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A Design Example

Objectives

A design competition example.

The typical engineering design cycle is made up of the following seven steps; 1) Define the objectives; 2) gather information; 3) choose a strategy; 4) make a first cut; 5) build, document, test; 6) revise (and revise again); and 7) test. Steps 5 and 6 form an iterative loop that may be repeated many times before the design effort is deemed successful. “Success” in this case is measured by how well the finished product meets the desired specifications. Imagine that your professor has announced a design competition as part of an introductory engineering course. The rules of the contest are outlined in the following flyer.

College of Engineering Peak-Performance Design Competition
Objective
The goal of the competition is to design and construct a vehicle that can climb a ramp under its own power, stop at the top of the ramp, and sustain its position against an opposing vehicle coming up the other side of the ramp. The illustration in Figure 7 shows the approximate dimensions of the ramp. The 30-cm width of the carpet-covered track may vary by ± 0.5 cm as the vehicle travels from the bottom to the top of the ramp. A vehicle is considered to be on “top of the hill” if its body, plus any extensions, strings, or jettisoned objects, lie completely within the two 120-cm lines after a 15-second time interval.

7. Ramp specifications for the Peak Performance Design Competition.Vehicle Specifications

The vehicle must be autonomous. No remote power, control wires, or remote-control links are allowed.
The vehicle's exterior dimensions at the start of each run must not extend beyond the sides of an imaginary 30-cm cube. A device, such as a ram, may extend beyond this limit once activated, but cannot be activated before the start of the run.
The vehicle must be started by an activation device (e.g., switch or mechanical release) on the vehicle. Team members may not activate any device before the start. Vehicles cannot have their motors running before the start and dropped after the start.
The vehicle can be powered by the following energy sources only:

One battery of up to 9 volts
Rubber bands (4 mm × 10 cm maximum size in their unstretched state)
Mouse-traps (spring size 3 cm × 6 cm maximum)

The vehicle's weight, including batteries, must not exceed 2 kg.
The vehicle must not use chemicals or dangerous substances. No rocket-type devices, CO₂ propulsion devices, or chemical reactions are allowed. No mercury switches are permitted. (Mercury is a toxic substance, and a risk exists that a mercury switch will break during the competition.)
The vehicle must not be anchored to the ramp in any way before the start. At the end of the run, the vehicle and all its parts, including jettisoned objects, extensions, etc., must lie completely within the top of the hill and the 30-cm track width.
The vehicle must run within the 30-cm-wide, carpet-covered track. The vehicle may not run on top of the guide rails.
The vehicle must compete in six 15-second runs against opposing vehicles. The vehicles with the most wins after six runs will be selected for the Grand Finale. The latter will determine the winner of the competition. Modifications to the vehicle are permitted between (but not during) runs.

1 Applying Design Principles to the Design Competition

Let's now examine the Design Competition problem in the context of the seven steps of the design cycle. Remember that the problem can be addressed in many different ways. Some design solutions, however, will work better than others.

2 Define the Overall Objectives

When faced with the task of designing something, an inexperienced engineer is tempted to begin with construction right away without taking the time for a careful planning stage. Building things is fun and satisfying, while estimating, sketching, calculating, simulating, and checking design parameters seem less glamorous. It's important, however, to begin any project by taking time to define its objectives. In this case, your objectives might take the following form:

Design for speed. The fastest vehicle will not necessarily be the winner, but in order to win, your vehicle must reach and maintain the center line well before the 15-second time limit. Otherwise, it may be blocked from reaching the top of the ramp by the other vehicle.
Design for defensive and offensive strategies. Not only must your vehicle reach the top of the ramp and stop on its own, but it also must maintain its position as your opponent tries to do the same. Although offensive and defensive strategies are not necessarily mutually exclusive, you've decided (somewhat arbitrarily) that defense will be given a higher priority than offense. You may need to modify this choice if tests show it to be infeasible.
Design for easy changes. The rules state that modifications to the vehicle are permitted between runs. During the contest, you may see things on other vehicles that will prompt you to make changes to your own vehicle. Similarly, you may choose to modify your vehicle in mid-competition should any of its features make it needlessly vulnerable. Adopting an easy-to-change construction strategy will facilitate on-the-fly changes to your vehicle. The likely trade-off in choosing this approach is that your car will be less durable and more likely to suffer a disabling failure.
Design for durability. The vehicle must endure six, and possibly more, trips up the contest ramp. Opposing vehicles and accidents can damage a fragile design. You must weigh the issue of durability against your desire to produce a vehicle that's flexible and easy to modify.
Design for simplicity. By keeping your design simple, you will be able to repair your vehicle quickly and easily. An intricate design might provide more performance features, but it also will be more prone to breakdowns and will be more difficult to repair.

Note that goals (1) through (5) are not independent of one another. For example, designing for easy changes may conflict with building a durable vehicle. Designing for both offensive and defensive strategies will lead to a more complicated vehicle that is harder to repair. Engineers typically face such trade-offs when making design decisions. Deciding which pathway to take requires experience and practice, but making any decision at all means that you've begun the design process.

3 Choose a Design Strategy

Many different design strategies will lead to a vehicle capable of competing. Building a winning design, however, requires making the right choices at each step in the design process. How can you know ahead of time what the right choices will be? In truth, you cannot, especially if you have never built such a vehicle before. You can only make educated guesses based on your experience and intuition. You test and retest your design choices, making changes along the way if they increase your vehicle's performance. This process of iteration is a crucial part of the design process. Iteration refers to the process of testing, making changes, and then retesting to observe results. Good engineering requires many iterations, trials, and demonstrations of performance before a design effort is completed. In the world of engineering, the first cut of a design seldom resembles the finished product.

In the case of the Design Competition described here, the rules provide for many alternatives in vehicle design. Regardless of the details of the design, however, all vehicles must have the same basic components: energy source, propulsion mechanism, stopping mechanism, and starting device. Although not required, a defense mechanism that prevents an opponent from pushing the vehicle back down (or off) the ramp will increase your chances of winning. After some discussion with your teammate, you develop the choice map shown in Figure 8.

8. Choice map that outlines the decision tree for the first phase of the design process.
This diagram displays some of the many design choices available to you and the consequences of making these design choices. For example, choosing battery power for propulsion constrains the stopping mechanism to one of the choices listed in the third box down. Although the diagram does not provide an exhaustive list of possible design choices, it serves as an excellent starting point for your design attempt. As your teammate points out, “We have to start somewhere.” You remark in return, “Let's begin with our best guess as to what will work.” The choice map of Figure 8 includes some of the following elements:
Energy Source
According to the rules of the competition, you may power your vehicle from standard nine-volt (9V) batteries, rubber bands, or mousetrap springs. Batteries are attractive because they require no winding or preparation other than periodic replacement. They will, however, be more expensive than the other two alternatives. Rubber bands will require much less frequent replacement, but will store the least amount of energy among the three choices. Like a rubber band, a mousetrap needs no frequent replacing. It stores more energy than a rubber band, but because of its physical form, it offers the fewest options for harnessing its stored energy.
Propulsion Mechanism
Your choice of the propulsion device, or prime mover, will depend entirely on your choice of energy source. If a battery is used as the energy source, an electric motor seems the obvious choice for turning the vehicle's wheels. Rubber bands can be stretched to provide linear motion or twisted for torsional energy storage that turns an axle or power shaft. Alternatively, a rubber band can be stretched around a shaft or spool like a fishing reel and used to propel the vehicle's wheels. A mousetrap can provide only one kind of motion. When released, its bale will retract in an arc, as depicted in Figure 9. This motion can be harnessed and used to propel the vehicle.

9. Harnessing the stored mechanical energy of a mousetrap. The bale retracts in an arc when the mousetrap is released.Stopping Mechanism
Your vehicle must stop when it arrives at the top of the ramp. This requirement can be met by interrupting propulsion power precisely at the right moment and relying on a combination of gravity and friction to stop the vehicle. A braking device to augment these forces also might be considered. If the vehicle is powered by batteries, there are many possibilities for a device to interrupt the flow of power to the vehicle. A simple tilt switch that disconnects the battery when the vehicle is level, but connects the battery when the vehicle is on a slope, will certainly do the job. For safety reasons, however, the rules prohibit the use of mercury switches (elemental mercury is toxic to humans), hence any tilt switch used in the vehicle must be of your own design. A metal ball bearing that rolls inside a small cage and makes contact with two electrodes, as illustrated in Figure 10, might serve as a suitable tilt switch. Other choices might be a spring loaded contact switch, such as the one shown in Figure 11, or a system that cuts off power to the wheels after the car has traveled a preset distance as measured by wheel rotations. This scheme will work well only if the wheels do not slip.

10. Tilt switch made from a small enclosure, a ball bearing, and two contact points.

11. Spring-loaded contact switch.
One interesting alternative to a mechanical switch would be to use an electronic timer circuit that shuts off power from the battery after a precise time interval. Through trial and error, you could set the elapsed time to just the right value so that the vehicle stops at the top of the ramp. One problem inherent with this open-loop timing system is that the vehicle does not actually sense its own arrival at the top of the hill, but rather infers it by precise timing. Because the speed of the vehicle may decrease with each successive run as battery energy is depleted, this timing scheme might cause problems. On the other hand, it is likely to be more reliable than solutions that involve mechanical parts.

If a rubber band or mousetrap is chosen as the power source, then stopping the vehicle will require something other than an electrical switch. One crude way of stopping a vehicle propelled by mechanical energy storage is simply to allow the primary power source to run out (e.g., by allowing the rubber band to completely unwind). However crude this method, it is reliable because power input to the vehicle will always cease when the source of stored energy has been depleted.
Starting Device
If the vehicle is powered by a battery, then an electrical switch becomes the most feasible starting device. A rubber-band power source will require a mechanical device such as a pin or trip lever to initiate power flow to the wheels. A mousetrap can make use of its built-in trigger mechanism or any other starting mechanism that you can devise.

4 Make a First Cut at the Design

The first design iteration begins with rough estimations of the dimensions, parameters, and components of the vehicle to make sure that the design is technically feasible. After discussing the long list of design choices, you and your teammate decide upon a battery-powered vehicle. This decision makes available the many choices for a stopping device and power train. You feel that this design flexibility far outweighs the advantages of mechanical propulsion schemes. You decide upon a defensive strategy and agree to build a slower-moving, wedge-shaped vehicle driven by a small electric motor. The advantage of this design strategy is that the motor can be connected to the wheels using a small gear ratio, thereby providing higher torque at the wheels and a mechanical advantage that would be unavailable to a very fast vehicle. You plan to use plastic gears and axles purchased from an on-line Web site. The gear box will reduce the speed of the wheels relative to the motor shaft speed, providing added mechanical torque that will significantly increase the force available to push the opposing vehicle off the ramp. Because your vehicle will be slower than the others, it may not reach the top of the ramp first, but its wedge-shaped design will help to dislodge the front of any opposing vehicle that arrives first at the top of the hill. If your vehicle should happen to arrive first at the top of the hill, your car's defensive wedge shape will cause your opponent's car to ride over your car's body, allowing you to maintain your place at the top of the ramp.

You decide to use one motor with a single driven axle attached to both rear wheels. An alternative strategy would be to drive each rear wheel separately, thereby allowing the driven wheels to turn at different speeds. Such differential capability is essential for vehicles that travel curved paths, but in this case the vehicle must travel along a straight path only. By driving the wheels from a common shaft, you will reduce slippage, because both wheels will have to lose traction before forward motion is impaired. You briefly consider front-wheel drive, because you assume from hearing many car advertisements that front-wheel drive is superior to rear-wheel drive. Your teammate is quick to point out, however, that the advantage of front-wheel drive lies in its ability to help the car negotiate curves. Despite the media-driven message of “better traction,” the advantages of front-wheel drive have nothing to do with your application. In fact, front-wheel drive may be a disadvantage to your wedge-shaped design, because it may cause your car to flip over forwards if another car travels on top of it (Figure 12). Your teammate draws the sketch of Figure 13 to illustrate this scenario. You abandon the idea of front-wheel drive.

12. Front- and rear-wheel drive options for a moving wedge vehicle.

13. Rough, preliminary sketch of a car for the Peak Performance Design Competition.
5 Build, Document, Test, and Revise

A rough preliminary sketch of your car is shown in Figure 13. You've entered this sketch into a notebook in which you've been recording all information relevant to the project. Included in your notebook are design calculations, parts lists, and sketches of various pieces of the car. Shown in Figure 13 are the car's wedge-shaped design, a single drive shaft driven by a motor, belt, and pulleys, and a single switch to turn off the motor when the vehicle arrives at the top of the hill. Your design concept represents a trade-off between several competing possibilities, but you and your teammate have decided that the car's electric motor drive and defensive shape have the best chance of winning the competition.

The sketch in Figure 13 represents a beginning, but it is not the finished product. You still have many hurdles to overcome before your vehicle will be ready to compete. The next step in the design process involves building and testing a “first-cut” prototype. To help you in this phase of the design process, your professor has built a test ramp available to all contestants. You begin by constructing a chassis shell in the form of a wedge, without a motor drive or stopping mechanism.

You run your wedge-shaped vehicle up the ramp by hand. You soon discover that the bottom of the vehicle hits the ramp at the top of the hill, as depicted in Figure (a)14. The change in angle of the ramp is large, and all four wheels do not always maintain contact with the ramp surface. You discuss several solutions to this problem with your teammate. One solution would be to increase the size of the wheels, as shown in Figure (b)14. This change would decrease the mechanical advantage between the motor and the wheels, requiring you to recalculate the torque required from the motor. Another solution would be to make the vehicle shorter, as in Figure (c)14, but you realize that this solution would lead to a steeper angle for the wedge shape of the vehicle and reduce its effectiveness as a defensive strategy. (The sharper the wedge, the more capable the car will be of wedging itself under opposing vehicles. However, the largest thickness of the vehicle must stay the same to leave space for the motor and gear box.)

14. Vehicle at the top of the ramp. (a) Bottom of vehicle hits the ramp; (b) vehicle with larger wheels; (c) a shorter vehicle.
6 Revise Again

Your teammate suggests keeping the wheels and shape of the wedge the same and simply moving the rear wheels forward, as depicted in Figure 15. You rebuild the vehicle by moving the rear shaft mount forward, and you test the vehicle again. The redesigned vehicle no longer bottoms out on the ramp, and you claim success. Your professor sees your design changes and suggests that you test your vehicle under more realistic conditions. For example, what will happen when another vehicle rides over the top of your wedge-shaped body? You proceed to simulate such an event by placing a weight at various positions on the top of the car. The results of these additional tests suggest that moving the wheel location may not be the best solution to your problem. When you move the rear wheels forward, you change the base of support for the car's center of gravity. You discover that if an opposing vehicle rides over the top of your car, the net center of gravity moves toward the rear, eventually causing your car to topple backwards, as depicted in Figure 16.

15. Moving the rear wheels forward.

16. Weight of opposing vehicle on top of rear end causes car to topple backwards.
7 Reality Check

These discoveries and setbacks may seem discouraging, but they are a normal part of the design process. Some things work the first time, while others do not. By observing and learning from failure and by building, testing, revising, and retesting, you can converge on the best solution that will meet your needs.

8 More Revisions

After some thought, you decide that increasing the size of the wheels may be the best option after all. Your teammate points out that you can simply change the ratio of the gear box to preserve the net mechanical advantage between the motor and the wheels. This change will allow you to accommodate larger wheels. You buy some new wheels and try them with success. With the rear axle moved to its original location and the larger wheels in place, your car no longer bottoms out on the ramp.

You next consider the motor that will provide mechanical power to the drive shaft. Motors of all sizes and voltage ratings are available, including some alternating current (ac) motors, as well as direct current (dc) motors. Given that your car will be powered by batteries, your obvious choice is a dc motor. What voltage rating should you choose? You find no motors rated at 9 volts at the local hobby shop. “I don't think anyone makes 9-volt motors,” says the salesperson behind the counter. You look up the Web pages of vendors,¹ scrutinize several catalogs, and find motors rated for 3, 6, 12, or 24 volts, but no 9-volt motors. Your professor explains that the rating of a motor specifies its operating voltage for continuous use. If a lower-than-rated voltage is used, the maximum torque available from the motor will be reduced. If the motor is connected to a higher-than-rated voltage, the excess current will heat the windings inside the motor and possibly damage it. During the competition, however, the motor will be energized only for about 15 seconds at a time. This interval may be short enough to allow a larger-than-rated voltage to be applied without damaging the motor. The feasibility of this intentional overloading must be verified by testing or by contacting the motor's manufacturer.

After hearing your professor's explanation, your team decides to purchase several different motors rated at 3 volts and 6 volts. You test each one by connecting it to a variable voltage supply and measure the current flow at several values of applied voltage.

You compute the power flow by multiplying the applied voltage by the current that actually flows to the motor. (Electrical power equals voltage times current.) You devise the apparatus shown in Figure 17 to measure the mechanical power delivered by the motor. Your contraption is a crude version of the industry-standard Prony brake used to measure motor torque.² The frictional rubbing of the weighted loop of string applies a mechanical load to the motor. You power each of your motors at the same voltage and add weights until the motor stalls. The motor that sustains the largest weight before stalling will have the highest torque. You check your mechanical loading measurements against your electrical measurements and use your data to determine which motor gives you the most mechanical torque when energized by a 9-volt power source.

17. Simple apparatus to measure motor torque.
As suggested by your professor, you and your teammate each keep a design notebook in which you record all your design decisions and the results of all your experiments and tests. Figure 18, for example, shows a page from your notebook in which you've recorded a list of the motors in your collection plus the results of the mechanical loading tests. You've also made a sketch of the loading apparatus of Figure 17 in your notebook. It's important to record the characteristics of the motors that you don't use, just in case your need to reconsider one of the rejected motors during a subsequent design revision. As your design proceeds, you record all calculations, specifications, and sketches pertinent to the drive train, including gear ratios, electrical power consumption, wheel diameters, weight of each part, and construction techniques. Your objective is to have a complete record of your design activities by the time you enter the vehicle competition.

18. Page from lab notebook that documents mechanical loading tests on various motors.

Practice!

Make a two-column list that outlines the advantages of the various power sources for the Design Competition.
Make a list of additional propulsion mechanisms not mentioned in this section that could be used to drive a Design Competition vehicle.
Make a two-column list that outlines the advantages of using gravity and friction versus an applied brake as a stopping mechanism for the Design Competition.
Determine the minimum energy needed to lift a vehicle of the maximum allowed weight (2 kilograms) from the bottom of the ramp to the top.
How much electrical energy (in joules) is needed to exert one newton of force over a distance of 1 meter (m)?
How much electrical power (in watts) is needed to exert one newton of force on a body over a distance of 1 m for 10 seconds?
Determine the number of turns per centimeter (cm) of wheel diameter that will be required to move a vehicle from the bottom of the ramp to the top of the ramp in the Design Competition.
Determine the diameter of the wheels needed to move a vehicle from the bottom of the ramp to the top in the Design Competition with 50 turns of the drive axle.

1 See, for example, www.robotics.com or www.hobby-lobby.com

2 The Prony brake was invented in 1821 by Gaspard de Prony, a professor and examiner at the Ecole Polytechnique in France, as a way to measure the performance of machines and engines.