When Dean Kamen unveiled the Segway® Human Transporter (HT) on ABC's Good Morning America, he described the machine as "the world's first self-balancing human transporter." When you look at the machine in motion, you get an idea of what he's talking about. Unlike a car, the Segway only has two wheels-it looks something like an ordinary hand truck-yet it manages to stay upright by itself.
To move forward or backward on the Segway HT, the rider just leans slightly forward or backward. To turn left or right, the rider simply turns the steering grip left or right.
The ability to balance on its own is the most amazing thing about the Segway HT, and it is the key to its operation. To understand how this system works, it helps to consider Kamen's model for the device-the human body.
If you stand up and lean forward, so that you are out of balance, you probably won't fall on your face. Your brain knows you are out of balance, because fluid in your inner ear shifts, so it triggers you to put your leg forward and stop the fall. If you keep leaning forward, your brain will keep putting your legs forward to keep you upright. Instead of falling, you walk forward, one step at a time.
The Segway HT does pretty much the same thing, except it has wheels instead of legs, a motor instead of muscles, a collection of microprocessors instead of a brain and a set of sophisticated tilt sensors instead of an inner-ear balancing system. Like your brain, the Segway knows when you are leaning forward. To maintain balance, it turns the wheels at just the right speed, so you move forward. Segway calls this behavior dynamic stabilization and has patented the unique process that allows the Segway HT to balance on just two wheels.
At its most basic, the Segway HT is a combination of a series of sensors, a control system and a motor system. In this section, we'll look at each of these elements.
The primary sensor system is an assembly of gyroscopes. A basic gyroscope is a spinning wheel inside a stable frame. A spinning object resists changes to its axis of rotation, because an applied force moves along with the object itself. If you push on a point at the top of a spinning wheel, for example, that point moves around to the front of the wheel while it is still feeling the force you applied. As the point of force keeps moving, it ends up applying force on opposite ends of the wheel-the force balances itself out.
Because of its resistance to outside force, a gyroscope wheel will maintain its position in space (relative to the ground), even if you tilt it. But the gyroscope's frame will move freely in space. By measuring the position of the gyroscope's spinning wheel relative to the frame, a precise sensor can tell the pitch of an object (how much it is tilting away from an upright position) as well as its pitch rate (how quickly it is tilting).
A conventional gyroscope would be cumbersome and difficult to maintain in this sort of vehicle, so the Segway HT gets the same effect with a different sort of mechanism. Segway HTs use a special solid-state angular rate sensor constructed using silicon. This sort of gyroscope determines an object's rotation using the Coriolis effect on a very small scale.
Simply put, the Coriolis effect is the apparent turning of an object moving in relation to another rotating object. For example, an airplane traveling in a straight line appears to turn because the Earth is rotating underneath it.
The Segway HT has five gyroscopic sensors, though it only needs three to detect forward and backward pitch as well as leaning to the left or right (termed "roll"). The extra sensors add redundancy, to make the vehicle more reliable. All of the tilt information, as well as information from additional tilt sensors, is passed on to the brain of the vehicle. The brain is made up of two electronic controller circuit boards, comprising a cluster of microprocessors. The Segway HT has multiple onboard microprocessors, which boast, in total, about three times the power of a typical PC. The vehicle requires this much brain power because it needs to make extremely precise adjustments to keep from falling over. If one board breaks down, the other will take over all functions so that the system can notify the rider of a failure and shut down gracefully.
The microprocessors run an advanced piece of software that controls the vehicle. This program monitors all of the stability information coming from the gyroscopic sensors and adjusts the speed of several electric motors in response to this information. The electric motors, which are powered by a pair of rechargeable nickel metal hydride (NiMH) batteries, can turn each of the wheels independently at variable speeds.
When the vehicle leans forward, the motors spin both wheels forward to keep from tilting over. When the vehicle leans backward, the motors spin both wheels backward. When the rider operates the steering grip to turn left or right, the motors spin one wheel faster than the other, or spin the wheels in opposite directions, so that the vehicle rotates.