Lecture # 11AUTOMATION TECHNOLOGIES

FOR MANUFACTURING SYSTEMS

1. Automation Fundamentals

2. Hardware Components for Automation

3. Computer Numerical Control

4. Industrial Robotics

Manufacturing Systems

A manufacturing system can be defined as a collection of integrated equipment and human resources that performs one or more processing and/or assembly operations on a starting work material, part, or set of parts

The integrated equipment consists of production machines, material handling and positioning devices, and computer systems

The manufacturing systems accomplish the value-added work on the part or product

Automation Fundamentals

Automation can be defined as the technology by which a process or procedure is performed without human assistance

Humans may be present, but the process itself operates under is own self-direction

Three components of an automated system:

1. Power

2. A program of instructions

3. A control system to carry out the instructions

Three Basic Types of Automation

Fixed automation - the processing or assembly steps and their sequence are fixed by the equipment configuration

Programmable automation - equipment is designed with the capability to change the program of instructions to allow production of different parts or products

Flexible automation - an extension of programmable automation in which there is virtually no lost production time for setup changes or reprogramming

Features of Fixed Automation

High initial investment for specialized equipment High production rates The program of instructions cannot be easily changed

because it is fixed by the equipment configuration Thus, little or no flexibility to accommodate product

variety

Features of Programmable Automation

High investment in general purpose equipment that can be reprogrammed

Ability to cope with product variety by reprogramming the equipment

Suited to batch production of different product and part styles Lost production time to reprogram and change the

physical setup Lower production rates than fixed automation

Features of Flexible Automation

High investment cost for custom-engineered equipment Capable of producing a mixture of different parts or

products without lost production time for changeovers and reprogramming Thus, continuous production of different part or

product styles Medium production rates

Between fixed and programmable automation types

Hardware Components for Automation

Sensors Actuators Interface devices Process controllers - usually computer-based devices

such as a programmable logic controller

Sensors

A sensor is a device that converts a physical stimulus or variable of interest (e.g., force, temperature) into a more convenient physical form (e.g., electrical voltage) for purpose of measuring the variable

Two types An analog sensor measures a continuous analog

variable and converts it into a continuous signal A discrete sensor produces a signal that can have

only a limited number of values

Actuators

An actuator is a device that converts a control signal into a physical action, usually a change in a process input parameter

The action is typically mechanical, such as a change in position of a worktable or speed of a motor

The control signal is usually low level, and an amplifier may be required to increase the power of the signal to drive the actuator Amplifiers are electrical, hydraulic, or pneumatic

Interface Devices

Interface devices allow the process to be connected to the controller and vice versa Sensor signals form the process are fed into the

controller Command signals from the controller are sent to

the process

Process Controllers

Most process control systems use some type of digital computer as the controller

Requirements for real-time computer control: Respond to incoming signals from process Transmit commands to the process Execute certain actions at specific points in time Communicate with other computers that may be

connected to the process Accept inputs from operating personnel

Programmable Logic Controllers (PLCs)

A PLC is a microcomputer-based controller that uses stored instructions in programmable memory to implement logic, sequencing, timing, counting, and arithmetic control functions, through digital or analog input/output modules, for controlling machines and processes

PLCs are widely used process controllers that satisfy the preceding real-time controller requirements

Major Components of a Programmable Logic Controller

Computer Numerical Control (CNC)

A form of programmable automation in which the mechanical actions of a piece of equipment are controlled by a computer program which generates coded alphanumeric data

The data represent relative positions between a workhead (e.g., a cutting tool) and a workpart

CNC operating principle is to control the motion of the workhead relative to the workpart and to control the sequence of motions

Components of a CNC System

1. Part program - detailed set of commands to be followed by the processing equipment

2. Machine control unit (MCU) - microcomputer that stores and executes the program by converting each command into actions by the processing equipment, one command at a time

3. Processing equipment - accomplishes the sequence of processing steps to transform the starting workpart into completed part

CNC Coordinate System

Consists of three linear axes (x, y, z) of Cartesian coordinate system, plus three rotational axes (a, b, c) Rotational axes are used to orient workpart or

workhead to access different surfaces for machining

Most CNC systems do not require all six axes

CNC Coordinate Systems

Coordinate systems used in CNC control: (a) for flat and prismatic work and (b) for rotational work

Two Types of Positioning

Absolute positioning Locations are always

defined with respect to origin of axis system

Incremental positioning Next location is

defined relative to present location

CNC Positioning System

Motor and leadscrew arrangement in a Computer numerical control positioning system

CNC Positioning System

Converts the coordinates specified in the CNC part program into relative positions and velocities between tool and workpart Leadscrew pitch p - table is moved a distance equal

to the pitch for each revolution Table velocity (e.g., feed rate in machining) is set by

the RPM of leadscrew To provide x‑y capability, a single-axis system is

piggybacked on top of a second perpendicular axis

Two Basic Types of Control in Computer Numerical Control

Open loop system Operates without verifying that the actual position

is equal to the specified position Closed loop control system

Uses feedback measurement to verify that the actual position is equal to the specified location

Two Types of Control System

(a) Closed loop and (b) open loop

Two Basic Types of Control in Computer Numerical Control

Operation of an Optical Encoder

Precision in Positioning

Three critical measures of precision in positioning:

1. Control resolution

2. Accuracy

3. Repeatability

Control Resolution (CR)

Defined as the distance between two adjacent control points in the axis movement

Control points are locations along the axis to which the worktable can be directed to go

CR depends on: Electromechanical components of positioning

system Number of bits used by controller to define axis

coordinate location

Statistical Distribution of Mechanical Errors

When a positioning system is directed to move to a given control point, the movement to that point is limited by mechanical errors Errors are due to various inaccuracies and

imperfections, such as gear backlash, play between leadscrew and worktable, and machine deflection

Errors are assumed to form a normal distribution with mean = 0 and constant standard deviation over axis range

Accuracy in a Positioning System

Maximum possible error that can occur between desired target point and actual position taken by system

For one axis:

Accuracy = 0.5 CR + 3

where CR = control resolution; and = standard deviation of the error distribution

Repeatability

Capability of a positioning system to return to a given control point that has been previously programmed

Repeatability of any given axis of a positioning system can be defined as the range of mechanical errors associated with the axis

Repeatability = 3

CNC Part Programming Techniques

1. Manual part programming

2. Computer‑assisted part programming

3. CAD/CAM‑assisted part programming

4. Manual data input Common features:

Points, lines, and surfaces of workpart must be defined relative to CNC axis system

Movement of cutting tool must be defined relative to these part features

Applications of Computer Numerical Control

Operating principle of CNC applies to many processes Many industrial operations require the position of

a workhead to be controlled relative to the part or product being processed

Two categories of CNC applications:

1. Machine tool applications

2. Non‑machine tool applications

Machine Tool Applications

CNC widely used for machining operations such as turning, drilling, and milling

CNC has motivated development of machining centers, which change their own cutting tools to perform a variety of machining operations

Other CNC machine tools: Grinding machines Sheet metal pressworking machines Thermal cutting processes

Non‑Machine Tool Applications

Tape laying machines and filament winding machines for composites

Welding machines, both arc welding and resistance welding

Component insertion machines in electronics assembly

Drafting machines (x-y plotters) Coordinate measuring machines for inspection

Benefits of CNC

Reduced non‑productive time Results in shorter cycle times

Lower manufacturing lead times Simpler fixtures Greater manufacturing flexibility Improved accuracy Reduced human error

Industrial Robotics

An industrial robot is a general purpose programmable machine that possesses certain anthropomorphic features

The most apparent anthropomorphic feature is the robot’s mechanical arm, or manipulator

Robots can perform a variety of tasks such as loading and unloading machine tools, spot welding automobile bodies, and spray painting

Robots are typically used as substitutes for human workers in these tasks

Robot Anatomy

An industrial robot consists of Mechanical manipulator

A set of joints and links to position and orient the end of the manipulator relative to its base

Controller Operates the joints in a coordinated fashion to

execute a programmed work cycle

Manipulator of an industrial robot (photo courtesy of Adept)

Manipulator Joints and Links

A robot joint is similar to a human body joint It provides relative movement between two parts

of the body Typical industrial robots have five or six joints

Manipulator joints - classified as linear or rotating Each joint moves its output link relative to its input

link Coordinated movement of joints enables robot to

move, position, and orient objects

Manipulator Design

Robot manipulators can usually be divided into two sections:

Arm‑and‑body assembly - function is to position an object or tool Three joints are typical for arm‑and‑body

Wrist assembly - function is to properly orient the object or tool Two or three joints are associated with wrist

Five Basic Arm‑and‑Body Configurations

1. Polar

2. Cylindrical

3. Cartesian coordinate

4. Jointed‑arm

5. SCARA (Selectively Compliant Assembly Robot Arm)

Basic Arm‑and‑Body Configurations

(a) Polar, (b) cylindrical, and (c) Cartesian coordinate

Basic Arm‑and‑Body Configurations

(d) Jointed-arm and (e) SCARA (Selectively Compliant Assembly Robot Arm)

Manipulator Wrist

The wrist is assembled to the last link of the arm‑and‑body

The SCARA is sometimes an exception because it is almost always used for simple handling and assembly tasks involving vertical motions A wrist is not usually present at the end of its

manipulator Substituting for the wrist on the SCARA is

usually a gripper to grasp components for movement and/or assembly

End Effectors

Special tooling that connects to the robot's wrist to perform the specific task

1. Tools - used for a processing operation Applications: spot welding guns, spray painting

nozzles, rotating spindles, heating torches, assembly tools

2. Grippers - designed to grasp and move objects (usually parts) Applications: part placement, machine loading

and unloading, and palletizing

A robot gripper: (a) open and (b) closed to grasp a workpart

Gripper End Effector

Robot Programming

Robots execute a stored program of instructions that define the sequence of motions and positions in the work cycle Much like a part program in CNC

In addition to motion instructions, the program may include commands for other functions: Interacting with external equipment Responding to sensors Processing data

Two Basic Robot Programming Methods

1. Leadthrough programming Teaching‑by‑showing - manipulator is moved

through sequence of positions in the work cycle and the controller records each position in memory for subsequent playback

2. Computer programming languages Robot program is prepared at least partially off-

line for subsequent downloading to robot controller

Where Should Robots be Used?

Work environment is hazardous for humans Work cycle is repetitive The work is performed at a stationary location Part or tool handling is difficult for humans Multi-shift operation Long production runs and infrequent changeovers Part positioning and orientation are established at the

beginning of work cycle, since most robots cannot see

Applications of Industrial Robots

Three basic categories:

1. Material handling Moving materials or parts (e.g., machine

loading and unloading)

2. Processing operations Manipulating a tool (e.g., spot welding, spray

painting)

3. Assembly and inspection May involve moving parts or tools