Robotics and Automation Copyright © Texas Education Agency, 2012. All rights reserved. 1.
INTRODUCTION TO ROBOTICS Part 2: Structural System Robotics and Automation Copyright © Texas...
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Transcript of 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 sub- systems 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
Copyright © Texas Education Agency, 2012. All rights reserved.
Photos Credit: VEX Robotics, Inc.
Copyright © Texas Education Agency, 2012. All rights reserved.
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
Copyright © Texas Education Agency, 2012. All rights reserved.
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.
Copyright © Texas Education Agency, 2012. All rights reserved.
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.
Base Material Considerations
• 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.
Copyright © Texas Education Agency, 2012. All rights reserved.
Base Material Considerations
• 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.
Copyright © Texas Education Agency, 2012. All rights reserved.
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
Copyright © Texas Education Agency, 2012. All rights reserved.
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
Copyright © Texas Education Agency, 2012. All rights reserved.
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.
Copyright © Texas Education Agency, 2012. All rights reserved.
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).
Copyright © Texas Education Agency, 2012. All rights reserved.
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
Copyright © Texas Education Agency, 2012. All rights reserved.
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
Copyright © Texas Education Agency, 2012. All rights reserved.
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.