Assignment 1 - Programming and Use of PLCs

8/9/2019 Assignment 1 - Programming and Use of PLCs

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Programme Title: HNC Mechatronics

Unit number / Title: Instrumentation and Control

Assignment Title: Assignment 1: Programming and use of PLCs

Tutor: Gareth Lambert

Student: Mark ONeill

Mark ONeill HNC Year 1Instrumentation and Control: Assignment 1


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Task 1

Describe the Following Terms used in process measurement; include any relevantdiagrams in your descriptions

Accuracy and Precision

In process measurement accuracy refers to the ability of a measuring device to

relay data as close to the actual quantity being measured, accuracy differs fromprecision as precision may supply data very close to each other in value but be

far away from the measured quantity,

For Example

If the quantity being measured is 100

And the values being measured by the device are100, 75, 70

This device is deemed as accurate but not precise

If another device measures the readings as50, 52, 55

This device is deemed to be Precise but not accurate

Accuracy and precision rely on being based on a known reference on which acomparison of the measured value can be made, ideally the measuring deviceshould be both Accurate and precise, with the values created clustering tightlyaround the required value

To communicate accuracy and precision effectively a target analogy can be used,



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Hysterisis Error

If a known quantity is beingmeasure but yielding differentoutput values each time this it isreferred as Hysterisis error, it

comes from the maximumseparation of loading and

unloading, in effect there is amemory this means that that the

loading and unloading yielddifferent values even though thesame force is being applied.

For Example, if we look at the extension of an elastic band, a setforce of lets say 5 Newtons is applied to the elastic band and the

length of the band Is increases to 10 cm, applying a further 5Newtons extends the length to 20 cm, However when 5 Newtonsis then removed the length of the Elastic band falls to 11cm, onewould have assumed that as the force has been returned to 5Newtons that the extension length would be 10 cm, this is due tothe memory of the material. The difference is the Hysterisis

error.



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Non Linearity Error

Many things we measure have a linear scale, i.e., numbers, 1,2,3,4,5,6,7,8,9,10if produced onto a graph the result would be a linear line due to the fact that ateach time period the increment rises by 1.

Non linearity occurs when the vales do not increase in an even fashion, such asvariable increases or exponential increases

Some examples of nonlinearity in graphical form

(Below). Analogue linearity error is thedeviation from the actual quantity, theamount in deviation from the desiredoutput is the linearity error

(Above). Digital linearity error is resultant ofthe switching of a digital signal to match themeasurement of a quantity, as a digital

signal can only be either high or low, thesignal tries to switch in order to try and

produce the desired output.

At certain values some materials expressvery good linearity where as the same material at other values posses poorlinearity, it is the role of the system designer to determine which sensors aresuitable for the correct applications. The more linear a sensor is the more it costs

Range

The range is the difference between the lower end and the higher end of a set ofvalues, for example if a thermocouple has a range of -270 degrees Celsius to+400 degrees Celsius the range is

400 + 270 = 670 degrees Celsius

Values Lower than -270 and higher than +400 degrees Celsius would require asensor with a larger range, however determining the correct sensor for the

correct application, then range would be taken into account when selecting asensor. Generally sensors with larger measurement ranges are proportionately

more expensive.



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D ead Band / Space

Refers to the unresponsiveness of a sensor, the amount that something needs tochange before this change is noticed by the sensor

For example the sensor detecting the teeth of a gear operates when the tooth is

high, the Angle distance between the teeth is Dead ban/space, in the firstinstance the bead band is 90 degrees, in the second instance the dead band is

reduced to 45 degrees.

If the first gear were to rotate 46 degrees clockwise the sensor would not noticethe change and therefore assume the gear had not moved, however in thesecond gear, if it rotated 46 degrees the sensor would then pick up the high toothand be aware that the gear had moved

Dead band in a pressure transmitter occurs when small changes in pressure dontprovide enough of a force change to allow the sensor to overcome friction andmove to provide the correct output



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Task 2

a) Select a modern sensing method for each of the following properties anddescribe their construction and operation

Pressure

Piezoresistive pressure sensors work on the principle of a Piezoresistive effect in

which a mechanical force applied to a material changes its electrical resistance,unlike the Piezorelectric effect where the application of a force on a material

create an electrical force.The Piezoresistive elements are arranged in a Wheatstone bridge, a measuringinstrument which compares known resistance to an unknown resistance createdby the pressure input, the relationship between the resistance results in anoutput.The output from the bridge is directly proportional to the pressure input

Piezoresistive strain gauges placed behind a silicon diaphragm convert the

pressure applied to the diaphragm in to an electrical resistance which can then beused to determine the unknown pressure

Level

Sensors can detect Light level through a photocell by means of producing anelectrical resistance directly proportional to the light input.

A Float switch is a means of measuring the level at which a liquid has reached,the principle of a float switch is very

simple indeed, the arrangement of afloat switch can differ in its construction

to produce different results

The floating element of the switch caneither float to make a contact or float tobreak a contact depending on the designintent of its application

Encased inside the float switch is a magnetic reed switch, which is operated,when the floating element containing a magnet passes it, when the liquid levelreaches such a level the floating element connecting the magnet raises away

from the magnetic reed switch cause the connection to break the circuit.

Although a float switch is very simple, its resolution is also limited a binary outputis all that is possible, the switch is either activated (high liquid level) or the switchis inactive (the level is not high)There is no way of accurately knowing whether the level is , or full

A proximity sensor with a floating armature could be used to determine the levelof the liquid



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Temperature

A thermocouple can be used for a wide rangeof temperature however accuracy of below 1Kelvin is hard to achieve, it works on theprincipal of an EMF (Electro magnetic Force)

being produced in the junction between twodissimilar metals, each metal that is subject

to a thermal source will generate a voltage,so by using dissimilar metals its possible to

measure the different between the twovoltages to determine the actual temperaturevalue. As with many types of sensors there are different types offering differentranges and characteristics depending on the application required, some types canoperate in temperatures as low as 270 degrees Celsius and some can operate intemperatures as high as +2600 degrees Celsius, selecting the correctthermocouple for the desired temperature range is important due to the fact thatthermocouples are nonlinear measurement devices past certain temperatures

An issue that occurs when using thermocouples is in their connection, special wire

that reduces any additional junctions being created that will generate their ownvoltage and interfere with the output.

See Appendix A for typical thermocouple types, their temperature Ranges andtheir voltage-temperature characteristics

Temperature Sensor Advantages and Disadvantages

Sensor Advantages Disadvantages

Thermocouple No resistance leadwire problems Fastest response

Simple, rugged

Inexpensive

High temperature operation

Point temperature sensing

Non-linear Low voltage

Least stable, repeatable

Least sensitive

RTD Most stable, accurate

Contamination resistant

More linear than thermocouple

Area temperature sensing

Most repeatable temperature

measurement

Current source required

Self-heating

Slow response time

Low sensitivity to small

temperature changes

Thermistor High output, fast

Two-wire ohms measurement

Economic

Point temperature sensing

Non-linear

Limited range

Fragile

Current source required

Self heating



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Flow

A rotating vane slow sensor can operate in two ways, buteither recording the flow of a liquid / gas by the rotating

speed of the shaft or by the veins interrupting an infra redbeam, the speed is then proportional to the flow.

In a vane rotating flow sensor turbine blades are used, the

pressure applied by the liquid / gas to the top of the fins, isgreater than the pressure on the back of the fins, thedifference in pressure causes the turbine to rotate, and infrared bean is aimed at the blades, as the beam is continuouslybroken the pulse repetition frequency output is directlyproportionate to the flow of liquid / gas being measured.To achieve accuracy the sensor will need to be calibrated byusing a known source of pressure.



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Application of sensors at place of work

Pressure Sensor

To test sub assemblies and components for leaks, we use a pressure sensor fromSMC pneumatics, this particular sensor measures in Kpa (kilopascal).

Manufacture Honeywell Honeywell

Model 24PCFFM6G 40PC100G2A

Pressure Range 0-100 psi 0-100 psi

Maximum permissibleover pressure

200 psi 200 psi

Operating Temp -40 to +85 degrees C -45 to +125 degrees C

Supply Voltage 10-12V (dc) 5V (dc)

Price 12.23 38.79

In the above table of 2 similar pressure sensors the selection must be made

based on the appropriateness of the sensor to meet the application for example ifpressures need to be measured in an environment with temperatures exceeding85 degrees Celsius then the Honeywell 24PCFFM6G would not be a suitablecomponent

However due to cost implications if the highest temperature acting upon thesensor would be below 85 degrees Celsius then the 24PCFFM6G should be chosenover the 40PC100G2A as would have a saving of 26.56 per unit, and wouldperform comfortably.



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Temperature Sensor

A thermocouple is used to measure the temperature of an inlet head, atemperature reading is required to ensure that combustion of process gas doesnot happen past a certain point but also to control the rate in which the processgas is passed through the inlet head, if the head is getting too hot then theprocess gas will be reduced

Manufacturer RS RS

Part Number 621-2170 455-4270Operating temp range -50 to 400 degrees C 0 to 760 degrees C

Standard Type Type K Type J

Price 7.00 16.32

The two components above primarily differ in their temperature range, if thetemperature required to be measured was 500 degrees C then the Kype Kthermocouple would not be suitable for the application, if the maximumtemperature being measured was 300 degrees C then the Type J thermocouplewould be over specification and un required for the application.



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Flow Sensor

These reed sensors are connected to a manifold which circulates water aroundthe system, its possible to determine the flow of water leaving the manifold to be

distributed around the system, the sum of the values obtained from the reedsensors would equal the total flow of water around the system.

This particular model is FM0102 it can measure 2 16 litres per minute obtaininga accuracy of 3.0% and a repeatability of 5.0%A similar product on the market is the PF2A710-F02-67 manufactured by SMC, itshas a measurement range of 1 10 litres per minute with a repeatability of 1%

or less, making it superior product to meet this application, however the cost ofthe SMC product although superior would be un-economically viable for use in our

systems as the level of repeatability is un required.

SMC PF2A710-F02-67Level Sensor



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On a product called Atlas Abatement the tank where the systems water is stored,

its level is controlled by the used of two float switches, a warning level switch isoperated when the water level rises to a level at which is deemed to be getting

higher, an additional switch located above indicates that the water level has risento high and the system shuts down

Manufacturer GENTECH GENTECH

Model LS304-11N LS404-11

Float Specific Gravity 0.85 0.65

Operating Temp -30 to 110 degrees C -30 to 105 degrees C

Medium Fuels, oils, alcohol Water

Switching Properties IDENTICAL IDENTICAL

Mechanical Specification IDENTICAL IDENTICAL

The important thing when selecting the correct float switch for the right

application is the medium, in which you require to measure, the specific gravity ofthe float which is compared to that of water (1.0) determines how the float willrespond to a certain liquids.

As Oils have a higher density to that of water, a higher specific gravity will makethe float respond quicker and should be used for such applications.



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b) Describe a typical application for each of the methods described in 2a

Pressure

There are many different applications where Piezoresistive sensors are used,

Although in essence the sensor is used only to measure pressure, it can also beused to measure height and depth

When submersed in a liquid to a depth, the pressure applied to the sensorcombined with the specific gravity and liquid volume of said liquid can result inthe depth

Use to determine altitude is also an application suitable for Piezoresistive sensors,

the higher the altitude the less pressure that is applied to the sensor.

Weather instrumentation: to determine the atmospheric and barometric pressure

which indicates the precipitation likelihood.

Level

An Application for a float level would be in a reservoir tank, if the tank is required

to be filled to a desired level at all times, then a float switch could facilitate this,when water is drained from the tanks outlet, the inlet will be operated by the

float dropping off, until such time as it floats again and shuts off the tank inlet.

Temperature

A thermocouple has a variety of applications within industry such as use in the

food industry where vats of liquid may been to be at a certain temperature foreither cooking to the correct temperature, also use in a ceramics company wherecontrol of the drying kiln will need to be monitored.

Flow

There are countless application in which a flow meter may be used, Gas flow

meters may be used to determine the speed of gas circulating around a heatingsystem where as a liquid flow meter may also be used in a heating system tomeasure the flow of water circulating.



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Task 3

a) Describe the main stages of analogue to digital conversion

Analogue signals are limitlessly variable where as digital signal can only choosefrom high or low states, analogue signal have a higher range and accuracy butunfortunately are electronically impossible to understand by electronic devices, sotherefore but be converted into a digital signal, this is done by matching up ananalogue wave form with binary bits, if we take a basic sinusoidal wave form forexample and convert it into a digital wave form you can see the steps taken tohow it was produced

The main stages of Digital to Analogue Conversion are

Sampling

Starting off with an analogue signal points at specific time periods must be

sampled to determine the values of the integers that will be made into a digitalsignal

Quantizing

Is the process of turning the values of the sampling process into whole integers,this process rounds up or down a number to the nearest whole integer for

example 6.4 would be rounded down to 6 and 6.5 would be rounded up to 7 as awhole integer



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Encoding

Converts the integers into words they are binary number sets with variableresolutions,i.e., a 2 bit binary word has 4 possible states

State Decimal Value Binary value1 0 00

2 1 01

3 2 10

4 3 11

Where as a 4 bit binary word has 16 possible states

State Decimal Value Binary value

1 0 0000

2 1 0001

3 2 00104 3 0011

5 4 0100

6 5 0101

7 6 0110

8 7 0111

9 8 1000

10 9 1001

11 10 1010

12 11 1011

13 12 1100

14 13 1101

15 14 111016 15 1111

The division of these words determines its resolution, as you can see the highernumber of bits the higher the resolution will be

Turns the integers produced from the previous two steps and converts them to a

digital value



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So for example

3 6 7 5 2

0011 0110 0111 0101 0010

In the graph the original waveform is represented as the BLUE line and the binaryequivalent is represented by the RED bar, going by this the wave form can be

digitally represented by the digital term

0011 0110 0111 0101 0010

b) A/D modules may have different word lengths giving different resolutions.Calculate the resolution of a 4 bit and a 12 bit A/D module

4 bit A/D module Resolution

to determine the resolution of a 4 bit number system we must raise the base of

the binary to a power of the number of bits

so:-

2^4 = 16

this means that there are 16 different configurations

12 bit A/D module resolution

to determine the resolution of a 12 bit number system we must raise the base ofthe primary to the power of the number of bits

so:-

2^12 = 4096



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Task 4

a) describe the Principles of 4-20mA current loop transmission

4 20 mA current loop is a sensor signalling standard that is widely used becauseit is largely unaffected by electrical noise, typically used to measure physical

factors such as temperature, Ph, pressure, etc4mA represents the lowest end of the range of current that can be used and

20mA represents the highest end of the range. The reasons that 4-20mA is sopopular are

- Easy Connectiono Only two wires are required for instillation, the supply voltage and

the measuring current.- Current immunity to electronic interference.- Sensor information can be transmitted long distances with little adverse

affect- The current loop can provide operating power to the devices.

b) Explain why 4mA represents the minimum value and not 0mA

If the minimum value was 0mA then if a problem in the system arose and nocurrent was being passed, components would read the current failure as a valueand continue to operate as normal, by setting the lowest value at 4mA it providesthe components with information that there is a problem if a value of


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Task 5

a) Draw an operational amplifier in a simple inverting configuration.

A Graphical representation of the input wave in comparison of the output waveform with the negative gain can be seen on the next page.

b) Calculate values of Rin & Rf to give a gain of -5

to achieve a gain of 5 the ratio between Rin and Rf needs to be 5:1

the equation to determine the gain (Av) is

Av = -Rf / Rin

so if Rin = 5 ohmsRf = 1 ohms

-5 killohms / 1 killohms = -5

The Gain (Av) for the circuit is -5



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Appendix A



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Bibliography

http://wpcontent.answers.com/wikipedia/commons/thumb/d/df/Elastic_Hysteresis.jpg/300px-Elastic_Hysteresis.jpg

http://www.tutelman.com/golf/measure/precision.php

http://www.visionic.co.uk/Thermocouple.htm

http://www.wellesley.edu/Chemistry/Chem105manual/Appendices/uncertainty_a

nalysis.html

Mark ONeill HNC Year 1
http://wpcontent.answers.com/wikipedia/commons/thumb/d/df/Elastic_Hysteresis.jpg/300px-Elastic_Hysteresis.jpghttp://wpcontent.answers.com/wikipedia/commons/thumb/d/df/Elastic_Hysteresis.jpg/300px-Elastic_Hysteresis.jpghttp://www.tutelman.com/golf/measure/precision.phphttp://www.visionic.co.uk/Thermocouple.htmhttp://www.wellesley.edu/Chemistry/Chem105manual/Appendices/uncertainty_analysis.htmlhttp://www.wellesley.edu/Chemistry/Chem105manual/Appendices/uncertainty_analysis.htmlhttp://wpcontent.answers.com/wikipedia/commons/thumb/d/df/Elastic_Hysteresis.jpg/300px-Elastic_Hysteresis.jpghttp://wpcontent.answers.com/wikipedia/commons/thumb/d/df/Elastic_Hysteresis.jpg/300px-Elastic_Hysteresis.jpghttp://www.tutelman.com/golf/measure/precision.phphttp://www.visionic.co.uk/Thermocouple.htmhttp://www.wellesley.edu/Chemistry/Chem105manual/Appendices/uncertainty_analysis.htmlhttp://www.wellesley.edu/Chemistry/Chem105manual/Appendices/uncertainty_analysis.html

Assignment 1 - Programming and Use of PLCs

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