[Vehicle Dynamics Blockset] Confused about coordinate system conventions
Hello everyone!
Recently I’m working on this Simulink example: https://www.mathworks.com/help/vdynblks/ug/braking-test.html
I also checked the setting of coordinate system in VDBS: https://www.mathworks.com/help/vdynblks/ug/coordinate-systems-in-vehicle-dynamics-blockset.html
In this simulation, I assume a few settings:
(1) the ground is completely flat and horizontal
(2) the origin of inertial (world-fixed) frame is attach at the ground surface, with Z-axe pointing downward.
(3) the origin of tire frame is attach at the ground surface, with Z-axe pointing upward.
I want to slightly modify the Simulink example shared above, by changing parameters in some blocks:
(1) Block 1 —- Vehicle Body 6DOF
Based on the figure below, the vehicle-fixed frame should attach at the center of mass (CoM)
In this block, I want to change the initial position of CoM in inertial frame (aka, world-fixed frame), as shown below:
Since the inertial frame is attached on the ground, for the initial vertical position of CoM, I set it to -0.30938-VEH.HeightCG = -0.44338. Here, the value 0.30938 is the unload tire radius, and VEH.HeightCG is the vertical offset between CoM and axle plane (h in figure "vehicle layout" above). From my understanding, this position can let the tire just slightly contact the ground.
(2) Block 2 —- Combined Slip Wheel 2DOF
In this block, I assign a small value to this parameter:
Based on my understanding, this value refers to the compression of tire due to vertical load. A positive value means that the tire is compressed.
(3) Block 3 —- Ground Feedback
Since I assume inertial frame is attach at the ground surface, so the ground height for all wheels G_xx_z (xx: FL, FR, RL, RR) are set to 0 all the time.
Then I start the simulation. During the first 2 seconds, the vehicle will stablize on the ground. I logged some relavent signals to check the vehicle states:
(1) Passenger Vehicle:1.Body.InertFrm.Cg.Disp.Z —- the vertical position of CoM in inertial frame
(2) Passenger Vehicle:1.Body.InertFrm.FrntAxl.Lft.Disp.Z —- the vertical position of front-left axle in inertial frame
(3) Passenger Vehicle:1.Body.BdyFrm.FrntAxl.Lft.Disp.Z —- the vertical position of front-left axle in vehicle-fixed frame
(4) Passenger Vehicle:1.Wheels.TireFrame.z(1) —- the vertical displacement of front-left axle in tire frame? (I’m not sure, just based on the description in https://www.mathworks.com/help/vdynblks/ref/combinedslipwheel2dof.html)
(5) Passenger Vehicle:1.Wheels.TireFrame.dz(1) —- the vertical displacement of front-left axle (wheel) in vehicle-fixed frame? (I’m not sure, just based on the computation in the model, as shown below)
The stablizing process for vehicle is shown below:
After the vehicle is stablized on the ground, we can see:
(1) the vertical position of CoM in inertial frame converges to around 0.0
(2) the vertical position of front-left axle in inertial frame is close to that value in vehicle-fixed frame, while the latter one is always a constant value equal to VEH.HeightCG
(3) TireFrame.z(1) and TireFrame.dz(1) converge to some small values near 0.0
(4) The initial value of TireFrame.z(1) is the opposite of the initial value of a parameter z0 in Block "Combined Slip Wheel 2DOF" (as shown above). It might be due to a flip of Z-axe direction.
It means that the CoM of the vehicle is near the ground, which means that the vehicle chassis penetrates and sinks under the ground. It doesn’t make sense to me. I have a few questions:
(1) What are the setting of coordinate systems for different components (CoM, axles, wheels, tires)? —- where should the origin of a specific frame attach?
(2) What are the exact meaning of TireFrame.z and TireFrame.dz? The vertical position of wheel center? In which frame?
I would be very grateful if someone could provide an answer.
Regards,
Zijun HeHello everyone!
Recently I’m working on this Simulink example: https://www.mathworks.com/help/vdynblks/ug/braking-test.html
I also checked the setting of coordinate system in VDBS: https://www.mathworks.com/help/vdynblks/ug/coordinate-systems-in-vehicle-dynamics-blockset.html
In this simulation, I assume a few settings:
(1) the ground is completely flat and horizontal
(2) the origin of inertial (world-fixed) frame is attach at the ground surface, with Z-axe pointing downward.
(3) the origin of tire frame is attach at the ground surface, with Z-axe pointing upward.
I want to slightly modify the Simulink example shared above, by changing parameters in some blocks:
(1) Block 1 —- Vehicle Body 6DOF
Based on the figure below, the vehicle-fixed frame should attach at the center of mass (CoM)
In this block, I want to change the initial position of CoM in inertial frame (aka, world-fixed frame), as shown below:
Since the inertial frame is attached on the ground, for the initial vertical position of CoM, I set it to -0.30938-VEH.HeightCG = -0.44338. Here, the value 0.30938 is the unload tire radius, and VEH.HeightCG is the vertical offset between CoM and axle plane (h in figure "vehicle layout" above). From my understanding, this position can let the tire just slightly contact the ground.
(2) Block 2 —- Combined Slip Wheel 2DOF
In this block, I assign a small value to this parameter:
Based on my understanding, this value refers to the compression of tire due to vertical load. A positive value means that the tire is compressed.
(3) Block 3 —- Ground Feedback
Since I assume inertial frame is attach at the ground surface, so the ground height for all wheels G_xx_z (xx: FL, FR, RL, RR) are set to 0 all the time.
Then I start the simulation. During the first 2 seconds, the vehicle will stablize on the ground. I logged some relavent signals to check the vehicle states:
(1) Passenger Vehicle:1.Body.InertFrm.Cg.Disp.Z —- the vertical position of CoM in inertial frame
(2) Passenger Vehicle:1.Body.InertFrm.FrntAxl.Lft.Disp.Z —- the vertical position of front-left axle in inertial frame
(3) Passenger Vehicle:1.Body.BdyFrm.FrntAxl.Lft.Disp.Z —- the vertical position of front-left axle in vehicle-fixed frame
(4) Passenger Vehicle:1.Wheels.TireFrame.z(1) —- the vertical displacement of front-left axle in tire frame? (I’m not sure, just based on the description in https://www.mathworks.com/help/vdynblks/ref/combinedslipwheel2dof.html)
(5) Passenger Vehicle:1.Wheels.TireFrame.dz(1) —- the vertical displacement of front-left axle (wheel) in vehicle-fixed frame? (I’m not sure, just based on the computation in the model, as shown below)
The stablizing process for vehicle is shown below:
After the vehicle is stablized on the ground, we can see:
(1) the vertical position of CoM in inertial frame converges to around 0.0
(2) the vertical position of front-left axle in inertial frame is close to that value in vehicle-fixed frame, while the latter one is always a constant value equal to VEH.HeightCG
(3) TireFrame.z(1) and TireFrame.dz(1) converge to some small values near 0.0
(4) The initial value of TireFrame.z(1) is the opposite of the initial value of a parameter z0 in Block "Combined Slip Wheel 2DOF" (as shown above). It might be due to a flip of Z-axe direction.
It means that the CoM of the vehicle is near the ground, which means that the vehicle chassis penetrates and sinks under the ground. It doesn’t make sense to me. I have a few questions:
(1) What are the setting of coordinate systems for different components (CoM, axles, wheels, tires)? —- where should the origin of a specific frame attach?
(2) What are the exact meaning of TireFrame.z and TireFrame.dz? The vertical position of wheel center? In which frame?
I would be very grateful if someone could provide an answer.
Regards,
Zijun He Hello everyone!
Recently I’m working on this Simulink example: https://www.mathworks.com/help/vdynblks/ug/braking-test.html
I also checked the setting of coordinate system in VDBS: https://www.mathworks.com/help/vdynblks/ug/coordinate-systems-in-vehicle-dynamics-blockset.html
In this simulation, I assume a few settings:
(1) the ground is completely flat and horizontal
(2) the origin of inertial (world-fixed) frame is attach at the ground surface, with Z-axe pointing downward.
(3) the origin of tire frame is attach at the ground surface, with Z-axe pointing upward.
I want to slightly modify the Simulink example shared above, by changing parameters in some blocks:
(1) Block 1 —- Vehicle Body 6DOF
Based on the figure below, the vehicle-fixed frame should attach at the center of mass (CoM)
In this block, I want to change the initial position of CoM in inertial frame (aka, world-fixed frame), as shown below:
Since the inertial frame is attached on the ground, for the initial vertical position of CoM, I set it to -0.30938-VEH.HeightCG = -0.44338. Here, the value 0.30938 is the unload tire radius, and VEH.HeightCG is the vertical offset between CoM and axle plane (h in figure "vehicle layout" above). From my understanding, this position can let the tire just slightly contact the ground.
(2) Block 2 —- Combined Slip Wheel 2DOF
In this block, I assign a small value to this parameter:
Based on my understanding, this value refers to the compression of tire due to vertical load. A positive value means that the tire is compressed.
(3) Block 3 —- Ground Feedback
Since I assume inertial frame is attach at the ground surface, so the ground height for all wheels G_xx_z (xx: FL, FR, RL, RR) are set to 0 all the time.
Then I start the simulation. During the first 2 seconds, the vehicle will stablize on the ground. I logged some relavent signals to check the vehicle states:
(1) Passenger Vehicle:1.Body.InertFrm.Cg.Disp.Z —- the vertical position of CoM in inertial frame
(2) Passenger Vehicle:1.Body.InertFrm.FrntAxl.Lft.Disp.Z —- the vertical position of front-left axle in inertial frame
(3) Passenger Vehicle:1.Body.BdyFrm.FrntAxl.Lft.Disp.Z —- the vertical position of front-left axle in vehicle-fixed frame
(4) Passenger Vehicle:1.Wheels.TireFrame.z(1) —- the vertical displacement of front-left axle in tire frame? (I’m not sure, just based on the description in https://www.mathworks.com/help/vdynblks/ref/combinedslipwheel2dof.html)
(5) Passenger Vehicle:1.Wheels.TireFrame.dz(1) —- the vertical displacement of front-left axle (wheel) in vehicle-fixed frame? (I’m not sure, just based on the computation in the model, as shown below)
The stablizing process for vehicle is shown below:
After the vehicle is stablized on the ground, we can see:
(1) the vertical position of CoM in inertial frame converges to around 0.0
(2) the vertical position of front-left axle in inertial frame is close to that value in vehicle-fixed frame, while the latter one is always a constant value equal to VEH.HeightCG
(3) TireFrame.z(1) and TireFrame.dz(1) converge to some small values near 0.0
(4) The initial value of TireFrame.z(1) is the opposite of the initial value of a parameter z0 in Block "Combined Slip Wheel 2DOF" (as shown above). It might be due to a flip of Z-axe direction.
It means that the CoM of the vehicle is near the ground, which means that the vehicle chassis penetrates and sinks under the ground. It doesn’t make sense to me. I have a few questions:
(1) What are the setting of coordinate systems for different components (CoM, axles, wheels, tires)? —- where should the origin of a specific frame attach?
(2) What are the exact meaning of TireFrame.z and TireFrame.dz? The vertical position of wheel center? In which frame?
I would be very grateful if someone could provide an answer.
Regards,
Zijun He vehicle dynamcis, coordinate system, vehicle dynamics blockset MATLAB Answers — New Questions
