The Smart Power-Assist Wheelchair: Aid To Seniors With Limited Mobility and Vision
Excerpts from the article “A prototype power assist wheelchair that provides for
obstacle detection and avoidance for those with visual impairments” by Richard Simpson,
Edmund LoPresti , Steve Hayashi , Songfeng Guo, Dan Ding, William Ammer, Vinod Sharma
and Rory Cooper
Editing and Commentary by Dr. Don Rose, Writer,
Life Alert
--
This article discusses power assistance for manual wheelchairs – a middle ground
between regular unassisted wheelchairs and powered mobility devices (like electric
scooters). The heart of the new technology described below is the Smart Power Assistance
Module (SPAM), which provides power assistance to the right and left rear wheels
of a manual wheelchair. The word “Smart” is key here. Although powered assistance
does already exist for the elderly who have trouble self-propelling themselves with
a manual wheelchair, the SPAM adds intelligence – like a brain for the chair to
go with the power-assist brawn. This intelligence helps with path planning -- detecting
and avoiding obstacles by modifying the forces applied to each wheel, and hence
the wheelchair’s speed or direction. The result is greater
protection in addition
to less strain. Imagine a senior citizen, in a regular wheelchair, ascending or
descending a steep hill while avoiding a bump or rock, and you can quickly see how
beneficial the SPAM can be.
Earlier related technology can also be helpful to the elderly, but they do not go
as far as the SPAM. For instance, the Wheelchair Pathfinder has limited sensor coverage,
and cannot alter the chair’s speed or direction of travel in order to avoid obstacles.
Another related innovation – the Collaborative Wheelchair Assistant -- restricts
the wheelchair's travel to software-defined "paths", whereas the SPAM is not limited
in this way.
As you will see below, even though evaluation of the SPAM prototype involved limiting
its movement (to straight forward, straight backward, and turning in place), the
results showed great promise for improving the mobility and
safety of seniors and
others who utilize such devices. At last, someone has created SPAM that is actually
useful.
--Dr. Don Rose
--
Background
Introduction
The concept of power assistance for a manual wheelchair is relatively new, and represents
a viable alternative for individuals who are unable to generate sufficient propulsion
force to use a manual wheelchair, but do not wish to use a traditional powered mobility
device. In a power assisted manual wheelchair, the traditional rear wheel hubs are
replaced with motorized hubs that serve to magnify or reduce (i.e., brake) the propulsive
force applied to the rear wheel push rims by the user. Power assistance is being
used as the basis for a Smart Power Assistance Module (SPAM) that provides independent
power assistance to the right and left rear wheels of a manual wheelchair. The SPAM
is able to sense the propulsion forces applied by the wheelchair user and provide
a smooth ride by compensating for differences in force applied to each wheel. The
SPAM is also able to detect obstacles near the wheelchair, and further modify the
forces applied to each wheel to avoid obstacles.
The user population for the SPAM consists of individuals with both a visual impairment
and a mobility impairment that makes it difficult or impossible to ambulate independently
using a white cane, guide dog, or other traditional mobility aid for the visually
impaired. The American Federation for the Blind (AFB) has estimated that 9.61% of
all individuals who are legally blind also use a wheelchair or scooter, and an additional
5.25% of individuals who have serious difficulties seeing (but are not legally blind)
also use a wheelchair or scooter (see Appendix). A large number of potential users
of the SPAM are expected to be elderly, since visual and physical impairments often
accompany the natural aging process. In 2000, approximately 13% of the total US
population, or an estimated 35 million people, were age 65 or older; with about
2% at least age 85. By 2030, the older population is projected to double, expanding
to 70 million. People age 85 and older are the fastest growing segment of the American
population and the US Census Bureau estimates that there are now 65,000 centenarians.
Relevant Research
Currently, the majority of non-ambulatory visually-impaired individuals are seated
in a manual wheelchair and pushed by another person. Depending on the extent of
useful vision remaining, individuals with low-vision can operate an unmodified manual
wheelchair, powered wheelchair or scooter, but the risk of an accident obviously
increases with increased visual impairment. There are reports of individuals using
a white cane or guide dog along with a wheelchair, but this is not common practice.
Despite a long history of research in smart power wheelchairs, there are very few
smart wheelchairs currently on the market. Two North American companies, Applied
AI and ActivMedia, sell smart power wheelchair prototypes for use by researchers,
but neither system is intended for use outside of a research lab. The CALL Center
of the University of Edinburgh, Scotland, has developed a wheelchair with bump sensors,
a single sonar sensor, and the ability to follow tape tracks on the floor for use
within a wheeled-mobility training program. The CALL Center smart power wheelchair
is sold in the United Kingdom (UK) and Europe by Smile Rehab, Ltd. (Berkshire, UK)
as the "Smart Wheelchair." The "Smart Box," which is also sold by Smile Rehab in
the UK and Europe, is compatible with wheelchairs using either Penny and Giles or
Dynamics control electronics and includes bump sensors (but not sonar sensors) and
the ability to follow tape tracks on the floor.
One common feature of all of these smart wheelchairs is that they are based on power
wheelchairs. Power wheelchairs are a convenient platform for researchers, but have
several disadvantages when compared with manual wheelchairs. In general, manual
wheelchairs are lighter and more maneuverable than power wheelchairs, and can be
transported in a car. Manual wheelchairs that make use of power assist hubs are
heavier than traditional manual wheelchairs, and can be more difficult to disassemble
for transport depending on how the hubs are attached to the frames, but still provide
many of the advantages of traditional manual wheelchairs.
In a search of the literature, only one other smart wheelchair was identified that
was based on a manual wheelchair. The Collaborative Wheelchair Assistant controls
the direction of a manual wheelchair with small motorized wheels that are placed
in contact with the wheelchair's rear tires to transfer torque. Unlike the SPAM,
however, the Collaborative Wheelchair Assistant restricts the wheelchair's travel
to software-defined "paths."
One of the few products that is commercially available and accommodates a manual
wheelchair is the Wheelchair Pathfinder, a commercial product sold by Nurion Industries
that can be attached to a manual or power wheelchair. The Wheelchair Pathfinder
uses sonar sensors to identify obstacles to the right, left or front of the wheelchair
and a laser range finder to detect drop-offs in front of the wheelchair. Feedback
is provided to the user through vibrations or differently-pitched beeps. The Wheelchair
Pathfinder differs from the SPAM in that the Wheelchair Pathfinder has limited sensor
coverage and cannot alter the speed or direction of travel of the wheelchair to
avoid obstacles.
Methods
The SPAM prototype has been implemented "on top of" a pair of Yamaha JWII power-assist
pushrim hubs (sold in the United States as the Quickie Xtender). The SPAM is able
to sense (1) the propulsive force applied to each rear wheel of the wheelchair,
(2) the magnitude and velocity of rotation of each rear wheel, and (3) the location
of obstacles relative to the wheelchair. Information from all sensors is collected
by a microprocessor which integrates information about the user's input and the
surrounding environment, and passes command signals to the JWII system's microprocessor.
Several types of sensors have been integrated into the SPAM. These sensors are used
for (1) tracking the state of the wheelchair (e.g., wheel velocity, torque applied
to each rear wheel by the user) and (2) locating obstacles in the wheelchair's environment.
Obstacles are identified using infrared rangefinders, sonar sensors and bump sensors.
The sonar sensors have a maximum range of 3.05 m and a minimum range of 2.54 cm.
The advantages of a smaller range are that (1) the frequency of sonar readings is
increased and (2) the sonar system is able to detect obstacles that are extremely
close to the wheelchair, which is important for passing through doorways. Infrared
range finders provide a focused, highly modulated infrared beam, providing absolute
ranging based on simple triangulation. The result is an accurate range value between
0.1 and 1.0 meters in a variety of circumstances. The infrared signal functions
at extremely steep angles, even exceeding sixty degrees, and does so both indoors
and outdoors, even in bright sunlight. The infrared rangefinders and sonar sensors
are housed in. 09 m × .06 m × .04 m boxes, which are referred to as "sensor modules."
Seven sensor modules are mounted on the current prototype. Bump sensors are attached
to both footrests and the "anti-tippers" of the manual wheelchair, and are implemented
using simple contact switches placed behind mechanical levers.
The SPAM's control software shares control of the wheelchair with the wheelchair
operator. The wheelchair operator is responsible for choosing when – and in which
direction – the wheelchair moves, while the SPAM modifies the speed of the wheelchair
based on the proximity of obstacles in the wheelchair's current direction of travel.
The algorithm currently employed by the SPAM forces the rear wheels to turn either
at exactly the same speed and direction (moving the wheelchair straight forward
or straight backward) or at the same speed and opposite directions (rotating the
wheelchair in place). This greatly simplifies the task of avoiding obstacles but
limits the wheelchair user's flexibility in choosing paths of travel.
User input (either forward, backward or turn in place) and sensor data are combined
into "cases" that are used to make obstacle avoidance decisions. The specific cases
that are in use at any one time vary, depending on the specific behavior that is
desired from the SPAM (for example, passing through a narrow doorway versus driving
quickly through a room with few obstacles). No single case can cause the software
to prevent both forward/backward movement and turning, but multiple cases can be
triggered at once and result in a situation in which the wheelchair will not move
in any direction. The motorized hubs can be turned off in these situations, at which
point the SPAM behaves like a normal (but heavy) manual wheelchair.
Results
Four able-bodied members of the investigative team, plus an individual who is blind
but does not have mobility impairment, took part in an evaluation of the SPAM prototype.
All participants used the SPAM to complete two obstacle courses. Able-bodied participants
were asked to complete each course three times blindfolded with navigation assistance
from the SPAM. The participant who is blind completed each course nine times, in
alternating sets of three trials. The sets of three trials alternated between the
SPAM providing navigation assistance and the SPAM acting as a normal manual wheelchair
(that is, the hubs were powered but the SPAM was not acting to avoid collisions).
All subjects completed trials with Course 1 first.
The SPAM did not completely eliminate collisions for able-bodied subjects. However,
three of four subjects had no collisions after the first trial on Course 1, and
only one of the four subjects had a collision in any trial on Course 2. Able-bodied
subjects generally completed both navigation tasks more quickly by the third trial.
The subject who was visually-impaired had no collisions in the first three trials
on Course 1 (with obstacle avoidance active) but did have collisions on Course 1
when obstacle avoidance was removed. On Course 2, where obstacle avoidance was not
active during the first three trials, the visually-impaired subject had collisions
in the first three trials but did not have collisions once obstacle avoidance was
introduced. There was not a consistent effect of experimental condition on time
in Course 1. In Course 2, time to complete the task was extremely consistent despite
experimental condition.
Observations and Evaluation
One clear observation from our preliminary evaluations of the SPAM is the distinct
difference between able-bodied, but blindfolded, individuals and individuals who
are completely blind. The participant who is blind was much better at localizing
the sound target and keeping track of his location in the course than any of the
able-bodied participants. The blind participant also found it much easier to learn
the layout of the course. One possible implication of these results is that the
SPAM may be more useful for individuals who are newly visually impaired. Another
possible implication is that the SPAM may be very useful in novel or frequently-changing
environments, but not particularly useful in well-known, static environments.
Our preliminary evaluation of the SPAM demonstrates that the SPAM can increase the
safety of visually-impaired manual wheelchair users. Of course, there is a large
difference between a constrained laboratory environment and real-world environments,
and much additional development and testing remains to be done. Our evaluation also
identified several shortcomings. In particular, navigation assistance increased
the time required to complete the navigation task. This was the result of an overly
conservative obstacle avoidance algorithm, which slowed the SPAM more than necessary.
An unanticipated benefit of using power assist hubs which emerged during development
was the ability to provide "haptic feedback" to the wheelchair user. As the SPAM
approaches an obstacle, the hubs provide greater resistance. This allows the user
to get an impression of the environment around the wheelchair through a series of
forward pushes and rotations in place. In addition to individuals with visual impairments,
this haptic feedback may also prove helpful for people with traumatic brain injuries.
Final Thoughts
The lessons learned from the first SPAM prototype are being incorporated into a
second generation prototype (currently under development). Most importantly, the
microprocessor used by the JWII hubs is being replaced with a new (programmable)
microprocessor, which will allow the SPAM to provide much smoother and more nuanced
control of the wheelchair. New enclosures have also been designed for the sensors
that provide increased mounting flexibility, and have increased the number of modules.
The additional sensor modules have forced us to abandon the case-based approach
to obstacle avoidance, and alternative algorithms are being pursued.
This article is based on the research article entitled,
“A prototype power assist
wheelchair that provides for obstacle detection and avoidance for those with visual
impairments” by Richard Simpson, Edmund LoPresti , Steve Hayashi , Songfeng Guo,
Dan Ding, William Ammer, Vinod Sharma and Rory Cooper . The article on this
Life
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