analog plcs

7/24/2019 Analog PLCs

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Programmable Logic Controllers

prv


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Contents

Analog Inputs Data Representation

Data Handling

Analog Outputs

Data Representation Data Handling

11-Oct-142 prv/PLC and its Applications


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What is Analog

PLC can understand

only discrete language

Examples of analoginputs:

Flow transducers

Humidity transducers Load cell transducers

Potentiometers

Pressure transducers

Vibration

transducers

Temperature

transducers



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Analog Input

Module

Analog input

modules digitizeanalog input signals

It uses an A/Dconverter for thepurpose.

PLC reads theinformation stored ineither BCD or Binaryformat.



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Steps in conversion

11-Oct-14prv/PLC and its Applications5


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An example

After the instruction is

executed, the contents of

register 1000 will be: 0000

1010 1100 1111 This

number corresponds to the

digitized value generated by

the module. Since the value

is represented in 12 bits,

the preceding bits are filled

with 0s. Note that the value

stored in register 1000 is in

binary. Its decimal

equivalent, forcomputational purposes, is

2767.


What will the contents of register 1000

be after the multi bit instruction shown inFigure is executed? Note that the

digitized value corresponding to the

analog transformation shown in the figure

is represented by 12 bits in binary format.


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Analog input data

representation



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Analog conversion

The field sensing device sends a very

low-level current or voltage analog input

to the transmitter.

The transmitter sends this information

to the input module as an amplified

current or voltage proportional to the

signal being measured.

Next, the analog input interface

digitizes the current or voltage by

converting it into an equivalent binary

number.

The interface then sends the digitized

signal to the controller. Thus, the binary

value that the PLC receives is the digital

equivalent of the incoming analog signal.



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A/D conversion

The A/D converter divides, or digitizes,

the input signal into many digital counts,

This division of the input signal is called

resolution.

An A/D converter transfers its digital-

equivalent values to the processor,

which in turn, makes them available for

use in register or word locations.

The format of these values varies

according to the format used by the

PLC; however, the most common

formats are binary and BCD. In BCD

format, the module or processor must

perform an extra linearity computation

to provide a valid BCD number.



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Try this!

(a) Find the equivalent

voltage change for eachcount change (the

voltage change per

degree Celsius change)

and the equivalent

number of counts per

degree Celsius, assuming

that the input module

transforms the data into

a linear 0 to 4095

counts, and (b) find the

same values for amodule with a 10-bit

resolution.


An input module, which is connected to atemperature transducer, has an A/D with a 12-bit

resolution. When the temperature transducer

receives a valid signal from the process (100 to

600C), it provides, via a transmitter, a +1 to +5 VDC

signal compatible with the analog input module.


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Solution for (a)

An input module, which is

connected to a temperature

transducer, has an A/D with a

12-bit resolution. When the

temperature transducer

receives a valid signal from

the process (100 to 600C), it

provides, via a transmitter, a

+1 to +5 VDC signal

compatible with the analog

input module.



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Solution for (b)

An input module, which is

connected to a

temperature transducer, has

an A/D with a 12-bit

resolution. When the

temperature transducer

receives a valid signal from

the process (100 to 600C),

it provides, via a transmitter,

a +1 to +5 VDC signal

compatible with the analog

input module.



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Question No.2



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Analog data

handling

During the input

reading section of thescan, the processorreads the value fromthe module andtransfers theinformation to alocation specified bythe user.

This location isusually a word orregister storage areaor an input register.



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Analog Input

Connections

Analog input modules usually

provide a high input impedance

(in the mega ohm range) for

voltage-type input signals. T

his allows the module to

interface with high source-

resistance outputs from input-

sensing devices (e.g.,

transmitters or transducers).

Current-type input modules

provide low input impedance

(between 250 and 500 ohms),

which is necessary to properly

interface with their compatible

field sensing devices.



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Analog Output

Channels

Analog output interfaces are

used in applications requiring

the control of field devices

that respond to continuous

voltage or current levels.

An example of this type of

field device is a volume adjust

valve (see Figure). This type of

valve, which is used in

hydraulic-based punch

presses, requires a 010 VDC

signal to vary the volume of

oil being pumped to the press

cylinders, thereby changing

the speed of the ram or

platen.



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Analog Output

Module

The controller

transfers the contentsof a register, generallyspecified by 12bits, tothe output moduleupon execution of theinstruction

The module thentransforms this value,whether BCD orbinary, from digital toanalog and passes it tothe field controldevice



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Question and

Answer

The value stored in register 2000 is

0000 0011 1111, which is equivalent

to decimal 63. Thus, the valve is open

approximately 1/65th, or 1.53%, of its

fully open position (63 4095 =

1.53%). Note that the position of the

valve is determined by the decimal

equivalent of the binary value, not

the number of 1s and 0sa binary

number with half 1s and half 0s does

not indicate that the valve is half

open. If the value in the register had

been in BCD, the output modulewould have converted the value to

decimal to determine the valve

position.


Figure illustrates the binary transfer of information to an analog outputmodule via a multi bit instruction. Assume that the module converts a digital

signal equal to the binary value 0000 0000 0000 (0 decimal) to an analogvalue that makes the control valve be completely closed, closed, while itconverts a value of 1111 1111 1111 (4095 decimal) to an analog value thatmakes the valve be fully open. What will the state of the valve be according tothe contents of register 2000?


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Analog Output Data

representation

An analog output interface operates

much like an analog input module,

except that the data direction is

reversed.

A D/A transforms the

numerical data (BCD or binary)

sent from the processor into an

analog signal.

This analog output value is

proportional to the digital numerical

value received by the module.

The magnitude proportional to the

minimum and maximum capable

analog voltages or currents of the

field device (e.g., 0 to 10 VDC)



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Analog Output

Data Handling

The storage or I/O table section of a PLCsdata

table area holds the data to be sent to an analog

output module (see Figure ).

This data comes from program computations

that, when sent to the module, will control an

analog output device. During the execution of the

output update, the processor sends the

register/word contents to the analog module

specified by the address in the instruction.

The module transforms the register/words

binary or BCD value into an analog output

voltage or current. Since the program calculates

the register/word value, the user should take

precautions during programming to avoid

computing or sending nonvalid ranges to the

module.



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Output data

handling



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References

W. Bolton, Programmable Logic Controllers, FourthEdition, Newnes, 2006. (ISBN-13: 978-0-7506-8112-4)

John W. Webb, Ronald A. Reis, Programmable LogicControllers, Principles and Applications, Fifth Edition,

Prentice Hall of India, 2003. (ISBN-81-203-2308-4) I.A. Bryan, E.A. Bryan, Programmable Controllers

Theory and Implementation, Second Edition, An IndustrialText Company Publication,1997, ISBN 0-944107-32-X

John R. Hackworth, Fredrick D. Hackworth Jr.Programmable Logic Controllers: Programming Methodsand Applications



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References


Madhuchandra Mitra, Samarjit Sen Gupta, ProgrammableLogic Controllers and Industrial Automation, anIntroduction, Penram International Publishing (India)PLimited, 2005, ISBN: 81-87972-17-3

Gary Duning, Introduction to Programmable LogicControllers, 2ndEdition, Thomson Delmar Learning, 2005,ISBN 0 7668 1768 -7


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Have any?


Thanks for listening


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A passing thought


Greatness knows itself

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