Design of a test method to evaluate the Forward Facing Wheelchair containment and occupant retention system subjected to low g accelerations

Loshna Krishnan, Erik Porach (University of Pittsburgh)

Abstract

Previous studies have shown the need to design new forward facing wheelchair containment and occupant retention systems (WCORS) in large accessible transit vehicles (LATVs). The purpose of this project was to design a test method that is able to replicate real world scenarios for a low g environment. A rotational acceleration system was developed that consists of a beam, rotary table, drive motor and support surface which effectively rotated the wheelchair creating a centripetal force similar to forces measured during real world LATV turning, braking and accelerating scenarios. A manual wheelchair occupied by a Hybrid II ATD was used for evaluation.  Results of this design project indicate a promising new test method to evaluate alternative wheelchair containment systems that are subjected to low g accelerations in large accessible transit vehicles (LATVs).

Introduction/Problem Statement

Since the advent of ADA in 1990, increasing numbers of people with disabilities use wheeled mobility devices such as manual, powered wheelchairs and scooters as a means of accessing public and private transportation¹. Due to this, the ADA requires that transit providers accommodate passengers who use “common wheelchairs” when traveling in a motor vehicle¹. This requires the installation of wheelchair securement devices and occupant restraints that will prevent injury to the wheelchair users in large accessible transit vehicles (LATVs). Currently there are three different securement devices in LATVs that act as an integral occupant protection system. Passengers rarely use these securement devices that are installed in LATVs due mostly to the fact that they are uncomfortable and are operator-dependent¹.

A recent study by van Roosmalen et al. has shown that accelerations that result from normal driving, hard braking and rapid turning on an LATV all remain below 1g and because of this, it is considered a “low-g” environment². Since wheelchair users rarely use the WCORS that are currently available, they are exposed to injury resulting from the movement of wheelchair during accelerations, braking and turning of the LATV². Due to the current issue with the WCORS system, there is a need to design and test new Forward Facing- WCORS (FF-WCORS) for the low g environment. Unfortunately no proper test method has been developed to evaluate the dynamics of an effective WCORS in a low g environment.

The objectives of this study was to evaluate the testing device, and use it to simulate accelerations at 0.8g, decelerations at 0.4g and turning at 0.45g that match the real world acceleration data previously obtained by Turkovich et al (figure 3). Another objective was to replicate incidents that frequently occur in LATVs such as wheelchair tipping, sliding and wheelchair seated passenger ejection from the wheelchair seat.

Literature reviews were carried out to evaluate potential test methods for the FF-WCORS. The rotational acceleration disc was determined best representative of the real world scenario and selected as the test method to investigate unsecured wheelchair dynamics subjected to low g accelerations. The idea of the rotational acceleration disc was suggested by Randy Johnson (Q’Straint, FL) and came from the basic idea of a roundabout. The disc creates a centripetal force equal to the desired acceleration when it is spun. It also simulates braking force when the wheelchair is placed facing away from the center of the disc. A turning force can be simulated when the wheelchair is placed tangentially on the disc and an acceleration force can be simulated when the wheelchair is placed facing the center of the disc.

Method

A triaxial accelerometer was used to determine the acceleration obtained in this study. To obtain the desired acceleration and estimate placement of the accelerometer on the prototype, an equation relating the angular velocity as well as the distance of the accelerometer from the center of the disc (prototype) was developed using excel spreadsheet. The equation also allows for the estimation of the prototype velocity when simulated.  A prototype was designed accordingly using Solidworks.

The prototype was built using a 6in x 6 in x16 ft  wooden support beam, a rotary table, caster wheels, a 4ft x 8 ft test platform and structure, a drive motor and speed controller (taken from an old power wheelchair) and a tether post (figure 1). To control and observe the acceleration of the prototype during testing, a laptop connected to the accelerometer was placed on the support surface and was controlled via remote desktop. Testing was carried out to evaluate the dynamics of the wheelchair during simulated acceleration, turning and braking forces.

During the three trials, the accelerometer was placed on the test platform at the center of gravity (COGxy) of the wheelchair and ATD. Rotation of the test platform was started out at speed power 1(out of 5) after which it was increased gradually to resemble a constant angular velocity and increased centripetal force (g-level). The speed of the motor was increased until there was more than 2 inches of wheelchair movement in anyd direction; after which the test was stopped. The acceleration trial involved placing the wheelchair and ATD radially inward, while for the turning trial, the wheelchair and ATD were positioned tangentially. As for the deceleration trial, the wheelchair and ATD were positioned facing outward. A tether was used to strap the wheelchair to the tether post to ensure it didn’t fall off completely off of the platform during testing.

Test setup and potential system movement of a Manual wheelchair and a Hybrid II ATD

Results

Data obtained from the accelerometer were then exported into MATLAB and filtered using a Butterworth Filter. The filtered data were then plotted for each trial against time. From the graphs, the maximum accelerations and time periods, during which there was movement of the wheelchair for each trial (acceleration, deceleration,turning) were determined (figure 2) and compared with previously obtained data from actual LATV turning and braking scenarios (figure 3). An average maximum acceleration was taken for the each trial plotted against time. Excessive wheelchair and occupant movement was recorded on video and final positions of the wheelchair-seated ATD were recorded (figure1-right).

Average maximum accelerations measured until excessive movement for acceleration, turning and deceleration over time using the prototype designed. Maximum acceleration for all three trials observed were below 0.4 g’s and has similar trend to in vehicle test data.

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: In vehicle testing data obtained for the turning and braking scenarios over time. Both trials show accelerations below 0.4 g

Evaluation

The prototype developed was capable of replicating and simulating real world accelerations, braking and turning scenarios. The dynamics of an unsecured wheelchair was determined using this prototype. When compared to the data from in-vehicle testing, it was found that dynamics of the three scenarios occurred at ranges lower than 0.75g’s. It was also observed at accelerations way below 0.45g’s, there were wheelchair movements for all three scenarios. For the acceleration trial, it was observed that the wheelchair occupied by an ATD tipped backward (figure 1) between the time period 1000 ms -1500 ms and the maximum acceleration obtained for this trial was 0.336 g.  As for the turning trial, there was movement of wheelchair and ATD between the time period 1400 ms – 1600 ms in which the wheelchair tipped sideways (figure 1). Maximum acceleration obtained for this trial was 0.261 g. Data for the deceleration trial revealed that there was movement of the wheelchair between 1300 ms -1600 ms and the maximum acceleration was at 0.18g. During the deceleration trial, the wheelchair crept outward over time until the tether straps holding it in place were in tension. Comparison with the in vehicle testing showed that movements for all three scenarios occurred at similar acceleration ranges. As there were no in-vehicle data for the acceleration trial, the trial done using this prototype could not be compared to a standard value

Conclusion

For future directions; a better motor needs to be used as the one used in this prototype was from an old power chair. The Hybrid II ATD used in this study was also very stiff and wasn’t very representative of the human movements, so a better dummy needs to be used for future testing. For this study, the tests were only carried out for a manual wheelchair. The dynamics of other mobility devices and alternative floor surfaces still needs to be determined. Testing also needs to be done for newly designed FF-WCORS. As a conclusion, the objectives of this study were met and new test method using the rotational acceleration prototype was successfully developed to evaluate and determine the dynamics of an unsecured wheelchair. With more adjustments to the prototype, it can then be used to design and validate a new FF-WCORS in LATVs. This would then be able to ensure safety and comfort of wheelchair passengers traveling in LATVS.

Acknowledgments

This project was done as part of BioEng 1002 at the Department of Rehabilitation Science and Technology/ Swanson School of Engineering. Prototype development and design tools were sponsored by National Institute on Disability and Rehabilitation Research (NIDRR). The authors would like to thank Dr. van Roosmalen, Michael Turkovich and the staff of the RERC on wheelchair transportation safety for their help in the development of this design.

References

1. Architectural and Transp. Barriers Compliance Board. Americans with Disability Act (ADA) Guidelines for Transportation Vehicles. 36 CFR Ch. XI (7–1–97 Edition): 392-425
2. van Roosmalen L., Bertocci, G.,Wolf, Peter (2007). Wheelchair Tiedown and Occupant Restraint system Issues in the Real World and the Virtual World: Combining Qualitative and Quantitative Research Approaches. Official Journal of RESNA 19.4:188-196
3. Turkovich,M., van Roosmalen L.,Hobson. D., Porach. E. Alternative Wheelchair Securement Systems- Performance During Normal and Emergency Driving in a Pubic Bus. Journal of Rehabilitation Research and Development. Submitted.