Introduction: Reading RC Receiver PPM Signal Using Arduino

RC transmitter is very well suitable for the projects requires a wireless link as it has encrypted link with good range.

All RC receiver is made to drive servo motors. so there are 3 pins for each channel: ground, Vcc, and signal. here special kind of signal is used. It sends pulses at some interval. When servo receives 1.5ms pulse it sets to 90 degrees and by varying this value from 1-2ms servo goes to minimum and maximum value.
(In above photo a 6 channel receiver is shown with PPM pin)

So easy method is to measuring pulse width on each pin and map that data as per requirement. but here problem is that:

  • For each channel of the receiver, we need to connect a wire to Arduino pin.
  • this not only requires lots of connection but also consume lots of pins on an Arduino.
  • Most of Arduino just has 2 interrupt pin so if we uses more than 2 channel read it add some delay to our code which may be problematic for some application.

To solve this problem many receivers comes with an extra pin called PPM. this PPM pin transmits data of all channel in a single signal.

Step 1: How PPM Pin Signal Composed:

This signal consists of data packets separated by blank space.
Here space in between peaks represents the value of the channel. in this case, I have used 6 channel receiver so there are 7 pulses.

So in our code first, we need to detect separation space and then start recording data from the pulses for each channel.

As can be seen in the second image,

all data is separated with approx 11500 microseconds of time. than 6 values are for each channel.

Step 2: Making and Using Code:

Here read_me() specified as function:

a=micros(); c=a-b; b=a; x[i]=c; i=i+1;

if(i==15){for(int j=0;j<15;j++) {ch1[j]=x[j];} i=0; } }

this part runs on interrupt pin and take 15 time values and store it in array.

another function read_rc()

this part look for any space which is higher than 10000microsecond, in our case it detects separation space and as it detects that space code moves to next section.

after this space next 6 values are for each channel position it is stored in array named ch[ channel no],
here ch[1], ch[2], ch[3], ch[4], ch[5], ch[6] represents value of each chanel.

Step 3: Finally...

As we are using PPM signal with interrupt

  • This code do not add delay to our main functions
  • It consumes just 1 pin from Arduino

Comments

author
alopez dominguez (author)2017-04-30

Very usefull code!! i had to modify it a little bit because my receiver is usually on high output and the spikes go down instad of up, got it working fast, just one little question , how do the variables a , b and c work? i didnt quite understand em

author

a,b and c is used to measure the spacing between two peaks in term of time.

when a new data comes we store it in "b" (last data already there as old data named: "a")
than take difference of "a" and "b" which is spacing in between two peaks.
now as we have used latest data it is not new data (which we store as "b"), so we update old data. and make value of "a" equal to "b".
now we again take new reading store it as "b".

...

..
so this process keep happens hundred of times per second.

author
nitbeat (author)2017-06-17

Thanks for the guide it's great, although I couldn't get it to work.

I'm using Flysky TGY-iA6C RX - https://hobbyking.com/en_us/turnigy-ia6c-ppm-sbus-... (PPM mode)

I tested first the pulseIn(3,HIGH) * had to set it to HIGH in order to get values, and it works fine, my separator is >6000 && < 9000.

When trying to use your full code example I only get -1000 values

Looking back at pulseIn values I noticed the number of values between separators change to somewhere between 8,9 even 12, so I don't know what's going on.

Is there any way to solve or address that ? thanks :)

author
abhilash_patel (author)nitbeat2017-06-19

hi...

Can please confirm that signal from ppm pin goes to any interrupt enable pin ..

if that is ok. go to read_me function and check the output for variable "c". as mentioned in the code it is spacing between two peaks and should be similar spacing and channel value as data you capture using pulseIn function. if data in c is right than there may be some problem in code.

author
JamesY59 (author)2017-04-19

Why do you store 15 time samples?

author
abhilash_patel (author)JamesY592017-04-20

hi...
My receiver (of 6 channels) sends PPM for 8 channel (out of 2 are meaningless). I have chosen 15 just to make coding easy. so that, in any case, there is one set of 8 values which represents a value of each channel.
for an example
(here each number represents a channel number)
2 3 4 5 6 7 8 1 2 3 4 5 6 7 8
7 8 1 2 3 4 5 6 7 8 1 2 3 4 5
5 6 7 8 1 2 3 4 5 6 7 8 1 2 3
8 1 2 3 4 5 6 7 8 1 2 3 4 5 6
here we can just pick 8 value after channel 1

if you use only 8 values it will become
6 7 8 1 2 3 4 5
1 2 3 4 5 6 7 8
8 1 2 3 4 5 6 7
so, in this case, we need to make rearrangement which is also not very difficult but I preferred easier approach.

hope now you are clear about it. If you have any query let me know.

author
JamesY59 (author)abhilash_patel2017-04-22

Thank makes perfect sense. Thank you for responding!

author
sfalconetti made it! (author)2017-03-20

Works great to me (Sanwa RDS8000 via trainer ppm port). Compared to other approaches, yours has no glitch. Only strange thing is the values: they differ on what I know coming out from the tx. Do you have an idea why ?

RDS8000 and Mega 2560.jpg
author

sorry, I don't have an exact idea. I have checked that values only on my Rx (avionics RZ6) and also on few websites.

author
BhagyeshG1 (author)2017-03-07

my receiver only has pwm outputs. is it okay if i add a ppm encoder between the receiver and the arduino?

author

Yes, It should work.
if it not works, just check signal and do modification in code as per separation (step 1).

If you don't have any problem with a delay in the reading channel (around 15milliseconds/channel) or do not wont to purchase PPM encoder you can directly use the pulseIn function for each channel.

author
PeterM138 (author)2017-02-13

Nice work. Really informative

author
abhilash_patel (author)PeterM1382017-02-14

Thanks lot... Hope this tutorial was helpful to you