RC Pulse standard vs PWM

[vidam]

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

[bens]

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

[vidam]

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! :-)

[bens]

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

[vidam]

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%?

[vidam]

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.

[bens]

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

[vidam]

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! :-)

[bens]

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

[vidam]

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

[bens]

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