5.2.3 Height Adjustment

The height-adjustment subsystem allows BoxBunny to accommodate users of different heights while preserving the robot's structural rigidity during operation. From a user perspective, the intended workflow is simple: the user selects a suitable training height, the robot adjusts vertically, and the robot must then remain stable throughout the session without visible wobble, jamming, or loss of alignment. Unlike a simple positioning stand, however, the height-adjustment stage lies directly beneath the main striking body of the robot. This means it must act not only as a lifting mechanism, but also as part of the structural load path that carries punching-induced lateral forces and overturning moments down into the rest of the robot.

The development of the subsystem progressed through several stages. Early ideas included fixed-height configurations, manually adjustable mast concepts, commercial telescopic columns, and a rear-linear-guide plus screw-jack arrangement. These concept stages gradually clarified the most important engineering principle: vertical lifting and lateral structural resistance should be separated in function, even if they are integrated into one mechanical assembly. This insight ultimately led to the final concept of a custom telescopic lift column actuated by a motorised travelling-nut screw jack, where the screw jack provides axial lift and the surrounding column structure resists side loading.

Requirements & Considerations

This subsystem addresses DO-5 (Modular Boxing Platform) and DO-2 (Intelligent Sparring System). Requirements were derived from the user height range, structural loading during punching, and the need for safe, self-locking actuation.

System Requirements

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

ID	Requirement	Source
RM-5	Provide ≥ 400 mm vertical stroke for height adjustment; full stroke completed in ≤ 32 s	User height range 150–190 cm (Section 4.3), DO-5 (Modular Boxing Platform), Appendix 6
RM-1	Remain upright and maintain alignment under combined axial and lateral punch loads without structural failure	Structural safety, Load & Stability Analysis (Section 5.2.1.3), Appendix 6
RM-2	Compact footprint: mechanism must not intrude into boxer's footwork zone during height adjustment or at rest	User journey (Section 4.3), boxing gym spatial constraints

Subsystem Acceptance Criteria

The following acceptance criteria were derived from design decisions specific to this subsystem that introduce failure modes not captured by the system-level requirements above. HA-AC-1 arises because the choice of a motorised screw-jack drive introduces a speed-limitation failure mode: RM-5 specifies stroke and time, but does not address whether the chosen actuator's back-calculated approximately 800 rpm input requirement can be reliably delivered under load by the selected DC motor. This is a sub-requirement of speed capability, not of stroke provision. HA-AC-2 arises because the Delrin wear-pad guide interface introduces a long-term wear failure mode: the column geometry and load-path separation may be correct, but if the pads wear excessively, the column will develop backlash and lose the stiffness that satisfies RM-1. This is not captured by any RM code.

ID	Acceptance Criterion	Derives From	Verification Test
HA-AC-1	Full 400 mm stroke completed in ≤ 32 s under full payload (22.5 kg design load); no stalling or drive degradation over 5 consecutive cycles	Screw-jack/motor drive (Section 5.2.3.3); speed capability failure mode not fully specified by RM-5 alone	Timed full-stroke test under load, 5 consecutive cycles
HA-AC-2	Delrin guide pad wear: < 1 mm surface wear after 200 adjustment cycles; column backlash < 2 mm lateral play at top of stroke	Delrin wear-pad guide design (Section 5.2.3.2); long-term wear failure mode not captured by RM-1 or RM-5	200-cycle endurance test; micrometer wear measurement and lateral backlash check

Table: Height Adjustment Subsystem Requirements and Acceptance Criteria

System Design Narrative

Following the Systems Engineering V-Model introduced in Section 3.2, RM-5, RM-1, RM-2, HA-AC-1, and HA-AC-2 were fixed before detailed actuator, column, or control decisions were made. The diagram below applies the V-Model to the Height Adjustment subsystem specifically.

Left Side of the V: Design Decomposition

RM-5 (stroke & speed) drove the concept selection of a guided lift column plus screw jack and the back-calculation of ~800 rpm motor input speed. RM-1 (structural stability) drove the architectural decision to separate the screw jack's axial load path from the telescopic column's lateral load path — a structural principle validated across Phases 2, 3, and 4 of the design evolution. RM-2 (compact footprint) drove the consolidation from dual rear linear guides (Phase 2) to a single integrated telescopic column (Phase 3), eliminating rail protrusion. HA-AC-1 drove the motor-sizing logic around 800 rpm speed capability. HA-AC-2 drove the choice of Delrin wear pads as a sacrificial, inspectable guide interface rather than a press-fit or fixed guide.

Base of the V: Integration Build

At integration stage, the telescopic column (8080 inner extrusion + welded outer tube + Delrin pads), HK2T screw jack, travelling-nut mount, and 24 V DC gear motor were assembled into a unified height-adjustment module. Motor coupling was confirmed, and initial stroke tests were performed to validate drive engagement.

Right Side of the V: Verification Closure

Verification is planned against the same requirements used during decomposition. Timed stroke testing will close RM-5 (400 mm in ≤ 32 s, 5 consecutive cycles under load). Load-path inspection will close RM-1 (lateral punch loads routed through column, screw jack kept axial). Wear testing will close HA-AC-2. Full test procedures are in Testing & Evaluation.

Design

Custom Telescopic Lift Column 400 mm Vertical Stroke

The final height-adjustment system consists of two coupled sub-assemblies: a mechanical structure in the form of a telescopic lift column, and a motorised screw-jack mechanism that drives the vertical motion.

The structural portion of the design comprises:

• an 8080 aluminium extrusion as the moving inner member,
• Delrin wear pads acting as low-friction sacrificial guide interfaces,
• and a welded steel outer tube with integrated plate forming the fixed sleeve of the telescopic column.

The actuation portion comprises:

• the travelling nut of the screw jack,
• the HK2T screw jack itself,
• and a dedicated screw jack–to–8080 mount that transforms the jack's axial motion into the vertical movement of the inner column.

Separation of Lift from Structure Axial vs Lateral Load Path

The most important design feature is the separation of functions between lift and structure. In the final concept, the screw jack is treated purely as an axial lifting device, operating mainly in compression. Lateral loads and overturning moments caused by punching are not intended to be carried by the screw. Instead, they are reacted through the 8080 inner column, across the Delrin guides, into the welded outer tube, and then into the lower support structure. This prevents side-loading of the screw jack, which would otherwise reduce efficiency, increase wear, and create binding risk. The subsystem was not selected simply because it could move the robot body up and down, but because it could do so without weakening the robot's resistance to operational loads.

Design Evolution

An earlier concept used two vertical linear rails at the rear with a lift carriage plate, with the screw jack near the centreline and a compliant interface for misalignment tolerance. That concept established the correct engineering principle but introduced too many precision-mounted components, alignment sensitivity, and tolerance stack-up.

Commercial off-the-shelf telescopic columns were also considered, but their side-load capability under boxing disturbances was uncertain. The project moved toward a custom telescopic column. This was not a change in structural logic from the rail concept, but an improvement in implementation—consolidating the guide function into one robust structural assembly.

Fabrication Considerations Durability + Safety Margin

As the design matured, fabrication realism became more important. Large SHS members were kept at ≥ 6 mm wall thickness, welded plates were sized to avoid distortion, and gusseted corner brackets were favoured where aluminium extrusion met steel platework to reduce joint flex. These details matter because the final performance of the subsystem depends not only on the actuator, but also on whether the surrounding structure preserves alignment under load.

Corrosion Protection Strategy Environmental Robustness

Rust was treated as a real design issue, not a cosmetic one. The robot operates in gym environments where humidity, sweat, and cleaning chemicals create a mildly corrosive atmosphere. Left unprotected, the mild steel structural members of the lifting column would develop surface rust rapidly, leading to progressive section loss and potential staining of gym floors.

The corrosion protection strategy differentiates between interior and exterior surfaces because they present fundamentally different treatment challenges.

Galvanising Rejection

Full hot-dip galvanising was considered but ultimately rejected as impractical at prototype scale. The high-temperature zinc bath (≈450 °C) can warp thin-wall welded assemblies. Furthermore, all enclosed sections would require drill holes for venting to prevent steam explosions during immersion, compromising the sealed tube integrity. It is also disproportionately expensive for a single prototype. Additionally, any post-galvanising modifications or welding would produce toxic zinc fumes and damage the protective layer at the weld zone.

Exterior Surface Protection

Exterior steel surfaces are fully accessible for conventional surface preparation and coating. The treatment sequence consists of:

1. Mechanical rust removal: Wire brushing and sanding to remove mill scale, weld spatter, and surface corrosion (Target standard: SA 2½).
2. Anticorrosion primer: Zinc-rich primer applied within hours of surface preparation to prevent flash rusting.
3. Paint topcoat: Polyurethane or enamel topcoat for UV resistance, abrasion protection, and aesthetic finish.

Interior Surface Protection

Enclosed SHS interiors cannot be fully cleaned or painted after assembly. Residual moisture, weld spatter, and mill scale remain trapped inside. The interior protection strategy utilizes penetrating, non-drying protective cavity waxes (or lanolin-based rust preventatives) applied by spraying into the tube ends before final sealing. These coatings creep along surfaces, displace moisture, and form a long-term barrier without requiring high-standard surface preparation. This approach is borrowed from automotive and marine practice, where box-section chassis members are routinely treated with cavity wax to prevent internal corrosion.

Low-Level Control Fail-Safe Actuation

The height adjustment is currently a motorised screw-jack system. The back-calculated motor speed requirement of approximately 800 rpm at the jack input drives the motor-sizing logic. The self-locking nature of the screw jack provides inherent fail-safe behaviour — the column holds its position even without motor power, ensuring safety during training.

Validation

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

ID	Requirement	Verification Method	Measured Result	Status
RM-5	Provide ≥ 400 mm vertical stroke; full stroke ≤ 32 s	Timed full-stroke test under 22.5 kg design load	400 mm stroke confirmed by design sizing; timed physical test not yet conducted	Partial
RM-1	Remain upright under punch loads; lift/structure load path separation	Load-path design review; lateral deflection inspection under 2.7 kN design load	Lift/structure separation confirmed by design (screw jack axial only, column carries lateral); quantitative deflection test pending	Partial
RM-2	Compact footprint — no intrusion into footwork zone	Physical package inspection; integration check with base and rotation module	Telescopic column consolidates guide into single profile; no lateral rail protrusion. Inspection confirmed.	Pass
HA-AC-1	Full stroke ≤ 32 s under load; no stall over 5 cycles	5-cycle timed stroke test with 22.5 kg payload	Motor speed requirement back-calculated at ~800 rpm; selected motor meets spec; physical cycle test not yet conducted	Partial
HA-AC-2	Delrin pad wear < 1 mm after 200 cycles; backlash < 2 mm at top of stroke	200-cycle endurance test; micrometer wear + lateral backlash measurement	Delrin wear-pad design rationale sound; long-term endurance test not yet conducted	Partial

At the current stage, the height-adjustment subsystem has been validated partly through concept selection, packaging logic, and load-path reasoning, and partly through the coherence of the final mechanical assembly. It has not yet been fully closed through every intended physical validation test.

The most important subsystem-level success is that the final design satisfies the core structural requirement: the lifting mechanism is decoupled from the lateral load-bearing path. Compared with the earlier rear-linear-guide concept, the telescopic lift column reduces part count, simplifies alignment, and consolidates the load path into fewer, better-integrated structural members. Compared with commercial lift columns, it provides much higher confidence that side loads from punches are being treated appropriately for a boxing robot application.

The architecture also meets the 400 mm stroke requirement, and the current movement target of approximately 32 seconds for full stroke is consistent with the inferred jack-speed requirement and the current actuation logic. This means the subsystem is functionally adequate for the current prototype, even if it does not yet achieve the more desirable 10-second user-experience target.

The main gaps in validation are: quantitative measurement of column deflection under lateral load, long-term wear monitoring of the Delrin interfaces, full repeated-cycle testing of the telescopic stage, and confirmation of whether future motor revisions can move the stroke time closer to the ideal target.

Aspect	Current status	Basis
400 mm stroke provision	Pass	Final concept sized around full required stroke
Separation of lifting and structural functions	Pass	Core design principle achieved
Telescopic column as guide structure	Pass	Cleaner load path and lower part count than earlier rail-based design
Current actuation speed (~32 s full stroke)	Pass (functional)	Meets present implementation target
Ideal user preference (~10 s full stroke)	Partial	Desired but not achieved with current screw-jack arrangement
Commercial off-the-shelf lift columns	Fail as final solution	Side-load confidence insufficient for boxing use
Long-term Delrin wear and stiffness	Partial	Strong concept logic, but long-term physical validation still needed
Full repeated-cycle and backlash testing	Partial	Further testing required

Detailed Documentation

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