force measurement attached with the end-effector of panda arm #2720

kylehuo8 · 2025-07-05T07:22:27Z

kylehuo8
Jul 5, 2025

Intro

Hi!

I'm a student and I'm trying to use MuJoCo for a project.

I'm currently considering how to obtain force and torque values. I am trying to utilize force and torque sensors. For my model, I've added two sensor in the ur5e/scene.xml (https://github.com/google-deepmind/mujoco_menagerie/blob/main/universal_robots_ur5e/scene.xml) from the MuJoCo Menagerie.

After that, I try to control the arm with mocap and controller it to collide with a wall. The steps are:

Ask the mocap to move in -Z direction. Then I measure the force, the result is: [0 0 440].
Ask the mocap to move in -Y direction. Then I measure the force, the result is: [0 0 -630].
Ask the mocap to move in -X direction to crash with the wall. Then I measure the force, the result is: [0 0 -270].

I also print out the contact info in this time, while the contact force along different directions are very large:

My questions are:

In step 1 and 2, why there are force measurements without contact?
In step 3, why the force measurement is smaller? And what is the coordinate frame of this force measurement?
Why the contact force is so large in each direction? What are the units of these measurements? I think only the value of z-axis should be large, right?

Thank you very much.

My setup

MuJoCo 3.1.6, python API.

My question

My questions are:

In step 1 and 2, why there are force measurements without contact?
In step 3, why the force measurement is smaller? And what is the coordinate frame of this force measurement?
Why the contact force is so large in each direction? What are the units of these measurements? I think only the value of z-axis should be large, right?

Minimal model and/or code that explain my question

If you encountered the issue in a complex model, please simplify it as much as possible (while still reproducing the issue).

Model:

minimal XML

<mujoco model="ur5e scene">
  <include file="ur5e.xml"/>

  <statistic center="0.3 0 0.4" extent="0.8"/>

  <visual>
    <headlight diffuse="0.6 0.6 0.6" ambient="0.1 0.1 0.1" specular="0 0 0"/>
    <rgba haze="0.15 0.25 0.35 1"/>
    <global azimuth="120" elevation="-20"/>
  </visual>

  <asset>
    <texture type="skybox" builtin="gradient" rgb1="0.3 0.5 0.7" rgb2="0 0 0" width="512" height="3072"/>
    <texture type="2d" name="groundplane" builtin="checker" mark="edge" rgb1="0.2 0.3 0.4" rgb2="0.1 0.2 0.3"
      markrgb="0.8 0.8 0.8" width="300" height="300"/>
    <material name="groundplane" texture="groundplane" texuniform="true" texrepeat="5 5" reflectance="0.2"/>
  </asset>

  <worldbody>
    <light pos="0 0 1.5" dir="0 0 -1" directional="true"/>
    <geom name="floor" size="0 0 0.05" type="plane" material="groundplane"/>

    <body mocap="true" name="mocap" pos="-0.817 -0.134 0.063" quat = "0. 1. 0. 0.">
            <site pos="0 0 0.075" size="0.003 0.003 0.1" type="box" name="mocap_left_site1" rgba="0 0 1 1"/>
            <site pos="0 0.075 0" size="0.003 0.1 0.003" type="box" name="mocap_left_site2" rgba="0 1 0 1"/>
            <site pos="0.075 0 0" size="0.1 0.003 0.003" type="box" name="mocap_left_site3" rgba="1 0 0 1"/>
    </body>
    <geom name="x-aixs" group="2" type="cylinder" contype="0" conaffinity="0"
			fromto="0 0 0 1 0 0" size="0.01" rgba="1 0 0 1" />
		<geom name="y-aixs" group="2" type="cylinder" contype="0" conaffinity="0"
			fromto="0 0 0 0 1 0" size="0.01" rgba="0 1 0 1" />
		<geom name="z-aixs" group="2" type="cylinder" contype="0" conaffinity="0"
			fromto="0 0 0 0 0 1" size="0.01" rgba="0 0 1 1" />

    <body name="wall" pos="-0.4 -0.45 0.4" quat = '1. 0 0 0'>
      <geom name = 'wall_geom' type="box" size="0.2 0.01 0.2" pos="0 0 0" rgba="0.1 0.5 0.8 1.0" mass="1"
                  condim="4"
                  friction="1 1 1"
                  solimp="1 1 1"
                  solref="0.00001 1"
                  contype="1"
                  conaffinity="1"/> 
    </body>

  </worldbody>

  <equality>
        <weld body1="mocap" body2="wrist_3_link"/>
    </equality>

  <sensor>
    <framepos objtype="site" objname="attachment_site"/>
    <force name="frceef0" site ='attachment_site'/>
    <torque name="trqeef0" site='attachment_site' />
  </sensor>

</mujoco>

Code:

import utils
import mujoco
import mujoco.viewer
import time
import numpy as np

def round_data(data):
    data = np.around(data, 3)
    return data

if __name__ == "__main__":    
    path_xml = ''
    model = mujoco.MjModel.from_xml_path(path_xml)
    data = mujoco.MjData(model)
    
    def get_force_torque_data():
        force = data.sensordata[id_force_sensor:id_force_sensor + 3]
        torque = data.sensordata[id_torque_sensor:id_torque_sensor + 3]

        force = utils.round_data(force)
        torque = utils.round_data(torque)

        return force, torque
    
    def step_simulation():
        mujoco.mj_forward(model, data)
        mujoco.mj_step(model, data, nstep=100)
        handle.sync()

    handle = mujoco.viewer.launch_passive(model, data)

    name_force_sensor = 'frceef0'
    name_torque_sensor = 'trqeef0'

    id_force_sensor = mujoco.mj_name2id(model, mujoco.mjtObj.mjOBJ_SENSOR, name_force_sensor)
    id_torque_sensor = mujoco.mj_name2id(model, mujoco.mjtObj.mjOBJ_SENSOR, name_torque_sensor)

    for i in range(30):
        data.mocap_pos[0][2] += 0.01
        step_simulation()
        time.sleep(0.1)

    force, torque = get_force_torque_data()        
    print("force\n", force)

    for i in range(30):
        data.mocap_pos[0][0] += 0.01
        step_simulation()
        time.sleep(0.1)

    force, torque = get_force_torque_data()        
    print("force\n", force)

    for i in range(30):
        data.mocap_pos[0][1] -= 0.01
        step_simulation()
        time.sleep(0.1)

    force, torque = get_force_torque_data()        
    print("force\n", force)

    list_contact = data.contact
    contact_pos = np.array([])
    contact_force = np.array([])

    for contact in list_contact:
        pos = contact.pos
        contact_pos = np.concatenate((contact_pos, pos))
        efc_address = contact.efc_address
        force = data.efc_force[efc_address:efc_address + 6]  
        contact_force = np.concatenate((contact_force, force))

    contact_pos = contact_pos.reshape(-1,3)
    contact_force = contact_force.reshape(-1,6)
    print("contact force\n", round_data(contact_force))
    print(contact_force.shape)

    time.sleep(10)

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force measurement attached with the end-effector of panda arm #2720

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force measurement attached with the end-effector of panda arm #2720

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kylehuo8 Jul 5, 2025

Intro

My setup

My question

Minimal model and/or code that explain my question

Confirmations

Replies: 0 comments

kylehuo8
Jul 5, 2025