Can you show the discrepancies that you are observing in the simulation?
Are you aware about the chaotic behaviour of the double pendulum?
Double_pendulum#Chaotic_motion
<https://en.wikipedia.org/wiki/Double_pendulum#Chaotic_motion>
A slight change in the initial conditions, little approximations, etc can
lead to totally different results and this is somehow expected.
Dario
Il giorno mercoledì 21 maggio 2025 alle 04:37:22 UTC+2 aa...@rice.edu ha
scritto:
> Hello
>
> I am trying to model a 2D double pendulum problem with the following
> geometry and initial conditions. However, the solution does not match the
> analytical results. I would like to verify whether my inputs are correct.
>
> Also, if I plan to make one of the links flexible, what would be an
> efficient way to implement it?
>
> The code is included below.
>
>
> ....................................................................................................................................................
>
> import pychrono as chrono
> import pychrono.irrlicht as chronoirr
>
> # Create system
>
> sys = chrono.ChSystemNSC()
> sys.SetGravitationalAcceleration(chrono.ChVector3d(0, -9.81, 0))
>
> # 1. Ground body (fixed)
> ground = chrono.ChBody()
> ground.SetFixed(True)
> ground.SetPos(chrono.ChVector3d(0, 0, 0))
> sys.Add(ground)
>
> # 2. First pendulum body
> pend1 = chrono.ChBody()
> pend1.SetMass(0.79)
> pend1.SetInertiaXX(chrono.ChVector3d(0.01, 0.01, 0.0658))
> pend1.SetPos(chrono.ChVector3d(0, -0.5, 0)) # CG in center of bar
> pend1.SetAngVelParent(chrono.ChVector3d(0, 0, 10))
> sys.Add(pend1)
>
> # Visual: 1m tall, half-dim = 0.5, shift visual down by 0.5
> shape1 = chrono.ChVisualShapeBox(chrono.ChVector3d(0.01,1, 0.01))
> shape1.SetColor(chrono.ChColor(0.5, 0.5, 0.8))
> pend1.AddVisualShape(shape1)
>
> # 3. Second pendulum body
> pend2 = chrono.ChBody()
> pend2.SetMass(0.3)
> pend2.SetInertiaXX(chrono.ChVector3d(0.01, 0.01, 0.0250))
> pend2.SetPos(chrono.ChVector3d(0, -1.5, 0)) # CG in center of second bar
> pend2.SetAngVelParent(chrono.ChVector3d(0, 0, 5))
> sys.Add(pend2)
>
> # Visual for pend2
> shape2 = chrono.ChVisualShapeBox(chrono.ChVector3d(0.01, 1, 0.01))
> shape2.SetColor(chrono.ChColor(0.8, 0.5, 0.5))
> pend2.AddVisualShape(shape2)
>
> # 4. Revolute joint between ground and pend1 at origin
> joint1 = chrono.ChLinkRevolute()
> frame1 = chrono.ChFramed(chrono.ChVector3d(0, 0, 0))
> joint1.Initialize(ground, pend1, frame1)
> sys.Add(joint1)
>
> # 5. Revolute joint between pend1 and pend2 at (0, -1, 0)
> joint2 = chrono.ChLinkRevolute()
> frame2 = chrono.ChFramed(chrono.ChVector3d(0, -1, 0))
> joint2.Initialize(pend1, pend2, frame2)
> sys.Add(joint2)
>
> # 6. Visualization setup
> vis = chronoirr.ChVisualSystemIrrlicht()
> vis.AttachSystem(sys)
> vis.SetWindowSize(1024, 768)
> vis.SetWindowTitle("Double Pendulum")
> vis.Initialize()
> vis.AddSkyBox()
> vis.AddCamera(chrono.ChVector3d(0.5, 1.5, 1.0)) # Camera looking in 3D
> vis.AddTypicalLights()
>
> # Simulation loop
> while vis.Run():
> vis.BeginScene()
> vis.Render()
> vis.EndScene()
> sys.DoStepDynamics(0.005)
>
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