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Projects: Space InformationFrom the Moon Rover to the Mars Rover
by Alexander Kermurjian The following is reprinted from the July/August 1990 issue of The Planetary Report. Alexander Kermurjian is the Chief Designer of the Industrial Transport Institute. Dr. Kermurjian has worked in the space program since 1963, when his institute was assigned the task of building the chassis for the Lunokhods, the robotic rovers that explored the Moon during the early 1970s. In 1990 the Lunokhod or Moon Rover is 20 years old. This
paper commemorates its anniversary. [Lunokhod I landed on the
Moon November 17, 1970; its mission lasted 11 months.] The Lunokhod was made up of an air-tight compartment containing equipment and a self-propelled undercarriage. It had a temperature-control system with an isotope heat source, a radio and television transmitter, a command receiver, a power plant with solar and back-up storage batteries, a remote control, a small-frame television, panoramic telophotometers [facsimile cameras, which compile an image by scanning back and forth] and a complex of scientific instruments. The self-propelled undercarriage gave mobility to this planetary rover. This undercarriage consisted of wheels, electric motors, suspension, automatic motion-control, a complex of onboard sensors to monitor the assemblies and systems, and a device to transfer data to the telemetry system. The undercarriage had eight rigid drive-wheels with perforated, cleated rims. The wheels did not swivel, so the rover was turned by imparting different velocities to the left or right side. The Lunokhod's total mass was 750 kilograms [1,650 pounds], with the undercarriage weighing 105 kilograms [230 pounds]. Its speed was 0.8 to 2 kilometers per hour [0.5 to 1.2 miles per hour].
As part of the undercarriage, the Lunokhod (Figure 1) carried the PROP (a Russian acronym for Surface Evaluation Instrument). Although it was a scientific instrument, the PROP (Figure 2) was also used as part of the safety system to determine the degree of wheel slippage and to forecast further motion [in response to commands.] The PROP system consisted of a free-rolling wheel (the "ninth wheel") and a penetrometer. The ninth wheel had spikes on its rim so that it could roll without slipping. The number of its revolutions provided information on the distance traveled. This wheel was attached to a lever so it could move up and down freely. Its vertical movement provided information on the unevenness of the terrain that the Lunokhod covered. Comparing the number of revolutions of the ninth wheel and the drive wheels (which were also counted) made it possible to calculate how much the rover slipped. Measuring simultaneously the slippage, the forces acting on the wheels and the average angle of the surface slope made it possible to determine the characteristics of the soil along Lunokhod's route. The penetrometer had a conical pressure-tool. During a measurement, the tool was implanted in the soil and turned 180 degrees. At the same time, the forces needed to penetrate the surface and to turn the instrument were measured. Lunokhod 2 landed on the Moon on January 16, 1973. The two Lunokhods operated for a total of 414 days, logged about 50 kilometers [30 miles], took several hundred panoramas and over 100,000 photographs of the Moon using the small-frame television. The special equipment on the self-propelled undercarriage, which itself acted as an independent scientific instrument, provided scientific and engineering data. For example, it has helped determine:
Creating the LunokhodIn creating the Lunokhod, many difficult scientific and technological problems had to be overcome. They must also be dealt with in developing the Marsokhod, or Mars Rover. There was no reliable information about the lunar surface until the Soviet Luna 9 madw the first soft landing on the Moon on February 3, 1966. This showed that the soil on the surface is sufficiently firm to support a spacecraft. The first panoramas made it possible for the soil structure and distribution of small stones to be evaluated. Special instruments on Luna 13 (a soil penetrometer and a radiation denisimeter) measured the bearing strength and density of the lunar soil. After that, designing the Lunokhod took a surer course and lunar-soil models were used for tests on Earth. For the Marsokhod the problem differs. The martian surface is known to have rock outcrops and soft sands. Gravity substantially affects the rover's cross-country capability and the stability and lubrication of its parts. This creates a problem in testing equipment on Earth since it is necessary to preserve the relationship between the mass and weight of the planetary rover. [On the Moon, a rover weighs only one-sixth as much as on Earth; on Mars it weighs a bit more than one-third.] In tests, terrains were chosen that mimicked the surfaces of the Moon and Mars. The equipment was built to allow simulation of the gravity of various planets and celestial bodies.
Closely associated with the problems of soil and gravity is the problem of choosing a means of locomotion. Experience in operating the Lunokhods, as well as differences in the conditions among the planets and their satellites, necessitated a search for better means of motion. The searches proceeded in the direction of improving the wheels (their shapes, dimensions, tread patterns and rigidity), trying a caterpillar-type mover (Figure 4) and trying other configurations. For [the martian moon] Phobos, because of its ultralow gravity) one two-thousandth that of Earth), hopping was found to be the best means of movement. Figure 5 shows the PROP-F (the Russian acronym for Mobile Robot for Evaluation of the Surface of Phobos) robotic spacecraft designed to travel over Phobos' surface. This "hopper" was installed on the Phobos 2 spacecraft. The spacecraft mass was 40 kilograms [90 ponds]. It was completely self-sufficient with its own power supply, radio transmitter and receiver, event programmer [a timer that turns components on and off], and an array of scientific instruments. The spacecraft was to move by hops 10 to 40 meters long. After each hop it would roll over into its operating position and perform its experiments. Data were to be transmitted to the Phobos 2 spacecraft for return to Earth. Unfortunately, the PROP-F hopper was not destined to operate on Phobos. [Contact with Phobos 2 was lost March 27, 1989 after it had reached the neighborhood of Phobos but before it had deployed the PROP-F. Phobos 1 had failed en route to Mars.] The search continues toward creating a walking rover for Mars. Figure 6 shows one of the mockups of a robot that walks. An apparatus that was to walk using skis (Figure 7) has been on the surface of Mars since 1971. This small rover was delivered to Mars by the Mars 2 and 3 spacecraft. [Mars 2 crash-landed on the planet on November 27, 1971. On December 2, Mars 3 was successfully deployed on the surface, but after 20 seconds of communications, transmissions ceased.] Installed on these walkers were dynamic penetrometers and radiation densimeters. They were to communicate via cable with the landing station. The walkers' distance traveled was intended to be 15 meters. They carried sensors and an automatic control system to enable them to move around objects. For the conditions on Mars, the "wheel-walking" mode of motion provides the best cross-country capability. A distinguishing feature of this mode is that the wheel can both roll and walk at the same time. Figure 8 shows the first mock-up of a wheel-walking robot. This vehicle is capable of going up a friable soil slope of 30 to 32 degrees, which would be inaccessible for a wheeled or tracked vehicle. When moving along a hillside, this vehicle will not slip as much as other types. A Mars rover of this design is also capable of changing the position of its body to accommodate the relief of the terrain (Figure 9). Control is one of the main problems in creating a Marsokhod. The problem of remote control also existed for the Lunokhods. A delay of three seconds [the round-trip light time between Earth and the Moon, see box below] in transmitting commands and receiving a response, in addition to the complexity of determining the sizes of surface features and distances to them required nonstandard modes of driving. The "crew" [that was to control the Lunokhods from Earth] had to acquire new habits. The Lunokhod crew was carefully trained in simulated conditions, which took account of the psychological and physiological characteristics of every member. As a result, the crew was able to remotely control the Lunokhods, and they avoided accidents. For Mars, this problem of delay between sending commands and receiving responses greatly increases. A signal can take 5 to 40 minutes to travel to and from Mars. [Distances between Earth and Mars change as the planets travel their different orbits about the Sun.] So it is not possible to control a Mars rover in the same manner that the Moon rovers were controlled. A Mars rover must be more independent. Controlling a robotic Mars rover presents difficulties, the principal one being limited or nonexistent information on the rover's situation. In this connection, the capabilities of the Marsokhod's information system are increasingly important. It must also be able to determine distance and to sense its surrounding by "touch." It is probable that the Marsokhod will be capable of independent motion and also of being remotely controlled from Earth by specifying a path over a certain time and distance. The navigation system has to be able to hold a heading to a finish point. Obviously, the Marsokhod must have a sufficiently developed onboard "brain" for data processing, decision making and command transfer in accordance with specific programs. To test these abilities an experimental prototype of a planetary roving robot has been developed and tested. It carries a laser-rangefinder technical-vision system (LTVS). [Technical vision produces an image that includes range and dimensional fidelity.] The robot also has an information and control complex built around a two-processor computer, a course-indication system and a displacement meter [odometer]. In motion tests, the robot traveled from a start point to a finish point. Motion was planned by processing the information from the LTVS to determine a possible path. The shortest paths were chosen. The control problem is closely related to that of the cross-country capability. A large safety margin in the rover's cross-country capability enhances the reliability of motion in severe conditions. This may decrease the number of maneuvers while reducing demands on the required resolution of the technical vision. Important to the efficient control of the rover's motion is its ability to maneuver. A possible configuration for a self-propelled undercarriage is one in which all wheels turn together at a specified angle. In this case, the rover may move transversely at an angle to its initial course without changing its body orientation. There are other problems, such as the wind, temperature, composition of the atmosphere and its low density. Modern Concept of the Marsokhod
The greatest cross-country capability can be ensured if the rover is a wheel-walker with a three-part configuration and a hinged frame. Such a rover has practically no road clearance. This is achieved by using conical wheels (Figure 10) that provide a continuous support surface for the rover, thus ensuring a cross-country capability for terrains full of obstacles and ruling out the rover's getting stuck on a high center obstacle. The hinged frame and a special drive for folding or raising the sections enable it to overcome obstacles whose height is twice the wheels' diameter. For overcoming small crevasses, the sections can clamp together to form a rigid frame. The sections can move alternately to enable the wheel-walker to creep up friable soil slopes with angles of 33 to 35 degrees. Such a Mars rover might have the specifications listed in Table A.
The many problems that might be solved using automated rovers as well as the inevitable restrictions [size, mass, power] on their placement on an interplanetary spacecraft suggest that it is expedient to create small mobile apparatuses (SMA). Such apparatuses may be used for the study of planetary surfaces with scientific instruments and or reconnaissance of landing sites. An SMA could also be used as a mobile radio beacon for landing the principal descent capsule and for exploration of hard-to-reach spots, where it could sample the soil and deliver the samples to a rocket for return to Earth. As an example, Figure 11 shows a full-scale mock-up created from the concepts for the planetary rover described above. Table B gives its possible specifications.
Human thoughts cannot stop. New thoughts, new ideas arise. Therefore, it would be no surprise if the Mars rover that ultimately appears bears little resemblance to what is presented in this paper. But today, we see it this way. Also from this issue: Light-Time and Robots: Communication Across the Solar System, by James D. Burke The First Rover on Mars: The Soviets Did It in 1971, by Charlene Anderson |
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