INTRODUCTION TO ROBOTICS Part 2: Structural System Robotics and Automation Copyright © Texas...

INTRODUCTION TO ROBOTICSPart 2: Structural System

Robotics and AutomationCopyright © Texas Education Agency, 2012. All rights reserved.

Robot Systems

• Structural System• Physical system that provides support and stability

• Propulsion System (motion)• Drive system includes motors, wheels, and gears

• Control System• Microcontroller, operating program, electrical power,

and joystick

• Tool and Actuator system• Arms, grippers, manipulators

• Sensor and feedback system• Perception, transducers

Copyright © Texas Education Agency, 2012. All rights reserved.

Structural System

• The structural subsystem of the robot is responsible for physical support.• Holds everything in place

• Also provides physical protection

• The durable “skeleton” of the robot to which all the other subsystems are attached

• The Structure and Motion subsystems are tightly integrated to form the chassis of the robot.

Copyright © Texas Education Agency, 2012. All rights reserved. Photo Credit: VEX Robotics, Inc.

Before You Begin

• Many types of tools and parts are necessary.

• Some tools and parts are required, others are simply nice to have.

• The larger the variety of supplies, the more creative the design can be.• Systems can look better and be more

structurally sound.

• Also increases the need for a tool/part inventory and management system.Copyright © Texas Education Agency, 2012. All rights reserved.

Recommended Tools

• Allen wrench set

(also called an L-wrench)

• Open ended wrench

• Screwdrivers• Flat head and Phillips

• Needle nose pliers and diagonal cutters

• Wire strippers

• Crescent wrenchCopyright © Texas Education Agency, 2012. All rights reserved.

Photo Credit: VEX Robotics, Inc.

Additional Tools

• Drill and drill bit set

• Saws for metal, wood, and plastic • Examples:

• Hacksaw, band saw, chop saw, scroll saw

• A variety of screws, nuts, bolts

• Vise

• Multi-purpose rotary power tool to cut and smooth metal

• Wire, soldering iron, electrical connectorsCopyright © Texas Education Agency, 2012. All rights reserved.

Example Parts

• Types of bolts

• 6-32 and 8-32

• Keps nuts

• Square drive shaft

• Bearing flat

• Spacers and friction reducers

• Metal sized by number of holes

5 X 15

1 X 25


Photos Credit: VEX Robotics, Inc.


Photos Credit: VEX Robotics, Inc.

The Robot Base

• The platform or base determines the stability, the durability, the maneuverability, and the functionality of the robot.

• Usually made from wood or metal

• Provides the support structure for the rest of the robot

• Everything connects and mounts here

• The frame or skeleton


Robot Stability

• Stability is defined as when the center of gravity is over the points of support for the base (called the support polygon).

• To increase stability, lower the center of gravity.

• Long arms need additional support.• With one point of support an arm will rotate.

• If the center of gravity is moved outside the support polygon the robot will tip over.


Base Material Considerations

• Common materials are wood, aluminum, sheet metal, or plastic (HDPE).

• Consider both weight and strength.• Metals have a high strength to weight ratio.

• Some plastics like plexiglass will crack or break when cut and drilled.

• Sheet metal and aluminum conduct electricity.

• Wood can splinter or split but is cheap.Copyright © Texas Education Agency, 2012. All rights reserved.


• Angled metal in an L or C shape will retain its shape under load.

• A solid square or round tube makes a very strong support structure.• Can be metal or PVC – be creative!

• Wood is great when load is not too great.• Wood can flex, bend, or break.

• Works well for a superstructure or platform.



• Even if you use wood or plastic for the base, you will need to use and cut metal.

• Motor mounts can be made from metal bent to a 90 degree angle (like angle iron).

• Wheels are an assembly with a mounting hub adapter to connect motor shaft to the wheel.

• 2 types of adapters: set screw and collet type

• Other types include keyway and D hubs.• These are usually for larger shafts.


http://www.robotgear.com.au/Product.aspx/Details/360

Wheels• Two drive wheels make control

easier.• Can be controlled with a single joystick

• The robot turning point will be between the two drive wheels.• Large turn radius

• Non-drive wheels will have to slide or slip

• Four drive wheels move the pivot point into the center of the robot.• Robot will turn in place.

• May require an additional joystick.

Pivot Point


Wheel Support

• Wheel axles should be supported at two points by the chassis or frame.

• Usually on each side of a wheel as shown

• Needed to keep the wheel straight and the axle from bending

Frame Axle

WheelBearing or hub


Types of Propulsion Systems

• Legs

• Motors, wheels, bearings

• Tank treads

• Gears and belts

• Supported by the structural system

• Includes mounting hardware

• Servos are used to hold a position, and are generally considered part of a system designed to complete specific objectives.


Movement

• Wheels and pulleys use DC motors.• A DC motor continuously rotates (360 ).⁰• Speed is controlled by the amount of DC voltage.

• Direction is controlled by polarity of DC voltage.

• Arms and grippers can use servos.• A servo goes to a position and holds there.

• Typically minus 90 degrees to plus 90 degrees

• Position is controlled by an electronic signal.

• Different forms of pulse width modulation are used for each (motors vs. servos).


DC Motor Control Signals

• Voltage amount given by pulse width modulation• Longer “on” time

means higher voltage

• Higher voltage equals higher motor speed

• Direction of rotation controlled by polarity


Servo Control Signals

• Pulses are always 20 ms apart • 50 pulses per second

• Pulse width varies between 1 and 2 ms• 18-19 ms of “dead time”

• A DC motor getting this signal would spin very slowly

• The pulse width determines the servo position

• 1.5 ms = middle (or null)

• 1 ms = full ccw (usually - 90 )⁰

• 2 ms = full cw (usually + 90 )⁰


Kit Versus Build From Scratch

• You can choose to build your robot completely from scratch using common and cheap parts.

• The advantage of a kit is that you get all of the parts you need.• The parts are designed to work together.

• There are standard designs and construction techniques.

• There is usually some help in the form of instructions and forums, plus corporate technical assistance.Copyright © Texas Education Agency, 2012. All rights reserved.

Kit Versus Build From Scratch

• If you choose to build from scratch you have lots of freedom for design and construction, but you need a large variety of parts.

• Although there are many different kinds of motors, brushed DC motors are popular because of cost and reliability.

• The motion and structural system must be designed to work together.• Functions are different but they are integrated to

form the chassis.Copyright © Texas Education Agency, 2012. All rights reserved.

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