3 Structure/Material

3.1 Structure

Due to the unpredictable terrain of the moon, a flexible rover is proposed. This allows for freedom of movement over a wide range of obstacles. The rover must also be strong enough to support itself and its payload while being as lightweight as possible. These and other factors helped to determine the best design for the structure of the rover.

3.1.1 Frame

In the initial design stages of the frame, two factors dominated the process: flexibility and simplicity. Flexibility is the basis for a versatile rover. Spending time and power to avoid minor obstacles could be detrimental to an experiment. Designing a very complex system for the frame could also prove detrimental. A system with minimal complexity and minimal number of parts reduces the probability of encountering unforeseen problems, and possible failure.

To maintain flexibility and allow for complete isolation of all critical instruments and components, the frame depicted in Figure 3.1 was chosen. It consists of a rectangular frame made of square tubing separated into three equal sections. The center section is enclosed on the bottom by a thin metal plate which helps provide the necessary support for the main body of the rover. The sections are hinged and supported by small torsional springs. These springs allow for a small amount of deflection at either end of the frame and bring an added damping force to the design.

The legs are constructed of rectangular tubing approximately nine centimeters in length. The tubing encloses the motor wires as well as the steering assembly discussed in a later section. Torsion springs also hinge the legs to the frame. These springs provide the main damping force of the suspension. The size and strength of the springs are undetermined at this time.

3.1.2 Main Body

The main body of the rover is rigidly attached to the frame along the center section, leaving the outer sections free to deflect. The main body is divided into smaller sections for isolating internal components. These small sections allow for the components to be maintained at different temperatures without interaction. The construction of all inner components is discussed in later sections. The battery array will be located in the front section of the body. Solar panels cover the top surface of the body. An Alpha Proton X-Ray Spectrometer (APXS) is located in the back section, attached to a small rack and pinion device that will allow the APXS to extend outward for various scientific experiments. All other devices are arranged as close to the center section as possible to ensure their safety from external forces. A collection of peltier devices will be attached to the underside of the box to aid in heat dissipation.

3.2 Material

3.2.1 Frame

Several materials were considered for use in the frame. Several materials were ruled out early in the design due to the fact that the overall loading does not demand a material with high strength characteristics. Low density, heat resistance and resistance to corrosion were the three primary factors in the selection of the proper material. Other factors such as strength and stiffness were also included in the selection process. Several materials were considered, including 356.0( or 413.0) aluminum alloy, ASTM-A36 structural steel, EZ33A magnesium alloy, titanium alloy and several engineering thermoplastics such as Polyphenylene Sulfide (PPS) and Phenolic. All relevant material properties are listed in Table 3.1.

Due to the density of steel, it was not considered for use. Magnesium alloys have many disadvantages which limit their use. They are difficult to cast because in the molten state they burn in air and thus fluxes must be used during casting. Also, magnesium alloys have poor resistance to creep, fatigue and wear.(Smith, 498) Dispite their drawbacks, the low density of magnesium alloys (1.74 g/cm3) and good heat conduction characteristics makes them a strong candidate. Aluminum alloys offer many appealing qualities. Their low density, good corrosion resistance and good heat conductivity make them an excellent candidate for use. Titanium alloys offer great strength with relatively low density, however, due to the cost and difficulty in machining titanium, these alloys were not considered further. The engineering thermoplastics inherently have insulative properties. PPS can be manufactured to have high conductive qualities, but the actual value of conductivity could not be determined. If PPS has a heat conduction coefficient similar to that of aluminum, it would be a valid candidate. Its density (1.34 g/cm3) is less than half that of aluminum (2.71 g/cm3) and 0.4 g/cm3 less than that of magnesium (1.74 g/cm3) while still maintaining adequate mechanical properties.

Table 3.1 - Materials Considered For Frame
Material Alloy Number Density
(g/cm3)
Yield Strength
(MPa)
Thermal Conductivity
(W/m K)
aluminum alloy 356.0-T6 2.71 152.0 175.0

After compiling the materials into a decision matrix, magnesium alloy was determined to be the best material (See Table 3.2). Its relatively high heat transfer coefficient combined with its low density (lowest of any metal considered) and adequate yield strength made it the obvious initial choice. Due to its poor resistance to creep, fatigue and wear, this material would require further study to determine its reliability under the given conditions.

Table 3.2 - Overall Rating of Materials
Material Alloy Number Yield Strength
Density (a=25%)
Heat Transfer Coeff
Density (a=75%)
Overall Rating
G=alR1+2R2
a1+a2
aluminum alloy 356.0-T6 56.0 1.00 64.6 0.72 0.790

All materials considered for use in construction of the frame were also considered for use in the legs and wheels. The material selected for the frame is also recommended for the legs and wheels, since the desired properties are identical. Referring to Table 3.2 for the overall ratings of each considered material, it can be seen that the magnesium alloy is the best choice of material for the legs and wheels as well. As was stated before, extensive testing is required to determine its reliability.

3.2.2 Main Body

Thermoplastics offer many attractive qualities for the design of the main body. Two types of thermoplastics have been considered: Polyphenylene Sulfide(PPS) and Phenolic. Both materials were discussed briefly in the previous section. Both have good heat resistance, mechanical properties and insulative properties. These thermoplastic materials maintain their strength to a temperature of approximately 260o C.(Smith, 319,332) Isolation of the inner components is an important issue. The properties of these materials provide insulation as well as a supporting structure. The density of PPS would be an asset. The main factor to consider in using PPS is its low yield strength. If the stresses applied are within its yield strength, the magnesium alloy might be a better choice.



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Last Modified: Fri Dec 18, 1998
CSR/TSGC Team Web