Polynomial stiffness and damping https://youtu.be/aKa3ZlEF9_Y

PiperOrigin-RevId: 884607673
Change-Id: Id336e65b65a899d676fe4444d85bf31a9e8b5a6f

Author: yuvaltassa

dm_control/mjcf/attribute.py

@@ -231,6 +231,8 @@ def to_xml_string(self, prefix_root=None,
       return util.to_native_string(out.getvalue())[:-1]  # Strip trailing space.
 
   def _check_shape(self, array):
+    if array.ndim == 0:
+      array = array.reshape(1)
     actual_length = array.shape[0]
     if len(array.shape) > 1:
       raise ValueError('Expect one-dimensional array: got {}'.format(array))

dm_control/mjcf/schema.xml

@@ -284,14 +284,14 @@
             <attribute name="solimplimit" type="array" array_type="float" array_size="5"/>
             <attribute name="solreffriction" type="array" array_type="float" array_size="2"/>
             <attribute name="solimpfriction" type="array" array_type="float" array_size="5"/>
-            <attribute name="stiffness" type="float"/>
+            <attribute name="stiffness" type="array" array_type="float" array_size="3"/>
             <attribute name="range" type="array" array_type="float" array_size="2"/>
             <attribute name="actuatorfrcrange" type="array" array_type="float" array_size="2"/>
             <attribute name="margin" type="float"/>
             <attribute name="ref" type="float"/>
             <attribute name="springref" type="float"/>
             <attribute name="armature" type="float"/>
-            <attribute name="damping" type="float"/>
+            <attribute name="damping" type="array" array_type="float" array_size="3"/>
             <attribute name="frictionloss" type="float"/>
             <attribute name="user" type="array" array_type="float"/>
             <attribute name="actuatorgravcomp" type="keyword" valid_values="false true"/>
@@ -424,8 +424,8 @@
             <attribute name="width" type="float"/>
             <attribute name="material" type="reference"/>
             <attribute name="rgba" type="array" array_type="float" array_size="4"/>
-            <attribute name="stiffness" type="float"/>
-            <attribute name="damping" type="float"/>
+            <attribute name="stiffness" type="array" array_type="float" array_size="3"/>
+            <attribute name="damping" type="array" array_type="float" array_size="3"/>
             <attribute name="user" type="array" array_type="float"/>
           </attributes>
         </element>
@@ -606,14 +606,14 @@
                 <attribute name="solimplimit" type="array" array_type="float" array_size="5"/>
                 <attribute name="solreffriction" type="array" array_type="float" array_size="2"/>
                 <attribute name="solimpfriction" type="array" array_type="float" array_size="5"/>
-                <attribute name="stiffness" type="float"/>
+                <attribute name="stiffness" type="array" array_type="float" array_size="3"/>
                 <attribute name="range" type="array" array_type="float" array_size="2"/>
                 <attribute name="actuatorfrcrange" type="array" array_type="float" array_size="2"/>
                 <attribute name="margin" type="float"/>
                 <attribute name="ref" type="float"/>
                 <attribute name="springref" type="float"/>
                 <attribute name="armature" type="float"/>
-                <attribute name="damping" type="float"/>
+                <attribute name="damping" type="array" array_type="float" array_size="3"/>
                 <attribute name="frictionloss" type="float"/>
                 <attribute name="user" type="array" array_type="float"/>
                 <attribute name="actuatorgravcomp" type="keyword" valid_values="false true"/>
@@ -741,8 +741,8 @@
                 <attribute name="width" type="float"/>
                 <attribute name="material" type="reference"/>
                 <attribute name="rgba" type="array" array_type="float" array_size="4"/>
-                <attribute name="stiffness" type="float"/>
-                <attribute name="damping" type="float"/>
+                <attribute name="stiffness" type="array" array_type="float" array_size="3"/>
+                <attribute name="damping" type="array" array_type="float" array_size="3"/>
                 <attribute name="user" type="array" array_type="float"/>
               </attributes>
             </element>
@@ -1405,7 +1405,7 @@
                 <attribute name="solimpfriction" type="array" array_type="float" array_size="5"/>
                 <attribute name="actuatorfrclimited" type="keyword" valid_values="false true auto"/>
                 <attribute name="actuatorfrcrange" type="array" array_type="float" array_size="2"/>
-                <attribute name="stiffness" type="float"/>
+                <attribute name="stiffness" type="array" array_type="float" array_size="3"/>
                 <attribute name="range" type="array" array_type="float" array_size="2"/>
                 <attribute name="margin" type="float"/>
                 <attribute name="ref" type="float"/>
@@ -1942,7 +1942,7 @@
             <attribute name="material" type="reference"/>
             <attribute name="rgba" type="array" array_type="float" array_size="4"/>
             <attribute name="springlength" type="array" array_type="float" array_size="2"/>
-            <attribute name="stiffness" type="float"/>
+            <attribute name="stiffness" type="array" array_type="float" array_size="3"/>
             <attribute name="damping" type="float"/>
             <attribute name="armature" type="float"/>
             <attribute name="user" type="array" array_type="float"/>
@@ -1977,7 +1977,7 @@
             <attribute name="actuatorfrcrange" type="array" array_type="float" array_size="2"/>
             <attribute name="margin" type="float"/>
             <attribute name="frictionloss" type="float"/>
-            <attribute name="stiffness" type="float"/>
+            <attribute name="stiffness" type="array" array_type="float" array_size="3"/>
             <attribute name="damping" type="float"/>
             <attribute name="armature" type="float"/>
             <attribute name="user" type="array" array_type="float"/>