basics plc
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Basics of PLC Programming
EE 100 – Intro to EEFall 2004Dr. Stephen Williams, P.E.
Overview How did we get where we are today?How does a project at GM in 1968
relate to the work of Henry Leland in the late 1800s?
Ford
DriveSensor
GMAutos
Bus
ABSLCPLC
VocabularyProgrammable Logic Controllers
Definite-purpose computers design to control industrial processes and machines
DrivesSolid-state devices designed to control
motorsSensors
Transducers used to obtain information
First Programmable ControllerGeneral Motors Corporation
Hydromatic DivisionReplaced relay-controlled systemPDP-8 minicomputers?MODICON 084
Modular Digital Controller
Information Flow
Genesis of AutomationOperation sheets
May date back to the 1830sListing of:
All machining operationsThe machine tools employedTools, jigs, fixtures, and gauges
Organization and flow of work
Industrial RevolutionHigh-volume production Interchangeable partsTransportation system Inexpensive energy (coal)Frederick W. Taylor
Scientific managementHenry Ford
Purpose of Automation Increase productivity Standardize
components or processes
Free workers from repetitive, and sometime dangerous, tasks
Early Automation Applications 1869 – Refineries in
Pennsylvania automatically covert crude oil to kerosene
1937 – Pictured is the loading and unloading of stators via an overhead conveyor for dipping in continuous process oven
The Case Against Automation Las Vegas Sun, August 2, 1961
Jimmy Hoffa saw a new industrial revolution forming with automation being a threat to his giant union more menacing than the Justice Department, Attorney General Bobby Kennedy and the president himself.
He felt he could cope with the Senate committees, the FBI, and all the new legislation being written, which he thinks is aimed at unionism. It is with automation that all his talents, energy and ability must be directed.
Forces Driving AutomationLower costsFaster productionBetter quality controlHow have they remained relevant
today?
Engineering Resources Why do you need all
of these engineers running around to make all of this stuff work?
Breakthroughs and PlateausWhere have we seen breakthroughs,
and then plateaus of technology?MicroprocessorsGraphical User InterfacesPower ElectronicsSoftware Systems
Brief Review of TechnologyTraditional (ancient?) devices
Still used in many plants If it ain’t broke …
Where are we going?
Traditional Relay LogicUsed since …Control via a series of relay contactsOn and off inputsRace conditions on the outputsVery expensive
Hard to design and constructDifficult to maintain
Traditional DevicesRelaysContactorsMotor StartersManually operated switchesMechanically operated switchesElectrically operated switches
CR1
CR1-1
RelaysOriginal control elementsNow used as auxiliary devices
The PLC is not designed to switch high currents or voltages
ContactorsUsed for heavy-duty switchingProvides isolation from high voltages
and large currentsUsefully for large inductive currents,
such as motor starting
Motor StartersContactors + Overload RelayOverload relays were usually heaters
and bimetal stripsThe bimetal strip separates when heated
Next steps: PLCs and motor startersElectronic overloads Intelligent starters
Manually Operated SwitchesPushbuttons
Normally openNormally closedBreak-then-makeMake-then-break
Selector switchesMaintained or spring return
Mechanically Operated SwitchesLimit SwitchesTemperature SwitchesPressure SwitchesLevel Switches
Electrically Operated SwitchesPhotoelectric SwitchesProximity Switches
What's ahead?Solid state devices to replace motor
startersDistributed smart sensorsMicro- and nanomachinesAdaptive controlSmart maintenance
SummaryA very brief history of industrial
automationOverview of some of the older
technologiesSome thoughts on the future
PLC Systems CPU
Processor Memory
One Module Power Supply
Part of the chassis or a separate module
Programming/ Monitoring Device
I/0 Modules
Small Logic Controllers
Input and Output Input Modules
Convert “real world” signal to PLC input 24 V, 120 V, Analog, etc.
Output ModulesConvert PLC signal to “real world” output
24 V, 120 V, Analog, etc.
Limiting valuesPLC power supply
ConfigurationsFixed I/O
Limited expandabilityRack
Many modules, with the possibility of chaining many racks together
SLC 500 is a fixed I/O deviceSLC 5/02 uses a rack configuration
Chassis Versus Rack One “Rack” is 128
inputs/outputs A chassis is the outer
shell of the PLC Chassis ≠ Rack SLC 5/02’s in S-340
have a ten-slot chassis Slots are numbered from
0 to 9
SLC Image TablesHex numberingAddressing
I1:2.0/01 I is for the file type 1 is the file number 2 is the element number .0 is the sub-element number (>16) /01 is the bit number
“Real World” Address I1:3.0/01
I is the module type 1 is redundant 3 is the slot number .0 is for terminals above 15 /01 is the terminal number
Remote Racks I/O racks located close to the equipment
being monitored Simplifies wiring Communication modules
Similar to LAN Fiber Optic Coaxial cable
Discrete I/O ModulesEither “on” or “off”Bit orientedVarious ratings
24 V 120 VTTL 4 – 20 mA
Special I/O ModulesAnalogHigh speed counterThumb-wheelTTLEncoderPIDServo
Memory OrganizationNot the same on all manufactures
Allen Bradley uses two main typesMemory Maps
Data tableUser program Internal registers
Memory allocation could be fixed or variable
SLC Program File Structure
Subroutines3-255
Main Program2
Reserved1
System Functions0
UseProgram File Number
RSLogix 500 Screen Define controller
attributes Model Memory Communication
Program files Main program Subprograms
SLC Data File Structure
Bit Table3
Status Table2
Input Image Table1
Output Image Table0
UseData File Number
SLC Data File Structure
Integer Table7
Control Table6
Counter Table5
Timer Table4
UseData File Number
SLC Data File Structure
Any combination of Bit, Timer, Counter, Control, or Integer Tables
10-255
Network Table9
Reserved (Floating Point Value Table)
8
UseData File Number
RSLogix 500 Screen Access to
input and output tables
Access to timer and control control files
Address FormatWhat type of device or moduleWhere is it located physically or in
memory For example, T4:0/DN is the done bit for
timer 0 in file 4 I:2.0 is an input module in slot 2Word versus bit addresses
I:3.0 is a word, I:3.0/04 is a bit
Multiword ElementsTimers, counters, and control elementsThree words used
Control word to store statusPreset word to store desired valueAccumulated word to store present valueControl file store a length and position
value (on functions other than counters and timers)
Counter Element Example
1240C5:0.ACCAccumulated Word
5000C5:0.PREPreset Word
C5:0/DNC5:0Control Word
ExampleAddressName
RSLogix 500 Screen Counter C5:0
Program Scan Each cycle through
the program and I/O process is called a scan
Scan times vary with the length of the program and the speed of the processor
Programming EnvironmentsLanguages available
Ladder logicBoolean Function chart
Ladder logic is the most commonFunction chart is the futureC, BASIC, etc., are also possible
TransducersConverts energy from one form to
another Input transducers
Real world into the PLCOutput transducers
PLC to real world
SensorsSensors are transducers used to
measure or detectConvert mechanical, magnetic, thermal,
or optical variations into electrical quantities
Sensor input is the basis for most of the decisions made in a large system
Proximity SensorsDetect the presence of a object (target)
without physically touching the objectSolid-state devicesCompletely encapsulatedUsed when:
Detecting small objectsRapid response is required
Inductive Proximity SensorsSenses a metallic objectA change in the magnetic field occurs
when a metallic object enters into rangeThis type of sensor can “see” through
cardboard boxes and other enclosuresCurrent-sourcing or current-sinking
output
Manually Operated SwitchesPushbuttons
Normally openNormally closedBreak-then-makeMake-then-break
Selector switchesMaintained or spring return
Counter InstructionsCount Up or DownSimilar to timers, but without an internal
sourceTwo methods used: block and coil
SLC 5/02s use the coil formatPREset and ACCumlated valuesRESet similar to RTO
How Counters Work Increment or decrement on a false to
true input transitionThey are retentive
The accumulated value remains when the rung goes false
PREset can be changed by the programMove a new value into C5:0.PRE
Control Bits
UAUNOVDNCDCU
101112131415
CU = Count UpCD = Count DownDN = DoneOV = Overflow, UN = Underflow
Integer LimitsPREset and ACCumulator values must
be integers Integers on the SLC 5/02 range from
32,767 to -32,768Cascade counters to go beyond these
limits
Cascading Example
Down CountersThe SLC 5/02 does not have a true
down counterThe counter does not start at a value and
become true when the ACCumulator is zero
The SLC 5/02 CTD works with another counter with the same address
Down Counter Example
Types of Data InstructionsMath Functions
Add, subtract, multiply, etc.Data Conversion and Comparison
Integer to BCD, Less than, Equal, etc.Logical Operations
Bits, Words, and FilesA bit is the smallest unit of information
T4:0/DN is a bitA “word” is another name for a register
T4:0.PRE is a wordA “file” is a block of words, also known
as a tableT4 is a file
Data Transfer – Move The move instruction takes a
value from a register, or a constant value, and places it in another register
BCD Move Into a Register Moves an integer value into a BCD
device. In lab, the LED Display
BCD Move From a Register Moves an BCD value into an integer
register. In lab, the thumb-wheel inputs
Comparisons Greater than, less than, equals,
etc. When true, output is true
Today’s Task Use what you have
learned to “break the code”
Each bench has a PLC program
The first bench to turn on all five lamps wins!
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