Low-reaction-force, misalignment-tolerant corers double as sensory probes.
Easy-to-use, low-power-consumption apparatuses capable of drilling to acquire core samples of thick layers of material and/or measuring physical and chemical characteristics of the layers are undergoing development. A major component of an apparatus of this type is an ultrasonic/sonic drill/corer (USDC) with integrated sensors. The USDC includes a hollow drill bit or corer, in which a combination of ultrasonic and sonic vibrations are excited by an electronically driven piezoelectric actuator. The corer can be instrumented with a variety of sensors for both probing the drilled material and acquiring feedback for control of the excitation (see figure). The potential uses of these apparatuses are so numerous that it is not possible to list them all here; a few representative examples include sampling rocks and soil, medical procedures that involve core sampling and/or probing, detecting buried land mines, and even extracting rock cores for use as small bricks.
The USDC advances into the material of interest by means of a hammering action and a resulting chiseling action at the tip of the corer. The combination of ultrasonic vibrations (typically at a frequency of ≈20 kHz) and sonic vibrations (typically at a frequency between 60 Hz and 1 kHz) gives rise to a hammering action that is more effective for drilling than is the microhammering action of ultrasonic vibrations alone. The hammering and chiseling actions are so effective that unlike in conventional twist drilling, a negligible amount of axial force is needed to make the USDC advance into the material. Also unlike a conventional twist drill, the USDC operates without need for torsional restraint, and can easily be made to drill into a material at an oblique angle.
In its role as a hammering mechanism, the USDC also acts as a sounding source for geophysical or physiological sonar to examine drilled objects and the surrounding ground or tissue. When the tip of the corer first touches an object, the acoustic impedance (and hence the electrical impedance) of the piezoelectric actuator changes; these impedances can serve as additional sensory quantities for probing the object and/or for feedback control of the excitation.
Unlike a conventional twist drill, the tip of the USDC corer need not be sharp. Because the corer can operate without rotation, the cross section of the corer and thus of the core samples can be square, round, or of any other convenient shape.
The corer vibrates transversely as well as longitudinally, causing the formation of a hole somewhat wider than the corer; consequently, unlike a conventional twist drill, the USDC resists jamming and is highly tolerant of misalignment.
At a location away from the tip of the corer, the hollow interior of the corer can be connected to a vacuum system via a tube. This connection can be used to extract drilling dust and gases emitted by the drilled material for analysis. The sensors in the corer can be used to determine various properties as functions of depth. Examples of sensors that could be integrated into the corer include accelerometers, acoustic transducers (in addition to the piezoelectric actuator) to measure mechanical properties, eddy-current sensors to measure electromagnetic properties, fiber-optic probes to examine the newly exposed surface, temperature sensors, and electrodes made of various materials to obtain measures of chemical reactivity.
This work was done by Yoseph Bar-Cohen, Stewart Sherrit, Benjamin Dolgin, Thomas Peterson, Dharmendra Pal, and Jason Kroh of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Machinery & Automation category.
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