How the Ansys platform may be used to analyze designs of systems, specifically how Ansys Spaceclaim and Ansys Static Structural were used to criticize the excavator claw design’s load capacity and pressure tolerance.

1 Analysis Using Ansys Spaceclaim

Once a design for the excavation sub-system of the lunar rover prototype was determined and finalized, a model was created in Ansys Spaceclaim to perform an analysis of the design’s performance capacity. Ansys Spaceclaim is a 3-D computer-aided design software that can be used with other tools in the Ansys platform to conduct several design engineering processes.

As the excavator claw was modeled, Ansys extracted metrics such as the volume and surface area of the excavator claw design. The Figure below is a picture of the excavator claw and its corresponding volume. 

Ansys Spaceclaim determined the design’s volume to be 127,372.9 mm3 and surface area of 14,862.1 mm2. This data will be used to determine the lunar rover prototype’s excavator maximum load capacity and pressure tolerance. 

1.1 Calculate Excavator Design Maximum Load Capacity

A crucial aspect of the excavator claw design is the amount of regolith it can support per dig during excavation on the surface of the Moon. To prevent the design from failing during the lunar rover mission, the maximum capacity was calculated using the volume data found by Ansys Spaceclaim. This maximum capacity was calculated by modifying the density formula, equation 1 below, and resulted in the excavator maximum capacity equation, equation 2 below:

1. d = M / V
2. M = d * V

When:

• M = Maximum Mass of Excavator Load Capacity

• d = Density of the lunar regolith at 4% concentration >>> 1.79 g/cm3

• V = Volume of the Excavator Claw >>> 127.37 cm3

The density of the regolith was acquired from the NASA Break the Ice Challenge. The volume of the excavator claw was found by analyzing the design Ansys Spaceclaim, as mentioned previously.

Using this excavator's maximum capacity equation, a maximum load mass of 227.99 g was calculated for the excavator claw design. This is the maximum mass the excavator claw can support per each dig of regolith during excavation on the lunar surface. 

2 Analysis Using Ansys Static Structural

Once the excavator design was analyzed in Ansys Sapceclaim to determine the maximum capacity of the excavator claw, the durability of the excavator claw was then also analyzed using Ansys Static Structural. In Ansys Static Structural, materials and their properties were added to the excavator model from Ansys Spaceclaim. 

Ansys Static Structural supported the lunar rover prototype by providing data for calculations to determine the excavator's maximum pressure tolerance given varying load capacities. These load capacities were found using percentages of the maximum load capacity previously calculated. 

2.1 Select Excavator Design Material

An aluminum alloy material was applied to the excavator design using Ansys Static Structural. Aluminum alloy was selected because it is the most common material used on past lunar rover systems that have traveled to the surface of the Moon. 

Aluminum properties consist of: 

• Young Modulus of Aluminum Alloy: 71000 Pa

• Poisson's Ratio of Aluminum Alloy: 0.33

These values are user preferences and can always be modified to perform multiple scenarios of the pressure analysis that follows.

2.2 Determine Load Capacity Scenarios

Since the excavator claw will not always be digging at maximum capacity, varying load capacities were considered and used during pressure tolerance calculations. These varying load capacities were determined to be 80% of the maximum capacity for an optimistic load capacity and 60% of the maximum load capacity for a pessimistic load capacity. These two load capacities are:

• 80% Load Capacity = 182.39 g

• 60% Load Capacity = 136.74 g

2.3 Calculate Excavator Design Pressure Tolerance

The lunar surface has an atmospheric pressure of 2.28x10^-12 torr which is similar to a hard vacuum. This data was provided by the Break the Ice Challenge. Since this pressure cannot be replicated here on Earth, Ansys Static Structural was used to evaluate the excavator design in the proper environmental conditions. To find the excavator's pressure tolerance, the pressure equation, P = M * g * A, was used when:

• M = Maximum mass of excavator load capacity

• g = Gravity of the Moon

• A = Area of Excavator

The pressure equation was used with each varied load capacity, 80% and 60%, to calculate the tolerance of pressure that can be exerted on the excavator claw design.

80% Load Capacity 

60 % Load Capacity

 The pressure exerted on the excavator design at 80% load capacity was calculated to be 4.39 Pa, and the pressure exerted on the excavator design at 60% load capacity was calculated to be 3.29 Pa. 

2.4 Analyze Consequences of Pressure Impacts

After the pressure values were found, they were inputted in Ansys Static Structural to further analyze the excavator design. A fix support point was identified where the excavator claw is attached to the robotic arm on the lunar rover prototype. This fix support defines the correct boundary conditions as in the physical model. The purple shaded area in the Ansys Static Structural drawing below highlights this support point.  

A pressure area was also defined in Ansys as the bucket area of the excavator claw. This is where the excavator design will take on the most pressure during excavation on the lunar surface. The red shaded area in the Ansys drawing below highlights this area. The direction of the pressure is expressed as the black arrow on the red area. The amount of pressure exerted was also recorded in Ansys as 3,000 MPa.

2.5 Evaluate Concerns Related to Pressure Impacts

Some concerns considered before the final evaluation of the excavator began were:

• Normal Stress

• Elastic Strain

• Deformation 

Normal stress is a force that acts perpendicular to the surface of an object. In the excavator design, this is the digging motion and loading of regolith on the excavator claw during each excavation. Stress continues to build as the excavator continues filling the bucket area. 

Elastic strain is caused by forces that are parallel to the surface of an object or lie in planes or cross-sectional areas. In the excavator design, this occurs when the excavator is holding and maneuvering the load of excavated regolith from the excavator claw to be dumped in the lunar rover's storage unit. 

Deformation is the physical transformation of an object brought by forces like gravity, mass, and temperature. In the excavator design, this occurs depending on the duration of the mission. The Ansys analysis was used to determine the regions where deformation will occur. 

3 Excavator Design Analysis Results

Once all the needed data inputs were calculated and collected for Ansys Static Structural, the final analysis of the excavator claw design was performed.

4 Fatigue Analysis vs Excavation Cycle

The results in the table above are the structural analysis of the excavator design. Evaluating the structural analysis was done to determine the strength and stability of the design under actual loading conditions. The various pressure impacts, i.e. Strain, Stress, and Deformation, were also considered during the analysis.

These impacts will lead to the excavator claw’s fatigue during excavation. Fatigue was determined by calculating each excavation cycle in the lunar rover mission. An excavation cycle is the process of the excavator claw digging and collecting regolith until the storage unit reaches its maximum capacity. This is when the most pressure will be endured on the excavator claw, and repetitive cycles can cause it to fail. The fatigue analysis determined the maximum strain, stress, and deformation occurs after several excavation cycles, depending on the varied load capacity.

 Knowing when the failure of the excavator claw could occur by counting the number of excavation cycles is crucial to maintaining the lunar rover prototype. Preventative maintenance can be planned to replace the excavator claw and not delay the mission completion.