A fast and effective way to step a relative position quantity by a reference interval. Each step in relative position includes open-loop and closed-loop control intervals. The invention contemplates an actuator capable of changing the relative position and a closed loop servo that acts on the actuator to keep the relative position centered on the nearest one of a series of reference values separated by the reference interval. The actuator is preferably capable of a fast response. Stepping the relative position is accomplished as follows, assuming an initial condition where the servo has locked the relative position to a particular initial reference value. First, the actuator is caused to change the relative position by an amount approximately equal to the reference interval in a manner that the servo cannot track the change, such as by disabling the servo. Servo control is then re-established, at which point the servo operates to keep the relative position centered on the nearest reference value. Servo control is maintained until a new step is required, at which time the process is repeated.