Discussion on Tendon Tensile Force and Extension Length #2604

ZnMnCr · 2025-04-29T07:29:19Z

ZnMnCr
Apr 29, 2025

Intro

Hi!

I am a ([graduate ] student in China, I use MuJoCo for my research on cable driven parallel robot. I hope to use the tendon function in MuJoCo to define ropes and perform control and feedback operations.

A cable-driven parallel robot is a device that controls the movement of a moving platform by retracting and releasing cables. The retraction and release of the cables are controlled by pulleys. It requires a certain range of tension.

My setup

mujoco 3.3.1 python ubuntu20.04

My question

However, I've encountered some confusion. First, how can I measure the tensile force on the tendon rope at a certain moment? Since ropes can only withstand tension and not exert thrust, cable-driven parallel robots require appropriate forces for normal control. I hope to read the rope's tensile force at any given moment through an interface. Moreover, I'm a bit confused about the driving mode of tendons. If I use position control to adjust the length of the rope, can it be understood that the rope is wound around a pulley, and the pulley actively retracts and releases the rope?

Here's what I've tried: I've attempted to use tendonlimitfrc, but it seems to only display the rope force reading when the rope length exceeds the specified range. I've also tried placing force sensors at the site points at both ends of the rope, but the readings are still incorrect.

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="Double_Pendulum">

    <!-- 基础视觉设置 -->
    <visual>
        <rgba haze=".3 .3 .3 1" />
        <quality shadowsize="16384" numslices="28" offsamples="4" />
    </visual>

    <!-- 编译器与物理引擎设置 -->
    <compiler angle="radian" coordinate="local" inertiafromgeom="true" />
    <option timestep="0.01" iterations="50" solver="Newton" tolerance="1e-10" gravity="0 0 -10"/>

    <!-- 材质资源定义 -->
    <asset>
        <texture type="skybox" builtin="gradient" rgb1="1 1 1" rgb2="0.6 0.8 1" width="256" height="256"/>
        <texture name="texplane" type="2d" builtin="checker" rgb1=".25 .25 .25" rgb2=".3 .3 .3" width="512" height="512" mark="cross" markrgb=".8 .8 .8" />
        <material name="matplane" reflectance="0.3" texture="texplane" texrepeat="1 1" texuniform="true" />
    </asset>

    <!-- 默认物理参数 -->
    <default>
        <joint armature="0.01" damping="0.01" limited="true" />
        <geom friction="0.5 0.1 0.1" density="1000" margin="0.001" rgba="0.8 0.8 0.8 1" />
    </default>

    <!-- 场景主体 -->
    <worldbody>
        <!-- 地面 -->
        <geom name="floor" pos="0 0 -1" size="0 0 1" type="plane" material="matplane" />
        <light directional="true" diffuse=".8 .8 .8" specular=".2 .2 .2" pos="0 0 5" dir="0 0 -1" />

        <!-- 锚点1（红色） -->
        <body name="anchor1" pos="-0.5 0 2">
            <geom type="sphere" size="0.05" rgba="1 0 0 1" /> <!-- 红色锚点 -->
            <site name="anchor1_site" pos="0 0 0" size="0.01" />
        </body>

        <!-- 锚点2（绿色） -->
        <body name="anchor2" pos="0.5 0 2">
            <geom type="sphere" size="0.05" rgba="0 1 0 1" /> <!-- 绿色锚点 -->
            <site name="anchor2_site" pos="0 0 0" size="0.01" />
        </body>

        <!-- 单摆质量块 -->
        <body name="pendulum_mass" pos="0 0 1">
            <joint type="free" />
            <geom type="sphere" size="0.15" mass="2" rgba="0 0 1 1" /> <!-- 蓝色质量块 -->
            <!-- 两个 site 分别连接两根绳索 -->
            <site name="mass_site1" pos="-0.1 0 0" size="0.01" />
            <site name="mass_site2" pos="0.1 0 0" size="0.01" />
        </body>
    </worldbody>

    <!-- 肌腱连接锚点与质量块 -->
    <tendon>
        <!-- 绳索1 -->
        <spatial 
            name="cable1" 
            limited="true" 
            range="0 1" 
            stiffness="100" 
            damping="1" 
            width="0.01" 
            rgba="0.5 0.5 0.5 1"
        >
            <site site="anchor1_site" />
            <site site="mass_site1" />
        </spatial>

        <!-- 绳索2 -->
        <spatial 
            name="cable2" 
            limited="true" 
            range="0 1" 
            stiffness="100" 
            damping="1" 
            width="0.01" 
            rgba="0.5 0.5 0.5 1"        
        >
            <site site="anchor2_site" />
            <site site="mass_site2" />
        </spatial>
    </tendon>
<actuator>
    <position name="act_cable1" tendon="cable1" kp="100" ctrlrange="-2 2" />
    <position name="act_cable2" tendon="cable2" kp="100" ctrlrange="-2 2"/>
</actuator>
    <!-- 可选：力传感器 -->
    <sensor>
    <tendonlimitfrc name="tendon1_limit_force" tendon="cable1"/>
    <tendonlimitfrc name="tendon2_limit_force" tendon="cable2"/>
    <force name="anchor_force_sensor" site="anchor2_site" />
    <force name="mass_force_sensor2" site="mass_site2" />
    </sensor>


</mujoco>

Code:

import mujoco
import numpy as np
import mediapy as media
import mujoco.viewer
import time
import matplotlib.pyplot as plt
# 加载 XML 模型
xml_path = "/home/znmncr/桌面/mujoco_py/cdprMujoco/Double_Pendulum.xml"  # # Replace it with the path of your XML file
model = mujoco.MjModel.from_xml_path(xml_path)
data = mujoco.MjData(model)
with mujoco.Renderer(model) as renderer:
  mujoco.mj_forward(model, data)
  renderer.update_scene(data)

  media.show_image(renderer.render())

# 初始化存储列表
times = []
tendon1_forces = []
tendon2_forces = []
anchor_forces = []
mass_forces = []

# 控制采样频率
sample_interval = 100  # 每隔100步采样一次
sample_counter = 0

# 启动 MuJoCo 可视化器
with mujoco.viewer.launch_passive(model, data) as viewer:
    start_time = time.time()
    while viewer.is_running() and time.time() - start_time < 10:  # 运行10秒
        mujoco.mj_step(model, data)

        sample_counter += 1
        if sample_counter >= sample_interval:
            elapsed_time = time.time() - start_time
            times.append(elapsed_time)

            # 读取传感器数据
            tendon1_forces.append(data.sensor("tendon1_limit_force").data)
            tendon2_forces.append(data.sensor("tendon2_limit_force").data)
            anchor_forces.append(data.sensor("anchor_force_sensor").data)
            mass_forces.append(data.sensor("mass_force_sensor2").data)

            sample_counter = 0

        viewer.sync()
plt.figure(figsize=(16, 10))

# 子图1: tendon1_limit_force
plt.subplot(2, 1, 1)
plt.plot(times, tendon1_forces, label='Tendon1 Limit Force', color='red')
plt.title('Tendon1 Limit Force Over Time')
plt.xlabel('Time (s)')
plt.ylabel('Force (N)')
plt.grid(True)
plt.legend()

# 子图2: tendon2_limit_force
plt.subplot(2, 1, 2)
plt.plot(times, tendon2_forces, label='Tendon2 Limit Force', color='blue')
plt.title('Tendon2 Limit Force Over Time')
plt.xlabel('Time (s)')
plt.ylabel('Force (N)')
plt.grid(True)
plt.legend()
# 自动调整布局
plt.tight_layout()
plt.show()
# 创建一个包含 6 个子图的画布（3 行 2 列）
fig, axes = plt.subplots(3, 2, figsize=(14, 12))
fig.suptitle('Force Sensors - X, Y, Z Components', fontsize=16)
# 将 anchor_forces 和 mass_forces 转换为 NumPy 数组便于切片
anchor_forces_array = np.array(anchor_forces)
mass_forces_array = np.array(mass_forces)
# 定义方向和颜色映射
directions = ['X', 'Y', 'Z']
colors = ['red', 'green', 'blue']

# 绘制 Anchor 的三个方向
for i in range(3):
    ax = axes[i, 0]  # 第一列：Anchor
    ax.plot(times, anchor_forces_array[:, i], label=f'{directions[i]} Component', color=colors[i])
    ax.set_title(f'Anchor {directions[i]} Force Sensor')
    ax.set_xlabel('Time (s)')
    ax.set_ylabel('Force (N)')
    ax.grid(True)
    ax.legend()

# 绘制 Mass 的三个方向
for i in range(3):
    ax = axes[i, 1]  # 第二列：Mass
    ax.plot(times, mass_forces_array[:, i], label=f'{directions[i]} Component', color=colors[i])
    ax.set_title(f'Mass {directions[i]} Force Sensor')
    ax.set_xlabel('Time (s)')
    ax.set_ylabel('Force (N)')
    ax.grid(True)
    ax.legend()

# 自动调整布局
plt.tight_layout(rect=[0, 0, 1, 0.96])
plt.show()

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Discussion on Tendon Tensile Force and Extension Length #2604

ZnMnCr Apr 29, 2025

Intro

My setup

My question

Minimal model and/or code that explain my question

Confirmations

Replies: 0 comments

ZnMnCr
Apr 29, 2025