Inaccurate acceleration values when applying friction #2037

wykang2 · 2024-09-02T00:47:31Z

wykang2
Sep 2, 2024

Intro

Hi!

I am a student using mujoco to simulate different basic systems. Currently, I am trying to simulate a basic system with friction applied and trying to get accurate acceleration values.

My setup

My first attempt at simulating friction was to slide a block down a 30 degree ramp with friction being applied.

Here is an image of what the basic setup looks like:

In addition to a sliding joint on the x axis (along the ramp), I added a z axis sliding joint (perpendicular to the ramp) to make sure that the block made contact with the ramp. The xml for my setup can be seen below. I also wrote a python script to simulate and graph some of the relevant values (shown below as well)

My question

My concern is with the measured acceleration. Since there was only 1 object moving, I first tried graphing the acceleration measured with data.qacc

The average acceleration from qacc was very close (within 0.01) to the theoretical acceleration that the block should have had. However, I am concerned about the flutuations in the values of the accelerations on the graph and worried that this may cause larger discrepancies for larger graphs.

I also graphed the acceleration recorded by an accelerometer (note that these values are negative of the previous values):

For this graph, we are looking at the acceleration with respect to the x axis labeled on the graph, since it is aligned with the ramp. The average acceleration differed by a larger amount from the the theoretical acceleration of the block (0.2). Additionally, there was the same issue of flutuation values on the acceleration values measured by the accelerometer

My questions are if there was any way I could fix my system so that the acceleration values measured (by the accelerometer mainly) would not flutuate so much and also so that the accelerometer would measure a more accurate value?

Minimal model and/or code that explain my question

Model:

minimal XML

<mujoco model="test">
  <compiler angle="radian" autolimits="true"/>
    <compiler meshdir="asset" texturedir="asset" assetdir="asset"/>
    <!-- <include file="scene.xml"/> -->
    <option gravity="0 0 -9.81"/>
    <default class="main">
        <geom rgba="0.9 0.7 0.1 1"/>
    </default>
    <visual>
        <headlight diffuse="0.7 0.7 0.7"/>
        <global offwidth="1000" offheight="1000"/>
        <quality offsamples="32"/>
    </visual>

  <option>
    <flag sensor="enable"/>
  </option>
    <option gravity="0 0 -9.81"/>
  <visual>
    <headlight diffuse="0.7 0.7 0.7"/>
  </visual>
  <default class="main">
    <joint limited="true" solimplimit="0.95 0.95 0.1 0.5 2" range="0 60"/>
    <geom size="0.01 0 0" rgba="0.9 0.7 0.1 1"/>
    <site rgba="0.9 0.9 0.9 1"/>
  </default>
  <asset>
    <texture type="2d" name="checks1" builtin="checker" rgb1="1 1 1" rgb2="0 0 0" width="256" height="256"/>
    <material name="test_color" texture="checks1" texuniform="true" texrepeat="2 2"/>
  </asset>
  <worldbody>
    <camera name="closeup" pos="0 -.025 .025" xyaxes="1 0 0 0 1 2"/>

    <!-- top of ramp -->
    <body name="angled_box" pos="0 0 0.15" quat="0 -0.2588 0 0.9659" gravcomp="0">
        <geom type="box" size="0.1 0.01 0.001" rgba="0 0 1 1" priority = "1" friction = "0.3"/>
    </body>

    <camera name="fixed" pos="0 -.6 .3" xyaxes="1 0 0 0 1 2"/>
    <camera name="track" pos="0 -.6 .3" xyaxes="1 0 0 0 1 2" mode="track"/>

    <!-- block -->

    <body name="lblock" pos="0 0 0.163" quat="0 -0.2588 0 0.9659" gravcomp="0">
      <joint name="b1" pos="0 0 0" axis="1 0 0" limited="false" type="slide"/>
      <joint name="b2" pos="0 0 0" axis="0 0 1" limited="false" type="slide"/>
      <geom size="0.01 0.01 0.01" pos="0 0 0" type="box" mass="10" rgba="0.2 0.2 0.5 1"/>
      <site name="bs1" pos="0 0 0"/>
    </body>



  </worldbody>
  <equality>
  </equality>
  <tendon>
  </tendon>
  <sensor>
    <accelerometer name="friction_accel" site="bs1"/>
  </sensor>
</mujoco>

Code:

import mujoco
import distutils.util
import os
import subprocess
import time
import itertools
import numpy as np

import mediapy as media
import matplotlib.pyplot as plt

np.set_printoptions(precision=3, suppress=True, linewidth=100)

from IPython.display import clear_output


#XML CODE

free_body_MJCF = """
<mujoco model="test">
  <compiler angle="radian" autolimits="true"/>
    <compiler meshdir="asset" texturedir="asset" assetdir="asset"/>
    <!-- <include file="scene.xml"/> -->
    <option gravity="0 0 -9.81"/>
    <default class="main">
        <geom rgba="0.9 0.7 0.1 1"/>
    </default>
    <visual>
        <headlight diffuse="0.7 0.7 0.7"/>
        <global offwidth="1000" offheight="1000"/>
        <quality offsamples="32"/>
    </visual>

  <option>
    <flag sensor="enable"/>
  </option>
    <option gravity="0 0 -9.81"/>
  <visual>
    <headlight diffuse="0.7 0.7 0.7"/>
  </visual>
  <default class="main">
    <joint limited="true" solimplimit="0.95 0.95 0.1 0.5 2" range="0 60"/>
    <geom size="0.01 0 0" rgba="0.9 0.7 0.1 1"/>
    <site rgba="0.9 0.9 0.9 1"/>
  </default>
  <asset>
    <texture type="2d" name="checks1" builtin="checker" rgb1="1 1 1" rgb2="0 0 0" width="256" height="256"/>
    <material name="test_color" texture="checks1" texuniform="true" texrepeat="2 2"/>
  </asset>
  <worldbody>
    <camera name="closeup" pos="0 -.025 .025" xyaxes="1 0 0 0 1 2"/>

    <!-- top of ramp -->
    <body name="angled_box" pos="0 0 0.15" quat="0 -0.2588 0 0.9659" gravcomp="0">
        <geom type="box" size="0.1 0.01 0.001" rgba="0 0 1 1" priority = "1" friction = "0.3"/>
    </body>

    <camera name="fixed" pos="0 -.6 .3" xyaxes="1 0 0 0 1 2"/>
    <camera name="track" pos="0 -.6 .3" xyaxes="1 0 0 0 1 2" mode="track"/>

    <!-- block -->

    <body name="lblock" pos="0 0 0.163" quat="0 -0.2588 0 0.9659" gravcomp="0">
      <joint name="b1" pos="0 0 0" axis="1 0 0" limited="false" type="slide"/>
      <joint name="b2" pos="0 0 0" axis="0 0 1" limited="false" type="slide"/>
      <geom size="0.01 0.01 0.01" pos="0 0 0" type="box" mass="10" rgba="0.2 0.2 0.5 1"/>
      <site name="bs1" pos="0 0 0"/>
    </body>



  </worldbody>
  <equality>
  </equality>
  <tendon>
  </tendon>
  <sensor>
    <accelerometer name="friction_accel" site="bs1"/>
  </sensor>
</mujoco>

"""

model = mujoco.MjModel.from_xml_string(free_body_MJCF)
model.opt.timestep = 1e-6
data = mujoco.MjData(model)


#SIMULATION
n_frames = 1000

frames = []
sensordata = []
times = []
accel = []

options = mujoco.MjvOption()
mujoco.mjv_defaultOption(options)
options.flags[mujoco.mjtVisFlag.mjVIS_CONTACTPOINT] = True
options.flags[mujoco.mjtVisFlag.mjVIS_CONTACTFORCE] = True
options.flags[mujoco.mjtVisFlag.mjVIS_TRANSPARENT] = True

model.vis.scale.contactwidth = 0.1
model.vis.scale.contactheight = 0.03
model.vis.scale.forcewidth = 0.05
model.vis.map.force = 0.3

mujoco.mj_resetData(model, data)
data.joint('b1').qvel = 0
data.joint('b2').qvel = 0


#PLOTTING CONTACT FORCES
n_steps = 100000
sim_time = np.zeros(n_steps)
ncon = np.zeros(n_steps)
force = np.zeros((n_steps,3))
velocity = np.zeros((n_steps, model.nv))
penetration = np.zeros(n_steps)
acceleration = np.zeros((n_steps, model.nv))
forcetorque = np.zeros(6)


mujoco.mj_resetData(model, data)
data.joint('b1').qvel = 0
data.joint('b2').qvel = 0

for i in range(n_steps): #simulate and save the data
  mujoco.mj_step(model, data)
  sim_time[i] = data.time
  times.append(data.time)
  ncon[i] = data.ncon
  velocity[i] = data.qvel[:]
  acceleration[i] = data.qacc[:]
  sensordata.append(data.sensor('friction_accel').data.copy())
  accel.append(data.qacc)
  for j,c in enumerate(data.contact):
    mujoco.mj_contactForce(model, data, j, forcetorque)
    force[i] += forcetorque[0:3]
    penetration[i] = min(penetration[i], c.dist)

_, ax = plt.subplots(4, 2, sharex=True, figsize=(10, 10)) #start plotting

lines = ax[0,0].plot(sim_time, force)
ax[0,0].set_title('contact force')
ax[0,0].set_ylabel('Newton')
ax[0,0].legend(iter(lines), ('normal z', 'friction x', 'friction y'));

ax[1,0].plot(sim_time, acceleration[:, 0], label = 'block accel')
ax[1,0].set_title('acceleration')
ax[1,0].set_ylabel('(meter,radian)/s/s')

#THEORETICAL VALUE OF ACCELERATION (NEED TO CHANGE FRICTION VALUE MANUALLY)
frictioned = 9.81*(np.sin(np.pi/6)- (0.3*np.cos(np.pi/6))) #friction value needs to be changed
print("ground truth:", frictioned)
ax[1,0].axhline(y=frictioned, color='red', linestyle='--', label='theoretical accel')

#MEASURED VALUE OF ACCELERATION (AVERAGE VALUE OF DATA POINTS)
average_acceleration = np.mean(acceleration[:, 0])
ax[1,0].axhline(y=average_acceleration, color='blue', linestyle='--', label='Average accel')
print("Measured average acceleration:", average_acceleration)
ax[1,0].legend(frameon=True, loc='lower right');

ax[2,0].plot(sim_time, velocity[:, 0], color = 'blue', label = "x axis (along slope)")
ax[2,0].plot(sim_time, velocity[:, 1], color = 'red', label = "z axis (prependicular to slope)")
ax[2,0].set_title('velocity')
ax[2,0].set_ylabel('(meter,radian)/s')
ax[2,0].set_xlabel('second')
ax[2,0].legend(frameon=True, loc='lower right');

ax[0,1].plot(sim_time, ncon)
ax[0,1].set_title('number of contacts')
ax[0,1].set_yticks(range(6))

ax[1,1].plot(sim_time, force[:,0])
ax[1,1].set_yscale('log')
ax[1,1].set_title('normal (z) force - log scale')
ax[1,1].set_ylabel('Newton')
z_gravity = -model.opt.gravity[2]
mg = model.body("lblock").mass[0] * z_gravity
mg_line = ax[1,1].plot(sim_time, np.ones(n_steps)*mg, label='m*g', linewidth=1)
ax[1,1].legend()

ax[2,1].plot(sim_time, 1000*penetration)
ax[2,1].set_title('penetration depth')
ax[2,1].set_ylabel('millimeter')
ax[2,1].set_xlabel('second')

#PLOTTING ACCELEROMETER VALUES (NEED TO MANUALLY CHANGE FRICTION VALUE IN EQ)
ax[3,0] = plt.gca()

ax[3,0].plot(np.asarray(times), np.asarray(sensordata)[:, 0], label = 'x')
ax[3,0].plot(np.asarray(times), np.asarray(sensordata)[:, 1], label = 'y')
ax[3,0].plot(np.asarray(times), np.asarray(sensordata)[:, 2], label = 'z')



frictioned = -9.81*(np.sin(np.pi/6)- (0.3*np.cos(np.pi/6))) #manually setting friction valur
print("accelerometer theoretical friction: ", frictioned)
ax[3,0].axhline(y=frictioned, color='red', linestyle='--', label='theoretical accel')

average_acceleration = np.mean(np.asarray(sensordata)[:, 0])
ax[3,0].axhline(y=average_acceleration, color='blue', linestyle='--', label='Average accel')
print("accelerometer Measured average acceleration:", average_acceleration)

ax[3,0].set_title('Accelerometer values')
ax[3,0].set_ylabel('meter/second^2')
ax[3,0].set_xlabel('second')
ax[3,0].legend(frameon=True, loc='lower right');

plt.tight_layout()
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.

mihirp1998 · 2024-09-02T01:08:23Z

mihirp1998
Sep 2, 2024

@Balint-H any help on this will be very helpful! thanks!

0 replies

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Inaccurate acceleration values when applying friction #2037

{{title}}

{{editor}}'s edit

{{editor}}'s edit

Replies: 1 comment

{{title}}

Select a reply

Inaccurate acceleration values when applying friction #2037

wykang2 Sep 2, 2024

Intro

My setup

My question

Minimal model and/or code that explain my question

Confirmations

Replies: 1 comment

mihirp1998 Sep 2, 2024

wykang2
Sep 2, 2024

mihirp1998
Sep 2, 2024