[Discuss-gnuradio] Help needed regarding implementing spring mass system in GRC

Hello,

I want to implement spring mass system with PID controller of example mentioned in following link 

[ http://ctms.engin.umich.edu/CTMS/index.php?example=Introduction&section=ControlPID ]

I want to create a flow graph exactly as given in the figure "Spring_mass_simulink" . Can you please help me how can i achieve this target? As i tried to implement it but unfortunately i always get an error of "Flow graph has loops" 

Thanks a lot

Afaq