Whatcha-ma-ThumperAn Interactive Musical ToyCreators: Nathan Wilcox and Jim CarlsonThings That Think, Spring 20041. IntroductionThe Whatcha-ma-Thumper is an interactive mechanical percussionist, which responds totiming of the user waving their hands over light sensors by playing various instruments ata related tempo. There are four instruments: a drum, a triangle, a clapper, and astrummer. Our concept was to create a musical toy that would engage the user bothvisually and through sound, combining familiar elements in an unfamiliar way.2. Mechanical elementsEach instrument has an mechanical apparatus that is mostly separate from those of theother instruments, which allowed for parallel and independent development and testing.The goal of these separate units is to make musical sounds from their associatedinstruments. The following sections describe the mechanics of each of these playermodules.2.1. The DrumThe goal of the drummer module is to strike the drum once when given the appropriatesignal from the Cricket. There are three main mechanical components that accomplishthis task: the hammer, the cam, and the motor.The hammerThe drum is struck by a hammer that ismounted on an axle. This allows one degreeof freedom: the head of the hammer maymove up or down. The resting positioncomes from the balance between gravity(pulling the head down) and a springopposite the head. If the hammer head islifted (or the tail is pushed down) and thenreleased, the weight of the head swings itdown until the tail contacts the spring. Thehead bounces a few times until it comes torest.The ideal operation is for the head to strike the drum only once upon the initial bounce.The spring, which is used to counteract the downward motion of the hammer, was madeadjustable for this reason. The spring is mounted in such a way that it can be raised orlowered slightly to accommodate the height of the drum, allowing this motion to be finetuned.The camIn order to cause the hammer head to be lifted and dropped using a simple motor, a snailcam is mounted on an axle such that it can press down on the tail of the hammer. As thecam turns it pushes the tail down, lifting the hammer’s head, until the cam's tooth passesthe end of the tail, at which point the tail is released and gravity causes the head to dropback down.One unanticipated issue in our design was that the cam was slightly too close to thehammer's tail. As the tail’s resting position was raised (by adjusting the spring), thetorque required of the cam to move the tail increased. The adjustability of the springunexpectedly affects the cam's operation, and not just the resting position of the hammerhead. Due to this problem, the resting position of the hammer head is now much moreconstrained that we had initially hoped. Fortunately, this did not prove to be a majorproblem, since we were able to adjust the height of the drum itself to accommodate thelimited range of motion.The motorWe had initially attempted to mount the motor at the base of the hammer mechanism,between and below the two main axles, using a rubber band as a friction belt to drive thesnail cam. Unfortunately, the rubber band pulled the cam's axle too tightly against themounting boards, increasing friction on the axel to the point where the rather weak motorcould no longer overcome it.Eventually we decided to do away with this extra mechanical problem by making themotor drive the cam axle directly. By making the motor coaxial with the cam, thefriction placed on the axel was reduced, and the motor was able to drive the cam withoutdifficulty.2.2. The ClapperThe clapper instrument consists of two broad flat pieces of wood(a.k.a. wings) that make a percussive noise when slappedtogether. To facilitate this, one wing is stationary, while theother can rotate a few degrees around an axle. The mechanicalplayer thus needs to move the mobile wing away from thestationary wing, then quickly slap it back to create the clappingsound. Apart from the mobile and stationary wings, themechanical components which execute a clap are a cam, a motor,and a rubber band belt connecting them.The mobile wingThe flat, wooden plane of the mobile wing is connected to acurve copper tube, at the end of which are a set of lead weights.The middle of the tube is mounted on a wooden disk, which hasa central hole through which the axel passes. The lead weightson the end of the tube act as a counterbalance to the weight ofthe wooden wing, so that in its resting position the mobile wingis pressed against the fixed wing.The camA snail cam is used to press out and up on the mobile wing, so that the two wings aredrawn apart. As the cam passes its peak, the counterbalance weights cause the mobilewing to snap back to its resting position against the fixed wing. This is the same basicdesign that is utilized by the drum player.The motorThe motor that drives the clapper is attached to the framework below the entire clappingmechanism. A rubber band provided a friction belt between the axles of motor and cam.2.3. The TriangleThe triangle is a simple suspended metal triangle, withone corner disconnected to allow reverberations. Themechanical playing mechanism strikes the triangle withsmall metal mallets.These mallets are attached to a wooden drum which,when turned via a rubber band belt (similar to thearrangement used for the clapper), strike the triangle.The triangle is suspended from the top of the automatonby thin elastic bands, allowing the triangle to resonatewhen struck, but holding it in place so that it can reliablybe struck again.2.4. The StrummerThe strummer is made from a taughtelastic string attached to a metal canwhich acts as a resonator. When thestring is plucked, a light twang emergesfrom the can.The strummer is plucked by a series ofplastic fins which are mounted on awooden drum. The drum itself isconnected by a rubber belt to a motor,so that when the motor turns, the drumrotates, and the fins pluck the string.3. Computational and Electronic ElementsIn addition to the motors, which drive the instrument player modules, the other electroniccomponents include the crickets for computation and sensors for interaction andfeedback.3.1. User interaction sensorsRather than using switches that rely on physical contact, three photocells were used toallow the user to control the actions of the instruments. The user can trigger changes intempo by waving his or her hands over the copper tubes in which the light
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