Real World Time control of Dynamixel motor

Dear All ,
I am a beginner in the usage of servo motors and am using MX-28AT with a U2D2. Am communicating the motors using Matlab.
My issue is regarding the increase in the time of motion of the motor with increasing time steps for the same time interval . And I wish to constrain the motion of motor (eg., number of to and fro motions=2.5) for fixed time intervals (say 10 secs) even if I change the time step values.

Please help me out how this can be done.