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FORCE CONTROL STRATEGIES
WHILE DRIVING ELECTRIC
POWERED WHEELCHAIRS WITH
ISOMETRIC AND
MOVEMENT-SENSING JOYSTICKS
Outline
 Introduction
 Methods
 Experimental Setup
 Subjects
 Data Collection
 Statistical Analysis
 Results
 Conclusion
Introduction
Introduction
 Approximately 220,000 Americans use
electric powered wheelchairs (EPWs) for
independent mobility.
 An additional 125,000 Americans with
disabilities desire power mobility but cannot
functionally drive an EPW.
Introduction
 Isometric joysticks (IJs)
 rigid post : theoretically reduces the amount of
dexterity required for control
 Movement sensing joysticks (MSJs)
Isometric joysticks
source : http://www.wheelchairnet.org/
Introduction
 Compared Human Engineering Research
Laboratories (HERL) IJ with two control
functions to a conventional MSJ (PML
Flightlink, Ltd).
 Subjects : experienced EPW users with upper
limb impairments from a variety of etiologies
Introduction
 Sophisticated signal processing
 statistically indistinguishable from the MSJ
 basic control function
 lower performance than using the MSJ
 Regardless of the control software installed,
users tended to exert more force than
necessary on the IJ
Methods
Experimental Setup
 Fitts’ Law
 The model is based on time and distance
 nine circular, black vinyl targets 155 cm in
diameter to the floor in a double semicircular
array
Experimental Setup
 Distance of targets from the starting position
 near targets : 305 cm
 far targets : 538.5 cm
 Mounted one of two test joysticks to a
Quickie P300 EPW (Sunrise Medical, 1994)
Experimental Setup
 Force sensing algorithm (FSA)
 program the IJ to operate as a simple IJ
 Variable gain algorithm (VGA)
 program the IJ to emulate an MSJ more closely
 dead zone : low amplitude inputs ( 0.9 N)
occurring from unintentional/resting movements
produces no output
Experimental Setup
Subjects
 Used an EPW with hand operated MSJ as the
primary means of mobility at least 20 h per
week
 Ages
 between 18 and 80 years
 Tolerate testing for 2.5 h
 Drive to each of nine targets three times with
each of three joystick
Subjects
 Subject numbers in each diagnostic category :
 4 (36.4%) with Cerebral Palsy (CP)
 2 (18.2%) with traumatic brain injury (TBI)
 2 (18.2%) with spinal cord injury (SCI)
 1 (9.1%) with Muscular Dystrophy
 1 (9.1%) with Spina Bifida (SB)
 1 (9.1%) with Polio
Cerebral Palsy (CP) : 腦性麻痺
spinal cord injury (SCI)脊髓損傷
Spina Bifida (SB) :脊柱分裂
Traumatic brain injury(TBI) :創傷性腦損傷
Muscular Dystrophy :肌肉萎縮症
Polio :小兒麻痺疫
Data Collection
 Accuracy
 successful trial
 halted the EPW within the target for at least 2 s
 with a total trial time of no more than 40 s
 Driving accuracy (DA)
 the percentage of successful trials completed with
each joystick
Data Collection
 Transducers
 forward and reverse
 “speed” axis
 left and right
 “direction” axis
Subjects
Data Collection
 Average applied force
 each trial by dividing the area under the force time
curve by the total trial time
 Excess force
 Each trial as the average of the differences
between the actual applied force
 control efficiency (CE)
 the Newton ‧ seconds (N‧s ) expended within the
operational range divided by the total N‧s of each
trial
Statistical Analysis
 Alpha at values equal to 0.05 a priori
 Used SPSS for univariate analyses and
nonparametric analyses
 nonparametric statistics(Friedman test)
 Average applied force and trial time were not
normally distributed
 learning effect for average applied force
subsequent trials
 nonparametric statistics(Mann–Whitney tests)
 Determine if average applied force for each trial was
related to DA
Statistical Analysis
 Spearman Rho correlations
 evaluate the relationship between average applied
force and trial time for successfully acquired
targets
Results
Results
 Average age
 37.8 ± 10.9 years
 Gender
 male : 6 (54.6%)
 female : 5 (45.5%)
 Race
 Caucasian : 6 (54.6%)
 African-American : 4 (36.4%)
 Asian-American : 1(9.1%).
Results
Results
 No learning effect seen for average applied forc
any joystick
 FSA p = 0.184, VGA p = 0.117, MSJ p = 0.804
 Average applied force negatively correlated with
trial time for all three joysticks for successfully
acquired near and far targets
 Average applied force was not related to DA for
any of the joysticks
 FSA p = 0.306, VGA p = 0.126, MSJ p = 0.304
Results
Conclusion
Conclusion
 The HERL IJ is a potential alternative control
interface for mobility for many individuals
with disabilities.
 The IJ required more force to operate than
the MSJ, but subjects’ driving performance
was not affected.
Thank You for Your Attention!