hi,everybody 
I try to simulate a sinusoidal motion for piston inside cylinder. I defined the piston as a rigid body and its velocity by a profile like that 
---------------------
piston 2 8 1
time v_x 
0 0
1 1
2 2
3 3
4 -3
5 -2
6 -1
7 0
-------------------
my question is why when I display the piston motion the fluent add the velocity values which mean that second (1 ) the velocity is 1 m/s and at sec (2) the velocity becomes 2+1=3 m/s and so on so the piston still moves in x direction and did not return back again 
I am so confused any help guys??