Vext Steps:
Foundational Concepts for Building Competition Robots with the Vex Robotics System
Table of Contents
Foreword
1. Design Basics
a) Chassis Design (Square/Rectangular, U/H-shaped, 3-Dimensional)
b) Speed, Torque, and Gear Ratios
c) Structural Snafus: 101 Ways to Build a Robot That Falls Apart
2. Tank-Type Drives (Differential Drives)
a) Direct Drive (2-wheel & 4-wheel)
b) Chain Drive (separated motors)
c) Gear-Coupled Drive
d) Chain-Coupled Drive
3. Omni-Directional Drives
a) Plus-Holonomic Drive
b) X- Holonomic Drive
c) H-Drive (Slide Drive)
d) Mecanum Drive
e) Kiwi Drives
4. Lifts
a) Hinge Lifts
b) Multi-Stage Lifts
c) Four-Bar Lifts, half-circle rotation
d) Four-Bar Lifts, full-circle rotation
e) Six-Bar Lifts (also 8/10, etc. bar-lifts)
f) Scissor Lifts
g) Linear Slides
h) Multi-stage Linear Slides
i) Reverse Double 4-Bar Lifts
5. Gripping/Grabbing Devices
a) The Claw
b) Continuous Conveyors
c) Top Rollers
d) Side Rollers
e) Passive intakes
6. Programming Basics
a) Autonomous Control - Multiple autonomous modes
b) Operator/Remote Control
c) Templates
7. Common Sensor Uses
a) Quadrature Encoder – Distance measurement
b) Potentiometer – Arm rotation
c) Limit & Bumper Switches – Programmed stops, object possession
d) Ultrasonic Sensor – Obstacle detection
e) Line Tracker – Line/light following
8. Some nifty mechanisms
a) Catapult
b) Gear Shifter
c)
9. Additional Resources
Section 2: Tank-Type Drives
A differential (or tank) drive is constructed with a motor (or motors) on each side. The difference in speed between the 2 sides produces; when the speeds on the 2 sides are equal, the vehicle drives straight. Commonly used differential drives include 2, 4, and 6-wheel drives, which are commonly programmed to be driven tank-style, with the left and right "sticks" controlling each side. A few examples of tank drives are shown below.
2-Wheel Drive (not commonly used in competition)
Advantages
· Easy to construct
Disadvantages
· Hard to maneuver (turning is difficult)
· Only 2 motors means not much power to wheels
4-Wheel Drive, Direct Drive (with 2 unpowered wheels) – motors are connected directly to the axle of each driven wheel
Advantages
· Easy to construct and repair if motors die
· Lack of gearing or chains means there is no power loss through energy transfer
Disadvantages
· Placement of motors is inflexible
· Speed is fixed at the motor speed
4-Wheel Drive, Chained motors (uncoupled) – each wheel has its own separate motor
Advantages
· Chain allows motors to be moved to other locations
· Speed can be modified by changing the sprocket ratios
Disadvantages
· Regular chain breaks easily. High strength chain is better.
· Slight power loss (chains and gears typically lose 3-5% with every energy conversion)
6-Wheel Drive (4 motors) – Gear Coupled
Advantages
· Gearing is more robust than chain (less breakable)
· If a wheel lifts off the ground, coupled motors/wheels divert power to the other wheel, so motor power is not wasted
Disadvantages
· Motor placement is less flexible – dependent on the gear spacing, though idler gears can be of different sizes than the drive wheels
6-Wheel Drive (6 motors) – Chain Coupled
(Rick TYler's image – get permission)
Advantages
· Motor placement is flexible
· 6 motors give more power
· Power is diverted to other wheels if one lifts off the ground
Disadvantages
· 6 motors on drive leave only 4 for manipulators
· Chain is breakable (though HS chain is more robust than regular)
· Multi-stage chain results in small (3-5%) power loss with each transfer, due to “slop”
Section 3 - Omnidirectional Drives
A holonomic (or omni) drive has motion with 3 degrees of freedom, unlike differential drives, which have only 2 degrees of freedom. As a result, a holonomic drive can shift from side to side or strafe diagonally without changing the direction of its wheels. It uses omniwheels to minimize drag. Two common configurations are the Plus-Holonomic Drive and X-Holonomic Drive, named to describe their axes of motion.
Plus-Holonomic (+Holonomic) - Wheels mounted parallel to the chassis sides
Advantages
· Construction and programming are relatively simple
Disadvantages
· Since forward/backward (rather than diagonal) motion tends to be the most common direction of motion, it uses only 2 motors much of the time, making it especially vunerable to imbalances in weight. With the manipulator shown, there most of the weight rides on the wheel to the left, giving poor traction
X- Holonomic Drive – wheels mounted at 45o angles to the chassis sides
Advantages
· Because forward/backward motion uses all 4 wheels, less vunerable to imbalances in weight
Disadvantages
· More difficult to construct, especially bracing diagonal pieces
· More complex programming required (except in EasyC v4)
H-Drive/Slide Drive (scidkelley’s picture, get permission)
Advantages
· Build, drive, and programming is similar to a 4-wheel drive
· 1 or 2 wheels in center allow for side-to-side motion and possibly strafing
Disadvantages
· Single motor drive means that side-to-side motion is weak
· Strafing is asymmetrical
· Requires use of all omniwheels, making robot susceptible to being pushed.
Mecanum Drive
Advantages
· Constructed like a 4-wheel drive (easier than X – holonomic)
· Strafes like a holonomic drive
Disadvantages
· Diagonal wheels direct motion at 45 degree angle to axles, resulting in power loss.
Kiwi Drive – 3-Wheeled Holonomic Drive (This is more of a fun exercise than a practical drive for competition)
(Foster's picture, get permission)
Advantages
· Fewer wheels means a smaller footprint, allowing better access to tight spaces
Disadvantages
· More difficult to construct, especially bracing diagonal pieces
· More complex programming required
(ignore this – it’s very incomplete)
Lifts
Hinge Lift – single axle
2-stage Gear Reduction Lift
4-bar lift - same plane
4-bar lift – staggered bars
6/8/10-bar lift; Scissor