The Wheelchair’s First Real Trip

Thursday, February 12th, ©2009 Marcus Brooks

DANGER: Before trying anything you see here, please consider your safety.

Now, whether it be bestial oblivion, or some craven scruple of looking too precisely on the event—a thought which quartered hath but one part wisdom and ever three parts coward—I do not know why yet I live to say, “this thing’s to do!,” sith I have cause, and will, and strength, and means, to do it! — Hamlet, Shakespeare

For a while this blog will touch frequently on the ATV Wheelchair that I’ve been building. We start the story in the middle, with the wheelchair built, but not finished. In short, it runs OK, and carries OK on its purpose-built trailer hitch adapter. So far, so good.

Today, however, the car and adapter were unavailable when I got called to an urgent meeting. I had planned, sooner or later, to begin using Austin’s Capital Metro system to get around. Apropos the quote above, my planning activity often pushes sooner off towards later. Suddenly I was amazed to find that my plans worked. Google Maps’ new Public Transit option, in Find Directions, instantly displayed the routes, stops, and times to get me where I had to be, when I had to be there. I made the meeting, but I learned some lessons.

First lesson: I must revise my joystick code. I use a little Zilog microcontroller to convert the joystick pot voltages to the R/C-style servo pulses understood by the motors’ speed controller. (More about all that in another post.) It works, but it’s a little jerky—OK, a lot jerky—so for now I’ve got the top speed regulated to two or three MPH. That’s fine indoors, but it gets tedious fast on a long city block. I’ll try a variable slew rate—speed changes take longer at higher speeds—to curb the bucking-bronco effect.

Second lesson: chain paths. I use ordinary turnbuckles between the motors to tension the first-stage drive chains. One turnbuckle is too thick for the space it’s in, so the chain rubs on it a little. Structurally it’s no big deal—the chain is steel and the buckle is aluminum. Nothing is going to wear enough to break. But in a bull-pen full of cubicles that little rub resonates into a very annoying squeal. I need to come up with a thinner tensioning plate. While I’m at it, I think I’ll reduce the gear ratio so the motors (and chains) don’t run so fast at walking speed. That will also increase my top speed for outdoors. 

Third lesson: brakes. The speed controller uses four-quadrant control for regenerative brakes—in short, the chair won’t coast. With the joystick centered, the chair is very hard to move—hard, but not impossible. The bus has tie-downs for wheelchair passengers, but the chair still tends to lurch around at stops and intersections. My wheel drive sprockets are machined inside for drum brakes, and I have the brakes, I just need to install them! While I’m at it, I should weld on more accessible tie-down loops.

There are other lessons. I want to make the seat back firmer, remount the water bottle carrier, and rig a laptop tray of some sort. Also I want to hook up the data logger before my next full-scale outing; I know about how much current the chair draws on carpet, but today was a far more thorough workout of grades, bumps, and obstacles. I missed all that great data because I didn’t have time to hook up the logger. Oh well, better luck next time!

3 Responses to “The Wheelchair’s First Real Trip”

  1. […] enough… (I use that phrase too often) …to catch a length of gauze bandage in my wheelchair’s right-hand drive sprocket. I heard a loud crack as the chair ground to a halt, and while […]

  2. Hello there! This is my first comment here so I just wanted to give a quick shout
    out and say I truly enjoy reading through your posts.
    Can you suggest any other blogs/websites/forums that cover the same topics?
    Thank you!

  3. marcus says:

    Dude, I was going to suggest something, but Google’s “I’m feeling lucky” link isn’t really random anymore.

Leave a Reply