Forum Proceedings

We seek innovative motor 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 and 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 softer materials 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 propulsion system of power wheelchairs typically consists of a pair of PM motors, one for each drive-wheel, and a drivetrain consisting of gears, belts, and other mechanical elements that couple the motor's shaft to the drive-wheel shaft.

PM motors have a linear torque-speed profile that makes them easy to control. A DC-DC converter drives each motor with a high-frequency, square-wave pulse-train that rapidly turns each motor on and off. A microprocessor-based "control unit" controls the speed and torque generated by each motor by independently modulating the pulse-width into each motor. Solid state relays are generally used to switch supply voltage polarity to change the running direction of PM motors.

Any PM motor, with its drivetrain, is maximally efficient at only one speed. With light loading, PM motors, with their drivetrains, can attain an efficiency of about 70%. Under loads typical for power wheelchairs however, PM motors and their drivetrains are about 45% efficient. Rare-earth permanent magnets are major improvements over iron permanent magnets but don't eliminate the speed/torque tradeoff. The typical current draw for power wheelchairs with PM motors is 6-8 amps with a typical range of 15 to 20 miles - when supplied by two fully charged, 12 Volt, group 24 batteries. A drop in motor efficiency increases the current drawn from the battery, decreases battery life, and decreases battery efficiency. High-quality manufacturing practices are needed to produce PM motors with matched performance characteristics (i.e., matched motors have near identical mechanical outputs for an identical electrical input).

A wheelchair's control module converts positional information from the joystick into power signals to the motors. Control modules are microprocessor-based and have many adjustable parameters. Many control modules use feedback to sense whether the motor is responding properly to the joystick position. These control modules adjust motor torque to maintain near-constant speed while the load varies in response to changes in the terrain (e.g., incline, bumps, linoleum, carpeting, concrete, grass, sand). Controllers automatically "step back" (i.e., reduce current to the motors) when motors are overloaded. However, repeated overloading through inappropriate use can still overheat motors and decrease motor life. Controllers are often designed to control a wide range of motors. Some controllers may cause one motor to fail before the other.

Recent innovations within the power wheelchair and scooter industry include the use of rare-earth magnets and brushless, gearless, and direct-drive motors. Motors that use rare-earth magnets can be smaller and lighter than analogous motors with iron magnets. Alternatively, motors that use rare-earth magnets can be more powerful than motors with iron magnets of a similar size. Brushless motor designs have better heat dissipation characteristics because windings are on the outside. These motors can be pushed harder before they are damaged by heat buildup and are more efficient because there is no power loss through the brushes. Gearing and belts in the indirect drivetrain are a source of noise and mechanical power loss.

Highly efficient, gearless, direct-drive, rare-earth magnet motors have recently appeared in the power wheelchair market. These motors can be mounted close to the drive-wheels and allow good access to the under-seat compartment. However, these motors tend to be relatively large and expensive. A European company has recently introduced a brushless, gearless, rare-earth magnet motor contained entirely within a power wheelchair's drive-wheel.

Other motor technologies suggested for power wheelchairs and scooters include pancake stepping motors; disc-armature DC motors; and alternating-current, three-phase, squirrel-cage induction motors (SCIM).

Technology Needs and Barriers

Motor and drivetrain specifications for power wheelchairs are unique - i.e., no similar motor specifications exist for other industries with the possible exception of electric bikes. For power wheelchairs, two-motor designs are strongly preferred over one-motor designs. It is expected that advanced motor design concepts and tools, now used in other industries, will be required to meet the unique and specific needs of the power wheelchair industry. During group discussions, motors with "redesigned pole structure," "poly-phase, AC motors" and "hydraulic motors" were all suggested.

Information gathered from users, manufacturers, clinicians, and other stakeholders has identified characteristics of an ideal motor. 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 motor...

1. Must have an average efficiency of at least 75% under typical power wheelchair loading. Near-constant motor efficiency, independent of loading, would be the ideal. The ideal motor would enable a power wheelchair to have a range of at least 30 miles (when supplied by two fully-charged, Group 24, 12 Volt batteries) while providing improved access to difficult terrain and surfaces.
2. Must have the performance characteristics of "a true electrical transmission" with "a continuously variable gear ratio."
3. Must not have complex control requirements. Each motor should be independently controllable.
4. Must generate a high startup torque.
5. Should incorporate sensors that provide information to compensate for motor imbalance, diagnostics, steering, acceleration, and wear status. Use of these diagnostics by technicians must not require extra training.
6. Must have good heat dissipation characteristics. In particular, the motor should not be easily damaged by heat buildup or likely to injure the user.
7. Should have the following physical characteristics:
   • reduced motor weight-a typical PM motor (with gearbox) weighs between 15 and 35 pounds.
   • reduced motor size-a typical PM motor has a length and width (diameter) from about 4 inches by 8 inches up to about 9 inches by 8 inches with a square or round cross-sectional profile.
8. Should require little maintenance and be very durable - comparable to PM motor maintenance and durability.
9. Should be located close to the wheels or be a part of the wheel design.
10. Should be compatible with portable/collapsible power wheelchair design. Motors should be easily removable by the user in order to facilitate folding of the wheelchair (for transportation) or to facilitate repair by a service technician.

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