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Author Topic: Motors question  (Read 1241 times)

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buildingmotors

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Motors question
« on: March 23, 2015, 09:09:01 am »
Hey all,

      Long time watcher, this is my first post.  I got the idea to build something, but its going to need a motor.  I have some tiny motors but they dont act the way that I need.  I am more of a programmer than a mechanic so I will try to describe it the best that i can.  The motors I currently have are "free flowing" so I can turn them on, reverse the way they spin, but when they stop, they can be turned freely.  I need to know what kind of motor acts more like a servo.  So that i can spin it in two directions, but when it is stopped, it stays stationary.  This motor will be used to turn a pully belt and servo isnt really feasable in the space. I hope this wasnt too vague.

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Re: Motors question
« Reply #1 on: March 23, 2015, 01:27:33 pm »
Hello buildingmotors, and welcome to the forum!
Sounds like you need some sort of "parking brake". This is a mechanical device that engages on the motor shaft when the motor is not being told to run. It is usually a friction type setup that "holds" the shaft and keeps it from turning. When motor is told to turn, the brake must be energized to release the shaft so it can turn.
Or you can use a motor that transfers it's rotation through a gearbox. The gearbox output will be considerably slower than the motor rotation, but it has much more torque.
This basically holds the device you are turning when the motor shuts off, due to being very hard for an external force to turn it.
you could also have a solenoid to engage and lock the rotation when the motor is turned off.
This is normally called a lock.
I think a servo can be braked also by leaving one step on, but im not sure how strong this method would be and it could cause excess heat in the motor.

lilshawn

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Re: Motors question
« Reply #2 on: March 23, 2015, 01:43:55 pm »
yes, a servo motor can be "locked" by stopping the motor on one step by energizing one of the coils. this will "lock" the shaft in place. (IE it will be difficult to turn, but not impossible) you wouldn't do it at full power for a long period of time... but just enough to stop the shaft sufficiently. maybe a 50 or 60% duty cycle. your servo motor documentation/datasheet would specifically address this.

my daughters electric wheelchair motor has a electromagnetic brake on the shaft of the motor. when the motor is energized, a solenoid pulls back a paul that is notched into the shaft keeping it from turning. to run the motor the solenoid energizes and the motor runs. at the end of run, the motor stops and the solenoid releases and the paul snaps via a spring mechanism into the motor shaft to keep it steady.

alternatively, connecting the motor leads together kind of acts like a self brake. it's not particularly strong though... it's good for flywheel driven motors to brake them to slow them in this fashion.

alternatively yet again, you could use a rotary encoder on a DC motor shaft and if your microcontroller sees the motor turning when it's not supposed to, it can reverse or forward the motor to keep it within a certain #of ticks of it's stopping point.

buildingmotors

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Re: Motors question
« Reply #3 on: March 23, 2015, 02:06:46 pm »
Thank you for your responses.   DaOld Man's parking brake is exactly what I was thinking.  But i couldn't think of a good way to word that to google.  The energized to release state is definitely what I had thought in mind.  I considered a gear box because speed isn't TOO big of an issue with what I am working on, but if my motor stop half way through a rotation, it will have about 17 pound of weight sitting on it at an angle.  I just figured this weight would be enough to engage the gear, and that is why I didn't look to far into it.  Then with the servo idea, i liked it, but they usually cant sustain alot of weight, and I would constantly worry if it was in an "always-on" state.

lilshawn, the electric wheelchair set up sounds like you and DaOld Man were on the same page, and I think thats the best way to go.  Energized lock release, but I would need it on a smaller scale.  Connecting the motor leads was also an option, but like you said, they arent strong and if I am getting near 20 pounds, i doubt it would sustain.  Lastly, the rotary encoder falls into the "always-on" scenerio.

Just so you guys know I didn't just blindly ask without wanting to do work myself, from the options I saw I thought a worm gear would be good.  But between the lubrication and the surety that they can't reverse without adding more components, I thought maybe it wasn't the best solution.  Thank you again for your guys' inputs.

bfauska

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Re: Motors question
« Reply #4 on: March 23, 2015, 02:53:30 pm »
I don't know how small they get but in my work when I need a motor that performs this way I purchase a "brake motor" they are what DaOldMan and lilshawn are describing. I just thought knowing the common term may help your search. I use them starting at 2hp so a bit bigger than you're looking for with a 20# load (unless you're moving it really quickly), but I suspect they come in smaller packages.

buildingmotors

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Re: Motors question
« Reply #5 on: March 23, 2015, 03:05:17 pm »
I don't know how small they get but in my work when I need a motor that performs this way I purchase a "brake motor" they are what DaOldMan and lilshawn are describing. I just thought knowing the common term may help your search. I use them starting at 2hp so a bit bigger than you're looking for with a 20# load (unless you're moving it really quickly), but I suspect they come in smaller packages.

Thank you.  I was actually afraid that they would all be huge, but if they start at 2hp, then i should be able to find one right where I need.  And speed is not an issue, as long as it reaches its destination, then im not going to complain.  Thank you again.

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Re: Motors question
« Reply #6 on: March 23, 2015, 10:03:16 pm »
You know, a motor with a screw gear box on it would be very hard to turn from the output end, possibly even to the point of breaking before it will turn.
But the speed would be quite a bit slower than the motor RPM.

I have used a wind shield wiper type motor on my rotating monitor (as have others too), and if what you are wanting to move doesn't have to be real fast, you might want to look into that type motor.
They are easy to find, only require 12 volts DC at about 2- 5 amps.
If you don't need to vary the speed, you can use relays to turn the motor on and off and reverse it. (See the thread in this forum that attempts to explain drives.)
Here are some motors to give you some ideas:

http://www.allelectronics.com/make-a-store/category/400400/motors/dc-gear-motors/1.html

Good luck with your project, and let us know what you decide to use.

buildingmotors

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Re: Motors question
« Reply #7 on: March 24, 2015, 08:22:07 am »
You know, a motor with a screw gear box on it would be very hard to turn from the output end, possibly even to the point of breaking before it will turn.
But the speed would be quite a bit slower than the motor RPM.

I have used a wind shield wiper type motor on my rotating monitor (as have others too), and if what you are wanting to move doesn't have to be real fast, you might want to look into that type motor.
They are easy to find, only require 12 volts DC at about 2- 5 amps.
If you don't need to vary the speed, you can use relays to turn the motor on and off and reverse it. (See the thread in this forum that attempts to explain drives.)
Here are some motors to give you some ideas:

http://www.allelectronics.com/make-a-store/category/400400/motors/dc-gear-motors/1.html

Good luck with your project, and let us know what you decide to use.

Those actually look really good.  I was looking into a normal gearbox on the same idea that it would be harder for the output to push back, but 17 pounds is quite amount of weight for a small motor.  But your idea sounds like it would almost garuantee no pushback.  Honestly, I wouldn't know until I physically had it, but it seems like a worth while investment.  I will grab one and let you know how it works out.  And the speed shouldnt be an issue, it only has to travel about 20" and as long as it doesnt take several minutes, I think i will survive :D, thank you.

  
 

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