Docs: Mentions setting the link mass in YAML

[matthias-mayr]

For my current setup, I am seeing better behavior during interactions with a smaller link mass.
It took me some minutes to find out how to set it in YAML. This should make it easier for other people.

[stefanscherzinger]

@matthias-mayr Thanks for this!

I like that it improves the documentation implicitly. I might add some specific hints on what the parameter actually does.

Note that your proposed 0.1 is already the default as specified (somewhat redundantly) here:

cartesian_controllers/cartesian_controller_base/include/cartesian_controller_base/ForwardDynamicsSolver.h

Line 138 in 42b7a26

std::atomic<double> m_min = 0.1;

and here:

cartesian_controllers/cartesian_controller_base/cfg/ForwardDynamicsSolver.cfg

Line 8 in 42b7a26

gen.add("link_mass", double_t, 0, "Virtual mass of the manipulator's links. The smaller this value, the more does the end-effector (which has a unit mass of 1) dominate dynamic behavior. Near singularities, a bigger value leads to smoother motion.", 0.1, 0.001, 1)

so maybe

I am seeing better behavior during interactions with a smaller link mass

is due to some other effect?

[matthias-mayr]

Note that your proposed 0.1 is already the default as specified (somewhat redundantly) here:

That is on purpose, because people often start with the example, so I want them to have the default value.

For my robot setup, I use these values:

solver:
    error_scale: 0.04
    iterations: 3
    forward_dynamics:
        link_mass: 0.025

since those were the best combination of some reactivity and stable contact in contact-rich situations.

iirc, higher (default) link mass gave me more low-frequency oscillations during contact. I might make my settings & the low-pass filter that I implemented available somehow. While it can be defined to be outside the scope of the library, it feels like it would be a great benefit if users can start out from some known well-working configurations for some standard robots like the UR5e.

I might add some specific hints on what the parameter actually does.

That'd be great