Resistance in a hinge joint #2366

shogan50 · 2025-01-17T01:39:57Z

shogan50
Jan 17, 2025

Intro

Hi!

I am a principal mechanical engineer working on a wind energy project.

My setup

Windows 11 / Linux, Mujoco, Python, Pytorch.

My question

There exists a resistance in a joint, which increases with inertia that I don't understand. I have a rotating body with high inertia via a armature parameter. I need the high inertia to mimic the physical system and make a PID controller happy. The behavior I would expect is that with initialized velocity, it would continue to spin, decaying slowly if there is damping under the hood somewhere, or maintain that velocity. What is causing this behavior?

Minimal model and/or code that explain my question

<body name="mast" pos="0 0 0">
<joint name="mast_joint" type="hinge" armature="4000" axis="0 0 1" />
  <geom name="mast_geom" type="cylinder" size=".1" fromto=".0 0 0 0 0 3.0" rgba="1 0 0 1"/>
  <site name="mast" pos = "0 0 0" size = "0.1"/>
  <body name="arm" pos="0 0 3">
  <!-- <joint name="torque" type="hinge" axis="0 0 1" springdamper=".005 1" /> -->
    <geom name="arm_geom" type="cylinder" fromto="0 0 0 1 0 0" size=".05" rgba="1 0 0 1"/>
    
    <body name="tether" pos="1 0 0">
      <joint name="tether_joint" type="hinge" axis="0 0 1"/>
      <geom name="tether_geom" type="cylinder" mass=".1" fromto="0 0 0 10 0 0" size=".025" rgba="0 1 1 1"/>
      
      <body name="kite" pos="10 0 0">
      <!-- <joint name="tether_spring" type="slide" axis="0 0 1"/> -->
        <geom name="kite_top" type="cylinder" mass="1.5" size="0.0625 .6" rgba="0 0.8 0 1"/>
        <site name="kite_cg" pos="0 0 0" size="0.1"/>
        
        <!-- <body name="alpha_vane" pos="0 0 0">
          <joint name="alpha_joint" type="hinge" damping="10" axis="0 0 1"/>
          <geom name="alpha_geom" type="cylinder" mass="0.01" fromto="0 0 0 0 3 0" size=".05" rgba="1 1 0 1"/>
          <site name="alpha_center" pos="0 0 0" size="0.1"/>
        </body>

        <body name="wind_vane" pos="0 0 0">
          <joint name="wind_joint" type="hinge" damping="10" axis="0 0 1"/>
          <geom name="wind_geom" type="cylinder" mass="0.01" fromto="0 0 0 0 3 0" size=".05" rgba="0 1 0 1"/>
        </body> -->
      </body>
    </body>
  </body>
</body>

Python:

import mujoco as mj
import numpy as np
from mujoco.glfw import glfw
from numpy.linalg import inv
import matplotlib.pyplot as plt
from mujoco_base import MuJoCoBase

integral = 0
previous_error = 0
ts = []
thetas = []

def controller(model, data):
global integral, previous_error
kp = 0.
ki = 0.
kd = 0.
dt = model.opt.timestep
pos = data.joint('mast_joint').qpos[0]

error =  data.time * 2 * np.pi - pos
integral += error 
p = kp * error * dt
i = ki * integral * dt
d = kd * (error - previous_error) * dt
pid = p + i + d

data.ctrl[0] = pid #  Spin the mast

model = mj.MjModel.from_xml_path("caro_test.xml")
data = mj.MjData(model)
mj.set_mjcb_control(controller)

data.joint('mast_joint').qvel[0] = np.pi * 2

simend = 5.0
glfw.init()

Create window

window = glfw.create_window(640, 480, "Kite Simulation", None, None)

glfw.make_context_current(window)

Initialize visualization data structures

cam = mj.MjvCamera()
opt = mj.MjvOption()
scene = mj.MjvScene(model, maxgeom=10000)
context = mj.MjrContext(model, mj.mjtFontScale.mjFONTSCALE_150.value)

cam.azimuth = 89.608063
cam.elevation = -30
cam.distance = 24.0
cam.lookat = np.array([0.0, 0.0, 1.5])

while not glfw.window_should_close(window):
simstart = data.time

while (data.time - simstart < 1.0/600.0):
    # Step simulation environment
    mj.mj_step(model, data)
    ts.append(data.time)
    thetas.append(data.time * 2 * np.pi - data.joint('mast_joint').qpos[0])

if data.time >= simend:
    break

# get framebuffer viewport
viewport_width, viewport_height = glfw.get_framebuffer_size(window)
viewport = mj.MjrRect(0, 0, viewport_width, viewport_height)

# Update scene and render
mj.mjv_updateScene(model, data, opt, None, cam, mj.mjtCatBit.mjCAT_ALL.value, scene)
mj.mjr_render(viewport, scene, context)

# swap OpenGL buffers (blocking call due to v-sync)
glfw.swap_buffers(window)

# process pending GUI events, call GLFW callbacks
glfw.poll_events()

glfw.terminate()
plt.plot(ts, thetas, label='theta_error')
plt.legend()
plt.show()

Confirmations

I searched the latest documentation thoroughly before posting.
I searched previous Issues and Discussions, I am certain this has not been raised before.

shogan50 · 2025-01-17T12:22:41Z

shogan50
Jan 17, 2025
Author

flag contact="disable" seems to have done the trick.

0 replies

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Resistance in a hinge joint #2366

{{title}}

Replies: 1 comment

{{title}}

Select a reply

Resistance in a hinge joint #2366

shogan50 Jan 17, 2025

Intro

My setup

My question

Minimal model and/or code that explain my question

Create window

Initialize visualization data structures

Confirmations

Replies: 1 comment

shogan50 Jan 17, 2025 Author

shogan50
Jan 17, 2025

shogan50
Jan 17, 2025
Author