From: ch@eiaut2.tuwien.ac.at (Christian Kral)
Subject: Re: ESC Electronics?
Organization: Technical University Vienna, AUSTRIA
Distribution: rec
Date: Fri, 6 Nov 1992 11:02:00 GMT

Hi Robert,

here is just a gerneral clue which will help you understanding the protocol between the receiver and servos, electric motor control, etc.

The receiver catches a certain number of high-states which depends of the number of channels your radio has. The signals are transferred one by the other and than the cycle starts again (the repetition time is constant, there is a variable break of a few milliseconds at the end of each run on which the start of the cycle is detected by the receiver). The length of each high-state correspond to the lever- or switch-position on your remote-control. 
Here is an example for a 4ch radio:

Input line of receiver(systematic diagram, the real lookout depends on the modulation your radion uses, only valid for PPM, on PCM systems is much more information on the line, but it is similar to this)


        |<-----always constant (about 20ms)------>|  
         
          ch1    ch2   ch3    ch4		    ch1    ch2    ...
         -----   ---   ---   -----                 -----   ---	
        |     | |   | |   | |     |               |     | |   |     
     ..-       -     -     -       ---------------       -     -...
               ^                          ^           ^
               |                          |           |
               |                          |       this time corresponds to
               |                          |       stick-position(nominal 1.5ms
               |                          |       in released condition,
               |                          |       +/- 0.5ms in endpositions)
               |                          |
         this gap is constant        this pause varies
         and depends on the radio    and is much longer
         (normally about 0.5ms)      than the others



Output lines to the servos (sorted by the receiver):


         -----                                     -----                   <- +5V
   ch1  |     |                                   |     |
   ..---       -----------------------------------       -------------...  <- GND
                 ---                                       ---
   ch2          |   |                                     |   |
   ..-----------     -------------------------------------     -------...
                       ---                                       ---
   ch3                |   |                                     |   |
   ..-----------------     -------------------------------------     -...
                             -----
   ch4                      |     |                                    
   ..-----------------------       -----------------------------------...                 

The only agenda of speed controls and servos is to translate the signal which is fed by the receiver. (Because all available devices work on this procedure (in Europoe they do) you can change from Robbe to Futaba, Graupner and so on by just changing your connectors.)
How is this done? A servo for example charges a capacitor while the input signal is high (of course, the first millisecond - which is a minimum; remember: 1ms+/-0.5ms! - is truncated first, done by a timer, like the NE555). This voltage is compared to the outcoming voltage of the turn-poti attached to the servo shaft. If the poti voltage, i.e. the angle, is higher than the resulting voltage on the capacitor the servo motor is turned left and vice versa. That results in a position according to the stick pos

ition on your control. 
All servos and engine controls have to understand this signaling. The way a motor steering is done is about the same.
A capacitor is charged with a constant current during the signal is high (minus the first millisecond, of course) and discharged with less current during the rest. Then there is an voltage detector (a simple operational ampl.) which turns the motor on above a compare voltage (can be adjusted) and off below. Then you need a timer (NE555, same as LM555) to detect the 1.5ms point. This is the point where the engine turn direction has to be changed via relais.

I hope this helps you understanding the way this stuff works. It would take me hours to explain circuit diagrams, but I am sure you can get some interresting books on this topic in radio shacks or at curtain book sellers.

greetings,

Christian.
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