Fix setting of hasdynamics attribute for URDF models

Author: petercorke

roboticstoolbox/robot/Link.py

@@ -15,8 +15,12 @@ def _listen_dyn(func):
     Use this decorator for any property setter that updates a parameter that
     affects the result of inverse dynamics.  This allows the C version of the
     parameters only having to be updated when they change, rather than on
-    every call.  This decorator signals the change by invoking the
-    ``.dynchanged()`` method of the robot that owns the link.
+    every call.  This decorator signals the change by:
+    
+    - invoking the ``.dynchanged()`` method of the robot that owns the link.
+      This assumes that the Link object is owned by a robot, this happens
+      when the Link object is passed to a robot constructor.
+    - setting the ``._hasdynamics`` attribute of the Link
 
     Example::
 
@@ -31,6 +35,7 @@ def m(self, m_new):
     def wrapper_listen_dyn(*args):
         if args[0]._robot is not None:
             args[0]._robot.dynchanged()
+        args[0]._hasdynamics = True
         return func(*args)
     return wrapper_listen_dyn
 
@@ -456,7 +461,11 @@ def hasdynamics(self):
         :return: Link has dynamic parameters
         :rtype: bool
 
-        Link has some assigned (non-default) dynamic parameters.
+        Link has some assigned (non-default) dynamic parameters.  These could
+        have been assigned:
+
+        - at constructor time, eg. ``m=1.2``
+        - by invoking a setter method, eg. ``link.m = 1.2``
 
         Example:
 

roboticstoolbox/tools/urdf/urdf.py

@@ -636,10 +636,10 @@ class Inertial(URDFType):
     """
     _TAG = 'inertial'
 
-    def __init__(self, mass, inertia, origin=None):
-        self.mass = mass
-        self.inertia = inertia
-        self.origin = origin
+    def __init__(self, mass=None, inertia=None, origin=None):
+        self._mass = mass
+        self._inertia = inertia
+        self._origin = origin
 
     @property
     def mass(self):
@@ -1579,7 +1579,7 @@ def inertial(self, value):
             raise TypeError('Expected Inertial object')   # pragma nocover
         # Set default inertial
         if value is None:
-            value = Inertial(mass=0.0, inertia=np.eye(3))
+            value = Inertial()
         self._inertial = value
 
     @property
@@ -1696,14 +1696,16 @@ def __init__(self, name, links, joints=None,
 
         # build the list of links in URDF file order
         for link in self._links:
-            elink = rtb.ELink(name=link.name)
+            elink = rtb.ELink(name=link.name,
+                m=link.inertial.mass,
+                r=link.inertial.origin[:3, 3] if link.inertial.origin is not None else None,
+                I=link.inertial.inertia
+            )
             elinks.append(elink)
             elinkdict[link.name] = elink
 
             # add the inertial parameters
-            elink.r = link.inertial.origin[:3, 3]
-            elink.m = link.inertial.mass
-            elink.inertia = link.inertial.inertia
+
 
             # add the visuals to visual list
             try: