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Definition of rotation axis in mujoco

2022-07-01 18:04:00 【Feisy】

Two definitions

    <body name="arm_low_l" pos="-.06 0 -.016" quat="0 0 0.707107 0.707107">
 
         <joint name="l_elbow" range="-1.3 1.57" type="hinge"/>
         <geom type="cylinder" size="0.001 0.008"/>

      </body>

		<body pos="0 0 2" euler="0 0 0">
			<joint name="pin" type="hinge" axis = "0 -1 0" pos="0 0 0.5"/>
			<geom type="cylinder" size="0.05 0.5" rgba="0 .9 0 1" mass="1"/>
		</body>

differences ：

To the first 1 The definition method of species , There is a quaternion quat=“0 0 0.707107 0.707107”, The first 2 There is no
To the first 2 The definition method of species ,body Yes euler, Subelement joint Yes axis,pos attribute , The first 1 There is no

reason

Any posture , Equivalent to rotating a certain angle around a specific rotation axis
—《 Introduction to mathematics of robot operation 》

The first 1 The definition method of species , It is based on the above theory , The rotation axis is represented by a quaternion , The quaternion here also contains the initial attitude .
The first 2 Methods , It's a more traditional method , Directly define a connecting rod , Then define the direction and position of the rotation axis

Some interpretations of the second method

Mujoco In the default body It's vertical , So if we need to build some level, it will be others
Formal body, Need to use euler as well as axis Attributes are combined

    <body name="shoulder_l" pos="0 0.06035 0" euler="0 0 0">
           <inertial diaginertia="0.0118742 0.00851644 0.00537304" mass="0.013" pos="-0.0135226 0.0102641 0.00139357" quat="0.89853 0.0814684 0.0409894 0.429332"/>
           <joint name="l_shoulder_pitch" range="-2.61799 2.61799" type="hinge" axis="1 0 0" pos="0 0 -0.03" />
           <geom type="cylinder"  size="0.001 0.03"/>
    </body>

1 euler： Defines the posture of the local coordinate system of the next joint in the local coordinate system of this joint

1.1 Example 1

axis=“1 0 0”
euler=" 0 0 0", Indicates that the attitude of the child coordinate system is the same as that of the parent coordinate system
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1.2 Example 2

axis=“1 0 0”
euler=" -1.57 0" Indicates that the child coordinate system revolves around the parent coordinate system x The axis rotates clockwise pi/2
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1.3 Example 3

axis=“0 0 1”
euler=" -1.57 0"

The rotation was originally pointing up , because euler The value of , It turns to point to the right
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1.4 Example 4

axis=“0 1 0”
euler=" -1.57 0"

The rotation was originally pointing to the right , because euler The value of , It becomes pointing down
Bold style

2 axis： Defines which axis of the local coordinate system the joint rotates about

3 pos: Define the joint at body Which position in , Be careful ,body The body coordinate system of is built on body Middle , therefore [0,0,0] Is to put the joint on body middle

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