Method and device for generating a control voltage for a position actuator of a disk drive system for displacing a lens of a pick-up unit to a given X-position. The actuator is operated by an open loop control system. First, parameters of the control system are calibrated so that the control system produces a control voltage (Va) from an input position signal (Xact setp). Then, the control voltage is processed in a feed forward system for generating a processed control signal (Va_p) which is fed to the position actuator for displacing said lens to the X-position. A Z-value corresponding to the axial distance of the lens from a calibrated axial position and a temperature of the pick-up unit are measured. The position actuator operates according to the formula: Xdc=Va_p*K(Temp,Z)/(R(Temp)*C(Temp,Z)) in which: Xdc=position signal for said given position Va_p=Va processed K=property of ‘actuator’ 21: Newton/Ampere R=resistance of ‘actuator’ coil C=Spring constant in Newton/meter. The Va_p is calculated according to the following formula: Va_p=Xact setp*Cal*S(Temp_cal,Z_cal)/S(Temp_write,Z_write) in which Cal is the result of the calibration S(Temp,Z)=(A_T+B_T*Temp+C_T*TempA2)*(A_Z+B_Z*Vz+C_Z*VzA2) wherein A T, B T, C T, A Z, B Z, C Z are constants; Vz is the voltage across the Z actuator, Temp cal and Vz cal are the temperature and Vz, respectively, during calibration, Temp write and Vz write are the temperature and Vz, respectively during writing.