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How is steering automatically straightened?

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How is steering automatically straightened?

Old 06-30-2016, 10:08 AM
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monkeyalien
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Question How is steering automatically straightened?

Forgive me if this is common knowledge in the RC community, but after several hours of research I cannot find an exact explanation for how steering is automatically straightened or centered when the user lets go of the steering control. Most search results are about how to adjust steering, not the nuts-and-bolts of how it actually works. I imagine it's a position controller using feedback from the servo.


The reason I ask is that I want to make an RC ATV without buying a heavy duty servo. Several components will be made from scratch and cheaper or readily available parts. I've thought about using a windshield wiper, power seat, or other such geared motor that I can attach a rotary potentiometer to. I've devised a circuit using comparators, op-amps, a trim pot, and limit switches that would allow a user to move a motor only so far in either direction and should automatically return a motor to a certain position if I had a way of getting feedback from a motor, and I think this might be how it's done in off-the-shelf RC cars.


Thank you.
Old 06-30-2016, 10:21 AM
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1QwkSport2.5r
 
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sorry... The lovely and wonderfully written RCU forum software ate my post when I tried to edit it. Oh well.

Morale of the story is IMVHO - making your own servo may be rewarding in one way, but it's a huge waste of time considering you're reinventing the wheel so to speak.

Last edited by 1QwkSport2.5r; 07-02-2016 at 11:57 AM.
Old 06-30-2016, 10:40 AM
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The Saylors
 
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Ya i have seen some of those massive servos for rc 1/1 scale vehicles for experiments and different things. Could you make one? well ya of course but there will be a lot of speed bumps and a steep learning curve if you don't have experience in large scale servos. Like qwksport said weight is gunna be a big hurtle for sure, another will be getting speed and torque at the same time. Servo companies spend a lot of time perfecting this balance so trying to be on that level without the experience is a recipe for a tough project at best. Have you priced out a large scale servo? May be more cost effective to buy one rather than spend weeks or months plus materials on it. I know it wouldn't be "built by you" but if its the difference between finishing and not then it may be worth it. just my $.02 as well. Huge kudos if you can pull it off! Not at all trying to be a downer, this is the place people come to for experimentation but i worry that it won't/can't pan out. Is there something you're trying to accomplish with this rc atv? just curious most people like to pilot those personally, myself included. If you do build one from scratch then keep us posted as it would be interesting to see a LS servo built from scratch! Good luck and welcome to the forum! A lot of nice people here!
Old 06-30-2016, 10:46 AM
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After posting i had a thought, what if you used a large high speed actuator instead of a servo? since a quad's bars only move forward or backward about 9-12 inches from the riders position then an actuator should work and be cheaper. I would use a robotics actuator if it were me. As long as its not a giant grizzly quad then the steering shouldn't need a crazy amount of torque to work and it would make setting up much less complicated. Just some food for thought.
Old 07-01-2016, 10:33 AM
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Maj_Overdrive
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Originally Posted by 1QwkSport2.5r View Post
Servos self-center themselves via a potentiometer. The pot basically tells the processor where the motor is.
This isn't quite correct. The potentiometer tells the servo's processor the position of the servo's output shaft. The processor then spins the motor whatever direction it needs to in order to get the output shaft position to match the signal it's receiving from the radio.

What you should use for steering really depends on the weight of the vehicle. There are some pretty powerful servos out there meant for projects like small rc ATV's from Tonegowa (correct spelling?) and I'm sure others. Whether they're in your budget or not is another story and if you're building a 100+ pound vehicle they might not be enough. I personally wouldn't want to put that expensive of a servo in something like that only to kill it. Busting a cheap used wiper motor sounds better to me.

There's plenty of instructions out there on how to build these and they can steer a large vehicle's wheels just as fast as any commercial servo. Just google "large scale rc wiper motor servo" and you'll get plenty of reading material. Most involve using a standard servo attached to the steering linkage for its potentiometer to determine the steering position and interface with a standard rc receiver. Then there's a H-bridge circuit that connects to the servo's board where the servo motor would with a wiper motor connected to the other end of the h-bridge. So the servo's processor does its thing determining position and sending instructions like normal just to the H-bridge instead of its normal motor. The H-Bridge functions like a couple of relays since the wiper motor will draw more power than the servo's processing board can handle. The hardest part will be adjusting the linkage of the rc servo to match the amount of steering throw you have so be sure to build some adjustability into the steering system for both the rc servo and the wiper motor.

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