Human Engineering Considerations in Product Design PDF

ME515 Integrated Product design and Prototyping Module 4 Human Engineering Considerations in Product Design INTRODUCTION Ergonomics or human factors engineering is the science of fitting tasks to man. The word “Ergonomics” is derived from the Greek words ergon and nomos, meaning “work” and “natur...

ME515 Integrated Product design and Prototyping Module 4 Human Engineering Considerations in Product Design INTRODUCTION Ergonomics or human factors engineering is the science of fitting tasks to man. The word “Ergonomics” is derived from the Greek words ergon and nomos, meaning “work” and “natural laws”, respectively. Ergonomics and human factors engineering are synonymous and cover a very wide field, including the human operator and his working environment. The areas of study covered under human factors engineering are one group comprising Anatomy, Physiology and Psychology, and the other group consisting of engineering sciences such as Physics, Mathematics, Materials Science and Design. Human factors engineering brings together two groups of specialists: those who know about machines and processes, and those who know about human capabilities. Thus, human factors engineering is the link between engineering sciences and social sciences. The role of human factors in product and equipment design assumes importance in three respects: 1. Man, as occupant of space, i.e. to operate a machine, the human operator should have adequate space, as dictated by human body dimensions or anthropometry. 2. Man, as reader of display from the machine. That is, based on the display data, man processes the data and takes action. 3. Man, as one who takes action through operating controls which form a part of the machine. Thus, man acts as applicator of force and controls the machine. It will be obvious that human engineering in design should consider application of forces and study of displays and controls. These aspects are covered in the subsequent sections. HUMAN BEING AS APPLICATOR OF FORCES Refer to Figs. 1 and 2. Energy is converted into useful work by the human motor system. The body can be regarded as a set of rigid members: the part of the limbs between joints, the main parts of the trunk and head. All these parts are quite heavy in relation to the available forces. Thus, available forces may be wasted in moving the parts of the body. Two consequences follow from this. First, action will not be efficient if the direction of force is such that the limb or any other part of the body is moving against the force of gravity. Second, the most effective way of generating force  Fig 1 Fig2 (i) Momentum. One other way in which body masses can be utilized is by the provision of forces from momentum. The force exerted is proportional to the mass of the moving part, as also to the relative velocity of the limb and the object on which the force is to be exerted. Machine operators are trained to use momentum to operate handwheels, etc. To achieve precision in limb movement, a worker uses two sets of muscles about each joint—those involved in causing the required movement (the agonists), and a complementary group opposing the movement (the antagonists). This is the best way to achieve high precision. Assuming that these two sets of muscles are required, the greatest precision will be achieved when the limbs are in the middle part of their total path of movement. (ii) Muscle contraction and energy release. Economy and spread of effort occur within a muscle. All muscles consist of a large number of fibres and bundles of fibres that can be regarded as motor units. The greater the force, or the faster the movement, the greater will be the number of motor units involved. A slight change in the movement of a particular joint may result in a drastic change in the order, number and identity of motor units involved. (iii) Isometric application of forces and isotonic action.  In mechanics, the principle that no work is done unless a force moves its point of application is used. The human operator is certainly capable of work, either by isometric (static) muscle action or by isotonic action, in which there is limb movement. Isometric activity probably is superior in reaction sensitiveness as it is called kinesthetic feedback. In kinesthetic feedback, the operator receives more information feedback from the feel of what he is doing than from observing the results of the activity. Isotonic action, in which muscle fibres are moving in relation to each other, facilitates the blood flow and thereby the supply of oxygen and the removal of waste products. Thus, for maximum precision, isometric action is often best, but for maximum power output and for postponing fatigue, isotonic action is superior. ANTHROPOMETRY: MAN AS OCCUPANT OF SPACE Refer to Figs. 3 and 4. The starting point of the design of work spaces must be the dimensions of the people who are going to operate within given spaces. Thus, one of the primary responsibilities of ergonomics is to provide data about body size. Such a study, which is part of the domain of the anatomist, is called anthropometry. Fig 3  Fig 4 The variations between individuals are usually large enough to be important and so the statement of averages is not enough. Ranges are required, but since human dimensions are distributed normally in a statistical manner, there are very few individuals at either extreme. The question, therefore, arises as to what proportion of actual ranges of size should be taken into account in design. For example, most adults are between 1.50 m and 1.85 m tall, but some men exist whose height is as much as 2.05 m. It would clearly be absured to make all doorways at least 2.50 m high, to cater to a few individuals in the total world population.  Anthropometric data are often expressed in the form of 5th, 50th and 95th percentiles. A dimension quoted at 5th-percentile level means that 5% of the population considered are estimated to be smaller than this. Correspondingly, only 5% of the population is larger than the 95th percentile. The range from 5th to 95th percentile covers 90% of the population. Even when anthropometric data has been gathered, there are problems of data presentation and application. The compilers of the data usually have background in Biology, but the users have a practical industrial or engineering background. It is of little use to the work-place designer to know that the poplietal height is 40 cm, unless this information is presented in such a way that he can appreciate its relevance to the height of seats. This problem of specific terminology can be overcome by the proper use of diagram or manikins. A manikin is a scale model of a human operator, usually in two dimensions only, with swivel points, corresponding to the sites, at which joints permit movement. These can be made for different percentile levels, so that the question of ranges is properly dealt with. Table 1 indicates anthropometric data Table 1 Two techniques for utilizing anthropometry have been developed. The first is to design and present data for a special problem. For example, a chart is available that indicates the area of acceptable centre-line heights for lathes as a function of certain design variables. The task of translating the raw data provided by anthropometrists into a form directly usable by lathe designers has been carried out by an ergonomist who took into account such factors as variations in size of likely user populations and the need to manipulate controls and materials. In this case, the range of application of the data has been reduced, in order to make the data more readily communicable. The second and a more general technique is to use ‘fitting trials’. This is effectively the use of live subjects instead of manikins. In its simplest form, a full-scale model of the work space is built and various people from the user population try it, and their opinion is taken. Figure 5 illustrates the relationship between human body dimensions and a chair and a table with it Fig 5 THE DESIGN OF CONTROLS Refer to Figs. 6–8 for the study of various types of controls. A control is a device that enables an operator to change the state of a mechanism. It converts the output of an operator into the input of a machine. Since controls are pieces of hardware, they are often regarded as parts of a machine, out for design purposes, they can be more effectively considered as connecting links between the machine and the operator. The following is a list of some controls: 1. Handwheel 2. Crank 3. Thumbwheel 4. Knob 5. Slider 6. Toggle switch 7. Joystick 8. Roller ball 9. Lever 10. Footpedal 11. Treadle 12. Detent switch 13. Handle  Fig 6 An important problem involves deciding as to which limb is to be used for which purpose. Broadly speaking, foot controls are best for the provision of powerful continuous forces and hand controls are best for speed and precision Fig 7  Fig 8 The variety of hand controls is commensurate with the versatility of the hands. The steering wheel is the hand equivalent of the rudder-bar, and has the corresponding advantage of achieving precision by balancing forces between the two hands. It makes possible a wide variety of positions and types of hand-grip and operations by either hand or by both hands, besides providing a large torque. As in the case of the bicycle, had it not been invented as a result of long experience, it would by now have been designed by the ergonomist. The handlebar is a specialized form of steering wheel with a reduced range of possible hand-grips. Cranks are the hand equivalent of rotating pedals, and since they are used to generate power, rather than information, it is usually pertinent to ask whether a pedal system might not be superior. The more precise hand controls such as selectors, toggles and buttons pose problems of identification and grouping, rather than of individual design. Population stereotypes and interaction between man and machine. The simples aspect of this relationship is in the direction of motion of controls. Particular populations have certain habitual expectations about how a control or a display or a control/display relationship should behave. These are called population stereotypes. The main control stereotypes are that an increase (e.g. in volume, speed, voltage, and height) should be effected by a clockwise rotation, a movement downwards for a pedal and upwards or to the right or away from the body for a hand level. These seem to be common to most populations  Fig 9 THE DESIGN OF DISPLAYS Refer to Figs. 10 and 11. A display is a part of the environment of the operator that provides him with information relevant to the task he is performing. Common examples of display are: (a) display on a TV screen, (b) dial type display with pointer, and (c) simple mechanical display, e.g an advertisement card on a visiting card. Various types of display (i) Pictorial display. This consists of some level of direct representation of the real situation, for example, a spot moving across a map representing position, or a tiny model aircraft and an associated line, with the model and line moving in synchronization with the real aircraft and the horizon. (ii) Qualitative display. This indicates a general situation, rather than a numerical description of it. This is often quite adequate for the needs of the operator. For example, a light showing whe the oil pressure is too low is satisfactory for most car drivers, rather than a gauge that indicates the pressure in units. (iii) Quantitative display. This presents a number denoting the value of some variable in the situation. There are two main types—the moving-pointer fixed scale display and the digital display. The fixed-pointer moving scale display—a hybrid of the two—is not commonly used, because of the confusing nature of the relationship between the scale direction, the scale movement and the pointer. Design guidelines of markers, distance between markers, graphics and visual communication are important areas of product design, recommended by Woodson and Conover. The height of the letter, h (mm) and the width of the letter, w (mm) can be expressed as a function of viewing distance s in metres. Wmrn = 0.15 mm Fig 10 Table 1 This last point is one aspect of the more general problem of population stereotypes, already mentioned in relation to controls. For displays, the expected patterns in Europe and USA are that scales should increase from down to up, from left to right and clockwise. There are only three important channels through which the operator receives information—the eves, the ears, and the sense of force and motion, or, to use more scientific terminology, the visual, the auditory and the kinesthetic sensory channels. The task of the ergonomist is to allocate the required information between these channels, according to their relative advantages and disadvantages and to ensure that, for each channel, the signals are above the threshold value MAN/MACHINE INFORMATION EXCHANGE Refer to Figs. 11 and 12 The man/machine interface is an imaginary plane across which information is exchanged between the operator and machine. Information is conveyed from the machine to the man by the display elements of the interface, and from the man to machine by the control elements of the interface. The separate problems of displays and control have already been discussed, but there are also the more general aspects of man/machine information exchange. Fig 11 This is inherently a difficult design problem that cannot be left to chance, to common sense or to tradition. The machine is fast, accurate, powerful, and inflexible; the man is slow, subject to error, relatively weak, and yet highly versatile. The nature of these properties explains why the man/ machine combination is so useful but only if these two fundamentally different units can be efficiently linked together. The higher the speed of the equipment, the more crucial it is to conform to the basic principles of interface design. Population stereotypes have already been mentioned for both displays and controls, but there are also stereotype aspects of control/display relationship. Optimal values of control/display of ratio of 2 have been recommended by ergonomists. Broadly speaking, these follow from the relationships established in natural situations. Movement should follow the line of force: if a control moves upwards, left or away from the body, then the corresponding display should move respectively upwards, left or away from the body. The relationship between rotary movements of controls and linear movements usually follow the logic of the simplest mechanical connection. For example, right-hand threads are such that a clockwise rotation causes an away movement, and this is the expected relationship even when there is no mechanical connection between the control and the display. Stereotypes for rotary controls adjacent to linear display follow simple ratchet or pulley/ belt type movements. Relationships between display movements at right angle to control movements are usually ambiguous because there are a variety of equally valid ways in which a mechanical connection could be envisaged. These should be avoided. Unexpected display/control relationships may result in longer response times, greater errors, longer training times and high fatigue rates. Even when a new relationship has been learnt for a new situation, there may be a regression to earlier habits when an emergency arises. For example, if the brake and accelerator pedals of a car were interchanged, it would be perfectly possible, with extra care, to drive such a car. However, if the car had to be stopped suddenly, there will be a tendency to press the accelerator pedal which would, of course, have the contrary effect. There have been many serious accidents resulting from situations similar to this. Given that almost all the power transmission occurs within the hardware, interface problems are almost entirely informational. The measurement of work or effort in functioning at an interface is, therefore, extremely difficult. Physiological measures of load are not appropriate, and even when used indirectly as measures of strain (e.g. muscle tension of sinus arrhythmia), it is not possible to make comparisons between systems. Psychological measures of information processing, and level of skill (e.g. required channel capacity or precision of timing) are obviously more relevant, but at present they are unreliable and tedious. Even if it is possible to quantify the amount of information crossing the interface, this does not incorporate the critical encoding and dynamics variables. Informational measures of difficulty have been proposed and used for simple laboratory situations, but they have not been extended to cover even simple practical interfaces. Obviously, there is considerable scope for interface design research Fig 12 WORKPLACE LAYOUT FROM ERGONOMIC CONSIDERATIONS A general purpose machine should be designed for the average person. However, if one considers an operator who may be of either sex, seated or standing at a workplace, there are very few persons with average dimensions for all limbs. Therefore, a workplace should be so proportioned that it suits a chosen group of people. Adjustment may be provided (on seat height for example) to help the situation. If one considers the workplace (driving control area) in a modern motor car which will be required to suit one or two people from a large population, the benefits of adjustment will be appreciated. Ideally, the seat should be adjustable for height, rake and position, the steering wheel for height and the control levers and pedals for length. Detailed anthropometric information is available for the different sexes, but simpler information could be obtained once the group of persons liable to use workplace is known. Figure 13 shows the maximum working area and the normal working area. These are defined as: 1. Normal working area: The space within which a seated or standing worker can reach and use tools, materials and equipment when his elbows fall naturally by the side of the body. 2. Maximum working area: The space over which a seated or standing worker has to make full-length arm movements (i.e. from the shoulder) in order to reach and use tools, materials and equipment. Fig 12 Introduction to Reverse Engineering Reverse engineering (RE) is now considered one of the technologies that provide business benefits in shortening the product development cycle. Engineering is the process of designing, manufacturing, assembling, and maintaining products and systems. There are two types of engineering, forward engineering and reverse engineering. Forward engineering is the traditional process of moving from high-level abstractions and logical designs to the physical implementation of a system. In some situations, there may be a physical part/product without any technical details, such as drawings, bills-of-material, or without engineering data. The process of duplicating an existing part, subassembly, or product, without drawings, documentation, or a computer model is known as reverse engineering. Reverse engineering is also defined as the process of obtaining a geometric CAD model from 3-D points acquired by scanning/digitizing existing parts/products. The process of digitally capturing the physical entities of a component, referred to as reverse engineering (RE), is often defined by researchers with respect to their specific task. RE described as, “the basic concept of producing a part based on an original or physical model without the use of an engineering drawing”. Yau et al.(1993) define RE, as the “process of retrieving new geometry from a manufactured part by digitizing and modifying an existing CAD model”. Reverse engineering is now widely used in numerous applications, such as manufacturing, industrial design, and jewellery design and reproduction For example, when a new car is launched on the market, competing manufacturers may buy one and disassemble it to learn how it was built and how it works. In software engineering, good source code is often a variation of other good source code. In some situations, such as automotive styling, designers give shape to their ideas by using clay, plaster, wood, or foam rubber, but a CAD model is needed to manufacture the part. As products become more organic in shape, designing in CAD becomes more challenging and there is no guarantee that the CAD representation will replicate the sculpted model exactly. Reverse engineering provides a solution to this problem because the physical model is the source of information for the CAD model. This is also referred to as the physical-to-digital process. Another reason for reverse engineering is to compress product development cycle times. In the intensely competitive global market, manufacturers are constantly seeking new ways to shorten lead times to market a new product. Rapid product development (RPD) refers to recently developed technologies and techniques that assist manufacturers and designers in meeting the demands of shortened product development time. For example, injection-molding companies need to shorten tool and die development time drastically. By using reverse engineering, a three-dimensional physical product or clay mock-up can be quickly captured in the digital form, remodeled, and exported for rapid prototyping/tooling or rapid manufacturing using multi-axis CNC machining techniques. Why Use Reverse Engineering? Following are some of the reasons for using reverse engineering: The original manufacturer no longer exists, but a customer needs the product, e.g., aircraft spares required typically after an aircraft has been in service for several years. The original manufacturer of a product no longer produces the product, e.g., the original product has become obsolete. The original product design documentation has been lost or never existed. Creating data to refurbish or manufacture a part for which there are no CAD data, or for which the data have become obsolete or lost. Inspection and/or Quality Control–Comparing a fabricated part to its CAD description or to a standard item.  Some bad features of a product need to be eliminated e.g., excessive wear might indicate where a product should be improved. Strengthening the good features of a product based on long-term usage. Analyzing the good and bad features of competitors’ products. Exploring new avenues to improve product performance and features. Creating 3-D data from a model or sculpture for animation in games and movies. Creating 3-D data from an individual, model or sculpture to create, scale, or reproduce artwork. Architectural and construction documentation and measurement. Fitting clothing or footwear to individuals and determining the anthropometry of a population. Reverse Engineering–The Generic Process The generic process of reverse engineering is a three-phase process as depicted in Figure 13. The three phases are scanning, point processing, and application-specific geometric model development. Reverse engineering strategy must consider the following: Fig 13 Phase 1–Scanning This phase is involved with the scanning strategy–selecting the correct canning technique, preparing the part to be scanned, and performing the actual scanning to capture information that describes all geometric features of the part such as steps, slots, pockets, and holes. Three-dimensional scanners are employed to scan the part geometry, producing clouds of points, which define the surface geometry. These scanning devices are available as dedicated tools or as add-ons to the existing computer numerically controlled (CNC) machine tools. There are two distinct types of scanners, contact and noncontact. Contact Scanners These devices employ contact probes that automatically follow the contours of a physical surface (Figure 14). In Ithe current marketplace, contact probe scanning devices are based on CMM technologies, with a tolerance range of +0.01 to 0.02 mm. However, depending on the size of the part scanned, contact methods can be slow because each point is generated sequentially at the tip of the probe. Tactile device probes must deflect to register a point; hence, a degree of contact pressure is maintained during the scanning process. This contact pressure limits the use of contact devices because soft, tactile materials such as rubber cannot be easily or accurately scanned. Fig 14 Noncontact Scanners A variety of noncontact scanning technologies available on the market capture data with no physical part contact. Noncontact devices use lasers, optics, and charge-coupled device (CCD) sensors to capture point data Although these devices capture large amounts of data in a relatively short space of time, there are a number of issues related to this scanning technology.  The typical tolerance of noncontact scanning is within ±0.025 to 0.2 mm. Some noncontact systems have problems generating data describing surfaces, which are parallel to the axis of the laser (Figure 15). Noncontact devices employ light within the data capture process. This creates problems when the light impinges on shiny surfaces, and hence some surfaces must be prepared with a temporary coating of fine powder before scanning. Fig 15 These issues restrict the use of remote sensing devices to areas in engineering, where the accuracy of the information generated is secondary to the speed of data capture. However, as research and laser development in optical technology continue, the accuracy of the commercially available noncontact scanning device is beginning to improve. The output of the scanning phase is point cloud data sets in the most convenient format. Typically, the RE software provides a variety of output formats such as raw (X, Y, Z values separated by space or commas). Phase 2–Point Processing This phase involves importing the point cloud data, reducing the noise in the data collected, and reducing the number of points. These tasks are performed using a range of predefined filters. It is extremely important that the users have very good understanding of the filter algorithms so that they know which filter is the most appropriate for each task. This phase also allows us to merge multiple scan data sets. Sometimes, it is necessary to take multiple scans of the part to ensure that all required features have been scanned. This involves rotating the part; hence each scan datum becomes very crucial. Multiple scan planning has direct impact on the point processing phase. Good datum planning for multiple scanning will reduce the effort required in the point processing phase and also avoid introduction of errors from merging multiple scan data. A wide range of commercial software is available for point processing. The output of the point processing phase is a clean , merged, point cloud data set in the most convenient format. This phase also supports most of the proprietary formats mentioned above in the scanning phase. Phase 3–Application Geometric Model Development In the same way that developments in rapid prototyping and tooling technologies are helping to shorten dramatically the time taken to generate physical representations from CAD models, current RE technologies are helping to reduce the time to create electronic CAD models from existing physical representations. The need to generate CAD information from physical components will arise frequently throughout any product introduction process. The generation of CAD models from point data is probably the most complex activity within RE because potent surface fitting algorithms are required to generate surfaces that accurately represent the three-dimensional information described within the point cloud data sets. Most CAD systems are not designed to display and process large amounts of point data; as a result new RE modules or discrete software packages are generally needed for point processing. Generating surface data from point cloud data sets is still a very subjective process, although feature-based algorithms are beginning to emerge that will enable engineers to interact with the point cloud data to produce complete solid models for current CAD environments. The applications of RE for generating CAD data are equally as important as the technology which supports it. A manager’s decision to employ RE technologies should be based on specific business needs. This phase depends very much on the real purpose for reverse engineering. For example, if we scanned a broken injection molding tool to produce a new tool, we would be interested in the geometric model and also in the ISO G code data that can be used to produce a replacement tool in the shortest possible time using a multi-axis CNC machine. One can also use reverse engineering to analyze “as designed” to “as manufactured”. This involves importing the as designed CAD model and superimposing the scanned point cloud data set of the manufactured part. The RE software allows the user to compare the two data sets (as designed to as manufactured). This process is also used for inspecting manufactured parts. Reverse engineering can also be used to scan existing hip joints and to design new artificial hips joint around patient- specific pelvic data. This creates the opportunity for customized artificial joints for each patient. The output of this phase is geometric model in one of the proprietary formats such as IGES, VDA, STL, DXF, OBJ, VRML, ISO G Code, etc. Advantages of reverse engineering RE typically starts with measuring an existing object, so that a solid model can be deduced in order to make use of the advantages of CAD/CAM/CAE technologies.  CAD models are used for manufacturing or rapid prototyping applications. Hence we can work on a product without having prior knowledge of the technology involved.  Cost saving for developing new products.  Lesser maintenance costs  Quality improvement  Competitive advantages Applications Some of the applications of reverse engineering are Manufacturing Field: To create a 3D virtual model of an existing physical part for use in 3D CAD, CAM, CAE or other software and to analyse the working of a product. Medical Field: Imaging, modelling and replication (as a physical model) of a patient's bone structure Software engineering: To detect and neutralize viruses and malware