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[Paparazzi-devel] problem with rotorcraft vertical guidance

From: alonso acuña
Subject: [Paparazzi-devel] problem with rotorcraft vertical guidance
Date: Tue, 17 Jun 2014 23:46:12 -0600

Hello. There is a problem in the way the altitude reference (desired altitude) is set when vmode is climb. This causes a situation where the aircraft takes off but then comes down when going to the STDBY point.

In the takeoff block we have <stay climb="0.5" vmode="climb" wp="CLIMB"/>
The problem is that the gv_z_ref variable is allowed to drift arbitrarily from the current altitude. So for example if the throttle is set to 0 in the RC the aircraft will not move but the desired altitude increases continuously. Once the throttle is increased and it goes to sufficient altitude to move into the Standby block the speed is set using the difference from the gv_z_ref to the new target altitude. The actual altitude is not taken into account. So this means that for example if the throttle was at 0 for enough time to allow the gv_z_ref to go to 30m and the STBDY is at 5m and the aircraft is at 2m, the speed is set for descent from 30 to 5m, which quickly makes the aircraft come back to the ground.

I am not sure if this is a general error and the entire reference calculation needs to be fixed or just a special case needs to be made for the takeoff.  There could be other situations, for example if the aircraft is in the air and the motors are not giving enough thrust to achieve the desired climb speed the gv_z_ref could go very high and then make the aircraft come down when it should be going up to a higher target altitude.

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