Development of an Ergonomic One Arm Drive Manual Wheelchair
Allen Hoffman, Sean Cassidy, Shaun LeMarbre, Tiffany Madsen and Holly Ault
Mechanical Engineering Department, Worcester Polytechnic Institute
Worcester, MA 01609
ABSTRACT
Current designs for one arm propulsion of manual wheelchairs have several deficiencies with regard to functionality, ergonomics and intuitiveness of operation. A prototype of a new design was created and evaluated. This lever action design incorporates a new ergonomic user interface for simultaneously controlling the functions of propulsion, steering and braking. Evaluation of the prototype revealed improved propulsion and braking characteristics. Steering performance was hindered by oversteer which can be addressed by relatively minor design modifications.
Keywords:
manual wheelchair propulsion; ergonomics; hemiplegia; stroke
BACKGROUND
One arm drive wheelchairs are used by persons with a variety of medical conditions such as amputation, cerebral palsy and stroke. Two fundamental designs have been developed to enable one hand operation. Lever-driven or ratchet arm wheelchairs are propelled by pushing or pulling on the end of a lever mechanism. Lever activated propulsion has the potential to be more efficient than traditional pushrim propulsion (1). In addition, the lever functions as a tiller which acts through a mechanical linkage to steer one of the front castors. Braking may be accomplished by pulling or pushing the lever to the extreme forward or rearward position. These wheelchairs can only function in a one arm drive mode. Several types of designs have been proposed (2, 3). The second type of design is the dual hand rim where two hand rims are mounted on the same side of the wheelchair. The outer pushrim is attached to the wheel in the normal manner. The inner pushrim is connected to the opposite wheel via an axle. The axle is removable and allows the wheelchair to be easily folded. The dual hand rim design is available as an accessory that allows adaptation of a standard wheelchair (4). Without the axle in place, the wheelchair can be operated in the normal manner.
There are deficiencies associated with both of these designs particularly with respect to the user interface. The lever drive design usually has a fixed mechanical advantage. The ergonomics of simultaneous propulsion and steering can be awkward and the operation of the brake is not intuitive. In the dual pushrim design, steering and propulsion can not be actuated simultaneously. For a large number of users, the overall ergonomics of operation are not efficient. Braking via the dual pushrims is more difficult than with a standard wheelchair since the user must simultaneously grasp both pushrims to avoid turning. The goal of our work was to develop a one arm drive wheelchair that when compared to existing designs would exhibit improved functionality, ergonomics and intuitiveness of operation.
DESIGN AND DEVELOPMENT
Development of the new design focused on the 3 major functional areas; propulsion, steering and braking. Design specifications were formulated and several preliminary designs were developed for each functional area. Combinations of the top ranked designs for each functional area were then ranked according to their potential to be integrated with each other. The final design consists of subassemblies for each of the 3 functional areas and is operated through a single, ergonomically efficient, user interface. A cylindrical grip is used to grasp the interface which is located at the upper end of the input lever.
Photo 1. User InterfaceThe propulsion subassembly is a four-bar double rocker mechanism. The mechanical advantage of the propulsion mechanism can be changed by moving the location of the pin connection between the input rocker and coupler link. To propel the wheelchair forward, the occupant pushes the input rocker forward. A ratchet mechanism allows the user to pull the input rocker back to prepare for the next power stroke. The direction of wheelchair travel is controlled by a forward-reverse switch which engages the appropriate pawl on a double pawl mechanism. Steering is accomplished by rotation of the wrist (pronation/supination). The rotary motion of the user interface is transferred via a cable system to one of the front castors. The braking system uses bicycle brake hardware. A lever attached to the steering mechanism acts through a cable doubler to simultaneously activate caliper brakes on each wheel (Figure 1).
EVALUATION
A prototype of the new lever action design was compared to a commercially produced lever action wheelchair (Meyra®) and a wheelchair with a dual pushrim adaptation (Quickie®). A group of six able bodied volunteers with no prior wheelchair experience were recruited and given minimal instruction about each of the wheelchairs. Each volunteer was then asked to propel each wheelchair along a predetermined indoor course that required turning and going up and down an incline. Then each volunteer rated the ease of operation of propulsion, steering, braking and the overall performance of each wheelchair. A 5 point scale was used where 5 was “easy” and “1” was “difficult” (Table 1)
Operating Characteristics |
Prototype |
Meyra® |
Dual Hand Rim |
Ease of Propulsion |
3.8 |
3.5 |
1.8 |
Ease of Steering |
3.2 |
4.0 |
1.8 |
Ease of Braking |
4.3 |
2.7 |
3.0 |
Overall Performance |
3.7 |
3.7 |
2.2 |
The prototype of the new design substantially outranked the dual pushrim wheelchair in all categories. The propulsion and braking systems of the new design were rated higher than the Meyra® wheelchair, while the steering on the Meyra® was rated higher. The overall performance ratings of the prototype and the Meyra® were equal.
DISCUSSION
Testing revealed that the steering on the prototype was overly sensitive. The steering ratio in the prototype design was that one degree of rotation at the user interface resulted in 1.33 degrees of rotation of the front castor. While this ratio could be potentially useful for persons with very limited wrist rotation, it resulted in oversteering by the test volunteers. This was the main reason that the volunteers rated the ease of steering of the prototype lower than that of the Meyra®. The steering ratio could easily be altered in a second generation prototype to eliminate oversteering. In addition, the following changes should be incorporated into the next design. Presently, the steering castor is non-trailing. While this design is satisfactory for occupant operated steering, the steering castor should be redesigned as a trailing castor in order to allow a caretaker to guide the wheelchair from behind. Increasing the size of the castors would increase the performance of the wheelchair on irregular terrain. Other goals for a next generation prototype are to reduce the weight and width of the mechanism and provide a more easily reached location for the forward-reverse lever.
In summary, a prototype of a one arm drive manual wheelchair with an improved user interface has been designed and evaluated. Improved propulsion and braking characteristics have been demonstrated. Redesign of the steering mechanism to eliminate oversteer will lead to improvement in that operating characteristic as well. Future evaluation and comparisons should be performed by persons currently using other types of one arm drive wheelchairs.
REFERENCES
- Bauer, S. M. & Stone, V. I. (1999). Manual Wheelchair Propulsion White Paper. Proceedings of the Stakeholders Forum on Wheeled Mobility.
- Watwood, B (2002). Wheelchair Drive Mechanism. U.S. Patent 20040104554.
- Wong, S. (2003). Manually-Propelled Vehicle and Related Systems. U.S. Patent 20040051272.
- Sunrise Medical. http://www.sunrisemedical.com
Author Contact Information
Allen H. Hoffman PhD, Mechanical Engineering Department, Worcester Polytechnic Institute, 100 Institute Road, Worcester, MA 01609, Phone: (508) 831-5217,
Email: ahoffman@wpi.edu