Simulation and hardware emulation software » libmrsim

Martin Jacquet wrote:

the default drag for the motor and the default allocation matrix G do not seem to match.

To be more explicit, the allocation matrix is computed using c=0.0154=cf/ct, hence cf=6.5e-4 implies ct=1e-5:
https://git.openrobots.org/projects/libmrsim/repository/libmrsim/revisions/master/entry/src/sim.c#L84

Also, the default value for cf in mrsim-gazebo is 6e-5:
https://git.openrobots.org/projects/mrsim-gazebo/repository/mrsim-gazebo/revisions/master/entry/src/plugin.cc#L150

Is it doable to set those default values in libmrsim, and have mrsim-gazebo and mrsim-genom3 rely on those?

Before merging the two constants, I would like to understand the
following discrepancy in the models:

• In the motor model, the friction is 'b.w' (linear in w)
• In the dynamic model, the Z torque is 'Kt.w²' (quadratic in w)

This is puzzling me ... shouldn't those two torques be similar?

Thinking about it, I think the motor model is missing was is called
the 'load torque' in the models, which is precisely the torque in
'Ct.w²' created by the propeller blades.

Furthermore, maybe the 'b.w' term can be neglected, and thus the motor
eq. would become:

dw/dt = -Ct.w² + K/2PI.i

What do you think?

• for w=20Hz: (b.w)/(ct.w²) = 2.5
• for w=1000Hz: (b.w)/(ct.w²) = 0.5

• best reference I know about motors and propellers is the PhD thesis by Bangura: https://openresearch-repository.anu.edu.au/bitstream/1885/112382/1/Bangura%20Thesis%202017.pdf
• for a quick shorter reference also the PhD Thesis by Tomic can be used https://elib.dlr.de/185788/1/2018_Teodor-Tomic_Thesis_Dissertation.pdf
• for an even quicker reference this ICRA 2014 paper by Bangura should be enough: https://openresearch-repository.anu.edu.au/bitstream/1885/11442/1/Bangura%20et%20al%20Aerodynamic%20power%20control%202014.pdf

I pushed a number of commits in libmrsim, mrsim-gazebo
and mrsim-genom3 to address (hopefully) all the issues.

• the rotor model has gained a quadratic drag term matching what is
  used in the rest of the software and also in the ref. above. The
  linear term is kept but set to 0. by default.

• a new set_gtmrp_geom in mrsim-genom3 was added, similar to
  what is found in nhfc-genom3.

If you can test it and it works for you, I'll make a release of those
projects soon.

I did some tests with both mrsim-genom3 and mrsim-gazebo, it seems to work fine, at least for the overall drone behavior.

So, in your opinion we should neglect the viscous friction term b*w? We can probably measure b empirically on the motor (removing the blade) if we can measure the current sent to the motor for rotating in steady state.
Anyhow, I agree that it's most likely less impactful than the aerodynamic drag term.

Martin Jacquet wrote in #note-7:

I did some tests with both mrsim-genom3 and mrsim-gazebo, it seems to work fine, at least for the overall drone behavior.

So, in your opinion we should neglect the viscous friction term b*w? We can probably measure b empirically on the motor (removing the blade) if we can measure the current sent to the motor for rotating in steady state.
Anyhow, I agree that it's most likely less impactful than the aerodynamic drag term.

it depends on the speeds, there is for sure a low enough speed for which is more impactful, if Bangura has it there is probably a reason, the best would be identify from real data and check if it is needed to estimate well the behavior

On Monday 26 Sep 2022, at 15:45, Martin Jacquet wrote:

So, in your opinion we should neglect the viscous friction term b*w?

In the simulator, I think this term is mostly irrelevant.
And if it's not, then it should be considered in the control software
as well :)

We can probably measure b empirically on the motor (removing the
blade) if we can measure the current sent to the motor for rotating
in steady state.

As a first approximation, you can check the current drawn on a power
supply by spinning the rotors (without props). Remove the ~100mA of
the odroid (you can also check that when not spinning the rotors),
divide by 4 and you're done :)