Princeton University - Council of Science & Technology

POPE PRIZE

Essay 1
A Sense of Time in the Fife Coalfields
Emily D. Johnson

Essay 2
Soapbox Derby for the Twenty-First Century
Adam Ruben

Soapbox Derby for the Twenty-First Century

In a bright, cluttered basement, an electrical hellhole of multicolored wires and tiny metal parts, intently, unblinking, Kinari Patel holds a steaming tool in her hand, presses it against a metal stick, and applies it to a thin wire. She is soldering the wire.

I know the verb "soldering." Beyond that, I have no idea what she's doing.

"This plugs into the computer," she explains. "The wires always fall out, so I have to solder them back together."

I would never, ever trust myself to plug anything homemade into an appliance, especially an expensive laptop computer. That sort of thing is better left to people who know what they're doing. But Kinari knows very well what she's doing.

She has to. For the last three months, she and her partner Rebecca Jones have been building a "smart" model car from raw parts. In the process, they have learned about everything from data processing to light detection to steering.

Kinari, Rebecca, and fifteen other pairs of students comprise Electrical Engineering 302, one of the most innovative-and time-consuming-courses at Princeton University. In February, on the first day of class, each pair of partners chooses a remote-controlled car from a commercial catalog. The goal: Rip out the car's guts, replace them with homemade guts, and teach the car to follow a black stripe on the floor as fast as it can.

The pairs each receive $150 to buy the kit and an additional $125 for parts thereafter. These parts come from a special vending machine in the basement that one student described as "an electrical engineer's dream." Simply log on to the web interface on a nearby computer, swipe your student ID through the machine, and it spits out the part you request and deducts money from your "account."

As soon as the students receive their kits, they assemble the car, make sure it works, and begin to dismantle the interior. Over the spring semester, they assemble the "system board," an on-board computer that communicates with the car, controls the steering, and reads and processes information from sensors to be mounted on the front bumper.

Most of the data that the car uses is pre-programmed via a special machine onto a ROM (Read-Only Memory) chip on the system board. Some of the parameters, though, can be fed in on the fly using a laptop cable connection. These parameters include information about how to steer, how to interpret the sensor data, and how fast to go. Typically, says Kinari, one partner focuses on writing

Once the students get the ROM to correctly store programs, they install a sensor to measure the car's speed. Usually they use a bunch of magnets embedded in the back wheel with a nearby Hall Effect Sensor, a little gadget that sends out a pulse every time a magnet goes by. So the faster the car goes, the faster the magnets spin, and the faster the Hall Effect Sensor tells the car it's going.

Now that the students can program the car to control its speed, they hook up a "steering servo" to the front, a tiny motor that can position-control the wheels. The servo sends data back to the circuit board about the position of the front wheels, telling the car the angle at which the wheels are turned. The circuit board correlates this data with information from the Hall Effect Sensor, causing the car to, for example, slow down around tighter turns.

The program stored in the ROM chip, then, dictates entirely how the car will behave. If the steering servo and Hall Effect Sensor are the car's arms and legs, the ROM chip is its brain. While I talked to Kinari, another student ran into the room.

"Chris just crashed his car," he said. "He took the ROM chip out and forgot to put it back in!" Essentially, the car had no idea where to go, like a goldfish suddenly dropped in the middle of a meadow. For lack of anything better to do, it crashed into a wall.

Tonight, the basic construction process is behind them. It is 1:00 AM. The students work feverishly, buried in their basement smorgasbord of wires and parts, to finish their cars before the race tomorrow.

There are two races. The later one is basically a performance for the public; roommates come to cheer, food is served, and some teams even show up in matching tee shirts. But the more imminent race, the one tomorrow, is more important. It counts for a grade.

Kinari unplugs one end of the soldered wire from Rebecca's laptop and the other end from their car. "All right," says Rebecca. "Let's go kick some butt." While they run upstairs to test their car on the track, junior Afsheen Afshar explains the final component of the car.

The track on which the cars must run is not really a track at all-it is merely a curvy black stripe on a white tarp. The car must somehow "sense" where the stripe is and adjust its speed and wheel position accordingly. Therefore, some kind of sensor is affixed to the underside of each car's front bumper. And therein, he says, lies the fun.

Though there are different kinds of sensors-and students staunchly defend their choice of sensor like a religion-all operate on the same basic principle. Hovering under the front of the car like a street sweeper's brush, the sensor shines light at the tarp. White and black, the two colors present on the tarp, reflect that light to different degrees. The sensor's photodetector then measures the reflected light and sends that information back to the circuit board.

The values it sends back are not strictly black and white. They may range over a field of over 65,000 possible values. It is up to the student to determine a numerical threshold above which all is effectively white and below which all is black.

Students typically place between four and eight sensors on their cars, each telling the circuit board whether or not it is over the black stripe.

So the entire car works like this:

"Hello," say the sensors to the ground. "Are you black, or white?"

The ground answers.

"Hey," say most of the sensors to the circuit board, "we're over white." Two of the sensors, however, say, "We're over black."

"Over black, eh?" asks the circuit board. "But you guys aren't the two center sensors. The car must not be centered on the stripe."

The ROM chip looks on at this point. "Hey, ROM chip," says the circuit board, "get a load of this. Sensors 6 and 7 say they're over black! Can you believe that?"

"All right," replies the ROM chip, "listen carefully. To center the car, move the front wheels this much."

The circuit board instructs the steering servo to do so, and it does.

"Hmm," adds the ROM chip as an afterthought, "another thing. Since the car is turning, we'll want to slow down a little."

"No problem," says the circuit board, and the car slows down.

All of this repeats several thousand times each second as the car cruises down the track.

Afsheen has a slightly different idea. Rather than a row of sensors, his car has two rows, one under the front bumper, and one hanging four inches ahead. The design should allow his car to anticipate upcoming turns. However, handling all the data has been more difficult than he'd planned, so he abandoned the front row of sensors and decided to just use the back like everyone else.

"These things happen," he says, staring at a flickering green line while he presses a metal rod into his sensor. "A lot." He tells the story of one engineer who, just days prior, accidentally connected his circuit board directly to "ground" and fried the entire thing.

Sheepishly I asked what that meant and received the following analogy. The ground is an endless source of electric current, which we can think of as water. Every board, to receive current, must be plugged into the equivalent of a high-pressure fire hose. But running current into the board is like trying to run the water through a tiny drinking straw. Therefore every board uses resistors, which lessen the amount of water coming into the straw.

The unfortunate student accidentally bypassed his resistors. He jammed the straw into the end of the fire hose and turned it on full blast.

Confident that his car only needs fine-tuning, Afsheen takes me up to the track itself. The tarp, the size of a medium-capacity lecture hall, has been spread out over the floor. Near it sits a pile of shoes and sandals-any marks left on the tarp could confuse sensors and make cars steer off onto something that's not the track. Still, the tarp is seven years old and therefore far from clean. It is easy to imagine a car calling some of the white patches black, especially in shadows.

There are two identical tracks, offset about a diagonal meter from each other. Each has one long straightaway and about ten turns to negotiate. Afsheen sets down his car and turns it on. It immediately turns to the side, preferring the vast white areas of the tarp to the black stripe. He grabs the car, sets it back on the track, and tries again. No luck.

"It wasn't doing this five minutes ago," he says. Another attempt, and this time the car stays on the straight track but skips an important turn. It stops inches short of a trash can when Afsheen pulls the trigger on a plastic gun.

That, apparently, is another good feature to have: a remote kill switch. When the students disassembled their commercial kits, some left a few controls intact, operable by the car's remote control. Though they are not allowed to use this equipment in the race, instant control can come in quite handy when your car careens toward a solid object.

The lack of such a feature is apparently the reason Kinari and Rebecca are here tonight. Their car worked perfectly-they only planned to come in half an hour a day, for fine-tuning-until disaster struck. "Three days ago," says Rebecca, "it rammed into a wall. Then it rammed into another wall, then it rammed into another wall, and then it rammed into another wall until it couldn't ram any more."

Now it chugs tentatively along the track like the Little Engine That Most Likely Won't, waving its front in an increasingly vain attempt to maintain straightness.

Afsheen picks up his car and cradles it like a baby orangutan-delicately, but without signs of ease. He curses graphically at it. Across the room, two identical, tall, athletic, blond guys chuckle. Afsheen returns downstairs to charge his battery.

The two blond guys, Shaun and Xander, talk to their car like a little brother. In fact, every team talks to its car in some way, some pleading, some chatting normally, and some almost treating the car like a hostage. Still, there is a very real and almost harmonious relationship between car and operator-after the race, it seems, a custody battle may ensue between partners to determine who keeps the car.

Shaun and Xander place their car on the track. They tower over the car as they follow or lead it, dancing to stamp out wrinkles in the tarp ahead of it. The car twitches a little, the front wheels responding to the stripe like a sniffing pig. This appears to be the effect of lag time between the sensors detecting the line and the car repositioning itself accordingly.

"Don't screw up, don't screw up," they chant as the car aims for its final turn. It pulls through the turn nicely, finishing where it started. Shaun and Xander cheer and high-five.

"It's our first time around the track!" says Shaun.

"Congratulations," mumbles Rebecca from the floor, bent over her disobedient vehicle. While Shaun and Xander plug the car into Shaun's laptop, Kinari and Rebecca try their car one last time.

Slowly, shuddering, pushing its way behind a bulky wooden bumper they affixed to protect the car from a new crash, the little vehicle sweeps around each turn, mercifully completing the track. Kinari cannot hide her glee as she says, "We're not sure it's gonna do it tomorrow, but we're calling it a night anyway."

As they exit, Shaun and Xander, high on success, try their car with different parameters. This time it moves swiftly and strongly around the curved stripe, the most graceful performance of the night.

"That was great!" says Shaun.

Xander sits down at the laptop, already furiously typing. "Faster!" he cries.

Most cars round the track in 20-60 seconds. The all-time record was set just last year by Mike Lindahl, now a teaching assistant for the class, whose car zipped down the straightaway and gobbled up the curves in just 15.5 seconds.

With new numbers, Shaun and Xander try another run. The car zooms even faster than before. Xander times it.

"Twenty three and a half," he says.

Best one I've heard so far is around twenty," says Shaun.

Already back at the laptop, Xander cries, "Faster!"

Afsheen gives his car one more try too. Fingers crossed at the first corner, he commands, "Turn!" The car veers off the tarp into nothingness.

At this point there are only four students left in the room. Strangely, everyone is barefoot and wears cargo shorts; it looks like there's been a shipwreck.

Gilligan's Island with four Professors.

The next day, sixteen teams line up to test their cars. Professor Bradley Dickinson, a seated man with a good-sized white beard, keeps the time on his wristwatch and records the speed of teams' attempts on a blackboard. The teams have three hours to test their car as many times as they want to. Obviously a faster time is preferable, but as long as a student has worked solidly all semester, and as long as the car makes it around the track eventually, a good grade is assured.

The race showcases more variety and innovation than the last-minute repair session the previous night. For one thing, some of the cars actually look like cars.

This is because the commercial kits included transparent plastic shells. Some teams, just for the sake of making their car look like a car, airbrushed the shells and fit them on their cars. Now instead of a fleet of cars that look like the Mars Lander, there is a jeep, a tank, a Volkswagen bug, and several BMW's (this is Princeton, after all). Kinari and Rebecca appear with a pickup truck painted fluorescent orange.

Many teams, though, have opted to leave their circuitry exposed. In some cases, this is a stylistic preference, and in others, it came down to a lack of time for decoration. However, in a few cases, the sensor rose too high for the shell to fit.

Why would a sensor rise too high? Wouldn't it be easier for it to detect reflected light close to the tarp?

For most conventional sensors, yes. However, a few adventurous teams tried using a far more complex sensor: a video camera.

The legendary Mike Lindahl, who holds the 15.5-second record, pioneered this method last year. The current students speak of him with as much reverence as they can muster for someone only a year older. Mike obtained special permission to buy a black-and-white video camera about the size of a bottle cap. Rather than receiving data from a few sensors across the front bumper, his circuit board chose a horizontal line from the video camera and analyzed the blackness or whiteness of every pixel in that line. So instead of four or eight side-by-side data points, Mike Lindahl's car handled about two hundred.

After Mike's success last year, says Professor Dickinson, five groups are trying cameras this year, mounted high above the car's front to capture a wide area. As he speaks, a car zips with a loud crunch into a folding table, its camera absorbing most of the impact. Professor Dickinson says, "Maybe four now."

Shaun and Xander arrive grinning, equipped with new parameters to impress the class. Their car fails, though it fails with astonishing speed. Bashfully they reenter their old numbers and astound the room with a time of 23.6 seconds.

Two seniors, former students of the Car Lab, wander in to watch the trials. "This isn't a real test," says one. "They don't have big splotches of sunlight messing everything up." Apparently, the previous year, the sun shone so brightly on the day of the race that everyone who set their parameters at 1:00 AM suddenly found their impressions of "white" and "black" very different. Even worse were the patches of sunlight that streamed through windows onto the tarp. Were the tarp brighter overall, one would only have to tweak the sensor threshold; but when the brightness varies dramatically on different areas of the tarp, very little can be done to tell the car, "Now change your definitions of black and white, and quickly change them back."

After a few attempts, Kinari and Rebecca hold their breath as their car rounds the track for the second time ever. Relieved, Rebecca jokes, "Do we get the slowest prize?"

"So far!" exclaims Professor Lyon. He writes "75.9 sec" on the board.

While Afsheen struggles to tame his car, Shaun and Xander attempt one final run. As their car finishes, Professor Lyon walks dramatically over to the board, erases their previous "23.6," and writes "23.5."

Watching the race, an alumnus standing next to me says, "You know, there's a lot of luck involved. Some cars are easier to work with than others, and you never know when you're ordering your kit. It's kind of like having kids."

I nodded, not quite listening. I was watching a student whose car had no remote kill switch, sprinting, yelling, diving shoulder-first into a cement wall to save a photoelectric toddler.