wheel velocity oscillate

I use gazebo7 with ubuntu16.04 to simulate a four wheel robot. The robot was controlled by a model plugin with PID. The problem is that the wheel velocity (radian per second) oscillates, see the attachment picture[C:\fakepath\wheel velocity.jpg](/upfiles/15251523224680721.jpg). The sdf file [C:\fakepath\model.sdf](/upfiles/15251523994071704.sdf)and plugin [C:\fakepath\topymodel_push.cpp](/upfiles/15251524082998635.cpp)is also in the attachments. Thank you in advance.