Basics of PLC Programming

1
Basics of PLC Programming

• EE 100 Intro to EE
• Fall 2004
• Dr. Stephen Williams, P.E.

2
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?

SLC
PLC
AB
GM
Ford
Autos
Sensor
Bus
Drive
3
Vocabulary

• Programmable Logic Controllers
• Definite-purpose computers design to control
  industrial processes and machines
• Drives
• Solid-state devices designed to control motors
• Sensors
• Transducers used to obtain information

4
First Programmable Controller

• General Motors Corporation
• Hydromatic Division
• Replaced relay-controlled system
• PDP-8 minicomputers?
• MODICON 084
• Modular Digital Controller

5
Information Flow
Process or Machine
Measure
Control
Programmable Controller
6
Genesis of Automation

• Operation sheets
• May date back to the 1830s
• Listing of
• All machining operations
• The machine tools employed
• Tools, jigs, fixtures, and gauges
• Organization and flow of work

7
Industrial Revolution

• High-volume production
• Interchangeable parts
• Transportation system
• Inexpensive energy (coal)
• Frederick W. Taylor
• Scientific management
• Henry Ford

8
Purpose of Automation

• Increase productivity
• Standardize components or processes
• Free workers from repetitive, and sometime
  dangerous, tasks

9
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

10
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.

11
Forces Driving Automation

• Lower costs
• Faster production
• Better quality control
• How have they remained relevant today?

12
Engineering Resources

• Why do you need all of these engineers running
  around to make all of this stuff work?

13
Breakthroughs and Plateaus

• Where have we seen breakthroughs, and then
  plateaus of technology?
• Microprocessors
• Graphical User Interfaces
• Power Electronics
• Software Systems

14
Brief Review of Technology

• Traditional (ancient?) devices
• Still used in many plants
• If it aint broke
• Where are we going?

15
Traditional Relay Logic

• Used since
• Control via a series of relay contacts
• On and off inputs
• Race conditions on the outputs
• Very expensive
• Hard to design and construct
• Difficult to maintain

16
Traditional Devices

• Relays
• Contactors
• Motor Starters
• Manually operated switches
• Mechanically operated switches
• Electrically operated switches

17
Relays

• Original control elements
• Now used as auxiliary devices
• The PLC is not designed to switch high currents
  or voltages

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Contactors

• Used for heavy-duty switching
• Provides isolation from high voltages and large
  currents
• Usefully for large inductive currents, such as
  motor starting

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Motor Starters

• Contactors Overload Relay
• Overload relays were usually heaters and bimetal
  strips
• The bimetal strip separates when heated
• Next steps
• PLCs and motor starters
• Electronic overloads
• Intelligent starters

20
Manually Operated Switches

• Pushbuttons
• Normally open
• Normally closed
• Break-then-make
• Make-then-break
• Selector switches
• Maintained or spring return

21
Mechanically Operated Switches

• Limit Switches
• Temperature Switches
• Pressure Switches
• Level Switches

22
Electrically Operated Switches

• Photoelectric Switches
• Proximity Switches

23
What's ahead?

• Solid state devices to replace motor starters
• Distributed smart sensors
• Micro- and nanomachines
• Adaptive control
• Smart maintenance

24
Summary

• A very brief history of industrial automation
• Overview of some of the older technologies
• Some thoughts on the future

25
PLC Systems

• CPU
• Processor
• Memory
• One Module
• Power Supply
• Part of the chassis or a separate module
• Programming/ Monitoring Device
• I/0 Modules

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Small Logic Controllers
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Input and Output

• Input Modules
• Convert real world signal to PLC input
• 24 V, 120 V, Analog, etc.
• Output Modules
• Convert PLC signal to real world output
• 24 V, 120 V, Analog, etc.
• Limiting values
• PLC power supply

28
Configurations

• Fixed I/O
• Limited expandability
• Rack
• Many modules, with the possibility of chaining
  many racks together
• SLC 500 is a fixed I/O device
• SLC 5/02 uses a rack configuration

29
Chassis Versus Rack

• One Rack is 128 inputs/outputs
• A chassis is the outer shell of the PLC
• Chassis ? Rack
• SLC 5/02s in S-340 have a ten-slot chassis
• Slots are numbered from 0 to 9

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SLC Image Tables

• Hex numbering
• Addressing
• I12.0/01
• I is for the file type
• 1 is the file number
• 2 is the element number
• .0 is the sub-element number (gt16)
• /01 is the bit number

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Real World Address

• I13.0/01
• I is the module type
• 1 is redundant
• 3 is the slot number
• .0 is for terminals above 15
• /01 is the terminal number

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Remote Racks

• I/O racks located close to the equipment being
  monitored
• Simplifies wiring
• Communication modules
• Similar to LAN
• Fiber Optic
• Coaxial cable

33
Discrete I/O Modules

• Either on or off
• Bit oriented
• Various ratings
• 24 V
• 120 V
• TTL
• 4 20 mA

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Special I/O Modules

• Analog
• High speed counter
• Thumb-wheel
• TTL
• Encoder
• PID
• Servo

35
Memory Organization

• Not the same on all manufactures
• Allen Bradley uses two main types
• Memory Maps
• Data table
• User program
• Internal registers
• Memory allocation could be fixed or variable

36
SLC Program File Structure
Program File Number Use
0 System Functions
1 Reserved
2 Main Program
3-255 Subroutines
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RSLogix 500 Screen

• Define controller attributes
• Model
• Memory
• Communication
• Program files
• Main program
• Subprograms

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SLC Data File Structure
Data File Number Use
0 Output Image Table
1 Input Image Table
2 Status Table
3 Bit Table
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SLC Data File Structure
Data File Number Use
4 Timer Table
5 Counter Table
6 Control Table
7 Integer Table
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SLC Data File Structure
Data File Number Use
8 Reserved (Floating Point Value Table)
9 Network Table
10-255 Any combination of Bit, Timer, Counter, Control, or Integer Tables
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RSLogix 500 Screen

• Access to input and output tables
• Access to timer and control control files

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Address Format

• What type of device or module
• Where is it located physically or in memory
• For example, T40/DN is the done bit for timer 0
  in file 4
• I2.0 is an input module in slot 2
• Word versus bit addresses
• I3.0 is a word, I3.0/04 is a bit

43
Multiword Elements

• Timers, counters, and control elements
• Three words used
• Control word to store status
• Preset word to store desired value
• Accumulated word to store present value
• Control file store a length and position value
  (on functions other than counters and timers)

44
Counter Element Example
Name Address Example
Control Word C50 C50/DN
Preset Word C50.PRE 5000
Accumulated Word C50.ACC 1240
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RSLogix 500 Screen

• Counter C50

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Program Scan
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

I/O Scan
47
Programming Environments

• Languages available
• Ladder logic
• Boolean
• Function chart
• Ladder logic is the most common
• Function chart is the future
• C, BASIC, etc., are also possible

48
Transducers

• Converts energy from one form to another
• Input transducers
• Real world into the PLC
• Output transducers
• PLC to real world

49
Sensors

• Sensors are transducers used to measure or detect
• Convert mechanical, magnetic, thermal, or optical
  variations into electrical quantities
• Sensor input is the basis for most of the
  decisions made in a large system

50
Proximity Sensors

• Detect the presence of a object (target) without
  physically touching the object
• Solid-state devices
• Completely encapsulated
• Used when
• Detecting small objects
• Rapid response is required

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Inductive Proximity Sensors

• Senses a metallic object
• A change in the magnetic field occurs when a
  metallic object enters into range
• This type of sensor can see through cardboard
  boxes and other enclosures
• Current-sourcing or current-sinking output

52
Manually Operated Switches

• Pushbuttons
• Normally open
• Normally closed
• Break-then-make
• Make-then-break
• Selector switches
• Maintained or spring return

53
Counter Instructions

• Count Up or Down
• Similar to timers, but without an internal source
• Two methods used block and coil
• SLC 5/02s use the coil format
• PREset and ACCumlated values
• RESet similar to RTO

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How Counters Work

• Increment or decrement on a false to true input
  transition
• They are retentive
• The accumulated value remains when the rung goes
  false
• PREset can be changed by the program
• Move a new value into C50.PRE

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Control Bits
15 14 13 12 11 10
CU CD DN OV UN UA

• CU Count Up
• CD Count Down
• DN Done
• OV Overflow, UN Underflow

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Integer Limits

• PREset and ACCumulator values must be integers
• Integers on the SLC 5/02 range from 32,767 to
  -32,768
• Cascade counters to go beyond these limits

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Cascading Example
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Down Counters

• The SLC 5/02 does not have a true down counter
• The 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

59
Down Counter Example
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Types of Data Instructions

• Math Functions
• Add, subtract, multiply, etc.
• Data Conversion and Comparison
• Integer to BCD, Less than, Equal, etc.
• Logical Operations

61
Bits, Words, and Files

• A bit is the smallest unit of information
• T40/DN is a bit
• A word is another name for a register
• T40.PRE is a word
• A file is a block of words, also known as a
  table
• T4 is a file

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Data Transfer Move

• The move instruction takes a value from a
  register, or a constant value, and places it in
  another register

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BCD Move Into a Register

• Moves an integer value into a BCD device.
• In lab, the LED Display

64
BCD Move From a Register

• Moves an BCD value into an integer register.
• In lab, the thumb-wheel inputs

65
Comparisons

• Greater than, less than, equals, etc.
• When true, output is true

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Todays 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!