ABC Wendesday for the letter "S" is a fun one - the Segway! I've never ridden one yet, but plan to sometime within the next year - they give tours at Disney World on them and there are also places around our town that give nature tours on the off-road version of the Segway (yes, they make an off-roading Segway, and also a golfing Segway to carry your golf bag and scorecard, but other types as well).
The Segway PT is a two-wheeled, self-balancing electric vehicle invented by Dean Kamen and unveiled in December 2001. It is produced by Segway Inc. of New Hampshire. The name "Segway" is a homophone of "segue" (a smooth transition, literally Italian for "follows"). PT is an initialism for personal transporter while the old acronym HT was an initialism for human transporter.
Computers and motors in the base of the device keep the Segway PT upright when powered on with balancing enabled. Users lean forward to go forward, lean back to go backward, and turn by using a "Lean Steer" handlebar, leaning it left or right. Earlier HTs used a twist grip to steer by twisting the grip left and right. Segway PTs are driven by electric motors at up to 5.6 m/s (12.5 mph/20 km/h). Gyroscopic sensors (see vibrating structure gyroscope) are used to detect tilting of the device which indicates a departure from perfect balance. Motors driving the wheels are commanded as needed to bring the PT back into balance. Segways do not have or need mechanical brakes; this makes their use illegal on public roads in jurisdictions which classify them as some form of motor vehicle.
In laws that regulate it, the applicable category is sometimes called "electric personal assistive mobility device" (EPAMD). A niche that Segways have been often adopted into is transportation across military bases, warehouses, corporate campuses or industrial sites, or neighborhoods in place of walking or bicycles.
The Segway PT has been known by the names Ginger and IT in the past. The name Ginger followed the name of the project the Segway branched from: the inspiration behind the Segway PT came from the balancing technology of Kamen's innovative wheelchair, the iBOT, a wheelchair which can climb stairs, and prop itself up to balance on two wheels, thus raising the user up to an eye-level position. The first iterations of balancing technology were done in early Segway models. It was called Fred Upstairs, after Fred Astaire — hence Ginger after Astaire's regular feature film partner, Ginger Rogers.
The advance buzz about the then-unknown product was, at times, hyperbolic. Steve Jobs claimed that it would be "as big a deal as the PC". Articles were written in major publications speculating on it , . The product was unveiled 3 December 2001 on the ABC News morning program Good Morning America 
In September 2003 the Segway PT was recalled because of the possibility of some PTs not being able to provide enough propulsive power to maintain balance while still in motion, particularly when the batteries are near the end of charge, allowing the rider to fall. With a new software patch to version 12.0, the PT will automatically slow down and stop in response to detecting that the battery's power capability is low, thus allowing a rider to dismount safely.
In August 2006, Segway discontinued all previous models and announced new second generation designs that upgraded many elements of the previous transporters. The Gen II PT, marketed under the two product lines, i2 and x2, allows users to steer by leaning the handlebars to the right or left, which matches the intuitive nature of leaning forward and backward to accelerate and decelerate. Another feature is wireless InfoKey access.
In September 2006, all 23,500 of the vehicles were voluntarily recalled. A glitch within the "speed limiter" created a risk of causing a rearward torque to be suddenly applied to the wheels, throwing off its rider. Segway released a patch to its software (version 14.2) to resolve the problem.
The invention, development, and financing of the Segway is the subject of a narrative nonfiction book, Code Name Ginger (in paperback as Reinventing the Wheel), by journalist Steve Kemper.
The dynamics of the Segway PT are identical to a classic control problem, the inverted pendulum. The Segway PT has electric motors powered by batteries which can be charged from household current. It balances with the help of dual computers running proprietary software, two tilt sensors, and five gyroscopes. (The gyroscopes do not affect the balance; they are merely used as sensors.) The servo drive motors rotate the wheels forwards or backwards as needed for balance or propulsion. The rider accelerates or decelerates by leaning forward or backwards in the direction they wish to travel. On older models, steering is controlled by a twist grip on the left handlebar, which simply varies the speeds between the two motors, rotating the Segway PT (a decrease in the speed of the left wheel would turn the Segway PT to the left). Newer models enable the use of "leaning" to steer as well as move forwards or backwards.
The Segway PT is built simply to stay balanced in one place. Designed to mirror the process of human walking, if the rider standing on an initially balanced Segway PT leans forward, therefore offsetting the balance, the PT moves forward to regain balance just as in walking a leg moves forward to retain balance. With the Segway PT, changes from a balanced status are first detected by the gyroscopes, and signals are passed on to the onboard computers which then direct motors to regain balance. This process occurs about 100 times per second, so small adjustments to maintain balance occur almost immediately after the balance is offset by the rider.
The side effect of this balancing system is that as the Segway PT balances itself the entire unit changes position in the direction it has moved to restore balance. (For example, if the rider leans forward, the entire Segway PT will move forward from its original position, until the rider restores an upright position on the unit.) This is precisely how the Segway PT is controlled - the balancing and movement is essentially one combined system.
The Segway PT features a governor (speed limiting) mechanism. When the Segway PT approaches the maximum speed allowed by the software, it intentionally begins to tilt slightly backwards. This moves the platform out in front, and leans the handlebars backwards towards the rider, eventually nudging the rider to lean back slightly and slow the Segway PT down. If not for the governor, riders would be able to lean farther than the motor could ever compensate for. The Segway PT also slows or stops immediately if the handlebar of the unit (or forward bag) nudges into any obstacle.
For the rest of the article on the Segway, its history, and uses, check out: http://en.wikipedia.org/wiki/Segway
For more ABC Wedesday fun, check out Mrs Nesbitts Place.