R. L. Norton, 11/5/93

Assigned 11/5/93 ----- Due 11/19/93 (14 days!)

    Convertible cars are returning to popularity and many are being made and sold in the U.S. They nearly were outlawed some years ago by proposed federal legislation that would have required rollover resistance for all vehicles. There is again talk of the institution of such rollover specifications for passenger cars. To protect the endangered convertible, our client, American Topless Corp. (ATC), has requested that we design an automatically deployable rollbar assembly to be retrofittable to existing convertibles.

    The design concept has already been developed by the Topless engineers and is that of a pair of identical, parallel, fourbar linkages which will carry the rollbar from its stored position in the vehicle to its erect and locked position in no more than 0.25 seconds. It should lock into position by geometry so that it can withstand the rollover forces without collapsing. It will automatically deploy if sensors in the vehicle detect a tip angle of greater than 30 degrees.

    The driving device for deployment will create a constant angular acceleration applied to the input rocker of each fourbar linkage over an angular displacement of your choosing. This will be accomplished by a pyrotechnically-fired cylinder driving a cable-wrapped capstan on your input rocker. Assume that the average pressure available during the stroke is 1000 psi. You must determine and specify the capstan diameter and the necessary diameter of the pyrotechnically-fired piston.

    To obtain a prototype design, we will target a particular convertible, namely the 1993-4 Chevrolet Cavalier. This vehicle will be available for inspection and measurement at Ragsdale Chevrolet, Rte 9, Spencer, MA between 9AM and 3PM on Saturday, 11/6 and on Monday 11/8, 9-4. The Service Manager, Mr. Gary Alfreds, will be available at the dealership to answer questions during that time. Ask for him when you aarrive. Bring a tape measure to make your measurements and a camera for pictures. You may carpool and work in groups to obtain the needed research data.

    Your assignment is to design a rollbar-deployment mechanism based on a fourbar linkage which will provide suitable motions to the rollbar, and to arrange the shapes, sizes and materials of the parts such that the reaction forces applied to the automobile are as small as possible. The driving-torque requirements are also of concern as they will require energy from the drive mechanism and thus should be minimized. The pivot pins must be sized against failure, and the dimensions and materials of the links specified. Also estimate the kinetic energy to be dissipated when the assembly hits the deploy stops. Note that you must also account for the external loads on the deploying rollbar due to wind loading up to the maximum speed of the vehicle (100 mph). To support the vehicle in a rollover, the steel rollbar itself needs a cross-section of at least 2.5 " outside dimension with at least 2 mm thick walls. It must deploy with the top in either the up or down position.

    Luckily for you, ATC has provided a computer program called DYNAFOUR (available in the Aptlab - note that the version supplied with your text has a bug which has been corrected in the lab versions). This program will calculate all the forces and torques on a fourbar linkage over multiple positions for a constant angular acceleration input. You must supply it the link dimensions, masses, and moments of inertia for your design's elements, and also supply the crank input angular acceleration, and any external forces or torques. Note that DYNAFOUR imposes a constant angular acceleration on that driving crank, which assumes that the device used has sufficient torque available at all speeds. You also have to specify what power will be required of this device. TKSolver and/or the Aries solids modeling system will be an invaluable aid for calculating masses and moments of inertia of your links.

    As with any design problem, there is an infinity of solutions possible. You are expected to come up with one solution which will work and, most importantly, to understand how it works. If it doesn’t work, you should at least be able to explain in good engineering detail why it doesn’t! To do this you will have to try out many alternate designs and iterate to your 'best' solution. You should expect to go through at least ten iterations before arriving at any acceptable solution. You will need to do some paper and pencil (or preferably TKSolver) calculations for masses and moments of inertia of your proposed links before going to the computer to use DYNAFOUR.

    You are also required to document your solution in a professional engineering report which adheres to the Project Report Specifications document which will be distributed. This report will document the process by which you iterated to your final design as well as the design itself. Do not just describe the final result. Rather show me how you arrived at it, including the failures encountered along the way. This will demonstrate to me that you understand the engineering concepts and the relevant course material. A working cardboard model of at least one plane (one side) of your design is required. Your report should include titled (labeled) plots and tables from program 'DYNAFOUR'. These graphics should preferably be imported to your word processor file using the GRAB.COM package. Remember, one purpose of the project report is to convince me that you understand your solution well enough to have been the real designer thereof!

Some suggestions to get you started:

1. Do research before trying to solve the problem! Don't 'shoot from the hip'. Avoid BFI (Brute Force and Ignorance). Engineer it.

2. Investigate the dynamic and air friction factors involved.

3. Linkage synthesis (ME3310) will definitely help.

4. Reading the textbook might even help.

5. Cardboard models make designing much easier.

6. THINK about the theory discussed in class and in your text. It IS applicable to this problem!

IMPORTANT! IMPORTANT!

IT IS CRUCIAL THAT YOU START THIS PROJECT RIGHT AWAY! Do not kid yourself that you can knock this off over the weekend before it is due! You cannot! This type of problem requires incubation periods. Work on it until stumped, then put it aside and do other coursework. Then come back to this problem after your subconscious has had a chance to work on it. You'll be surprised how effective this 'time-sharing' of your tasks can be. Read The Design Process in Chapter 1 for more information on this phenomenon. Incubation really does work. You should plan to have all the design work done at least 2 days before the due date, and use that time to write it up. It will take about three times longer to write up the report than you think it will. Allow at least two days for the write-up.

    The report must be word processed and spell checked! WordPerfect (with built-in spell checker) is available in the ADP Word Processing Lab in Fuller Laboratories. Use the GRAB.COM package to incorporate graphics into the report. If you have your own PC and word processor, that's fine too. Letter quality output is NOT required, but DARK type is. Use a good printer ribbon, or better yet, take your disk to CCC and laser print the final draft.

Regarding cooperation between students: This is a very gray area. I do not object to your discussing the problem with your classmates or others. Much learning can take place by 'bouncing' ideas off other technically competent people (including your instructors). So you do not need to work on these projects in a vacuum, BUT, and this is a very large BUT, the final result must be your own. Any duplication of results or designs in the final reports will be quite obvious and will result in a very tense confrontation between you and me. So, brainstorm ideas among yourselves if that helps, but make sure that the final result is your own and that you fully explain its intricacies in your report. This is NOT a group project.