Mechanism Simulation With Working Model

Transcription

1 Mechanism Simulation With Working Model Shih-Liang Wang Department of Mechanical Engineering North Carolina A&T State University Greensboro, NC Introduction Kinematics is a study of motion and force of linkages, gear trains and cams. To many students, visualizing the motion of a mechanism is very challenging. Physical models can be built to illustrate the motion, and modular models using Tinkertoy or Lego are also effective. However, the link lengths of these models are fixed to some extent, and the physical models can not represent many problems closely. Many software packages are available for linkage animation and analysis. Nevertheless few can easily promote an active learning environment. Felder and Silverman [1] have synthesized five categories of learning style models: sensing/intuitive, visual/verbal, inductive/deductive, active/reflective, and sequential/global. Most textbooks and classroom teaching are intuitive, verbal, deductive and sequential, and this environment can not meet the needs of the second tier students [2,3] who are sensing, visual, inductive, active, and global learners. This paper presents the author s effort to develop a learning environment to reach this group of students based on an easy to use, powerful, and robust software Working Model [4]. Many PC programs for kinematics developed in the past are written in FORTRAN or BASIC languages. They have limited graphics capability by using sticks (line segments) to model links, and this representation is often too abstract for some mechanisms. For example, each part of the compound snips in Figure1 is difficult to be visualized as a stick. Another drawback of traditional programs is that animation and analysis are limited to four links and can not be extended to a sixbar linkage, like the quick return mechanism shown in Figure 2. Advanced motion analysis software like DADS or ADAMS is run on mainframes and workstations. These software packages are expensive and their procedures are complex. They are good for research and industrial environment but not ideal for an active learning environment in undergraduate education Working Model (WM) is a powerful engineering analysis and motion simulation software on PC. A user can sketch a problem using a variety of simple geometric primitives. Then sketch additional constraints (joints, springs, and dampers) and actuators (cylinders and motors). With a click of the mouse, WM uses its simulation engine to put the model in motion. WM applies Newton s law along with physical constraints and external forces to calculate the internal forces and acceleration of each rigid body. Using numerical integration, the velocity and position of each body can be calculated. Since no closed form formula is used in WM for the animation, students can use WM to verify answers obtained from using loop closure equations. For a complex control problem, WM can establish real-time links with Excel, MATLAB, or other programs that support Windows DDE to carry out calculations. WM can import drawings with AutoCAD DXF format. Alternatively AutoMotion, developed by WM's vendor Knowledge Revolution, can be used within AutoCAD to create each part s geometry and assign physical property to the geometry. AutoMotion can also create and attach WM constraints to AutoCAD geometry. All this information can be exported directly to Working Model at the click of one button. Once a mechanism is constructed and run in WM, a user can play "what if" scenarios by using the Smart Editor to manipulate objects on screen and change the property and appearance of each body. This flexibility gives an active learner a virtual prototyping tool. A Learning Environment for Kinematics A companion software for a kinematics textbook by Robert Norton [5] is developed as a learning environment for kinematics. This software includes many helpful animation and analysis programs for linkages, gear trains, and cams so that students can visualize their motion with realistic model. The software should promote students' interests in kinematics, and should motivate them to learn Working Model that can later be used in other class projects as a design tool. In the linkage program shown in Figure 3, a fourbar linkage is constructed and can be animated. The lengths of links and the orientation of the ground link can be modified from input boxes. Changing the link lengths, a linkage can be transformed from a Grashof to non-grashof linkage and vice versa. These dimensions can also be changed from property boxes. The velocity

2 and acceleration of any point or link can be displayed by using output boxes. Figure 2 (similar to Figure 3-14 in [5]) shows a quick return mechanism. In the animated version, a user can visualize that the return stroke is faster than the forward stroke, and therefore can understand the meaning of "quick-return." Figure 4 (similar to Figure 2-15 in [5]) shows four inversions of a Grashof fourbar linkage. Kinematics inversions are obtained by grounding different links of a linkage, and some students have trouble visualizing the difference, especially visualizing the Grashof double rocker. Clicking the run button in WM, one can visualize all four inversions in motion simultaneously, and the contrast becomes obvious. This animated program is easily constructed in WM but will be a fairly large programming task in FORTRAN. Figure 5 (similar to Figure 2-13 in [5]) shows four inversions of a crankslider mechanism. The velocity instant center of a linkage is another difficult concept to grasp and visualize. An instant center is a point common to two bodies having the same velocity at the instant. In Figure 6, a WM program shows the location of instant centers and the instant center's velocity vector during animation. In Figure 7, two fixed points are specified in two different bodies. In the animation process, a user can observe that only at the instant center location, the velocities of these two points are identical. Planetary gear trains are sometimes difficult for some students to visualize because there are four components (sun gear, planet gear, ring gear, and the arm) in a gear train. In a WM program shown in Figure 8 (similar to Figure in [5]), all components are shown in different colors and rotated at different speeds. The mechanism has two degrees of freedom, and two input boxes are used to modify the velocity of the sun gear and the arm. Output velocities of the planet gear and ring gear are also shown in boxes. Most PC based cam programs can plot cam profiles very accurately, but can not show the coordinated motion between a cam and its cam-follower. The animation of a cam system (a cam with its cam-follower and return spring) is important for the second tier students and can be easily accomplished by WM. Working Model can create cams of force or form closure (as shown in Figure 9), and cam followers can be modeled with a roller or a flat face (as shown in Figure 10). The cam profile can be inputted from a spreadsheet file. The spring constant can be specified using the Smart Editor. Animated Mechanism Library In addition to the learning environment, a related effort using WM is to compile a library of animated mechanisms. This library will be a useful resource for students working on design projects and for practicing engineers to develop new products. Text files can be linked to the Working Model using Visual BASIC to provide features and detail information of each mechanism. One completed module is compound shears that can provide a large mechanical advantage as compared to simple shears. These problems have been introduced as statics problems [6,7], or kinematics problem [8]. For a design engineer, the stroke and the mechanical advantage over its motion range are important. WM can provide the analysis and animation of these mechanisms as shown in Figure 11 to Figure 14. A user can play what if scenarios to modify these hand tools. A designer may want to adapt this type of mechanism in applications requiring a high mechanical advantage. Modules to be developed in the future include mechanisms used in automobiles, robot grippers, and labeling machines. Summary The WM programs presented in this paper will eliminate the frustrations some inexperienced WM users faced when building sophisticated models in the beginning. They can start running these programs right away and then learn to use the Smart Editor as their interests grow. Although this companion software is intended for [5], the software is generic enough that students using other textbooks can also benefit. Educational discounts for multiple copies of WM make it affordable. Student edition of Working Model [9] is sold at a very reasonable price if a student prefers his or her own copy. Addison Wesley also includes WM into AutoDesk Collection Plus for collegiate users. All figures in this paper are generated by WM. Animated versions of these figures are also included in the Proceeding CD with the same figure numbers and captions. Acknowledgments WM programs illustrated in this paper are developed by Zhidong Liao, Fan Mo, Maja Jankov, Amir Naghshineh-Pour, and other former students in my MEEN 440 and MEEN 565 classes.

3 References 1. Felder and Silverman How Students Learn: Adapting Teaching Styles to Learning Styles, Proceedings of Frontiers in Education Conference. ASEE/IEEE, Santa Barbara, CA., p. 489, Felder, R.M. Reaching the Second Tier- Learning and Teaching Styles in College Science Education, Journal of College Science, 23(5), , Tobias, S., They Are Not Dumb, Thet Are Different: Reaching the Second Tier, Tucson Research Corporation, Working Model User s Manual, Knowledge Revolution, San Mateo, CA, Norton, R., Design of Machinery, McGraw-Hill, Riley, W.F., and Sturges, L.D., Engineering Mechanics- Statics 2nd ed., Prentice Hall, Hibbeler, R.C., Engineering Mechanics- Statics 7 th ed., Prentice Hall, Erdman and Sandor Mechanism Design: Analysis and Synthesis, 2 nd ed. Prentice Hall, Rubin, Tutorial Manual for The Student Edition of Working Model, Addison Wesley, 1995 Figure 2 The Quick Return Shaper Figure 4 Four Inversions of a Grashof Fourbar Linkage Figure 1 Compound Snips Figure 5 Four Inversions of a Crank-Slider Mechanism Figure 3 A Fourbar Linkage Figure 6 The Location of Instant Centers

4 Figure 7 Two Points Away From Instant Center Location Figure 11 Compound Shears Figure 8 A Planetary Gear Train Figure 12 A Bolt Cutter Figure 9 A Cam of Form Closure Figure 13 Toggle Pliers Figure 10 A Cam With a Flat-Faced Follower Figure 14 Compound Pliers

5 Links to animations with demonstrations : Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 Figure 14