When Dean Kamen unveiled the Segway® Personal Transporter (PT) 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 PT only has two wheels, yet it manages to stay upright by itself.
To move forward or backward on the Segway PT, the rider just leans slightly forward or backward. To turn left or right, the rider simply moves the LeanSteer frame left or right.
How dynamic stabilization works
The ability to balance on its own is the most amazing thing about the Segway PT, 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 PT 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 and gyroscopic sensors instead of an inner-ear balancing system. Like your brain, the Segway PT 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 PT to balance on just two wheels.
The brains and the brawn
The Segway PT is controlled by an intelligent network of sensors, mechanical assemblies, propulsion, and control systems. The second you step on, five micro-machined gyroscopic sensors and two accelerometers sense the changing terrain and your body position at 100 times per second – faster than the brain can think.
Segway PTs use a special solid-state angular rate sensor constructed using silicon. This type of gyroscopic sensor determines an object's rotation using the Coriolis effect on a very small scale.
Simply put, the Coriolis effect is the apparent turning of a moving object in relation to another rotating object. For example, an airplane trying to travel in a straight line can appear to turn because the Earth is rotating underneath it.
The Segway PT has five gyroscopic sensors, though it only needs three to detect leaning forward or backward (termed “pitch”), leaning to the left or right (termed "roll") and steering to the left or right (termed “yaw”). The extra sensors add redundancy, to make the product more reliable. All of this leaning and steering information, as well as information from additional tilt sensors, is passed on to the brain of the device.
The brains and brawn are made up of two identical and redundant sets of microprocessor-based electronic controller circuit boards, batteries and motor windings that operate together and share the load of driving the wheels. The Segway PT has a number of additional onboard microprocessors. The vehicle requires this much brain power because it needs to quickly make precise adjustments to keep from falling over. If one controller board (or it’s associated battery, motor windings or wiring) breaks down, the other set 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 Segway PT. This program monitors all of the stability information coming from the gyroscopic sensors and adjusts the speed of the electric motors in response to this information. The electric motors, which are powered by a pair of rechargeable lithium-ion batteries, can turn each of the wheels independently at variable speeds.
When the PT leans forward, the motors drive both wheels forward to keep the PT from tilting over. When the PT leans backward, the motors drive both wheels backward. When the rider moves the LeanSteer frame to turn left or right, the motors drive one wheel faster than the other, or if traveling slowly enough, drive the wheels in opposite directions, so that the Segway PT rotates (steers). When under way, the Segway PT will generate precisely the right radius of turn so that your lean is balanced by the centripetal acceleration generated by the turn.