Chapter 7. PROGRAMMABLE LOGIC CONTROLLER

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Chapter 7. PROGRAMMABLELOGIC CONTROLLER
Dr. T.C. Chang School of Industrial
Engineering Purdue University
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FUNCTIONS OF CONTROLLERS

• 1) on-off control,
• 2) sequential control,
• 3) feedback control, and
• 4) motion control.

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CONTROL DEVICES

• 1) mechanical control - cam, governor, etc.,
• 2) pneumatic control - compressed air,
  valves,etc.
• 3) electromechanical control - switches, relays,
  a timer, counters, etc,
• 4) electronics control - similar to
  electromechanical control, except uses electronic
  switches.
• 5) computer control.

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PROGRAMMABLE LOGIC CONTROLLER
Invented in 1968 as a substitute for hardwired
relay panels.

• "A digitally operating electronic apparatus which
  uses a programmable memory for the internal
  storage of instructions by implementing specific
  functions such as logic sequencing, timing,
  counting, and arithmetic to control, through
  digital or analog input/output modules, various
  types of machines or processes. The digital
  computer which is used to perform the functions
  of a programmable controller is considered to be
  within this scope. Excluded are drum and other
  similar mechanical sequencing controllers."

National Electrical Manufacturing Association
(NEMA)
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VENDORS

• MODICOM - GOULD
• ALLEN-BRADLEY
• GE
• SQUARE-D
• etc.

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PLC
Input
CPU
Input Module
Flag
System
Output Module
Output
User Ladder Diagram
Working memory registers
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An Large Size PLC

• The main module measures 19 x 20 x 14.5.
• have upto 10,000 I/O points
• supports all functions
• expansion slots to accommodate PC and other
  communication devices.

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A Small Size PLC

• Measures 4.72x 3.15 x 1.57.
• 32 I/O points
• Standard RS 232 serial communication port

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SWITCHES
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DPST
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SPDT
P
2
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P
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T
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TERMS

• Throw - number of states
• Pole - number of connecting moving parts (number
  of individual
• circuits).
• SPDT

A serial switch box (A-B box) has two 25 pin
serial ports to switch from.
A
B
Output
Input
DPST
Knob
How is this switch classified?
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TYPES OF SWITCHES

• Selector switches
• Pushbutton switches
• Photoelectric switches
• Limit Switches
• Proximity switches
• Level switches
• Thumbwheel switches
• Slide switches

• RATING
• 24 Volts AC/DC
• 48 Volts AC/DC
• 120 Volts AC/DC
• 230 Volts AC/DC
• TTL level
• (Transistor-to-transistor
• 5V)
• Isolated Input

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RELAYS
A switch whose operation is activated by an
electromagnet is called a "relay"
contact
coil
input
Relay coil
R1
Output contact
R1
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COUNTER

• Digital counters output in the form of a relay
  contact when a preassigned count value is reached.

5
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TIMER

• A timer consists of an internal clock, a count
  value register, and an accumulator. It is used
  for or some timing purpose.

Time 5 seconds.
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AN EXAMPLE OF RELAY LOGIC

• For a process control, it is desired to have the
  process start (by turning on a motor) five
  seconds after a part touched a limit switch. The
  process is terminated automatically when the
  finished part touches a second limit switch. An
  emergency switch will stop the process any time
  when it is pushed.

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PLC ARCHITECTURE

• Programmable controllers replace most of the
  relay panel wiring by software programming.

A typical PLC
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PLC COMPONENTS

• 1. Processor Microprocessor based, may allow
  arithmetic operations, logic operators, block
  memory moves, computer interface, local area
  network, functions, etc.
• 2. Memory Measured in words.
• ROM (Read Only Memory),
• RAM (Random Access Memory),
• PROM (Programmable Read Only Memory),
• EEPROM (Electric Erasable Programmable ROM),
  
• EPROM (Erasable Programmable Read Only Memory),
• EAPROM (Electronically Alterable Programmable
• Read Only Memory), and
• Bubble Memory.

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PLC COMPONENTS

• 3. I/O Modular plug-in periphery
• AC voltage input and output,
• DC voltage input and output,
• Low level analog input,
• High level analog input and output,
• Special purpose modules, e.g., high speed
  timers,
• Stepping motor controllers, etc. PID, Motion
• 4. Power supply AC power
• 5. Peripheral hand-held programmer (HHP)
• CRT programmer
• operator console
• printer
• simulator
• EPROM loader
• graphics processor
• network communication interface
• modular PC

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Discrete I/O

• AC Voltage I/O
• DC Voltage I/O
• numerical input and output
• special-purpose modules, for example, high-speed
  timers, and stepping motor controllers

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Analog I/O

• Analog inputs
• Flow sensors
• Humidity sensors
• Potentiometers
• Pressure sensors
• Temperature sensors
• Analog outputs
• Analog meters
• Analog valves and actuators
• DC and AC motor drives

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Some Special I/O

• Thermocouple input
• Low level analog signal, filtered, amplified, and
  digitized before sending to the processor through
  I/O bus.
• Fast input
• 50 to 100 microsecond pulse signal detection.
• ASCII I/O
• Communicates with ASCII devices.
• Stepper motor output
• Provide directly control of a stepper motor.
• Servo interface
• Control DC servo motor for point-to-point control
  and axis positioning.
• PID control
• The Proportional Integral Derivative is used for
  closed loop process control.
• Network module

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A Hand Held Programmer
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LADDER DIAGRAM

• A ladder diagram (also called contact symbology)
  is a means of graphically representing the logic
  required in a relay logic system.

Rail
Rung
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PLC WIRING DIAGRAM
Output
PLC
Input
A
11
01
C
01
02
20
B
12
02
20
03
11
20
External switches
Stored program
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Input Connections
AC DC
TTL
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Output Connections
AC DC
TTL
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SCAN

• A PLC resolves the logic of a ladder diagram
  (program) rung by rung, from the top to the
  bottom. Usually, all the outputs are updated
  based on the status of the internal registers.
  Then the input states are checked and the
  corresponding input registers are updated. Only
  after the I/Os have been resolved, is the program
  then executed. This process is run in a endless
  cycle. The time it takes to finish one cycle is
  called the scan time. In some controllers the
  idle state is eliminated. In this case, the scan
  time varies depends on the program length.

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PLC Programming

• Ladder Diagram - most common
• Structure Text Programming (ST)
• Functional Block Programming (FB)
• Instruction List (IL)
• Sequential Function Chart (SFC)

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PLC Ladder DiagramINSTRUCTIONS

• 1) Relay,
• 2) Timer and counter,
• 3) Program control,
• 4) Arithmetic,
• 5) Data manipulation,
• 6) Data transfer, and
• 7) Others, such as sequencers.

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LOGIC STATES

• ON TRUE, contact closure, energize, etc.
• OFF FALSE, contact open , de-energize, etc.

Do not confuse the internal relay and program
with the external switch and relay. Internal
symbols are used for programming. External
devices provide actual interface.
(In the notes we use the symbol "" to represent
negation. AND and OR are logic operators. )
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AND and OR LOGIC
PB2
R1
PB1
R1 PB1.AND.PB2

AND

PB4
PB3
R2

R2 PB2.AND.PB4
R1
PB1
R1 PB1 .OR. PB2
OR
PB2
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COMBINED AND OR
R1 PB1 .OR. (PB2 .AND. PB3)
R1
PB1
PB2
pb3
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RELAY
A Relay consists of two parts, the coil and the
contact(s).

• Contacts
• a. Normally open - -
• b. Normally closed -/-
• c. Positive transition sensing -P-
• d. Negative transition sensing -N-
• Coil
• a. Coil -( )-
• b. negative coil -(/)-
• c. Set Coil -(S)-
• d. Reset Coil -(R)-

( )
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Relay (continue)
Coil e. Retentive memory Coil -(M)- f.
Set retentive memory Coil -(SM)- g. Reset
retentive memory Coil -(RM)- h. Positive
Transition-sensing Coil -(P)- h. Negative
Transition-sensing Coil -(N)- (set coil
latches the state, reset coil deenergize the set
coil. retentive coil retain the state after power
failure.)
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TIMERS AND COUNTERS
Input

• Timers
• a. Retentive on delay -(RTO)-
• b. Retentive off delay -(RTF)-
• c. Reset -(RST)-
• Counter
• a. Counter up -(CTU)-
• b. Counter down -(CTD)-
• c. Counter reset -(CTR)-

True False True
RTO counting stop counting
resume RTF
stop counting stop
RTO reach PR value, output ON RTF reach PR value,
output OFF
PR value in 0.1 second
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SEQUENCER
Sequencers are used with machines or processes
involving repeating operating cycles which can
be segmented into steps.

• Output
  
• Step A B C Dwell time
• 1 ON OFF OFF 5 sec.
• 2 ON ON OFF 10 sec.
• 3 OFF OFF ON 3 sec.
• 4 OFF ON OFF 9 sec.

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A-B PLC

• I/O points are numbered, they correspond to the
  I/O slot on the PLC.
• For A-B controller used in our lab
• I/O uses 1-32
• Internal relays use 033 - 098
• Internal timers/counters/sequencers use 901-932
• Status 951-982

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PROGRAMMING EXAMPLE 1

• id description state explanation
• MSI microswitch 1 part arrive
• R1 output to bar code reader 1 scan the part
• C1 input from bar code reader 1 right part
• R2 output robot 1 loading cycle
• R3 output robot 1 unloading cycle
• C2 input from robot 1 robot busy
• R4 output to stopper 1 stopper up
• C3 input from machine 1 machine busy
• C4 input from machine 1 task complete

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SOLUTION

• Rung 1. If part arrives and no part is stopped,
  trigger the bar code reader.
• Rung 2. If it is a right part, activate the
  stopper.
• Rung 3. If the stopper is up, the machine is not
  busy and the robot is not busy, load the part
  onto the machine.
• Rung 4. If the task is completed and the robot
  is not busy, unload the machine.

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Structured Text Programming

• a high level language
• used to express the behavior of functions,
  function blocks and programs
• In IEC 1131-3 standard, it has a syntax very
  similar to PASCAL
• strongly typed language
• Functions
• assignments
• expressions
• statements
• operators
• function calls
• flow control

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Data Types
SINT short integer 1 byte INT integer 2
bytes DINT double integer 4 bytes LINT long
integer 8 bytes USINT unsigned short integer 1
byte UINT unsigned integer 2 bytes UDINT unsigned
double integer 4 bytes ULINT unsigned long
integer 8 bytes REAL real 4 bytes LREAL long
real 8 bytes TIME time duration DATE calendar
date TOD time of day DT date and time of
day STRING character strings
BOOL boolean 1 bit BYTE byte 1 byte WORD 16 bit
bit string 16 bits DWORD 32 bit bit string 32
bits LWORD 64 bit bit string 64 bits
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Derived Data Types
TYPE ( user defined data types, this is a
comment) pressure REAL temp
REAL part_count INT END_TYPE Structure
TYPE data_packet STRUCT input BOOL t TI
ME out BOOL count INT END_STRUCT END_
TYPE
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Variable Declarations
Local variable
Use VAR, VAR_INPUT, VAR_OUTPUT, VAR_IN_OUT,
VAR_GLOBAL, VAR_EXTERNAL for different variable
types.
VAR I,j,k INT v REAL END_VAR
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Operators and Expressions

• ( ) parenthesized expression
• function( ) function
• exponentiation
• - negation
• NOT Boolean complement
• - / math operators
• MOD modulus operation
• lt gt lt gt comparison operators
• equal
• ltgt not equal
• AND, Boolean AND
• XOR Boolean XOR
• OR Boolean OR

Y X1.0 y a AND b v (v1 v2
v3)/3 output (light open) OR (door shut)
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Condition Statements

• IF a gt 100 THEN
• redlight on
• ELSEIF a gt 50 THEN
• yellowlight on
• ELSE
• greenlight on
• END_IF

CASE dial_setting OF 1 x 10 2 x
15 3 x 18 4,5 x 20 ( 4 or 5
) ELSE x 30 END_CASE
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Iteration Statements

• FOR I 0 to 100 BY 1 DO
• lightI ON
• END_FOR
• I 0
• WHILE I lt 100 DO
• I I 1
• lightI on
• END_WHILE

I 0 REPEAT I I 1 lightI
on UNTIL I gt 100 END_REPEAT
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Functions

• FUNCTION add_num REAL
• VAR_INPUT
• I,J REAL
• END_VAR
• add_num I J
• END_FUNCTION
• Call a function
• x add_num(1.2, 5.6)
• Built-in Functions
• ABS, SQRT, LN, LOG, EXP, SIN, COS, TAN, ASIn,
  ACOS, ATAN, ADD, MUL, SUB, DIV, MOD, EXPT, MOVE),
  logic functions (AND, OR, XOR, NOT), bit string
  functions (SHL, SHR shift bit string left and
  right , ROR, ROL rotate bit string), etc.

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Programs

• PROGRAM example7.1
• VAR_INPUT
• MSI BOOL
• C1 BOOL
• C2 BOOL
• C3 BOOL
• C4 BOOL
• END_VAR
• VAR_OUTPUT
• R1 BOOL FALSE
• R2 BOOL FALSE
• R3 BOOL FALSE
• R4 BOOL FALSE
• END_VAR

R1 MS1 AND (NOT R4) R2 R4 AND (NOT C3)
AND (NOT C2) R3 C4 AND (NOT C3) R4
C1 END_PROGRAM
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Functional Block Programming

• Functional block (FB) is a well packaged element
  of software that can be re-used in different
  parts of an application or even in different
  projects. Functional blocks are the basic
  building blocks of a control system and can have
  algorithms written in any of the IEC languages.

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An Up Counter Function Block

• The algorithm in Structured Text
• FUNCTION BLOCK CTU
• VAR_INPUT
• CU BOOL
• R BOOL
• PV INT
• END_VAR
• VAR_OUTPUT
• Q BOOL
• CV INT
• END_VAR
• IF R THEN
• CV 0
• ELSEIF CU
• AND (CV lt PV) THEN
• CV CV 1
• END_IF
• Q (CV gt PV)

CU input to be counted R reset PV
preset value Q contact output CV counter
value.
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A PID Control Function Block
block diagram
SP set point PV sensor feedback KP proportional
error gain TR integral gain TD derivative
gain AUTO calculate XOUT output to
process XO manual output adjustment cycle time
between execution
control algorithm
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Instruction List Programming

• A low level language which has a structure
  similar to an assembly language. Since it is
  simple, it is easy to learn and ideally for small
  hand-held programming devices. Each line of code
  can be divided into four fields label, operator,
  operand, and comment.
• e.g.
• LD MS1
• ST R1
• loop ANDN C3

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Operators
GT ( greater than GE ( greater than and equal
to EQ ( equal NE ( not equal LE ( less than
and equal to LT ( less than JMP C, N jump to
label CAL C, N call function block RET C,
N return from function or function
block ) execute last deferred operator

• Operator Modifiers Description
• LD N load operand into register
• ST N store register value into operand
• S set operand true
• R reset operand false
• AND N, ( Boolean AND
• N, ( Boolean AND
• OR N, ( Boolean OR
• XOR N, ( Boolean XOR
• ADD ( addition
• SUB ( subtraction
• MUL ( multiplication
• DIV ( division

Modifier N means negate. ( defers the
operator. C is a condition modifier, the
operation is executed if the register value is
true.
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Instruction List for Example 1

• PROGRAM example7.1
• VAR_INPUT
• MSI BOOL
• C1 BOOL
• C2 BOOL
• C3 BOOL
• C4 BOOL
• END_VAR
• VAR_OUTPUT
• R1 BOOL FALSE
• R2 BOOL FALSE
• R3 BOOL FALSE
• R4 BOOL FALSE
• END_VAR

LD MS1 ANDN R4 ST R1 LD R4 ANDN C3 ANDN C2
ST R2 LD C4 ANDN C3 ST R3 LD C1 ST R4 END_
PROGRAM
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Sequential Function Chart(SFC)

• A graphics language used for depicting sequential
  behavior. The IEC standard grew out of the
  French standard Grafcet which in turn is based on
  Petri-net. A SFC is depicted as a series of steps
  shown as rectangular boxes connected by vertical
  lines. Each step represent a state of the system
  being controlled. The horizontal bar indicates a
  condition. It can be a switch state, a timer,
  etc. A condition statement is associated with
  each condition bar. Each step can also have a
  set of actions. Action qualifier causes the
  action to behave in certain ways. The indicator
  variable is optional. It is for annotation
  purposes.

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SFC

• Action Qualifiers
• N non-stored, executes while the step is active
• R resets a store action
• S sets an action active
• L time limited action, terminates after a given
  period
• D time delayed action.
• P a pulse action, executes once in a step
• SD stored and time delayed
• DS time delayed and stored
• SL stored and time limited

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SFC for Example 1