The motion of a micromachined bi-directional electro-thermal vibromotor (BETV) is analyzed using an impact model which takes into account of friction and contact, as well as the compliance of the slider and the impact head. This model is shown to address several important issues on the reliable usage of BETV. These include (a) how to achieve the push or pull mode (b) avoiding multiple impacts in one actuation cycle (c) the existence of periodic solution with the same period as the thermal actuator. To test our theory, we fabricated and conducted experiments on several BETV with different geometries. Preliminary results showed good agreement between theory and experiments.