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Electric motor switch
2

Electric motor switch

Electric motor switch

(OP)
My wife is an elementary school teacher and Ive agreed to help her with a project. They are putting on a play and need help with some stage props. They have a house and they want a cardboard cutout to move back and forth in a window. I have the basic idea of what I want to do and most of the materials. Im a machinist/engineer by trade so the design and fabrication aren't really the issue. I need help with the electrical stuff though. Basically what I plan on doing is taking a small electric motor and attaching a long threaded rod to the shaft. As the motor turns the shaft, the screw turns, and in so it will move the "nut" along the threaded rod. What Im having trouble with is when it gets to the end. I need it to come back and it would be great if I could make it pause before it changes directions. I would like to use a pressure switch at each end to change the direction of the motor rather than trying to do something with the timing. Any thoughts? Also, any suggestions on a motor? Id really like less than 10 rpms and it doesn't need a lot of torque or anything like that. Id like to be able to use around a 3/4" threaded rod on the shaft.

Thanks for any input.

RE: Electric motor switch

Why not a crank attached to the motor, and a crank rod to move the cutout back and forth? If you really are stuck on the threaded rod, I'd suggest buying a programmable "smart relay" and get a couple of microswitches to detect the end-of-travel position, and program the smart relay to switch motor direction.

You can buy low rpm, low torque gearmotors from a variety of sources for under $50 or so, mcmaster.com has them (but not necessarily the cheapest option). If you need to reverse direction, you will need a dc version of them, again you can find them at mcmaster, or search for them. Surplus houses have them for cheap, but you may not find the exact speed/rpm you are looking for there.

RE: Electric motor switch

(OP)
I was thinking cheap. With the design in my head I can make pretty much everything except for the motor and switch and Im just not sure what kind to get and the wiring.

A quick search found this switch. Its the style I was picturing in my head.

http://www.ebay.com/itm/TEMCo-Adjustable-Rod-Arm-L...

Any thoughts.

RE: Electric motor switch

Go to the local junk yard and pull out windshield wiper motor assembly.

Connect to a battery.

Done.

RE: Electric motor switch

So, the switch opens or closes when the drive hits one end stop...and you use that to reverse the motor direction...the drive then starts to move away and the switch closes reversing the motor again...and the unit just sits there rattling back and forth a few tenths of an inch or so from that one end stop. To fully move from one stop to the opposite stop some distance away will require you to flip and latch a relay, or maybe two relays to reverse the motor wiring. It won't be $6 cheap using just relays and diodes, nor with the programmable relay I suggested. But a crank arm is pretty cheap, and you don't need any switches or relays, and you can buy a 110v AC gearmotor for under $50 that will give you enough torque to do the job. But, that isn't YOUR idea, it's mine, and so there it lies.

RE: Electric motor switch

12v car window motor, (10 rpm is CRAWLING, so you will need gear reduction to shaft or less than 12V DC)
Limit switch at end of travel that is actually a toggle switch to reverse motor polarity (see: useless machine). You could also have a kill switch at start position that will turn it off once it returns.

Pause when changing directions could be tricky if you want more than 1 second or so, but depending on slop in the gear box and the motor speed, you could achieve desired affect with just reversing the motor.

RE: Electric motor switch

(OP)
I like my idea only because I have almost everything, it works in my head, and I can adjust the speed just by changing out the threaded rod but Im not stuck on it. I guess Im having trouble picturing what you are describing. Can you draw a quick diagram or give me a link to something that might make it clearer to me?

Thanks

RE: Electric motor switch

(OP)
Last post was intended for btrueblood.

thanks

RE: Electric motor switch

(OP)
1gibson -

Will a toggle switch at each end reverse it back and forth? Im not trying to stop it either end and the pause isn't really necessary.

RE: Electric motor switch

Quote (I like my idea)


Everybody does :)

Look at some "useless machine" schematics. Basically a toggle switch on top of a box. You flip the switch, something comes out of the box to flip the switch back, the something reverses and goes back into the box, until you flip the switch again. That's all it does, so you can see where the name comes from.

Power supply, use a wall wart (whatever is lying around) and if necessary a potentiometer to reduce voltage and slow down/speed up the motor. Think wireless home (not cell) phone charger, rechargeable battery charger, pretty much anything that blocks more than 1 power outlet when you plug it in. Just read the fine print and it will tell you voltage and current.

RE: Electric motor switch

Why not just have it run on a pulley system so the motor runs at the same speed and direction the entire time. It would also appear to pause as it rounds each end, just have it mounted so it swivels to keep the side facing the same direction the entire time. A bonus would be all you need to do is stop the swivel action and an human cutout would give a general appearance of pacing.

RE: Electric motor switch

I had to build some mechatronic assemblies years ago for a tradeshow, a scotch yoke might be just the thing. You get the reciprocal action and a pause at each end. As a plus, it can be handcranked by a willing volunteer.

Just a thought.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

RE: Electric motor switch

What's your deadline? The show must go on!

Best to you,

Goober Dave

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RE: Electric motor switch

(OP)
DRWeig

Last week of June. Any suggestions? Design or wiring

RE: Electric motor switch

Sorry, SpecialtyMFG --

I dwelt on doing this electrically. Our friend btrueblood is correct, you can't really do that on the cheap. Assuming DC motor, I could work it with a DPDT alternating relay plus an interval timer relay, and two of your limit switches you found.

The alternating relay can reverse the polarity of the motor with each contact opening, so as your object hits the limit switch the motor will be stopped and the polarity reversed. But it won't start again by itself. At that point the inteval timer relay would wait a second or two, then short out your open limit switch contact to start the motor with the now-reversed polarity, and keep the contact closed until the object has time to move away from the limit switch and let it close. Same thing happens at the other end. You'd only need one interval timer relay.

You can pick up DPDT alternating relay on eBay for $25 or so, but the interval timer is going to be $50 plus and I didn't find any on eBay.

If it were for an educational program, my former company might donate a timer -- but I don't think they'd do that for a school play.

Best idea: ornerynorsk's. Only moving a couple feet across a window, a scotch yoke is ideal.

Best to you,

Goober Dave

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RE: Electric motor switch

why not use a DPDT relay with a latching wire? Have a look at the attached and let me know if I've messed something up.

The idea is that you have a power switch, when you turn it on the relay is deenergized and the motor goes in reverse
the moving thing eventually bumps the "forward" pushbutton, which energizes the relay and reverses the motor direction (forward). relay latches "energized" until circuit is broken.
the moving thing moves to the other end of the screw where it hits the "deenergize" button and the relay goes back to the original state and the motor again goes "reverse"

flip the power switch to stop.
sorry, no pauses - but you can always run the object past the end of the stage before it reverses, right?

RE: Electric motor switch

That's a nice shot at it, ivymike. But there's a short circuit in there at the instant you push the NO (go forward) pushbutton. Follow the green highlighter in the attached.

You gave me a thought that your scheme might be fixable with a second relay -- but we still won't get the delay at the ends electrically. Your idea of just running the object out of sight at each end might do?

Best to you,

Goober Dave

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RE: Electric motor switch

just put a small resistor in the "latch" wire then?

RE: Electric motor switch

heh.. or a diode?

RE: Electric motor switch

Relay seems overkill for what I assume is a small amount of current. Try this on:

You have a locking push button switch with 6 contacts on the bottom (dual pole, dual throw) that you use as a limit switch.

1-2-3
4-5-6

You jumper from 1-6 and 3-4
Power supply in goes to the middle: 2(+ wire) and 5(- wire)
You can wire the motor (or the center of another identical switch at the other end) to 1 and 4.

Toggle switch left (really a locking push button, but calling it toggle to illustrate the diagram)
2+ connects to 1 which is +
5- connects to 4 which is -
Motor turns clockwise

Toggle switch right
2+ connects to 3+ which is jumpered to 4, now outputs +
5- connects to 6- which is jumpered to 1, now outputs -
Motor turns counter clockwise


Every time a limit switch is hit, it flips the polarity and changes motor direction. If you have a switch on each end, the motor will change directions at each limit, and run back and forth all day. Add a main on/off power switch in front of all this stuff.

Maybe 10 bucks in wire and switches.

Alright, now somebody poke some holes in that idea!

RE: Electric motor switch

that ought to be workable with a pair of rocker switches, but if it's a locking pushbutton at each end when does it pop back out?

The relay w/diode setup should also be around $10.

RE: Electric motor switch

Click in and stay in, click again and it comes out. Like a pen. A lot of possible solutions.

RE: Electric motor switch

I'd do this with a clothesline and a volunteer. But I'm cheap like that.

RE: Electric motor switch

Very good, 1gibson. That's not a very pokeable-holeable-innable idea. You've made a cross-wired alternating relay with maintained-contact switches instead of a coil. We still have the no-pause issue, but that may not be a problem at all.

I'll volunteer a quick sketch (attached).

Best to you,

Goober Dave

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RE: Electric motor switch

1gibson,

What toggles either switch back to its normal position? I.e., what device, mechanical or electrical, resets the toggle/locking pushbotton switch to the "sense" mode after it switches polarity to reverse the motor? A push-and-stay locking pushbutton switch stays in the left-drive mode as the motor moves left, and suddenly hits the right hand switch - which now connects directly across the closed left switch - zap! Little brown star for you, to counteract the pink one you got...

Without latching relays it won't work, or you have to wire a single switch with a long mechanical hysteresis device to make it toggle as the motor drive nut swings from one side to the other, and that's not simple $10 stuff either. Latching relays can be made to do the job with momentary contact limit switches...but by the time you are done, you'll spend at least $50. Plus you still have to buy the motor.

SpecMFG, a window wiper mechanism is a pretty good analogy to what I was trying to describe earlier. An eccentric (crank, cam, whatever) drives a lever back and forth, the object could either be directly attached to the lever, or slide on a rail being pushed/pulled by a slider attached to the oscillating lever.

RE: Electric motor switch

This is the switch type I am talking about, can't seem to find the correct name:



Bruteblood, I just don't see the "zap."


Here is the logic:

Power directly to motor would turn it CW, both switch 1 (S1) and switch 2 (S2) in position A turns motor CW

Power -> S1 A -> S2 A -> motor is CW, cutout moves to the right
Hits S2 right limit switch
Power -> S1 A -> S2 B -> CCW, cutout moves to the left
Hits S1 left limit switch
Power -> S1 B -> S2 B -> CW, moves to right
Hits S2 right limit switch
Power -> S1 B -> S2 A -> CCW, moves to left
Hits S1 left limit switch
Power -> S1 A -> S2 A -> CW, moves to the right

Rinse and repeat.

Goober Dave hit the nail on the head with the sketch.

RE: Electric motor switch

It's a maintained-contact pushbutton switch. Also known as ON-ON. btrueblood, it is a latching relay, but without a coil. Each time the object rams into it, the polarity of the motor wires will change.

Exactly like a ballpoint retractable pen in action. Push-click - one side goes ON, Push-click, other side goes ON. The button is always extended, waiting to be pushed.

The reset occurs the next time either switch is hit. Take a look at the attached.

I can use these diagrams in a training class I'm putting on next month -- but I might pretty them up a bit.



Best to you,

Goober Dave

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RE: Electric motor switch

Dang it! I'm glad it's Friday. Hit the submit button again instead of preview.

Pay no attention to the picture of the SPST model in this web page. It's a Thorsen Model #35-405, which is DPDT ON-ON.

http://www.frys.com/product/2874604?site=sr:SEARCH...


Best to you,

Goober Dave

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RE: Electric motor switch

ok, got it. Brown star retracted. Should not have had the 2nd beer at lunchtime.

RE: Electric motor switch

I've got one more suggestion, then I plan to catch up with btrueblood who has a 2 beer head start on me!

You have a single DPDT toggle switch, wired as described previously, but this time it is attached to the back of the cutout.

You have a bump stop at each end (dowel rod attached to window frame?) that aligns with the toggle switch. When the switch runs into the dowel, it flips, switches polarity and changes direction.

RE: Electric motor switch

Now as far as a motor goes, I'd propose that you get ahold of a power wheels motor and gearbox, used. A 6V model should be something you can find cheap on craigslist.

RE: Electric motor switch

By the way, ivymike -- your resistor-in-the-latching-wire idea might have worked too. I don't think a diode would do, though -- current has to go both ways through the Y portion of your blue wire.

Best to you,

Goober Dave

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RE: Electric motor switch

Wrong spot...use the diode for the latch wire instead of the red loop.

RE: Electric motor switch

blush My bad, ivymike.

Best to you,

Goober Dave

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RE: Electric motor switch

there is a thread / nut system that is bi-directional, it winds the nut along its length and at the end of the threaded rod it hits a stop and then changes direction to return. Where / how / why to get the info I don't know - leave it to the OP to source it ..
I did find this http://www.practicalmachinist.com/vb/general/how-m... which might give a starting point.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: Electric motor switch

I would go with a pulley or sprocket at each side of the opening. Run an endless chain or belt between them and attach a link from a spot on the chain to the object to be moved.

Bill
--------------------
"Why not the best?"
Jimmy Carter

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