Author Topic: RC Pulse standard vs PWM  (Read 26422 times)

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Offline vidamTopic starter

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RC Pulse standard vs PWM
« on: June 02, 2008, 01:22:55 PM »
I want to understand the fundamental difference between PWM and standard signals output from an RC transmitter? What is the duty cycle for low medium, high of an RC signal. What is the deadband of the RC signal? What is the frequency of the RC signal or is it based on the timer of the MCU?

Thanks,

Melanie
« Last Edit: June 02, 2008, 01:24:07 PM by vidam »

Offline bens

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Re: RC Pulse standard vs PWM
« Reply #1 on: June 02, 2008, 01:43:24 PM »
Standard hobby RC servo pulses are just a specialized form of PWM.  Typically the signal repeats every 20 ms (so 50 Hz PWM frequency), but this can vary.  The pulse itself most often ranges from 1 ms to 2 ms, with ~1.5 ms being the neutral position.  For servos with larger ranges, you might see pulses between 0.5 and 2.5 ms (neutral will still be around 1.5 ms).

Deadband of the RC signal is determined by your speed controller (ESC).  I don't think there's any official deadband range (at least not that I know about).  Basically you implement your own deadband by keeping motor speed at zero or servo position at neutral for servo pulses between (deadband - dt) and (deadband - dt), where dt might be a few percent of the total range (so maybe 20 - 40 us?)

- Ben

Offline vidamTopic starter

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Re: RC Pulse standard vs PWM
« Reply #2 on: June 02, 2008, 02:22:14 PM »
Standard hobby RC servo pulses are just a specialized form of PWM.  Typically the signal repeats every 20 ms (so 50 Hz PWM frequency), but this can vary.  The pulse itself most often ranges from 1 ms to 2 ms, with ~1.5 ms being the neutral position.  For servos with larger ranges, you might see pulses between 0.5 and 2.5 ms (neutral will still be around 1.5 ms).

Deadband of the RC signal is determined by your speed controller (ESC).  I don't think there's any official deadband range (at least not that I know about).  Basically you implement your own deadband by keeping motor speed at zero or servo position at neutral for servo pulses between (deadband - dt) and (deadband - dt), where dt might be a few percent of the total range (so maybe 20 - 40 us?)

- Ben

Thanks Ben. This has confirmed what I was sort of already knew, but just needed outside support! :-)

Offline bens

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Re: RC Pulse standard vs PWM
« Reply #3 on: June 02, 2008, 04:01:19 PM »
No problem.  One thing I neglected to mention is that I think some RC transmitters can implement their own deadband, but I'm not sure exactly how this works.  I'm guessing they just output the neutral pulse value when the channel stick is within a certain "neutral range".

- Ben

Offline vidamTopic starter

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Re: RC Pulse standard vs PWM
« Reply #4 on: June 05, 2008, 01:08:30 PM »
Standard hobby RC servo pulses are just a specialized form of PWM.  Typically the signal repeats every 20 ms (so 50 Hz PWM frequency), but this can vary.  The pulse itself most often ranges from 1 ms to 2 ms, with ~1.5 ms being the neutral position.  For servos with larger ranges, you might see pulses between 0.5 and 2.5 ms (neutral will still be around 1.5 ms).

Deadband of the RC signal is determined by your speed controller (ESC).  I don't think there's any official deadband range (at least not that I know about).  Basically you implement your own deadband by keeping motor speed at zero or servo position at neutral for servo pulses between (deadband - dt) and (deadband - dt), where dt might be a few percent of the total range (so maybe 20 - 40 us?)

- Ben

Is the duty cycle 50%?

Offline vidamTopic starter

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Re: RC Pulse standard vs PWM
« Reply #5 on: June 05, 2008, 01:27:20 PM »
duh, ... the pulse ranges from 1 ms to 2 ms ON and the rest of the 50 Hz pulse is off. So it is not a 50% duty cycle all of the time.


Offline bens

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Re: RC Pulse standard vs PWM
« Reply #6 on: June 05, 2008, 03:02:33 PM »
It's never a 50% duty cycle.  If you use the standard 50 Hz frequency, it's a 5% duty cycle for 1 ms pulses and a 10% duty cycle for 2 ms pulses.  The space between pulses is always much greater than the pulse length.

- Ben

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Re: RC Pulse standard vs PWM
« Reply #7 on: June 06, 2008, 09:02:42 AM »
Ok, one more question...I hope... I'm generating the RC Pulse from an FPGA and wanted to make sure whether an amplitude of 3 Volts for the RC PWM is the standard?

Thanks again! :-)

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Re: RC Pulse standard vs PWM
« Reply #8 on: June 06, 2008, 11:27:09 AM »
Typically it's a 5V pulse, but 3V might work depending on the high threshold whatever you're controlling.  If not, you will need to convert that 3V signal to a 5V one using external circuitry, or by using your FPGA in a creative way.  For example, if your FPGA has an analog output, maybe you can generate 5V pulses with that as RC pulse signals do not have to deliver much current.

- Ben

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Re: RC Pulse standard vs PWM
« Reply #9 on: June 06, 2008, 12:46:57 PM »

If the RC Pulse Standard is 5V ttl logic then 3.3 Volt amplitude PWM signals will still work.


Offline bens

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Re: RC Pulse standard vs PWM
« Reply #10 on: June 06, 2008, 01:10:49 PM »
Probably, but be careful about making assumptions.  Ultimately it depends on the microcontroller that is processing the signals.  On AVRs 3.3 V is just barely above the guaranteed-high threshold, so you would be ok.  Other microcontrollers might have this threshold at a higher or lower voltage, so at least test it out to make sure it works reliably with your target device.

- Ben

 


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