Question about RC servos

[zeldazork]

I am building a RC land sailor and I need a sail winch. I like to make my stuff so I have a question about the servo. I have a non-proportional controller but I want a proportional one ....so Will this work????

what if I take a servo apart and put a bigger motor in (a new case) with more and bigger gears. would the electronics work on a bigger motor like an old goldfire or is it to big?

 

[The Mad Professor]

Give us a little more info and we will all pitch in and help.
But first can you tell us just exactly what controller you do have
make / model , with that we can help you much quicker.

 

[dknguyen]

I am going to make a distinction here when I say servo and RC servo. Where a servo is ANY motor with electronics and feedback to control it's position, and an RC servo is a servo specifically designed to be used in RC planes, cars, and boats.

Do you even know what proportional control means (you did say that you wanted it after all)? If you have a non-proportional controller, what control scheme does it use exactly? PD? PI? PID? All of these are better than proportional, so why would you want proportional? The ________ control just means that the math is done differently to try and get the motor positioned as fast as possible while minimizing how much it overshoots the desired position by.

Is your servo controller a "general-purpose any motor with feedback" controller? Or is it supposed to be used to drive many RC servos? I say this because RC servos already have a control electronics built in so it doesn't need a controller.

I think it's a bit strange that you say you like building your own stuff, so then you plan to go out and buy an undersized fully-built RC servo and then try to beef it up for your task (which won't work since you are discarding the motor, gears, and case while only keeping the electronics which cannot handle the higher current of the stronger motor you plan to use).

Are you building the servo motor from scratch? Or are you thinking about modifying an existing RC servo for more power (when you could just go out and buy a bigger servo)? No, you cannot replace the motor and gears inside an RC servo without replacing the electronics designed for the smaller motor- they can't handle the current. But then again, why on earth would you need to buy an RC servo just to scavenge electronics? You already have your motor controller.

 

[zeldazork]

sorry - communications

Sorry I guess I said it wrong. Let me start over ....I have a digital Futaba 3 channel RC radio, also a 6 channel Futaba "skyport" system too. I also have the schematics for a non-proportional sail winch controller which is my second choice to use. The Futaba FP-S148 servo isn't strong enough to control the sail winch.......so I want to take the motor out and put a larger more powerful "hobby" motor in/with it to increase the power....I probably will change the gear train to metal gears too. I'm on a fixed income and don't have the $50-$90 to buy one. But I can buy electric can openers for $1 and use those gears. I also have on hand can motors from dust busters too. I just figure that its basically an electronic controller in the 148 servo so exchanging the original motor with a larger one would work....Its just I want to check with knowledgeable people before I take my RC servo apart and possibly ruin it.

 

[hjames]

One issue is that the motors you describe need currents of >5-10Amps to drive, which are about 100x as large as the motor in the S148. Making a buffer isn't too difficult - you attach another h-bridge to the S148 pcboard to amplify the built-in motor controller($5 - $10 of parts), which will take up ~1 sq" of space. A related issue is that these motors you describe are *huge*, not terribly efficient and will probably drain any receiver style battery pretty quickly.

The second issue is that without the feedback potentiometer, this modified servo is pretty much going to be on-off control. To make the servo control it's speed, you'll need to keep the potentiometer and couple it to the output somehow (or make an electronic equivalent). Without some mechanical work, or some EE knowledge, this modified servo will end up being non-proportional.

James

 

[zeldazork]

OK ...it sounds like I'm better off just using the non-proportional winch. It works with two snap switches that the swing sail control arm hits as it enters its longest swinging point in ether direction, and then use my RC servo to control a double pole-double throw reverse switch. These things are used as an alternative to the more expensive RC servos and seem to be popular or at least use to be. I'll have to play with this thing and hope its not as clumsy as it appears. thanks for your help

 

[hjames]

Well, as a consolation prize, I can imagine setting one up so that it isn't much larger than the servo itself.

It's pretty easy to get 3 terminal snap switches with arms/rollers on them. Take one of the round servo control arm disks (no idea what they're called), clip off most of the disk, leaving > 180 deg. in place. Mount two of the snap switches to a servo, one on each side with the arms extending up so that they will be contacted if the disk touches them. In the neutral position, both switches should be pushed down, if the disk rotates a little bit in either direction, one of the switches will be released.

Next, wire both NO contacts to the + of the battery, the NC contacts to the - of the battery, and wire the motor between the two remaining terminals.

good luck

 

[zeldazork]

That sounds great hjames , I'll do that.
thanks

 

[zeldazork]

3 snaps?

I was drawing up the design of the servo thing you described but ...in your distription hjames you forgot to say what the 3rd snap switch is for.

 

[hjames]

word games - qty 2 x (3-terminal snap switches) i.e. snap switches with NC, NO and COMmon terminals

 

[zeldazork]

Do you mean - not 3 snaps sws, but 2 snaps with the 3 parts (NC,NO,CO)
right? I get confused easly.

 

[hjames]

Do you mean - not 3 snaps sws, but 2 snaps with the 3 parts (NC,NO,CO)
right? I get confused easly.

yes, that's the ticket