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How do RC car/plane "servo motors" act as servos with only three leads and two w

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brucehoult:

--- Quote from: Beamin on February 15, 2020, 11:00:02 am ---
--- Quote from: Nerull on February 15, 2020, 10:46:34 am ---I'm curious what you think a servo is and why hobby servos don't count.

It's a motor with position sensing and a closed loop control IC that drives the motor to the position received on the data wire. That's...what a servo is.

Servomotors and stepper motors are completely different things and often competing technologies.

--- End quote ---

I remember opening one up as a kid and just finding a motor inside no IC or positional sensor. I'm talking the 15.00$ motor that adjusts the steering or advances the throttle on the engine. Real simple device. Plus the receiver is a totally analog device that uses higher/lower tones to control it. Very simple too simple it seems to have a IC sending data back.

I shouldn't have said stepper motor, but I did read somewhere that stated the RC car servo motors weren't even servo motors too simple. The third wire measures resistance or something, cant find a good explanation of how they work.

--- End quote ---

No, that's exactly how even the $1.70 SG90 RC servos work. e.g.

https://www.amazon.com/Cenrkay-Micro-Helicopter-Aircraft-Control/dp/B07N1C8D2F

You send an analogue 1.0 to 2.0 ms pulse for the desired position on the 3rd wire at regular intervals. It doesn't really matter what the interval is, but a single pulse isn't enough to move the servo from one extreme to the other. I think 100 or 200 Hz is the recommended repetition speed for the pulse but I've found 20 Hz to be enough at least with these ones. I've used a bunch of them.

The slightly more expensive MG90S -- $2 instead of $1.70 -- uses metal gears instead of plastic ones and can produce (and take without stripping gears!) more torque.

Unless you have a pretty big model there's little point in paying $15 or $30 for a servo these days.

rstofer:
The repetition rate for the pulses is around 20 ms or 50 Hz.  A 1 to 2 ms pulse every 20 ms.  If the pulse train fails, the servo will shut down electronically and there won't be any holding torque.

brucehoult:
It depends how much torque you need to resist. These things are pretty hard to force to turn even turned off, due to the high gearing of the motor. But of course actively holding position is stronger.

beanflying:
While this is heading off the topic this is why those of us playing with higher performance R/C spend way more than $30 a servo  ::)

Proper high quality so called Digital Servos have very high holding torques partly due to the motors and also the electronics they will hold position for a period of time without a signal. The first of these I brought in the early 90's were about $150USD each and hand assembled in Germany. By comparison an equivalent sized servo now would have 4-5 times the power and cost under half of that. These are still nothing like the Hobbyking junk end of the market which I also use a bunch of in appropriate aircraft.

brucehoult:
Exactly. It depends on the application.

Over on the stepper motor side, the 28byj costs around $2.50 WITH a driver board (in qty 5 on Amazon).

Those things are used for example to drive the louvers on hundreds of millions if not billions of air conditioning units, which they appear to do just fine.

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