Mars Constraints for Designing a Better Moving Robot 50

Mars is a harsh environment; the vehicle that will operate in this environment will have to withstand the difference in temperature between –120ºC and 10ºC, high ultraviolet radiation and CO2 atmosphere, there is a high exposition to dust daily for a period depending on the mission that can range from 1-5 years. As seen in the last chapters mars rovers have been a wheeled design with low power consumption but high constraints:

• Movement in harsh terrain.

For the wheeled rovers the problems start with the landing site, which needs to be adequate for the rover movement capabilities. Due to the low capacity of the wheeled rovers to overcome obstacles, the obstacle height has to be less the half of the wheel radius REF.   42, it is required to find alternative routes which complicates the daily operations in Mars.

The legged rovers adapt better to any type of terrain with a higher capacity of overcome obstacles. This makes a lot easier to select the landing site and the future exploration of the zone. As the legged robots are intrinsically omni- directional systems they can change direction independently of the main body, they can move and orientate its body while maintaining the footholds.

The planning trajectory of the legged robot is easier because it is able of overcome obstacles just by stepping into them.

• Speed requirement.

This has never been the priority in a mission. Wheeled rovers “can be” faster than legged rovers, but as they have to plan such complex trajectories to avoid obstacles, the total time to cover one trajectory may be slower than a legged robot.

Wheeled robots require continuous and regular surfaces to move efficiently; when the terrain is more uneven the rover speed decreases rapidly. On the other hand legged robots do not require a regular or continuous terrain, they can move in a wide variety of terrains with the same efficiency.

• Very stable positioning.

During the scientific operation some very precise instrument will be handled, the legged rover will adapt better to the terrain reducing vibrations as much as possible, the ability to keep the rover deck with little inclination will make easier to the payload instruments to actuate on any terrain.

A legged robot provides intrinsically active suspension by adapting the leg lengths to terrain irregularities. A legged robot can cover highly irregular terrain with the body levelled; in contrast, the body of a wheeled robot is always parallel to the terrain and adopts similar tilts to the ground, in order to overcome this problem the system for a wheeled rover needs to be very complex.

• Power consumption.

Legged robots are power consuming. For the last missions and future missions to Mars RTG sources of energy have been used, minimizing the weight of the power subsystem and maximizing its longevity. The use of RTG generators is a mature technology to send to Mars and assures the operation of a legged rover. It is proven that legged systems under very irregular terrain are more efficient than wheeled systems, REF.  43.

• Minimum environment degradation.

The use of large rovers able to carry heavy payloads will require the use of big wheels for a wheeled rover configuration. This is not convenient due to the fact that all the sediment could be distorted.

The use of legged robots degrades less the terrain, due to the fewer contact area required.

Legged vehicles require discrete contact points with the ground, while wheeled rovers use a couple of continuous paths along the ground. Therefore, legged robots touch the ground less than traditional vehicles do, causing less environmental damage.

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• Mechanical robustness and viability

A robust design will be needed to survive this kind of mission. Conceptually a wheeled robot is more robust than a legged one but with a good design a similar grade of robustness could be achieved for a legged one.

This limitations and constraints leads to the idea that once the problems of power and mass has been solved, MSL rover weighed 500kg and works on a nuclear RTG generator, a better concept of rover will be presented in this thesis that will try to solve the problems of mobility in this harsh environment.

The next table summarizes the advantages of the legged robots vs. wheeled rovers.

Table    2  Advantages  of  legged  Vs  wheeled  robots.  REF.  44  

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7. WALKING  MACHINE  DESIGN  CRITERIA  

On this chapter is going to be presented the initial design configuration, which started with a moving table concept. This design evolved to a six leg walking machine. The requirements for the design of the hexapod robot and the possible actuator configurations will be explained along this chapter.

7.1. Moving  Table  Design  

Having reviewed the state of the art in autonomous robots for Earth and planetary missions I am ready to introduce here a first design that was thought best for the scientific goals, but later discarded.

Keeping in mind that the rover will be a platform for scientific purposes with no special speed requirements, the first option was a “moving table” concept design that has already been introduced. It consists of two platforms with four legs each, the legs can move independently and the platforms slide between each other. With this selection is possible to achieve a very stable scientific platform with low power requirements as it is shown In the next figure.

Figure  37  Moving  Table    

Due to the large scientific payload that is going to be carried the idea was to use this very stable platforms by sliding them with linear actuators, this configuration