Cube PX PWM signals issue

Dear Guys, when doing a motor test I noticed that PWM output of our Pixhawk cube was different in the no 1 motor output, being approx 0.68 V at low value, when moving the throttle up it reached only 1.15 Volt. When the motor was increasing speed there were brusk changes and shocks, which were quite scary. Also when slowing down these shocks occurred. All the other motor outlets were doing 1.41 V when throttle down and with a max of ca 1.8V. Here the motors (we tested a number of motors) were smoothly reacting when increasing, decreasing speed. It seems to be a hardware thing? Does anyone know how this failure ( low voltage at PWM 1 motor outlet can be caused and if it can be corrected?

Hope to hear, Best Winfried, Drone4agro.com

Try temporarily swapping ESC1 and ESC2 wires and see if the problem follows the ESC or stays with the Flight Controller motor1 output.

Hi Shawn, thanks and I did it with different motors (that are combined with ESC’s), so I think it is a hardware thing. So possibly a damaged system… However wonder if it can be repaired or just have to accept that this is damaged… Thanks again, Winfried

Did you figure this out from Motor Test page?
Can you recall anything you did that may have damaged the output?

Dear Alvin, the measurement of voltage output from the different esc outlets was done after the incident, so might also have been caused after the incident. But pls let us know as cube expert:

  1. what is the max distance the PWM signal can reliably travel (with eg 22 AWG shielded cable): with only Black and White Signal wire connected
  2. does the signal lenght improve if the positive on the rail is fed with 5 Volt

Hope to hear and thanks for your reaction!

These problem has nothing to do with the Cube.

PWM signal strength depreciation depends on the quality of cable and the length. Normally the servo drivers are specified to drive a 50pF servo input load over 2m of 26AWG servo cable, as mentioned here.

Supplying external voltage to the rail will not improve the signal strength, as the servo rail is not powering the Cube in typical situations. The logic voltage level is irrelevant to rail voltage.

Thanks Alvin, Servo drivers, OK that is clear: The PWM ESC motor output is possibly doing max of 100 mA thus having 12.5 mA max average? We used shielded wire (not grounded) with 24AWG over 5 meters for the end motors, Is that distance too long for a ESC.motor PWM signal, in your opinion? Cheers, Winfried

Distance too long or not depends on the voltage depreciation. You need to measure the signal voltage and latency at the end, then decide whether the outcome is acceptable.

You may also try CAN
https://ardupilot.org/copter/docs/common-uavcan-escs.html#uavcan-escs

Great suggestion, will start with that. a voltage depreciation of 5% should be acceptable, I presume? Thanks! Winfried

Just did some tests PWM signal when armed (red button) 1.43V when start command given 1.54 V. When short cable (20 cm) then 2.47 V when full throttle, when long cable ( 4.2 m) at same motor ESC outlet 2.46V Cannot measure latency, but visually same behavior in reaching end values…

Cheers Winfried

You may need an oscilloscope to measure the latency.

Are you really measuring the peak voltage or they were given by the duty cycle different?