5.2.5.1 Design & Ideation
This section traces the iterative design process that led to the final Coaxial Differential 2-DOF joint mechanism. The narrative follows the engineering methodology from initial motion requirements analysis, through three concept iterations, to the motor selection journey that culminated in the adoption of the Damiao DM-J4310-2EC integrated BLDC actuator. Each design decision is justified against the performance specifications established in the Arm Actuation overview.
Degrees of Motion Analysis
Industrial robotic arms are typically engineered with six or more degrees of freedom to achieve arbitrary end-effector positioning. However, for a boxing training application, such complexity is unnecessary and counter-productive: each additional actuator increases cost, weight, control complexity, and the number of potential failure points.
A systematic analysis of the required strike motions, derived from the padwork research documented in the Arm Actuation overview, identified that all target strikes (jab, hook, uppercut) can be decomposed into combinations of exactly two rotational degrees of freedom about the wrist joint. The first degree of freedom is pitch (vertical tilting), which produces the downward jab motion and the upward component of the uppercut. The second is roll (axial rotation), which produces the lateral sweeping motion required for the hook and the horizontal component of the uppercut.
This finding established a fundamental design constraint: the mechanism must provide precisely two motorised degrees of freedom. Any solution requiring more than two motors per arm was eliminated from further consideration.
Joint Concept Selection
Three candidate joint configurations were generated and evaluated against four weighted criteria: training fidelity, mechanical complexity, component cost, and user zone obstruction. The full concept sketches and scored decision matrix are presented in Appendix 1.
Concept 1: Multi-Actuator Rig
The first concept allocated a dedicated actuator for each individual strike type. Covering both left and right striking zones would require a minimum of six motors. While this approach theoretically maximises strike fidelity by permitting independent optimisation of each motion, the solution was deemed prohibitively complex, high-cost, and likely to physically obstruct the user's striking targets.
Concept 2: Windmill Actuator
The second concept consolidated two strike motions into a single motor by mounting training sticks on both ends of a rotating shaft (a "windmill" configuration). However, analysis revealed a critical fidelity failure: in the neutral position, the extended uppercut component would directly obstruct the liver strike zone, preventing the user from accessing a primary training target.
Concept 3: 2-DOF Coaxial Differential (Selected)
The selected concept employs a 2-Degrees-of-Freedom coaxial differential mechanism using only two motors per arm. By combining coordinated pitch and roll rotations, this single mechanism reproduces all required strike patterns from a compact form factor. The concept scored highest across all four evaluation criteria, offering the optimal balance of training fidelity, mechanical simplicity, cost-effectiveness, and minimal zone obstruction.
Proximal Motor Mounting — Avoiding Weight on Moving Parts
A critical design constraint for the coaxial configuration is that no motors or heavy components may be mounted on the moving arm itself. In a high-velocity striking mechanism, any additional mass on the rotating arm directly increases the torque required to achieve target punch speeds and amplifies the impact forces transmitted back through the 3D-printed drivetrain during deceleration.
The coaxial gear-rail architecture solves this by mounting both motors side-by-side at the stationary base of the arm assembly. Each motor drives a pinion gear into the central coaxial gear stack, transmitting torque to the arm without adding any weight to the moving structure. The result is an arm assembly composed only of lightweight 3D-printed gears, a training stick, and padding, keeping the rotating mass to a minimum and ensuring the mechanism can achieve the required strike velocities within the motor's torque envelope.
The closed-form kinematic equations governing motor-to-joint space conversion, including the software decoupling term for the bevel coupling effect, are derived in full in Drivetrain Logic (Section 5.2.5.2.2).
Interim Joint Design — 2-DOF Concept Validation
With the 2-DOF coaxial differential selected, the next step was to validate the concept through physical prototyping. Inspiration for the gear-rail implementation was drawn from a coaxial gear-rail concept (reference video). This approach implements the proximal motor mounting strategy: both motors are mounted side-by-side at the base of the assembly, each driving a pinion gear into a central coaxial gear stack, with a bevel gear converting the gear stack output into pitch axis rotation.
Using SolidWorks, a parametric 3D model was created and iteratively refined through multiple design revisions. Each iteration addressed assembly constraints that became apparent only after physical prototyping, demonstrating that geometric fit alone is insufficient; the method of connection and assembly sequence must be considered as a first-order design criterion for 3D-printed mechanisms.
Physical Validation
The final SolidWorks iteration was fabricated using PLA FDM printing. The physical prototype confirmed that the two degrees of freedom (pitch and roll) were sufficient to replicate all three target strike types (jab, hook, uppercut) when operated manually, validating the 2-DOF concept selection.
Post-Interim Design Review
During the interim presentation, the examining panel raised two critical concerns regarding the gear-rail mechanism: backlash and impact-induced damage. Both were addressed through targeted design modifications before proceeding to the final concept.
Backlash Mitigation
Backlash, defined as the angular play between mating gear teeth, introduces a dead zone during direction reversals, degrading positional repeatability. For a high-velocity striking mechanism where rapid bidirectional motion is fundamental, this represents a critical performance limitation. Three mitigation strategies were evaluated:
| Strategy | Mechanism | Trade-off |
|---|---|---|
| Reduced tooth module | Smaller teeth reduce inter-tooth clearance | Reduced load capacity; finer teeth are more fragile in PLA |
| Helical gears | Angled tooth profile provides continuous meshing pressure | Introduces axial thrust load requiring bearing support |
| Timing belts | Zero backlash by design; positive engagement | Requires precise tensioning; larger physical envelope |
The helical gear approach was selected for the final design as it offered the best balance of backlash reduction and structural compatibility with the existing coaxial form factor.
Impact Damage & the BLDC Motor Rationale
The predecessor Box Bunny prototype employed standard hobby servo motors for joint actuation. During testing, repeated sparring impacts transmitted directly through the drivetrain caused progressive wear and eventual fracture of the servo motors' internal reduction gears, rendering the prototype inoperable.
This failure mode established a critical requirement: the selected motor must be capable of absorbing external torque without mechanical damage. Brushless DC (BLDC) motors satisfy this requirement because they do not rely on internal reduction gears and can freely spin (back-drive) when external force is applied. However, the application simultaneously demands precise rotational position control with closed-loop feedback, capabilities that standard ESC-controlled BLDC motors do not provide. This dual requirement (impact resilience + position control) directly motivated the motor selection journey documented in Electrical Integration — Motor & Actuator Selection.