For a while now, I have been trying to secure a few old wheel chairs to use as big robot platforms. At first I tried searching the Craigs List, but as it turns out there is nothing better than actually asking for it! While searching, the best I could find was $200. But as soon as I asked, a chair for $75 was offered. Then a second for $150, but I actually managed to get it for $100. So for $175 I got two chairs, as opposed to one chair for $200. Looks like my large scale robot platform is in bussiness!
Why are they so cheap? As it turns out, mobility chairs are very expensive and are often subsidized by Medi Care. So Medi Care comes up with the 5K per chair and the users ride it until they become a burden. Replacing batteries is not chea! But guess what? Selling these chairs used is practically impossible as Medi Care will not subsidized used equipment. So now you have a big hunk of a chair, taking up precious space and nobody will pay what it is really worth. Enter us robotics enthusiasts, with some cash to spend on the next platform.
What will you get? Of course a chair, but this is not what we are interested in, so feel free to discard this portion of the rig as soon as you get home. You may want to hold onto it, though as they are pretty comfy and after cleaning, it may work for something else. What we can use are the two humongously large DC motors, already attached to wheels. This by itself is worth the $100.00. Inside you will also find a battery charger (it can not get more convenient than this!), up to four casters (I removed the two back ones), drained batteries (this is the only unfortunate part of the deal, as you may be stuck with a toxic piece of junk, although you can use it to buy new ones and skip some of the fees), and a controller. The controller is pretty much useles as it is wired and I just do not see a reason to employ a joystick based contraption, but you are free to differ.
The DC Motors
This is what we came looking for. These two DC motors are already attached to two awesome wheels. I always have a lot of problem with properly attaching wheels to a motor shaft, so this is a golden scenario for me. The only thing the motors have you may want to get rid of, is the brake. The brake is in place in case the wheel chair needs to stay positioned on an inclined surface. It sounds like a cool gadget but it implies you need to apply 24V at all times the motor is to move. Since I am not in the move of adding a relay per wheel and more microcontroller GPIO for this purpose, the brakes are gone. I will detail on a later post how to remove the brake.
There are four wires coming into the DC motors. Two are for the motor winding and two are for the brake mechanism. Of course, I will only use the two for the motor winding. I will soon be hooking the AE-MDL-7960 power stage boards and remote controlling the chairs. I will use the same rig I had on my R2D2 replica.
When I used the R2D2, one of the hassles was removing and placing the batteries on a continuous basis. As it turns out, on R2, there was no charger for the batteries other than my car battery charger. Good thing with the wheelchair robot is the battery charger is included and I will never have to tamper with the batteries. But of course, only until they are replaced.
There is a book out there that explains how to take the wheel chair motors and how to build the chassis. I think that if you have your own idea of how you want this to be, and are good at welding (which I am not), then that may be the way to go. But these wheelchairs have been designed to carry heavy loads! (AKA us). Why not take advantage of this?
More to come…