define u03b4u on the basis of empirical values moreover u03b4u can be defined the same or differently for adjusting the upper withstand voltage limit and the lower withstand voltage limit after the initialization a determination is made of whether a movement of the actuator occurs under the present voltage u curr to that end the current position x curr is compared with the starting position and in step s 113 the difference between the two positions is compared with a minimum movement distance u03b4x min if an actuator movement is detected it is concluded that the assumed withstand voltage limits were set too broadly so that in step s 121 when testing in reference to the upper withstand voltage limit u up the latter is reduced by an increment u inc starting from the prior value or when testing a lower withstand voltage limit u dn the prior value is increased by an increment u inc so that the withstand voltage range as a whole is defined more narrowly the withstand voltage test ends in this case after the change to the characteristic map in step s 121 if there is still no movement detected in step s 113 then in steps s 114 s 115 corresponding to steps s 14 and s 15 from fig1 a check is performed of whether a wait time u03b4t has already expired if appropriate steps s 113 through s 115 are repeated until the wait.