Bryan L. Programmable controllers. Theory and implementation

509

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Table 11-31. Internal output assignment.

Figure 11-64 shows the method used to interpolate psi values based on the

contents of each register used. The PLC program shown in Figure 11-65

implements the linear interpolation by finding the high and low counts and

psi values. Using the two pointers, the PLC obtains the high and low values

for the counts and psi through table-to-register instructions. The PLC pro-

gram compares its current analog counts with the count values in the table

registers to find a table location with a greater value. This comparison starts

with the lowest count values in the table. If the actual value (analog counts)

is more than the value pointed to by the pointer, the PLC increments the

pointer (adds 1) and tests a new table value. Once the program finds a register

in the table that contains a value greater than the analog reading, it stores the

pointer value associated with this register in register 4050, thus pointing to the

4050 is decreased by one and stored in register 4000 to point to the register

at point P – 1 in Figure 11-64. These two registers (4000 and 4050) point to

the low psi/low counts and high psi/high counts, respectively, which will be

used to complete the interpolation.

In the software program presented, we have considered that the actual count

value may be 0 counts. When the count value is 0, the equivalent psi is 0

and the program does not perform the subroutine shown in Figure 11-66. If

it did perform the subroutine, the program would enter into a loop error.

lanretnI noitpircseD

0001 delbanerefsnartgolanA

1001 stnuoc0=tupniroferapmoC

2001 0erastnuoctupnI

3001 retsigerispdetupmocot0tnatsnocevoM

4001 )delbane(retniopsa2tnatsnocevoM

•

0011 retsigerotelbatevom,noitalopretnibusoG

1011 delbaneerapmoC

2011 stnuochgiH ≥ )stchgiH(stnuoctupni

3011 delbaneretniop)DDA(tnemercnI

•

0021 retnioperots,htametaluclacotoG

1021 delbaneretniopmorf1tcartbuS

2021 )stcwoL(delbaneretsigerotelbatevoM

3021 )ispwoL(delbaneretsigerotelbatevoM

4021 )isphgiH(delbaneretsigerotelbatevoM

5021 delbanetcartbuS

6021 delbanetcartbuS

7021 delbaneylpitluM

0121 delbanetcartbuS

1121 delbaneediviD

2121 delbaneddA

510

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Figure 11-64. Interpolation method.

Figure 11-65. Main program.

(psi)

4450 points to high psi

4150 holds computed psi

4400 points to low psi

Counts

psi

P – 1

Low

Counts

Actual

Counts

High

Counts

4500Count Register 4100 4550

Actual Counts – Low Counts

High Counts – Low Counts

Computed psi – Low psi

High psi – Low psi

(Actual Counts – Low Counts)(High psi – Low psi)

High Counts – Low Counts

+ Low psiComputed psi =

isp elbaT stnuoC elbaT

retnioP

retsigeR stnetnoC retsigeR stnetnoC

retsigeR

0044 ispwoL 0054 stnuocwoL 1–P@0004

0514 ispdetupmoC 0014 stnuoclautcA

0544 isphgiH 0554 stnuochgiH P@0504

XFER IN

psi read

1000

Slot 7

Rack 0

Reg 4100

Length 1

psi read

1000

CMP =

CMP 0 counts

1001

psi = 0 counts

1002

psi = 0 counts

1002

Sub interpolation

GO SUB 1100

Reg 4100

Reg K 0

psi = 0 counts

1002

MOVE

psi Reg = 0

1003

Reg K0

Reg 4150

Length 1

psi = 0 counts

1002

MOVE

Pointer P – 1 = 2

1004

Reg K2

Reg 4000

Length 1

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Figure 11-66. Interpolation subroutine.

(continued at right)

High Cts ≥ input Cts

1102

Subroutine

1100

High Cts ≥ Input Cts

1102

Found High Psi

Go To 1200

Reg 3000

Length 1

Pointer

4000

Reg 4550

High Cts ≥ Input Cts

1102

Go To

1100

MOVE

TBL REG

MOVE

TBL REG

Done

1101

Reg 4550

≥

Reg 4100

CMP

Inc Pointer P – 1

1103

Reg 4000

Reg K1

Reg 4000

ADD

Store High Pointer P

1200

Reg 4000

Reg 4050

Length 1

MOVE

Store Pointer P – 1

1201

Reg 4000

–

Reg K1

Reg 4000

SUB

Get Low Cts for P – 1

1202

Reg 3000

Length 1

Pointer

4000

Reg 4500

MOVE

TBL REG

MOVE

TBL REG

Get Low Psi

for P – 1

1203

Reg 3100

Length 1

Pointer

4000

Reg 4400

Get High Psi

for P

1204

Reg 3100

Length 1

Pointer

4050

Reg 4450

Input Cts –

Low Cts

1205

Reg 4100

–

Reg 4500

Reg 4600

SUB

High Psi –

Low Psi

1206

Reg 4450

–

Reg 4400

Reg 4601

SUB

Numerator

Multiplication

1207

Reg 4601

Reg 4600

Reg 4602

Scale 0

MULT

High Cts –

Low Cts

1210

Reg 4550

–

Reg 4500

Reg 4603

SUB

Numerator

multiplication

1211

Reg 4602

Reg 4603

Reg 4604

Scale 0

DIV

Result +

Low Cts

1212

Return

Reg 4604

Reg 4400

Reg 4150

ADD

512

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LARGE BATCHING CONTROL APPLICATION

This example explains the automation of a large batching process. It

includes the process description, controller requirements, flowchart, logic

diagrams for each output sequence, assignment of I/O, and program coding.

Figure 11-67 shows the process flow diagram illustrating the elements this

batching application will control. Two ingredients, A and B, will be mixed in

the reactor tank. The reactor tank must be empty (indicated by the normally

closed liquid level switch LLS) and at a temperature of 100°C before

ingredient A can be added. The mixer motor must be off to avoid liquid

precipitation, and the finished product tank should be in a set position, which

the limit switch detects.

Once the reactor tank reaches an initial temperature of 100°C, the controller

will add ingredient A by opening solenoid valve 1 (SOL1) until 100 gallons

of ingredient A have been poured into the tank. LLS1, which is normally open,

detects the quantity of ingredient A in the tank. This switch closes when

ingredient A reaches the proper level. At this point, the controller will add

Figure 11-67. Process flow diagram of the batching application.

Finished Product

Tank

SOL1

Valve

SOL2

Valve

Mixer

Motor M1

Level Switch

LLS2

Level Switch

LLS1

Temp

Empty LLS

Switch (NC)

Heater

SOL3

Valve

LS Tank Position

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ingredient B by opening SOL2. LLS2 detects the quantity of ingredient B,

which should be 400 gallons. The temperature should be at 100°C ± 10%

during the Add Ingredients step. If the temperature drops, the PLC will turn

ON the heater automatically while the process continues.

When the reactor tank contains both ingredients, the controller will raise the

temperature to 300°C ± 10% and turn ON the mixer for 20 minutes. The

PLC will control the temperature automatically at predefined set points

during the process.

SOL3 will activate the drain valve when the mixing is completed. This

operation will reset the process until another finished product tank is placed

in position, and the cycle starts again. The system should incorporate pilot

light indicators to alert the operator to the status of the batching process.

This application must be capable of reading analog signals from the process.

In this case, the voltage comes from a temperature transducer (0–10 volts),

which has a range of 0 to 500°C (50°C/volt). The heater coil’s ON/OFF

control switch controls the temperature. The application also requires stan-

dard 110 VAC input and output modules. Figure 11-68 shows a flowchart of

this process. It illustrates what has been described in the control task

definition and serves as a preparation for the logic diagrams.

Figure 11-69 shows the logic diagrams for this example. The logic diagrams

map the initial implementation of the logic required to control each of the

process sequences. These diagrams represent the conditions required for a

rung to be energized. Real I/Os are marked with an X. The first logic diagram

shows the initial requirements for starting the process. The start push button,

when pressed, enables the Start Mix output if the following conditions are

satisfied: the tank is in position, SOL3 is closed, and the stop push button is

not pressed. Pilot lights PL1 and PL2 indicate that the tank is in position and

that the system Start Mix signal is enabled.

Logic diagram 2 in Figure 11-69 sets the initial temperature (T1) at 100°C.

The logic indicates that the mixer motor (M1) must be off, the Start Mix input

enabled, and the reactor tank empty. The Ready to Mix input in diagram 2 is

an interlock from logic diagram 6; it disables T1 when T2 is being set. Note

that the OR function uses the Empty signal with the initial Set to T1 signal to

ensure that, even when the tank is still adding ingredients, the temperature

control will maintain the temperature at T1.

Logic diagram 3 controls the Ready to Add signal, which allows ingredient

A to be added. Here, the output Temp OK1 (T1 = 100°C) indicates that the

tank is at the proper temperature. As long as the Start Mix signal is still

enabled, the process is ready to add the first ingredient.

514

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Figure 11-68. Flowchart for the large batching application process.

START

Back To Start

Yes

Check For

Initial

Conditions

Finish

Ingredients

All

Conditions

Set?

T1 = 100˚C

Set Temp to

T1 = 100˚C

Yes

T2 = 300˚C

Elevate Temp to

T2 = 300˚C

Keep T2 = 300˚C

Start Mixer

For 1200 Sec

Release Mix To

Holding Tank

And Reset

Add Ingredients A & B

Keep T = 100˚C

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Figure 11-69. Logic diagrams for the batching application.

Start PB

Mix

Motor

Start Mix

Temp

OK 1

Empty

Stop PB

SOL3

Tank In Pos

PL1

PL4

PL5

PL3

PL2

Start Mix

Empty

Ready

to Mix

Read

Temp

and Set to

100˚± 10%

High 1

OK 1

Low 1

Set to

T1 (100˚)

Ready

to Add

SOL3

LLS1

Open

Valve

SOL 1

PL6

Open

Valve

SOL2

Ready to Add

Finish Ingr. A

PL7

Finish Ingr. B

Finish Ingr. A

LLS2

SOL3

Finish Ingr. A

Finish Ingr. B

Read

Temp

and

Set to

300˚± 10%

High 2

OK 2

Low 2

Ready To Mix

Set to T2 (300˚)

Temp. OK2

Ready to Mix

Start Mixer

PL8

PL9

Set to T1

High1

OK1

Open Valve

SOL3

PL10

Heater

Coil ON

20 Min.

Finish

Mixing

Mixer ON

Set to T2

High2

OK2

Set to T1

High1

PL11

Temp High

Temp Low

Temp OK

Set to T2

High2

Set to T1

Low1

PL12

Set to T2

Low2

Set to T1

Temp OK1

PL13

Set to T2

Temp OK2

Empty

Finish Mixing

516

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In logic diagram 4, the Ready to Add A signal enables SOL1 to open. This

action occurs while LLS1 is still open (less than 100 gallons) and the drain

valve (SOL3) is not energized. When the liquid reaches the proper level,

LLS1 closes and, according to the logic, SOL1 de-energizes. The last part of

the logic diagram indicates that the addition of ingredient A is finished.

In logic diagram 5, SOL2 opens to add ingredient B until LLS2 closes (500

gallon level), indicating that 400 additional gallons have been added to the

reactor tank. The remainder of the logic indicates that the addition of

ingredient B is finished.

Logic diagram 6 shows that when both ingredients are in the reactor tank

(the Finish Ingredient A and Finish Ingredient B signals are both ON), the

Ready to Mix control signal is enabled. This condition will start a new

temperature control block, raising the temperature to 300°C. It will also

disable the other temperature control (T1).

In logic diagram 7, after the temperature is at 300°C and the Ready to Mix

(Set T2) signal is ON, the mixer will turn ON, enabling the timer at the same

time. After 20 minutes (1200 seconds), the timer will time out and reset the

mixer motor logic. The timer output sends the Finish Mixing signal, which is

used to energize SOL3. SOL3 opens the drain valve to discharge the mixed

ingredients (logic diagram 8). The valve remains open until the empty switch

returns to its normal state (closed).

Logic diagram 9 turns the heater ON if the temperature is low. The heater

can be turned on from either of the two temperature control function block

outputs. Sequences 10, 11, and 12 provide the operator with the status of the

temperature inside the tank.

The controller will perform the logic for reading the temperature using

compare functional block instructions. Once the command, or logic, indicates

temperature control, the compare functional block will be enabled. This block

will perform three comparisons to determine if the temperature is more than

110°C, equal to 100°C, or less than 90°C. The compare block must include

a limit (LIM) compare function, since the ingredients must be added at 100°C.

The output of this functional block will be OK1. The logic for the pilot light

tells the operator that the temperature is OK. This logic is an AND combina-

tion of the NOT greater than 110°C and NOT less than 90°C functions.

Thus, the range is within the tolerances as specified (100°C ± 10%). Figure

11-70 shows this logic. The limit instruction also applies to the control of T2

(temperature), with the exception of the set point.

Figure 11-70. Logic diagram for Temp OK signal.

CMP High

Temp. OK

CMP Low

517

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The assignment of real I/O begins by addressing the real inputs and outputs.

Table 11-32 illustrates the assignment of I/O for this application example.

Note that the modularity for the digital I/O is four points per module. The

analog module contains two input channels, which occupy one half of a group

(four locations).

Table 11-32. I/O address assignment.

Table 11-33 shows the assignment of internals. This table lists several

internal coil addresses representing coil relay conditions related to the logic

diagram. The coils associated with the compare functional blocks are

internals, which are used to describe the temperature conditions, such as high

sserddAO/I

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epyT kcaR puorG lanimreT noitpircseD

tupnI 0 0 0 BPximtratS

0 0 1 BPpotS

0 0 2 SLnoitisopknaT

0 0 3 )CN(SLLytpmE

tupnI 0 0 4 )lag001(1SLL

0 0 5 )lag005(2SLL

0 0 6 desutoN

0 0 7 desutoN

tuptuO 0 1 0 noitisopniknatLP

0 1 1 )LPxiMtratS(metsystratS

0 1 2 knatrotcaerytpmE

0 1 3 )Atneidergni(1evlavdioneloS

tuptuO 0 1 4 nepo1evlavLP

0 1 5 AhsiniF

0 1 6 2evlavLOS

0 1 7 2evlavLP

tuptuO 0 2 0 BhsiniF

0 2 1 )1M(rexiM

0 2 2 NOreximLP

0 2 3 )niard(3evlavdioneloS

tuptuO 0 2 4 nepo3evlavLP

0 2 5 liocretaeH

0 2 6 NOretaehLP

0 2 7 hgihpmetLP

tuptuO 0 3 0 wolpmetLP

0 3 1 KOpmetLP

0 3 2 desutoN

0 3 3 desutoN

golanA 0 3 4 rettimsnartotdetcennoc43tupnI

tupnI 0 3 5 4303retsigerO/IotsdnopserroC

0 3 6 sisatfel63tupnI

0 3 7 6303retsigeR

518

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and low. I/O register 3034 stores the analog value of the temperature. This

values of the temperature ranges.

Table 11-33. Internal output assignment.

Translating the logic diagrams into PLC diagrams is the next step after

tabulating the input and output assignments. The program coding follows the

logic diagram sequences previously specified and uses the information from

the I/O and internal tables as references for the addresses. Figure 11-71

shows the program coding for this example.

The ladder logic shown in the program coding is the implementation of each

logic diagram. The internals are assigned as specified in the internal assign-

ment table. Several storage registers, added in the compare blocks, hold preset

values. These values correspond to the equivalent temperature set points

used, including the tolerances (i.e., 110°C, 100°C, 300°C, 270°C, etc.).

The voltage received from the temperature transmitter ranges from 0 to 10 V,

representing 0 to 500°C. Thus, each volt represents a change of 50°C. The

controller used in this example receives the voltage signal and converts it to

a count ranging from 0000 to 9999. These counts are proportional to the

voltage and, therefore, to the temperature.

The first set point (register 4000) contains the count value 2200, which is

proportional to 110°C (100°C ± 10%); register 4003 contains the value 1800,

which is equivalent to 90°C (100°C ± 10%). Registers 4001 and 4002 contain

the values of 2040 (102°C) and 1960 (98°C), respectively. The controller

uses these values to detect a small range in which the temperature is very

close to 100°C, thus starting the ingredient addition. These two registers are

used in a LIM compare block that detects when the temperature is between

98 and 102°C. The PLC does not compare the value to 100°C because the

value may never be exactly 100°C during the time it is being read. The preset

values of the other compare blocks are specified in the same manner.

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cigoL 0001 1TotteS

kcolbPMC 1001 1hgiHPMC

MIL 2001 1KOegnarrofPMC

kcolbPMC 3001 1woLPMC

cigoL 4001 1KOpmeT

cigoL 5001 ddaotydaeR

cigoL 6001 2Tottes/ximotydaeR

kcolbPMC 7001 2hgiHPMC

MIL 0101 2KOegnarrofPMC

kcolbPMC 1101 2woLPMC

cigoL 2101 2KOpmeT

cigoL 3101 reximtratS

remiT 0002 retsiger)ces0021(nim02teserpremiT

)1004retsigerdetalumucca(0004

Bryan L. Programmable controllers. Theory and implementation

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