This paper presents a novel measurement method for angular displacement of an oscillation-assisted micro drill device, Ros-Drill (c). The device is driven with a brushless dc motor (BLDC) which is desired to track a sinusoidal position reference. The measurement method is based on the principle of monitoring the back-emf voltage that is induced on the non-fed winding of the brushless motor. It offers sensorless analog measurement of the angular displacement of the oscillatory motion which is not possible with optical encoders. The measurement methodology and control algorithms are implemented utilizing a digital signal processor. Experiments reveal that the method is feasible for measuring angular displacements of rotational oscillations during cellular piercing operations. Furthermore, it presents fair performance for frequencies up to 1000 Hz and provides early diagnosis for a potential malfunction of the micro drill.