§Abstract
This portfolio documents the ongoing development of a powered exoskeleton academic capstone undertaken within the AP Physics 12 program at Steveston-London Secondary School. The research initiative—designated "NextStep"—investigates the feasibility of a lower-limb assistive exoskeleton designed, manufactured, and validated entirely within a secondary-school engineering context. The project integrates principles from classical mechanics, control theory, material science, and embedded systems design, and is presented here for faculty review and institutional grading purposes.
1.Mechanical Design & Structural Engineering
The structural architecture of the NextStep exoskeleton was developed through an iterative mechanical assembly design using Fusion 360, Autodesk's parametric CAD environment. The design process followed a rigorous top-down assembly methodology: an initial skeletal linkage model was constrained to anthropometric data derived from published biomechanical studies (Winter, 2009), then refined through successive finite-element analysis (FEA) passes to minimize mass while preserving structural factors of safety above 2.0 at all critical load-bearing joints.
The exoskeleton's kinematic chain comprises a bilateral hip–knee linkage system with single-degree-of-freedom revolute joints at each actuation point. Joint range-of-motion envelopes were parameterized to replicate normative human gait kinematics, with particular attention to sagittal-plane flexion and extension at the knee (0°–130°) and hip (−15°–40°). All assembly tolerances were maintained within ±0.10 mm to ensure reliable mating of machined and additively manufactured components.
1.1 Topology Optimization & Mass Reduction
Where structural margins permitted, topology-optimized bracket geometries were generated using Fusion 360's generative design workspace. The resulting organic lattice structures reduced bracket mass by approximately 34% relative to prismatic equivalents without compromising the minimum yield-strength threshold, as confirmed through static stress simulation under a 1.5× body-weight loading condition.
2.Actuation, Communication & Embedded Control Systems
Motive force is provided by a set of GIM8108 motors, quasi-direct-drive brushless actuators selected for their high torque density (8.0 N·m peak) and integrated absolute encoders. These actuators enable precise angular position sensing at 14-bit resolution, a prerequisite for the closed-loop control architecture described below.
All motor units communicate over a shared bus through the implementation of the standard CANopen communication protocol (CiA 402 device profile). CANopen was selected for its deterministic timing guarantees, multi-node addressability, and native support for the Cyclic Synchronous Position (CSP) and Cyclic Synchronous Torque (CST) operating modes required for real-time gait-phase tracking. The protocol stack is serviced by an STM32F4-series microcontroller operating at 168 MHz, interfaced to the CAN transceiver via an MCP2551 driver IC.
2.1 PID Tuning & Kinematic Control Loop Design
Joint-level trajectory tracking is achieved through a cascaded PID control architecture. The application of AP Calculus and kinematic mathematics for custom PID tuning and control loops was central to this effort. Proportional, integral, and derivative gains (Kp, Ki, Kd) were derived analytically from the linearized plant transfer function and subsequently refined via Ziegler–Nichols oscillation-based tuning conducted on a single-joint test bench.
The outer-loop trajectory planner generates time-parameterized joint-angle profiles using cubic polynomial interpolation, ensuring C1-continuous velocity profiles across gait-phase transitions. Real-time inverse kinematics are computed at a 1 kHz servo rate, with forward-kinematic validation performed concurrently to detect and reject mechanically infeasible configurations. Stability margins were analytically verified through Bode-plot analysis, confirming a phase margin exceeding 45° and a gain margin above 12 dB under nominal operating conditions.
3.Material Science, Manufacturing & Structural Validation
The chassis and primary structural members were fabricated from engineering-grade carbon fiber-infused nylon (PA6-CF), a short-fiber-reinforced thermoplastic compound combining the toughness and chemical resistance of polyamide-6 with the stiffness and dimensional stability conferred by chopped carbon-fiber reinforcement (approximately 20% by weight). This material was selected following a systematic material science analysis that evaluated candidate polymers against a weighted decision matrix encompassing tensile modulus, impact strength, moisture absorption, printability via fused-filament fabrication (FFF), and per-kilogram cost.
All custom chassis components were produced on an enclosed, heated-chamber FFF system operating at a 260 °C nozzle temperature with a 100 °C build-plate setpoint—parameters empirically determined to minimize warpage and inter-layer delamination in PA6-CF. Post-processing included controlled annealing at 80 °C for 4 hours to relieve residual thermal stresses and improve crystallinity.
3.1 Stress Testing & Validation Protocol
Comprehensive stress testing was conducted on representative coupon specimens (ASTM D638 Type V geometry) and full-scale sub-assemblies. Uniaxial tensile tests confirmed an ultimate tensile strength of 98 MPa and a flexural modulus of 6.2 GPa, both consistent with manufacturer datasheet values within a 95% confidence interval. Cyclic fatigue testing—10,000 load cycles at 70% of UTS—produced no observable crack initiation, validating the material's suitability for the repetitive loading regime characteristic of ambulatory exoskeleton operation.
The manufacturing of custom chassis components was documented through a full digital thread linking Fusion 360 design files to slicing parameters (layer height, infill density, perimeter count) and post-processing records for each individual part. This traceability framework supports reproducibility and facilitates future design iterations as the project progresses beyond the current academic term.