It’s taken me the best part of the day to filing away so that the motors fit onto the forks, but it’s done, and all six motors are now sitting in the forks. Shoulder aching, I decided to push on and managed to get another wheel laced, then much to Ada’s delight, mocked up part of the frame (a bogie) for the actual Mountain Wheelchair!
Recently, whilst building the larger prototype, one of the things that has caused delay is one of 3D printers being busy with other projects and the other one completely out of action. I spent most of last week stripping it down and rebuilding it and gave the working printer an overhaul whilst I was at it. It was a huge relief once both printers were up and running again and the quality of the prints are now better than ever.
Although it works, the syringes aren’t perfect and small air bubbles are being drawn in. As a proof of concept though, it’s great, and I think it has more potential than both the differential gears and the differential bar. The reasons being that there is less wear and tear on the hydraulics (than the other proposed methods) and therefore less maintenance with increased reliability, but also it has a dampening affect which should result in smoother ride for Ada.
In a previous post I had demonstrated that the bogie often flips upside down:
Obviously, if this happened in the mountains with a full size “vehicle” there could be serious consequences. For the moment, I’ve come up with a simple solution.
In the image below you can see that I’ve added a small protrusion above the pivot point of the bogie.
In the full size version, I don’t think this would be a viable solution without the use of springs/dampers but for this early prototype it should suffice.
Today all those years of playing with Lego as a child was finally put to good use as I made a couple of quick mockups of possible mountain wheelchair solutions. One is the obvious choice of caterpillar tracks, the other is a rocker-bogie mechanism inspired by NASA’s Mars rovers.
I really like the simplicity of the caterpillar tracks and pushing this simple Lego model you can feel that it wants to go over obstacles. You can see that I’ve angled the front of the tracks to help it climb over larger obstacles. My main concern with this design is that it looks like it could tip over backwards quite easily.
The rocker bogie mechanism looks like it has excellent potential for overcoming obstacles but it seems finicky and things can go wrong in lots of different ways. I imagine that once there is drive to each of the wheels (as opposed to being pushed by hand) that some of these issues will be resolved.
The rocker bogie mechanism is worth exploring because even in the Lego models it looks like a comfier ride and in theory a greater ability to overcome larger obstacles. Looking in these two pictures, you can see imagine that as the caterpillar track version travels over the obstacles the seat would bounce all over the place, whereas with the rocker bogie, a differential mechanism keeps the seat level.
The following images show the differential. As the rocker on one side of the bogie tilts, the differential forces the other side in opposite direction. This results in the seat having an angle which is an average of the two sides. Most of the time then, the seat will remain parallel to the ground no matter what the wheels are doing.
Another advantage of the rocker bogie mechanism is that it removes the need for suspension. As you can see in the following images as the wheelchair comes down off an obstacle there is a gradual process and the seat remains relatively level throughout.
Whereas in the caterpillar track design the wheelchair pivots over the apex of the obstacle and then comes down with a thud.
By comparing the angle of the seat between the two designs as they step off the obstacle have left me feeling that the rocker bogie design is worth exploring first.