Introducing Kart-Bot!

Yeah, so I vanished last weekend when I had promised more updates. My bad. I half lost my motivation, and half just want to take this real slow and careful to avoid shorting out these motor controllers. It's a lot of voltage, and a lot of current coming out of the batteries. I'm just trying to be very careful.

In the mean time I made a decision on what direction to go with this robot. I'm no longer thinking of building an upright robot like say, "Johnny 5", instead I've decided to make a semi-autonomous vehicle. So it can still be a roving robot, but it can also be a vehicle that you ride.

Let's go over my design idea. Use the electric wheelchair base WITH a go-kart frame. This electric wheelchair base used what's called "differential steering" to maneuver. This works a lot like a tank, drive the right side faster than the left side to turn left, and vice versa. It had swivel wheels on the front that would just simply swing around to whatever direction the chair was going. This is a great system and it's something I plan to implement in the Kart. I'll need to relocate the front wheels of the wheelchair frame to the front of the Go-kart frame.

I plan to acquire a video game steering wheel/pedals assembly for input to the microcontroller that will drive the motor controllers. Steering while driving the motors will be nice and easy, take the steering wheel position and calculate what percentage left or right from center it's in, and make that a modifier to the gas pedal input. It'll go the direction you intend. The part that gets interesting is if you let off the gas in a corner or otherwise want to turn while coasting. I'm not quite sure if I can PWM the braking feature of the motor controllers. Let's assume I can. In that case, I need to tune a transition curve so that rolling on and off the throttle while cornering results in smooth operation without jerkiness. And finally, the brakes. Stepping on the brake pedal will need to not only be modified by the steering input, there'll have to be a certain amount that is covered by the motor controller braking, and then transition to the wheelchair's build in "electric brakes". This gets even more interesting, because I'm not sure if I can PWM those, either. I've seen a couple pictures online that make the wheelchair brakes look more like an electric clutch, like you might find on an AC compressor in a car. If that's what this has, it'll be real interesting to see if they can actually be used for braking. I'm sure they were originally only intended to be used to hold the wheelchair in place when stopped. I may have to come up with a way to install real brakes.

This is the GoKart frame I'll be using. It's a tired old frame that has been parked outside in the rain for the last four years after I used it in my last electric GoKart project. I picked it up on CraigsList about 4 1/2 years ago for $25. It's in better shape now than it was when I got it, sadly.

I shot some video of me welding the two together, but it's really pretty boring, and the audio was pretty bad. I started by cutting the back end off the go-kart frame so that the wheelchair base could butt up right against the back of the seat. I removed the front wheels from the wheelchair base, and then propped the back of the go-kart up at the right height, the front of the wheelchair base at it's correct height, did a whole bunch of measurements to make sure I had them as square as possible (I say as square AS POSSIBLE, because it seems like nothing on the go-kart from is really square, or ever was), then I tacked the seatback to to the upper tube on the wheelchair frame. (By tacked, I mean welded with a Mig welder).

I have some 1" gas pipe from the hardware store, it seems to be the same material this whole go-kart was made out of. I cut a piece of it that was long enough to go across the back of the go-kart frame, it is on the inside of the "tongue" of the wheelchair frame. I welded that in place and went back and finished welding the seat back. From there, I did a load test. It seemed to bow a bit more than I felt comfortable with, in the middle. So I decided to add some triangulation for support.

I started with these on the side, coming straight up from the kart's original tubes to the wheelchair's upper tube. This helped, but it seemed to put all the tension right at the end of that tube, so I wanted something farther forward.

I came up with these. They give me some excellent triangulation, I think. They come down from a higher point on the tongue (which ties the upper and lower half of the wheelchair base together, then it comes as far forward as I could possibly get away with, actually extending into the seat area, but doing it down low enough that I'll be able to work around it. This really took care of the vast majority of the flexing issue in the middle of the frame (with my 160lb body bouncing on it).

Here's a shot of the inside of the wheelchair frame, showing that first tube I ran across inside the tongue. Needless to say, I'm not a professional welder. But it gets the job done. I'm using flux core wire, if that's any excuse.. ;) Anyways, you can see all of the tubes I added, from this angle.

On some other notes, I setup a TIP122 transistor to release the brakes. I wasn't really sure how the original controller had them wired, but I figured it was in parallel, at 24v. I was a bit concerned about them at 36v, not the least of because of how much current they would be drawing at 36v. So I wired them in series. They draw .3 amps wired in series at 36v. I powered up my Arduino and confirmed that the Arduino can control them, at least in an on or off fashion. When I get the video game controller, I'll try some downhill coasting experiments to see if I can PWM them and use them as brakes.

I'm sure there's quite a bit of concern about the safety of a fly-by-wire Go-Kart.. So last weekend I ordered an emergency kill-switch:

push button

It'll be mounted somewhere that's easy for the driver to get to. Maybe under the steering wheel, or between the legs. Just punch/slap/kick that button and the motor controllers shut down and the electric brakes lock up. Should bring the thing to a stop very quickly in an emergency. It's only rated 10amps, so I won't be cutting power to the motor controllers completely, I'll simply be cutting power to the control chip that runs the mosfets on the motor controller. Good enough.

I measured the battery tray on the wheelchair base, and then I measured an 18ah SLA battery I had sitting around. Crazy enough, there's precisely enough room to fit 6 of these on that battery tray. So that's what I'll be doing. I'll wire three in series, twice, and then parallel the two sets of three. I haven't ordered the batteries yet, waiting on funding for that. For now, I've got 3 12ah SLA's from a chinese electric scooter. I taped the 3 together into one single "battery pack":

I put ring terminals so I can use the original battery cables with these until I get the real battery pack installed.

I've done some work with the original wiring inside the wheelchair base, I'll get pictures of that up soon. I've also got fans on the motor controllers now, I'll get pictures of that soon, too. I still need to come up with a mount for the motor controllers. Once I have a safe way for those to mount, and have tracked down a steering wheel/pedal setup for this, I can start trying to get this thing to move under it's own power. 

© Una 2011