Lately we had a problem with one of our VTOL planes. One of the multirotor motors suddenly stalled and we crashed. It happened before and there were no indications of any component malfunctioning.
I started to investigate and noticed that the output pwm voltage level is 3.3V. Frankly speaking, I expected it to be 5V as most components like servos, esc’s and others are designed to work with 5V pwm signals.
I tried to find information about what kind of logic levels are standard and very much to my surprise I cannot find any definitions.
We are flying a relatively big VTOL with a wingspan of more than 2 meters. The wire from the flight control to the esc is almost 2 meters. Then I also noticed there is a 100 ohm resistor between the flight control output and the output signal. If you add up everything, the resitor, the length of the wire and the input resistance of the esc, the chances are that you will not reach the required voltage level for a logic “1” on the input of the esc.
Since I suspected that to be the reason of the stall, I shorted the 100 ohm resistors since I cannot reduce the length of the cable or increase the output voltage.
The most modern controllers are work on 3.3V so also the FCs.
The most used logic levels are TTL which is LOW < 0,8V and HIGH > 2V
The parameters of your specific FC and ESC I can’'t say as you don’t give information of these models
What do you mean with “100 ohm resistor between the flight control output and the outpit signal”? What is the “outpit” signal? Do you mean a serial 100 ohm between FC PWM output pin and ESC PWM input pin?
On your long lines this resistor will be really not needed? Why you added this?
On such long lines you also have the high risk of getting noise and disturbanc which can effect your ESC? Are the lines straight or drilled or shielded?
Sorry, outpit was a typo, I corrected it to output. My flight controller is the CUAV V5+, my esc is T-Motor. The 100ohm resistor is according the reference schematic, both CUAV and Pixhawk Cube use them as a standard. The wires from the FC to the esc are shielded.
Ok, if the 100 ohm is internal of your FC I recommend don’t to change this. In this case the 100 ohm is to ensure that the controller itself is safer against misshandling.
Did you messured the level on the ESC input with ESC connected. Normally is the input resitance of the ESC on the PWM input relativ high compared to your 100 ohm plus 2 meter wire. The ESC draws only very low current on such pin so that this will not drop the input voltage so dramatically.
In case the input high voltage on the ESC is really near or below 2 V you need a extra linedriver at the FC output.