Dynamics Simulation Inconsistencies in SimMechanics Model

I have been learning how to use SimMechanics and Simulink to model an F450 quadcopter system. I used SimMechanics Link for SolidWorks to export the model to a 2nd Generation SimMechanics model. However, when I apply a torque to the motors, the dynamics of the system doesn't follow the laws of physics. For example, when I apply a torque to one of the motors, only the yaw of the system changes. I have not been able to trace the root cause of this issue. More experienced advice/feedback would be much appreciated!

Here is a link my SimMechanics/Simulink model.

Answer 1

[Sebastian Castro]

This makes sense, actually. Have you made any connection between the angular velocity of the motor and the thrust provided? SimMechanics only knows that you're spinning a propeller but can't figure out the aerodynamics for you.

There is usually a relationship between propeller speed and thrust which you have to implement yourself, whether it be a curve fit, lookup table, etc. The goal is to measure the rotational speed of each motor and use an "External Force and Torque" block to apply a force that is normal to each of the corresponding propeller.

Once you've set up all that, SimMechanics will apply the 4 thrust forces to the system, which will make the quadcopter translate and rotate because of that 6-DoF joint.

- Sebastian

[Jonathan]

Aw, perfect! I was overlooking this. I added an "External Force and Torque" block with a force input that is proportional to the torque generated by the motor.

I should also be able to use the table for my motors to model this. So, for example, I can generate a function modeling the thrust (G) for a given propeller speed using the table for my motors below. Does this sound right?

[Sebastian Castro]

Exactly that!

If you already have the table, get yourself a PS Lookup Table block from the Simscape -> Foundation Library --> Physical Signals library and connect it between the motor speed measurement and that External Force & Torque block.

[Jonathan]

Man, these toolboxes are so powerful! I am falling in love with these toolboxes.

Quick follow up.. I don't have all of the information from the manufacturer to model the Brushless motors. For example, I don't have a list of maximum torque values, the electrical loss information, and the rotor inertia. What is typically the best way to model this given this constraint?

[Sebastian Castro]

If you have a real motor at hand, you can collect data from it and use it to estimate parameters, as in the following video:

http://www.mathworks.com/videos/estimating-dc-motor-parameters-97057.html

Otherwise, if you have different parameterizations in your datasheet, you can try convert them to a format that works with premade blocks, or make your own blocks using the Simscape language:

http://www.mathworks.com/help/physmod/simscape/lang/creating-custom-components.html

- Sebastian