toward solution for issue #226, need to deal properly with static joints

Author: petercorke

roboticstoolbox/robot/ERobot.py

@@ -2102,7 +2102,10 @@ def rne(robot, q, qd, qdd, symbolic=False, gravity=None):
                 Xtree[j] = SE3(np.eye(4, dtype="O"), check=False)
             else:
                 Xtree[j] = SE3(link.Ts, check=False)
-            s[j] = link.v.s
+            if link.v is not None:
+                s[j] = link.v.s
+            else:
+                s[j] = None
 
         if gravity is None:
             a_grav = -SpatialAcceleration(robot.gravity)