Post on 22-Dec-2015
transcript
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