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In the forty years of Solar System
exploration with unmanned probes, many of the missions that landed on
the surface of other celestial bodies carried manipulators. Two
different typologies of manipulators can be distinguished:
To the first kind belong the arms carried on the Moon by Luna-13, on
Mars by Mars-2, -3 and -6 and on Venus by Venera-13 and -14.
Luna-13 was an E-6 type lunar lander
carrying two manipulators actuated by torsion springs, each of which
had six degrees of freedom. The first arm carried a small quantity of
Cesium-137 to study the density of the terrain, while the second
carried a wedge shaped penetrometer. Mars-2, -3 and -6 carried the PrOP-M microrover on their six degrees of freedom
booms. Venera-13 and -14 carried a two degrees of freedom spring loaded
manipulator carrying a penetrometer.
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Of course, manipulators of the second kind, with actuation and control system are much more interesting. This kind of manipulators were flown on the lunar Surveyor-3, -4 and -7 probes, on Luna-15, -16, -18 and -20, on the two Viking Mars landers and on the ill-fated Mars Polar Lander (MPL).
The robotic arms mounted on the american Surveyor probes (known as SMSS, Soil Mechanics Surface Sampler), which landed on the moon in 1967 and 1968 were the most complex ones for what concerns the mechanical configuration. They consisted of a pantograph arm (of varying length) with three degrees of freedom: azimuth (112 degrees total angular movement), elevation (42 degrees total) and of course length (from a minimum of 58 to a maximum of 150 cm). The "end effector" of this robotic arm was a soil sampler with fixed orientation. A fourth actuator commanded the opening or closing of the sampler. This robotic arm was used to dig trenches, test the density of the soil, weight rocks, break pebbles to measure their mechanical characteristics and to move the alpha ray spectrometer from one position to another during the Surveyor-7 mission.
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The Soviet E-8-5 probes named Luna-15,
-16, -18 and -20 (1969-1972) all carried on the Moon a robotic arm
whose mission was to bring an hollow drill in direct contact with the
soil to collect samples and then to move the samples from the sampling
area to an Earth return capsule. The manipulator mounted on these
missions had two degrees of freedom: elevation (100 degrees of angular
excursion) and wrist rotation (180 degrees).
The arm was carried to the Moon in a stored position alongside the body
of the probe. Once on the surface, the arm was moved to a vertical
configuration and the wrist was rotated 180 degrees. The drill was then
lowered to the surface. The whole manoeuvre took 17.5 minutes. The wrist
apparently could be used to clear the drilling area of small rocks by
moving it some degree clockwise and anticlockwise of the reference
position. Because of the simplicity of the arm, the system could only
collect samples from a well defined position with respect to the probe.
The sample unloading sequence was the exact reverse of the arm
deploying one. Luna-15 and -18 failed, while Luna-16 returned 101 g of
lunar soil to Earth and Luna-20 some 30 g from the lunar highlands. The
later Luna-23 and -24 missions carried a completely redesigned sample
collecting system using a rail mounted 2 meters long drill instead of
the robotic arm.
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Below: The deploying sequence of the E-8-5 robotic arm from the stowed position to the sample drilling position |
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The two Viking probes, which landed on
Mars in 1976 all carried a robotic arm to collect samples from the
terrain and to hand them to various scientific instruments. The
manipulator had four degrees of freedom: azimuth (120 degrees of
angular excursion), elevation, wrist rotation and length. The arm was
made of telescoping sections, enabling it to vary its length to a
maximum of three meters. The end effector was a small sampler using
some simple instruments to measure the mechanical characteristics of
the soil and to verify the presence of iron using small magnets.
The manipulator was extended to collect samples and then retracted and
positioned over one of the many scientific instruments of the probe.
Sample unloading was accomplished by rotating the sampler wrist by 180
degrees. The Viking robot arm was a critical component for the mission
since it was to hand over samples to the most important scientific
instruments for analysis (five instruments, including the three
dedicated to organic life detection).
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The Mars Polar Lander (MPL) probe, lost while landing in december
1999 carried the most complex robotic manipulator ever flown on a
planetary mission. The main characteristics of this robotic arm were:
Many samples collected by the manipulator were then handed to an
instrument designed to find traces of water.
The manipulator manoeuvres were extremely slow (tenths of degrees per
second) to minimize the size of the actuators and the energy
requirements. Depending on the posture, the arm was able to reach a
cartesian end effector speed of up to 7 cm/s and to exercise a 80 N
force.
Beside an "operational" role, the MPL robotic arm was conceived as a
real scientific instrument, for it could be used to perform mechanical
soil characterization.
A very similar manipulator to this one was to have been carried on the
canceled Mars Surveyor 2001 Lander.
In this case, the possible end effector included a sample collector, a
rake, an electrometer to measure triboelectric charges caused by the
rubbing of the collector against the Martian surface and to measure
ionization of the atmosphere and an hook to move a small rover similar
to Sojourner from its flight position on top of
the lander to the surface. The rover deploying manoeuvre was considered
very critical and was to have been preceded by a 3D photographic survey
of the terrain on which the rover would "land". The manoeuvre was to
have been accomplished by lowering the rover 3 cm at a time until the
manipulator would have signaled its "unloaded" status.
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Right: The kinematic configuration of the MPL robotic arm |
Another manipulator is mounted the Beagle 2 European Mars Lander. The robotic arm weighs 2.64 kg and is designed to bring many scientific instruments in contact with the ground, including the sample collecting "mole".
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An image of the British Beagle II Mars
lander showing the robotic arm. Image Copyright Paolo Ulivi |
And recently, China has announced to be studying a robotic manipulator for collecting samples on the Moon. The analysis of this statement is on my Chinese planetary program page.
Finally, a very complex manipulator was to have flown on NASA's Mars Sample Return mission in 2003.
This was to have been built in Italy and was a four degree of freedom
manipulator, just like the Mars Polar Lander One. As an end effector,
however, the manipulator was to have carried a drill box, inside which
there was a 50 cm long drill and sample retriever, the drill rotation
and translation electric motors, some scientific instrument and a
carousel cache for sample storage. The arm was abel to hand its samples
to either the MAV (Mars Ascent Vehicle) rocket for Earth return or to
scientific instruments part of the ASE (Additional Science Envelope)
and IPSE (Italian Package for Science Experiment) packages. The total
weight of the manipulator was approximately 15 kg, most of which was
made up by the drill box. Unfortunately, the mission was canceled after
the loss of Mars Polar Lander.
The MSR probe, showing the four degrees of freedom robotic manipulator and drill box in dark brown.
BIBLIOGRAPHY
Beagle 2:
Robotic Arm
Bonitz, R.G., Nguyen, T.T., Kim, W.S.: The Mars Surveyor '01 Rover
and Robotic Arm
Bonitz, R.G.: Mars Surveyor '98 Lander MVACS Robotic Arm Control
System Design Concepts, Proceedings of the IEEE International
Conference on Robotics and Automation, 1997, pp. 2465-2470
Bonitz, R.G:: MVACS Robotic Arm. Available on line
Ezell, E. C., Ezell. L. N. "On Mars", NASA SP-4212, Available
on-line
Wilson, A.: Solar System Log, Jane's, London
Ulivi, P.: Studio di Ottimizzazione ad Energia Minima per un
Manipolatore per l'Esplorazione di Marte. Master Thesis, Politecnico
di Milano, 2000
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