07. Discrete Controllers

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Unit 7 Discrete Controllers

Sections:

1. Discrete Process Control

2. Ladder Logic Diagrams

3. Programmable Logic Controllers

4. Personal Computers Using Soft Logic


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Discrete Process Control

Control systems that operate on parameters andvariables that change at discrete moments in time orat discrete events, usually binary (0 or 1, off or on,open or closed, etc.)

Sensors Limit switch Contact/no contact

Photo-detector On/off

Timer On/off

Actuators Motor On/off

Valve Open/closed

Clutch Engaged/not engaged


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Categories of Discrete Control

Logic control event-driven changes

Sequencing time-driven changes

Logic Control - a switching system whose output atany moment is determined exclusively by the values ofinputs

No memory

No operating characteristics that depend on time

Also called combinational logic control


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Elements of Logic Control

Logic gates:

AND output = 1 if all inputs = 1, zero otherwise

OR output = 1 if any input = 1, zero otherwise

NOT output = 1 if (single) input = 0, and vice versa

NAND combination of AND and NOT

NOR combination of OR and NOT


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Boolean Algebra & Truth Tables

AND function is expressed as

Y = X1.X2

OR function is expressed asY = X1+X2

NOT function is expressed as

Y=X1


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NAND and NOR Functions


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Symbols for Logical Gates


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Example

Write the Boolean logic expression for the pushbutton switchsystem below using the following symbols:

X1 = START, X2 = STOP, Y1 = MOTOR, and Y2 =POWER-TO-MOTOR.

X1

X2Y2

Y1


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Solution

Truth Table

X1

X2Y2=

Y1

X2

X1+Y1

( )X Y X1 1 2


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Sequencing

A switching system that uses internal timing devices todetermine when to initiate changes in output variables

Outputs are usually generated open loop

No feedback that control function is executed

Sequence of output signals is usually cyclical, as in ahigh production work cycle

The signals occur in the same repeated patternwithin each regular cycle

Common sequencing devices: Timer output switches on/off at preset times

Counter counts electrical pulses and stores them
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Ladder Logic Diagrams

A diagram where logic elements aredisplayed along horizontal rungs

1. Contacts - logical inputs

(usually), e.g., limit switches,photo-detector

2. Loads - outputs, e.g., motors,lights, alarms, solenoids

3. Timers - to specify length ofdelay

4. Counters - to count pulsesreceived


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Ladder Logic Diagram


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Solution

If X1 and X2 remain open then C coil is unexcited and Ccontact remains closed, therefore Y is on.

If X1 or X2 are closed then C coil is excited and C contactis opened and Y is off

C

Y

X1

X2

C


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Exercise

The production process may only be performed when theoperator activates two spring activated safety switches.The switches have to be depressed and held closedtogether by the operator using both hands.

(a) What is the truth table? (b) What is the Boolean logicexpression? (c) What is the logic network diagram (in ISOsymbol terminology)? (d) What is the ladder logicdiagram?

Assume X1 and X2 are the first and second switches andY is the output

(a) Truth table for the operation is as follows:


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Solution

(a) Truth table for the operation is as follows:

Where X1 is first switch, and X2 is second switch, and Y is the output ofswitch activation.

Inputs Output

X1 X2 Y0 0 0

0 1 01 0 01 1 1

(b) Boolean logic expression for this operation corresponds to the AND logicgate, thus:

Y = X1 . X2

(c) The logic network diagram for the operation (in ISO symbol terminology) isas follows:

X1

X2

Y&

(d) The ladder logic diagram for the system:

X2X1 Y


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Exercise

Create ladder logic diagram for Push Button switch


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Solution

( )X Y X1 1 2


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Programmable Logic Controller (PLC)

A microcomputer-based controller that uses storedinstructions in programmable memory toimplement logic, sequencing, timing, counting,and arithmetic functions through digital or analogmodules, for controlling machines and processes


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Components of a PLC

I/O

ModuleOutputs

Inputs

Processor

& Memory

Programming

Device


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Advantages of PLCs Compared toRelay Control Panels

Programming a PLC is easier than wiring a relay controlpanel

PLC can be reprogrammed

PLCs take less floor space

Greater reliability, easier maintenance

PLC can be connected to computer systems (CIM)

PLCs can perform a greater variety of control functions


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Typical PLC Operating Cycle

1. Input scan inputs are read by processor and stored inmemory

2. Program scan control program is executed

Input values stored in memory are used in the controllogic calculations to determine values of outputs

3. Output scan output values are updated to agree withcalculated values

Time to perform the three steps (scan time) variesbetween 1 and 25 msec


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PLC Programming

Graphical languages:

1. Ladder logic diagrams most widely used

2. Function block diagrams instructions

composed of operation blocks that transforminput signals

3. Sequential function charts series of steps andtransitions from one state to the next (Europe)

Text-based languages:1. Instruction list - low-level computer language

2. Structured text high-level computer language


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PLC Programming
http://www.stp-stampaggi.com/equipments/equipments.htm


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Personal Computers Using Soft Logic

Sturdy enclosures for plant environment

Membrane-type keyboards

I/O cards and other hardware to connect to

machines and processes Windows for implementing control

applications

Soft logic software emulating PLCs


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NXT Controller and Program

07. Discrete Controllers

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