An apparatus and method for controlling an oscillating device is presented. In particular, a controller for adjusting the frequency of a drive signal driving a haptic actuator is presented. The controller contains a calculator adapted to receive a first and a second measurement of the electrical parameter and to calculate a difference between the first measurement and the second measurement. The first measurement is obtained at a first sampling time and the second measurement is obtained at a second sampling time. The controller is adapted to provide a control signal to adjust a frequency of the drive signal based on the difference.