how to make a ramp function

I want to perform a tensile test using a ramp function. I want to pull one end of my sample and hold the other end. The problem is that I want to apply a ramp velocity to the moving end starting from zero and reach a certain value and the keep this value. I tried many different ways like introducing a variable using function like “ramp” but everyone of them has a problem, for example when I use ramp, it errors:“you cannot use ramp between run commands”. Can anybody help me.

Mohammad Aramfard

Try the fix move command which will adjust the velocity
of atoms during a run (not between them). You can write
a varialee which specifies velocity (or position) as a function
of time for the atoms at the end and use it as in input

parameter to fix move.