High Thrust Calculation in Motor Model Plugin

[Kametor]

Environment

• OS Version: Ubuntu 22.04
• Gazebo Harmonic

Description

• There may be a misunderstanding or misuse of the Gazebo motor model plugin parameters. Specifically, the “maxRotVelocity” parameter, which is described as the “Maximum rotational velocity command in units of rad/s”, seems to be leading to unintended results in the thrust calculation.
  The thrust force is computed using the following equation:

thrust = realMotorVelocity * realMotorVelocity * this->motorConstant;
Here, the variable realMotorVelocity is defined as:

double realMotorVelocity = motorRotVel * this->rotorVelocitySlowdownSim;
Consequently, at full throttle, the resulting force calculation is:

(maxRotVelocity * 10)^2 * motorConstant.
Based on my calculations, the resulting force in Gazebo appears significantly higher than expected. I am uncertain if I am misinterpreting a specific aspect of the model or if there is an issue with the parameter setup. In my calculation the thrust force is very high. Also drone climbs very fast. I think there is a problem. Any guidance or insight into this discrepancy would be greatly appreciated.

[Kametor]

As observed, the ESC PWM values are approximately 250, while the joystick input remains below the halfway mark. Despite this moderate input, the drone exhibits rapid ascent behavior. The custom parameters currently in use are as follows:

<plugin filename="gz-sim-multicopter-motor-model-system" name="gz::sim::systems::MulticopterMotorModel">
  <jointName>rotor_0_joint</jointName>
  <linkName>rotor_0</linkName>
  <turningDirection>ccw</turningDirection>
  <timeConstantUp>0.0125</timeConstantUp>
  <timeConstantDown>0.025</timeConstantDown>
  <maxRotVelocity>313.0</maxRotVelocity>
  <motorConstant>830.0e-06</motorConstant>
  <momentConstant>0.03</momentConstant>
  <commandSubTopic>command/motor_speed</commandSubTopic>
  <motorNumber>0</motorNumber>
  <rotorDragCoefficient>0.001</rotorDragCoefficient>
  <rollingMomentCoefficient>0.001</rollingMomentCoefficient>
  <rotorVelocitySlowdownSim>10</rotorVelocitySlowdownSim>
  <motorType>velocity</motorType>
</plugin>

Additionally, the drone has a total weight of 45 kg.

[azeey]

This is my understanding:

Joint velocity assignment

1. Commanded rotor velocity is assigned to refMotorInput

   gz-sim/src/systems/multicopter_motor_model/MulticopterMotorModel.cc

   Lines 528 to 530 in 83b30c2

   	this->refMotorInput = std::min(
   	static_cast<double>(msg->velocity(this->actuatorNumber)),
   	static_cast<double>(this->maxRotVelocity));

2. This velocity is filtered (refMotorRotVel) to fullfil the time constant parameters specified

   gz-sim/src/systems/multicopter_motor_model/MulticopterMotorModel.cc

   Lines 673 to 675 in 83b30c2

   	double refMotorRotVel;
   	refMotorRotVel = this->rotorVelocityFilter->UpdateFilter(
   	this->refMotorInput, this->samplingTime);

3. refMotorRotVel/rotorVelocitySlowDownSim is assigned as the actual commanded velocity on the joint

   gz-sim/src/systems/multicopter_motor_model/MulticopterMotorModel.cc

   Lines 677 to 680 in 83b30c2

   	_ecm.SetComponentData<components::JointVelocityCmd>(
   	this->jointEntity,
   	{this->turningDirection * refMotorRotVel
   	/ this->rotorVelocitySlowdownSim});

Thrust Calculation

1. The current joint velocity is obtained from the ECM and multiplied by rotorVelocitySlowDownSim to get the reference motor velocity (realMotorVelocity). In steady state, this should be the same as refMotorInput. In full throttle, this would be motorRotVel.

   gz-sim/src/systems/multicopter_motor_model/MulticopterMotorModel.cc

   Lines 555 to 565 in 83b30c2

   	const auto jointVelocity = _ecm.Component<components::JointVelocity>(
   	this->jointEntity);
   	double motorRotVel = jointVelocity->Data()[0];
   	if (motorRotVel / (2 * GZ_PI) > 1 / (2 * this->samplingTime))
   	{
   	gzerr << "Aliasing on motor [" << this->actuatorNumber
   	<< "] might occur. Consider making smaller simulation time "
   	"steps or raising the rotorVelocitySlowdownSim param.\n";
   	}
   	double realMotorVelocity =
   	motorRotVel * this->rotorVelocitySlowdownSim;

2. Thrust is calculated as a constant times a signed realMotorVelocity^2.

   gz-sim/src/systems/multicopter_motor_model/MulticopterMotorModel.cc

   Lines 570 to 572 in 83b30c2

   	double thrust = this->turningDirection * realMotorVelocitySign *
   	realMotorVelocity * realMotorVelocity *
   	this->motorConstant;

One difference in your analysis is

Consequently, at full throttle, the resulting force calculation is:

(maxRotVelocity * 10)^2 * motorConstant.

I think it should be (maxRotVelocity)^2 * motorConstant since the velocity used in the thrust calculation is in the same order of magnitude as the input rotor velocity refMotorInput, i.e, rotorVelocitySlowdownSim cancels out when calculating realMotorVelocity.