How did Rosie the Robot, the beloved cyber-domestic on The Jetsons, balance on those three tiny wheels? Well yes, I know, she's a cartoon, but that's not really the point here, is it? I've noticed that the idea of a robot servant with magical balance seems to have real staying power. There is, for example, the very recent example of Flo the waitress droid in "Attack of the Clones." Obviously when George Lucas was a teenager he used to watch Hazel, and now he's borrowed his future from Shirley Booth by way of Hannah-Barbera. Who knew? Anyway, back to the original question: how do female working-class robots in the food services industry balance like that? I've been inspired to try to make something that would do the same. Rather than make a full fledged house-hold robot maid, complete with voice-chip and bourgeois-baiting working-class chutzpah, I figured I'd start with simple things - a robot that rolls around on one wheel, or a table that balances on three or four tiny wheels. You could put your glass of Perrier (or can of Budweiser) on this table, and have it roll across the room to serve your guests. As you put things on the table and took them off again, the table would magically stay poised and balanced. Why? Because it's cool for a serving table to be smart enough to balance itself on closely spaced little wheels (just like Rosie). In that spirit, here are some research projects along these lines...

Keeping the weight centered:

Inside the tabletop will be several little counterbalancing mini-vehicles, each of which runs around the inside the tabletop so as to keep the weight distribution balanced.

Between the tabletop and the support pillar beneath it is a set of relative tilt sensors. An imbalance in these sensors causes the mini-vehicles to roll "uphill", so as to keep the weight even.

Each mini-vehicle is really simple: just a motor and a battery. When switched on, the motor turns a little wheel, which makes the motor move within the table. If the motor is with positive, the mini-vehicle goes one way; if driven with negative voltage, it goes the other way. There are several possible arrangements for the sensors and mini-vehicles. I'll describe some of them.

Click on this image to see one method

Click on this image to see another method

Keeping the table from toppling over when it moves

Source: tilt.java

When the table moves, a torque will be developed (because of the forward push on the base) that will tend to make the vehicle fall over backward. We need to counter this force. One way to do this is to actively tilt the table base forward, just enough so that the force on the table is always pointing straight down the now slanted support pillar. To do this, we can hang a pendulum on a rigid rod inside the (hollow) support pillar. A relative tilt sensor detects whether the pendulum is tilted with respect to the support pillar (which will happen when the table accelerates or decelarates). This information is used to power a motor which tilts the base so as to align the base with the pendulum: When the pendulum pushes backward, the base is tilted forward; when the pendulum pushes forward, the base is tilted backward. In the applet on the left, which shows a schematic side view, drag your mouse left or right to move the table. Watch how the table tilts as it moves.

I started here asking the question: If a robot could only balance on one wheel (essentially a single point), how could that robot stay upright? Clearly there is a need for very fast, responsive active control. There are some precedents. The "parallel bicycle" developed in 1987 by Kazuo Yamafuji (far left), and its famous recent variant the Segway, developed by Dean Kamen (near left), both balance on two wheels. The vehicle rocks forward and back under active control to effect a sort of balancing pencil trick. But what about one wheel? How do you stop a single wheel from falling sideways? Click on the sketch below to find out.