To address the aggressive goals of the 2009 Mars Science Laboratory (MSL) mission, Honeybee Robotics Spacecraft and Mechanisms Design Co is designing a Mini-Corer device for analysis of rock specimen composition. A NASA-approved provider of space flight hardware, Honeybee has completed over 90 projects for NASA since 1986.
According to systems engineer Mike Maksymuk, the mechanism will manipulate and move around 74 cups of Mars dirt that will be presented to various science stations -- primarily two ovens. Each cup can be inserted and hermetically sealed in an oven, which cooks the sample and analyzes the gases that burn off. This instrument represents about 85% of the science the MSL will perform.
Onerous Space Flight Demands
To be space qualified, devices are tested to meet the conditions they will encounter in space. Material shrinkage due to cold temperatures has to be taken into account, as well as vibration. Of course, to make it to Mars, the designs must withstand the 62 Gs produced by the launching. Honeybee does the calculations and analysis as well as tests in a thermal-vac chamber with temperatures that go down to -70°C.
The rock-coring device that NASA''s Jet Propulsion Laboratories (JPL) wants would be part of a mobile robot that could withdraw rock samples from Mars'' surface without being disabled by environmental dust. The MSL''s capacity and all the other equipment it has to carry dictate size and weight.
Tom Myrick, Honeybee''s chief engineer, has been designing for space at Honeybee since 1989. A device that he originally invented for the 2003 Mars Exploration Rover''s Mission, the Mini-Corer may now have its chance to make its intended journey, since the vagaries of space project shrinkage caused it to be left off that mission. Double-wide linear bearings from NB Corp are integral to its functioning.
Its Time Is Coming
The Mini-Corer resides in an aluminum shoebox-sized enclosure which, for the MSL Rover, will be made out of albemet (a combination of aluminum and beryllium). This outer box seals the interior from dust and acts as a very rigid structural housing. Inside it is a smaller box, a little larger than a man''s fist. These boxes double seal the Mini-Corer to prevent dust from getting into the gears and bearings. The motors and gears are inside the smaller box; the shaft that spins and cores the drill is attached to the fist-shaped box.
The smaller box slides up and down about 6 in. or so inside the big box on two linear bearings. When the Mini-Corer drills into rock, the big box doesn''t move at all: the guts inside of it push the drill shaft into the rock. With a coring bit that''s 5 cm (about 2 in.) long and 8 mm in diameter, it takes a 1 in. core. It has two linear slide rails and one threaded acme screw, which moves the small box within the larger box. Two NB Corp SMS10WUU double-wide linear bearings guide the slide rails.
NB bushings are available not only in single widths but also double-wide and double-wide with flange. (Flanges come round, square, or two-side cut.) The double-wide bushings have shaft diameters that range from 3.175-50.8 mm, 0.125-2 in. Their standard width bushings come in lengths from 12.7-203.2 mm or 0.5-8 in.
"What If" Forces
To make the Mini-Corer lighter and stiffer, Honeybee chose the double-wide linear bearing because it provided more stiffness and more rigidity than they could achieve with two standard width bushings. Plus, if you put two bushings into a housing, there may still be ''give'' at the break, which is not a problem with the double-wide bushing.
"This robot is a coring tool to core rock and to take sample cores out of rock. In order to do that, we wanted to have a stable linear slide," notes Mike Maksymuk. "When we created the Mini-Corer, our engineers started researching places that would give us the highest quality precision slide rails. The forces and torques that the two NB Corp double-wide linear bearings can handle were enough that we didn''t need to look further. (Anyway, NB is the only manufacturer of the double-wide bushing.) That''s because, being a double linear slide, it gives us more distributed load. Imagine coming down on a rock with your drill moving forward without cutting rock perpendicular to its surface. That''s going to put a lot of side loads back into the tool, and, if the drill shaft flexes too much, it will not be able to drill the rock efficiently. We needed something really rigid to prevent the tool from flexing. Or, imagine the torques involved if you drill into rock and for some reason the drill gets stuck and these loads get transmitted back into the whole unit."
Autonomous Bit Changer
The Mini-Corer can change its own bits if they get dull. Honeybee figured out a way to incorporate bit changeout without actually adding any motors to the design -- the same mechanisms that primarily function as a coring tool were tweaked to allow the Mini-Corer to drop worn bits and put on new bits. The drill has been tested and fares well in harsh desert environments. Testing indicates that 8 mm carbide drill tips start to dull after drilling ten 30 mm cores into basalt with a compressive strength of over 100 Mpa.
The Mini-Corer can acquire, break, retain, and transfer consolidated and unconsolidated cores and changeout different end-effectors and bits. The Mini-Corer uses its bit changeout system to replace worn bits so it can maintain high coring and abrasion penetration rates. It is an arm-mounted, standalone device, requiring no additional arm actuation once positioned and preloaded. A compact internal low-mass transmission provides all the motor power used by the tool mechanisms.
Weighing less than 2.8 kg (6 lb) and measuring less than 270 mm (10.6 in.) x 112 mm (4.4 in.) x 100 mm (4 in.), the Mini-Corer will be expected to obtain two, 25 mm long and 8 mm diameter cores from a single hole of a strong rock. Surprisingly, the compact, low mass device can drill 25 mm into strong basalt in less than 6 minutes while consuming under 20 watt-hours of power. The Mini-Corer''s carbide cutting teeth require a minimum of torque, therefore keep the unit''s power needs low.
Hoping To Be There For Blast-Off
"In 2003," Tom Myrick says, "there were no problems with the rock abrasion tool on the Mars Lander, but there were problems with the rocket, so I actually missed the launch." Hopefully, when the Mini-Corer heads to Mars, Tom will have a front row seat at blast-off.