Yuval Tassa
a264d0bc8b
https://youtu.be/GioWwB36XHI The new geom attribute adhesion (units of force, signed; pair-level override) translates the contact friction cone along its normal so that the force origin lies strictly inside it. Consequences: each contact can pull with up to the given force before breaking, and the tangential friction budget becomes mu*(f_N + adhesion) -- the Mohr-Coulomb yield condition with cohesion c = mu*adhesion -- so lightly-squeezed grasps retain a guaranteed friction floor. A translated cone factors exactly into {constant attractive force} + {original cone}, so no solver kernels change. The implementation is this factorization: a constant attraction along contact normals accumulated into the new mjData.qfrc_adhesion (summed into qfrc_passive), plus a bias of adhesive contact rows' reference acceleration (aref += R*adhesion), which makes resting penetration exactly independent of adhesion. Contacts of adhesive pairs remain active throughout the gap zone, producing rows with positive violation whose reference acceleration pulls: a tether that resists pull-off smoothly, captures objects released within the band into steady contact, and detaches at the specified force. Adhesion values of the two geoms combine by sum; explicit pairs override. mj_contactForce reports the net interface force (cone force minus the adhesive pull), whose normal component can now be negative. Negative adhesion is allowed and produces a repulsive offset (air hockey). PiperOrigin-RevId: 950858148 Change-Id: I879c08eba7ae501e5c0f8c2f807167344da4c2bc
116 lines
4.4 KiB
XML
116 lines
4.4 KiB
XML
<mujoco model="Sand castles">
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<!-- Adhesion is Mohr-Coulomb cohesion: the shear strength of a contact is
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mu*(N + adhesion), nonzero even without confining pressure, and contacts
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resist tension up to the summed adhesion of the two geoms. Three identical
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cubes of grains differ only in adhesion: dry sand avalanches to its angle
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of repose, damp sand slumps but holds a mound, wet sand stands as a castle.
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The small gap is the moisture film: grains within it form tensile bridges. -->
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<option solver="CG" timestep="4e-3"/>
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<statistic center="0 0 .15" extent="1.2"/>
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<visual>
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<headlight diffuse=".8 .8 .8" ambient=".3 .3 .3"/>
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<quality shadowsize="8192"/>
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</visual>
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<asset>
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<texture type="skybox" builtin="gradient" rgb1=".4 .6 .8" rgb2=".9 .9 .95" width="32" height="512"/>
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<texture name="grid" type="2d" builtin="checker" width="512" height="512" rgb1=".1 .2 .3" rgb2=".2 .3 .4"/>
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<material name="grid" texture="grid" texrepeat="6 3" reflectance=".2"/>
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</asset>
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<default>
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<geom type="sphere" size=".01" friction="1 .005 .005" solref=".01 1"/>
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<default class="dry">
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<geom rgba=".87 .78 .55 1" adhesion=".002" gap=".001"/>
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</default>
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<default class="damp">
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<geom rgba=".72 .58 .38 1" adhesion=".02" gap=".001"/>
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</default>
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<default class="wet">
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<geom rgba=".5 .38 .25 1" adhesion=".05" gap=".001"/>
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</default>
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</default>
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<worldbody>
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<light pos=".5 -1 1.5" dir="-.3 .6 -.8" diffuse=".7 .7 .7"/>
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<geom name="floor" type="plane" size="1.2 .6 .01" material="grid"/>
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<geom name="rail-x" type="plane" size=".6 .05 .01" pos="-1.2 0 .05" xyaxes="0 1 0 0 0 1" rgba=".5 .5 .5 .15"/>
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<geom name="rail+x" type="plane" size=".6 .05 .01" pos="1.2 0 .05" xyaxes="0 -1 0 0 0 1" rgba=".5 .5 .5 .15"/>
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<geom name="rail-y" type="plane" size="1.2 .05 .01" pos="0 -.6 .05" xyaxes="-1 0 0 0 0 1" rgba=".5 .5 .5 .15"/>
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<geom name="rail+y" type="plane" size="1.2 .05 .01" pos="0 .6 .05" xyaxes="1 0 0 0 0 1" rgba=".5 .5 .5 .15"/>
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<body name="ramp1" mocap="true" pos="-.4 0 .02" euler="0 18 0">
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<geom type="box" size=".1 .12 .06" rgba=".35 .42 .28 1"/>
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</body>
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<body name="ramp2" mocap="true" pos="0 0 .02" euler="0 18 0">
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<geom type="box" size=".1 .12 .06" rgba=".35 .42 .28 1"/>
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</body>
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<body name="ramp3" mocap="true" pos=".4 0 .02" euler="0 18 0">
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<geom type="box" size=".1 .12 .06" rgba=".35 .42 .28 1"/>
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</body>
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<site name="adhesion = 0.002" size="1e-5" pos="-.45 0 .4"/>
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<replicate count="3" offset="0 0 .02828">
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<replicate count="6" offset=".02 0 0">
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<replicate count="6" offset="0 .02 0">
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<body name="a" pos="-0.45 -.05 .29" childclass="dry">
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<freejoint/>
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<geom/>
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</body>
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</replicate>
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</replicate>
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<replicate count="5" offset=".02 0 0">
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<replicate count="5" offset="0 .02 0">
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<body name="a2" pos="-0.44 -.04 .30414" childclass="dry">
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<freejoint/>
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<geom/>
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</body>
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</replicate>
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</replicate>
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</replicate>
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<site name="adhesion = 0.02" size="1e-5" pos="-.05 0 .4"/>
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<replicate count="3" offset="0 0 .02828">
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<replicate count="6" offset=".02 0 0">
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<replicate count="6" offset="0 .02 0">
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<body name="b" pos="-0.05 -.05 .29" childclass="damp">
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<freejoint/>
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<geom/>
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</body>
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</replicate>
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</replicate>
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<replicate count="5" offset=".02 0 0">
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<replicate count="5" offset="0 .02 0">
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<body name="b2" pos="-0.04 -.04 .30414" childclass="damp">
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<freejoint/>
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<geom/>
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</body>
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</replicate>
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</replicate>
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</replicate>
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<site name="adhesion = 0.05" size="1e-5" pos=".35 0 .4"/>
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<replicate count="3" offset="0 0 .02828">
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<replicate count="6" offset=".02 0 0">
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<replicate count="6" offset="0 .02 0">
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<body name="c" pos="0.35 -.05 .29" childclass="wet">
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<freejoint/>
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<geom/>
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</body>
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</replicate>
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</replicate>
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<replicate count="5" offset=".02 0 0">
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<replicate count="5" offset="0 .02 0">
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<body name="c2" pos="0.36 -.04 .30414" childclass="wet">
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<freejoint/>
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<geom/>
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</body>
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</replicate>
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</replicate>
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</replicate>
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</worldbody>
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</mujoco>
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