The Nomad Robot
Nomad is a four wheeled robot designed to traverse planetary analogous
terrain. Fully deployed, it is 2.4 x 2.4 x 2.4 meters, and it weighs 725
kg. It can travel up to 50 centimeters per second, and has the capability
to traverse over large obstacles. On this expedition, the robot will be
powered by a gasoline generator, will use studded tires for friction on
Antarctic ice, and determine its location using GPS (Global Positioning
System). Finally, Nomad serves as a sensing and computing platform that
allows effective remote science to be performed.
To find out about Nomad, click below:
Nomad's unique mechanical configuration is the result of extensive testing
and evaluation of wheel, chassis, steering, and body designs. Nomad used
metallic wheels with grousers to grip into the desert terrain on the Atacama
Desert Trek, but this year studded snow tires will be used to provide greater
friction. Nomad uses three other mechanical features to provide greater
mobility, stability, and control: a transforming
chassis, internal body averaging,
and in wheel propulsion.
Nomad features a transforming chassis
that can expand or compact by driving two pairs of four-bar linkages
with two electric motors, one on each side of the robot. Compacting, or
stowing, the wheels allows Nomad to fit within a 1.8 meter square. Nomadís
transforming chassis enables increased stability by allowing the robot
to deploy its wheels out to a 2.4 meter square footprint.
The transforming chassis enables skid steering as well as explicit steering
by using the same motors to deploy the wheels and affect wheel heading.
The transforming chassis is based on the motion of four bar linkages connected
to each wheel. The wheels are actuated in pairs such that the two right
wheels move synchronously (as do the two left wheels) to achieve double
Skid steering is used in vehicles with treads, such as bulldozers, and
involves differential wheel velocities for turning. The wheels are left
pointing straight; therefore, skid steering can be performed while Nomad
is in its stowed position. However, the wheels are not pointed, so skid
steering requires overcoming friction and slipping the wheels across the
surface. This involves a large amount of power to the wheel motors.
Less wheel power is required in explicit steering, which involves activating
both steering mechanisms to allow the robot to arc in a particular direction.
Nomad's explicit steering is like that of car except only a car's front
wheels actually steer. This method therefore gives the robot greater steering
control than skid steering.
Finally, the transforming chassis can move both steering mechanisms
past deployed mode to allow the robot to perform a point turn. This allows
Nomad to stop and change direction to avoid an obstacle.
Internal Body Averaging
In order to distribute the normal forces on the wheels, Nomad has two floating
side frames, called bogies. Each bogie is a structure that supports and
deploys two wheels (left or right). By allowing the side frames to pivot
on a central axle, the wheels can conform to uneven terrain and maintain
even ground pressure. In order to stabilize the sensors mounted to the
body, the two side frames are connected by means of a passive mechanical
mechanism, enclosed in the chassis above the central axle. The central
pivot of the averaging mechanism has a degree of freedom in the vertical
direction, which is needed to allow the link to follow the bogies through
a maximum wheel excursion of 50 cm. Body averaging of pitch and roll allows
Nomad to have greater mobility while maintaining a high level of stability
for accurate sensor readings.
In Wheel Propulsion
Nomad features individual propulsion drive units that reside inside the
wheel. This is unlike typical all-terrain vehicles, which have a central
drive unit that distributes power to each of the wheels. The advantages
of in-wheel propulsion include: sealed drive units, identical drive components,
simplicity, and improved motion control.
The in-wheel propulsion unit is independent of the steering and suspension
systems; no geometric or operational interferences occur between the systems.
No electromechanical components are needed for propulsion beyond those
enclosed in the wheel (with the exception of the motor wires, which are
routed to the body fuselage through the deployment/steering linkages).
This allows the drive components to be sealed within the wheel.
The motor and drivetrain assembly is at an offset distance below the
wheel axle, which lowers the center of gravity of the wheel and simplifies
its structural design and bearing selection. Triangular brackets suspend
the drive assembly from the stationary axle. The motor is accessible for
ease of removal and replacement if necessary. In the drive unit a brushless
DC motor transmits torque and power to the wheel hub through a harmonic
drive and a single stage gearing reduction. The output gear is mounted
on the inside face of the outward facing wheel hub.
Eliminating mechanical transmission components and coupling assemblies
encourages simplicity and thus reliability. Only two bearings are needed
to decouple the stationary wheel parts from the moving parts. The simplicity
of the propulsion system also imposes fewer constraints on the design of
the chassis and the steering mechanism.
Nomad will use three types of cameras, a laser rangefinder, and a spectrometer
to navigate terrain and perform remote science. These sensors are described
Nomad uses two pairs of two wide angle CCD cameras for navigation. These
camera pairs are spaced apart on Nomad's camera mast. Simulating eye position,
the cameras are set apart to provide two different images. An object in
one image is slightly displaced in the other. This is called disparity,
and it gives the robot depth perception. The robot creates a "disparity
map" of objects several meters ahead of it to warn of any potential obstacles
in its path. In Antarctica, this technique is experimental, as there is
very little texture on the bright white snow. However, in the event of
problems, the laser rangefinder (described below) will detect obstacles.
For more information about computer vision, check out CMU's
Computer Vision Home Page.
High Resolution Pan/Tilt Camera
This CCD camera, used to identify interesting rocks for classification,
is mounted on a pan/tilt unit next to the stereo cameras on the sensor
mast. The user will simply point the camera at a rock, zoom in, and take
a high resolution image. This image can be used to determine rock size,
color, and texture, characteristics necessary for autonomously classification.
Traditional cameras provide limited resolution and field of view for remote
human operators who are teleoperating robots.Nomad's
panoramic camera, mounted on top of Nomad, captures imagery 360 degrees
around the robot by combining a camera with a convex optical mirror. Spherical
images of a complete horizon provide operators and observers with wider
imagery for driving through and viewing planetary terrain.
This year, panoramic images will also be used to track objects surrounding
the robot in order to implement landmark
based navigation. In the future, the panoramic camera may also help
the robot autonomously select interesting rocks to examine.
A laser rangefinder mounted on the stereo camera mast is used by the navigation
system to detect obstacles in the robot's path. It scans several meters
in front of the robot to find features that the stereo cameras may miss.
Reflection spectroscopy is a powerful technique for deducing the chemistry
of a rock sample. The sample is lit by incandescent light, and the reflected
spectrum is recorded over a fiber optic cable. However, it is necessary
to get close to a sample in order that good quality spectra may be obtained.
This year, the spectrometer will be manually deployed. A sample therefore
requires a much larger amount of effort to acquire than a camera image.
The spectrometer projects light from a small Tungsten-halogen bulb through
a reflection probe onto the surface of a target. The same probe collects
the light and transmits it via optical fibers to the spectrometer. Inside
the spectrometer, a special diffraction grating spreads the light's spectrum
onto a CCD chip. The chip then generates electrical signals with strengths
proportional to the amount of light that hits its surface. This raw signal
is then passed through an analog to digital converter (ADC) that translates
it into information a computer can understand.
Raw signals are not very useful to scientists because sampling conditions
can vary. Therefore software is used to callibrate the spectrometer that
provides scientists and computers with standardized data regardless of
what sampling conditions.
Nomad is a powerful computing platform. Its size allows all necessary processing
to be performed on the robot. There are four computers on Nomad during
this expedition. Two PCs running Windows NT control the panoramic camera,
perform landmark based navigation, and run the autonomous classification
software. A third computer running Red Hat Linux coordinates robot navigation
and obstacle avoidance with the stereo cameras and the laser rangefinder.
Finally, a VME processor cage with a Motorola 68060 processor controls
Nomad's real-time processing, such as translation of driving commands into
servo motor movements and the monitoring of all systems on Nomad.
The Atacama Desert Trek 1997
Nomad navigated over 200 kilometers of the Atacama Desert in June and July
of 1997, proving that Nomad's design is capable of traversing terrain similar
to that of Mars and the Moon. The following accomplishments were also made
during the trek:
Autonomous and safeguarded navigation results are available.
Researchers in North America were able to perform science remotely with
the use of Nomad's sensors.
Planted meteorites were detected with magnetic / eddy current sensors on
Nomad performed over 50 kilometers of autonomous patterned searches.
Nomad's sensors allowed discovery of in situ meteorites.
Click here for the NASA
Ames Research Center Atacama Trek Home Page.
A few pictures from the Atacama Desert, showing its terrain and how Nomad's
mechanical design its traverse:
Valley of the Moon in the Atacama Desert
Atacama Desert, the driest place on Earth
during its autonomous traverse
executing a patterned search
traversing very uneven terrain
deploying magnetometer and metal detector
And some of the sensors Nomad used:
raw panoramic image taken in the Atacama Desert showing Nomad and surrounding
processed panoramic view of the desert surrounding the robot
Robotic Search for Antarctic Meteorites 1998
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Send comments, questions, or suggestions to Dimitrios
This document prepared by Michael