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Boring the Planets
The JPL-developed Gopher (seen in the image on the left) created a 1.76-m deep borehole in the ice Lake Vida, Antarctica in Dec. 2005. The Gopher’s planned successor will drill autonomously.
Future planetary missions will place increasing importance on in-situ sampling and analysis of surface and subsurface materials. But designing a drill for use by robotic probes on other worlds presents a suite of challenges. Auguring into solid rock or ice requires a relatively large amount of torque and axial force, which puts stability constraints on the robotic probe and consumes precious power. The rotating bit can dull, jam or break millions of miles from the reach of engineers. An advanced solution will be required to operate on the hellish surface of Venus or to bore into the frozen crust of Europa.
Long a leader in the field of electroactive materials, JPL’s Advanced Technologies Group, under Senior Research Scientist Yoseph Bar-Cohen, has spent the past few years developing an ultrasonic drill that could be just what a future planetary mission is looking for. Their Ultrasonic-Sonic Driller Corer (USDC) has no rotating bit, requires very little axial force or torque to push into a hard surface and can operate in extreme temperature environments.
The USDC uses a stack of piezoelectric discs, which convert electric currents into physical force. The ultrasonic vibrations of the discs are converted to sonic-frequency waves that pound the bit into rock or ice like a mini jackhammer. Using multiple thin piezoelectric plates keeps the required voltage low, which is more compatible with flight hardware requirements. And because the bit does not have to rotate, the active end of the drill can be fitted with sensors including thermocouples and optical fibers, and it can impart seismic sounding for probing the subsurface environment on planetary exploration missions.
Typical ultrasonic drills acoustically couple the bit to a horn that is in contact with the piezoelectric actuator as a means for focusing the vibrational energy. Bar-Cohen, members of his group and engineers from the company Cybersonics realized they could interpose a free mass between the horn and bit to turn high frequency, low-force vibration into low frequency, high force vibration. The free-flying mass bounces between the horn and the drilling-coring bit, converting the ultrasonic impacts to hammering at sonic frequencies.
Bar-Cohen’s lab has demonstrated piezo drills in low temperature environments, testing one called the “Gopher” in Antarctica. In this test, conducted by the two members of his group, Mircea Badescu and Stewart Sherrit, the Gopher drilled to a depth of 1.76 meters into the ice. The team intends to soon push even deeper, with a gopher that can bore autonomously up to five meters. They will provide the piezoelectric actuation mechanism for the Auto-Gopher under a recently awarded ASTEP (Astrobiology Science and Technology for Exploring Planets) technology development project. Once the ASTEP funding is issued by NASA, the Auto-Gopher will be developed under the leadership of Honeybee Robotics, which provided the Rock Abrasion Tool for NASA’s Mars Exploration rovers Spirit and Opportunity.
Because some piezoelectric actuator crystals have a high Curie point, such actuators continue to operate at very high temperatures, and potentially could be employed by a future Venus lander, like the Venus In-Situ Explorer concept NASA is considering. The Advanced Technologies Group has an ongoing NASA Planetary Instrument Definition & Development Program (PIDDP) task to develop such a drill, and they have demonstrated that their mechanism can indeed operate at the 500 degree Celsius temperature of the Venusian surface.
Ultimately, adding rotation to the USDC bit can be a good thing. Although the Ultrasonic-Sonic Driller Corer does not require a rotating bit to chew into materials, it turns out that percussive action makes rotating drills much more efficient. This principle is employed in hammer drills used for drilling masonry. Bar-Cohen’s group demonstrated a significant enhancement to the drilling rate of a rotating drill last year, as part of an ASTEP task.