FEA Load Simulations

This document compiles the finite element analysis (FEA) setup variables, simulation load cases, and structural stress/deflection outcomes of the Smart Agri Four Legged Bot.

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1. FEA Setup & Material Properties

Simulations were performed using a solid tetrahedral mesh model in SolidWorks Simulation to represent both isotropic metals (mild steel) and anisotropic plastics (3D-printed PETG and PLA).

Material Input Variables

Mild Steel

• Used For: Central chassis base, structural rib reinforcement plates
• Yield Strength: 220–250 MPa
• Young’s Modulus: 210 GPa
• Poisson’s Ratio: 0.30

PETG (FDM)

• Used For: External covers, probe bracket, housing enclosures
• Yield Strength: 50 MPa
• Young’s Modulus: 2.1 GPa
• Poisson’s Ratio: 0.38

PLA+ (FDM)

• Used For: Leg joints, internal component base plate standoffs
• Yield Strength: 60 MPa
• Young’s Modulus: 3.2 GPa
• Poisson’s Ratio: 0.35

Rubber Pads

• Used For: Sensor mounts, vibration dampers
• Yield Strength: —
• Young’s Modulus: 0.01 GPa
• Poisson’s Ratio: 0.45

Mesh Density Metrics

• Element Type: SOLID187 (10-node quadratic tetrahedral element)
• Average Element Size: 5.0 mm
• Minimum Element Size: 1.0 mm (locally refined around bolt holes and corners)
• Average Aspect Ratio: < 3.5
• Mesh Quality: High (adaptive refinement active)

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2. Load Cases & Simulation Results

Three critical load cases representing the worst-case operating environments were simulated prior to fabrication.

graph LR Chassis["Chassis Assembly"] --> Case1["Case 1: Vertical Gravity Load<br>98N downwards"] Chassis --> Case2["Case 2: Lateral Bump Impact<br>30N side force"] Chassis --> Case3["Case 3: Soil Probe Insertion<br>20N vertical resist"] Case1 --> Res1["FOS: 5.17<br>Deflection: 0.313 mm"] Case2 --> Res2["FOS: 2.04<br>Deflection: 3.600 mm"] Case3 --> Res3["FOS: 14.40<br>Deflection: 0.780 mm"]

Case 1: Static Vertical Load

• Physical Representation: Chassis fully loaded with electronics (approx. 10 kg total weight) resting stationary.
• Boundary Conditions: Fixed constraints at all 4 shoulder joints; a 98 N vertical downward force distributed on the chassis deck.
• Simulation Metrics:
  • Maximum Von Mises Stress: 42.58 MPa
  • Maximum Deflection: 0.313 mm (located at the geometric center of the base plate)
  • Maximum Strain: $1.238 \times 10^{-4}$ (well within elastic boundaries)
  • Factor of Safety (FOS): 5.17

Figure 3.19: Von Mises stress distribution — static vertical load

Case 2: Dynamic Lateral Bump Load

• Physical Representation: A sudden bump or minor impact hitting one leg laterally during traversal.
• Boundary Conditions: Underside of the chassis fixed; a 30 N lateral side force applied directly to a leg mount.
• Simulation Metrics:
  • Maximum Von Mises Stress: 108.00 MPa (localized around the mounting flange bolt holes)
  • Maximum Deflection: 3.60 mm (deflected in the lateral axis)
  • Maximum Strain: $1.451 \times 10^{-4}$
  • Factor of Safety (FOS): 2.04

Strain Distribution

Figure 3.20: Strain under lateral load

Stress Concentration

Figure 3.21: Von Mises stress under lateral load

Bending Deflection

Figure 3.22: Displacement under lateral load

Case 3: Soil Probe Insertion Resistance

• Physical Representation: Downward reaction force exerted by hard-packed loamy soil on the PETG mount during probe docking.
• Boundary Conditions: Rear enclosure attachment point fixed; 20 N vertical downward force acting at the probe bracket.
• Simulation Metrics:
  • Maximum Von Mises Stress: 43.00 MPa (localized at the bracket neck)
  • Maximum Deflection: 0.78 mm
  • Maximum Strain: $3.844 \times 10^{-5}$
  • Factor of Safety (FOS): 14.40 (designed with a high safety margin to resist cyclic fatigue)

Stress Concentration

Figure 3.23: Local stress concentration at probe clamp slot

Deflection Gradient

Figure 3.24: Vertical displacement of probe bracket under load

Strain Distribution

Figure 3.25: Strain in probe bracket under load

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3. Simulation Feedback & Optimization

Based on the FEA results, several structural optimizations were identified to improve future design revisions:

Leg-to-frame flange

• Current Status: Single thickness plate
• Suggested Design Improvement: Add gusset plate or tapered transition to improve lateral bending resistance.

Probe mount neck

• Current Status: Flat profile bracket
• Suggested Design Improvement: Integrate a curved fillet/radius at the neck region to further distribute localized stress.

Base ribs

• Current Status: CNC-cut steel plate
• Suggested Design Improvement: Implement thickness reduction in non-critical zones to optimize overall weight.

Chassis cutouts

• Current Status: Decorative X-pattern cutouts
• Suggested Design Improvement: Add minor rounding to inner corners to lower localized stress spikes.