Small-scale simulation problem 2

[AaronRapopot]

Hello, I found that when conducting small-scale simulations, setting the radius too small can lead to numerical errors in the position value. The rod parameters I set refer to "Real-time control of an octopus arm - NNES". I print the position of the rod. The shape of the position is (250, 3, 101). There are a total of 250 steps. In the initial dozens of steps, numerical values are still present (although highly unstable), but all subsequent values displays "nan". If I set the radius from 0.0125 to 0.1, everything is ok.
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'''Setup Simulation'''
class BaseSimulator(BaseSystemCollection, Constraints, Connections, Forcing, CallBacks, Damping):
pass

'''Create Rod'''
Thermal_drawing_robot = BaseSimulator()

n_elem = 100
L0 = 0.2
radius = 0.012
damp_coefficient = 0.1
rod1 = CosseratRod.straight_rod(
n_elements=n_elem,
start=np.zeros((3,)),
direction=np.array([0.0, 0.0, 1.0]),
normal=np.array([0.0, 1.0, 0.0]),
base_length=L0,
base_radius=radius,
density=700,
youngs_modulus=1e4,
)
Thermal_drawing_robot.append(rod1)

'''Define Boundary Conditions'''
Thermal_drawing_robot.constrain(rod1).using(
OneEndFixedRod,
constrained_position_idx=(0,),
constrained_director_idx=(0,)
)

'''Define torque'''
torque = 1.0
torque_direction = np.array([0.0, -1.0, 0.0])
Thermal_drawing_robot.add_forcing_to(rod1).using(
UniformTorques, torque, direction=torque_direction
)

'''Apply damping'''
time_step = 1.0e-5
nu = damp_coefficient
Thermal_drawing_robot.dampen(rod1).using(
AnalyticalLinearDamper,
damping_constant = nu,
time_step = time_step,
)

'''Add callback'''
class RodCallBack(CallBackBaseClass):
def init(self, step_skip: int, callback_params: dict):
CallBackBaseClass.init(self)
self.every = step_skip
self.callback_params = callback_params

def make_callback(self, system, time, current_step: int):
    if current_step % self.every == 0:
        self.callback_params["time"].append(time)
        self.callback_params["radius"].append(system.radius.copy())
        self.callback_params["position"].append(system.position_collection.copy())
        self.callback_params["director"].append(system.director_collection.copy())
        self.callback_params["velocity"].append(system.velocity_collection.copy())
        return

callback_data_rod1 = defaultdict(list)
Thermal_drawing_robot.collect_diagnostics(rod1).using(
RodCallBack, step_skip=1000, callback_params=callback_data_rod1)

'''Finalize simulator'''
Thermal_drawing_robot.finalize()

'''Set timestepper'''
timestepper = PositionVerlet()
final_time = 2.5 # seconds
total_steps = int(final_time / time_step)
integrate(timestepper, Thermal_drawing_robot, final_time, total_steps)

print(callback_data_rod1['position'])
position_shape = np.array(callback_data_rod1["position"]).shape
print(position_shape)
And the result (just show one step, others are same):
array([[ 0., nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan,
nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan,
nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan,
nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan,
nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan,
nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan,
nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan,
nan, nan, nan, nan, nan, nan, nan, nan, nan, nan],
[ 0., nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan,
nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan,
nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan,
nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan,
nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan,
nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan,
nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan,
nan, nan, nan, nan, nan, nan, nan, nan, nan, nan],
[ 0., nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan,
nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan,
nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan,
nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan,
nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan,
nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan,
nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan,
nan, nan, nan, nan, nan, nan, nan, nan, nan, nan]])
`

[AaronRapopot]

I solved it. Set youngs modulus from 1e4 to 1e7, it works.