A mass measurement system using the self-excited oscillation of a nonlinear system is developed. The system contains an on-off relay with hysteresis and switches force acting on the object in relation to the velocity of the object. The mass of the object is determined from the period of the oscillation. An apparatus with two voice coil motors (VCM's) is developed for experimental study. The VCM's work as a force generator and a velocity sensor. The effects of system parameters on measurement accuracy are studied experimentally. It is proposed to use one VCM as both forcer and sensor.