Lean on me

Feb. 22, 2007
The Segway Personal Transporter now uses body language to control steering.

Ivan Masek
Novotechnik U.S.
Southborough, Mass.

The introduction of the Personal Transporter (PT) in 2001 by Segway Inc. created great excitement. It was the product that everyone wanted to try. The second-generation PT, unveiled last August, breaks new ground with the addition of LeanSteer technology.

Riders steered the first PT with a twist of the wrist. LeanSteer gives a more intuitive steering control that is almost instinctive.

Whether riding a bicycle, running, or walking, a person instinctively leans in the direction they want to go before actually moving in that direction. The PT now senses that lean and reacts by turning in the corresponding direction. It's as if the rider wears the PT rather than drives it. The reaction is almost like steering by mere thought.

The PT combines microprocessor controls, sensors, and feedback loops with wheels and motors in an operation described as "active mechanics in everyday use." Together the components form a tightly integrated system using Segway Smart Motion (SSM) and LeanSteer.

SSM dynamically stabilizes the PT so it self-balances even as the rider mounts or dismounts. It's this ability that distinguishes the Segway from other two-wheel transportation systems.

Bicycles and traditional scooters typically need forward speed for balance. But the PT maintains its balance even when standing still. Segway's model for the PT's dynamic stabilization is the human body. Most people walk without conscious thought. They don't think, "left foot, right foot, left foot." Segway's goal was to make PT operation just as instinctive, for the best user interface is no user interface.

Chances are a person won't fall on their face when they stand up and lean forward because their brain knows they're out of balance. Fluid in their inner ear shifts, triggering a signal to put one leg forward for stability. If they keep leaning forward, their brain keeps placing one leg ahead of the other. They begin walking forward one step at a time.

The Segway PT works in a similar fashion, though it has wheels instead of legs, motors instead of muscles, and a collection of microprocessors instead of a brain. Sophisticated tilt and gyroscopic sensors monitor its balance just like the inner ear. As the rider leans forward, the PT starts rolling in that direction. When they lean back it slows down, stops, or even backs up.

As the PT approaches top speed, the handlebar pushes back against the driver. This gentle nudge forces the driver to straighten up and keeps the PT from going too fast.

Instead of twisting a control on the handlebar, LeanSteer adds the same lean-to-control idea to steering the Segway. Unlike a bicycle where the handlebar turns to go left or right, the Segway rider leans in the direction of desired travel. "Lean left to go left, lean right to go right," states Mike Gansler, Segway product development and systems engineer.

The secret is in the new control or handlebar shaft. As riders lean, they tilt the Segway handlebar and control shaft in the direction they're leaning. The PT responds to that input by turning in that direction. It has been likened to skiing without snow. It's the mechatronics in the PT that keeps the rider from falling.

The handlebar shaft acts as a lever exerting force at the pivot on the base where the rider stands. A Novotechnik SP 2800 Series position sensor continuously measures the angle of the handlebar shaft. It sends the realtime data to both the balance and LeanSteer systems. The measurement spans an 80° range centered on the upright position of the handlebar shaft.

Segway's Gansler says the key challenges in creating LeanSteer were the selection of a position sensor and developing the pivot assembly for the handlebar shaft. The position sensor had to be highly reliable and accurate. At the same time the pivot assembly had to withstand high transmitted forces yet still feel smooth and friction-free to the user. A rubber bushing attached to the shaft assures smooth pivot performance. The bushing deforms as the rider leans from one side to the other, pivoting the shaft left or right. This imparts a light return-to-center force to the handlebar.

The new design eliminates all wiring from the handlebar shaft, replacing it with wireless technology. Riders can unlock and start the Segway, arm or disarm the security system, and check battery life, speed, and performance using the wireless InfoKey controller. The controller is small enough to slip into a pocket; but, when the user is ready to roll, it docks on the LeanSteer frame.

The LeanSteer Segway Model i2 hits a top speed of 12.5 mph and can travel up to 24 miles, or about 480 city blocks, on a single charge of its Saphion lithium-ion batteries. The Segway is environmentally friendly with zero emissions and an energy efficiency equivalent to 450 mpg.

The i2 weighs just 105 lb. Its adjustable handlebar eases stowing in the trunk of a car. One feature helps keep the machine water resistant. Segway engineers use the machine's motor to generate alarm sounds eliminating a speaker and its sound vent holes.

Another PT, the x2, is designed especially for the off-road. It sports a wider track than the i2 and has all-terrain 21-in. tires to travel confidently over dirt, gravel, grass, or sand.

Vital to the LeanSteer system is the compact position sensor that measures the angle of the handlebar shaft as it moves side to side.

The angle of the handlebar shaft effectively matches the rider's body angle. The PT uses this angle information to generate the lateral acceleration needed to balance the tilted body of the rider.

Sensor reliability and accuracy are critical to the safe operation of the transporter. The sensor must maintain that reliability over an extended lifetime while sensing at high-bandwidth rates to support the real-time control system.

Novotechnik produced a potentiometer-based sensor integrating two independent measuring systems in a single housing with a diameter of 28 mm. The SP2880 sensor features dual-tracks, redundant multifingered precious-metal wipers, and dual output to ensure uninterrupted operation. It delivers the absolute tilt-angle of the handlebar shaft as a continuous proportional output to steer the Segway while it maintains balance.

The SP2880 measures 17 mm from flange to back and has a push-on shaft coupling for quick installation. The sensor is rated for an extended life of more than 50 million movements with a resolution of less than 0.01° over a 140° electrical range. Linearity of the sensor is ±0.3% with a repeatability ±0.03%.

Engineered to resist vibration, the sensor uses a fiber-filled, temperature-resistant, and dustproof plastic body sealed to an IP65 rating. It's designed for reliable operation in temperatures from –40 to 150°C. Sensor mounting flanges contain elongated holes that simplify installation and allow easy mechanical adjustments. The push-on mechanical coupling simplifies linking the sensor to the handlebar pivot and also minimizes any hysteresis from excess play.

If the PT traveled only on flat surfaces, then any leaning by the rider would command a turn. However, a person may "lean" while standing upright as when the Transporter travels along a sloped bank or hits a bump in the road. So the balance sensor assembly (BSA) must sense the angle of the terrain as well as the rider's body position. The dual-controller system has two digital signal processors, two dual-axis accelerometers for tilt-angle measurement, and five angular-rate sensors using MEMS-based gyroscopes.

The gyroscopic angular-rate sensors monitor rotation by measuring acceleration from Coriolis effect. A micromachined oscillating accelerometer within the angular-rate sensor experiences this effect when it is rotated. The output of the accelerometer is then processed to provide the rate of rotation.

The BSA detects forward or backward lean, left and right lean, and rotation left or right. These motions are often referred to as pitch, roll, and yaw respectively. It checks, cross-checks, and acts on all inputs 100 × a second — many times faster than a rider can react.

Cross-checks assure that the BSA is operating properly. If one controller board, or its associated battery, motor windings, or wiring breaks, the second controller takes over all functions. The system notifies the rider of the failure and shuts down gracefully bringing the rider to a safe stop.

Novotechnik U.S., (508) 485-2244,
Segway Inc., (866) 473-4929, segway.com

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