Here is the output from Print, in the above code to illustrate what is going on.
As you can see it vibrates between 2 positions constantly.
0.567266405 8.30943100e-006 0.567266405 8.30943100e-006 0.567266405 8.30943100e-006 0.567266405 8.30943100e-006 0.567266405 8.30943100e-006 0.567266405 8.30943100e-006 0.567266405 8.30943100e-006 0.567266405 8.30943100e-006 0.567266405 8.30943100e-006
As you can see it vibrates between 2 positions constantly.