At its most basic, the Segway 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. (See How Gyroscopes Work to learn more).
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 gets the same effect with a different sort of mechanism. Segways 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.
A typical solid-state silicon gyroscope consists of a tiny silicon plate mounted on a support frame. The silicon particles are moved by an electrostatic current applied across the plate. The particles move in a particular way, which causes the plate to vibrate in a predictable manner. But when the plate is rotated around its axis (that is, when the Segway rotates in that particular plane), the particles suddenly shift in relation to the plate. This alters the vibration, and the change is in proportion to the degree of rotation. The gyroscope system measures the change in vibration, and passes this information on to the computer. In this way, the computer can figure out when the Segway is rotating along particular axes. (Check out this site for more information on solid-state silicon gyroscopes).
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. Additionally, the Segway has two tilt sensors filled with electrolyte fluid. Like your inner ear, this system figures out its own position relative to the ground based on the tilt of the fluid surface.
All of the tilt information is passed on to the "brain" of the vehicle, two electronic controller circuit boards comprising a cluster of microprocessors. The Segway has a total of 10 onboard microprocessors, which boast, in total, about three times the power of a typical PC. Normally, both boards work together, but 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 Segway requires this much brain power because it needs to make extremely precise adjustments to keep from falling over. In normal operation, the controller boards check the position sensors about 100 times per second. The microprocessors run an advanced piece of software that monitors all of the stability information and adjusts the speed of several electric motors accordingly. The electric motors, which are powered by a pair of rechargeable nickel metal hydride (NIMH) or Lithium-ion (Li-ion) 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 handlebar control 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.
This is certainly an amazing machine, but is it really as important as the Internet, as some have claimed? In the next section, we'll see what sort of impact this machine might have on the modern world.