To: Nascent Swanlings Design Group
From: Black Swan, First Assistant Chief Engineer
Subject: Grandma’s Rockin’ (ME 3310 Project 2 Term D-96)
Date: Assigned 4/5/96 – Final report due on 4/19/96 @ 4:00 PM

    Grandma is off her rocker —again! Junior has run down to the Bingo parlor to fetch her, but we need to do something about her rocking chair before she gets back. Her arthritis makes it difficult for her to push the rocker, but she just loves to rock and roll. So, we are going to design a motorized rocking chair for her 90th birthday in 2 weeks. She likes the motions of her Boston Rocker and detests the motions of Grandpa’s platform rocker. (See Figure 6-16 on p. 210 of your text for definitions of these terms.)

    Our marketing group thinks that there may be a large, untapped market for a motorized Boston Rocker. Designs are scarce if they exist at all. After meeting with our board of directors (over pizza at Gompei’s) to discuss how to approach this market, we have defined some general constraints (task specifications) for the problem. You are expected to develop many more appropriate performance (task) specifications.

- The device should provide comfortable and safe coupler motions to the occupant, similar in character to those of the Boston Rocker.

- The device must not injure the occupant in any way nor expose any parts of the body to unacceptable accelerations or velocities.

- The device should accommodate any adult of reasonable size.

- The device must be a pin-jointed or slider-crank linkage, either a fourbar, sixbar, or a geared fivebar (so we can use the recently purchased software programs Fourbar, Fivebar, Sixbar, and Slider.)

    Junior came back from the Bingo parlor without Grandma. It appears that she took off for Las Vegas on the Greyhound bus with her Bingo winnings. She’ll be back in exactly two weeks and she will expect this chair to be ready for her then. Don’t be late with it because she’s a terror when she’s off her rocker!

    You must follow the Design Process to fully define and constrain the problem. You are expected to do Background Research into the problem and any extant solutions. You must create a general Goal Statement. You must generate a list of at least 16 additional Task Specifications. As with any design problem, there is an infinity of solutions possible. You are expected to come up with one solution which will work. To do so you will have to try out many alternate designs and iterate to your 'best' solution. You should expect to typically go through at least ten iterations before arriving at an acceptable one.

    There will be a need for extensive kinematic analysis in this project in addition to the synthesis activities. You are expected to do a complete velocity and acceleration analysis of your design to determine its feasibility and its effects on the occupant at various points on her body. In order to make it possible for you to accomplish this task in the short time available, the AARP has generously made available to you their computer programs, Fourbar, Fivebar, Slider, and Sixbar. You may use these programs to analyze your potential design solutions. However, to ensure that you understand your results and to discourage a BFI approach, we require that you include a complete derivation of all equations necessary to the analysis of your design. These must be a general (i.e., non-numeric) derivation and you must use your derived equations to numerically check the computer program solutions for a minimum of two positions of your device. (Remember, you can’t trust any software written by anyone over thirty!)

    You are also required to document your solution in a professional engineering report which adheres to the Project Report Specifications document previously distributed. The background data should be written up in the Background Research section. The 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. Note that unreferenced and undiscussed computer or other illustrations will be considered to be report "filler" and be ignored. Do not put anything in the report unless you discuss its meaning. A working cardboard model of at least one plane (one side) of your design is required.

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. Some suggested background research areas are:

- investigate human factors and ergonomics (see material on reserve)

- investigate any existing similar devices

- Determine Grandma’s preferences (There are lots of Grandmas at any nursing home and they will be delighted to see you. They might even pinch your cheeks!)

2. There are several suggested linkage configurations to investigate, the fourbar, sixbar, slider-crank, and the geared fivebar. Consult the Hrones and Nelson 4-Bar atlas and the Zhang et al Geared 5-Bar atlas on reserve in Gordon Library, and select some linkages whose coupler curves look like they might do what you want. Note the link ratios and other parameters of these linkages.

3. Input these data to the appropriate program in the computer labs to investigate the velocities and accelerations of the points on the coupler corresponding to various points on the occupant's body.

4. Redesign the linkage as necessary to improve the desired output parameters. Note that cardboard models make designing much easier.

5. Pay particular attention to the velocity and acceleration polar diagrams for the coupler points corresponding to important locations on the user’s body These will, along with the printed data, show you the velocity and acceleration at any point of interest along its path.

6. Iterate steps 1-5 as necessary to obtain a reasonable solution.

7. Check the computations of the computer program for velocity and acceleration of one point using any manual methods, graphics or analytical. Note that in engineering practice, you should never rely on the output of anyone’s computer program being correct until you have personally checked it by another method.