Petruzella F.D. Programmable Logic Controllers

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Data Manipulation Instructions Chapter 10 213

timers. However, the same circuit can be programmed

using only one internal timer along with data compare in-

structions. Figure 10-32 shows the program required to

implement the circuit using only one internal timer. The

operation of the program can be summarized as follows:

• The momentary stop button is closed.

• When the momentary start button is pressed,

SOL A output energizes immediately to switch on

solenoid A.

• SOL A examine-on contact becomes true to seal

in output SOL A and to start on-delay timer T4:1

timing.

• The timer preset time is set to 15 seconds.

• Output SOL D will energize (through the timer

done bit T4:1/DN) after a total time delay of 15 sec-

onds to energize solenoid D.

• Output SOL B will energize after a total time delay

of 5 seconds, when the accumulated time becomes

equal to and then greater than 5 seconds. This, in

turn, will energize solenoid B.

• Output SOL C will energize after a total time delay

of 10 seconds, when the accumulated time becomes

equal to and then greater than 10 seconds. This, in

turn, will energize solenoid C.

Figure 10-33 shows an application of an on-delay

timer program implemented using the EQU instruction.

The operation of the program can be summarized as

follows:

• When the switch (S1) is closed, timer T4:1 will

begin timing.

• Both EQU instructions’ source A s are addressed to

get the accumulated value from the timer while it is

running.

• The EQU instruction of rung 2 has the value of 5

stored in source B.

• When the accumulated value of the timer reaches5,

the EQU instruction of rung 2 will become logic

true for 1 second.

• As a result, the latch output will energize to switch

the pilot light PL1 on.

• When the accumulated value of the timer reaches

15, the EQU instruction of rung 3 will be true for

1second.

• As a result, the unlatch output will energize to

switch the pilot light PL1 off.

• Therefore, when the switch is closed, the pilot light

will come on after 5 seconds, stay on for 10 sec-

onds, and then turn off.

• The mask must be the same element size (16 bits) as

the source and compare addresses.

• You must set mask bits to 1 to compare data. Bits

in the compare address that correspond to 0s in the

mask are not compared.

• If you want the ladder program to change mask

value, store the mask at a data address. Otherwise,

enter a hexadecimal value for a constant mask

value.

• The instruction is true because reference bits XXXX

are not compared.

10.4 Data Manipulation Programs

Data manipulation instructions give new dimension and

 exibility to the programming of control circuits. For ex-

ample, consider the hardwired relay-operated, time-delay

circuit in Figure10-31 . This circuit uses three electrome-

chanical time-delay relays to control four solenoid valves.

The operation of the hardwired circuit can be summarized

as follows:

• When the momentary start pushbutton is pressed

solenoid A is energized immediately.

• Solenoid B is energized 5 s later than solenoid A.

• Solenoid C is energized 10 s later than solenoid A.

• Solenoid D is energized 15 s later than solenoid A.

The hardwired time-delay circuit could be implemented

using a conventional PLC program and three internal

Stop

Start

1TD

2TD

3TD

L2L1

(5 s)

SOL D

SOL A

SOL B

SOL C

3TD

2TD

1TD

Figure 10-31 Three electromechanical time-delay relays

used to control four solenoid valves.

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214 Chapter 10 Data Manipulation Instructions

Figure 10-33 Timer program implemented using the EQU instruction.

T4:1

1.0

TON

TIMER ON DELAY

Timer

Time base

Preset

Accumulated

Input

Ladder logic program

T4:1.ACC

EQU

EQUAL

Source A

Source B

T4:1.ACC

EQU

EQUAL

Source A

Source B

PL1

Output

PL1

SOL A

T4:1

1.0

TON

TIMER ON DELAY

Timer

Time base

Preset

Accumulated

Ladder logic program

SOL A

SOL B

T4:1.ACC

GEQ

GREATER THAN OR EQUAL

Source A

Source B

SOL C

T4:1.ACC

GEQ

GREATER THAN OR EQUAL

Source A

Source B

Inputs

SOL D

T4:1

SOL A

Outputs

SOL B

SOL C

SOL D

Stop

Start

Figure 10-32 Controlling multiple loads using one timer and the GEQ instruction.

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Data Manipulation Instructions Chapter 10 215

• While the vessel  lls, the PLC performs a compari-

son between the vessel’s current weight and a pre-

determined constant programmed in the processor.

Figure 10-34 Counter program implemented using the LES instruction.

Source: Photo courtesy Turck, Inc., www.turck.com.

C5:1

CTU

COUNT UP

Counter

Preset

Accumulated

Sensor

Input

Ladder logic program

C5:1

RES

SOL

C5:1.ACC

LES

LESS THAN

Source A

Source B

Output

SOL

Figure 10-34 shows an application of an up-counter

program implemented using the LES instruction. The op-

eration of the program can be summarized as follows:

• Up-counter C5:1 will increment by 1 for every

false-to-true transition of the proximity sensor

switch.

• Source A of the LES instruction is addressed to the

accumulated value of the counter and source B has a

constant value of 20.

• The LES instruction will be true as the long as

the value contained in source A is less than that of

source B.

• Therefore, output solenoid SOL will be energized

when the accumulated value of the counter is be-

tween 0 and 19.

• When the counter’s accumulated value reaches 20,

the LES instruction will go false, de-energizing

output solenoid SOL.

• When the counter’s accumulated value reaches its

preset value of 50, the counter reset will be ener-

gized through the counter done bit (C5:1/DN) to

reset the accumulated count to 0.

The use of comparison instructions is generally

straightforward. However, one precaution involves the

use of these instructions in PLC programs used to control

the  ow in vessel  lling operations ( Figure10-35 ). This

control scenario can be summarized as follows:

• The receiving vessel has its weight monitored con-

tinuously by the PLC program as it  lls.

• When the weight reaches a preset value, the  ow is

cut off.

Figure 10-35 Vessel ﬁ lling operation.

Source: Courtesy Feige Filling.

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216 Chapter 10 Data Manipulation Instructions

performed by the LED display device. The BCD output

module is used to output data from a speci c register or

word location in memory. This type of output module en-

ables a PLC to operate devices that require BCD coded

signals.

Figure10-38 shows a PLC program that uses a BCD

input interface module connected to a thumbwheel switch

and a BCD output interface module connected to an LED

display board. The program is designed so that the LEDs

display the setting of the thumbwheel switch. Both the

MOV and EQU instructions form part of the program. The

operation of the program can be summarized as follows:

• The LED display board monitors the decimal set-

ting of the thumbwheel switch.

• The MOV instruction is used to move the data from

the thumbwheel switch input to the LED display

output.

• If the programmer uses only the equal instruction,

problems may result.

• As the vessel  lls, the comparison for equality will

be false. At the instant the vessel weight reaches the

desired preset value of the equal instruction, the in-

struction becomes true and the  ow is stopped.

• However, should the supply system leak additional

material into the vessel, the total weight of the ma-

terial could rise above the preset value, causing the

instruction to go false and the vessel to over ll.

• The simplest solution to this problem is to program

the comparison instruction as a greater than or equal

to instruction. This way, any excess material enter-

ing the vessel will not affect the  lling operation.

• It may be necessary, however, to include addi-

tional programming to indicate a serious over ll

condition.

10.5 Numerical Data I/O Interfaces

The expanding data manipulation processing capabilities

of PLCs led to the development of I/O interfaces known

as numerical data I/O interfaces. In general, numerical

data I/O interfaces can be divided into two groups: those

that provide interface to multibit digital devices and those

that provide interface to analog devices.

The multibit digital devices are like the discrete I/O

because processed signals are discrete (on/off). The dif-

ference is that, with the discrete I/O, only a single bit is

required to read an input or control an output. Multibit

interfaces allow a group of bits to be input or output as a

unit. They can be used to accommodate devices that re-

quire BCD inputs or outputs.

The thumbwheel switches (TWS), shown in Fig-

ure10-36 , are typical BCD input devices. Each one of the

four switches provides four binary digits at its output that

correspond to the decimal number selected on the switch.

The conversion from a single decimal digit to four binary

digits is performed by the TWS device. The BCD input

module allows the processor to accept the 4-bit digital

codes and input their data into speci c register or word

locations in memory to be used by the control program.

Data manipulation instructions can be used to access the

data from the input module allowing a person to change

set points, timer, or counter presets externally without

modifying the control program.

The seven-segment LED display board, shown in Fig-

ure10-37 , is a typical Binary Coded Decimal (BCD) out-

put device. It displays a decimal number that corresponds

to the BCD value it receives at its input. Conversion of the

four binary bits to a single decimal digit on the display is

Bit

address

BCD

input module

1s units

Decimal

10s units

100s units

1000s units

Thumbwheel

switch

(TWS)

BCD

Figure 10-36 BCD input interface module connected to a

thumbwheel switch.

Source: Photo courtesy Omron Industrial Automation, www.ia.omron.com.

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Data Manipulation Instructions Chapter 10 217

• Setting of the thumbwheel switch is compared to

the reference number 1208 stored in source B by the

EQU instruction.

• Pilot light output PL is energized whenever the

input switch S1 is true (closed) and the value of the

thumbwheel switch is equal to 1208.

Input and output modules can be addressed either at

the bit level or at the word level. Analog modules con-

vert analog signals to 16-bit digital signals (input) or

16-bit digital signals to analog values (output). An analog

I/Owill allow monitoring and control of analog voltages

and currents. Figure10-39 illustrates how an analog input

interface operates. The operation of this input module can

be summarized as follows:

• The analog input module contains the circuitry nec-

essary to accept analog voltage or current signals

from  eld devices.

• The input signal is converted from an analog to a

digital value by an analog-to-digital (A/D) converter

circuit.

• The conversion value, which is proportional to the

analog signal, is passed through the controller’s

data bus and stored in a speci c register or word

location in memory for later use by the control

program.

An analog output interface module receives numerical

data from the processor; these data are then translated into

a proportional voltage or current to control an analog  eld

device. Figure10-40 illustrates how an analog output in-

terface operates. The operation of this output module can

be summarized as follows:

Figure 10-37 BCD output interface module connected to

a seven-segment LED display board.

Source: Photo courtesy Omron Industrial Automation, www.ia.omron.com.

Bit

address

BCD

output module

1s units

Decimal

10s units

100s units

1000s units

LED

display board

BCD

Figure 10-38 Monitoring the setting of a thumbwheel switch.

Thumbwheel switch

Inputs Ladder logic program

TWS

1208

EQU

EQUAL

Source A

Source B

MOV

MOVE

Source

Destination

(All 16 bits)

TWS

LED

LED display

Outputs

(All 16 bits)

LED

12 08

1 2 0 8

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218 Chapter 10 Data Manipulation Instructions

10.6 Closed-Loop Control

In open-loop control, no feedback loop is employed

and system variations which cause the output to deviate

from the desired value are not detected or corrected. A

closed-loop system utilizes feedback to measure the ac-

tual system operating parameter being controlled such as

temperature, pressure,  ow, level, or speed. This feedback

signal is sent back to the PLC where it is compared with

the desired system set-point. The controller develops an

error signal that initiates corrective action and drives the

 nal output device to the desired value.

PLC set-point control in its simplest form compares

an input value, such as analog or thumbwheel inputs,

to a set-point value. A discrete output signal is provided

if the input value is less than, equal to, or greater than

the set-point value. The temperature control program of

• The function of the analog output module is to ac-

cept a range of numeric values output from the PLC

program and to produce a varying current or voltage

signal required to control a connected analog output

device.

• Data from a speci c register or word location in the

CPU memory are passed through the controller’s

data bus to the digital-to-analog (D/A) converter.

• The analog output from the D/A converter is then

used to control the analog output device.

• The level of the analog signal output is based on the

digital value of the data word supplied by the CPU

and manipulated by the control program.

• These output interfaces normally require an external

power supply that meets certain current and voltage

requirements.

Figure 10-39 Analog input interface module.

Thermocouple

Data

bus

A/D

Variable voltage

or current input

BCD conversion value

Analog output module

Analog

input

device

Figure 10-40 Analog output interface module.

Isolation

Word

data

from

CPU

Digital

analog

converter

Voltage

and

current

driver

Load

Amplifier

Analog

output

Servo

motor

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Data Manipulation Instructions Chapter 10 219

on and sealed-in through the heater examine-on

instruction.

• Once the temperature increases to 598°F the LEQ

instruction goes false but the heater output remains

on until the temperature rises to 603°F.

• At the 603°F point the GEQ instruction and B3:0/2

will both be true and the heater will be switched off.

Several set-point control schemes can be performed by

different PLC models. These include on/off control, pro-

portional (P) control, proportional-integral (PI) control,

and proportional-integral-derivative (PID) control. Each

involves the use of some form of closed-loop control to

maintain a process characteristic such as a temperature,

pressure,  ow, or level at a desired value. When a control

system is designed such that it receives operating infor-

mation from the machine and makes adjustments to the

machine based on this operating information, the system

is said to be a closed-loop system.

The block diagram of a closed-loop control system is

shown in Figure 10-42 . A measurement is made of the

variable to be controlled. This measurement is then com-

pared to a reference point, or set-point. If a difference

Figure10-41 is one example of set-point control. In this

application, a PLC is to provide for simple off/on control

of the electric heating elements of an oven. The operation

of the program can be summarized as follows:

• Oven is to maintain an average set-point tempera-

ture of 600°F with a variation of about 1 percent

between the off and on cycles.

• The electric heaters are turned on when the temper-

ature of the oven is 597°F or less and will stay on

until the temperature rises to 603°F or more.

• The electric heaters stay off until the temperature

drops to 597°F, at which time the cycle repeats

itself.

• Whenever the less than or equal (LEQ) instruction

is true, a low-temperature condition exists and the

program switches on the heater.

• Whenever the greater than or equal (GEQ) instruc-

tion is true, a high-temperature condition exists and

the program switches off the heater.

• For the program as shown the temperature is

595°F so the LEQ instruction and B3:0/1 will

both be true and the heater output will be switched

Figure 10-41 Set-point control program.

Ladder logic program

B3:0/1

(Internal)

B3:0/2

(Internal)

LEQ

LESS THAN OR EQUAL

Source A

Source B

GEQ

GREATER THAN OR EQUAL

Source A

Source B

MOV

MOVE

Source

Destination

Low temp.

B3:0/1

Heater

High temp.

B3:0/2

Inputs

Thermocouple

LED

Heater

Outputs

LED Display

LED

59 5

597

603

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220 Chapter 10 Data Manipulation Instructions

the output to degrade closing the valve by different per-

centages, adjusting the valve to maintain a set-point.

Proportional-integral-derivative (PID) control is the

most sophisticated and widely used type of process control.

PID operations are more complex and are mathematically

based. PID controllers produce outputs that depend on the

magnitude, duration, and rate of change of the system error

signal. Sudden system disturbances are met with an aggres-

sive attempt to correct the condition. A PID controller can

reduce the system error to 0 faster than any other controller.

A typical PID control loop is illustrated in Figure10-44 .

The loop measures the process, compares it to a set-point,

and then manipulates the output in the direction which

should move the process toward the set-point. The termi-

nology used in conjunction with a PID loop can be sum-

marized as follows:

• Operating information that the controller receives

from the machine is called the process variable

(PV) or feedback.

• Input from the operator that tells the controller the

desired operating point is called the set-point (SP).

• When operating, the controller determines whether

the machine needs adjustment by comparing (by

subtraction) the set-point and the process variable

(error) exists between the actual and desired levels, the

PLC control program will take the necessary corrective

action. Adjustments are made continuously by the PLC

until the difference between the desired and actual output

is as small as is practical.

With on/off PLC control (also known as two-position

and bang-bang control ), the output or  nal control ele-

ment is either on or off—one for the occasion when the

value of the measured variable is above the set-point and

the other for the occasion when the value is below the

set-point. The controller will never keep the  nal control

element in an intermediate position. Most residential ther-

mostats are on/off type controllers.

On/off control is inexpensive but not accurate enough

for most process and machine control applications. On/

off control almost always means overshoot and resultant

system cycling. For this reason a deadband usually ex-

ists around the set-point. The deadband or hysteresis of

the control loop is the difference between the on and off

operating points.

Proportional controls are designed to eliminate the

hunting or cycling associated with on/off control. They

allow the  nal control element to take intermediate po-

sitions between on and off. This permits analog control

of the  nal control element to vary the amount of energy

to the process, depending on how much the value of the

measured variable has shifted from the desired value.

The process illustrated in Figure10-43 is an example

of a proportional control process. The PLC analog output

module controls the amount of  uid placed in the holding

tank by adjusting the percentage of valve opening. The

valve is initially open 100 percent. As the  uid level in the

tank approaches the preset point, the processor modi es

Figure 10-44 Typical PID control loop.

Error

Set-point

(SP)

PID equation

Control

variable

(CV)

Process

variable

(PV)

Level

detector

Flow

rate

Figure 10-42 Closed-loop control system.

PLC

controller

Measurement of

variable to be

controlled

(sensor)

Control

element

(heater-valve)

Set-point

(potentiometer)

Figure 10-43 Proportional control process.

Ultrasonic

level sensor

Analog output

4 to 20 mA

analog input

Adjustable

valve

PLC

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Data Manipulation Instructions Chapter 10 221

to produce a difference (the difference is called the

error ).

• Output from the loop is called the control variable

(CV), which is connected to the controlling part of

the process.

• The PID loop takes appropriate action to modify the

process operating point until the control variable

and the set-point are very nearly equal.

Programmable controllers are either equipped with

PID I/O modules that produce PID control or have suf-

 cient mathematical functions of their own to allow PID

control to be carried out. Figure10-45 shows an SLC 500

PID instruction with typical addresses for the parameters

entered. The PID instruction normally controls a closed

loop using inputs from an analog input module and pro-

vides an output to an analog output module. Explanation

Figure 10-45 SLC 500 PID instruction.

PID

Control

Process

Control

Block

Variable

Block length

N10:0

N10:28

N10:29

of the PID instruction parameters can be summarized as

follows:

• Control Block is the  le that stores the data required

to operate the instruction.

• Process Variable (PV) is an element address that

stores the process input value.

• Control Variable (CV) is an element address that

stores the output of the PID instruction.

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222 Chapter 10 Data Manipulation Instructions

Figure 10-46 Logic rungs for Question 14.

(a)

(b)

(c)

GEQ

GREATER THAN OR EQUAL

Source A

Source B

N7:601

N7:600

Output

LES

LESS THAN

Source A

Source B

N7:500

250

Input

Ladder logic program

EQU

EQUAL

Source A

Source B

Output

N7:400

N7:401

Input

1. In general, what do data manipulation instructions

allow the PLC to do?

2. Explain the difference between a register or word

and a table or  le.

3. Into what two broad categories can data manipula-

tion instructions be placed?

4. What takes place with regard to a data transfer

instruction?

5. The MOV instruction is to be used to copy the

information stored in word N7:20 to N7:35.

What address is entered into the source and the

destination?

6. What is the purpose of the mask word in the MVM

instruction?

7. What is the purpose of the bit distribute

instruction?

8. List three types of data shifts used with  le

instructions.

9. List the six parameters and addresses that must

be entered into the  le arithmetic and logic (FAL)

instruction.

10. Assume the ALL mode has been entered as part of

a FAL instruction. How will this affect the transfer

of data?

11. What is the advantage of using the  le copy (COP)

or  ll  le (FLL) instruction rather than the FAL in-

struction for the transfer of data?

12. What are data compare instructions used for?

13. Name and draw the symbols for the six different

types of data compare instructions.

14. Explain what each of the logic rungs in

Figure10-46 is instructing the processor to do.

15. What does the limit test (LIM) instruction test

values for?

16. How are multibit I/O interfaces different from the

discrete type?

17. Assume that a thumbwheel switch is set for the

decimal number 3286.

a. What is the equivalent BCD value for this

setting?

b. What is the equivalent binary value for this

setting?

CHAPTER 10 REVIEW QUESTIONS

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