Question about applying forces to links in gazebo

[manumerous]

While looking through the gazebo_motor_model.cpp I had a few questions coming up.

Am I right to assume that link_ will specify the propeller as an actual physical body which is linked through the rest through the revolute joint joint_? What is the difference between adding relative and normal forces?

e.g. link_->AddRelativeForce(ignition::math::Vector3d(0, 0, force * scalar));

e.g. link_->AddForce(air_drag);

How do we know the direction the force is applied to in both cases? Is the first one relative to a local bodyframe of link and the second one represented in the "main body frame"?

I guess it would be possible to find that somewhere in the gazebo repo but i hope it is okay to ask, since you might already know part of the answers I guess.

[Jaeyoung-Lim]

@manumerous In general, you can look up the gazebo api: http://osrf-distributions.s3.amazonaws.com/gazebo/api/11.0.0/classgazebo_1_1physics_1_1Link.html

As it is said in the API the relative force applies force on a specific location while the addforce applies forces at the link origin.

They are both not related to the main bodyframe and apply forces at the link frame

[manumerous]

Okay thank you for your answer. I am still a bit confused by it.

According to the documentation it seems that AddRelativeForce() also adds the force at the links origin, but the force is expressed in the link frame:

Could it be that you were refering to AddForceAtRelativePosition() instead of AddRelativeForce()?

I guess in contrast the AddForce probably has a force specified in bodyframe, what would make since since the airspeed used to calculate the drag force is most likely expressed in the bodyframe. Unfortunatly it is not specified directly.

[manumerous]

Anyways that setup seems to be slightly overkill for our application. I guess I will just calculate everything in the body frame since we do not have that many links and joints in the UAV case.

[Jaeyoung-Lim]

@manumerous

Could it be that you were refering to AddForceAtRelativePosition() instead of AddRelativeForce()?

I think you are right. The truth is in the API docs :smile:

Since those are component wise plugins, they don't generally know what the "body" is in the simulation. It is applying forces to the link entity, therefore a "body" means link.

Those are referred from two different objects (model and link) as they are on different pointers. I think that is where your confusion on the coordinate frames are coming form.

[manumerous]

Hey Jay, I have a few questions about the lift drag plugin. Would you have time for a quick sparring session on jitsi?

[Jaeyoung-Lim]

@manumerous Sure

[manumerous]

https://jitsi.bastli.ethz.ch/SA_Meeting

[Jaeyoung-Lim]

@manumerous I am in the meeting - unfortunately I need to leave before 4