The ultrasonic/sonic anchor (U/S anchor) is an anchoring device that drills a hole for itself in rock, concrete, or other similar material. The U/S anchor is a recent addition to a series of related devices, the first of which were reported in “Ultrasonic/Sonic Drill/Corers With Integrated Sensors” (NPO-20856), NASA Tech Briefs, Vol. 25, No. 1 (January 2003), page 38. There are numerous potential uses for U/S anchors, especially in enabling walking robots and humans to climb steep rock faces for diverse purposes, including scientific exploration, recreational rock climbing, military maneuvers, and search and rescue.
Like the prior devices in this series, the U/S anchor drills a hole by means of hammering and chiseling actions of a tool bit excited with a combination of ultrasonic and sonic vibrations. The U/S anchor also contains an actuator that includes a piezoelectric stack at the upper end of a rodlike horn that serves to mechanically amplify the piezoelectric displacement. In addition, as in the previously related devices, the tool bit is mounted at the lower end of the horn. The piezoelectric stack is electrically driven at its resonance frequency (an ultrasonic frequency), and a bolt holds the stack in compression to prevent fracture during operation.
In a typical prior related device, the sonic vibrations are generated with the help of upper and lower mass that is denoted the free mass because it is free to move axially through a limited range within the actuator/tool-bit assembly. In the U/S anchor, there are two free masses: one above and one below the lower tip of the horn (see figure). Each free mass bounces between hard stops at the limits of its range of motion at a sonic frequency. The impacts of the free masses on the hard stops create stress pulses that propagate along the horn, to and through the tool bit, to the tool-bit/rock interface. The rock becomes fractured when its ultimate strain is exceeded.
A major advantage of the U/S anchor (or of any device in this series) is that it is not necessary to apply a large axial force to make the tool bit advance into the drilled material. Similarly, during operation, only a small force suffices to extract the tool bit from the drilled hole. Hence, a human or robotic rock climber could easily insert and withdraw a U/S anchor at successive positions during traversal of a rock face.
This work was done by Yoseph Bar-Cohen and Stewart Sherrit of Caltech for NASA’s Jet Propulsion Laboratory.
This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to the Patent Counsel, NASA Management Office–JPL. Refer to NPO-40827.