Wheelchair Controller Died!

Tuesday, September 22nd, ©2009 Marcus Brooks
Cheap, Simple E-Vehicle Controller

Cheap, Simple, Dangerous? E-Vehicle Controller (For Emergencies)

The other week my homemade wheelchair’s speed controller died; sort of an odd failure, it just stopped delivering power—both channels dead.

I was using a RobotPower Sidewinder, a high-power two-channel proportional speed controller, and I’m at a loss to explain what killed it. It had a rough day the day before, but came through in fine shape—I thought. The next morning I rolled about 20 feet, stopped to do something, and couldn’t start again. Very unexpected.

I couldn’t bear staying in or near bed for a week or so, waiting for the replacement. I needed a simple, cheap wheelchair controller, fast!

My solution: bang bang controls! I got my brother to find me a couple of heavy-duty center-off DPDT toggle switches at Home Depot (sorry, I’ve misplaced the SKU number). Then I wired them up as a pair of motor-reversing switches, one for each of my two 24V motors. I wired the switches (and hence motors) in series to halve the motor voltages, and used only one 12V battery to halve the voltage again.

Two-Switch Controller Schematic Diagram

Two-Switch Controller Schematic Diagram

This creates a very limited controller with only five control states: Forward, Spin Left, Spin Right, Backward, Off; and only one speed: a slow walk. In the early R/C days this was called bang-bang control.

Controller Switch States

Controller Switch States

I have gotten used to this scheme now, so it’s pretty easy to maneuver. (Perhaps all that time driving vector-graphic tanks in BattleZone was well-spent.) There’s a rhythm to throwing switches in the right order to stop, turn, go straight, or sometimes “blip” one side for a course correction. The path tends to zig-zag, a sort of navigation by successive approximations, but it gets smoother with time. Still, I imagine I move like an oversize Dr. Strangelove, or one of the cheesier ’60’s TV robots.

If you ever try this setup on a vehicle be very careful! Slow as mine is, it dumped me on the landing the first time I tried to cross a threshold. That could have been extremely bad. As it was, it was damned embarrassing.

Remember, this setup has no brakes! Most good speed controllers have regenerative braking, which slows the motors on a slope by feeding power back into the batteries. In a switched setup like mine, the motors disconnect entirely and can spin freely. My chair’s steep gearing offers some resistance, but not much, so the chair can more or less run away on a slope.

That’s what happened when the door threshold dumped me: the chair seemed to be running away; I reversed automatically, and the abrupt control change jerked the chair out from under me! I would’ve fallen straight to the floor if I wasn’t wearing the seat belt. As it was, I was lowered more or less gently while the belt squeezed my belly and the chair rode up my back. A thinner person might have done a full-on face plant.

Now that I know better, I stay off the controls and push through thresholds; there’s not enough drop to really run away. On a ramp, I drag one foot to slow down. Still, I don’t think it would be safe to go out on the streets with this rig; there’s no real way to maneuver on a slope, and it’s just too easy to lose control.

Two-Switch Controller Wiring

Two-Switch Controller Wiring

One motor in this application is wired in reverse to run “tank style.” If your motors are “advance timed” to turn faster in one direction, you may notice a tendency to turn gradually when you expect to go straight. Sometimes my chair seems to do this, but I think it’s actually because one of the back casters has a sticky swivel. Another thing to fix!

My motors and battery happen to be pretty well sized for a slow, controllable speed with this setup. I’m using one 12V 35AH SLA battery and two brushed 500W 25A 2500RPM motors (wired in series) geared about 35:1 with 16″ dia. tires. Running weight is almost 600 lbs. With a charged battery I can go a couple of days or more before our 1:12 back-door ramp gets too steep to climb comfortably.

My chair is sturdy enough so I don’t think the jerky nature of this control scheme will stress it too much. My arthritic hips are another matter entirely. The setup works, but it can get very uncomfortable after a while.

There’s nothing special about the wiring, really. See the diagrams. I used three lengths of some jacketed 2-wire 18 gauge cable that I had handy; one length each for the battery and two motors. I braided them together to make them easier to manage. I don’t think the motors draw more than two or three amps for any length of time, so the gauge is adequate. Anything heavier would be hard to manage.

I found a neat way to daisy-chain the switches. I simply removed one center-pole lug screw from each switch and turned one of the removed screws around to fasten together the two lugs (and hence the switches). This makes for tight clearance between the switches’ adjacent throw lugs, so I laid a piece of heavy plastic insulation between them. After wiring I wrapped the whole assembly with a piece of large heat shrink tube, and mounted it on my chair arm with a scrap of aluminum.

I count myself lucky that the Sidewinder controller failed at home, and not out on the streets the day before. Even after I get the replacement controller working, I intend to stash this rig somewhere on the chair. That way I won’t be utterly stranded if the next controller fails out in the field.

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2 Responses to “Wheelchair Controller Died!”

  1. […] Robot Power Sidewinder speed controller died, and that I’ve been using a simple bang-bang controller instead. I have finally remedied the situation, but it took far longer than I […]

  2. marcus says:

    I believe one could add auto-braking to this circuit by connecting the outer center-pole terminals to each other.

    This dumps all the chair’s momentum as electric current, however, so the link should be through a low-value, high-current power resistor. Remember, the lower the value, the higher the current!

    I suspect you could route the braking current back to the battery (through diodes, of course) for regenerative braking, but if you get it wrong expect a fire or explosion or worse. YMMV.

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