plc - georgia institute of technologyume.gatech.edu/mechatronics_course/me4447_6405/plc.pdf ·...

George W. Woodruff School of Mechanical Engineering, Georgia TechGeorge W. Woodruff School of Mechanical Engineering, Georgia Tech

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Section Objectives:

Before the invention of the Programmable Logic Controller (PLC), most industrial control was done using relay control panels.

Switches and relays can be arranged in circuits to make logical decisions. Output from these circuits can be used to drive “loads” such as motors, heaters, or electromagnetic coils. A relay control panel is comprised of a single to thousands of these circuits.

In this Section, relay control panels will be presented.


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Relay Control Panel Components : Switch

Pins 1 and 2 are “normally closed” since they are connected when the switch is off. T Pins 1 and 2 are not connected when the switch is on.

Pins 1 and 3 are “normally open” since they are not connected when the switch is off. Pins 1 and 3 are connected when the switch is on.

(Note: Although this is a toggle switch, this switch can symbolize any type of input source such as push button switches, sensors, power supplies, etc. in this lecture.)

2

3

1

2

3

1

Off: contacts 1 and 2 connected On: contacts 1 and 3 connected


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Relay Control Panel Components : Coil

(Note: Although this is really an electromagnetic coil, this can symbolize any “load” such as a pump, dc motor, heating element, light, etc. for this lecture.)

Coil off Coil on


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Relay Control Panel Components : Relay

A relay is a combination of coil and switch.

With coil off, the switch goes to its normal position off.

With coil on, the switch is pulled by electromagnetic force to its on position.

Off: Coil off, contacts 1 and 2 connected

1

3 2

1

3 2ON: Coil on, contacts 1 and 3 connected


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Relay Logic : NOTUsing one switch, a logical “NOT” operation can be constructed. An example is given below:

“NOT” Switch 1 = Coil

V+ Switch 1 Coil2

3

1


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Relay Logic : NOT (Continued)“NOT” Switch 1 off = Coil on

V+ Switch 1 Coil2

3

1

V+ Switch 1 Coil2

3

1

“NOT” Switch 1 on = Coil off


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Relay Logic : AND Using two switches, a logical “AND” operation can be constructed. An example is given below:

Switch 1 “AND” Switch 2 = Coil

V+ Switch 1 Switch 2 Coil

2

3

1

2

3

1


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Relay Logic : AND (continued) Switch 1 off “AND” Switch 2 off = Coil off


2

3

1

2

3

1

Switch 1 on “AND” Switch 2 off = Coil off


2

3

1

2

3

1


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Relay Logic : AND (continued) Switch 1 off “AND” Switch 2 on = Coil off


2

3

1

2

3

1

Switch 1 on “AND” Switch 2 on = Coil on


2

3

1

2

3

1


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Relay Logic : ORUsing two switches, a logical “OR” operation can be constructed. An example is given below:

Switch 1 “OR” Switch 2 = Coil

2

3

1

V+ Switch 1

Switch 2

Coil2

3

1


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Relay Logic : ORSwitch 1 off “OR” Switch 2 off = Coil off

2

3

1

V+ Switch 1

Switch 2

Coil2

3

1


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Relay Logic : ORSwitch 1 on “OR” Switch 2 off = Coil on

2

3

1

V+ Switch 1

Switch 2

Coil2

3

1


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Relay Logic : ORSwitch 1 off “OR” Switch 2 on = Coil on

2

3

1

V+ Switch 1

Switch 2

Coil2

3

1


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Relay Logic : ORSwitch 1 on “OR” Switch 2 on = Coil on

2

3

1

V+ Switch 1

Switch 2

Coil2

3

1


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Relay Logic : XORUsing two switches and four relays, a logical “XOR” operation can be constructed. An example is given below:

Switch 1 “XOR” Switch 2 = Coil

V+

Coil

2

3

1

Switch 1

Switch 2

2

3

1

1

2

1

3 2

1 1

3 23 2

V+ V+

3


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Relay Logic : XOR (continued)

V+

Coil

2

3

1

Switch 1

Switch 2

2

3

1

1

2

1

3 2

1 1

3 23 2

V+ V+

3

Switch 1 off “XOR” Switch 2 off = Coil off


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2

1

2

2


V+

Coil

2

3

1

Switch 1

Switch 2

2

3

1

1

2

1

3

1

33

V+ V+

3

Switch 1 on “XOR” Switch 2 off = Coil on


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2

3

1

1

23

1

3 2

1

3 2

1

3 2


V+

Coil

Switch 1

Switch 2

2

3

1

V+ V+

Switch 1 off “XOR” Switch 2 on = Coil on


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2

3

1

1

23

1

3 2

1

3 2

1

3 2


V+

Coil

Switch 1

Switch 2

2

3

1

V+ V+

Switch 1 on “XOR” Switch 2 on = Coil off


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Problems with relay control panels:Mechanical Relays and switches failed regularly (coil failure, contact wear and contamination, etc.)

Difficult to diagnose problems and replace relays and switches

Difficult to change hardwired logic (example: changing an “OR” circuit to “XOR”)

Consumed a lot of power

To address these problems, Richard E. Morley of Bedford Associates invented the first PLC as a consulting project for General Electric in 1968. Bedford Associates is currently named Modicon and is a supplier of PLCs.


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Section Objectives:

Basic PLC Components needed to replace relay control panels will be presented. These include:

For this lecture, Siemens A&D S7 314C-2 PtP PLC installed in the Mechatronics Laboratory will be used as an example.

Siemens 314C-2 PtP

Isolated Power Supply

Micro-controller

(Note: Advanced features such as Timers, Interrupts, Counters, etc. will not be discussed in this lecture)

Digital Input and Output pins ( DI/0)

Memory


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Basic PLC: Isolated Power SupplyEvery PLC has an external or internal Isolated Power Supply.

Isolated Power Supplies can have more than one isolated output.

One isolated output is reserved for the PLC micro-controller. The rest is reserved for other components such as DI/O.

Normally Power supplies are high voltage. Typically 24 Volts forindustrial PLCs.

The S7 314C-2 PtP PLC uses the Siemens A&D PS307 5A power supply. The PS307 5A can source 5 amps of current at 24 volts. The PS307 5A has 3 isolated outputs.

Siemens PS307 5A


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Basic PLC: Micro-controller

Every PLC at least one micro-controller

The S7 314C-2 PtP PLC uses a custom micro-controller. Designed by Siemens A&D and manufactured by InfineonTechnologies AD.

Part Number:InfineonSiemens A&DIBC 16SXA1020A-ES7 Controller

Specifications not given in documentation


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Basic PLC: Digital Inputs and Outputs (DI/Os)

DI/Os are electrically isolated from the micro-controller

The number of DI/Os can be increased by adding additional DI/O modules.

Example:

The S7 314C-2 PtP PLC has 16 digital outputs and 24 digital inputs. Can be expanded up to 1024 DI/Os by adding additional DI/O modules.

SM232 DI/O module


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Basic PLC: Memory

Memory on a PLC is separated into 3 main areas:LOAD Memory• Can be RAM (dynamic) or EEPROM (retentive)

• Used to store user programs

• For S7 314C-2 PtP PLC : LOAD Memory located on memory card

WORK Memory• Memory is RAM

• When PLC starts, Program is copied from LOAD memory to WORK memory. The program is then executed from Work memory.

• For S7 314C-2 PtP PLC: 48K bytes of WORK memory


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Basic PLC: Memory ( Continued)

SYSTEM Memory Memory is RAM

Is used by micro-controller to implement counters, timers, interrupt stacks, etc..

Contains a bit for each D I/0

Contains “Marker Memory”. Marker memory is a free area of RAM that can be used by the programmer. (In S7 314C-2 PtP, 258 bytes are available as Marker Memory)

Contains “Process Input and Output Images.” Periodically the PLCwill store the states of the inputs to the Process Input Image and Process Output Image to the output. (In S7 314C-2 PtP, this is limited to the first 128 bytes of input information and 128 bytes of output information.)


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Section Objectives:

Initially PLCs were used to directly replace relay control panels. To directly replace relay control panels based on mechanical relays with PLCs based on a micro-controller presented challenges. These challenges and solutions will be discussed.


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Transition:A Simplified Programmer’s Model

I1,I2, … ,Im

I1,I2, … ,Im

I1,I2, … ,Im

Q1

Q2

Qn

In the simplified programmer’s model of relay logic, all inputs I1, I2, .., Im go into each relay logic section. Each relay logic section then produces an output Q.

Relay Logic Section 2

.

.

.

Relay Logic Section n



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Transition: Relay control panel execution of Model


.

.

.

I1,I2, … ,Im changes at t0





Q1 changes at t1

Q2 changes at t1

Qn changes at t1

A relay control panel will execute all relay logic sections in parallel since each switch is capable of powering many coils at a time. If any input changes at time t0 then all the relay logic sections will update the outputs at time t1.


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Transition: PLC execution of Model


.

.

.






Q1 changes at t1

Q2 changes at t2

Qn changes at tn

A PLC will execute all relay logic sections in series since a micro-controller can execute only one instruction at a time. If any input changes at time t0 then relay logic section 1 will update Q1 at t1, relay logic section 2 will update Q2 at t2, …. , and relay logic section n will update Qn at tn.


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Transition: Differences in Relay Control Panel vs. PLC execution of Model

Difference 1:Relay Control Panel – The maximum time any change in input is reflected in any output is t1.

PLC – The maximum time any change in input is reflected in any output is t1+t2+…+tN.

Difference 2:Relay Control Panel – Since this is made from analogue components. It is possible to replace

a logic section without stopping execution of other logic sections if wired correctly.PLC – This is made with a digital micro-controller. The micro-controller must be halted to

replace a logic section. All other logic sections will stop operation.

Difference 3:Relay Control Panel – Since parallel execution of logic sections, all outputs are a function of

one set of inputs.PLC – Since serial execution of logic sections, all outputs may not be a function of one set of

inputs. (example: input I2 may change as the micro-controller is processing Logic section

2. Therefore Q1 and Q2 are based on different inputs)


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Transition: PLC Operation To minimize the effects of differences between the Relay Control Panel and PLC execution of the programming model, the PLC operates in the following manner:

Warm Restart

Update Process Image Input

User Program

Update Process Image Output

PLC System Processesscan cycle

Steps:• PLC Restarts (Warm Restart)

• Reads Inputs and updates Process Input Image

• Executes User Program Once

• Writes Process Output Image to Outputs

• Take care of system processes ( such as communications with other PLCs, updating user program, etc..)

• Loop Back to step 2

Steps 2 through 5 is called a scan cycle. (Note: some people may refer to a PLC as a Programmable “Loop” Controller because of the scan cycle loop.)


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Transition: PLC OperationTo Minimize Difference 1:Time to complete a scan cycle can be set by user. If PLC violates the scan cycle, an interrupt

routine can be run or the PLC will halt execution. (For S7 314C-2 PtP, maximum scan cycle allowed is 6 sec)

To Minimize Difference 2:If a part of the user program is replaced, the new part is written first to LOAD memory. During

step 5, PLC System Processes, the new part is copied into WORK memory from LOAD Memory. During the next scan cycle, the new part of the user program will be executed.

To Minimize Difference 3:If the programmer uses the inputs stored in the Process Input Image, the user program will

have access to the same inputs per scan cycle. Also if the programmer, writes outputs to the Process Output Image, all the outputs will be updated simultaneously during step 4, Update Process Output Image.


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Section Objectives:

The biggest transition from relay control panels to PLCs was the transition from the hard wired relay logic to logic defined by user program. In order to allow established relay logic users to program the PLC, a visual programming language that looks like a relay control panel was created. This visual programming language is called “Ladder Logic”.

In this section, basic Ladder Logic will be presented.


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Ladder Logic: System Memory Addressing

To address a bit of memory

To address a byte, word, or double word

___ ___ . ___

Memory AreaNotation

Byte Address Bit Number

___ ___ ___

Memory AreaNotation

Size of Addressed Memory Notation

Byte Address


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Ladder Logic: System Memory Addressing (continued)Memory Area Notations:

Local Memory of current Data BlockL

Data Block MemoryDB

Counter Storage AreaC

Timer Storage AreaT

Peripheral Output ( Actual Output Pins)PQ

Peripheral Input ( Actual Input Pins)PI

Marker MemoryM

Process Output ImageQ

Process Input ImageI

Memory AreaNotation

(Note: Advanced features such as Timers, Counters, Data Blocks will not be discussed in this lecture)


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Ladder Logic: System Memory Addressing (continued)Size of Addressed Memory Notations:

Double Word (32 bits)D

Word (16 bits)W

Byte (8 bits)B

Size of Addressed MemoryNotation

Byte Address:

Each Memory Area is addressed in one byte increments starting at byte 0.

Bit Number:

MSBit is 7 and LSBit is 0


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Ladder Logic: System Memory Addressing (continued)Examples:

Byte 5

Byte 6

Byte 7

Byte 4

Byte 3

Byte 2

Byte 1

Byte 0

Marker Area

M1.3 (Note: only bit 3 ofMarker Area byte 1)

MB0

MW1

MD4

MD3


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Ladder Logic: System Memory Addressing (continued)Examples:

Byte 5

Byte 6

Byte 7

Byte 4

Byte 3

Byte 2

Byte 1

Byte 0

Peripheral Input Area

PI2.5 (Note: only bit 5 ofPeripherial Input Area byte 2)

PIB1

PID4


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Ladder Logic : The LadderA ladder logic program has a “ladder” look to it. The sides of the ladder are the power rail on the left and ground rail on the right. The rungs of the ladder consists of Virtual Relay Components. (Note: Rungs are called “Networks” in Step 7)

Virtual Relay Components



Pow

er Rail

Ground R

ail


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Ladder Logic : Virtual RelaysAny Marker or Function Block memory bit can be one or more virtual relays. If memory bit is 0, the coils of virtual relays associated with the bit are off. If memory bit is 1, the coils of virtual relays associated with the bit are on.

Any D I/O memory bit ( Peripheral or Process Image) is a virtual relay for a digital input or output pin of the PLC.

Mechanical Relay

1

3 2

Virtual Relay Components:

Normally Open Switch ( equivalent to pins 1 and 3 of Mechanical Relay. If this switch is closed for a virtual digital output relay, the digital output pin is high. If this switch is open for a virtual digital output relay, the digital output pin is low )

Normally Closed Switch ( equivalent to pins 1 and 2 of Mechanical Relay)

Coil ( equivalent to coil of Mechanical Relay. Not available for virtual digital input relays)


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Ladder Logic: Rules for converting Relay Logic to Ladder Logic

Each external switch must be connected to an input pin of a PLC.

Each external coil or load must be connected to an output pin ofa PLC.

The relay logic must be recreated using virtual input and outputrelays associated with the input and output pins.

Only possible paths from power to ground though virtual relays need to be recreated in Ladder Logic.


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Ladder Logic : NOT

“NOT” Switch 1 = Coil

V+ Switch 1 Coil2

3

1

From Relay Logic:


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Ladder Logic : NOT (continued)

Relay Logic rewired to include Virtual Input and Output Relays:

V+Coil

3

Switch 12

1 1

3 2

1

3 2

V+V+

Inside PLC

Virtual OutputRelay at Q0.0

Virtual InputRelay at I0.0

(Note: Wired to PLC Input Pin Associated with Virtual Input Relay I0.0) (Note: Wired to PLC

Output Pin Associated with Virtual Output Relay Q0.0)


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Ladder Logic : NOT (continued)

Ladder Logic Equivalent:

Switch 1 is wired to PLC input pin associated with Virtual Input Relay I0.0Coil is wired to PLC output pin associated with Virtual Output Relay Q0.0

I0.0 Q0.0


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2

3

1

2

3

1

Ladder Logic : AND

From Relay Logic:

Switch 1 “AND” Switch 2 = Coil


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Ladder Logic : AND (continued)Relay Logic rewired to include Virtual Input and Output Relays:

Coil

3

Switch 12

1

1

3 2

V+

V+

V+

1

3 2

Inside PLC



(Note: Wired to PLC Input Pin Associated with Virtual Input Relay I0.0)

Switch 22

1

V+


(Note: Wired to PLC Output Pin Associated with Virtual Output Relay Q0.0)

1

3 2


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Ladder Logic : AND (continued)


Switch 1 is wired to PLC input pin associated with Virtual Input Relay I0.0Switch 2 is wired to PLC input pin associated with Virtual Input Relay I0.1Coil is wired to PLC output pin associated with Virtual Output Relay Q0.0

I0.0 Q0.0I0.1


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2

3

1

V+ Switch 1

Switch 2

Coil2

3

1

Ladder Logic : OR

From Relay Logic:



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Ladder Logic : OR (continued)Relay Logic rewired to include Virtual Input and Output Relays:

Coil

3

Switch 12

1

1

3 2

V+

V+

V+

1

3 2

Inside PLC




Switch 22

1

V+


(Note: Wired to PLC Output Pin Associated with Virtual Output Relay Q0.0)

1

3 2

V+


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Ladder Logic : OR (continued)



I0.0 Q0.0

I0.1


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V+

Coil

2

3

1

Switch 1

Switch 2

2

3

1

1

2

1

3 2

1 1

3 23 2

V+ V+

3

Ladder Logic : XOR

From Relay Logic:



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Ladder Logic : XOR (continued)Relay Logic rewired to include Virtual Input and Output Relays:

Coil3

Switch 12

1

V+

Inside PLC




Switch 22

1

V+


(Note: Wired to PLC Output Pin Associated with Virtual Output Relay PQ0.0)

V+

1

3 2

1

3 2

V+

1

3 2

V+

1

3 2

1

3 2




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Ladder Logic : OR (continued)



I0.0 Q0.0

I0.0

I0.1

I0.1


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Section Objectives:

A micro-controller can be used for more than relay logic with virtual relays. Ladder logic has components that take advantage of the micro-controller. These components can be categorized as follows: bit logic,comparator, converter, counter, data base calls, jumps, integer functions, floating point functions, move, program control, shift/rotate, status bits, timers, and word logic.

It is impossible to cover all of the components in one lecture. This lecture will first explain formatting of constants. Then, only a few categories and examples of components will be shown.


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Constants


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Bit Logic

Available Bit logic components:

Normally Closed Switch

Normally Open Switch

Output Coil

Midline Output

Set Coil

Reset Coil

Invert Power Flow

Save RLO into BR Memory

Bit Exclusive OR

Positive Edge Detection

Negative Edge Detection

Address Positive Edge Detection

Address Negative Edge Detection

Set-Reset Flip Flop

Reset-Set Flip Flop

Immediate Read

Immediate Write


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Bit Logic example: Set Coil and Reset Coil

Description:Set Coil is executed only if power flows to the coil. When executed, the specified <address> of the element is set to "1". It will remain set even if power is removed from the coil.


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Description:Reset Coil is executed only if power flows to the coil. When executed, the specified <address> of the element is reset to "0". No power flow to the coil has no effect and the state of the element's specified address remains unchanged.(Note: can be used to reset timers and counters)


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Example:Switch 1 connected to Input 0.0Switch 2 connected to Input 0.1Coil connected to Output 0.0

If Switch 1 turns on then turn on Coil and keep it on even if Switch 1 is released. If Switch 2 turns on then turn off the Coil.

I0.0 Q0.0

I0.1

S

Q0.0

R


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Comparator

Available Comparator components (Note: Integer is Word, Double Integer is Double Word)

Integer: Equal to

Integer: Greater than

Integer: Less than

Integer: Greater than or Equal to

Integer: Less than or Equal to

Double Integer: Equal to

Double Integer: Greater than

Double Integer: Less than

Double Integer: Greater than or Equal to

Double Integer: Less than or Equal to

Real: Equal to

Real: Greater than

Real: Less than

Real: Greater than or Equal to

Real: Less than or Equal to


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Comparator example: Integer Compares


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Comparator example: Integer Compares

Example: Coil connected to Output 0.0

If MW0 and MW2 are equal then turn on coil.

Q0.0

CMP== I

MW0

MW2

IN1

IN2


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Jumps

Available Jump components (Note: called Logic control in Step 7 Help)

Label

Unconditional Jump

Conditional Jump

Not conditional Jump


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Jump example: Conditional Jump

Description Conditional Jump:The micro-controller will goto the specified Label if power flows into the JUMP. (Note: a label can be assigned to any Network)


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Jump example: Label and conditional JumpExample:

Switch 1 connected to Input 0.0

If Switch 1 turns on then jump to label “END”

I0.0 “END”

I0.1

JMP

Q0.0

Components

Components

END


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Integer Math

Available Integer Math components:

Integer: Add

Integer: Subtract

Integer: Multiply

Integer: Divide

Double Integer: Add

(Note: Integer is Word, Double Integer is Double Word)

Double Integer: Subtract

Double Integer: Multiply

Double Integer: Divide

Double Integer: Modulus


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Math example: Integer Add

Description:IN1 and IN2 are added and the result is stored in OUT when power is applied to EN . Power flows out of EN0 when power is applied to EN unless the addition results in overflow.


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Math example: Integer AddExample:

Add 5 and integer stored at MW0. Store the result in MW2.

ADD_I

5

MW0

IN1

IN2

EN EN0

OUT MW2


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Move

Available Move components:

Move


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Move example:

Description:IN is moved to Out and power flows out of EN0 when power is applied to EN.


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Move example:Example:

Move 5 to MW2.

MOVE

5 IN1

EN EN0

OUT MW2


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Timer

Available Timer components:

Pulse S5 Timer

Extended Pulse S5 Timer

On-Delay S5 Timer

Retentive On-Delay S5 Timer

Off-Delay S5 Timer

Pulse Timer Coil

Extended Pusle Timer Coil

On-Delay Timer Coil

Retentive On-Delay Timer Coil

Off-Delay Timer Coil


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Timer example: Extended Pulse S5 Timer

Description:A power transition from OFF to ON on S will restart the timer. Power flows from Q while timer is running. The timer will run for a preset time TV. (Note: 256 timers allowed in S7 314C-PtP PLC)


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Timer example:Example:

Switch 1 connected to Input 0.0Coil is connected to Output 0.0

Turn on coil for 10 seconds if Switch 1 is turned on.

S_EXt

S5T#10s TV

S Q

BI

R BCD

I0.0 Q0.0T 0


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Word Logic

Available Word Logic components:

“AND” Word

“OR” Word

“XOR” Word

“AND” Double Word

“OR” Double Word

“XOR” Double Word


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Word Logic example: “AND” Word

Description:IN1 “AND” IN2 is stored in OUT when power is applied to EN . Power flows out of EN0 when power is applied to EN unless the addition results in overflow.


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Word Logic example: Integer AddExample:

“AND” MW0 and MW2. Store the result in MW4.

WAND W

MW0

MW2

IN1

IN2

EN EN0

OUT MW4


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Section Objectives:

In this section two example ladder logic programs will be given.


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Example 1 :

Switch 1 connected to Input 0.0Coil connected to Output 0.0

If Switch 1 is on then turn on and off a coil at 2 second intervals (Note: 2 second interval means a period of 4 seconds and 50% Duty cycle).


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Example 1 (Continued)

S_EXt

S5T#2s TVS Q

BIR BCD

I0.0

M0.0

T 0

S_EXt

S5T#2s TVS Q

BIR BCD

I0.0T 1

Q0.0

Time: Scan cycle right before t = 0sUser Action : None

Q0.0M0.0


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Example 1 : Continued

S_EXt

S5T#2s TVS Q

BIR BCD

I0.0 Q0.0T 0

S_EXt

S5T#2s TVS Q

BIR BCD

I0.0T 1

Q0.0 M0.0

M0.0

Time:Scan cycle at t = 0User Action: User turns Switch 1 on

(Note:Time left: 2 s)


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S_EXt

S5T#2s TVS Q

BIR BCD

I0.0 Q0.0T 0

S_EXt

S5T#2s TVS Q

BIR BCD

I0.0T 1

Q0.0 M0.0

M0.0

Time: Scan cycle right before t = 2sUser Action: None

(Note:Time left: ~0)


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S_EXt

S5T#2s TVS Q

BIR BCD

I0.0 Q0.0T 0

S_EXt

S5T#2s TVS Q

BIR BCD

I0.0T 1

Q0.0 M0.0

M0.0

Time: Scan cycle at t = 2 sUser Action: None

(Note:Time left 0 s)

(Note: There is an inconsistency in this picture. The power is still flowing though the normally closed contact for M0.0 on the first rung even though the coil on the second rung for M0.0 is on. This is due to the serial nature of the PLC micro-controller. Since the first rung is evaluated first, the coil was still off when the micro-controller evaluated the normally closed contact for M0.0)



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S_EXt

S5T#2s TVS Q

BIR BCD

I0.0 Q0.0T 0

S_EXt

S5T#2s TVS Q

BIR BCD

I0.0T 1

Q0.0 M0.0

M0.0

Time: Scan cycle right after t = 2 sUser Action: None

(Note: Inconsistency from the previous slide resolved)

(Note:Time left: 2 s – 1 scan cycle time)


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S_EXt

S5T#2s TVS Q

BIR BCD

I0.0 Q0.0T 0

S_EXt

S5T#2s TVS Q

BIR BCD

I0.0T 1

Q0.0 M0.0

M0.0

Time: Scan cycle right before t = 4 sUser Action: None

(Note:Time left: ~0 s)


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S_EXt

S5T#2s TVS Q

BIR BCD

I0.0 Q0.0T 0

S_EXt

S5T#2s TVS Q

BIR BCD

I0.0T 1

Q0.0 M0.0

M0.0

Time: Scan cycle at t = 4 sUser Action: None



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Example 1 : ContinuedTime: Scan cycle right after t = 4 sUser Action: None

S_EXt

S5T#2s TVS Q

BIR BCD

I0.0 Q0.0T 0

S_EXt

S5T#2s TVS Q

BIR BCD

I0.0T 1

Q0.0 M0.0

M0.0


(Note: A once scan cycle error has been introduced in the timing. The reason is that the coil of M0.0 on the second rung was turned off during the scan cycle at t = 4s. The normally closed switch of M0.0 is not evaluated again until the scan cycle after the scan cycle at t = 4 s. Therefore, Timer T0 starts one scan cycle after t = 4. This error will propagate and similar errors will accumulate. )


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S_EXt

S5T#2s TVS Q

BIR BCD

I0.0 Q0.0T 0

S_EXt

S5T#2s TVS Q

BIR BCD

I0.0T 1

Q0.0 M0.0

M0.0

Time: Some time laterUser Action: User turns Switch 1 off


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Example 1 :

Comments:

As this example illustrates, consistent timing is difficult to achieve with a PLC due to the scan cycle. This is the reason why PLC’s are not used to control systems with very fast time constants such as CNC machines, chemical mixers, etc….


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Example 2 :

Switch 1 connected to Input 0.0A Hall effect switch is connected to Input 0.1

(Note: a Hall effect switch will turn on when a magnetic object comes in close proximity)

The motor for a conveyer belt is connected to Output 0.0 (Note: As previously mentioned, a coil can be any “load” such as a motor during these lectures.)

If Switch 1 is turned on, the conveyer belt will transport 1000 magnetic SHAFTS to Georgia Tech Students. Switch 1 must be turned off then on to send another 1000 magnetic SHAFTS. The hall affect switch is positioned right under the conveyer belt and can be used to count the SHAFTS as they pass by.


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Example 2 (Continued)

I0.1

Time: Scan cycle right before t = 0sActions : no part near hall effect switch

Q0.0M0.0I0.0S

I0.1

M0.1R

ADD_I

IN1

IN2

EN EN0

OUTMW1

1

MW1

Move

IN1

EN EN0

OUT0 MW1

M0.1 M0.1

S

Q0.0R

CMP== I

IN1

IN2MW1

1001

M0.0

S

Q0.0

M0.0R

I0.0


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Example 2 (Continued)Time: Scan cycle at t = 0sActions : Switch 1 is turned on,

no SHAFT near hall effect switch

(Note: There is an inconsistency. Power is still flowing though normally closed contact for M0.0 even though the coil M0.0 is on. Since the components on a rung is evaluated from left to right, coil for M0.0 when micro-controller evaluated the normally closed contact for M0.0 was still off. Same for PQ0.0)

I0.1

Q0.0M0.0I0.0S

I0.1

M0.1R

ADD_I

IN1

IN2

EN EN0

OUTMW1

1

MW1

Move

IN1

EN EN0

OUT0 MW1

M0.1 M0.1

S

Q0.0R

CMP== I

IN1

IN2MW1

1001

M0.0

S

Q0.0

M0.0R

I0.0


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Example 2 (Continued)Time: Scan cycle right after t = 0sActions : no SHAFT near hall effect switch

(Note: Inconsistency from previous slide resolved. The conveyer is still moving because of the “Set” coil.)

I0.1

Q0.0M0.0I0.0S

I0.1

M0.1R

ADD_I

IN1

IN2

EN EN0

OUTMW1

1

MW1

Move

IN1

EN EN0

OUT0 MW1

M0.1 M0.1

S

Q0.0R

CMP== I

IN1

IN2MW1

1001

M0.0

S

Q0.0

M0.0R

I0.0


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Example 2 (Continued)Time: t = t1Actions : SHAFT approaches hall effect switch, 1 is added to MW1

(Note: Similar Inconsistency between normally closed switch of M0.1 and coil of M0.1 as seen with normally closed switch of M0.0 and coil of M0.0)

I0.1

Q0.0M0.0I0.0S

I0.1

M0.1R

ADD_I

IN1

IN2

EN EN0

OUTMW1

1

MW1

Move

IN1

EN EN0

OUT0 MW1

M0.1 M0.1

S

Q0.0R

CMP== I

IN1

IN2MW1

1001

M0.0

S

Q0.0

M0.0R

I0.0


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Example 2 (Continued)Time: t = t1 + 1 scan cycleActions : SHAFT passes over hall effect switch

(Note: Inconsistency from previous slide resolved.)

I0.1

Q0.0M0.0I0.0S

I0.1

M0.1R

ADD_I

IN1

IN2

EN EN0

OUTMW1

1

MW1

Move

IN1

EN EN0

OUT0 MW1

M0.1 M0.1

S

Q0.0R

CMP== I

IN1

IN2MW1

1001

M0.0

S

Q0.0

M0.0R

I0.0


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Example 2 (Continued)Time: t = t1 + 2 scan cycleActions : no SHAFT near hall effect switch

(Note: Inconsistency between the “set” and “reset” of M0.1. That is because coil is still set when the third rung is evaluated.)

I0.1

Q0.0M0.0I0.0S

I0.1

M0.1R

ADD_I

IN1

IN2

EN EN0

OUTMW1

1

MW1

Move

IN1

EN EN0

OUT0 MW1

M0.1 M0.1

S

Q0.0R

CMP== I

IN1

IN2MW1

1001

M0.0

S

Q0.0

M0.0R

I0.0


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Example 2 (Continued)Time: t = t1 + 3 scan cycleActions : no SHAFT near hall effect switch

I0.1

Q0.0M0.0I0.0S

I0.1

M0.1R

ADD_I

IN1

IN2

EN EN0

OUTMW1

1

MW1

Move

IN1

EN EN0

OUT0 MW1

M0.1 M0.1

S

Q0.0R

CMP== I

IN1

IN2MW1

1001

M0.0

S

Q0.0

M0.0R

I0.0

(Note: Inconsistency between the “set” and “reset” of M0.1 resolved.)


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Example 2 (Continued)Time: t = t2Actions : the 1001th SHAFT approaches hall effect switch (so 1000 have been delivered)

I0.1

Q0.0M0.0I0.0S

I0.1

M0.1R

ADD_I

IN1

IN2

EN EN0

OUTMW1

1

MW1

Move

IN1

EN EN0

OUT0 MW1

M0.1 M0.1

S

Q0.0R

CMP== I

IN1

IN2MW1

1001

M0.0

S

Q0.0

M0.0R

I0.0


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Example 2 (Continued)Time: t = t2+ 1 scan cycleActions : the conveyer is stopped with 1001th SHAFT over the Hall effect switch

I0.1

Q0.0M0.0I0.0S

I0.1

M0.1R

ADD_I

IN1

IN2

EN EN0

OUTMW1

1

MW1

Move

IN1

EN EN0

OUT0 MW1

M0.1 M0.1

S

Q0.0R

CMP== I

IN1

IN2MW1

1001

M0.0

S

Q0.0

M0.0R

I0.0

(Note: Inconsistency between the “set” and “reset” of PQ0.0. That is because coil is still set when the first rung is evaluated.)


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Example 2 (Continued)Time: t = t2+ 1 scan cycleActions : the conveyer is stopped. Switch 1 must be turned off and on to deliver 1000 more

I0.1

Q0.0M0.0I0.0S

I0.1

M0.1R

ADD_I

IN1

IN2

EN EN0

OUTMW1

1

MW1

Move

IN1

EN EN0

OUT0 MW1

M0.1 M0.1

S

Q0.0R

CMP== I

IN1

IN2MW1

1001

M0.0

S

Q0.0

M0.0R

I0.0

(Note: Inconsistency from previous slide resolved.)


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Example 2 :

Comments:

This and the previous example illustrates that the serial nature of the PLC micro-controller can still affect program execution.

Also, this program can be simplified using an positive edge detection coil. This was not done because the positive edge detection coil was not an example in Section 5.


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So far we have looked at topics applicable to all PLC’s. Further Study Should focus on:

Topics applicable to some but not all PLC’s:

Interrupts

Counters

Communication Protocol:Profibus

How to use communications to communicate with other PLC’s, smart actuators and sensors, etc…

A/D

Function Blocks

plc - georgia institute of technologyume.gatech.edu/mechatronics_course/me4447_6405/plc.pdf ·...

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