RC Engine Speed controller

[csa3d]

Hey all,

So I'm messing around with a 5v RC Motor to drive my rotating monitor rig.  My first attempts were swiveling poorly, and left me wanting a serious torqued engine and a super slick lazy susan settup.  Well, I've created such.  Now I'm finding that my monitor goes from 0-90 in 1.5 seconds!  While that might sound good, it's not.  I would like to decrease the time to about 5 seconds, or something much gentler then achieved.

Can I put an electronic equivalent of a "dimmer switch" between the motor leads to drop the voltage to make it spin slower... or is this bad and are there better/safer ways to do this.  I don't want to burn the house down after all.

Thanks in advance.
-csa

[csa3d]

Searching more, I'm considering limiting the speed of my rotating monitor motor with a setup like so and a 5K-Ohm Linear Taper Potentiometer like this one.  Thoughts?   

-csa

[danny_galaga]

short answer: no.

electric motors like those used in rc vehicles develop very little torque when using a resistor to reduce the speed. you really need to look at gearing it so that it can run at peak revs. however, having said that, you could look at making an electronic speed controller. or modifying an RC one so you dont have to use a radio. they control speed buy 'chopping' up the power. basically it is switching full on, then full off at various frequencies. the net result is that you have full torque, even at low speeds.

which is actually something like the dimmer switch you talked of. thats how they work. they work on a much higher voltage of course, but same principle...

[protokatie]

Gearing > pulsing IMHO. If you can get (or make) the gear box with the correct ratios, it would be a much smoother and possibly longer lasting solution to pulsing (I may be wrong on that last point).

As per what I think I got from your question: If you undervolt an electric motor, it can increase the risk of shorting it out. This may sound counter intuitive, but an electric motor that is undervolted under load can heat up quite a bit as an electric motor is not much more than a longish short circuit. If it isn't turning at a decent rate, or if it is held in a stop position for enough time, the coils will just simply get hot.  For a small RC motor, it doesnt take much heat to melt the very thin plastic coating around the wires in the coil, or even heat the very very thin wire in the coil to the point of breaking it (thus killing the motor).

[danny_galaga]

very true with the under-volting thing. this is how starter motors get burnt out. when the battery is half dead...

in this case though, csa3d is proposing to use a resistor to slow the motor down. voltage will be the same, but current will be limited. due to lack of torque, it just wont do  much of anything!

[protokatie]

very true with the under-volting thing. this is how starter motors get burnt out. when the battery is half dead...

in this case though, csa3d is proposing to use a resistor to slow the motor down. voltage will be the same, but current will be limited. due to lack of torque, it just wont do  much of anything!

The caveat here is that if the motor "doesnt do much of anything" means that it will heat up. I prefer to think in watts when it comes to stuff like this (as opposed to volts or amps exclusivly)

A basic (but not fully correct) way of looking at this is:

Input Watts(electric) = Output Watts(mechanical) + Output Watts(heat).

If you limit the current that a circuit will take you may not get much real world effect, but since a motor is based on induction, electricity will still flow and heat can build up.

Like both of us said; go with gears or pulsing. (gears are what I would suggest personally).



Edit: Online friend of mine asked me to post this: http://en.wikipedia.org/wiki/Pulse-width_modulation   apparently this is a good way to use pulsing.

[csa3d]

Hmm.. well, thanks to the both of you for the input!  I suppose I'll quit being lazy and rewire the circuit for pulse modulation and see how that goes.   

-csa

[danny_galaga]

can you post some pics of what youve done so far? what sort of monitor is it? what sort of gearing, if any, do you have. if its no heavier than a 15" CRT, you can use a tamiya planetary gearbox kit, as is. this is what i used for my monitor. heres some pics so you can see what ive done (about half way down)

http://dannygalaga.com/page3.htm

personally, i think it would be easier (and more successful) to gear it than try a speed controller. and by gearing, it doesnt have to be 'gears' of course, just something to step down the speed. for instance, from the drive wheel to the turntable on my machine is maybe 8:1'gearing'. the gearbox is 400:1 which makes the final drive 3200:1!  to see how fast that is, check out the little vid at the bottom of the link.

now, its probably likely that you have a heavier monitor than mine (unless its LCD). if you are using a 540 sized motor (way bigger than what im using), youve already noticed that its plenty powerful enough, but way too fast. guess what size motor they use in small battery drills? pick up a cheap or second one of those and youve got yourself the ideal gearbox. then its a matter of figuring out how to hack it. i had one years ago (like 20 years ago!) and back then the whole gearbox was nicely built into a unit with motor attached. therefore it could be taken out of the drill casing. im guessing that some of the modern ones will have the gears supported by the casing itself and therefore cant exist as a gearbox without the casing. either way, im sure youd figure out how to support it.

rotating monitors rule, so persevere!

[protokatie]

guess what size motor they use in small battery drills? pick up a cheap or second one of those and youve got yourself the ideal gearbox. then its a matter of figuring out how to hack it. i had one years ago (like 20 years ago!) and back then the whole gearbox was nicely built into a unit with motor attached. therefore it could be taken out of the drill casing. im guessing that some of the modern ones will have the gears supported by the casing itself and therefore cant exist as a gearbox without the casing. either way, im sure youd figure out how to support it.

Oh oh oh oh oh! Damn you for giving me a simple but totally useful idea! Get a modern battery drill/screwdriver and leave it in its case. Hack the trigger pot/"multi resistance" switch and have a two state switch control regulated DC power to the drill. Stick a "blank" bit into the chuck, and weld whatever "gear" or sprocket etc you need to that. Then just mount the drill (still in its case) in the cab, and adjust the trigger multi-switch to get the speed you need. Added benefit is that if you need a powered drill/screwdriver, you could remove it and the bit and use it as such

[danny_galaga]

yeaeh! i was only thinking of using the motor/gearbox. but of course, you have a perfect speed controller with it! if the drill is still too fast, all you have to do is find what setting of the trigger is right. then measure the resistance of the trigger pot. now just replace the trigger pot with a regular resistor of the same value. the forward/reverse switch of course replace with something mountable on the cab, probably a toggle. all of these things can be done whether the drill has been pulled apart or not of course...

[csa3d]

can you post some pics of what youve done so far? what sort of monitor is it? what sort of gearing, if any, do you have. if its no heavier than a 15" CRT, you can use a tamiya planetary gearbox kit, as is. this is what i used for my monitor. heres some pics so you can see what ive done (about half way down)

I used the Solarbotics Gearmotor combo as I've documented here.  This is my old setup but I'm using the same motorized setup on the new version.  I've setup my new version very similar to your cocktail if I recall... where there's a disk with four down facing wheels at 90 degrees, and 2 bumper wheels for the platter to rest on.  The wheel is set up to ride along the circumference of the platter, and limit switches under the platter cause it to stop.

I have a few more bugs to work out.. and I've all ready posted two non-working rotating versions.  I want to get this last one right before I post again

-csa

[Ed_McCarron]

Can I put an electronic equivalent of a "dimmer switch" between the motor leads to drop the voltage to make it spin slower...

Yes.  Like PK said, check out PWM controllers.

http://www.ramseyelectronics.com/cgi-bin/commerce.exe?preadd=action&key=MSC1C

Theres tons of speed controllers out there, but you'd want one with no R/C interface.  If you're controlling it from a computer or PIC check out:

http://www.dimensionengineering.com/SyRen10.htm