plc memorandum

5/28/2018 PLC Memorandum

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To: Dr. David Dixon

From: Anh Vo Hai

Date: December 18th

, 2013

Subject: Case study for programmable logic control

The applications and basic information about PLC control

Introduction:

For the last 50 years, there have been many changes in different types of industrial process

control systems. More and more complex systems were developed in order to satisfy the quality of

production manufacturing in term of safety as well as productivity of the system. In this memorandum,

the main idea introduced is the basic understanding about PLC or Programmable Logic Controller. The

skill to utilize and develop a broad understanding about PLC will greatly enhance the process control of

many systems such as mechanical, electrical as well as chemical industrial scenarios. PLC systems havebeen developed to be able to handle data processing even large quantity of manufacturing processes.

Programmable Language Controller is good at optimizing production of process. The

development of PLC has moved from a small single unit to large complex systems. However, there are

always draw-back for certain controller. For PLC, it is required that controller must adapt its operation

for each product instead of changing the program as the product is changed. The installing cost as well

as maintained cost for certain operation unit might increase if there is a change of product system. On

the other hand, the built processor is proved to be long lasting in a long period of time.

Basic operation theory:

The figure below sum up the progression of how a PLC process should progress:


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From the diagram above, first a programed of different steps or response of different situation of the

process design are programmed and stored into a processor card a data. The external programing can

be the algorithms in which describes the response for example a valve position as the level of a liquid

tank increase or reaches the limit height of the tank.

As the level sensor detects the momentarily height of the liquid level in the tank, the data is sent toinput box. This information is then processed by the central core of processor card that has the memory

of algorithms and response that will result in the output. In our example, as the liquid level becomes

really high, the risk of overflowing the tank makes it logical that the valve of the outlet must be open or

the closing of the inlet feed.

PLC Parts and components:

It is important to recognize different parts and components of the PLC, there are many models of PLCs in

the industrial markets. Allen Bradly is a very popular PLC products provider. In fact, Allen Bradley have

been producing many different type of control system. Here are some pictures of different type of PLC

parts that are provided by Allen Bradley ControlLogix:

http://epub1.rockwellautomation.com/images/web-proof-large/GL/1036082.jpg

Allen Bradly definitely produce some of the most feasible and reliable parts for various PLC controller.

To take a look at a better understanding of what a PLC controllers are made of, the image below show a

detail explanation of different part of a PLC controller. The information is given by the textbook:

Programmable Logic Controller (4th

ed) by Frank D. Petruzella

PLC programs consist of a logic sequence of different types of instructions. When the step and changes

are programmed, they are stored in a special, electronic memory processor card. This will be important

because this is considered the brain of system.

The processor will process different signals that come from the input signals when they reach the PLC

via sensor. Signals carry information about the status of the system to be controlled. According to the

text programmable logic controllers (4th

ed) these signal can be input using input binary, digital and

analogue signals.
http://epub1.rockwellautomation.com/images/web-proof-large/GL/1036082.jpghttp://epub1.rockwellautomation.com/images/web-proof-large/GL/1036082.jpghttp://epub1.rockwellautomation.com/images/web-proof-large/GL/1036082.jpg


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As we can see from here, we can see that a PLC model is equipped with a set out input module running at 24 VDC and a set of

output module running at 240 VAC. Each of the input modules is labelled with number for indication such as push button,

temperature, and pressure or level sensor. On the other hand, the output module is corresponding with light turn on, motor,valve position change. Here are some of the examples of input and output devices used in PLC controller:

Sources: Programmable Logic Controller (4thed) by Frank D. Petruzella

Even though there might be many different types of designs that has been but a general PLC controller

requires the same amount of different part of the system. In the figure, some of the input signals are

shown such as pushbuttons, selector switch, and limit switch. On the other hand, the output signals are

indicator light, signaling column etc.


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Programmable Logic Control notation:

PLC works alike to much different types of control process equipment; it operates on the binary system.

In fact, binary principle means that many things can be thought of as existing in only one or two states.

These states are denoted as numeric number 1 and 0 and these 1 and 0 represents statuses of the

systems such as on/off, true/false, high/low and etc.

And because PLC works just like many different types of equipment, the operations performed by digital

equipment are based on three fundamental logic functions: AND, OR and NOT.

The mathematical study of the binary number system and logic is called Boolean algebra which provides

a simple way of writing complicated combination of logic statements. Below are some of the common

Boolean Instruction and Function:

Understanding Ladder Logic Control:


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Input and out-put element is the basic method of ladder logic control. In a ladder logic programming

control, there are two rungs. The one on the left is for the input while the one on the right is the output

rung.

PLC control originally came from the study of circuit. When the circuit is made, current flows and the

output element are then triggered on. There are two types of contact, for example, when the contact isopen, the output is switched on. While the second type is the close contact, which mean that the output

is normally on through a closed contact.

In another word, each rung is a combination of input condition symbols connected from the left side of

the ladder toward the right side with the symbol represents the output.

Latches:

Latches are formed when there is more than one condition that can make up the output elements to beturned on. Latches used different functions as shown as the table from above with different examples of

symbol.

Example Problems:

PLC is very useful for industrial system such as level sensor of a water holding tank or any material

holding tank. This is a very common problem for industrial because understanding PLC for certain


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material can save millions of dollars from spilling or over flow in the real industry. The logic is quite

simple to map out as a flow chart of outcomes as follow:


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A diagram can be draw for the set-up of the water tank as follow:

Using a flow chart will carry out different responses to the sensors and different outcomes base on the

situation of the set up. Use the flow chart and the diagram above, we can figure out different situations

of the liquid level.

For example, if the sensor of our low level of the tank detects and indicates that the liquid level in the

tank is low, then the result would be that the valve to the outlet of the tank should be off and the pump

to pump more water to the tank to be switched on.

At the same time, if the sensor of the low level is off, and the sensor of the high level is on, now we have

a different situation. Because the level is low, the motor for pump is switched off. The valve of the out

let of the tank is on until the indicator sensor of the lower level is on. This again indicates that the level

is low so there for the valve should be off and the pump should be switch on again.

A proposed model for different part of the hardware units are:


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The figure above can be interpreted as the low level and high level proximity sensors serve as DC input

to the PLC at two different inputs I1 and I3, after the sensor and signals has been processed, the

information is sent to outputs Q1 and Q2 that is the motor to fetch water and solenoid valve to drain

the water. A drag and drop simulation of this system can be easily generated as follow:

From here, we can interpret the ladder program, noting that Ladder Logic Program has many different

versions with similar way to express the same symbols. From here, I use Logix Pro Simulator. For certain

Allen Bradly PLCs, the usual program to be used is the compatible versions of RSlogix 5000 simulator

that the manufacturer has already developed for different models.

Q1 or pump has two conditions due to the latches set up:

- It will turn on if the I1-low sensor turns on. This indicates that the liquid level is low.

-Or it will turn on if Q-2 (outlet valve) is off while I3 is on. This indicates that the pump is on as

long as the outlet valve is off because it shows that the tank need more water due to the draining.

The ladder diagram shows that Q1 (pump) will only start when either of the inputs I1 (lower sensor) or

I3 (upper sensor) is in on. As soon as they both turn off Q2 (valve) will turn on and pour out the water.

Conclusion:

My personal goal for this was to understanding the basic of PLC; however, PLC can be used for many

different processes. In fact, if I was working on the same project next semester, I can work on such

system as concentration control. For concentration can be measured using mole fraction, and the

disturbance might be the amount of certain different other components, therefore alternating the flow

rate of inlet or the out let valve can be useful for concentration control.


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The water PLC control can become more effective if I work on implementing different aspect of process

control such as gain, integral time etc. An algorithm can be set up for such implementations.

PLC systems have been proving that it can be strong feasible, safe control system for the future of

industrial process control. With the improving and further development of intelligent technology, PLC

will have higher capacities for bigger processes. This will make understanding about PLC becomes veryimportant.

Work Cited:

1.Petruzella, Frank D. Programmable Logic Controllers. New York: Glencoe/McGraw-Hill, 1989. Print.

2.Rehg, James A., and Glenn J. Sartori. Programmable Logic Controllers. Upper Saddle River, NJ: Pearson

Prentice Hall, 2009. Print.

plc memorandum

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