[lammps-users] How to set up the moving periodic boundary connected by spring

Hi folks,

I have a question to set up periodic boundaries with springs.(no pair-wise potential w/o spring)
Stationary boundary can be set by spring/self directly as

fix 6 lower spring/self 50.0

However, I have big difficulties to construct moving boundary using springs.
Could you let me know the way or trick to make the spring-connected periodic boundary with constant speed?

Thank you.

What is a periodic boundary with a spring?