5.2.2 Rotation

The rotation subsystem reproduces yaw re-orientation, one of the most important footwork behaviours in live boxing drills. A coach does not only move forward and backward; they also pivot and re-angle to create new attack lines, defensive responses, and counter-punch opportunities. For BoxBunny, this required a dedicated yaw axis capable of rotating the upper structure on the spot while remaining stable under impact. The subsystem therefore had to be treated as both a motion mechanism and a structural support interface.

The development of the rotation subsystem progressed through several major design stages. The earliest serious concept used a large geared slewing ring that combined structural support and direct drive. This was later refined into a smaller non-geared slewing ring with an external transmission, and then further developed into the current low-profile yaw module comprising a 010.10.120 slewing bearing, an off-axis timing-belt drive, cam-follower edge supports, and integrated rear transport features. This final arrangement is more buildable, more appropriate to the realistic load case, and better integrated with the wider lower-mechanism architecture.

Requirements & Considerations

This subsystem addresses DO-2 (Intelligent Sparring System) and DO-4 (Adaptive Fight Intelligence). Requirements were derived from the boxing coaching motion repertoire, structural loading under punch impact, and the need to integrate cleanly with the base and height-adjustment modules.

System Requirements

The following system-level requirements are inherited from the Robot Mechanism (Section 5.2) requirements table and govern all rotation subsystem design decisions:

Subsystem Acceptance Criteria

The following acceptance criteria were derived by identifying which design decisions in this subsystem introduce new failure modes not covered by the system-level requirements above. ROT-AC-1 arises because the choice of a timing-belt transmission introduces a tooth-skip failure mode under shock loads and rapid reversals that RM-4 (which specifies speed only) does not capture: the belt could technically reach 150°/s and still fail mechanically. ROT-AC-2 arises because the wireless-only UDP command path (eliminating physical cables through the rotating joint) introduces a packet-loss failure mode that no system-level requirement previously addressed, since all RM codes are concerned with physical performance, not communication reliability.

Table: Rotation Subsystem Requirements and Acceptance Criteria

System Design Narrative

Following the Systems Engineering V-Model introduced in Section 3.2, RM-4, RM-1, RM-2, RM-3, ROT-AC-1, and ROT-AC-2 were fixed before detailed bearing, transmission, or electrical decisions were made. The diagram below applies the V-Model to the Rotation subsystem specifically.

Left Side of the V: Design Decomposition

RM-4 (yaw speed) drove the selection of the BLDC servo motor with a 25:1 planetary gearbox and the S8M timing-belt transmission at a 1:3.5 pulley ratio. RM-1 (structural stability) drove the choice of the 010.10.120 four-point contact slewing ring over a lighter turntable bearing, and the addition of discrete cam-follower edge supports. RM-2 (compact footprint) constrained the drive system to remain off-axis on the fixed base, keeping the rotating stage low-profile. RM-3 (portability coexistence) ensured the rear transport interfaces were positioned away from the bearing and belt routing. ROT-AC-1 and ROT-AC-2 drove the tooth-skip mitigation strategy and the WiFi UDP communication architecture respectively.

Base of the V: Integration Build

At integration stage, the slewing ring, timing-belt drive, cam-follower supports, motor controller (Arduino Uno R4 WiFi), and AS5047P encoder were assembled into a unified yaw module. Belt tension was set, cam-follower track contact was confirmed, and the wireless command interface was brought online against the Jetson host.

Right Side of the V: Verification Closure

Verification is planned against the same requirements used during decomposition. Physical speed testing will close RM-4 (sustained ≥ 150°/s confirmed against encoder logs). Lateral punch testing will close RM-1 (≤ 5° deflection, recovery within 500 ms). Transmission integrity testing will close ROT-AC-1 (zero tooth-skip under shock and reversal). UDP packet-loss testing will close ROT-AC-2 (< 1% loss). Full test procedures are in Testing & Evaluation.

Design

The final rotation design is a low-profile yaw module built around a non-geared slewing ring bearing, an external timing-belt drive, and cam-follower roller supports at the perimeter of the rotating plate. This pair of concentric circular plates: a top plate that interfaces with the upper structure and a bottom plate that mounts to the slewing bearing, form the primary rotary body. The motor is mounted off-axis on the fixed base, drives a small pulley, and transmits torque via a timing belt to a larger pulley that is integral to the bottom circular plate, causing the entire plate assembly to rotate about the central bearing. The rotating plates are additionally stabilised by discrete outboard cam followers, while the welded base members provide the fixed reaction structure for the bearing, motor, belt drive, and bearing support.

Interact with the 3D model to explore its structure. Toggle annotations to view key components. Hide Annotations

Rotation Motor

Slewing Ring Bearing

Circular Top Plate

Circular Bottom Plate

Rollers

Driving Pulley

Driven Pulley

Idler Pulley

View in your space

_{Interactive 3D Model: Rotational motion mechanism showing slewing bearing, drive system, and motor.}

Non-Geared Slewing Ring High Stiffness + Compact

Another major design decision was the move away from the earlier large integrated geared slewing ring (011.25.400). Although that concept was structurally strong, it was too heavy, too expensive, too difficult to handle, and too demanding in motor power relative to the actual needs of the robot. The final selected 010.10.120 non-geared slewing ring still provides the required combined-load capacity, but with much better design appropriateness. This change also forced the drive system to become an explicitly designed subassembly rather than something inherited from the bearing.

Modular Timing-Belt Drive Flexible Ratio Tuning

The timing-belt drive was selected because it gives positive torque transmission, keeps the motor on the fixed structure, offers flexible ratio selection through pulley sizing, and is easier to package into the welded base than a bulky direct-drive or integrated gear solution. In the current design, the timing-belt transmission converged to the following selected parameters:

These values came from the timing-belt selection process and represent a practical balance between the ideal reduction target and the real packaging constraints of the current rotating-base geometry.

Cam-Follower Edge Supports Tilt Stabilisation

The outer cam-follower supports were added because the yaw stage is loaded not only by torque demand but also by plate-edge rocking under elevated off-axis punches. These supports do not replace the central slewing bearing; rather, they share the anti-tilt function by increasing the effective support radius of the rotating plate. This makes the stage feel more planted and reduces local rocking at the edge.

Rear Transport Integration Tip-and-Roll

The final design also incorporates rear transport interfaces as part of the wider lower-mechanism architecture. Within the full base assembly, these are intended to accommodate the rear-wheel hardware and provide the structural connection points needed for the robot to be tipped and rolled during movement between locations. From a mechanical perspective, locating these features at the rear is sensible because that region already provides the larger footprint, greater structural support, and more separation from the user's immediate footwork zone. However, because this transport function has not yet been physically tested, it should currently be treated as an integrated design provision rather than a fully verified operational capability.

Electrical & Control Architecture Wireless Command + CAN Bus

The rotation axis is driven by a Z55BLD400-24GU 400 W BLDC motor with a 25:1 planetary gearbox, controlled via the ZBLD C20-800LRC CAN motor driver. Position feedback is provided by an AS5047P 14-bit magnetic encoder (SPI).

The motor controller is an Arduino Uno R4 WiFi, operating as a separate controller from the Teensy 4.0 that manages the arm motors. Communication with the Jetson host uses WiFi UDP — eliminating physical cable routing through the rotating base joint, which would otherwise fatigue and fail. The CAN bus operates at 125 kbps (vs 1 Mbps for the arm motors), necessitating a separate bus to avoid firmware complexity. Full details are documented in the Electrical & Control sub-page.

Validation

The following table summarises the system-level requirement mapping for the rotation subsystem, corresponding to the Robot Mechanism verification matrix.

At the current stage, the rotation subsystem has been validated partly through concept selection, load-path reasoning, and transmission calculations, and partly through the coherence of the realised CAD assembly. It has not yet been fully validated through all intended physical tests.

The most important completed success is that the final concept retains the correct system-level architecture: yaw and translation remain decoupled, the yaw stage stays at the bottom of the lower mechanism, and the main rotary support is based on a slewing-bearing approach capable of carrying combined loads. This means the subsystem is mechanically aligned with the realistic needs of a strike-receiving boxing robot rather than with a generic mobile-robot platform.

The timing-belt transmission has also been validated to the extent that it has been sized through a structured engineering process rather than by approximation alone. The pulley ratio, belt series, belt width, centre distance, and belt length were all developed through real selection calculations. This gives confidence that the drive system is mechanically credible and manufacturable. However, the final selected 1:3.5 pulley ratio is also known to be a packaging-driven compromise relative to the earlier ideal reduction target of about 4.8:1, so the actual integrated yaw speed should still be measured physically to confirm that the system still achieves the desired re-angling performance.

The current design also succeeds in structural intent. The load path is clearly divided between:

• the motor and belt, which provide controlled rotation,
• the slewing bearing, which carries the main rotary support load,
• the cam followers, which improve anti-rocking behaviour,
• and the welded base, which closes reaction loads back into the floor.

The main items that remain incomplete are:

• physical confirmation of the actual yaw speed and acceleration,
• physical assessment of belt compliance, tooth-jump margin, and tension stability,
• contact-stress and stiffness validation of the cam-follower supports,
• and physical confirmation that the rear transport arrangement does not interfere with planted operation.

A concise subsystem validation summary is given below.

Detailed Documentation

The following sub-sections provide in-depth design documentation for the rotation subsystem. Select a section to view its contents.