Simulation in the presence of a wind field. about gym-pybullet-drones

[JacopoPan]

Hi @dipaco,

can you help me understand what you want to implement? Could a time-varying force contribution do the trick? The way the robot model is specified in the URDF file/simulation, one could apply external forces on its center of mass or at the site of each motor.

E.g.,

env = CtrlAviary(...)
PYB_CLIENT = env.getPyBulletClient()
p.applyExternalForce(env.DRONE_IDS[0],
                     -1,  # -1 for the base, 0-3 for the motors
                     forceObj=[w_x, w_y, w_z], # a force vector
                     posObj=[0, 0, 0], flags=p.WORLD_FRAME,  physicsClientId= PYB_CLIENT)

Note that external forces are reset at each tilmestep, so you should repeatedly apply one, if that's the intended behaviour.

[dipaco]

Thank you @JacopoPan for your quick answer. This might work, I need to look into it, but so far it looks pretty much like what I need. Yes, what I meant by f: R^3 x t -> R^3 is a time-varying force that gives me a wind velocity at every position x. I can convert that wind field into a drag force using some knowledge of the geometry of the quadcopter. However, I wasn't sure if it was possible to exert that external force into the quadcopter at simulation time. I will try your example.

Thank you so much.

[JacopoPan]

No prob.
Note that you should call p.applyExternalForce() before every p.stepSimulation().
env.step() might contain multiple p.stepSimulation(), if you created env with argument aggregate_phy_steps > 1.
In that case, you might want to modify BaseAviary directly, to apply the external force just before p.stepSimulation().

[chijiokekechi]

Hi @JacopoPan, thank you for answering this. Just for clarification, the p.applyExternalForce() adds this force to the total forces acting on the drone. What units are the forces and how do I distinguish this from the drones internal forces (i.e. thrust and the likes)? Thank you very much.

[JacopoPan]

Hi @chijiokekechi,

I would refer you to the PyBullet manual for the use of applyExternalForce (or search for it in BaseAviary.py). The unit is Newton. In the method call, you would specify one of the links composing the robots (4 for the center of the drone).