The project that we have decided upon, after extensive "thought experimentation" with other ideas, is one that our group had discussed previously in theory but not fully defined. Explicitly, the project concept is the design of a tracking device that would, once it had the target "in its sights", be able to accurately fire a projectile that would hit the target. There would be emphasis on programming and analysis of sensor feedback to control the rotation of the tracking device, as well as the challenge of constructing the mechanical gearing of the tracking station to achieve a desired speed and position. In addition, basic concepts of projectile motion and potential energy in a spring would need to be understood and incorporated by the freshmen students in development of the firing mechanism. Though not explicity geared toward one single field of engineering, definite aspects of programming, mechanics, problem solving and teamwork will be required of students attempting this project.
The basic schematic of the project's physical layout is shown in Figure 1. The target -- a small receptacle attached to a larger backboard -- is attached to circular platform of thin, light, yet fairly sturdy material (yet to be determined) which has a concentric circle removed from its center to allow room for the stationary tracking station. Rotation of the target and platform will be achieved by use of one of the provided motors, programmed either ahead of time by the project developers or by the lab groups with assistance, to make use of a random number generator to control motor speed and direction. Gears could be attached to the motor shaft that would interlock with a ring of "teeth" attached to the platform, in essence making the platform a large gear. The radius of the platform will need to be determined by the design team based upon such considerations as sensor sensitivity and the accuracy and distance with which a projectile can be fired. The main physical design portion of the lab would be the construction of the tracking device, using the parts supplied in the Lego kit, that must meet the following requirements:
1. Be free-standing and structurally sound and, making use of the motors provided, be able to rotate in a controlled and quantifiable manner 360 degrees.
2. Using feedback from the given phototransistor sensors, which detect changes in the intensity of incoming light at a specific frequency, first locate (in one dimension) and then "hone in" on the target, which will be moving at varying speeds and alternating directions at a set distance away. Our preliminary determination is that two photo-resistor sensors would accomplish the task of both determining the target's presence and also which direction the object is moving.
3. Upon successful pinpointing of location and velocity, fire a projectile (possibilities include a ball bearing or a ping-pong ball) at the target. A successful shot will propel the ball into the mounted hoop.
To enable students to track the target we propose the following setup: mount a colored light source upon the tracking device to "broadcast" light in all directions, which would be reflected back from a mirror mounted at the appropriate level on the target. The intensity of the light detected by the sensors would indicated the position of the target relative to the direction the sensors were pointed, since the maximum intensity value would be achieved when the target was directly in front of a sensor. If the reflected light proves too difficult to sense, a second possibility would be to mount a light on the target itself which could be sensed by the phototransistors. A reflected beam would seemingly be more "real", resembling radar or sonar, and for this reason the design team prefers this option. However, a target-mounted light source may prove easier to detect, since small variations in mirror angle could make the former option less reliable. In addition, if target-mounted, only one light source would need to be purchased, whereas a reflected light would need to be purchased for each lab group to incorporate into their design.
1. Encourage emphasis on the engineering method -- identification of problem, developing solutions, and evaluating and testing these solutions for effectiveness.
2. Encourage teamwork and drawing upon the resources of all group members to achieve the most effective and innovative means of construction.
3. Provide opportunity for first-hand observation of the basic equations and laws of projectile motion involved in aiming the "gun".
4. Introduce elementary concepts of energy conservation and transfer, i.e. from potential to mechanical if a spring is incorporated into the launching mechanism.
5. Demonstrate C programming, and the integral role of programming in engineering of absorbing feedback from the environment and providing adequate physical response.
6. Provide the opportunity to develop an understanding of motors and gearing.
Additional equipment required:
- Concentrated source of red, visible light, since the phototransistors are tuned to a wavelength near visible red. The number needed will depend upon the tracking scheme implemented.
- Material to construct the rotating platform, including thin board and gear teeth.
- Small hoop and mirror for the construction of the target
- Small spring for each kit
- Small number of projectiles for each kit
Two measures of performance are envisioned by which the design skills and basic understanding of each group could be tested in a competitive format. First, each group could be given a limited number of projectiles to be fired, and part of the score would be a measure of how many actually made it into the hoop. Second, a more efficient and well-written code would enable the tracker to "lock-in" on the target more quickly, suggesting that a second score be given as a measure of how quickly the projectiles were fired. To eliminate the temptation some might have to fire at will in order to win the speed race, the accuracy test should be more carry a heavier weighting in the final score.
The development of a one-dimensional tracking station capable hitting a target with a projectile introduces several key aspects of engineering in general -- group work, creative problem solving and innovation, and making use of limited materials. In addition, basic concepts of physics and mechanics, as well as a few of the capabilities of computer programming, are introduced. Finally, the concrete measure of results, through competition, provides motivation to come up with the best design, in addition to enabling concrete determination of success or failure.
