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Design Education, pt. II

March 8, 2009

As I explained in my last blog post, I am currently a teaching assistant for an engineering design class at Georgia Tech.  In that post, I showed an email which I had sent to my students, imploring them to think more clearly about the design process (and design tools), and I explained how understanding epistemology and possessing clear definitions of design concepts can enable this.  I received another round of reports from my students again this week, and they were MUCH better this time around.  I even saw that some of the students had explicitly adopted some of the ideas I had shared with them, and I’ll admit that it was exciting to see that I had helped them to think more clearly.  Amidst the usual frustration which is inherent to teaching a subject you love to questionably interested students, design education can be rewarding when you see you have gotten through to them.

However, unclear thinking was still evident in most of these new reports.  Specifically, most of these reports showed that the students did not really understand what constitutes the definition of the challenges of a design problem (this was a skill which should have been learned earlier in the semester).  In this latest set of reports they had to identify the technical challenges inherent to a design/build/test competition which they have to compete in.  The problem entails making a device which can A) knock over and clear two bowling pins from their “zone” in the competition arena, and B) deposit four plastic “salary” balls into a “bank” (A cylindrical rotating aluminum drum, two feet in diameter, one foot high, with a one foot diameter hole in the middle of the top…  There are also “swindler” bowling pins affixed to the rim of this rotating drum to complicate getting the balls into the “bank”).

Here is an email I wrote to my students to help them differentiate between “problem definition” and “definition of challenges” (as well as the joys of McMaster-Carr, the four-bar linkage, and ‘flexible design.’):


I went through your papers, and they were far better than last week’s.  However, I saw some epistemological sloppiness in most of the write-ups, particularly in the Introductions.  Briefly, the sloppiness related to the unclear definition of “problem challenges,” and the problem can be corrected by understanding the following statement:
“Design constraints (the problem statement–the ‘givens’) are NOT the challenges of a design problem…  Design challenges are your technical interpretation and understanding of the demands imposed by the constraints.”
Many teams just restated the problem statement/constraints when they thought they were describing the “specific design challenges” in the Introduction.  Challenges are what result from constraints.  They are your technical interpretation and evaluation of the problem’s “givens.”
This was the same problem which many teams had in the Pasta Tower Lab from earlier this semester…  Asked to describe the challenges, many teams just restated the constraints: “We had a limited amount of time to build the spaghetti tower and only one square foot of space to work in and we are only allowed to use dry spaghetti and tape to build the tower.”  Foo!  EVERY problem is ‘limited in time’!  And parroting the customer’s wish for a pasta tower back to him does not begin to show him you understand how to solve his technical problem for him!!! 
We engineers do not get paid to restate the obvious while pretending that’s what comprises engineering acumen.  We get paid to exercise the intelligence which makes the most with what we’ve got!
And the first step in “making the most of what you’ve got” is understanding the limitations of what you have.  Accordingly, when asked to state the challenges in that pasta tower lab write-up, I wanted to see things like: “Pasta is a brittle material and it can buckle easily when compressed.  Buckling can be counteracted by lashing several pasta strands together, which increases the second moment of area of a pasta structural member.  Tape cannot bear compressive loads on its lengthwise axis, but can serve as the means to bind together spaghetti strands and, as a structural member, can be used to create ‘tension cables.'”  THAT is recognizing the technical challenges which result from the problem’s constraints.  Restating the constraints does NOT demonstrate an understanding of the problem’s technical challenges from the perspective of an engineer. 
For this most recent write-up, when listing the challenges inherent to this problem, I would have liked to see things like:
– The geometric constraints require that we develop a machine which can fit in a small space but perform over a large area.  Consequently, our machine will have to have the ability to either move itself, reach out or expand itself, or launch items in order to be able to operate over a large area from a small space.
– The time constraints of the competition require that the device EASILY fit inside the Go/No-Go Box, as well as having mechanisms which A) can be easily reset and B) which are resistant to the inevitable jostling which will bump the machine after it is reset.  Time cannot be wasted during setup on false starts or a boxing procedure which requires an inhuman level of box-positioning-accuracy.
– The elimination format of the competition requires that our machine perform reliably through multiple elimination rounds.  The machine must also bear up to being transported around campus for several weeks while it is under development.  Both of these challenges demand a machine which is structurally strong so that it can perform relibably with no variation stemming from structural weakness and deformation, and in order to bear some abuse in transport.
– The competition itself is a well-defined problem, but variables in the machine’s operating environment still exist and must be accounted for.  The design must account for slight uneveness of plywood surfaces as well as interference in the machine’s operation stemming from other competitors.  A machine with design flexibility–the ability to accomodate these variables impinging on it while still operating correctly–or the ability to repel or avoid the interference must be incorporated (see end of the email about this).
THOSE are some of the challenges of this design competition problem.  Defining the problem’s challenges is really the most abstract level at which you begin to technically interpret the problem.  Since design proceeds from your abstract definitions to concrete solutions, defining the challenges is thus a very crucial step…  If your initial abstract definitions are messed up, everything down-stream from them in the design process will likely be equivalently messed up.

My personal experience in design classes tells me that, whenever it’s possible, it’s worth buying components instead of making them yourself.  The importance of this principle is in direct relation to the complexity of the system you are designing.  Though the systems you are designing are relatively simple, I’d make the most of the $100 spending limit which is available to you, whenever it can save you the trouble of having to hack something together yourself.  A good resource if you haven’t heard of it already (the place to order and get ANY type of hardware that exists in this world) is McMaster-Carr.  Sheet metal, pvc/metal rods/tubing, wheels, bearings, fasteners, etc, etc.  If you need it, and it is hardware, then they’ll probably have it.  They have a really good search engine, fast shipping, and good customer service.  I just got myself a McMaster-Carr tattoo and I’d probably keep McMaster-Carr if I had to choose between keeping it and my mother (sorry Mom!).
Four-Bar Linkages
I’ve seen several machines executing actions which could be more effectively accomplished by the mechanism known as a “four-bar linkage.”  I have not yet watched the videos of class competions from previous years, so I do not know if you guys have already seen what a four-bar linkage COULD do for your machine…  Though I’m sure you have already seen a four-bar in one form or another already: the four-bar linkage is the principle behind vice grips, and it is the base unit which is repeated to make up the scissor arm which was used by one of the teams in our class.  What four-bar linkages enable you to do is take a very small input displacement (such as the extension of the pneumatic cylinder from your mechatronics kit) and translate it into a very large output displacement (such as lifting something over the top of the ‘swindler’ obstacles you have to get beyond, or hurling a catapult arm to launch things over them).  See the following two websites and the image I drew up in order to get an idea of what you can do with this useful mechanism (particularly that second website–you’ll see you can get some pretty wild displacements out of these things.  and check out the rest of that website, too.  it’s chock full of magical mechanisms which might get you thinking.):

Design Flexibility

If your machine is relying on devices which must achieve precise positioning in order to function correctly, I’d recommend stepping back from the problem for a moment to  consider the promise of alternative designs which have more design flexibility, which enables them to operate more successfully.  What is ‘design flexibility’?  First, consider two concrete examples which will contrast the concept: one machine possesses the trait, while the other machine does not…  Which of the following devices will more likely succeed in a competition to drop balls in the bank:
DESIGN A: a device which must be positioned precisely on the starting platform before it rolls forward off of a bumpy ramp, through a field in which bowling pins and projectiles are flying all over the place (hopefully not getting turned off course), and then must align itself juuuuuust right with the bank so that a tiny sensor will (hopefully) align with and sense the swindlers, telling the machine when it can drop the balls into the bank.

DESIGN B: a device which slides forward on rails–OVER the crazy field of flying object insanity–and lifts the salary balls OVER the swindlers, to drop them into a relatively (at one foot in diameter) cavernous hole.
In other words, Design B has the ability to function correctly under a wider range of operating circumstances–it possesses design flexibility.  Both of the machine’s targets are to “drop the balls into the bank” but Design B is not limited by having to get a sensor into a small volume of space near the bank in order for the machine to operate correctly, and its design inherently avoids much of the chaotic activity which will be occurring in the competition arena by operating above it on sliding rails.  Design A is limited by too many “choke points” in its order of activity and these “choke points” are where its successful operation can be derailed: its path rolling off the ramp can be messed up by bad positioning of the ramp or variability in the plwood surface, it can get knocked off course by bowling pins or interference from other machines, its reliance on the sensor adds another mechanism which can malfunction, etc…

Design B is inherently more robust to environmental variables through avoidance of them.  The scenario I proposed omits considering design execution: a cleverly designed and well constructed rolling-cart concept can outperform a shoddily constructed sliding-rail design.  But I think the “design flexibility / robustness to environment” principle is worth pondering: if it removes “variables” from your problem’s “equation,” (and thus: headaches and time from your design process) then it may be worth thinking about.  That is my two cents.

– Justin Ketterer

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