Question regarding publishing motor speed to Gazebo & some theoretical parameters

wonggwan commented 1 year ago

Hi,

Thanks for the great repo! As I sent the email a few days ago that probably was not clear enough to state my question, I am sending all the related issues I have using this repo over here:)

My setup

I am using Ubuntu 20.04 with ROS Noetic and Gazebo 11 version and have already managed to setup & install all the things now

Issue 1: publishing motor speed to Gazebo via rostopic

After I managed to show the CrazyS robot in Gazebo, I was trying to control the drone via Python script by publishing the corresponding motor speed to the ROS topic.

My questions are as follows for the first issue:

As there seems to be multiple topic that is related to motor speed, which is the one that I should publish my information to and what is the unit of the motor_speed we publish (rpm or rad/s)?

/crazyflie2/command/motor_speed
/crazyflie2/gazebo/command/motor_speed
/crazyflie2/motor_speed
/crazyflie2/motor_speed/0
/crazyflie2/motor_speed/1
/crazyflie2/motor_speed/2
/crazyflie2/motor_speed/3

I tried to publish my motor speed to the topic /crazyflie2/command/motor_speed which is using the message type mav_msgs/Actuators, but when I find the corresponding information about this message type in ROS webpage, it shows

float64[] angular_velocities # Angular velocities of the actuator in [rad/s].
                             # E.g. "rpm" of rotors, propellers, thrusters

But I think the motor speed has a different unit rpm compared to the angular velocity of the actuator rad/s, how do we specify this when we publish our motor speed to Gazebo?

What is the correct max angular velocity of the CrazyS in Gazebo? Is it 2618 rad/s?
When I try to publish some motor speed to the CrazyS, but as I subscribe the topic /crazyflie2/motor_speed, I found that the actual motor speed of the drone is not exactly following my command, e.g. I specified all 4 motors with 2418 rad/s, yet the drone motor speed kept bouncing around. Could you tell me what might be the issue?

Issue 2: question regarding some coefficient & parameter setting to calculate the system dynamics of the CrazyS

My questions are as follows for the second issue:

As I found in your publication on page 6:

Does the parameter match the physical configuration of the drone being used in Gazebo?

What is the correct MASS of the drone as a whole (including the motors)? I checked in Gazebo it showed as 0.03 kg, yet the CrazyS/rotors_description/urdf/crazyflie2.xarco showed that:

  <xacro:property name="mass" value="0.025" /> <!-- [kg] -->
  <xacro:property name="mass_rotor" value="0.0005" /> <!-- [kg] -->

which means the total mass will be 0.025 + 4*0.0005 = 0.027 kg.

Thank you so much!

welcome[bot] commented 1 year ago

Thanks for opening your first issue here! Make sure that no other issues on the same topic have already been opened!

dingthe4th commented 1 year ago

Hi! Think of my 'answers' with a grain of salt, I am no expert regarding this repo. I am just someone who used this repo recently as well.

For Issue 1, you publish the motor speed to /crazyflie/command/motor_speed. I used the Matlab version of this repo, so I am not entirely sure how to publish via script, but in Matlab, /crazyflie/command/motor_speed needs input from mav_msgs/Actuators and Propellers Angular Velocities (4) from Omegas (motor speed), see Fig. 11 (p.22) of the documentation that you refer to. As for the units, I already forgot but I think it's in the docu as well.

As for Issue 2, I think it's correct as maybe the ones showed in Gazebo is rounded-up to 0.XX significant digits? Not entirely sure.

If you wait for a bit, your issues maybe resolved by the author of this repo, good luck!

wonggwan commented 1 year ago

Update for Issue 1 regarding the motor speed:

I found that the /CrazyS/src/CrazyFlie/rotors_control/src/nodes/roll_pitch_yawrate_thrust_crazyflie_node.cpp is also publishing some sort of motor speed commands to gazebo apart from the /crazyflie2/command/motor_speed one, what should we do about this? E.g. does these two node interfere each other during simulation in gazebo?

wonggwan commented 1 year ago

Hi! Think of my 'answers' with a grain of salt, I am no expert regarding this repo. I am just someone who used this repo recently as well.

For Issue 1, you publish the motor speed to /crazyflie/command/motor_speed. I used the Matlab version of this repo, so I am not entirely sure how to publish via script, but in Matlab, /crazyflie/command/motor_speed needs input from mav_msgs/Actuators and Propellers Angular Velocities (4) from Omegas (motor speed), see Fig. 11 (p.22) of the documentation that you refer to. As for the units, I already forgot but I think it's in the docu as well.

As for Issue 2, I think it's correct as maybe the ones showed in Gazebo is rounded-up to 0.XX significant digits? Not entirely sure.

If you wait for a bit, your issues maybe resolved by the author of this repo, good luck!

Hi! Thanks for the help!

For Issue 1, now I also agree with you that we should publish the motor speed to the /crazyflie2/command/motor_speed, but as my comments above mentioned, there is one more node named as /CrazyS/src/CrazyFlie/rotors_control/src/nodes/roll_pitch_yawrate_thrust_crazyflie_node.cpp that is also publishing something towards the same topic, so I am not sure how is this affect my result.

As for Issue 2, since the drone is too light-weighted (roughly 30 grams), so any small change in the value will result in a very huge change in the calculation result. Since I am trying to use pure math calculation to 'simulate' how the drone fly, so I think the numbers will be crucial to the result.

gsilano commented 1 year ago

Hi wonggwan! Glad to hear from you that you found my work helpful. Here are my answers:

Motor speeds are expressed in radians per second [rad/s]. If you want to control the drone, without using the provided controller, just run roslaunch rotors_gazebo crazyflie2_without_controller.launch. Then, your code should publish on the /crazyflie2/gazebo/command/motor_speed topic.
ROS Messages are defined in the mav_msgs repository. Specifically, here you can find the definition of Actuators. You can find the maximum angular velocity in the the crazyflie2_base.xacro and crazyflie2_forster.xacro files. Since I have never tested the simulator with the Crazyflie, comparing the behaviors, I suggest you find your own parameters or rely on those in Forster's thesis.

I hope this helps. If not, let me know.

wonggwan commented 1 year ago

Hi wonggwan! Glad to hear from you that you found my work helpful. Here are my answers:

Motor speeds are expressed in radians per second [rad/s]. If you want to control the drone, without using the provided controller, just run roslaunch rotors_gazebo crazyflie2_without_controller.launch. Then, your code should publish on the /crazyflie2/gazebo/command/motor_speed topic.

ROS Messages are defined in the mav_msgs repository. Specifically, here you can find the definition of Actuators. You can find the maximum angular velocity in the the crazyflie2_base.xacro and crazyflie2_forster.xacro files. Since I have never tested the simulator with the Crazyflie, comparing the behaviors, I suggest you find your own parameters or rely on those in Forster's thesis.

I hope this helps. If not, let me know.

Thank you so much for your help! I think that solves my current problem! Closing this issue now:)

gsilano / CrazyS