Design of a Power-Assist Wheelchair for Persons With Hemiplegia---IEEE2011

7/31/2019 Design of a Power-Assist Wheelchair for Persons With Hemiplegia---IEEE2011

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Design of a Power-Assist Wheelchair for Persons

with HemiplegiaAllen H. Hoffman and Keith N. Liadis

Mechanical Engineering Department, Worcester Polytechnic Institute

Worcester, MA USA 01609emails: [email protected], [email protected]

AbstractPersons with hemiplegia often require a one arm drivewheelchair. Current designs exhibit substantially degraded

performance when compared to a standard manual wheelchair.

A power-assist, one arm drive wheelchair was developed that was

maneuverable, foldable and easy to operate. A motor powers the

wheel on the users affected side, encoders on both rear wheels

track wheel positions and a rotary heel interface mounted on a

footrest controls steering. A control system analyzes wheel and

steering positions and responds to the motion of the hand-driven

wheel. The prototype met and exceeded predetermined design

specifications based on standard industry test procedures. Thepower-assist components could be attached to a wide range of

manual wheelchairs with only minimal modifications.

Keywords- power-assist wheelchair; hemiplegia; rehabilitation;

assistive technology

INTRODUCTIONCommon causes of hemiplegia include stroke, cerebral palsyand trauma with stroke being the most common. Within theUnited States there are over 4.5 million people managingstroke with 700,000 new cases occurring annually [1].Individuals with severe hemiplegia require specialized manualwheelchairs. Powered wheelchairs are not a viable solution,since they are expensive, difficult to transport and do not

encourage an appropriate level of physical activity. Manualwheelchair users implement propulsion, steering and braking

by interacting with pushrims attached to each wheel.Achieving these control functions using only one hand is morecomplex. Two common commercial designs for one-armdrive manual wheelchairs are the lever arm and the dual

pushrim. Both designs exhibit substantially degradedperformance when compared to the standard wheelchair.

The lever arm design attaches a lever to the hub of the wheelon the unaffected side and propels the wheelchair through aone-way clutch. A thumb switch reverses the direction of

propulsion. The lever creates an approximate 2:1 mechanicaladvantage compared to propulsion with the pushrim. Thewheel rotation per stroke is decreased by the same ratio.Twisting the handle of the lever controls steering via a linkageconnected to a front caster. This design increases the turningradius. Braking is counter intuitive and is activated by movingthe lever to either the extreme forward or rearward positions.Engaging the brake in the rearward position may be beyondthe range of motion of some users.

The dual pushrim design is an accessory that is added to astandard wheelchair. A second, smaller diameter pushrim ismounted on the side of the unaffected arm and a removableaxle transmits power to the wheel on the affected side. Thewheelchair can be folded by removing the transverse driveaxle; however this operation is difficult even for an able-

bodied person. The propulsion system lacks any mechanicaladvantage and when compared to a standard wheelchairapproximately doubles the required power input from the

unaffected arm. Propulsion is cognitively challenging and ittakes considerable ability to coordinate the motion of the two

pushrims rims to maneuver the wheelchair. A wheelchair hasbeen developed that alters the functions of the dual pushrims[2]. One pushrim propels both wheels in the forward/reversedirection, while the second pushrim is used to perform

pirouettes in place. Steering is achieved by coupling a frontcaster to rotation of a footrest.

A one arm drive power-assist wheelchair has also beendeveloped where the direction of travel is controlled byvarying the pressure in two vinyl tubes attached to each side ofthe hand-driven pushrim [3]. The differential pressure is usedto control steering. Thus, the steering input signal is

intermittent during propulsion. It is noted that this wheelchairis difficult to track along curves with small radii of curvature.

OBJECTIVEThe objective of this work was to produce a functional

prototype of a power-assist manual wheelchair for personswith hemiplegia that would exhibit the ease of operation andmaneuverability associated with a standard wheelchair andremains foldable. The major design goals were;

1. To produce an add-on system of components thatwould allow a manual wheelchair to be converted toone arm drive.

2. To retain the existing folding feature that allows thewheelchair to be easily transported in an automobile.

3. To provide an intuitive and appropriateuser/technology interface [4, 5].

4. To meet appropriate ISO and ANSI/RESNAwheelchair testing standards [6].

5. To allow a typical user to travel at speeds above 2.25m/s (5.00 mph).

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FINAL DESIGNThe design of the wheelchair incorporates modularcomponents that can be attached to a variety of folding manualwheelchairs. These modular components include a singlemotor and gearbox that attaches to the existing wheelchairframe, an electronic control unit, a 24 V rechargeable battery

pack, and various sensors for tracking wheel motion and

steering. The design allows for the wheels and footrests to beeasily removed for transport. A control system was developedto effectively achieve the three fundamental wheelchaircontrol functions of propulsion, braking, and steering usingthree rotary encoders mounted to the wheelchair. Encodersare attached to the frame adjacent to the wheel hubs to trackindividual wheel position and velocity. The third encoder isattached to a rotating heel plate on the footrest on theoccupants unaffected side and is used for steering (Fig. 1).

Figure 1. Prototype showing hand-driven wheel and backpack containingelectronic control unit and batteries.

The control system constantly monitors the velocity of the two

drive wheels and signals the motor driven wheel tosynchronize with the speed of the hand-driven wheel. Thismimicking of the wheel motion generated by the usersfunctioning arm effectively replaces the function of the userssecond arm without requiring additional exertion from theuser. The control algorithm uses the difference in wheelvelocities to determine the voltage to be applied to the motoramplifier. The control system uses a discreet-time derivativesof the position data received from the encoders and sums thedifference over a set period of time. Discrete-time integrationconverts these data into the voltage which is applied to themotor (Fig. 2).

Figure 2. Block diagram of control system.

The steering encoder attached to the rotary heel plate acts asan amplifier or de-amplifier whose signal is dependent on theangle from the centerline of the wheelchair (Fig. 3). Thecontrol algorithm converts the steering encoders position intoa multiplication factor which is applied to the velocity signalfrom the hand-driven wheel. When the heel plate is pointedstraight, the encoder signal is converted into a multiplication

factor of 1 resulting in true synchronization of the wheels. Asthe foot rotates, the multiplication factor either increases ordecreases. This signal effectively modifies the control

Figure 3. Prototype showing rotary heel plate for steering and motor drivenwheel.

systems perceived velocity of the hand-driven wheel which inturn commands the motor driven wheel to rotate

proportionally faster or slower to maintain synchronization.This steering differential successfully provides the user withturning capabilities. The steering interface is intuitive sinceturning the foot in the direction of desired travel produces the

expected result. Since steering is decoupled from propulsion(hand and foot operate separate functions), it is easier tocognitively operate than a system requiring a single hand tooperate both functions. To further increase maneuverability,the control system was also designed with a pirouette mode,allowing the user to turn in place. This feature is enabled whenthe heel plate encoder is turned fully to either side andoperates by reversing the synchronization of the motor-drivenwheel (multiplication factor of -1). As a safety measure, thecontrol system restricts the user from entering pirouette modewhen the sum of the wheel velocities exceeds 0.34 m/s.Reversing wheel direction while travelling at higher speedscould result in a safety hazard.

TESTING AND EVALUATIONTests were performed based upon ISO 7176 andANSI/RESNA WC standards [6] that apply to power-assistwheelchairs. These tests included static stability, dynamicstability, braking, dimensions, mass and turning space, andobstacle-climbing ability. Additional qualitative tests were

performed to evaluate driving performance in a straight lineand over a figure eight course. The ability of the wheelchair

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to be folded and stowed was evaluated through a timed test.Tests were performed using either a 113 kg (250 lb) ISO testdummy or a human occupant. With a human occupant,additional mass was added to the wheelchair to achieve a totalmass of 113 kg.

The prototype was able to achieve speeds of up to 2.8 m/s (6.2mph) on level surfaces. The static stability tipping angle

exceeded 14o

in all directions (10o

required). Dynamicstability tests were performed by driving the wheelchair downa ramp having an adjustable slope at maximum speed (2.8m/s) and executing left and right turns. Three wheelsremained in contact with the ramp at slopes up to 8o. Braketesting showed that the motor would hold the wheelchairstationary in all directions on a 10o slope (8o required). Brakestopping distance tests were not performed because brakingdistance is dependent on the hand strength of the individualuser. The prototype retained the same overall envelopedimensions of the original manual wheelchair. The addition ofthe encoder block beneath the footrest decreased groundclearance beneath by 19 mm. The backpack containing the

batteries and electronic control unit was hung behind the seat,

but still remained within the original footprint of thewheelchair. Prior to modification, the mass of the wheelchairwas 10.9 kg (24 lb). The mass of mechanical components toconvert the wheelchair to power-assist was 2.7 kg (6 lb). Themass of the control system including the battery was 8.2 kg(18 lb) and was suspended in a backpack behind the chair.These components raised the total mass of the wheelchair to21.8 kg. The pirouette feature allowed the prototype to retainthe same turning space (1.27 m) as the unmodified wheelchair;however, the time to perform this maneuver was increased.Since the user powers one of the wheels, obstacle-climbing isdependent on the users strength in the unaffected arm as wellas the motor power. With younger tests subjects, the torque inthe motor driven wheel was limiting. Two test methods were

employed. In the first method (no run up) the rear drive wheelswere positioned flush with the obstacle and the occupantattempted to climb the obstacle. In the second method (withrun up) the front casters were on top of the obstacle and theoccupant accelerated prior to the rear wheels reaching theobstacle. The minimum obstacle height required by ISO 7176is 12.5 mm (0.5 in). The wheelchair was able to traverseobstacles up to 19 mm (0.75 in) with no run up and up to 51mm (2.00 in) with run up.

Several qualitative tests were used to evaluate drivingperformance. The general performance of the wheelchair wasevaluated on several common surfaces; including high pilecarpet, brick, concrete, tile, and dirt and found to be similar to

a standard manual wheelchair. The following specific testswere conducted. The ability of the wheelchair to track in astraight line was evaluated on a 9.1 m (30 ft) course on a tilefloor using a marker attached to the wheelchair centerline thatmarked the path. Three trials were conducted at a walkingspeed of 1.6 m/s (3.5 mph). The maximum deviation from thestraight line recorded in the three trials was 0.14 m (5.5 in).

Maneuverability was evaluated using a figure-eight test trackwith two obstacles spaced 0.76 m apart. The obstacles werelow enough to the floor so that the footrests could pass overthem. The tests were performed using four differentwheelchairs: the prototype, a standard manual wheelchair, adual-rim hemiplegic wheelchair, and a lever-arm hemiplegicwheelchair. The driver trained on each wheelchair for at leastthirty minutes before performing the tests. The test was

repeated 5 times with each wheelchair and the times wereaveraged (Table 1). As expected, the two-handed manualwheelchair demonstrated the best maneuverability. However,the maneuverability of prototype hemiplegic power-assistwheelchair was clearly superior to the lever action and dual

pushrim wheelchairs.

Table 1. Times for figure-eight performance tests.

Wheelchair Test Run # Time (s) Average (s)1 17

2 20

3 16

4 14.55 16

1 132 10

3 9.5

4 9

5 11

1 35

2 40.53 28.54 35

5 30

1 48

2 55

3 37.54 44

5 50

Dual-Rim

Hemiplegic33.8

Lever Arm

Hemiplegic46.9

Hemiplegic

Prototype16.7

Two-Handed

Manual10.5

FOLDING AND TRANSPORTABILITYA CAD model (Pro/Engineer) was used in the design

process to insure that the additional components would notinterfere with folding. Careful visual inspection of theconstructed prototype confirmed this result. The followingsteps must be performed to fold and stow the wheelchair fortransportation in an automobile. Three electrical connectionsmust be uncoupled, the backpack must be removed from

behind the chair, the two legrests must be removed and thewheelchair must be folded. Timed tests revealed that thesesteps were typically accomplished in one minute. Deployingthe wheelchair using the stowed components took a similaramount of time.

DISCUSSIONThe user interface of the prototype retains similarcharacteristics to a standard manual wheelchair. Propulsioneffort on the unaffected side is unchanged. Steering, which in

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a standard wheelchair requires two hands, is replaced by arotary input using the unaffected foot. The wheelchair

performance was evaluated using a combination of industrybased standards (ISO 7176) and qualitative driving tests. Thewheelchair demonstrated superior maneuverability whencompared to existing one-arm drive wheelchairs. Less strengthand endurance were required of the user, thereby potentiallyincreasing the range of travel. The performance and

maneuverability characteristics of the prototype weregenerally similar to a standard manual wheelchair. Duringstraight-line driving and normal turning, the prototype canmaneuver efficiently at speeds up to 2.8 m/s (6.2 mph), whichfar exceeds the average walking speed of 1.6 m/s (3.5 mph). Inaddition, the folding ability of the wheelchair was retained.The user interfaces for propulsion and steering are separateand intuitive. The propulsion interface is identical to astandard wheelchair. The steering interface is controlled bythe unaffected foot and is continuous. This represents asubstantial improvement over a previously reported one armdrive, power-assist design [3] that combined the propulsionand steering interfaces in the hand-driven wheel.

The components added to the wheelchair to create thehemiplegic prototype were incorporated within thewheelchairs original footprint and were also located in areasthat were clear from interference during folding. The

wheelchair may be pushed by an assistant when powereddown. Standard manual wheel locks can be used to hold thewheelchair in place on inclines when it is powered off and toassist in chair transfers. The additional components have amass of 10.9 kg. It is estimated that 2-3 kg, could beeliminated in a 2

nd generation design.

ACKNOWLEDGMENT

DEKA Research provided facilities, resources, and extensivemachine shop time throughout the development of the

prototype.

REFERENCES

[1] American Stroke Association, Learn about stroke, 2005.[2] S. Lesley, One-Handed Wheelchair, University of Cambridge, Dept.

of Engineering, news item, June, 2001.

[3] S. Aoshima, H. Kaminaga, M. Shiraishi, One-hand drive-type power-assisted wheelchair with a direction control device using pneumaticpressure, Advanced Robotics, 16, 773-784, 2002.

[4] A. M. Cook and S. M. Hussey, Assistive Technologies Principles andPractice, 2nd ed., Mosby, St. Louis, 2002.

[5] A. H. Hoffman, The role of robotics in the design of devices to assistpersons with disabilities, Proceeding of the 2009 IEEE Conference onTechnologies for Practical Robot Applications, 1-4, 2009.

[6] International Standards Organization, ISO 7176 Parts 1-22, Geneva,Switzerland, 1986-2003.

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Design of a Power-Assist Wheelchair for Persons With Hemiplegia---IEEE2011