I have a px4 quadcopter , and i am a control engineering student and i want to do system identification , where my inputs are PWM signals and the output is the angle θ , and i want to mesure the effect of sending a pwm to the system and applying one of the identification methods that we’ve been learning . and i wonder how can one send PWM using qgroundcontrol to each motor , Thanks
Hi @mosaab, welcome to the Blue Robotics forum 
Questions about the Pixhawk 4, especially when used in a quadcopter, are likely better suited to the more general ArduPilot forum (this forum is more specifically for underwater robotics usage and application, including ArduSub - the submarine variant of ArduPilot).
Most likely you’re after the normal Motor Test functionality of QGC. If you for some reason happen to be running ArduSub on your pixhawk then there’s also a specific ArduSub Motors page.