The rotary-percussive ultrasonic drill (RPUD) employs vibrations on two sides of a piezoelectric stack to drive a drill tool to achieve simultaneous rotary-percussive motion. It has the advantages of being small and requiring low power, low axial load, and small holding torque, making it suitable for extra-terrestrial rock sampling, especially for a minor planet with a weak gravitational field. This paper presents the impact dynamics prediction of the percussive system of RPUD, which is composed of a piezoelectric actuator, a free mass, and a drill tool. Considering the vibration of the RPUD and the weight on bit, the interactions between these three components before and during drilling are analyzed separately. The effects of various parameters (i.e., the coefficient of restitution between the actuator and the free mass, the damping ratio of the RPUD, the weight of the free mass, and the weight on bit) on the contact force between the free mass and the drill tool, a number of collisions per second, kinetic energy transferred to the drill tool per second, and reacting force are simulated. Simulation results show that the free mass converts the high-frequency harmonic vibration of the actuator into lower frequency impacts on the drill tool. Furthermore, the contact force and the kinetic energy transferred to the drill tool per second can be enhanced by increasing the coefficient of restitution, free mass, and weight on bit or decreasing the damping ratio of the RPUD.
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