The math behind the motor model plugin

[AllenHuangDongyue]

As mentioned by issue110 this issue, I got some questions about this model,

1. What are the units of rotational speed we used in the model, rad/s?
2. Why do we need the parameter (rotorVelocitySlowdownSim)?

I am going to contructure my own model based on this one, any suggestion will be welcomed!!

Many thanks!

[JohnTGZ]

The best place to start looking is the source code.

It seems that this line tells me that double force = real_motor_velocity * std::abs(real_motor_velocity) * motor_constant_; Since units of Force = [Mass] [ Length] [Time^-2], and motor_constant_ is [mass][Length] (This is derived in these notes), we can conclude that real_motor_velocity is [s-1], which would be in units of (rad/s).

As for the rotorVelocitySlowdownSim, I believe that when motor speed is higher than what the simulation can step through, it is used as part of anti-aliasing during simulation at (line 197)[https://github.com/PX4/PX4-SITL_gazebo-classic/blob/20ded0757b4f2cb362833538716caf1e938b162a/src/gazebo_motor_model.cpp#L197]:

  motor_rot_vel_ = joint_->GetVelocity(0);
  if (motor_rot_vel_ / (2 * M_PI) > 1 / (2 * sampling_time_)) {
    gzerr << "Aliasing on motor [" << motor_number_ << "] might occur. Consider making smaller simulation time steps or raising the rotor_velocity_slowdown_sim_ param.\n";
  }
  double real_motor_velocity = motor_rot_vel_ * rotor_velocity_slowdown_sim_;

[DronecodeBot]

This issue has been mentioned on Discussion Forum for PX4, Pixhawk, QGroundControl, MAVSDK, MAVLink. There might be relevant details there:

https://discuss.px4.io/t/gazebo-sdf-file-motor-parameters-definitions/34747/1