L ARGE B AT C H I N G C ONTROL A P P L I C AT I O N

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

Mixer

SOL1

Motor M1

Level Switch LLS2

Temp Level Switch

LLS1

Empty LLS Switch (NC)

SOL3 Valve

Finished Product Tank

LS Tank Position

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

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S ECTION PLC System Programming C HAPTER 3 Programming

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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

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.

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

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START

Check For Initial Conditions

All

Conditions Set?

Yes

Set Temp to T1 = 100˚C

No T1 = 100˚C

? Yes

Add Ingredients A & B Keep T = 100˚C

Yes Elevate Temp to

T2 = 300˚C Keep T2 = 300˚C

No T2 = 300˚C

? Yes

Start Mixer For 1200 Sec

Release Mix To Holding Tank And Reset

Back To Start

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

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M1 3 C T Start PB

PL1

Ready to Mix

Temp. OK2

Start Mixer

Tank In Pos

PL2

Mixer ON

N IO

1 Start Mix

7 20 Min.

ro L

Stop PB

Mix g Motor

Finish Mixing

PL9

Start Mix

In Set to Empty

Open Valve

8 SOL3

T1 (100˚)

u s Read

lT PL10

t& Heater

3 Ready to Add

Start Mix

9 Set to T2

w Coil ON V w High2 w OK2 id

d PL11 C

tri High1 a p

LLS1

SOL 1

a Finish Ingr. A

Temp High

LLS1

t.c High2 o

-8 0 Low1 LLS2 SOL2

Finish Ingr. A

Finish Ingr. B

Temp Low

9 Finish Ingr. A

8 6 Ready To Mix

High 2

e n ra m

Finish Ingr. B

Set to T2 (300˚)

Read

Temp OK1

Temp OK tio in

Temp OK2

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

S ECTION PLC System Programming C HAPTER 3 Programming

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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.

CMP High

Temp. OK

CMP Low

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

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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).

0 2 5 H e a t e r c o li 0 2 6 P L h e a t e r O N 0 2 7 P L t e m p h i g h

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

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and low. I/O register 3034 stores the analog value of the temperature. This register will be compared to the storage registers that hold the equivalent 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 assignment 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, etc.). The voltage received from the temperature transmitter ranges from 0 to 10 V,

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

representing 0 to 500 °

C. Thus, each volt represents a change of 50 °

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

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.

98 and 102 °

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Program Coding

Tank LS

Tank PL

Tank In Position

Start System/

Stop Tank LS SOL3

Start Mix PL

Start Mix (System)

Tank in

Empty LS

Empty PL

Empty Ready

M1 System

003 Empty LS

Temp CMP

Set to T1

Range OK 1

Temp Temp

Temp

High 1 Low 1

Range Start

Ready

OK 1 System

Ready to Add LLS1 SOL3

Open PL

Valve 1 Open 014

SOL1 LLS1

Finish Ingr. A 015

(continued on next page)

Figure 11-71. PLC implementation of the batching application.

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SOL2 Valve 016

SOL2

Valve 2 Open

PL017

Valve 2 Open 017

Finish Ingr. B 020

Fin A

Fin B

Ready to Mix

CMP>

CMP High 2

Ready to Mix

LIM

Range OK 2

High 1 Low 1

Range Ready

Mix CMD 1013

PL Mixer ON

Mixer ON 022

Discharge SOL3

SOL3 Valve 023

SOL3 023

(continued on next page)

Figure 11-71 continued.

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Open PL

Valve 3 Open

High 1 Low 1

to Mix High 2 Low 2

PL Heater ON

Heater ON 026

PL Temp High

Temp High 027

Ready

CMP to Mix High 2

PL Temp Low

Temp Low

PL Temp OK

Temp OK

Ready Temp

Figure 11-71 continued.

L ARGE B AT C H I N G C ONTROL A P P L I C AT I O N