trackedbot

Mechanical Chassis & Platform - tracked_bot

Mechanical system overview, physical chassis specifications, and assembly documentation for the tracked_bot autonomous tracked robot platform.

📋 Table of Contents

Mechanical System Overview
Platform Visual Feature Breakdown
Top Deck & Chassis Geometry Diagram
T101 Chassis Specifications
Skid-Steering Kinematics
Mechanical Documentation & Guides
Media & Diagrams Index

🚜 Mechanical System Overview

The mechanical locomotion platform of tracked_bot is built upon the T101 Mini Tracked Tank Chassis. Designed for structural rigidity, stability on uneven terrain, and high payload-to-weight ratio, the platform employs a high-grade anodized aluminum alloy chassis driven by two independent DC metal gearmotors coupled to modular continuous caterpillar tracks.

T101 Tracked Platform

The platform operates via differential skid-steering (tank steering), enabling 360° in-place zero-radius turns by driving opposing tracks in reverse directions, as well as smooth variable-radius turning arcs.

🔍 Platform Visual Feature Breakdown

Referencing the physical platform photograph above (images/body_platform.png), the mechanical architecture consists of seven primary functional sub-assemblies:

  (1) Front Bearing Idler Wheels (Dual Ball Bearings + 5-Screw Hub Flange)
  (2) High-Traction Modular Tracks (Engineering Plastic Links with Inner Guide Lugs)
  (3) Formed 6061 Aluminum Deck & Side Chassis Walls (2.0 mm Anodized Alloy)
  (4) Central Circular Cable Pass-Through Port (Clean Vertical Wiring Harness Route)
  (5) 4x Longitudinal Accessory Mounting Slots (Sliding Brackets & Sensor Mounts)
  (6) Standardized M3 Controller Standoff Pattern (Arduino Uno R3 / Mega Footprint)
  (7) Rear Toothed Drive Sprockets (CNC Aluminum Keyed to Motor D-Shafts)
  (8) Under-Deck 25 mm Metal DC Gearmotors & Protected Battery Bay

1. Monolithic 6061 Aluminum Chassis Body

The main chassis plate is precision-cut and CNC-bent from 2.0 mm thick 6061 sandblasted and anodized aluminum alloy. The downward-folded structural side flanges form an inverted U-channel profile that exhibits exceptional torsional stiffness against skid-steering shear stresses while maintaining an overall chassis weight of only ~530 g.

2. Top Equipment Deck & Mounting Slots

As shown in images/body_platform.png, the upper deck incorporates:

Central Cable Pass-Through Hole ($\varnothing \approx 18\text{ mm}$): Allows motor power leads, battery cables, and encoder wires to route directly up from the lower equipment bay to the controller shield without exterior hanging wires.
Four Longitudinal Adjustment Slots ($4 \times 35\text{ mm}$): Permit continuous linear positioning for sensor brackets, camera pan-tilt gimbals, ultrasonic distance sensors, or battery securing straps.
M3 Mounting Hole Array: Pre-tapped and clearance holes matching the standard Arduino Uno R3 / Mega 2560 standoff pitch ($53.3\text{ mm} \times 27.9\text{ mm}$), ensuring rigid mounting of the Arduino Uno R3 and L293D motor driver stack.

3. Drive Sprockets vs. Idler Wheels

Toothed Drive Sprockets (Rear): Machined from solid aluminum with radial weight-reduction cutouts. The outer circumference features positive-engagement gear teeth that mate precisely with the drive windows of the caterpillar track links. Each sprocket hub contains two M3 set-screws (grub screws) that lock onto the flat facet of the motor D-shaft.
Bearing Idler Wheels (Front): Machined aluminum wheels featuring dual miniature deep-groove ball bearings and a 5-screw outer retaining ring. The idlers maintain track alignment, tension, and provide an elevated obstacle approach angle.

4. Continuous Caterpillar Tracks

The track belts consist of modular, high-durability engineering plastic / POM (polyoxymethylene) interlocking tread links connected with stainless steel hinge pins:

Outer Surface: Aggressive chevron-patterned anti-skid ribs for high friction on carpet, concrete, tile, and packed dirt.
Inner Surface: Double-ridge center guide horns that straddle the sprocket teeth and idler wheel rims, preventing track derailment during aggressive in-place skid-steering maneuvers.

5. Lower Equipment & Motor Bay

Located directly beneath the upper aluminum deck, the central chassis cavity encloses:

Dual 25 mm High-Torque DC Gearmotors: Cylindrical metal gearmotors (visible in images/body_platform.png under the deck edge) secured with four M3 machine screws into the structural side walls.
Low Center-of-Gravity (CG) Battery Bay: Accommodates a 2S 18650 Li-ion cell holder or 7.4V LiPo battery. Placing the heaviest component below the wheel axle line ensures maximum incline stability and eliminates chassis roll during fast braking.

📐 Top Deck & Chassis Geometry Diagram

The following ASCII schematic illustrates the physical top-down layout and component distribution observed in images/body_platform.png:

flowchart TD
    subgraph Front ["<b>FRONT (Obstacle Approach Side)</b>"]
        direction LR
        IDLER_L["<b>Left Bearing Idler</b><br/>Dual Ball Bearings + Flange"]
        APPROACH["<b>Approach Clearance</b><br/>~18 mm Ground Clearance"]
        IDLER_R["<b>Right Bearing Idler</b><br/>Dual Ball Bearings + Flange"]
    end

    subgraph ChassisDeck ["<b>T101 Top Equipment Deck (2.0 mm Aluminum)</b>"]
        direction TB

        subgraph UpperSlots ["Accessory Mounts (Front)"]
            SLOT1["<b>Mounting Slot 1</b><br/>4 x 35 mm"]
            SLOT2["<b>Mounting Slot 2</b><br/>4 x 35 mm"]
        end

        subgraph ElectronicsBay ["Electronics Mounting Pattern"]
            STANDOFFS["<b>M3 Controller Standoffs</b><br/>53.3 mm x 27.9 mm Uno Pitch"]
            WIRE_PORT["<b>Central Wire Port</b><br/>Ø 18 mm Pass-Through"]
        end

        subgraph LowerSlots ["Accessory Mounts (Rear)"]
            SLOT3["<b>Mounting Slot 3</b><br/>4 x 35 mm"]
            SLOT4["<b>Mounting Slot 4</b><br/>4 x 35 mm"]
        end
    end

    subgraph InternalBay ["<b>Internal Lower Bay (Below Deck)</b>"]
        MOTOR1["<b>DC Gearmotor 1</b><br/>Right Track Drive"]
        BATTERY["<b>Low-CG Battery Bay</b><br/>2S Li-Ion / LiPo Pack"]
        MOTOR2["<b>DC Gearmotor 2</b><br/>Left Track Drive"]
    end

    subgraph Rear ["<b>REAR (Motor Drive Side)</b>"]
        direction LR
        SPROCKET_L["<b>Left Drive Sprocket</b><br/>Keyed Aluminum Teeth"]
        SPROCKET_R["<b>Right Drive Sprocket</b><br/>Keyed Aluminum Teeth"]
    end

    subgraph Tracks ["<b>Differential Track Belts</b>"]
        TRACK_L["<b>Left POM Track</b><br/>Interlocking Links"]
        TRACK_R["<b>Right POM Track</b><br/>Interlocking Links"]
    end

    IDLER_L --- TRACK_L --- SPROCKET_L
    IDLER_R --- TRACK_R --- SPROCKET_R

    Front --> ChassisDeck
    ChassisDeck --> InternalBay
    InternalBay --> Rear

    classDef wheelStyle fill:#1e293b,stroke:#94a3b8,stroke-width:2px,color:#ffffff;
    classDef slotStyle fill:#0f766e,stroke:#2dd4bf,stroke-width:2px,color:#ffffff;
    classDef elecStyle fill:#1e3a8a,stroke:#60a5fa,stroke-width:2px,color:#ffffff;
    classDef motorStyle fill:#831843,stroke:#f472b6,stroke-width:2px,color:#ffffff;
    classDef trackStyle fill:#4c1d95,stroke:#a78bfa,stroke-width:2px,color:#ffffff;

    class IDLER_L,IDLER_R,APPROACH,SPROCKET_L,SPROCKET_R wheelStyle;
    class SLOT1,SLOT2,SLOT3,SLOT4 slotStyle;
    class STANDOFFS,WIRE_PORT elecStyle;
    class MOTOR1,BATTERY,MOTOR2 motorStyle;
    class TRACK_L,TRACK_R trackStyle;

    style Front fill:none,stroke:#94a3b8,stroke-width:2px,stroke-dasharray: 4 4
    style ChassisDeck fill:none,stroke:#60a5fa,stroke-width:2px,stroke-dasharray: 4 4
    style UpperSlots fill:none,stroke:#2dd4bf,stroke-width:1px
    style ElectronicsBay fill:none,stroke:#60a5fa,stroke-width:1px
    style LowerSlots fill:none,stroke:#2dd4bf,stroke-width:1px
    style InternalBay fill:none,stroke:#f472b6,stroke-width:2px,stroke-dasharray: 4 4
    style Rear fill:none,stroke:#94a3b8,stroke-width:2px,stroke-dasharray: 4 4
    style Tracks fill:none,stroke:#a78bfa,stroke-width:2px,stroke-dasharray: 4 4

⚙️ T101 Chassis Specifications

Parameter	Specification	Engineering Details & Observations
Model	T101 Mini Tracked Platform	Standard desktop-scale autonomous robotics platform
Chassis Material	6061 Aluminum Alloy	2.0 mm thickness, sandblasted, anti-oxidation anodized
Track Material	Engineering Plastic (POM / Nylon)	Modular links linked by stainless steel hinge pins
Wheel Material	CNC Machined Aluminum	Anodized finish, dual ball bearings on idlers
Overall Dimensions ($L \times W \times H$)	~190 mm $\times$ 165 mm $\times$ 60 mm	Compact desktop footprint
Track Width	~40 mm	Large contact patch distributing ground pressure
Track Gauge ($W$)	~125 mm	Centerline distance between left and right tracks
Ground Contact Length ($L$)	~110 mm	Effective track surface in ground contact
Unladen Weight	~530 g	Bare chassis including motors, tracks, and wheels
Maximum Payload	~2.0 kg – 3.0 kg	Ample margin for controller, batteries, and sensor payloads
Operating Voltage	6.0 V – 9.0 V DC	Nominal 7.4 V from 2S Li-ion / LiPo pack
Ground Clearance	~18 mm	Minimum underbody clearance over obstacles
Locomotion Mode	Differential Skid-Steering	Zero-radius in-place spin ($R = 0$) capability

🔄 Skid-Steering Kinematics

Differential tracked locomotion relies on differing track velocities ($v_L$ and $v_R$) to effect steering:

flowchart TD
    subgraph CHASSIS ["<b>Differential Drive Platform (Track Gauge W = 125 mm)</b>"]
        direction LR
        subgraph LEFT_SIDE ["<b>Left Drive System</b>"]
            TL["<b>TRACK LEFT</b><br/>Motor 3 (D6 PWM)"] --> VL["<b>Linear Velocity: v_L</b>"]
        end

        TL <-->|"Track Gauge W = 125 mm"| TR

        subgraph RIGHT_SIDE ["<b>Right Drive System</b>"]
            TR["<b>TRACK RIGHT</b><br/>Motor 1 (D11 PWM)"] --> VR["<b>Linear Velocity: v_R</b>"]
        end
    end

    subgraph KINEMATICS ["<b>Kinematic Equations of Motion</b>"]
        direction TB
        V_LIN["<b>Linear Forward Velocity</b><br/><code>v = (v_R + v_L) / 2</code>"]
        OMEGA["<b>Angular Turn Rate</b><br/><code>ω = (v_R - v_L) / W</code>"]
        RADIUS["<b>Steering Arc Radius</b><br/><code>R = (W / 2) · (v_R + v_L) / (v_R - v_L)</code>"]
    end

    CHASSIS --> KINEMATICS

    classDef trackStyle fill:#065f46,stroke:#34d399,stroke-width:2px,color:#ffffff;
    classDef velStyle fill:#0284c7,stroke:#38bdf8,stroke-width:2px,color:#ffffff;
    classDef eqStyle fill:#4338ca,stroke:#818cf8,stroke-width:2px,color:#ffffff;

    class TL,TR trackStyle;
    class VL,VR velStyle;
    class V_LIN,OMEGA,RADIUS eqStyle;

    style CHASSIS fill:none,stroke:#38bdf8,stroke-width:2px,stroke-dasharray: 4 4
    style LEFT_SIDE fill:none,stroke:#34d399,stroke-width:1px
    style RIGHT_SIDE fill:none,stroke:#34d399,stroke-width:1px
    style KINEMATICS fill:none,stroke:#818cf8,stroke-width:2px,stroke-dasharray: 4 4

Straight Forward/Reverse: $v_L = v_R$ (Both tracks driven at equal PWM duty cycles).
Pivot Turn (One Track Locked): $v_L = 0$ or $v_R = 0$ (Robot pivots around the stationary track; turning radius $R = W/2$).
Zero-Radius Spin (In-Place Turn): $v_L = -v_R$ (Robot spins about its geometric center with zero turning radius $R = 0$).
Curved Arc Turn: $v_L \ne v_R$ with both non-zero (Gradual steering arc of radius $R = \frac{W}{2} \cdot \frac{v_R + v_L}{v_R - v_L}$).

[!NOTE] Because skid-steering induces lateral track scrub against the ground, the motor torque demand increases significantly during turning. Ensure the L293D Motor Shield is supplied from a dedicated high-current battery rail (not USB).

📖 Mechanical Documentation & Guides

assembly_guide.md: Detailed step-by-step mechanical construction manual, covering frame assembly, motor faceplate mounting, sprocket set-screw alignment, track pin sizing and tensioning, and M3 standoff placement.
Hardware & Electrical Architecture (../hw/): Master electrical interconnect matrix, board schematics, motor shield pin mappings, and power supply design.
Software Architecture (../sw/): Embedded C motion control algorithms, skid-steering kinematics, slew rate acceleration limiters, and hardware drivers.

🖼️ Media & Diagrams Index

Platform Assembly Reference Photo: images/body_platform.png

This site is open source. Improve this page.