Understanding an RF remote control

[Curt Zerr]

Hello!

This question may be a little too specific but if there are any drone enthusiasts out there they might be able to help me out! I'm trying to understand the transmitted signals from a remote control to an air drone. When the user of a remote control, let's say, pushes up on the joystick for thrust, a signal of pulses are transmitted at a specific rate acting as a code. This code of pulses is then received by the receiver and sent through an encoder which then goes to the designated channel on the micro controller of the drone telling it to either send power to the motor or take away power? Am I on the right track here?

Also, are these codes specific to every drone manufacturer or is there a standard for these kind of commands?

Any help would be greatly appreciated!

Thank you

Curt

 

[CraigHB]

I think what you are talking about is the PPM stream (pulse period modulation) sent to the receiver/controller and then on to the ESCs which control motor output. What the transmitter does is encode a stream of PPM data on a carrier, typically 2.4GHz. Older RC transmitters used frequencies in the MHz range, but since the last decade 2.4GHz is pretty much across the board now for RC transmitters.

Once the receiver/controller decodes the data stream, it breaks it out into a PPM signal to each servo or ESC for a multi-copter. PPM frequencies range from 50Hz to serveral thousand Hertz. For quite a long time a 50Hz stream was used across the board, but with multi-copters came the need for faster response times. There's various protocols now like One Shot and One Shot 125. These use a higher frequency PPM stream. It's a system wide thing, if you have a receiver/controller than puts out a higher frequency PPM stream you need an ESC (or servo) that can accept it.

Generally the PPM used by servos and ESCs is standardized with the caveat you need compatibility for higher frequencies. All servos and ESCs can use a 50Hz signal, but you have to check servo and ESC specs for anything higher. What's proprietary is the protocol used between the transmitter and receiver. Before the advent of 2.4GHz it was fairly standard, pretty much any transmitter would work with any receiver. With 2.4GHz came digital encoding which is a proprietary thing. For example if you use a Spektrum transmitter, you have to use a Spektrum receiver.

 

[Curt Zerr]

Thank you so much!

This answered a lot of questions. So I guess the only way to know these control protocols is to talk to the manufacturers of the transmitter/receivers themselves. I imagine this is information that is not easily obtained...

 

[CraigHB]

I imagine this is information that is not easily obtained...

Yes it is not easily obtained. They keep their protocols close. It's actually something that is a major complaint with RC modelers. But, that's the way these makers do it so that's what we get. There are some companies reverse engineering protocols to offer components in a cheaper brand. For example the protocol used by Spektrum is called DSMX. A company by the name of Lemon offers DSMX compatible receivers for a lot less than what Spektrum charges. They work just as well, but to design them Lemon had to reverse engineer the protocol. There's others like that. Another example, Walkera offers a transmitter that runs firmware called Deviation allowing the transmitter to use various protocols designed by other makers. So there's ways around this situation, but generally it's best to buy your transmitter and receivers to go with it under the same brand.

 

[Curt Zerr]

Ok I see. That makes sense. Thanks a lot for all the information!