plugin not defined in gazebo ?

edit

hello every today when i try to work with my SDF model in gazebo , i find my plugin could't use any more , does any one see this before ?

Error [parser.cc:697] XML Element[plugin], child of element[joint] not defined in SDF. Ignoring.[joint]
Error [parser.cc:688] Error reading element <joint>
Error [parser.cc:688] Error reading element <model>
Error [parser.cc:348] Unable to read element <sdf>
Error [parser_urdf.cc:2691] Unable to load file[/home/tiange/.gazebo/models/my_robot/model.sdf].