Back to Rotation Subsystem

Design Ideation

The ideation process for the rotation subsystem was not treated as a single component-selection exercise. Instead, it progressed through a sequence of design questions, where each stage of the yaw system was compared against the most important needs of BoxBunny: realistic re-angling behaviour, positional stiffness under punching, compact packaging, manufacturability at prototype scale, and practical integration with the wider lower mechanism. To make those trade-offs explicit, selection matrices were used as a decision-support tool.

In this subsystem, the role of the selection matrix was not simply to identify the "best" component numerically. Rather, it was used to structure the progression from system-level motion architecture down to bearing selection, then to drive-transmission strategy, and finally to supporting refinements for stability and deployment. Each matrix narrowed the solution space and informed the next stage of design refinement.

V-Model Traceability: This page documents the concept-selection rationale that satisfies RM-4 (Yaw rotation ≥ 150°/s for rapid re-angling) and RM-1 (Structural stability under punching loads). Each selection matrix records why a specific concept was chosen, providing an auditable link from the system requirement to the physical realisation in Mechanical Design.

1. Rotary Support / Bearing Concept

Concept Combined axial/radial/moment capacity Rotational stiffness Availability / cost Ease of integration Suitability for punching environment Final decision
Lazy-Susan / turntable bearing Low Low High High Poor Rejected
Cross-roller bearing Excellent Excellent Poor Moderate High Not selected
Four-point contact slewing ring High High High High High Selected

This matrix was used to decide the type of rotary support for the yaw stage. The lazy-Susan option was rejected because it is too loose and too weak in overturning resistance. A cross-roller bearing was attractive in stiffness but less favourable in cost and accessibility. The four-point contact slewing ring was selected because it provides the best balance of structural performance, prototype practicality, and appropriate stiffness for the punching environment.

2. Geared vs Non-Geared Slewing-Ring Strategy

Concept Structural capacity Mass Cost / shipping Ease of handling Motor power demand Design appropriateness Final decision
Geared slewing ring (011.25.400) Excellent Very poor Very poor Poor Poor Poor Rejected
Non-geared slewing ring (010.10.120) Adequate High High High High High Selected

This matrix captured the most important refinement in the design journey. The geared slewing ring was structurally excellent but far too heavy, costly, and power-hungry relative to the robot's real needs. The 010.10.120 non-geared slewing ring was selected because it satisfies realistic load requirements while greatly improving manufacturability and design appropriateness. This marks the point where the design shifted clearly from maximum capability to fit-for-purpose engineering.

3. Rotation Drive-Transmission Strategy

Concept Positive torque transmission Packaging simplicity Ratio flexibility Fabrication tolerance Serviceability Final decision
Direct pinion on external gear High Moderate Moderate Moderate Moderate Early concept only
Friction-drive / wheel-drive concept Low High Low Poor Moderate Rejected
Timing-belt drive to inner rotating surface High High High High High Selected

Once the non-geared bearing had been selected, the drive system had to become a separately designed subassembly. The timing-belt drive was selected because it preserves positive torque transmission while improving packaging flexibility, allowing the motor to remain on the fixed structure and the ratio to be tuned through pulley selection. Crucially, the belt acts as a mechanical fuse, permitting elastic give under shock punches to protect the motor's gearbox, and is highly forgiving of fabricated mounting tolerances. This made it the most buildable and maintainable solution for the welded-base architecture.

4. Outboard Support and Deployment Refinement

Concept Edge stability under overturning Added part count Packaging cleanliness Transport compatibility Final decision
Central bearing only Moderate High High Moderate Not preferred alone
Ring of many support rollers High Poor Poor Poor Rejected
Discrete cam-follower supports + rear transport wheels High High High High Selected

This final matrix was used to refine the subsystem beyond its minimum working form. The selected approach used discrete cam-follower supports to improve edge stability without excessive part count, together with rear transport integration to ensure the module remained practical for workshop and demo handling. This is where the rotation subsystem matured from a pure motion axis into a more complete base module.

5. Motor Selection

Concept Low-speed torque Position control & accuracy Disturbance rejection Motion smoothness Final decision
Brushed DC gearmotor High Poor (backlash) Poor Moderate Rejected
Closed-loop stepper (hybrid servo) High High Moderate Poor (cogging/ripple) Not selected
BLDC servo motor with encoder High Excellent Excellent Excellent (FOC) Selected

The yaw motor must rotate the full upper body about the vertical axis while resisting punching-induced moments. It must achieve a target speed of ≈150°/s with accurate, repeatable positioning and good disturbance rejection.

For the yaw axis, user perception and stability are critical: the robot should rotate smoothly and feel "solid" under punches, without rattling or drifting. While brushed DC gearmotors and closed-loop steppers can meet basic torque needs, their drawbacks in backlash and torque ripple make them less suitable. A BLDC servo motor with encoder was therefore chosen because it provides smooth, sinusoidal torque production, high positional stiffness, and active disturbance rejection.

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