Linda van Roosmalen PhD and Jason Sellers BS
Department of Rehabilitation Science and Technology
University of Pittsburgh, Pittsburgh, PA, USA

ABSTRACT

Wheelchair users with limited upper extremity function have difficulty in using belt and buckle type safety restraints when they ride motor vehicles while seated in their wheelchairs. A wheelchair-anchored pelvic-belt restraint with a user-friendly buckle mechanism was designed to enable independent occupant restraint use by individuals with reduced upper extremity function. Design criteria included usability and robustness during daily use as well as compliance with ANSI/RESNA WC19 (WC19). An early prototype of the design was mounted onto a manual wheelchair and evaluated by individuals with limited upper extremity function. The final design was optimized based on user input and a solid model of the design was evaluated using Finite Element Analysis (FEA).

Key words:

wheelchair, transportation, restraint, safety, usability, independence, buckle

BACKGROUND

Wheelchair users often use their wheelchairs as motor vehicle seats when riding as a passenger or driver in a motor vehicle. The use of wheelchair tiedowns and occupant restraint systems (WTORS) provide wheelchair occupants with crash protection during motor vehicle travel. Wheelchair tiedowns limit wheelchair movement and upper torso and pelvic safety restraints prevent wheelchair occupants from sliding forward from their wheelchairs and/or being ejected from their wheelchair seat.

Figure 1: Photo of typical automotive seatbelt buckle, with push-button receptacle and latch plate. (Click image for larger view)

A study conducted by Van Roosmalen, et al. shows that wheelchair-anchored occupant restraints are a feasible solution to improve wheelchair transportation safety, and suggests that further development of wheelchair-anchored pelvic belts in conjunction with improved belt fit and usage would be beneficial to the safety and independence of wheelchair users [1]. Attaching the pelvic-belt portion of the occupant restraint system to the wheelchair is the first step to improved independence and safety of individuals riding in vehicles. However, results from a study conducted among wheelchair users indicate that typical buckle and latch plate-style belt restraints are difficult to use independently by individuals with limited upper extremity function [2] (Figure 1).

This study proposes a solution to the (user) problems that exist with buckle mechanisms on occupant restraints. In addition a pelvic safety restraint design is proposed to connect with a vehicle anchored shoulder belt, to provide optimal belt fit and comfort to potential users and to not cause user injury during a motor vehicle incident or accident.

STUDY AIMS

The aim of this study is to design and evaluate a wheelchair-anchored pelvic belt for use by wheelchair users with limited upper-extremity function. The pelvic belt should also provide for attachment of an upper torso belt by means of a standard pin connector to comprise a three-point belt restraint system. It should also be designed so as to not interfere with the wheelchair user’s activities of daily living (e.g. transfers, postural support use, reaching) and must be cost effective and aesthetically pleasing to potential users.

DESIGN METHOD

Results from a usability study formed the basis for the design of the improved pelvic-belt restraint [2]. Suggestions to improve pelvic belt usability of existing buckle and latch-plate included the following:

1. Place buckle within reach
2. Place latch plate within reach
3. Fixate buckle receptacle during connecting
4. Make it easier to release latch plate from buckle receptacle
5. Reduce release spring force on buckle
6. Make it easier to connect latch plate with buckle
7. Make latch-plate grabbing and belt tensioning easier
8. Integrate pelvic belt with postural support system
9. Provide ability to snug the belt
10. Have less tension on belt when performing belt-donning

Several pelvic belt restraint ideas were generated and the most promising was built into a test prototype and evaluated by potential wheelchair users with a range of upper-extremity limitations (Figure 2). Individuals were asked to buckle up and release the prototype pelvic belt several times. Based on this preliminary user study several design changes were made to the pelvic restraint components to improve its usability.

An improved pelvic restraint concept was developed using Solid Works. FEA using COSMOS was used to evaluate the strength of various key components during impact loading. A load of 3000 lbs was used for the FEA and represents the load that can act onto a wheelchair-anchored pelvic belt and vehicle-anchored upper-torso belt during a 20-g/30-mph sled impact with a manual wheelchair occupied by a 50th percentile Hybrid III Anthropomorphic Test Device [3].

DESIGN SOLUTION

Figure 3 shows the assembly of the improved pelvic restraint when attached to the rear securement points on a manual wheelchair. The rear securement points were slightly modified and re-evaluated for their use as pelvic-belt  anchor points. The wheelchair and use of a wheelchair-anchored pelvic belt was previously evaluated during a 20-g/30-mph sled impact test.

Wheelchair occupants strive for independence, and a restraint device that is simple to operate, preferably with one hand, is a must. This means that the restraint must be able to be buckled and unbuckled with one hand using minimal effort.  The push-button mechanism present on common buckle designs can only be activated by fingers or knuckles. A rotational buckle mechanism gives the user the choice to disengage with the use of the fingers, the palm of the hand, or even the distal forearm. The rotary buckle knob has rather smooth and rounded edges so that clothing or other items cannot hook onto the buckle and inadvertently open it. However, it does have three edges to allow a hand to create enough moment to release the buckle (Figure 4).

Figure 4: Rotary buckle with several funneled slots to allow for latch plate misalignment. (Click image for larger view)

Subjects had difficulty aiming the latch plate into the buckle slot. Funneled slots in the rotary buckle design make it easier to connect and release the latch plate by allowing the latch plate to enter and exit under different angles (Figure 4). Individuals from the user study also had difficulty grasping and holding a flat metal latch plate. They often used the little latch hole of the plate to pull with one finger. However, this hole is fairly narrow and sharp at the edges and is not always present on latch plates. Therefore, to make the latch plate easier to reach, grab, and hold onto, a flexible grasp hook was designed to fit over the latch plate (Figure 5). This way, the user can grab, pull, and guide the latch plate into the rotary buckle. The latch plate is attached to webbing which is housed in a wheelchair-mounted retractor that provides automatic adjustability of the pelvic belt. To make it easy for individuals who have use  of only one hand to connect the latch plate with the buckle, the buckle mechanism is fixed onto a stiff curved rod that goes around the user’s abdomen. The length of the rod can easily be altered by a caregiver to adjust the buckle properly for each adult wheelchair user (Figure 6). The adjuster attaches with a bolt connection to the rear securement points of the wheelchair.

Figure 6: Drawing of buckle adjuster mechanism attached to the wheelchair’s rear securement point.(Click image for larger view)

When the rotary buckle, the mount, and adjuster components were quasi-statically loaded to 3000 lb (1500 lb times a with safety factor of 2.0), COSMOS results indicate that the material stresses stayed below the yield strength, and system components showed no points of failure or excessive deformation during loading (see Figure 7).

CONCLUSIONS AND FUTURE STEPS

An easy-to-use rotary-style pelvic belt restraint system was designed to attach to the securement points of a crashworthy manual wheelchair frame. Early user evaluations show that a rotary buckle is easier to connect and release independently than a push-style buckle, and a hook on the latch plate side of the seat belt can assist users during the latching process. FEA results of the pelvic restraint show that it can withstand the loading up to 3000 lb. Future considerations include a user study to evaluate the torque required to release the rotary buckle and to look at overall product usability and comfort during normal use of the device. The prototype pelvic belt restraint design needs to be evaluated under dynamic loading conditions. Since this restraint was designed to attach to the rear securement points of a wheelchair, it can easily be placed on other type wheelchairs that comply with WC19.

Figure 7: FEA results for the adjuster (Al7176) and the rotary buckle and latch component when loaded at 3000 lb.(Click image for larger view)

The study focus was only on the pelvic belt of the occupant restraint system.  However, to safely restrain a wheelchair occupant during transportation in motor vehicles, a vehicle-anchored shoulder belt needs to be used and connected (currently by pin-bushing) to the pelvic belt near the hip of the user. The ease of use and safety of the standard pin-bushing connector on the pelvic restraint design needs further investigation.

REFERENCES

1. Van Roosmalen, L., Bertocci, G. E., Ha, D. and Karg, P. (2002). Wheelchair integrated occupant restraints: feasibility in frontal impact. Medical Engineering & Physics 23(10): 685-698.
2. Van Roosmalen, L., Reed, M., Bertocci, G. E. Pilot Study of Safety Belt Usability For Vehicle Occupants Seated In Wheelchairs. Assistive Technology Journal 17(1), 2006.
3. American National Standards Institute (ANSI)/Rehabilitation Engineering Society of North America (RESNA), (1998). WC/Vol. 1-Section 19: Wheelchairs used as seats in motor vehicles. ANSI/RESNA.

ACKNOWLEDGEMENTS

This project was funded by the Rehabilitation Engineering Research Center on Wheelchair Transportation Safety, Grant #H133E010302 from the National Institute on Disability and Rehabilitation Research, U.S. Department of Education. We thank IMMI (J. Chinni) and the VA-HERL (J. Puhlman) for their assistance with prototyping and Sunrise Medical for donating a manual wheelchair. Opinions are those of the authors only and do not necessarily reflect those of the funding agency.

Linda van Roosmalen
University of Pittsburgh
Department of Rehabilitation Science and Technology
Suite 1311, 2310 Jane St
Pittsburgh, PA 15203
412-586-6911
Lvanroos@pitt.edu

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