A haptic simulation method determines a location of a needle assembly within a magneto-rheological fluid. Each opening is closed by an end closing device, such as a curtain. The pneumatic servo valve further comprises a spool holding means including a ball nut 60 which is engaged, via a ball, with a helical groove formed on a connection rod of the spool connected to the torque motor and rotates when the spool moves linearly, and an electronic brake for holding the ball nut not to rotate when the spool is stopped.