Keeping a block in a certain height #2202

adawne · 2024-11-03T15:47:41Z

adawne
Nov 3, 2024

MuJoCo: 3.2.2
API: Python
Ubuntu 20.04

Hi, I am trying to lift a block to a certain height and try to make it float on that height by applying external force F = W = mg, but when I try to apply that, the block's height keeps increasing, instead of remaining constant

minimal XML

<mujoco model="block scene">
    <asset>
        <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" rgba="1 1 1 0.7"/>
    </asset>

    <worldbody>
        <light pos="0 0 1.5" dir="0 0 -1" directional="true"/>
        <camera name="fixed" pos="0 0 0" xyaxes="1 0 0 0 0 1"/>
        <geom name="floor" size="0 0 0.05" type="plane" material="groundplane" solimp="0.995 0.999 0.0000001 0.5 2"/>
        <body name="block_0" pos="0 0 .03">
            <joint name="block_joint" type="free" damping="0"></joint>
            <inertial pos="0 0 0" mass="0.1" diaginertia="0.001 0.001 0.001" />
            <geom name="blue_subbox" size="0.030 0.03 0.03" pos="0 0 0" type="box" rgba="0.3 0.5 0.8 1" 
            solimp="0.99 0.995 0.0000001 0.5 2" solref="0.005 2" friction="5 0.3 0.001" />
        </body>
    </worldbody>
    
    <sensor>
        <framepos name= "block_pos" objtype = "body" objname="block_0" />
    </sensor>
</mujoco>
</details>

Code:
model = mujoco.MjModel.from_xml_string(xml_string)
data = mujoco.MjData(model)

mujoco.mj_resetData(model, data)
#viewer = mujoco.viewer.launch_passive(model, data)

n_frames = 1000
height = 240
width = 320
frames = []
times = []
heights = []
fps = 60.0

with mujoco.Renderer(model, height, width) as renderer:
  for i in range(n_frames):
    while data.time < i/fps:
        block_mass = model.body('block_0').mass[0]
        gravity = -model.opt.gravity[2]
        mg = block_mass * gravity
        block_pos = data.sensor('block_pos').data.copy()
        block_height = block_pos[2]
        block_body_id = model.body('block_0').id
        print(f"Block height: {block_height}")

        if block_height < 0.7:
            data.xfrc_applied[block_body_id] = [0, 0, mg + 0.01, 0, 0, 0]

        else:
            data.xfrc_applied[block_body_id] = [0, 0, mg, 0, 0, 0]
        mujoco.mj_step(model, data)
        times.append(data.time)
        heights.append(block_height)
    renderer.update_scene(data, "fixed")
    frame = renderer.render()
    frames.append(frame)

plt.plot(times, heights)
plt.xlabel('Time (s)')
plt.ylabel('Block Height (m)')
plt.title('Block Height vs Time')
plt.grid(True)
plt.savefig("block_height_vs_time.png")
print("Plot saved as 'block_height_vs_time.png'.")
![block_height_vs_time](https://github.com/user-attachments/assets/af24f679-37c9-4a3e-9635-b51f8b2268e4)

yuvaltassa · 2024-11-04T12:35:34Z

yuvaltassa
Nov 4, 2024
Maintainer

This is due to Newton's Second Law of motion. You are accelerating the body and then not accelerating any more, so it keeps moving.

Also, read about <body gravcomp>

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Keeping a block in a certain height #2202

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Keeping a block in a certain height #2202

adawne Nov 3, 2024

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yuvaltassa Nov 4, 2024 Maintainer

adawne
Nov 3, 2024

yuvaltassa
Nov 4, 2024
Maintainer