Mechanics

From Bespoke Robot Society
Jump to navigation Jump to search
Mechanics
Understanding what actuates and what is actuated, what the software models, and where the electronics signals end
Difficulty Range Beginner to Advanced
Time to Basic 1-3 weeks
Essential Tools Calipers, screwdriver set, hex keys, ruler
Optional Tools CAD software (FreeCAD, Fusion 360), torque wrench, 3D printer
Get Started Mechanical Design
Unlocks (Basic) Capability:Differential Drive, Capability:Line Sensing
Unlocks (Advanced) Capability:Omni Drive, Capability:Gripper Control, Capability:Servo Control

Mechanics is the competency of understanding physical motion, forces, and mechanical systems in robotics. Mechanics is where electronic signals become physical movement - motors spin wheels, servos rotate arms, and gears multiply force. It encompasses understanding kinematics (motion), dynamics (forces), power transmission, and structural design.

Mechanics is distinct from 3D Printing (the fabrication technique) and CAD (the design software). This competency focuses on understanding mechanical systems: how they move, how forces propagate, and how to design mechanisms that work reliably.

Why Mechanics Matters for Robotics

Mechanics is the body and muscles of a robot:

  • Actuators convert electrical energy into motion (motors, servos, solenoids)
  • Transmissions modify speed and force (gears, belts, linkages)
  • Chassis provide structural support and mounting points
  • Wheels and drive systems enable locomotion
  • End effectors interact with the environment (grippers, arms, tools)

Without mechanics knowledge, you can't design effective robots, predict their behavior, or troubleshoot motion problems. The software can send perfect signals, and the electronics can deliver perfect power, but if the mechanics are wrong, the robot won't work.

Skill Progression

Beginner (SimpleBot Level)

Skills you need to build and understand SimpleBot:

  • Basic kinematics - Understand how wheels create motion, turning radius, velocity
  • Differential drive - Two-wheel steering (forward, backward, turning)
  • Motors - DC motor basics, direction control, speed control
  • Wheels and traction - Wheel diameter, friction, slipping
  • Center of mass - Weight distribution, stability, tipping
  • Mechanical assembly - Using screws, nuts, standoffs, spacers
  • Basic measurements - Using calipers, measuring wheel diameter, gear pitch

Unlocks:

Tutorials: Mechanical Design, Motor Control Basics

At this level you can:

  • Build SimpleBot from parts or a kit
  • Understand how differential drive works
  • Mount sensors and motors to a chassis
  • Troubleshoot basic mechanical issues (loose wheels, misaligned sensors)
  • Calculate turning radius and basic motion planning

Intermediate (Expanding Capabilities)

Skills for designing more complex mechanisms:

  • Gear ratios - Speed reduction, torque multiplication, efficiency
  • Torque calculations - Motor torque, load torque, stall conditions
  • Kinematics analysis - Forward kinematics, wheel velocities, robot velocity
  • Structural design - Load paths, stress, strain, material selection
  • Bearings and bushings - Reducing friction in rotating joints
  • Belt and pulley systems - Timing belts, GT2 belts, tensioning
  • Linkages - Four-bar linkages, parallel linkages, mechanical advantage

Unlocks:

Tutorials: Gear Ratio Calculation, Kinematics Tutorial

At this level you can:

  • Select motors based on torque and speed requirements
  • Design gear trains for speed reduction or torque multiplication
  • Calculate robot velocity from wheel speeds and encoder counts
  • Design custom mounts and brackets in CAD
  • Integrate servos and multi-DOF mechanisms
  • Troubleshoot binding, friction, and alignment issues

Advanced (Custom Designs)

Skills for designing robots from scratch:

  • Inverse kinematics - Calculate joint angles for desired end effector position
  • Dynamics - Forces, inertia, acceleration, momentum
  • Motor selection - Match motor to application (torque-speed curves, efficiency)
  • Mechanical advantage - Leverage, gear ratios, compound machines
  • Finite element analysis - Simulate stress and deformation in parts
  • Closed-loop control - PID control for position, velocity, torque
  • Advanced drive systems - Omni wheels, mecanum wheels, swerve drive

Unlocks:

Tutorials: Inverse Kinematics, PID Control for Motors

At this level you can:

  • Design multi-DOF robot arms with IK solvers
  • Optimize mechanical systems for efficiency and performance
  • Perform stress analysis and select materials appropriately
  • Design custom transmissions (gearboxes, differentials)
  • Implement advanced control algorithms for mechanical systems
  • Troubleshoot complex mechanical vibration and resonance issues

Learning Paths

Path 1: SimpleBot Builder (Beginner)

  1. Start with Mechanical Design - Learn basic concepts and measurements
  2. Build SimpleBot - Hands-on experience with differential drive
  3. Experiment with wheel changes and sensor mounting
  4. Study Motor Control Basics - Understand motor behavior

Result: You can build SimpleBot and understand how differential drive works.

Path 2: Mechanism Designer (Intermediate)

  1. Complete Path 1 (SimpleBot Builder)
  2. Study Gear Ratio Calculation - Learn about gear trains
  3. Study Kinematics Tutorial - Calculate velocities and motion
  4. Design a custom gripper or sensor mount in CAD
  5. Add encoders or servos to SimpleBot

Result: You can design custom mechanical systems and calculate their behavior.

Path 3: Robot Architect (Advanced)

  1. Complete Path 2 (Mechanism Designer)
  2. Study Inverse Kinematics - Multi-DOF robot arms
  3. Study PID Control for Motors - Closed-loop position control
  4. Design a robot arm or custom drive system
  5. Implement inverse kinematics in software

Result: You can design complex robots with multi-DOF mechanisms from scratch.

Essential Concepts

Differential Drive

Differential drive is the simplest robot steering method - two independently-controlled wheels:

  • Forward - Both wheels rotate forward at same speed
  • Backward - Both wheels rotate backward at same speed
  • Turn right - Left wheel faster than right wheel (or right wheel backward)
  • Turn left - Right wheel faster than left wheel (or left wheel backward)
  • Rotate in place - One wheel forward, one wheel backward

Turning radius depends on wheel speeds:

  • Equal speeds = infinite turning radius (straight line)
  • Opposite speeds = zero turning radius (rotate in place)
  • Different speeds = turning radius between zero and infinity

SimpleBot uses differential drive with two DC motors and a caster wheel for balance.

Kinematics: Motion Without Forces

Kinematics describes motion without considering forces:

  • Linear velocity (v) - Speed in a straight line (m/s or cm/s)
  • Angular velocity (ω) - Rotational speed (rad/s or degrees/s)
  • Wheel velocity - Linear velocity at wheel edge: v = ω × r (radius)

For differential drive robots:

  • Robot velocity = average of left and right wheel velocities
  • Robot turning rate = difference of left and right wheel velocities divided by wheel separation

Example: If left wheel is at 10 cm/s and right wheel is at 6 cm/s:

  • Robot moves forward at (10 + 6) / 2 = 8 cm/s
  • Robot turns right at (10 - 6) / wheel_separation rad/s

Dynamics: Forces and Torque

Dynamics describes motion considering forces:

  • Force (F) - Push or pull measured in Newtons (N)
  • Torque (τ) - Rotational force measured in N·m or oz-in
  • Newton's Second Law - F = m × a (force = mass × acceleration)

For motors:

  • Motor torque determines acceleration and maximum load
  • Stall torque - Maximum torque when motor is not rotating
  • Free-run speed - Maximum speed when motor has no load
  • Operating point is between stall and free-run

Why this matters:

  • Underpowered motor can't move the robot (insufficient torque)
  • Oversized motor wastes battery and adds weight
  • Steep ramps require more torque than flat ground

Gear Ratios and Mechanical Advantage

Gear ratio = Output teeth / Input teeth (or output speed / input speed)

  • Speed reduction - Large gear ratio (e.g., 30:1) decreases speed, increases torque
  • Speed increase - Small gear ratio (e.g., 1:2) increases speed, decreases torque
  • Power is conserved (minus losses) - Speed × Torque is constant

Example: Motor with 100 oz-in torque at 300 RPM, 30:1 gearbox:

  • Output torque ≈ 100 × 30 = 3000 oz-in
  • Output speed = 300 / 30 = 10 RPM
  • Power is approximately conserved (ignoring 5-10% friction losses)

Common applications:

  • High-speed motors with gearboxes for robot wheels
  • Servo motors have internal gearboxes for high torque
  • Multi-stage gearboxes for extreme ratios (100:1, 200:1)

Center of Mass and Stability

Center of mass (COM) is the average position of all the robot's mass:

  • Low COM = more stable (less likely to tip)
  • High COM = less stable (easier to tip)
  • COM should be centered between support points (wheels)

Stability rules:

  • Robot tips when COM moves outside support polygon
  • Add caster wheels to expand support polygon
  • Keep heavy components (batteries, motors) low and centered
  • SimpleBot uses front caster for three-point stability

Friction and Traction

Friction determines how much force wheels can transfer to the ground:

  • Static friction - Maximum force before slipping starts
  • Kinetic friction - Force during sliding (less than static)
  • Coefficient of friction (μ) - Material property (rubber on wood ≈ 0.7)

Maximum force = μ × Normal force (weight on wheel)

Why this matters:

  • Wheels slip if motor torque exceeds traction limit
  • More weight on drive wheels = more traction
  • Rubber wheels have better traction than plastic
  • Encoders may give false readings if wheels slip

Mechanical Design Principles

Tolerances for 3D Printing

3D printed parts need clearance for fit:

  • Press fit - 0.0 to 0.1mm clearance (interference fit)
  • Sliding fit - 0.2 to 0.3mm clearance
  • Loose fit - 0.5mm or more clearance
  • Horizontal holes print smaller than designed (add 0.2mm)
  • Vertical holes print more accurately

Fasteners and Mounting

  • M3 screws - Most common robot fastener (3mm diameter)
  • Standoffs - Spacers to mount PCBs and separate layers
  • Heat-set inserts - Metal threads in 3D printed parts
  • Thread-forming screws - Cut threads directly into plastic (one-time use)

Material Selection

  • PLA - Stiff, brittle, easy to print (good for prototypes)
  • PETG - Flexible, strong, impact-resistant (good for functional parts)
  • ABS - Strong, heat-resistant (hard to print)
  • Acrylic - Laser-cut sheets for chassis (cheap, rigid)
  • Aluminum - High strength-to-weight, machineable (expensive)

Tools and Equipment

Essential Tools (Start Here)

  • Calipers ($15-30) - Measure parts accurately (0.01mm precision)
  • Screwdriver set ($10-20) - Phillips and flathead
  • Hex key set ($5-15) - Allen wrenches for socket head screws
  • Ruler ($5) - Quick measurements and alignment
  • Pliers ($10) - Hold nuts, bend wire, grip small parts

Intermediate Tools

  • CAD software (Free - $500/year) - FreeCAD (free), Fusion 360 (hobbyist free)
  • 3D printer ($200-500) - Print custom parts
  • Drill and bits ($30-60) - Make holes in chassis
  • File set ($10-20) - Clean up burrs and adjust fit

Advanced Tools

  • Torque wrench ($30-100) - Precise fastener tightening
  • Dial indicator ($20-50) - Measure runout and alignment
  • Tap and die set ($20-40) - Create or repair threads
  • Lathe or mill ($500+) - Machine custom metal parts

Common Pitfalls

  • Underpowered motors - Calculate required torque before selecting motors
  • Wheel slippage - Encoders are useless if wheels slip on smooth surfaces
  • Loose wheels - Vibration loosens set screws; use threadlocker
  • Misaligned sensors - Line sensors must be parallel to ground and perpendicular to line
  • Over-constrained assemblies - Too many attachment points cause binding
  • Ignoring tolerances - 3D printed parts won't fit together without clearance
  • Top-heavy designs - High center of mass causes tipping
  • Ignoring wire routing - Wires get caught in wheels or mechanisms

Tutorials and Resources

BRS Tutorials

Component Pages

External Resources

Related Competencies

  • Electronics - Provides power and signals to actuators
  • Software - Sends commands to move mechanical systems
  • 3D Printing - Fabricates mechanical parts
  • CAD - Designs mechanical parts and assemblies
  • Soldering - Assembles electronics that control mechanics

See Also

Retrieved from "https://wiki.bespokerobotsociety.org/index.php?title=Mechanics&oldid=84"