Hello Everyone,

I’m new to your forum and only have the most basic electrical background (home repairs, garage wiring, and an automotive electrical class). I’m venturing into a new project, a motorized forward and reverse revolving chair. I have some preliminary ideas and sure could use some expert advice at this point.

Function:
The chair, upon manually pressing a button, is to rotate one revolution to the right and stop automatically. Then, pressing another button it is to rotate in reverse (left) one revolution and stop automatically. Each rotation should take about a minute. Weight capacity could be 200 pounds and as high as 300 pounds. Ideally the start-up should be pretty smooth and steady as it completes a rotation and a smooth stop.

I’m thinking a low speed high torque motor with a worm-gear on it. This would drive a gear that is fixed to the shaft of a rotatable chair. Controlled by forward and reverse buttons.

Does this look like a job for a DC motor?

Which would be easiest and lower cost: using mechanical or induction limiters attached to the gear affixed to the chair shaft or would a 555 timer be better (would I need two, one for each direction?).

Attached is a crude line drawing.

Thank you for your time and ideas.
Dmend

 

I advise that you keep the motor under manual control, or provide an emergency stop function.

This circuit will function similarly to power window controls in an auto, but latching:

For non-latching operation, remove D3/D4.

Motor reverse-EMF (flywheel) diodes are not shown to keep the schematic less cluttered.

 

Could not bring up your diagram. If worm wheel has 60 teeth, them worm turns at 60 RPM ,requiring about a 50 to 1 gear reduced DC motor of about 50W.

 

Look at The Electronic Goldmine # G 17355 & others, or All Electronics, Cat # DCM-360

 

Thanks for the diagram. I understand the general concept depicted, but, I'll have to spend some time in the site reference pages before I can ask further questions on this.

The diagram is a MSword document.

Thanks for the web store suggestions. The All Electronics, Cat # DCM-360 motor has potential. Can you suggest web stores where worm gear sets or spur gears can be found?

Thanks,
dmend

 

Can you suggest web stores where worm gear sets or spur gears can be found?

McMaster-Carr
Grainger
Surplus Center

(I didn't see gears listed at Grainger or Surplus Center, but they often have other electrical, mechanical and hydraulic bits, and Surplus Center often has very good prices, as well.)

 

I suggest using a stepper motor, not a D.C. motor. If you use the latter, you'll need a sensor to tell you when you've reached a full revolution, whereas the stepper inherently controls its location. To start and stop smoothly, you could use hardware-built electronics, but it's a lot simpler in this day and age to use a little microprocessor, like an Arduino.

In fact you'd most likely need a processor whichever kind of motor you use, just to do the starting and stopping and reading a sensor etc.

Note also that in the D.C. motor case, you would have to use two sensors--one to decide when to slow down and one to decide when to stop. Maybe three sensors, if the stopping point must be approached from either side. And then, will you want the stopping point to be exactly the same every time? So many decisions.

 

You could also attach a hall-effect sensor to the chassis side of the chair, with a magnet on the wheel besides it (passing each other one time per revolution). Then make the electronics to spinn the motor until voltage increases on hall-effect sensor. Sort of like a bicycle speedometer I had when I was younger.

 

I don't think the hall-effect sensor will work with this application: I'm only making one chair revolution, actually, it should be stopped at the end of one chair revolution, as compared to a motor revolution.

Stepper Motor:
Once power is cut, wouldn't a stepper motor naturally come to a halt faster than other other motor types?
A sensor to slow the motor down? Wouldn't just cutting the power (just short of one chair revolution) with one sensor be enough? If the motor has a brake on it it should stop , maybe even too fast. Also, I've added a tapered roller bearing to the vertical chair shaft to reduce resistance, thus reducing the amount torque necessary to move the chair.


AC Gearmotor:
At Mcmaster.com page 1008, there are low rpm, high torque ac gearmotors. With sensors or timer to cut the power, couldn't this be a simpler set-up? What do you see as the trade-offs compared to the stepper motor?

Thanks,
dmend

 

I think an AC motor is unusable, as they're almost always made for just one direction of rotation. There are exceptions, but I didn't see them in McMaster-Carr. DC motors are much easier to control, and for the speed you're talking about, you don't need a big one.

You ought to plan how to connect the final drive to the chair. Whatever the drive is, the chair's support must pass through the hub of the gear or pulley, and I'd bet that it's an inch or so in diameter, so the bore of the pulley has to be large, and large gears are very expensive, plus they always need precision mounting. But if you always turn the chair one way and then the other, never many turns the same way in succession, you could build a pulley in two sections out of plywood, clamp it around the shaft and drive it via a cord wrapped around the edge and pinned. The cord would be pulled by being wrapped/unwrapped on a motor-driven drum. That's a cheap solution!

A lot depends on how precise you want to be with your one turn. If it's acceptable to start a motor and run it for a fixed time, that's quite easy. But if it needs more than that, you have to think about sensors and controls. A stepper motor needs an electronic drive system to control its speed--you wouldn't want to just "cut the power". The electronics would gently accelerate the motor, run it at constant speed and then bring it to a gentle stop in (hopefully) exactly the right place.

But with a system where the chair takes one full minute per turn, maybe the difference between full speed and stop is so small that you don't need to worry about acceleration at all!

 

Harbor Freight has a hand crank worm drive wench, might need some machineing. A belt drive with more reduction might be easier. A code disc could be placed on gear or pulley, read with reflective sensor, which would allow input for position & speed.

 

I like John P's idea. It would work quite well with the relatively simple limit switch circuit I posted early on.

I don't see why this project needs lots of machining or lots of expensive parts. I suggest that anything involving high voltage is out from the safety aspects alone.

Just a simple low-voltage DC-motor driven system with pulley reduction and limit switches will do fine.

 

The chair post I plan on using is 3/4" in diameter. I've a couple of options with gear sets, worm and spur. I'll probably use a stepper motor. Now I've got to work all this out on paper before jumping in and buying the parts. I thank you all for your help, I'll let you know how my final design turns out.

Until then...,
Dmend