Development of CAD models from sketches: a case study for automotive applications
R Vignesh,R Suganthan, and K Prakasan*
Department of Production Engineering, PSG College of Technology ,Peelamedu,Coimbatore,India
The manuscript was received on 6 April 2006 and was accepted after revision for publication on 2 August 2006.
DOI:10.1243/09544070JAUTO331
Abstract: Today products are designed not only for their functional requirements but also for aesthetics. In the automotive industries, styling has become a major part of the design process with class-A surfaces Class-A surfaces are freeform surfaces with a continuous curvature。The process of engineering any component or system begins by generating a concept that actually describes the product in terms of its form, function, and fit. Concept sketches help the designers to arrive quickly and easily at a stage where a satisfactory design can be specified for detailed design. These concept sketches can be used for development of the digital concept design and analysis of the curves and surfaces. Software such as Alias exists and thus can be used with writable hardware for sketching the concept of the car body on a computer screen. In this paper a systematic procedure is discussed for generating class-A surfaces from the images of concept sketches which are manually prepared on paper. These images are imported into the sketch tracer module of CATIA V5.A designer can use the image as the reference and produce a digital sketch by tracing the image using CATIA V5 software without adding any special hardware. Later，interrogation of these surfaces for improved aesthetics can be attempted。This method will be useful for the users of CATIA V5 to improve their design practices and skills.
Keywords: concept sketches，clay modeling，class-A surface, reflection lines,
1 INTRODUCTION
Product styling is carried out to create visual attractiveness in products. Styling is widely accepted as an important way to add value to a product without changing its technical performance.
As new product quality rankings converge，styling is emerging as a key differentiator for consumers。The role of styling is to create fresh and exciting design concepts that are not just contemporary but trend setting as well. Styling enhances the visual appeal of the vehicles and at the same time develops innovative designs and components for customers. Today, not only are products designed considering the functionality but also special considerations are given to their aesthetics which can produce a desire in a person’s mind to own that product。This is the reason for the evolution of class-A surfaces and their importance。Class-A surfaces are those aesthetic freeform surfaces that are visible to us and that have an optimal aesthetic shape and high surface quality .
Restyling of existing products ids frequently performed in the automotive industries, since engineers usually prefer the evolution of product to a complete redesign. Customer feedback, client assistance suggestions, and market directives usually influence the decision on redesign or restyling of a product.
A traditional car development process involves the development of many handmade rendering sketches in order to offer a vision of the model’s look. From these sketches, a few are selected and，by pasting the four views of a constrained drawing on the wall. Which are made on a 1:10 scale-down clay model is developed. One scaled-down clay model is finalized and the model is scanned using three-dimensional scanners. Then a 1:1clay model of the finalized sketch is developed. In this phase all the changes require much time. A comparison between concept development and styling by the conventional method and the approach based on computer aided design (CAD) is provided in Fig. 1.
The objective of this work is to present a case in which the images of manually prepared conceptual sketches are used in CATIA V5softwareto trace the curves for developing CAD models and surface interrogation. An attempt is also made to compare a surface that is not checked for curvature continuity for their aesthetics and appearance. The approach used here can reveal the discontinuity in the surface which is difficult to notice otherwise although both are created in the same software The procedure adopted here can be practiced by the users of CATIA V5 to improve their innovations. A sketch on paper by an artist is made use of in this procedure. This appears to be a novel approach. From the experience of the present authors it was found that awareness of the capability of CATIA V5 for surface interrogation is limited among users.
2 LITERATURE REVIEW
To determine the industry practices and surface interrogation techniques for styling, a literature review was carried out and relevant observations are presented here.
Tovey [1] discussed sketching and its role in the concept design of automotives. automotive designers use sketches to support the styling activity through its two phases of concept design and design development before handing over to the downstream development processes. The importance of computer aided styling’ over CAD is highlighted.
Tovey [2] described the work on sketch mapping as a usable tool and contrasted it with the more conventional direct modeling approach which is seen to have limitations. He also gave a brief description of work in progress on deriving forms directly from sketches. The concept design methodology was also discussed.
Sunner et al.[3] discussed the use of reflection lines and specular highlights for the quality control of car body surfaces. A prototype with a highly reflective surface was built and viewed in a cubing room, which has many parallel arranged lights. The reflections of the lights are a good measure for surface quality, i.e. for detecting surface curvature discontinuities and for discussing the character of the shape.
Hagen et al.[4]presented a detailed survey of surface interrogation methods. Orthotomics are used for convexity tests. Focal surfaces, a new tool for analyzing freeform curves and surface, are special line congruences that are used to analyse the quality of the surface before sending the data to CNC machines.
Hahmann [5] gave a general method for surface interrogation which includes reflection lines, isophotes, curvature plots, highlight lines, and isolines. These tools are mainly used in computer graphics and found application in analyzing freeform surfaces in CAD.
Theisel [6] showed that isophotes and reflection lines are different tools for surface interrogation Isophotes were used to identify curvature discontinuities between surfaces, and reflection lines were used to measure the quality of the surface.
Monacelli[7] discussed the virtual reality (VR) applications for reducing the time and cost involved in the vehicle development process. VR can help to change the initial concept sketching from a manual method to a digital environment. VR has applications in styling and the designer can visualize the digital model in a better way.
Barone [8]explained the need for renovating the traditional styling process to a CAD-based styling process and addressed the difficulties faced during the traditional styling process.
3 STYLING OF A CAR BODY
A study was carried out on the styling of automotive exteriors because the first impression that a customer has of a car is the design of the car body. To be competitive in the market, not only is it necessary to have a good design but also it is crucial that the stylists guidelines will be accurately implemented when building the car.
The disadvantages with the traditional car development process are the time taken to make changes and the inability of the clay model to create variants of the new product。As the concept model can be seen only at the final stage of the design process, making changes to the concept model in the final stages is difficult.
The methodology adopted and the step-by-step procedures of the CAD-based development process for the car body using CATIA V5 are explained in Fig.2.
3.1 Importing concept sketches
The input for concept development is sketches. Concept sketches help the designers to arrive quickly and easily at a stage where a satisfactory design can be specified for development of detailed design. The sketches are imported into the sketch tracer module of CATIA V5. They are aligned such that the front view of the car will lie on the front plane. Similarly all the views of the car are aligned to lie on the corresponding plane. The sketches will form a bounding box and the model will form a bounding box. Figure 3 show the aligned concept sketches imported into CATIA V5 using the images (scanned) of manually prepared concept sketches.
3.2 Tracing and translating curves
Two sketches that will best represent the curve to be generated are selected and curves are traced above the sketch. Both curves will lie on the corresponding plane of sketch.。To generate a single curve from the curves traced, an intersection technique is used; the curves traced are extruded and intersected so that the resulting curve will be the required curve for surface generation.
If all three sketches represent a curve, then points are plotted in one view and with the other two sketches as references the points are translated such that the points will appear to lie on all three sketches.
The generated sketches are mirrored to generate a grout of curves as shown in Fig.4, which are suitable for generating patches.
3.3 Curve interrogation
Good curves will result in good surfaces. Therefore, before generating surfaces the curves are analysed。for their quality. Porcupine curvature analysis in CATIA V5 checks for a smooth variation in curvature (plots) throughout the curve. If the curves will result in smooth surfaces. If it is not smooth, then the control points representing the curves are translated so that a smooth variation in curvature is obtained. Figure 5 represents an irregular variation in curvature and Gig.6 shows a smooth variation in curvature. The curvature comb plot will show variation in the magnitude of the radius of curvature. When the magnitude of the spikes is small, this indicates a flat region on the curve. The magnitude must vary smoothly, or also useful in analyzing the continuity between the curves. For curvature continuity it is necessary that there is no step in the curvature comb.
3.4 Generating surfaces and variants
Surfaces are generated from the curves as patches. Figure 7 show s the concept model generated in CATIA V5.The surfaces representing a particular component in a car (e.g. the hood and the top are separate components )are placed in separate open bodies because of the ease in restyling. Figures 8 and 9 show some of the variants generated during the present study.
3.5 Surface interrogation
Surfaces are interrogated for class-A quality. Isophotes and reflection lines are the most commonly used tools for surface interrogation. Isophotes are used to identify the discontinuity between surfaces. They highlight the behaviour of the form or shape of a surface when light reflects from the surface. This reflection of light gives the user an understanding about the curvature discontinuity. This reflection should be natural and streamlined and should have uniformity. It is used to identify curvature discontinuities and to locate the dents in surfaces. Curvature discontinuities are represented by discontinuous highlight plots. Figure 10 shows the isophote plot of one of the models.Dents are represented by convergence of the highlight plot at the point of dent. Figure11 shows the isophote plot at on a dent on the surface. Reflection lines are used to a family of parallel lines on the light plane on the surface. Figure 12 shows the reflection line plot on the car surface.
3.6 Rendering
Rendering of the models is carred out to improve the visualization aspects of the model. Real-life rendering is mainly performed to make the customers appreciate the look of the final models. A VR environment is used for visualization and real-life rendering. Figure13 shows the rendered model generated in CATIA V5.
4 CONCLUSIONS
This paper tries to provide a systematic approach in developing styles from the images of the manually prepared concept sketches made by an artist with the aid of CATIA V5 software .It also interrogated two surfaces specifically to reveal the discontinuities that satisfied while designing a surface conventionally,the surface may have discontinuities and aesthetics will have to be improved.This approach is useful for any product development that needs class-A surfacing . The users of CATIA V5 can implement the same technique in their practice without adding any costly hardware.
REFERENCES
1 Tovey, M.I., Newman, R.M., and Porter, S. Skeching, concept development and automotive design 。Des. Stud.,2003,24,135-153.
2 Tovey,M.I., Concept design CAD for the automotive Industry.。J.Engng Des.,2002,13,5-13
3 Sunner, G., Greiner, G., and Augustiniack, S. Interactive examination of surface quality on cau bodies. Computer Aided Des., 2004,36,425-436
4 Hagen,H., Hahmann, S., Schreiber, T., Nakajima, Y., Wordenweber,, B., and Hollemann-Grundstedt, P. Surface interrogation algorithma. IEEE Computer Graphics Applic.,1992,12(5),53-60
5 Hahmann, S. Visualization techniques for surface analysis. In Advanced visualization techniques (EDC. Bajaj), 1999,pp.1-26(John Wiley, New York).
6 Theisel, H. Are isophotes and reflection lines the same? Computer Aided Geometric Des., 2001,18,711-722
7 Monacellli, G. VR applications for reducing time and cost of veicle developmet process. In Proceedings of the Eighth International Conference ATA on Vehicle Architectures: Products, Processes and Future Developments, Florence, Italy, May 2003(Eurographics Assoceation, Switzerland).
1. Brake dynamometer model predicting brake torque variation due to disc thickness variation
Jaeyoung Kang and Sungjin Choi
Department of Mechanical Engineering, Purdue University, West Lafayette, IN, USA
Body and Chassis Engineering Centre, Korea Automotive Technology Institute, Chonan, Chungnam, Republic of Korea
The manuscript was received on 11 May 2005 and was accepted after revision for publication on 5 June 2006.
DOI: 10.1243/09544070JAUT091
Abstract: In this study, the mathematical model for a brake system in a dynamometer is proposed to identify brake torque variation (BTV), and correlations between the dynamo test and simulation are determined. The interaction between the pads and the disc on the contact patch is simply modeled as a point-contact element and the transient response analysis in the time domain is introduced. The model indicates that the amount of BTV is linearly proportional to the pad stiffness, the friction coefficient, the disc thickness variation, and the effective radius of the brake pads. To verify the analytical model. A brake dynamometer is used and additional material tests are carried out. The dynamo test shows that the suggested model has reasonable accuracy in predicting the BTV of a brake system.
Keywords: brake dynamometer model, brake torque variation, disc thickness variation
1INTRODUCTION
The brake torque variation (BTV) induced by the disc thickness variation (DTV) and thermal distortion is well known as the excitation source of brake judder. Since it is hard to predict the quantitative thermal effect due to frictional forces（摩擦力）, thermal distortion will not be covered here. Instead , this paper considers the relatively low-temperature test condition of a brake dynamometer to reduce the thermal effect. The dynamometer is to reduce the thermal effect. The dynamic characteristics of brake motion during brake dynamometer operation are sought and any vibration analysis is omitted because the modes of the brake component and the brake dynamometer are too high to be excited by rotation of the brake disc. Time-domain analysis itself can effectively uncover the mechanism of the low-frequency excitation induced by brake engagement. The dynamic model includes the normal load variation [1] on the contact between the pads and the disc and allows the BTV to be expressed in an analytical form. Some papers [2] have shown through experiment that the BTV depends on several factors such as the temperature, the number of revolutions, and the DTV. In this paper, the DTV is assumed to be time invariant and the friction coefficient is simplified as a given function of only the angular velocity of the disc. The braking test in the brake dynamometer and some mechanical tests are conducted to verify the dynamic model.
2 BRAKE DYNAMIC FORMULATION
The braking-mode test of the brake dynamometer is used to measure the BTV level of the brake system; this is called the BTV test. The braking-mode dynamo test consists of the inertia equivalent to the quarter weight of the vehicle and brake system. When the inertia reaches a certain angular velocity, the driving force is stopped and then the brake system is engaged. During engagement（约束）of the brake system, several dynamic parameters are measured. To describe the BTV test in an analytical way, the following model is suggested.
2.1Brake model without BTV
The general equation of motion for the brake system is
f=2µN
Iθ=-2µNReff
The normal load is generated by the brake fluid pressure Pn acting on the brake cylinder cylingder with a section area Acylinder and is expressed as
N=PnAcylinder
The brake rorque (BT) is equivalent to Iθ and can be measured in the brake dynamo test . In reality, μ varies with the angular velocity, temperature, etc., but it is often regarded as a constant design parameter. Gigure 1 shows the result of dynamic simulation
2.2 Modified brake dynamic model
To account for the brake torque, the normal load variation should be included in the brake dynamic model.
2.2.1 Normal load variation
The normal load at the contact between the pad and the disc can be modeled by contact stiffness [3,4]. The pad lining is composed of soft non-metallic materials such as fibre or rubber, and the contact stiffness has a non-linear function of the normal load.
The disc profile has thickness variation in the circumferential direction, namely the DTV, which is defined as the difference between the maximum thickness and the minimum thickness of the disc. Since the peak value of the DTV has a key role in the brake dynamic model, the disc thickness can be simplified as a sinusoidal function (Fig.2) with first-order rotation
tv=tmax-tmin=tmean=DTV
Also the run-out value of the brake disc does not produce the net normal load on the contact patch between the pads and the disc because the outward normal load on one side of the disc cancels out the inward normal load on the other side. Therefore, the normal perturbation function u on the contact can be simply expressed as the complex exponential function according to
U=
For viscous elastic pads, the function of the normal load variation becomes (Fig.3)
Nvariation=Kpadu+Csystemu
