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Old Aug 16, 2008, 02:43 AM
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Need help with rc system

Hello,
I am trying to build an AM rc system for my self. Can some body help me out. I have attached two waveforms that i get from my encoder and the other one is the output of the receiver, are they correct?? will a ppm decoder be able to decode this signal??

thanks!

P.S. sorry for the bad english
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Old Aug 16, 2008, 03:12 AM
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decoder

looks pefectly OK to me, Robin Anbdrew, UK
KEEP up the good work
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Old Aug 16, 2008, 04:11 AM
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Is the rx output generated from this decoder as the rx pulse widths seem different to the encoder output - maybe I'm misreading the scope settings.
The pulse train looks fine, I assume this feeds a slicer to square-up the rx pulses before they are fed to the decoder (maybe a comparator or schmitt-trigger?)
Just as a matter of interest, I wonder why only 3 channels and what you are using for RF?
Good luck
Phil
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Old Aug 16, 2008, 04:20 AM
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Quote:
Originally Posted by robin andrew
looks pefectly OK to me, Robin Anbdrew, UK
KEEP up the good work

Thanks a lot!
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Old Aug 16, 2008, 04:30 AM
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Quote:
Originally Posted by phil_g
Is the rx output generated from this decoder as the rx pulse widths seem different to the encoder output - maybe I'm misreading the scope settings.
The pulse train looks fine, I assume this feeds a slicer to square-up the rx pulses before they are fed to the decoder (maybe a comparator or schmitt-trigger?)
Just as a matter of interest, I wonder why only 3 channels and what you are using for RF?
Good luck
Phil
Yes the scope settings were different for the two readings. But I dont understand, if everything is fine then when i connect the servos to the decoder they start moving on their own (I can even feel them vibrate upon touching the outer case). I am building a rc system for the first time so to make it less complex i used a three channel encoder, i will expand for more channels once i perfect this. I will mention again the scope readings are for the rec o/p which will be fed to the decoder. will a schmitt trigger help here and i am using a RF circuit similar to the one in the tx2/rx2 ic datasheet (ones used for toy car).do u think the signal at the o/p needs conditioning and amplification? if so how do i achieve this??

thanks!

P.S. i am using a superregen rec on 27mhz.
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Old Aug 16, 2008, 05:42 AM
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I think perhaps your channel timing is too long, so the servos could jammed at one end of their travel, maybe stalled against the stop. This would account for the vibration & jittering.
Looking at your second trace, channel one seems to read 2ms, channel 2 reads 2.5ms and channel 3 reads nearly 3 ms. Over 2ms is too long for standard servos. At rest with the controls at centre I would expect to see 1.5ms spacings between the pulses, with the controls changing the spacing by half a ms either direction.
The other possibity is that your decoder isnt clocking cleanly from the rx output. Is is a shift-register or a processor? The rx output as shown in your second trace isnt clean enough to drive the decoder directly, it will need to be squared. PICs have schmitt-trigger inputs, if your decoder is a different processor maybe you could see if that is an option for inputs. Otherwise say a cmos 4093 between the rx & decoder?
Super-regen is an unusual choice of receiver, is 27mhz fairly quiet in your locality?
Cheers
Phil
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Old Aug 16, 2008, 10:33 AM
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I am using a 4017 encoder for my system. I would like to diagnose the problem step by step. I am attaching the waveforms for different pot positions (1ms,1.5ms,2ms and then the 20 ms delay) This is standard for any rc system right?? .If these waveforms (at encoder output) are correct then tell me i will take the output at the receiver end and compare that way I will be able to find out the problem. I am using a superregen because i am learning and this is my first system. I am studying communication techniques at engineering college so I thought that this would be a good exercise for me and i also wanted to build my own rc system.

thanks!
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Old Aug 16, 2008, 10:38 AM
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Thats perfect. However the RX output (in your first post) shows different timing - how is this possible?
Phil
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Last edited by phil_g; Aug 16, 2008 at 10:48 AM.
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Old Aug 16, 2008, 10:42 AM
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Quote:
Originally Posted by phil_g
I think perhaps your channel timing is too long, so the servos could jammed at one end of their travel, maybe stalled against the stop. This would account for the vibration & jittering.
Looking at your second trace, channel one seems to read 2ms, channel 2 reads 2.5ms and channel 3 reads nearly 3 ms. Over 2ms is too long for standard servos. At rest with the controls at centre I would expect to see 1.5ms spacings between the pulses, with the controls changing the spacing by half a ms either direction.
The other possibity is that your decoder isnt clocking cleanly from the rx output. Is is a shift-register or a processor? The rx output as shown in your second trace isnt clean enough to drive the decoder directly, it will need to be squared. PICs have schmitt-trigger inputs, if your decoder is a different processor maybe you could see if that is an option for inputs. Otherwise say a cmos 4093 between the rx & decoder?
Super-regen is an unusual choice of receiver, is 27mhz fairly quiet in your locality?
Cheers
Phil
The decoder I am using is the 4 channel PIC version on bruce abbot's web site. There are not many fliers where I stay and 27mhz is free freq. Please explain in a littel detail how to square and amplify the pulses at the rec o/p. and can I use an audio amp (lm386) to amplify the signal before giving it to the decoder??

thanks!
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Old Aug 16, 2008, 10:43 AM
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Quote:
Originally Posted by phil_g
Thats perfect. However the RX output (in your first post) shows different timing - how is this possible?
Phil
Thanks phill, these were new readings I just took. I will take new o/p readings from the receiver for the same pot positions and compare.I will post the wave forms in some time.

thanks!
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Old Aug 16, 2008, 10:50 AM
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Sorry I was editing my post as you replied. See my previous post with the rx timing query!
Will look at new traces when you're ready!
Cheers
Phil
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Old Aug 16, 2008, 12:37 PM
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phil,here are the receiver outputs for 1ms,1.5ms,2ms and 20ms delay. They correspond to the encoder outputs posted before .Are they correct ? If so what signal conditioning will be required for feeding these pulses to the decoder?

thanks!
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Old Aug 16, 2008, 12:45 PM
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sorry,forgot this one!!
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Old Aug 16, 2008, 02:10 PM
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Thats better, the channel timing is spot on.
You need to get rid of the underlying capacitive voltage effect, it looks like the negative-going pulses are only about a volt superimposed on that curve. I'd try just a pull-up first, say 10k then 1k, just to see if the output can be cleaned easily, as it could be just a high impedance floating voltage that will disappear when even a small load is applied.
If not I'd suggest a comparator set to just above the minimum voltage of the negative peaks. Is the test point connected to the decoder when you take these traces? If not, try that, the decoder input may load it sufficiently to discharge that capacitance.
The pic can optionally have a weak pull-up programmed on pin 4, I'm not sure if Bruce implemented that. Unfortunately it cant be set as a schmitt trigger when acting as GP3 input so we need a clean pulse train at logic voltages.
Cheers
Phil
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Old Aug 16, 2008, 02:40 PM
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here are the results with the 10k pull up res. a change is noticed in the amplitude of the received signal but it is still not clean enough to be fed into the decoder.The test point is connected to the rx o/p and not the decoder's o/p.
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Last edited by fm_head; Aug 16, 2008 at 02:46 PM.
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