A NALOG I NPUT C OMPARISON AND D ATA L INEARIZATION

A NALOG I NPUT C OMPARISON AND D ATA L INEARIZATION

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

High Alarm

Low Alarm

Set Point

Set Point

ON ON TWS data read

PL1

PL2

Low Temperature

OK Range

High Temperature

Alarm

Alarm

0 ° C 1000 ° C 4 Low Set Point 4 High Set Point

1 2 Figure 11-48. Elements in an analog input comparison system.

Set

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S ECTION PLC System Programming C HAPTER 3 Programming

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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.

° C Y = mX + b

X counts + 500

= 4 ( 1221 10 − ° C )( ) X counts + 500

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

START Read TWS and

convert to decimal (binary)

GO SUB to check for correct range

NO

TWS correct range?

YES Read analog

input GO SUB to

linearize to ˚C

Is

YES

Energize low

temp < low

alarm PL

Is

YES

Energize high

temp > high

alarm PL

? NO END

Figure 11-50. Flowchart of process steps.

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S ECTION PLC System Programming C HAPTER 3 Programming

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START

Compare TWS

START

low and TWS high settings with range

100 ° C ≤ TWS ≤ 850 ° C MULT 0.1221 times the input

(result in ° C)

Is TWS high

Yes

Set high OK

high OK, then range OK; else range not OK

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.

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S ECTION PLC System Programming C HAPTER 3 Programming

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1 1 0 0 S u b r o u t i n e t o c h e c k f o r v a il d r a n g e s ( C M P f o r L o w )

1 2 0 0 G o t o s u b r o u t i n e t o il n e a r i z e i n p u t c o u n t s t o ° C

1 2 0 1 A d d i t i o n o f 5 0 0 e n a b l e d — r e g 4 1 0 2 h a s t e m p v a l u e Table 11-23. Internal output assignment.

Table 11-24. I/O address assignment.

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S ECTION PLC System Programming C HAPTER 3 Programming

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Start

XFER IN

TWS Read

Slot 1 Rack 0 Length 2 Reg 4000

TWS Read

BCD-BIN

BIN Done

Reg 4000 Reg 4010 Length 2

Check TWS Ranges

BIN Done

Range OK

XFER IN

Temp Read

Slot 7 Rack 0 Length 1 Reg 4100

Linearize Temp

Temp Read

Range OK

Temp < Low

Reg 4010

Temp < Low

Low Temp Alarm

Range OK

CMP High

Temp > High

Reg 4010

Temp > High

High Temp Alarm

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

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S ECTION PLC System Programming C HAPTER 3 Programming

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CMP

CMP Low TWS Temp 10

Reg 4010

Low TWS ≥ 100 ° C

Reg K100

CMP High TWS Temp

High TWS ≤ 850 ° C

Reg K850

Range OK Low TWS ≥ 100 ° C High TWS ≤ 850 ° C 100 ° C ≤ TWS ≤ 850 ° C

Low TWS ≥ 100 ° C Range not OK

1104 U

High TWS ≤ 850 ° C 1103

RET

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

Temp Read

MULT Done

Reg 4100 x Reg K 1221 = Reg 4101 Scale –4

MULT Done

ADD

ADD Done 1201

Reg 4101 + Reg K 500 = Reg 4102

RET

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

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S ECTION PLC System Programming C HAPTER 3 Programming

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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.