Bryan L. Programmable controllers. Theory and implementation

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Figure 11-46. Elements of the moderization example system to be left hardwired.

Table 11-20. I/O address assignment.

sserddAO/I

eludoM

epyT kcaR puorG lanimreT noitpircseD

tupnI 0 0 0 BPputeS—5BP

0 0 1 )CNderiw(teseR—6BP

0 0 2 hctiwserusserpciluardyH—1SP

0 0 3 CN(hctiwsrotceleselbanE—1SS

)detcennocnutfeltcatnoc

tupnI 0 0 4 noitisop1tceleS—1LES

0 0 5 noitisop2tceleS—2LES

0 0 6 )1noitisop(puhctiwstimiL—1SL

0 0 7 )2noitisop(puhctiwstimiL—2SL

tupnI 0 1 0 tesnoitacoL—3SL

0 1 1 elcycdaoltratS—6BP

0 1 2 )CNderiw(parT—4SL

0 1 3 hctiwsnoitisoP—5SL

tupnI 0 1 4 BPdaolnU—7BP

0 1 5 )ONderiw(pukcab/niaM—3SS

0 1 6 tcetedhtgnelmumixaM—6SL

0 1 7 pukcabhtgnelmuminiM—7SL

tuptuO 0 3 0 KOputeS—2LP

0 3 1 1tceleS—3LP

0 3 2 2tceleS—4LP

0 3 3 drawrofecnavdA—1LOS

tuptuO 0 3 4 egagnE—2LOS

0 3 5 NOegagnE—5LP

0 3 6 rotomnuR—2M

0 3 7 NOnurrotoM—6LP

tuptuO 0 4 0 potstsaF—3LOS

0 4 1 NOpotstsaF—7LP

0 4 2 pukcabhtiwdaolnU—4LOS

0 4 3 NOpukcaB—8LP

PB1

Master

Start

PB2

Master

Stop

PB3

Emergency

Stop 1

PB4

Emergency

Stop 2

SCR1-2

SCR1-1

OL1 OL1

4FU

X1 X2 X3

H2 H3

L1 L2

SCR1

PL1

PLC Fault

Contact 1

PLC Fault

Contact 2

PLC Fail Alarm

PLC

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1700 accomplishes this same function (for this example, MCR1700 is the

first available address for MCRs). If the MCR coil is not energized, the PLC

will not execute the ladder logic that is fenced between the MCR coil and the

END MCR instruction.

An internal will not replace the control relay CR2 in line 9 since the PL3

contacts in line 10 can be used instead. This technique can be used whenever

a control relay is in parallel with a real output device. Moreover, we do not

need to separate the coils in lines 17 and 18 of the hardwired logic. This has

already been done, since the PLC used here does not allow rungs with

multiple outputs. Using separate rungs for each output is always a good

practice.

The normally closed inputs that are connected to the input modules are

programmed as normally open, as explained in the previous sections. The

limit switch LS4 has two contacts—a normally open one and a normally

closed one in lines 17 and 19, respectively, of Figure 11-45. However, only

one set of contacts needs to be connected to the controller. In this example,

we have selected the normally closed contact LS4. Although the normally

open contact is not connected to the controller, its hardwired function can

still be achieved by programming LS4 as a normally closed ladder contact.

Applications such as this one also require timers with instantaneous

contacts, which are not available in most PLCs. An instantaneous contact is

one that opens or closes when the timer is enabled. In most PLCs, an internal

coil is used as a substitute for an instantaneous contact. Line 15 in the

hardwired logic shows that, if PB6 is pressed and CR4 is closed, the timer

TDR2 will start timing and contact TDR2-1 will seal PB6. This arrangement

requires special PLC implementation. If we use software timer contacts, the

Table 11-21. Internal address assignment.

eciveDlanretnInoitpircseD

1RC0001)ydRputeS(1RC

1RDT0002detalumucca(0003retsigerces01teserpremiT

)1003retsiger

RCM0071RCMsserddaRCMtsriF

2RC— sserdda3LPsaemaS

3RC— sserdda4LPsaemaS

4RC— 1LOSsaemaS

—1001fotcatnocsuoenatnatsniroflanretniputesoT

2RDT

2RDT1002detalumucca(2004retsigerces5teserpremiT

)3004retsiger

—2001fotcatnocsuoenatnatsniroflanretniputesoT

3RDT

3RDT2002detalumucca(4004retsigerces21teserpremiT

)5004retsiger

491

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Figure 11-47. PLC implementation of the circuit in Figure 11-45.

L1 L1 L2L2

PB5

PB6*

PS1

000

001

Inputs Program Coding Outputs

002

003

004

SS1

SS2

PL2

030

PL3

031

PL4

032

033

PB5

000

CR1

1000

PB6

001

CR1-Setup Ready

1000

CR1

1000

TDR1

2000

PS1

002

TDR1

2000

SS1

003

SEL1

004

PL3

031

005

SS1

003

SEL2

005

PL4

032

036

TDR2

2001

LS4

012

036

MCR 1700

TDR1

2000

PL2

030

015

SS3

Trap TDR3

1002

TDR3

2002

013

035

SOL2

034

PL5

035

010

LS1

006

PL3

031

LS1

006

LS2

007

LS3

010

SOL1

033

SEL1

SEL2

END MCR 1700

*Wired NC

Programmed NO

034

LS4

012

TDR2

2001

LS4

012

LS5

013

PL4

032

SOL2

034

LS2

LS3

011

Trap TDR2

1001

TDR2

2001

PB6

LS5

PB7

007

SOL1

033

PB6

011

Trap TDR2

1001

Trap TDR2

1001

Trap TDR3

1002

014

TDR2

2001

LS4

012

PB7

014

Trap TDR3

1002

SOL1

SOL2

040

LS6

016

SS3

015

LS6

016

TDR3

2002

SOL3

040

SOL3

PL5

037

036

PL6

037

PL6

017

041

SOL3

040

PL7

041

LS7*

PL7

PL8

043

SOL4

042

PL8

043

042

SS3

015

LS7

017

TDR3

2002

SOL4

042

SOL4

ONOFF

492

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timer will not seal until it has timed out. If PB6 is released, the timer will reset

because PB6 is not sealed. To solve this problem, we can use internal coil

1001 to seal PB6 and start timing timer 2001 (TDR2). Lines 9, 10, and 11

of the PLC program coding show this technique. The time delay contacts

(2001) are used for ON delays.

This section deals with the organization and implementation of analog

readings and controls. The examples presented describe new additions to a

system, assuming no existing electrical ladder logic is available. The ex-

amples are based on the PLC specifications described in Section 11-5.

Throughout these examples, the internal address assignments and register

assignments will develop as the program coding from the flowchart or logic

diagram is implemented. Flowcharting is important during this stage because

it defines the task to be performed and the steps required to perform it.

ANALOG INPUT COMPARISON AND DATA LINEARIZATION

In this example, we will compare the input signal from a temperature

transducer (0°C to 1000°C) with two alarm set points (a low alarm and a high

alarm). The PLC receives set point data via two sets of 4-digit switches

(BCD). The valid range of this set point data is 100 to 850°C. The analog input

module receives a signal, which is proportional to the temperature, that ranges

between –10 and +10 VDC. When the signal is over or under either of the two

set points, an indicator light is illuminated. Figure 11-48 illustrates a simple

Figure 11-48. Elements in an analog input comparison system.

5 8 6 7 5 8 6 7

High Alarm

Set Point

Low Alarm

Set Point

Start

TWS1

TWS2

EN1

EN2

EN3

EN4

Set 1 2

Low

Temperature

Alarm

High

Temperature

Alarm

OK Range

Low Set Point

100°C

High Set Point

850°C

0°C 1000°C

PL1 PL2

OFF

TWS data read

11-6 ANALOG I/O CONTROL PROGRAMMING

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Figure 11-49. Relationship between counts and degrees Celsuis.

Figure 11-50. Flowchart of process steps.

diagram of the elements used in this system. The thumbwheel switches

(TWS) are connected to a register input module with multiplexing (MUX)

capability. The TWS inputs are read only once when the spring-loaded, key-

operated switch is turned ON. Figure 11-49 shows the analog input relation-

ship between counts and degrees Celsuis. Figure 11-50 illustrates a

flowchart of the required steps for this example, while Figure 11-51 shows a

flowchart of the subroutines used in the program.

START

END

YES

Read TWS and

convert to

decimal (binary)

GO SUB to

check for correct

range

GO SUB to

linearize to ˚C

Read analog

input

TWS

correct

range?

temp < low

Energize low

alarm PL

YES

Energize high

alarm PL

temp > high

°C

1000

500

–4095 +40950

Counts

YmXb

−

counts

1000

8190

500

1221 10 500

()()

494

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Figure 11-51. Subroutine flowcharts: (a) check for correct range and (b) linearize to °C.

Tables 11-22 and 11-23 present the register and internal output address

assignment tables, respectively. Table 11-24 lists the I/O address assignment,

while Figure 11-52 illustrates the final PLC circuit implementation.

Moreover, Figures 11-53 and 11-54 show the subroutine circuit programs.

A block transfer input instruction, used to read the TWS, selects slot location

1 and reads 8 bits (2 digits). It then automatically goes to the next slot (slot

2) to get the other 8 bits.

Table 11-23. Register assignment.

retsigeR noitpircseD

0004 )DCB(SWTmralatimilwoL

1004 )DCB(SWTmralatimilhgiH

•

0104 lamicedmralatimilwoL °C

1104 lamicedmralatimilhgiH °C

•

0014 )erutarepmet(stnuoctupnigolanA

1014 tluserpmetnoitacilpitlumnidesU

2014 nierutarepmettluseR °C

START

END

MULT 0.1221

times the input

count

Compare TWS

low and TWS high

settings with range

100°C ≤ TWS ≤ 850°C

If low OK and

high OK, then range

OK; else range not OK

Set low OK

ADD 500

(result in °C)

START

Return

TWS low

≥ 100?

TWS high

≤ 850?

Yes

Set high OK

Yes

(a) (b)

495

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Table 11-24. I/O address assignment.

Table 11-23. Internal output assignment.

eciveDlanretnInoitpircseD

—0001)daeRSWT(delbane)XUM(SWTnirefX

—1001)enoDNIB(delbanenoisrevnocyranib-ot-DCB

—2001)daeRpmeT(delbanetupnigolananirefX

—3001mralawolhtiwpmeterapmoC

—4001)woL<pmeT(noitidnocmralawoL

—50011LPmralapmetwoL

—6001mralahgihhtiwpmeterapmoC

—7001)hgiH>pmeT(noitidnocmralahgiH

—01012LPmralapmethgiH

—0011)woLrofPMC(segnardilavrofkcehcotenituorbuS

—1011001nahteromtupnipmetwoL°C

—2011)hgiHrofPMC(egnarhgihroferapmoC

—3011058nahtsseltupnipmethgiH°C

—4011KOtonegnaR=0,KOegnaR=1

—0021otstnuoctupnieziraenilotenituorbusotoG°C

—1021eulavpmetsah2014ger—delbane005fonoitiddA

sserddAO/I

eludoM

epyTkcaRpuorGlanimreTnoitpircseD

tupnI000tupniSWTdnagnidaergolanatratS

001

002

003

tuptuO004desutoN

005

006

007

retsigeR010wolfostigidowttnacifingistsaeL

tupnI011;stnioptesmralahgihdnamrala

)etybwol(012stupnihtobnisexelpitlumretsiger

013

014

015

016

017

retsigeR020wolfostigidowttnacifingistsoM

tupnI021;stnioptesmralahgihdnamrala

hgih(022.nidexelpitlumerahtob

)etyb023

024

025

026

027

tuptuO030rotacidni1LPmralawoL

031rotacidni2LPmralahgiH

032

033

golanA070pmettupnigolana1lennahC

tupnI071eraps2lennahC

072eraps3lennahC

073eraps4lennahC

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Figure 11-52. PLC implementation of the analog input comparison system.

Start

000

XFER IN TWS Read

1000

Slot 1

Rack 0

Length 2

Reg 4000

TWS Read

1000

BCD-BIN BIN Done

1001

Reg 4000

Reg 4010

Length 2

BIN Done

1001

Check TWS Ranges

Go Sub

1100

Range OK

1104

XFER IN Temp Read

1002

Slot 7

Rack 0

Length 1

Reg 4100

Range OK

1104

CMP

CMP Low

1003

Reg 4102

Reg 4010

Temp < Low

1004

Temp Read

1002

Linearize Temp

Go Sub

1200

Range OK

1104

CMP

CMP High

1006

Reg 4102

Reg 4010

Temp > High

1007

Temp < Low

1004

Low Temp Alarm

1005

Temp > High

1007

High Temp Alarm

1010

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Figure 11-53. Subroutine 1100—check for valid TWS range and convert to decimal.

Figure 11-54. Subroutine 1200—convert analog counts to degrees.

CMP

CMP Low TWS Temp

1100

Reg 4010

≥

Reg K100

Low TWS ≥ 100°C

1101

CMP

CMP High TWS Temp

1102

Reg 4011

≤

Reg K850

High TWS ≤ 850°C

1103

Low TWS ≥ 100°C

1101

Range OK

100°C ≤ TWS ≤ 850°C

1104

RET

High TWS ≤ 850°C

1103

Low TWS ≥ 100°C

1101

High TWS ≤ 850°C

1103

Range not OK

1104

≥

≤

Temp Read

1002

MUL

MULT Done

1200

Reg 4100

Reg K 1221

Reg 4101

Scale –4

MULT Done

1200

ADD Done

1201

RET

ADD

Reg 4101

Reg K 500

Reg 4102

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The check range subroutine compares the values entered for the low and high

temperature alarms with the constants 100 and 850. If the values are within

that range, the program latches internal 1104, indicating that the range is OK.

If the values are not within the acceptable range, internal 1104 remains OFF

(0). The latch is required because the subroutine is not executed during every

scan, yet the main program uses the OK or not OK signal during its regular

program execution. The check range subroutine is only executed when the

key switch is turned ON.

ANALOG POSITION READING FROM AN LVDT

Figure 11-55. LVDT analog position reading system.

A linear variable differential transformer (LVDT) provides position feed-

back for the moving mechanism of a machine. Figure 11-55 illustrates a block

diagram of an LVDT application. The LVDT has a range of ±10 inches from

its null position; therefore, the effective total range is 20 inches from a zero

reference. The LVDT provides a ±10 VDC signal and is connected to an

analog input module, which transforms the –10 to +10 VDC voltage swing

into counts ranging from –4095 to +4095.

Motor

0 inches 20 inchesVirtual

Position

(V.P.)

LVDT

LVDT Attachment

5 8 6 7

Virtual Position

TWS

Start PB ( )

Reset PB ( )

Stop PB ( )

+ 4095

– 4095

Counts

10 20

Displacment (inches)

counts

inches

– 4095

counts

= (4095)(10

–1

)

inches

– 4095

8190