Back to Main Report

Appendix 2: Lower Mechanisms (Jeanette)

2.1. Footwork Movement

2.1.1. Motion Axis Selection

Having to accomplish two motions, decision has to be made to have both controlled within one mechanism or decouple those axes into separate control. One possible collective system is using omnidirectional wheels. The omnidirectional wheel concept provides full planar mobility of the robot base. A single drive system enables translations in both the X and Y directions, as well as on-the-spot rotation, by appropriately commanding the individual wheel speeds. At the control level, this appears attractive because any desired planar velocity (forward, lateral, yaw) can be synthesised from the same set of actuators.

Benefits of Omni-wheels for Boxing Application

These advantages are well aligned with general mobile robotics applications, where the primary design objective is flexible navigation in cluttered environments.

Limitations of Omni-wheels for Boxing Application

However, this comes with limitations for a punch-receiving boxing robot. For a boxing trainer robot that must repeatedly absorb impulsive loads from punches while maintaining a stable stance, several limitations arise when using omnidirectional wheels:

Most boxing gyms do not provide the ideal hard, polished surface assumed in many mobile robotics applications. Instead, they commonly use interlocking foam mats, rubber tiles, or sprung or cushioned flooring systems.

On these surfaces:

• Wheels can sink slightly into the surface, changing the effective rolling radius and increasing rolling resistance.
• The floor itself adds another compliant layer beneath the robot, amplifying pose creep under repeated impacts.
• Small shifts at the base accumulate over rounds, making it difficult to maintain a consistent stance reference relative to the user.

Consequently, maintaining repeatable stance alignment, which is critical for structured drills and quantitative training metrics, is challenging with an omnidirectional base on typical gym flooring.

Benefits of Decoupled Axes for Boxing Application

A design that explicitly decouples the primary motions, e.g. a linear rail for the forward–backward axis combined with a slewing bearing for rotation, sacrifices omnidirectional mobility but introduces significant advantages for this application:

Selection Matrix – Forward-Backward Range Motion

Table shows the decision matrix created to aid the linear footwork movement axis selection, assuming rating scale 1-5 (5 = best performance for BoxBunny’s needs). Omnidirectional wheels score better on cost/simplicity as one system that does everything. The linear rail is much stronger on stiffness, repeatability, and independence from floor compliance, which are critical for a punch-receiving training robot. Thus, for forward-backward motion, the decoupled linear rail is the preferred solution.

Selection Matrix – Yaw Rotation (On-Spot Turning)

Table B10 shows the decision matrix created to aid the rotational footwork movement axis selection. Omnidirectional wheels can rotate on the spot, but yaw stiffness and angle repeatability are limited by wheel slip and floor compliance. A slewing bearing driven by a motorised pinion offers high rotational stiffness, well-defined backlash, and high moment capacity, all independent of floor conditions. For on-spot rotation, the decoupled rotating base is the preferred solution.

Motion Axis Selection for BoxBunny Footwork Movement

Across both motions, the selection matrices show that omnidirectional wheels are attractive for general mobile robots but are penalised here by low stiffness, pose creep, and strong dependence on floor conditions. Decoupled axes (linear rail + slewing bearing) provide the stiff, repeatable, floor-independent geometry needed for a boxing trainer that must reliably hold stance under repeated punches. Therefore, for this application, the project adopts a decoupled-axis architecture: Motorised linear rail for forward–backward range, and slewing bearing with external gear and pinion-driven motor for yaw rotation.

2.1.2. Linear Motion Selection

2.1.2.1. Power Transmission Drive Selection

The power transmission drive for the linear stage must reliably move the entire BoxBunny robot (≈ 20–25 kg including arms and torso) forward and backward over a short stroke (~300 mm), while withstanding dynamic punching loads. Key requirements for this axis are:

• Load and thrust capacity: Sufficient to accelerate the robot mass and resist horizontal reaction forces during punches.
• Positional stiffness and repeatability: The base must return to the same stance positions for drills without noticeable drift.
• Moderate travel speed: Fast enough to simulate a boxer stepping in/out, but not a high-speed shuttle (short strokes, <1 m).
• Robustness under repeated duty: Frequent starts/stops and impulse loads during training sessions.
• Compact integration: Drive must fit under the rotating base and vertical structure without excessive footprint.

Three common drive types were evaluated: lead screw, ball screw, and timing belt.

Lead Screw Drive

Lead screws convert rotary motion to linear motion via a threaded rod and nut. They are generally low cost, mechanically simple, and can use self-locking polymer or anti-backlash nuts, which reduces drift in vertical axes and improves repeatability for light loads. However, for BoxBunny’s application they present several drawbacks:

• Lower efficiency and higher friction, leading to more heating at higher speeds or duty cycles.
• Limited load and speed capability with typical polymer nuts; not ideal when combining moderate loads, repeated motion and dynamic impacts.
• Moderate accuracy and life compared to ball screws, especially under higher thrust and duty.

Lead screws are therefore more suited to light-load, low-speed or low-duty applications, and were not selected as the primary drive for BoxBunny’s main footwork axis.

Ball Screw Drive

Ball screws use recirculating balls between the screw and nut, providing rolling contact instead of sliding. For BoxBunny, they offer several key advantages:

• High thrust and load capacity appropriate for moving a ~20–25 kg robot and resisting horizontal reaction forces.
• High stiffness, accuracy and repeatability, due to low backlash and well-controlled preload, which supports repeatable stance positions.
• High mechanical efficiency, reducing motor torque requirements and heat build-up under frequent moves.
• Good durability under repeated duty, making them suitable for regular training sessions with many cycles.

The main limitations are higher cost and the need for periodic lubrication, but these are acceptable trade-offs given the short stroke and performance requirements.

Belt Drive

Belt-driven actuators use a toothed belt and pulleys to generate linear motion. Their strengths lie in:

• High speed and long travel capability, typically over several metres.
• High efficiency and relatively simple maintenance for long-stroke, lower-load systems.

However, for BoxBunny’s short-stroke footwork axis they are less suitable:

• Lower positional stiffness and accuracy compared to screw drives, due to belt elasticity.
• Susceptibility to elongation and shock loads, which can degrade repeatability under punching impacts.
• Need for periodic belt re-tensioning, and potential back-driving / drift in horizontal or inclined applications.

Given the short stroke and high stiffness requirement, these disadvantages outweigh the speed benefits.

Selection Matrix – Power Transmission Drive in Linear Rail

Table shows the decision matrix created to aid the power transmission drive in linear rail selection.

Drive Selection for BoxBunny Linear Stage

Considering BoxBunny’s specific requirements of moderate stroke, significant mass, repeated dynamic loads, and the need for high positional stiffness and repeatability, the ball screw drive is selected for the linear stage. It provides:

• Sufficient thrust and load capacity for the full robot mass,
• High stiffness and repeatability for consistent stance positions, and
• Robust performance under repeated use, with manageable lubrication requirements.

This makes a ball screw–driven linear rail the most appropriate power transmission solution for the BoxBunny footwork axis.

2.1.2.2. Motor Selection

The motor driving the ball-screw linear stage must translate the full BoxBunny mass (~20–25 kg) over a short stroke (≈200–300 mm) while maintaining precise, repeatable stance positions under punching disturbances. Key requirements are:

• High torque at low–medium speed to drive the ball screw without stalling.
• High positioning accuracy and repeatability so stored “stance distances” are consistent.
• Good disturbance rejection and stiffness to resist position loss when the robot is punched.
• Realistic cost and integration complexity for a prototype.

Three motor families were evaluated: Open-Loop Stepper Motor (NEMA-Frame), Closed-Loop Stepper / Hybrid Servo (Stepper + Encoder), and Industrial AC/BLDC Servo Motor (Servo Pack).

Open-Loop Stepper Motor

Standard NEMA-frame stepper motors, driven in microstepping mode, are widely used in low-cost linear stages.

Advantages

• High holding torque at low speed, matching the ball screw’s operating range.
• Simple step/direction interface; no encoder required.
• Very low cost and excellent availability of motors, drivers, and mounting hardware.

Limitations for BoxBunny

• Open-loop control: missed steps under sudden loads or aggressive acceleration are not detected.
• Loss of steps leads to silent position drift, which directly undermines stance repeatability.
• Resonance and vibration can occur if not carefully managed.

Open-Loop Stepper Motor is therefore an attractive baseline for cost and simplicity, but its vulnerability to undetected position loss is a concern for a punch-receiving system.

Closed-Loop Stepper / Hybrid Servo (Stepper + Encoder)

Closed-loop steppers (often sold as “hybrid servos”) combine a stepper motor with an integrated encoder and a driver that closes the position/velocity loop.

Advantages

• Encoder feedback allows detection and correction of missed steps, improving reliability under shock loads.
• Similar NEMA form factor and mounting as a standard stepper; easy mechanical integration with the ball screw.
• Retains high low-speed torque while significantly improving positioning accuracy and stiffness.
• Many drives still accept simple step/direction commands, minimising software changes .

Limitations for BoxBunny

• Higher cost than open-loop steppers.
• Some tuning of current limits and loop gains may be required, though typically simpler than full industrial servos .

Closed-Loop Stepper / Hybrid Servo strikes a good balance between performance and complexity for BoxBunny’s linear stage.

Industrial AC/BLDC Servo Motor (Servo Pack)

Industrial servo systems pair an AC/BLDC motor with a high-resolution encoder and a matched servo drive.

Advantages

• Excellent torque–speed characteristics and disturbance rejection.
• Very high positioning accuracy, repeatability, and configurable stiffness.
• Well suited to demanding industrial positioning applications.

Limitations for BoxBunny

• Highest cost among the three options (motor, drive, cables).
• Greater integration complexity: power wiring, controller interface, and tuning of control loops.
• Over-specified for a short-stroke, low-speed axis in a student prototype unless hardware is already available.

Industrial AC/BLDC Servo Motor offers the best raw performance, but at a cost and complexity level that may not be justified for this application.

Selection Matrix – Motor in Linear Rail

Table shows the decision matrix created to aid the motor in linear rail selection.

Motor Selection for BoxBunny Linear Stage

Considering BoxBunny’s need for high stiffness and repeatability under punching disturbances , while keeping integration feasible for a student project, Closed-Loop Stepper / Hybrid Servo is selected to drive the ball-screw linear stage. It provides a substantial improvement in robustness over an open-loop stepper without the full cost and integration burden of an industrial servo pack.

2.1.3. Rotational Motion Selection

2.1.3.1. Bearing Selection

The yaw rotation system must support the full BoxBunny robot mass (≈ 20–25 kg), plus dynamic punching loads and overturning moments from the elevated torso and arms. The bearing must:

• Carry combined axial, radial and moment loads from punches and body weight.
• Provide high rotational stiffness so the robot does not “wobble” or twist when struck.
• Remain durable under repeated shock and vibration.
• Allow reasonably precise positioning when driven by an external pinion and motor.
• Be cost-effective and readily available for a student prototype

· Fit within limited height of the lower structure and interface cleanly with the rotating base plate.

Four bearing families were evaluated: a low-cost lazy-Susan bearing and three types of slewing-ring bearings.

Lazy-Susan Bearing

Lazy-Susan (turntable) bearings are inexpensive, thin-section assemblies commonly used in furniture and light-duty turntables.

Advantages

• Very low cost and widely available.
• Simple mounting between two plates.
• Compact axial height.

Limitations for BoxBunny

• Intended mainly for light axial loads, with limited overturning-moment capacity.
• Minimal preload → noticeable play in tilt and rotation; unsuitable for precise yaw positioning.
• Not designed for repeated impacts; looseness and wear are expected under punching loads.

Lazy-Susan bearing is mechanically attractive for a toy turntable, but does not offer the stiffness, moment capacity, or durability required for BoxBunny.

Slewing-ring Bearing

Four-Point Contact Ball Slewing Ring Bearing

Four-point contact ball slewing rings use a single row of balls in a specially profiled raceway, allowing each ball to carry load at up to four contact points.

Advantages

• Designed to support combined axial, radial and overturning moment loads , including varying and shock loads.
• Can be preloaded to reduce internal clearance and improve stiffness.
• Lower friction than cross-roller designs → adequate speed with smooth motion (although high speed is not critical here).
• Widely used in light–medium duty slewing applications, readily available in standard sizes and generally cheaper than cross-roller slewing rings.
• Provides sufficient stiffness and load capacity for a 20–25 kg robot with moderate punching loads, with safety margin.

Limitations for BoxBunny

• Rotational stiffness is lower than cross-roller bearings for the same size and preload; small elastic deflections are still present under heavy impact.

Thisoffers a good balance of load capacity, stiffness, availability and cost, and satisfies BoxBunny’s yaw requirements with adequate margin.

Cross-Roller Slewing Ring Bearing

Cross-roller slewing rings use cylindrical rollers arranged in alternating orientations (X-pattern) between inner and outer rings.

Advantages

• Very high tilting and rotational stiffness due to line contact and crossed layout.
• Excellent for combined axial, radial and moment loads in compact envelopes.
• Commonly used in robot joints and precision rotary stages requiring minimal play and high rigidity.

Limitations for BoxBunny

• Generally, more expensive than four-point ball slewing rings.
• Somewhat less readily available in suitable sizes from mainstream suppliers compared to four-point ball types.
• Higher performance than strictly required for this application.

This is technically the stiffest option, but its cost and availability are not justified when Four-point contact ball slewing rings already meets the design requirements.

Thrust Ball Slewing Rings

Thrust ball slewing rings are optimised mainly for axial loads , with limited radial and moment capacity.

Advantages

• Efficient support for vertical loads with low friction.

Limitations for BoxBunny

• Not optimised for large overturning moments from an elevated torso and punching impacts.
• Usually requires additional bearings to handle radial and moment loads, increasing complexity.

This does not match the combined-load and high-moment demands of BoxBunny’s yaw joint without extra components.

Selection Matrix – Bearing in Yaw Rotation System

Table shows the decision matrix created to aid the bearing in yaw rotation system selection.

Bearing Selection for BoxBunny Yaw Rotation

Given that Four-Point Contact Ball Slewing Ring Bearingalready meets BoxBunny’s load and stiffness requirements with margin, and offers better cost-performance and procurement practicality , it is selected as the yaw bearing for the system.

2.1.3.2. Gear Reduction Selection (Baseline)

The gear reduction for the yaw axis is largely dictated by the choice of slewing ring bearing with external gear teeth. Once the bearing diameter and tooth form are fixed, the mating pinion and resulting gear ratio follow naturally. The calculations in this section are presented as a baseline template: when the final torque, speed, and load analysis for BoxBunny are complete, the same steps can be repeated with updated values.

External Gear and Pinion Selection

Selected bearing: Four-Point Contact Ball Slewing Ring, externally geared

Outer diameter (approx.): 400 mm

Example model: LILY Bearing MTE-265X (externally geared)

Gear mesh: Module 5 external gear

From the bearing datasheet, the external ring (driven gear) is specified with:

• Driven gear (slewing ring): 84 teeth
• Driving gear (pinion): 17 teeth

The pinion is mounted on the motor shaft (or via a short shaft + coupling), driving the external gear on the slewing ring.

Gear Ratio Calculation

The gear ratio (GR) is defined as:

where

Ndriven = number of teeth on the external slewing ring,

Ndriving = number of teeth on the pinion.

For this configuration:

• Ndriven = 84
• Ndriving = 17

This means the pinion (and motor) must rotate approximately 4.94 times faster than the slewing ring.

Baseline Motor Speed Requirement

Assume a target yaw speed at the slewing ring of:

• Desired output speed (slewing ring):

The required motor (pinion) speed is:

Convert to RPM:

So, for an output yaw speed of 150°/s, the motor driving the 17-tooth pinion must run at approximately 124 rpm with this gear ratio. This is a reasonable speed range for many commercial motors and servo systems. When final requirements are known, the same equations can be reused with updated ωout and tooth counts.

Baseline Torque Relationship

Once the required yaw torque at the slewing ring is known from the load analysis (see below), the required motor torque can be estimated as:

where

Tslewing = required torque at the slewing ring (from loads),

GR = 4.94 (gear ratio),

η = efficiency of gear mesh (typically 0.90-0.95 for a well-lubricated spur gear pair).

This formula provides the baseline motor torque requirement once the yaw moment demand has been quantified.

Load Calculation

To size the slewing ring and motor torque properly, the main loads on the yaw bearing must be identified. Figure B1 illustrates the free-body diagram of loads acting on the bearing.

• Fa: Axial Load
• Total vertical load acting along the axis of rotation.
• Primarily the weight of all rotating components above the bearing (torso frame, arms, padding, sensors, etc.).
• Fr: Radial Load
• Forces acting perpendicular to the axis of rotation in the horizontal plane.
• Includes horizontal components of punching forces and any off-centre loads due to asymmetric mass distribution.
• Mk: Tilting Moment (Overturning Moment)
• Moment about a horizontal axis through the bearing centre.
• Arises from:
• The weight of the robot’s upper structure acting at a vertical offset from the bearing centre (lever arm).
• Punching forces applied away from the bearing centre (e.g. at the head or torso pad).
• Typically, the most critical load for a slewing ring.
• Calculated from the centre of gravity (CG) location and impact points:

where F is the force (weight or impact) and d is the perpendicular distance from the bearing centre to the line of action.

• Impact/Shock Loads
• Boxing involves dynamic impacts. A peak punch force (e.g. up to ~1.5 kN for an average user) acting at a certain radius from the bearing centre will generate a short duration tilting moment.
• Such impacts are typically accounted for using a service factor (SF) , multiplying the nominal static/dynamic loads by SF to ensure the bearing and gear are sized with sufficient margin.
• Mr: Friction Torque
• Internal resistance to rotation within the bearing assembly, dependent on bearing diameter, preload, rolling element type, and lubricant.
• Contributes to the baseline torque the motor must overcome even without external loads.

These load components form the input to the manufacturer’s rating charts (axial load, radial load, tilting moment, friction torque). Once the final BoxBunny mass distribution and impact assumptions are fixed, the above framework can be used to:

1. Compute Fa, Fr, and Mk
2. Check the slewing ring against its static and dynamic load ratings .
3. Derive the required yaw torque at the bearing, and from that, the motor torque using the gear ratio and efficiency relations described earlier.

2.1.3.3. Motor Selection

The yaw motor, together with the pinion and slewing ring, must rotate the full BoxBunny upper body about the vertical axis while resisting punching-induced moments. Key requirements are:

• Sufficient torque at the pinion to overcome:
• Tilting moment transmitted through the gear mesh
• Bearing friction torque
• Inertial torque during acceleration/deceleration
• Target speed: ≈150°/s at the slewing ring (≈124 rpm at the motor with GR ≈ 4.94).
• Accurate, repeatable positioning for defined yaw angles (e.g. open/closed stance modes).
• Good disturbance rejection so the robot does not “twist” or lose position under impacts.
• Reasonable cost and manageable integration for a student prototype.

Three realistic motor options were evaluated: Brushed DC Gearmotor , Closed-Loop Stepper / Hybrid Servo, and BLDC Servo Motor with Encoder .

Brushed DC Gearmotor

Brushed DC motors with an integrated gearhead are common in low-cost rotary stages.

Advantages

• Simple control: speed is roughly proportional to applied voltage; direction via polarity.
• High torque at low output speed due to the gearhead, potentially matching the required 124 rpm.
• Widely available and relatively low cost.

Limitations for BoxBunny

• Poor inherent position control: without an encoder and closed-loop controller, position is estimated indirectly (via time/voltage), which is not precise enough for repeatable yaw angles.
• Even with an encoder added, most low-cost gearmotors exhibit gearbox backlash , causing play at the pinion and visible yaw “looseness” when punched.
• Brushed motors have brush wear and may require more maintenance in long-term use.

Thisis attractive for torque and simplicity, but the combination of backlash and weaker position control makes it unsuitable as the primary yaw actuator for BoxBunny.

Closed-Loop Stepper / Hybrid Servo

A closed-loop stepper (hybrid servo) combines a stepper motor, encoder and smart driver. It accepts step commands but internally closes the loop on position/velocity.

Advantages

• Good low-speed torque, suitable for directly driving the pinion through the 4.94:1 reduction.
• Encoder feedback allows detection and correction of position error under disturbances.
• Relatively simple command interface (often still step) compatible with typical motion controllers.
• More compact and affordable than many industrial AC servos.

Limitations for BoxBunny

• Steppers have torque ripple and microstep non-linearity , which can produce small angle “cogging” in very slow or static positions; this can be felt as slight chatter when the user manipulates the robot.
• Dynamic performance is good but not as smooth as a sinusoidally-commutated BLDC servo under rapid direction changes.
• Acoustic noise and vibration can be higher than BLDC at certain operating points.

This is a solid option with closed-loop control and adequate torque, but its torque ripple and dynamic smoothness are not ideal for a “human-facing” yaw motion that should feel smooth and responsive.

BLDC Servo Motor with Encoder

This option uses a brushless DC motor paired with a rotary encoder and a FOC/servo driver that closes the loop on current, velocity and position (i.e. a BLDC servo).

Advantages

• Smooth, sinusoidal torque production via field-oriented control, reducing cogging and vibration at low speed.
• Good torque–speed envelope easily meets ≈124 rpm at the motor with margin for faster moves if desired.
• Closed-loop position control using encoder feedback → high yaw accuracy and repeatability, with active disturbance rejection under punching.
• High efficiency and low maintenance, since there are no brushes to wear out.
• Well suited to continuous rotary motion with frequent accelerations and reversals.

Limitations

• Higher integration complexity than a simple DC gearmotor: requires encoder wiring and a suitable BLDC servo driver.
• Typically, more expensive than basic brushed motors, though often comparable to or slightly above closed-loop steppers depending on brand.

This offers the best combination of smoothness, torque, precision and disturbance rejection for BoxBunny’s yaw axis, at a complexity level that is still manageable for a student build using off-the-shelf BLDC servo drivers.

Selection Matrix – Motor in Yaw Rotation System

Table shows the decision matrix created to aid the motor in yaw rotation system selection.

Motor Selection for BoxBunny Yaw Rotation

For the yaw axis, user perception and stability are critical: the robot should rotate smoothly and feel “solid” under punches, without rattling or drifting from its commanded angle. While brushed DC gearmotors and closed-loop steppers can meet basic torque and speed needs, their drawbacks in backlash and torque ripple/noise make them less suitable.

Given these requirements, BLDC Servo Motor with Encoder is chosen as the yaw motor. It provides smooth and controlled yaw motion, high positional stiffness and repeatability, and robust behaviour under impact-induced disturbances, all of which align well with the BoxBunny design goals.

2.2. Height Adjustment

The height adjustment system must vary the BoxBunny torso/head height by approximately 300–400 mm to accommodate different user heights, while:

• Carrying the axial load of the upper structure (~20–25 kg plus padding and arms)
• Resisting lateral forces and overturning moments from punches
• Remaining stiff and “non-bouncy” during training (once set, the height should not oscillate)
• Being safe and simple for users to adjust occasionally (not continuously during a round)
• Fitting within the available vertical envelope above the linear stage

Three concepts were explored.

Concept #1: Office Chair Mechanism + Vertical Linear Guide

The first idea was to adapt the office chair mechanism (Figure B2): a gas lift cylinder to carry the axial load, combined with vertical linear guides at the back to resist lateral punching forces.

Advantages

• Gas lifts are readily available and compact.
• Can carry the static axial load of the robot mass.
• Vertical linear guides at the back can be sized to take lateral loads and overturning moments.

Limitations for BoxBunny

• Gas cylinders are inherently compliant – they behave like a spring/damper. Under repeated punches, this leads to vertical “bouncing” of the torso, which feels unrealistic and reduces strike rigidity.
• The internal gas spring force varies with stroke; stiffness is not easily tuned for the specific mass and dynamic loads.
• Conventional office-chair gas lifts are designed for human seating comfort , not for resisting repeated impact loads at an elevated lever arm.

Conclusion: While mechanically simple, Concept #1 does not provide sufficient vertical stiffness; the gas lift introduces unwanted bounce under impact.

Concept #2: Electric Gas Strut + Motorised Vertical Linear Guide

The second concept combined an electrically actuated mechanism for height control with a gas strut at the bottom to assist in carrying axial load (Figure B3):

• A motorised drive (e.g. screw or belt) to move the upper carriage along vertical linear guides.
• A gas strut mounted beneath to partially support the robot’s weight.

Advantages

• Allows push-button height adjustment, potentially even mid-session.
• The gas strut can reduce the required motor torque by counterbalancing part of the robot’s weight.
• Vertical linear guides again provide lateral and moment support .

Limitations for BoxBunny

• Mechanically and electrically complex: coordinating motor drive, gas strut support, and guide loads is non-trivial.
• Multiple elements share the load path (motor, screw/drive, gas strut, guides), making stiffness and reliability harder to predict .
• For an application where height is adjusted occasionally (between users, not continuously), the added complexity and cost of powered adjustment is not justified.

Conclusion: Concept #2 offers convenience but at the cost of high complexity and a more difficult load path. It is unnecessarily sophisticated for a height setting that is changed only occasionally.

Concept #3: Manual Screw Jack + Vertical Linear Guides

The final concept uses a manual screw jack with handwheel mounted on top of the linear stage, combined with vertical linear rails as the primary guides.

• The screw jack carries the axial load of the robot and provides precise height adjustment via handwheel rotation.
• Two vertical linear rails (each with multiple long blocks) mounted behind the robot carry lateral forces and overturning moments from punches, ensuring the structure does not tilt.

Advantages

• High axial load capacity and stiffness: screw jacks are designed for heavy-duty industrial applications; they carry the full weight with minimal vertical compliance.
• Self-locking: with an appropriately chosen screw lead and efficiency, the jack is non-backdrivable, so the height does not drift or bounce under impacts. No extra brake is required.
• Simple load path: axial loads go directly through the screw jack; lateral and moment loads are taken by the vertical rails → easier to analyse and design.
• Manual but ergonomic: height is adjusted via a handwheel; this is acceptable because height changes occur between users or rounds, not continuously.
• Mechanically robust and maintainable, using standard industrial components (jack + linear guides).

Limitations

• Requires manual effort to adjust height; not as convenient as push-button systems.
• Adjustment speed is slower than an electric actuator, though still acceptable for occasional use.

Conclusion:

Concept #3 provides high holding stiffness, self-locking behaviour , and a clear separation of axial vs lateral/moment support . It meets BoxBunny’s functional requirements with moderate cost and manageable complexity and is therefore selected as the height adjustment solution.