Ultrasonic drill head suits Martian environment

Magna Parva is optimising the design of an ultrasonic drill tool which hopes to improve the efficiency of drilling in the challenging Martian environment.

When searching for extinct or extant life on Mars it is, without a doubt, highly desirable to drill deep below the rock or regolith surfaces to obtain samples for later analysis.

With the extinction horizon for oxidants in the subsurface being several centimetres and damaging ionising radiation penetrating to depths of around 1m precision drilling takes on great importance.

Previously hammering or percussion have both been used to improve the drilling performance in hard and brittle materials, however existing hammering drilling techniques still have disadvantages, including the high axial (thrust) forces required and tendency for "drill walk" when initiating cutting.

Conventional rotary corers that, for example, produce 10mm diameter cores may require up to 150N or more of axial preload.

A typical rotary corer that produces 10mm cores in hard rock requires 30W or more of power and operates at low efficiencies in the duty cycle.

Motor-operated drills can demand up to four times surge current upon start up than for continuous operation.

During core initiation, drill walk can induce torques on the drilling platform that may exceed 30nm and tangential forces of 100N.

The drill chatter delivers low-frequency (typically 2 - 10Hz), high-force perturbations on the drilling platform, requiring massive platforms for stability.

For Mars missions, where gravity is only 38% of that on earth, clearly such characteristics are undesirable.

There is no ideal drill bit for hard and soft rocks.

In hard rocks convention drillers stop drilling by shearing and spoliation and become grinders.

The grinding process is accompanied by at least a 300% increase in consumed energy per unit length of the core.

In addition, because the grinding mechanism is determined by the compression failure of the rock the sharpness of the bits must be monitored, otherwise the heat generation at the tip may increase by a factor of ten.

This increase is accompanied by a concomitant drop in drilling efficiency and often causes temperature degradation of the drill bit and thermal alteration of the drilled sample.

"Clearly, traditional percussion drills are not ideally suited to power and mass constrained applications such as those prevalent in planetary exploration missions", says Andrew Bowyer, Commercial Director at Magna Parva.

"Our solution is to use ultrasonic axial excitation of the drill head to achieve cutting".

The principle of ultrasonic drilling is to axially oscillate a cutting head at high frequencies (~20-30kHz) to produce a small axial motion (~10 um) at relatively high velocity (typically 1-20m/s).

The impact of the drilling head against the rock surface causes micro-cracking of the rock along the crystal and mineral fracture planes causing the surface to break off and allow cutting/drilling to be achieved.

The progress of the drill bit through the medium is achieved by the application of a modest preload to the drill bit.

"We contributed to a recent European Space Agency study to investigate the feasibility of an ultrasonic drilling technique for the collection of samples from basalt and other types rocks".

"That study, along with others in ultrasonic machining, have indicated that the benefits of ultrasonic technology for planetary drilling are manifold", adds Bowyer.

Indeed ultrasonic technology benefits include: low axial (thrust) force required; a lower power consumption than conventional percussion drilling; the possibility of operation from far less massive and stable drill platforms; low drill bit wear; good material removal rates; lower bit temperatures, reducing the possibility of damage to the sample; potentially higher efficiency; and a smaller envelope.

"In addition to these benefits the technology we're developing at Magna Parva with Loughborough University has a further two potential benefits".

"The first is low sensitivity to axial (thrust) force variations, which is very significant when considering an autonomous, relatively flexible drilling platform; and the second is higher efficiency of the ultrasonic cutting process", says Bowyer.

Bowyer is the first to admit potential difficulties.

"The potential benefits of ultrasonic machining are not a given".

"The gains are only accrued with very careful design of the dynamics of the whole ultrasonic system".

"It's very easy to design an ultrasonic drilling system that has far worse performance than conventional drilling".

"What we're working on right now is optimising the design of the Ultrasonic Drill Tool (UDT) to ensure maximum benefits".

This approach has just been given the green light by the European Space Agency.

Bowyer says "We recently won a contract with the European Space Agency to design, develop and build UDTs which we hope will be used in the Exomars Pasteur Rover and Mars Sample Return missions in the Aurora Exploration programme".

"Both missions aim to collect surface and subsurface samples, making the drill system a critical key element".

"The drill system must be able to penetrate and obtain samples from well-consolidated formations such as sedimentary rocks and evapouritic deposits".

"Looking at the job at hand and the benefits UDT offers, we absolutely believe that we have designed and are developing the right solution".

The UDT is a 'pure' ultrasonically actuated tool.

The power electronics subsystem consists of an efficient, compact and lightweight bi-directional switching amplifier circuit, drawing from a 28V supply.

Excitation is carried out via a stack of piezoelectric transducers.

The horn is of a novel design that maintains low stress and efficient performance.

The tool head and bit are dynamically tuned to effectively become part of the horn.

The control mode uses auto-resonance, which is more efficient and offers several other advantages over conventional control.

The tool head can be used to collect a core or unconsolidated material.

It incorporates a redundant pair of thermocouple temperature sensors.

The removal of cuttings is essential and is effected via an auger.

The tool head can be connected to the UDT via simple screw or bayonet interface that is ideal for automated tool swapping.

The ultrasonic tool will be connected to the rest of the drill system via a simple mechanical/electrical coupling that is based on current work taking place on the drill system and down-hole hammering mechanism, to ensure compatibility.

The electrical interface is simple.

It consists of power supply and bi-directional serial lines only.

The ultrasonic tool has a fitting to the rest of the drill system that dynamically isolates it from the rest of the drill system via the nodal points.

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