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We seek innovative transmission technologies for power wheelchairs and scooters.
Description of the Problem
Power wheelchairs are used predominantly by people with both lower and upper
extremity impairments resulting from cerebral palsy, high-level spinal cord injury, or muscular
dystrophy. There are more than 93,000 power wheelchair users in the United States. The "standard
power wheelchair" accounted for $166 million in Medicare expenditures in 1997.
Scooters are used mostly by people with the ability to walk short distances but who
require assistance when shopping or interacting within the community-wishing to remain active despite
growing physical limitations. Scooters are used commonly by elderly persons. It is estimated that
64,000 scooters are currently in use in the United States.
Currently, the brushed, direct current, internally rotating, permanent magnet motor
(PM motor) is the industry standard. Under light loading, the PM motor and its drivetrain can have an
overall efficiency of about 60% to 70%. Under loads typical of power wheelchairs, the overall motor
and drivetrain efficiency can drop to about 45%. The standard PM motor with drivetrain places
undesirable constraints on the performance and design of power wheelchairs and scooters.
Motor and drivetrain efficiency impacts battery performance (e.g. capacity, peak
current, life span, and time between recharge) and the overall performance (e.g., range, speed) of
the wheelchair system. Whether a power wheelchair or scooter is appropriate for general
transportation or short distance mobility in "non-challenging" environments is determined
by the characteristics of its motor, drivetrain, battery and power management systems.
The speed and torque delivered by wheelchair motors and drivetrains have a major
impact on a user's ability to access home, work, recreational, and educational environments. The
ability to negotiate inclines (e.g., ramps, curbs) and difficult surfaces (e.g., gravel, soft
soil, sand) is limited by available torque.
The size and configuration of motors, drivetrains and batteries constrain the
physical dimensions of a wheelchair or scooter (e.g., weight, width, height). Seat height, which
is dependent upon power base configuration, limits access to desks, tables, and transportation.
Vans often require extensive and costly modification in order to accommodate the seated height
of a power wheelchair user. Increased under-seat space would allow users to better transport
ventilators and oxygen tanks, thus improving their independence.
Maintenance costs for a power wheelchair are estimated to be in excess of $1,000
over a 5-year period. Motor and drive system repairs often cannot be completed by technicians
"in the field" and must be returned to the manufacturer for service. The chatter and
swiping of (some) gears and the friction associated with motor and idler bearings are potential
sources of vibration and noise. Brushes wear out, cause noise, and need to be replaced regularly.
Brushes made from soft material are quieter but wear faster. Many users elect to operate their
motor until failure and then purchase a replacement motor rather than be inconvenienced and pay
for expensive maintenance.
Current State of Technology
The drivetrain is a mechanical system that transfers power from the motor to the
drive-wheels. Power wheelchairs use direct and indirect transfer drivetrains. The drivetrain is
composed of gears, belts, chains, and other mechanical elements that serve to reduce motor speed
while proportionally increasing motor torque.
For direct-drive transfer, the motor is directly coupled to the drive-wheel -
i.e., through a gear train. Direct-drives require a low-speed, high-torque motor and are
mechanically efficient. However, gears in a direct-drive system are prone to wear and/or break and
can be expensive to repair. For indirect-drive transfer, the motor is coupled to the drive-wheel
through a gear train and flexible machine elements, such as belts or chains. These mechanical
elements act as a "shock absorber" when the drive-wheels become "stuck" or are
under a "heavy load." Excessive stress on the drive system can cause an increase in
noise and misalignment. However, realignment is usually easy and the cost of maintaining the
system is relatively low if adjustments are done regularly.
Involute gear drives have a power transfer efficiency of 90 to 95% but are larger,
heavier, and noisier than worm gear drives. Involute gear drives are used on fixed-frame power
wheelchairs with both motors perpendicular to the orientation of the drive-wheels. Worm gear
drives are relatively smaller, lighter, and quieter than involute gear drives and are approximately
70 to 80% efficient. With worm gear drives, both motors are oriented parallel to the drive-wheels.
Worm gear drives are used on folding-frame power wheelchairs. Because of their smaller size, worm
gear drives enable more space to be available for respirators, power seating, and storage.
In bench testing, transmissions have been shown to double the average efficiency
of PM motors under typical loads. Patents have been granted for power wheelchairs that use
transmissions in their drivetrains. However, there is only a single, top-of-the-line power
wheelchair now in the marketplace that uses a transmission-like drive system. All other power
wheelchairs use direct-drive, indirect-drive, or gearless-drive systems. Variable-pitch belt
technology can act like a transmission but is inefficient. Users can now select either a
"high" or "low" operating speed for most wheelchairs. This capability is not
equivalent to a true transmission and provides no gain in overall efficiency.
Technology Needs and Barriers
Industry and technical experts suggest that the need for transmissions is great.
These experts believe that transmissions have not succeeded because of concerns for control,
safety, complexity, reliability, maintenance, and cost. The motor, drivetrain, controller, and
power base are typically designed as an integrated system. A transmission may add to the design
and performance complexity of this system; however, a transmission could enable manufacturers
(if desired) to use smaller motors because of improved motor efficiency under high loads. Multiple
transmissions are likely to be needed in order to match drivetrain capabilities to user
performance requirements.
Information gathered from users, manufacturers, clinicians, and other stakeholders
has identified characteristics of an ideal transmission. Minimum requirements are identified as
items that "must" be addressed to a make a design acceptable; other attributes that will
benefit the design are identified as items that "should" be addressed. Specifically, an
ideal transmission...
- Must improve the average overall system efficiency - i.e., motor plus
drivetrain - to at least 70%. Near-constant drivetrain efficiency, independent of loading, would
be ideal. In particular, the transmission should deliver high torque under high loads.
- Must provide a coordinated transmission gear shifting for both wheels. The
control of steering and acceleration for wheelchairs that incorporate transmissions must be safe
and reliable. Diminished user safety is not acceptable.
- Should have the following performance characteristics:
- Short-term: a manual two-speed transmission in which the user shifts gears
would constitute a reasonable solution. For a manual transmission, shifting should be very
smooth and a reasonable mechanism should exist whereby persons with varied and limited
functional abilities could perform such shifting.
- Intermediate-term: an automatic two-speed transmission (perhaps where gears
shift in response to torque or other mechanical demands) would constitute a reasonable
solution.
- Long-term: an automatic transmission, employing a continuously variable
gear ratio, would constitute a step up in performance and complexity over both manual and
automatic two-speed transmissions.
- Must be similar in size to current power wheelchair gearboxes (i.e., roughly
5 inches by 5 inches).
- Must not require the power base to be redesigned. In particular, incorporation
of the transmission should not require an increase in the height or width of the wheelchair.
- Must be durable, reliable, and quiet.
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