Cannot get ChBody and ChGravityLoader to work together

[Yavor Vale]

Hi,

My previous post (https://groups.google.com/g/projectchrono/c/A7GlGUYS1Ik) failed to garner any engagement, so I did the only thing I could think of in the situation and moved to C++ to remove any Python/SWIG-related issues.

As much as I try, I can't get ChBody's to work with ChGravityLoader.

I started from demo_IRR_visualize_collision.cpp and made only two notable changes:

* I removed the built-in gravity by calling SetGravitationalAcceleration with the zero vector

* I added back the gravity individually to each object through a ChLoadContainer and a ChLoad for each object.

The simulation does not run at all when I try to use ChSystemNSC.

It starts with ChSystemSMC, but it looks like the gravity is working only until the first collision (or some other undetermined condition that I can't recognize).

I can't use ChCollisionSystem::Type::MULTICORE, because I didn't build Chrono to support it. Would this collision system solve my issues?

The full source is available at https://pastebin.com/fXEmB9Bg

These are the changes I've made to the original source code:

diff --git a/src/demos/irrlicht/demo_IRR_visualize_collision.cpp b/src/demos/irrlicht/demo_IRR_visualize_collision.cpp

index 022d906c7..fc04cba6b 100644

--- a/src/demos/irrlicht/demo_IRR_visualize_collision.cpp

+++ b/src/demos/irrlicht/demo_IRR_visualize_collision.cpp

@@ -17,6 +17,8 @@

 #include "chrono/collision/bullet/ChCollisionSystemBullet.h"

 #include "chrono/physics/ChSystemNSC.h"

+#include "chrono/physics/ChSystemSMC.h"

+#include "chrono/physics/ChLoadContainer.h"

 #include "chrono/physics/ChBodyEasy.h"

 #include "chrono/core/ChTimer.h"

 #include "chrono/core/ChRealtimeStep.h"

@@ -59,10 +61,13 @@ int main(int argc, char* argv[]) {

     std::cout << "Copyright (c) 2017 projectchrono.org\nChrono version: " << CHRONO_VERSION << std::endl;

     // Create the Chrono system, bodies, and collison shapes

-    ChSystemNSC sys;

+    ChSystemSMC sys;

+    //ChSystemNSC sys;

     sys.SetCollisionSystemType(csys_type);

+    sys.SetGravitationalAcceleration(ChVector3d(0, 0, 0));

-    auto mat = chrono_types::make_shared<ChContactMaterialNSC>();

+    auto mat = chrono_types::make_shared<ChContactMaterialSMC>();

+    //auto mat = chrono_types::make_shared<ChContactMaterialNSC>();

     auto ground = chrono_types::make_shared<ChBodyEasyBox>(10, 3, 10, 100, mat);

     ground->SetFixed(true);

@@ -95,6 +100,14 @@ int main(int argc, char* argv[]) {

     mesh->GetVisualShape(0)->SetColor(ChColor(0.2f, 0.3f, 0.6f));

     sys.AddBody(mesh);

+    auto load_container = chrono_types::make_shared<ChLoadContainer>();

+    for (auto obj : sys.GetBodies()) {

+        auto custom_loader = chrono_types::make_shared<ChLoaderGravity>(obj);

+        auto custom_load = chrono_types::make_shared<ChLoad>(custom_loader);

+        load_container->Add(custom_load);

+    }

+    sys.Add(load_container);

+

     // Create the Irrlicht visualization system

     auto vis = chrono_types::make_shared<ChVisualSystemIrrlicht>();

     vis->AttachSystem(&sys);

I feel like I'm hitting something fundamental, but I don't know what it is and where to read more about it.

I'm using a 1 week old build of the `main` branch of Project Chrono, with Irrlicht support enabled. The example runs fine if I return the code to the original state, as do all other examples, so I have no reason to believe this is caused by a bad build of Project Chrono.

Can someone please provide any sort of insight as to what is happening?

I'm simply trying to add the simplest ChLoaderUVW force available and it is absolutely not working.

Does this even work for ChBody's? I've seen it works with demo_FEA_loads_dynamic, but according to the docs, ChBody is loadable.

Best regards, Yavor

[Yavor Vale]

One thing I failed to mention in my previous post is that the following message appears in the console even before the simulation starts running:

warning cbtCollisionDispatcher::needsCollision: static-static collision!

As far as I understand, this is coming from within Bullet.

I'm surprised there's static-static collisions, since after consulting the code, it seems to me that by providing a material to the constructors of the Easy* objects, I'm initializing them as non-static.

What could be the objects that triggered the static-static collision? Is there a hook for me to find out?

Best, Yavor

[Alessandro Tasora]

hi

ChLoaderGravity was meant for ChFiniteElement objects, to automate some computations inside ChMesh container, but was NOT meant for ChBody. In theory it could be extended to be compatible with them, but it would require some workaround because bodies do not have a xyz domain for Gauss quadrature, and their mass comes precomputed by the user. In future, if I have time, I'll try to see what happens and how to circumvent this limitation. The ChLoad stuff is quite complicate and I'd need to look again at it from ground up.

regards

Alessandro Tasora

[Radu Serban]

Hi Yavor,

 

You can safely ignore the warning about static-static collision. This is a red herring coming from the fact that, while we only use its collision detection piece, Bullet has its own dynamics and this warning (issued only in Debug mode) is part of an internal Bullet coupling between the two which is irrelevant for us.  Since we anyway have custom Chrono modifications to the Bullet collision, I pushed another one to just disabled this misleading warning.

 

--Radu

--
You received this message because you are subscribed to the Google Groups "ProjectChrono" group.
To unsubscribe from this group and stop receiving emails from it, send an email to projectchron...@googlegroups.com.
To view this discussion visit https://groups.google.com/d/msgid/projectchrono/d7f69dab-d61a-4857-a55f-181796badbb2n%40googlegroups.com.