Servos go to extreme position at power up


I’ve been using a Pixracer. Now I have a Pixhawk 4 Mini and they show different (unwanted) behaviour even with the same settings.

Original post:

There are two problems (both on the Pixhawk Mini now):

  1. At power up, the servos go to their extreme position (2000 PWM) for a couple of seconds, before they move to the DISARMED PWM value.

  2. Once the Pixhawk has started, I cannot move the control surfaces of the airplane with the RC sticks while the Pixhawk is not armed. The CBRK_IO_SAFETY is pulled.

On the Pixracer before, the servos never went to an extreme position. They didn’t move at all, then they went to my manual control input. I want this behaviour again, I’ve the same PX4 settings on both devices. So why do they behave so differently? What can I do?
In any case, I need to prevent those servos to go to the extreme, as it will induce damage on the aircraft.

Thanks in advance!

EDIT: 1. The servos also go to the extreme position when the Pixhawk is unpowered, so it is a servo problem. Every time I power on the servo, it moves up a bit (about half the way). My conclusion is then, that Pixhawk Mini just needs much longer to boot up and send correct PWM than Pixracer. Apparently, Pixracer is so fast sending the first PWM signals, that the servo doesn’t get to the extreme position. The only solution I see here is to use a hardware switch to power the servos on after the Pixhawk.

Problem 2. stays however. How do I need to configure PX4 such that all channels except thrust are open before arming? I thought it was CBRK_IO_SAFETY, but it’s not.

What is the Failsafe PWM set to for the servo pins?

It’s the default (-1) for each PWM MAIN OUT, on both Pixracer and Pixhawk Mini.

Regarding Problem 2, you have to set param COM_PREARM_MODE=2


Thank you! I looked for something like that in the Parameter Reference, but didn’t find it - it’s not in the 1.10 stable version yet.

However, I understand now that the ‘difference’ between Pixhawk Mini and Pixracer came from having a GPS module connected, including the safety button. Having a safety button caused the different behaviour.