The Nomad Robot

Overview

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:

Mechanical Design

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.

Transforming Chassis

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. Nomad’s 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 Ackerman steering.

Steering

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.

Sensors

Nomad will use three types of cameras, a laser rangefinder, and a spectrometer to navigate terrain and perform remote science. These sensors are described below.

Cameras

Stereo Cameras

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.

Panoramic Camera

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.

Laser Rangefinder

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.

Spectrometer

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.

Computing

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.
Click here for the NASA Ames Research Center Atacama Trek Home Page.

Photo Gallery

A few pictures from the Atacama Desert, showing its terrain and how Nomad's mechanical design its traverse:
 
The Valley of the Moon in the Atacama Desert The Atacama Desert, the driest place on Earth Nomad during its autonomous traverse
Nomad executing a patterned search Nomad traversing very uneven terrain Nomad deploying magnetometer and metal detector
 
And some of the sensors Nomad used:
 
A raw panoramic image taken in the Atacama Desert showing Nomad and surrounding terrain The 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 Apostolopoulos.
This document prepared by Michael Wagner.