ohio, 1981. 6. Waters, Putz-Anderson, Garg, «Aplication Manual for the Revised NIOSH Lifting Equation», U.S. Department of Health and Human Service - Centers for Disease Control and Prevention, 1994. 7. Hignett, S. and McAtamney, L. Rapid Entire Body Assessment: REBA Applied Ergonomics, 31, 201-5, 2000. 8. NTP 601 Evaluación de las condiciones de trabajo: carga postural. Método REBA (Rapid Entire Body Assessment). Instituto Nacional de Seguridad e Higiene en el Trabajo, España. 9. ISO 11228-3:2007. Ergonomics. Manual handling. Part 3: Handling of low loads at high frequency.
Page 7 of 7 10. PhotoModeler. http://www.photomodeler.com/