Home - All Terrain Wheelchair

All Terrain Wheelchair

SPRING 2015 MAE 156B SPONSORED PROJECT

UNIVERSITY OF CALIFORNIA, SAN DIEGO

sponsored by Scott Carlson

(Figure 1) Original wheelchair, Permobil C300 (Figure 2) Redesigned wheelchair

Background

Scott Carlson is a graduate student at UCSD. He was born with a form of muscular dystrophy and currently uses a Permobil C300 model wheelchair, that is fitted for smooth surfaces and has extremely limited abilities in outdoor terrains. Scott is planning a trip to Cancun, Mexico this August/September where he plans to bring his chair and use it on the beach. Because the current state of his wheelchair dramatically limits his mobility in outdoor terrains, he has requested the modification of his current wheelchair to function in both soft and rigid outdoor surfaces while maintaining the current indoor usability.

Objective

The primary objective of this project is to modify Scott's wheelchair so that it will be able to perform in various outdoor terrains (sand, snow, and gravel) while maintaining indoor usability. The modified system should be able to generate more traction and torque in order to traverse freely between soft terrains like sand and harder surfaces such as concrete. The modified wheelchair must also be able to fit through both interior and exterior doors ranging between 28-36 inches.

Functional Requirements

• Modify the existing wheelchair, Permobil C300, for outdoor accessibility. The modified wheelchair should be able to traverse through sand, snow and loose gravel easily

• The modified wheelchair should have reasonable dimensions and turning radius to allow for indoor use (28" wide to be able to fit the smallest door frame)

• The modification should meet or exceed the safety standards of the current wheelchair

Final Design

Design Features

• All Terrain Wheels

• Modified Chassis

All Terrain Wheels

During initial testing of the unmodified wheelchair’s performance in sand, it was found that the wheelchair would easily become immobilized in sand deeper than 1”. The drive wheels would get stuck in sand and lose traction. Applying power to the drive wheels would worsen the situation as the wheels would free spin and dig deeper into the sand, making the wheelchair sink further and becoming harder to move.

The original drive tires were foam-filled. Their rigidity and narrow width cut through sand easily, which made them ineffective in sand. The tires did not allow the wheelchair to float over the sand. Therefore, the tires were replaced with ones that could improve traction and reduce sinkage in sand.

We decided to go with the All-terrain tires because it was able to give the wheelchair significantly more traction than before. The wider tires also provided flotation in the sand which prevented the chair from sinking into the sand. Figure 4 is a preview of what the wheelchair looks like with the new tires.

(Figure 3) All-terrrain tires used (Figure 4) CAD model of wheelchair base with new tires

Modified Chassis

With the new tires, the wheelchair has a width of 29 inches. The United States American Disability Act states that door width should be a minimum of 32''; however, the international standards for door widths are much smaller, around 28 inches. In order for the modified wheelchair to fit within doorways, the wheels need to be placed closer together. However, the tires could not be put any closer because of the motor and batteries are in its way (See the red-battery and blue-motor in Figure 4). Therefore, the current chassis needs to be smaller in width in order to have the wider tires fit within the door-size constraint. To create room for the tires, the front battery will be rotated 90 degrees seen in Figure 5.

(Figure 5) The transition of the wheelchairs width

Finite Element Analysis (FEA) Comparison between the Original, Modified, and New Chassis Design

We chose the modified chassis design based on its factor of safety from finite element analysis and ease of construction based on the time constrain we have for our project. Overall, the original wheelchair had a maximum stress of 101.6 MPa and a maximum displacement of 1.28mm under normal vertical loading conditions; this comes out to a factor of safety of 2.7 in stress. The modified chassis design was chosen because it had a factor of safety of 3.73 in stress with a max stress of 94.2 MPa and 0.79mm of maximum displacement, respectively. The modified chassis out performed the new chassis design in terms of factor of safety in the vertical loading direction (the most realistic loading conditions) ; that was a very important aspect of our design decision since we had to make sure any design considerations we made had to be stronger than our original design in order to account for the safety of our sponsor who will be using the chair.

Fabrication Steps For Chassis

1) Measure parts that need to be cut*

2) Cut up old chassis into two pieces

3) Machine reinforcement part

4) Assembly of modified chassis

5) Fasten parts together

* The measuring step was repeated frequently in each step of the fabrication process.

Below are CAD drawings of the modified chassis that the team constructed.

Performance

executive summary