Friday, April 23, 2010

Inch by Inch...

Small progress and no pictures this time ... tonight I mounted the on-board battery charger and sorted through the power cables. The original cables connect the batteries in series and include a 50 amp circuit breaker. I'm going to add a "kill switch" using an automotive headlight switch in the main battery feed ... this is recommended by several builders so there's a way to stop things quickly if the robot starts misbehaving.

I've been researching how rotary encoders work. I think I've settled on a design but haven't built it yet. I really should build those before mounting the motors, but I'm tempted to move ahead so I can have something running.

Hopefully I can get some time this weekend...

Monday, April 19, 2010

First Parts Go Back Together

A small step forward tonight getting the frame back inside the garage before the rains start and putting the rear caster wheels on. They still need a better washer under the axle nut, but it's good to connect a few parts together.

I also weighed many of the parts to get a better idea what the final monster will be like.

The frame tops the scales at 70 lbs. This thing is heavy. As you can see though, it's really solid. It's made to carry around up to 300 pounds in a comfy chair.

Other heavy stuff:
  • Frame : 70 lbs
  • 2 large wheels : 24 lbs
  • 4 small wheels : 4 lbs
  • Single 12v 35 amp-hour battery, 22 lbs or 44 lbs total
  • Two motors with 32:1 reduction gearbox : 25 lbs combined
  • On board 24v battery charger : 5 lbs
For those of you not doing the math, that comes to 172 pounds. Call it about 175 when the cables, switches and motor controller are added.

I threw on one motor to take a look at it's mount and how it interacts with the front suspension. The two smallest wheels stick out in front and normally ride a few inches above a flat surface. These can prevent the unit from tipping forward, or help ride up a curb or small obstacle.

The front and main wheels are mounted on a small bogie bracket. This can pivot slightly from a point just in front of the center drive wheels, and is held in place by the large spring shock unit. When the front wheels hit a curb, they are pushed up, which in turn pushes down on the main wheels and thus raises the frame to a climbing angle.

Sunday, April 18, 2010

Dissecting The Wheelchair

After bringing home my wheelchair unit, I took it apart to figure out what exactly I have. Since I didn't have the whole wheelchair, it took some digging at the Pride Mobility site to come to the conclusion that I probably have a Pride Jazzy 1121. I based this on the frame style and location of the serial number.

I pulled the unit apart until it was broken down into the basic parts : main frame, motor units detached, front and rear wheels, battery charger, motor controller, plastic fairings and interconnect cables. I didn't have any batteries.

After digging around in the internet, I came to the conclusion that the motor controller didn't have simple inputs, but was interfaced to the digital joystick controller (which I don't have) with some unknown control protocol. Pride is unfortunately very tight-fisted with their service info, so I couldn't find any manuals or interface information on the web. Thus the motor controller wasn't going to be easily useful, so it went up on eBay and actually paid for a third of the wheelchair cost.

I found the Sabertooth 2x25A controller looks perfect for this application ... lots of power, and a few different control modes to play with. This seems much better than trying to hack the Jazzy controller.

The motors were dissected to remove the power brake. All electric wheelchairs apparently must have these brakes, which are engaged unless the unit is powered. I hooked them up to a power supply and found they drew about 8 watts each, so with a few bolts removed (and a big internal spring popping out) they are gone.

In playing around with my motors, I unfortunately found one gearbox was stripped. It had a weak spot where the motor would start slipping. Damn - the motors were the main component I needed. I managed to find a reasonably priced replacement on eBay, however, so a week later I was back in business.

Finally, once everything was stripped down, a few touch-ups with flat black spray cleaned it up nicely. It's now ready to start going back together.

My first goal is a basic running chassis ... two batteries feeding the Sabertooth 2x25A motor controller, hooked up with dual variable resistors for left and right motor control.

Once that's running, I'd like to add Arduino motor control, plus some sensors such as motor shaft encoders, wifi link, a camera, etc etc etc... too many ideas here!

Sunday, April 11, 2010

In The Beginning...

A couple of weeks ago I started a project that's been sitting in the back of my mind for years ... build a robot based on an electric wheelchair. The idea certainly isn't new or original, but I wanted to get my hands dirty with some hardware that can move around in the world. I want to get my kids excited too, and show them the fun of building and experimentation.

I'll use this space to track the project, and keep a record of what I'm doing in the hope it will be interesting to others tackling similar projects.

So the line is drawn in the sand, and there's a pile of parts in the garage that will hopefully become something fun and exciting... I guess I should next explain the story behind that pile of parts, and how they might fit together.