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Balance
Sensor Assembly (BSA)
The BSA, supplied by Silicon Sensing Systems, is an elegantly
designed, extremely robust, and yet incredibly sensitive piece
of equipment.
This small cube, 3 inches on a side, is packed with five solid-state,
vibrating-ring, angular-rate sensors ("gyroscopes")
that use the Coriolis effect to measure rotation speed. These
tiny rings are electromechanically vibrated in such a way that
when they are rotated, a small force is generated that can be
detected in the internal electronics of the sensor. Each "gyro"
is placed at a unique angle that allows it to measure multiple
directions. Segway's onboard computers constantly compare the
data from all five gyros to determine if any of the five is
supplying faulty data--in this condition, it can compensate
and use data from the remaining sensors to continue balancing
through a controlled safety shutdown. Two tilt sensors filled
with an electrolyte fluid provide a gravity reference in the
same way your inner ear does for your own sense of balance.
The BSA is monitored by two independent microprocessors and
is split into two independent halves for redundancy. Even the
communication between sides is performed optically to avoid
electrical faults on one side propagating to the other.
Motor
The Segway HT's motors are unique in a number of respects. Produced
by Pacific Scientific, a division of Danaher, they are the highest-power
motors for their size and weight ever put into mass production.
Each motor is capable of maintaining a power output of 1.5 kilowatts--that's
2 horsepower! The motors use brushless servo technology, meaning
there are no contacts to wear, arc, and reduce performance.
The magnets are constructed of an incredibly powerful rare-earth
material: neodymium-iron-boron. Each motor is constructed with
two independent sets of windings, each driven by a separate
board and motor. Under normal conditions, both sets of windings
work in parallel, sharing the load. In the event of a failure,
the motor is designed to instantly disable the faulty side and
use the remaining winding to maintain control of the Segway
HT until it can be brought to a stop. The motor is carefully
balanced to operate up to 8,000 rpm, allowing it to produce
very high power levels in a small package. Feedback from the
motor back to the Segway HT is provided by redundant, noncontact
analog hall sensors that sense the positions of magnets with
no moving parts other than the motor shaft itself.
Gearbox
The Segway HT's gearbox, a joint effort between Axicon Technologies
and Segway, is constructed more like a precision Swiss watch
than a traditional gear drive from an automobile. A two-stage
reduction system provides a 24:1 reduction, allowing the motor
to operate at powerful, efficient speeds throughout the full
range of speeds of the Segway HT. Each gear is cut to a helical
profile, which creates a spiral engagement to minimize noise
and increase the load capability of the gears. The number of
teeth on each gear is chosen to produce noninteger gear ratios.
This means that the teeth will mesh in a different location
each revolution, maximizing the life of the gearbox. Our engineers
were so obsessed with the details on the Segway HT that they
designed the meshes in the gearbox to produce sound exactly
two musical octaves apart--when the Segway HT moves, it makes
music, not noise.
The gearbox is pre-assembled and lubricated, and is designed
to require no maintenance over the life of the Segway HT. It
has been tested for thousands of miles under severe conditions.
Wheel
and Tire
Michelin was one of the earliest partners in the Segway HT design
project. The tires on the Segway HT have been designed specifically
for this product, using a unique tread compound (a silica-based
compound instead of traditional carbon-based materials), giving
enhanced traction and minimized marking on indoor floors, and
a specially engineered tubeless construction that allows low
pressure for comfort and traction while minimizing rolling resistance
for long range.
The tire is mounted on an equally unique wheel design: the wheel
is constructed of a sophisticated engineering-grade thermoplastic
chosen in partnership with GE Plastics. This composite material
allows light weight and excellent durability, and actually reduces
noise transmitted from the drive system. The wheel is molded
around a forged steel hub, eliminating fasteners that can loosen
over time. The attachment between this hub and the transmissions
incorporates a unique (patent pending) taper and hex design,
which allows a single nut to attach or remove the wheel while
retaining the security of multiple-bolt systems more typically
seen in conventional applications.
Batteries
The Segway HT uses twin NiMH battery packs, designed in partnership
with SAFT (a division of Alcatel), running at a nominal 72 volts.
These nickel-metal hydride cells deliver the highest power of
any currently available chemistry, optimized to maintain the
Segway HT's balance under severe conditions. These are not your
cell phone batteries!
Each pack consists of an array of high-capacity cells and a
custom-designed circuit board that constantly monitors the temperature
and voltage of the pack in multiple locations. This assembly
is enclosed in another unique application of GE thermoplastics--the
battery box is sealed using a vibration welding technique that
makes the outside of the pack a single, continuous structure--sealed
from moisture and strong enough to survive the most extreme
tests our durability engineers could throw at it.
The internal electronics in the battery incorporate "smart"
charging--the customer need only plug the Segway HT into the
wall and the battery will choose the appropriate charge rate
based on temperature, voltage, and level of charge. The batteries
will quick charge, then automatically transition into a balance
and maintenance charge mode. The Segway HT customer does not
need to worry about memory or timing their charges--just plug
it in.
Under normal operation, the Segway HT carefully monitors both
batteries and automatically adjusts to drain the batteries evenly.
In the unlikely event of a battery failure, the system is designed
to use the second battery to operate the machine and allow it
to continue balancing until it is brought to a safe stop.
Controller
Boards
Two sophisticated controller boards from Delphi Electronics
provide both brains and brawn for the system. Delphi Electronics
was chosen as a partner based on their track record in the production
of high-volume, high-quality automotive electronics for such
demanding applications as airbag modules.
Each board contains a Texas Instruments digital signal processor,
monitoring the entire Segway HT system and checking 100 times
per second for any faults or conditions that might require immediate
response. It reads the information from the BSA to determine
if the rider is leaning forward or backward, and instantly uses
this information to deliver power from the batteries to the
motors through a set of 12 high-power, high-voltage field-effect
transistors (FETs). These calculations take place 100 times
a second, and the motors are adjusted at up to 1,000 times per
second, responding far more quickly than the human body is capable
of perceiving. Although each board is capable of operating the
Segway HT after a failure (each board, in fact, powers electrical
circuits on both motors), under normal conditions they share
the load.
Inventors' Note
The dictionary defines segue as "a smooth transition from
one place or idea to another." While the Segway HT represents
a smooth transition from one physical place to another, we chose
the name Segway for our company because we believe the Segway
HT also represents an important transition to a new idea: an
energy efficient, environmentally friendly, productivity enhancing,
and fun alternative for short distance travel.
Segway enthusiasts who choose to purchase the first Segway
Human Transporters through this program will have the world
watching them. Hopefully, you will represent Segway with the
same passion and vision we had in bringing the idea to reality.
Why such great expectations? Because we believe that when you
are among the first to purchase and embrace new technology,
you personify the spirit that allows inventions to become innovations,
and that spirit can change the world.
So, welcome to Segway, and enjoy the ride!
Thanks, Dean Kamen and the team at Segway
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