3.1 Monitoring technologies

3 Monitoring and control

3.1 Monitoring technologies

Monitoring, or measurement as it is sometimes called, involves the use of

a computer or microprocessor-based device to monitor or measure physical

variables over a period of time. It is important to know which sensors would

be appropriate in a given situation to measure physical variables such as light,

temperature, atmospheric pressure, humidity, moisture, sound, blood pressure

and pH, among others.

In Chapter 1 in the section on data logging, we saw that a sensor is a device

that is used to collect (input) data. The data usually relates to physical changes

in the environment that are being monitored. A sensor converts the physical

characteristic, such as temperature, light or pressure, into a signal which can

be measured electrically. Monitoring systems are different to

control systems

regarding their outputs. Control systems process the data and then make

decisions based on the data regarding which actions to take, as we saw in

Chapter 1 in the section on real-time processing. Monitoring systems only

process and record data so that people can see trends in the changes that are

taking place in the environment being monitored.

Sensors continually send data back to the computer or microprocessor. Students

often misunderstand this process, with some thinking that sensors send data

every few seconds or even minutes or that sensors only send data when there

is a change in the environment. The process is actually continuous and never

ends. Most sensors used in monitoring systems are analogue, which means the

data sent to the computer is in analogue form. However, the computer can only

process data in digital form, so the data has to be converted using an analogue-

to-digital converter (ADC). This is so the computer, which can only understand

digital data, is able to process it.

The output from the system is usually on a screen or printed out, but can be a

warning sound if the monitoring that is taking place is critical, for example an

overheating nuclear reactor.

In this chapter you will learn:

+ aboutthesensorsandcalibrationusedin

monitoring technologies

+ about the uses of monitoring technologies

+ aboutthesensorsandactuatorsusedin

control technologies

+ how to write an algorithm and draw a

flowchart.

Before starting this chapter you should:

+ be familiar with the terms ‘sensor’, ‘physical variable’ and

‘microprocessor’.

3 MONITORING AND CONTROL

Sensors

Below is a table showing the uses of some of the more commonly occurring

sensors. In reality there are very few basic sensors, with several sensors actually

being combinations of the others.

Type of sensor Description and use

Light/ultraviolet Light sensors, as the name suggests, measure the amount of light. There are many types of

light sensor, but they all follow the same principle. When light falls on this type of sensor,

it generates electrical energy. The amount of energy is then converted to give a value for

the amount of light. These sensors can be used in weather stations to measure the amount

of sunshine.

Ultraviolet sensors are used to measure the amount of UV (specifically UVB radiation, which

can be dangerous to humans, sometimes causing skin cancer).

Temperature The components of different types of temperature sensor either change their electrical

resistance or generate a voltage according to temperature. Whichever type is used,

electrical signals are generated which are converted into values to represent temperature.

These sensors are used to feed data back to the computer about, for example, the

ambient

temperature in a weather station, the temperature in a river or air quality.

Pressure A pressure sensor converts the force applied to its surface to generate electrical energy

which is then converted into values to represent the applied pressure.

These sensors are used to measure atmospheric pressure in weather stations.

Humidity/moisture Humidity sensors are often a combination of a moisture sensor and a temperature sensor

in one unit. This is because humidity can only be calculated by knowing how much water

there is in the atmosphere together with the temperature. A moisture sensor is actually

a combination of a light sensor and a light emitter, as the amount of light transmitted

depends on the moisture content of the air.

Humidity sensors are used to measure the air humidity in a weather station. Moisture

sensors are also used when monitoring soil quality.

Sound These sensors convert sound waves into voltages or electrical signals which are converted

by the computer into values to represent sound.

Sound sensors can be used in environmental monitoring systems to measure noise

pollution.

Infrared All bodies possess thermal energy and therefore emit infrared radiation. This radiation is

converted into electrical signals as a result.

These sensors can be used in environmental monitoring, for example the Earth’s surface

temperature can be monitored by satellites.

Oxygen, carbon dioxide,

pH, turbidity

These are sensors used in environmental monitoring and water pollution monitoring.

Oxygen (O

) sensors measure the level of oxygen in soil and water.

Carbon dioxide (CO

) sensors measure the level of carbon dioxide in the air or in water and

are basically an adaptation of an infrared sensor.

pH sensors measure the acidity of soil and acidity in rivers, lakes, etc. They are very similar

to a simple battery and generate electricity depending on the number of hydrogen ions in

the solution, which causes an electrode to generate a voltage.

Turbidity sensors measure the cloudiness of water in a river that is affected by pollution. A

turbidity sensor is actually a light sensor usually placed at right angles to a light emitter.

The greater the number of particles in the water, the greater the amount of light reflected

off them on to the sensor.

Reed switch Strictly speaking, a reed switch is not a sensor, but it is used to help measure rainfall (see

the next section about the uses of monitoring technologies in weather stations).

V Table 3.1 Uses of common sensors

3.1 Monitoring technologies

Uses of monitoring technologies

Weather stations

There are many types of weather station, ranging from very sophisticated and

complex systems to a basic home-based system. We will be considering the type

of station that might exist in a school. This type of weather station could be

used to monitor the weather in terms of temperature, rainfall, hours of sunlight,

atmospheric pressure, humidity, and UV radiation. For this purpose, it would

need:

» Temperature sensors to measure the ambient temperature. When referring

to the weather, ambient temperature means the temperature of the

surrounding air of the weather station.

» Pressure sensors to measure atmospheric pressure, which is the pressure

of the air above us. Weather forecasters use pressure readings to help them

formulate weather forecasts. High pressure is usually an indication of fine

sunny weather, whereas low pressure tends to be associated with wet and

windy weather.

» Humidity sensors to measure absolute and relative humidity. Absolute

humidity is the amount of moisture in the air, measured without taking

temperature into account. It is usually measured as the number of grams of

water per cubic metre of air. Relative humidity is also a measure of moisture

but does consider the temperature of the air and is actually a percentage

value. It is the fraction of the amount of moisture in the air compared to

the maximum amount of water that could be held in air at that temperature.

It is used by weather forecasters to predict the likelihood of rain or snow

occurring.

» Light sensors to measure the number of hours of sunlight. Measuring

sunlight requires an array of light sensors which collectively measure the

intensity of the light radiation.

» A tipping bucket and a reed switch to measure rainfall. Most non-

automated weather stations use a bucket into which the rain falls. When the

bucket reaches a certain weight (usually after a very small amount of rain) the

bucket mechanism causes it to tip over and empty the water. It then tips back

to collect further rainfall. Originally, the tipping bucket was connected to a

rotating, mechanical, graph plotter that would make a mark every time the

bucket was tipped. By counting the number of marks on the graph human

operators could work out the amount of rain by multiplying this count by

the amount it took to tip the bucket. In modern automated systems, the

tipping of the bucket activates a reed switch which sends a signal back to

the microprocessor. The microprocessor, counting the number of times the

bucket tips, performs the same calculations that human operators used to do.

Wind speed and direction can also be measured using sensors or

combinations of sensors.

When the weather station is operating, the readings from the sensors are fed

back to an ADC and then sent to the computer. The ADC converts the data

from analogue to digital so that the computer can understand and process it. On

receiving the digital data, the computer stores the data in the form of a table,

which could be done using a spreadsheet or database package, so that it can be

processed. The processing might consist of calculating, for each day, the highest,

lowest and mean temperature, the level of UV radiation, total rainfall, hours of

sunshine, highest and lowest value of atmospheric pressure, and wind speed and

direction. These values can also be calculated for the month and year to date.

3 MONITORING AND CONTROL

Results can be output in the form of graphs, either to a monitor or printed out.

This all happens automatically without the need for human intervention.

Monitoring water pollution

Studies of water pollution usually happen with reference to bodies of water such

as rivers, lakes and, sometimes, seas. There are basically two ways of carrying

out the study.

One is to compare the readings with those that would normally be expected.

This requires the lowering of one set of sensors into the river or lake.

The other relates usually to industrial pollution, though it can also be used to

measure pollution from a farmer’s field. This involves inserting two sets of sensors,

one upstream from the suspected site of pollution and the other downstream,

immediately after the site, whatever that may be, farm or factory. The readings

from the two sets of sensors are compared to see if there are any differences so

that a conclusion can be reached as to whether the site is causing pollution.

The system operates the same as the weather station, with sensors feeding data

to an ADC and then the computer processing the digital data. The sensors

involved are temperature sensors, pH sensors, turbidity sensors, O

and CO

sensors. The processing carried out is a comparison of the readings with normal

values if it is the first method, or a comparison of the readings from the two sets

of sensors if the second method is being used.

Environmental monitoring

Environmental monitoring is the collection of data relating to our environment.

It can include:

» monitoring sound in cities, in addition to other pollution produced by motor

vehicles

» monitoring soil quality in outdoor gardens and also in greenhouses/

glasshouses, using pH, moisture and temperature sensors, among others

Environmental monitoring is closely linked to climate study, so can also include:

» detecting abnormally high temperatures using temperature sensors, so that

people can be warned of the dangers

» monitoring the level of air pollution using O

sensors and CO

sensors; pH

sensors can also help in this regard as they can provide acidity readings

» monitoring ultraviolet levels; excessive amounts can cause skin cancer, so

these are often monitored by governments in high-risk countries to judge

whether people should be advised to wear skin protection cream.

Activity 3a

Explain how you would set up a study to monitor pollution in your school or

college grounds.

Calibration

When sensors measure physical variables, people believe that the results are

always going to be accurate because a computer is involved. However, for this to

be the case, sensors have to be calibrated.

Calibration is making sure that when,

for example, a temperature sensor is used to measure the temperature of boiling

water, it actually causes the computer to output a value of 100ºC. It is done by

comparing the value a sensor produces to a known measurement.

3.1 Monitoring technologies

The importance of calibration

The accuracy of all sensors reduces after a period of time. This is often caused

by constant use and exposure to the atmosphere or liquids that are being used.

Slight erosion of the material the sensor is made out of is also bound to occur.

It depends on the type of sensor and how it is being used as to how quickly this

occurs but whatever the cause, regular calibration helps to maintain accuracy.

The sensor is only one component in the monitoring system and devices like the

ADC may also deteriorate over time, so calibration is important for that reason

too. If a sensor is being used as part of a sensing system, as with a humidity

sensor, then if that sensor loses accuracy, the whole system will need calibrating.

Methods used to calibrate devices

Let us take a look at how to calibrate a temperature sensor as an example. In

order to calibrate the sensor, there needs to be some way of creating a known

temperature. This can be done by using a heat source that generates an accurate

temperature or using a mixture of ice and water which can give an accurate

reading for 0 °C. The sensor, when activated, generates a voltage which is

converted into a temperature reading. The algorithm to do this can be refined

so that the temperature reading matches the actual temperature. The alternative

is to take a reading from a sensor that has already been calibrated and use this to

compare the temperature reading of the sensor that is being calibrated.

One-point calibration

This type of calibration is the easiest to perform. Only one measurement point

is needed and this makes it a particularly appropriate method to use with sensors

which have to measure a set temperature or temperature range that is constant,

that is, it never changes. The sensor need not be calibrated against any other range

of temperatures since it is not going to be required to measure these. This type

of calibration is often needed for sensors which are constantly used at very high

temperatures and after some time lose accuracy. Accuracy can be checked by doing

a one-point calibration every so often and comparing the result with the previous

calibration. In order to carry out a one-point calibration, a reading is taken from the

sensor in the range being measured and it is compared with either a pre-calibrated

sensor or a known value. The sensor reading is subtracted from the known value

which gives the ‘offset’. In the algorithm, this offset is then added to every reading

in the temperature range being measured.

Two-point calibration

Figure 3.1 shows a graph of a sensor’s readings plotted

against a standardised (pre-calibrated) sensor for a range

of pH values. A number of solutions with different pH

values are being used. The readings shown in the graph

are exaggerated as it is most unlikely that even the most

inaccurate sensor would produce readings this far out.

The sensor being calibrated is different to the standardised

sensor by +1 at the lower end and -5 at the higher end.

The calibration cannot be carried out by just adding an

offset value in this case, since the offset values are different

for every pH value. There is however a linear relationship

between the two and to calibrate the sensor the algorithm

would need to multiply each reading by 2 and subtract 3.

This calibration compensates for both offset errors and

what are slope errors, so called because the slope of the

graph has to be considered.

Sensor

Key

Standard

V Figure 3.1 Two-point calibration

3 MONITORING AND CONTROL

To arrive at this conclusion, obviously we need to compare more than one reading.

With a pH sensor, it is recommended to place sensors in solutions which are neutral

(pH 7) and either one which is acidic (say, 4.0) or one that is alkaline (say, 10.0).

The use of the two values will enable the relationship between the sensor and the

standard to be established.

Multipoint calibration

Figure 3.2 shows a graph of a sensor’s readings plotted

against a standardised (pre-calibrated) sensor, but this

time for a range of temperatures. The temperature

readings are taken from a hot liquid as it cools down.

Although the sensor being calibrated in Figure 3.2 has

the same reading as the standardised sensor at the lower

end, it is markedly different for other readings. The

calibration cannot be carried out by just adding an offset

value or by allowing for the slope. There is no longer a

linear relationship between the two and to calibrate the

sensor, the algorithm would need to include some form

of what is called ‘curve fitting’. This is because it is a

nonlinear relationship and needs to be described using a

quadratic function of the form y = ax

+ bx + c, where y

is the standardised reading and x is the reading from the

sensor that needs calibration. Do not worry if you find this complicated as all you

really need to understand is that the values from the sensor are a curve, whereas they

should be a straight line and the algorithm needs to be amended to deal with this.

To carry out the calibration, at least three known temperatures need to be created

and the sensor readings at those temperatures need to be taken. Good practice

would be to use temperatures of 100ºC, 0ºC and say, 50ºC. This should provide

the relationship between the sensor and the standardised sensor.

Advantages and disadvantages of monitoring technologies

Although computers are now used in all aspects of monitoring, there is still the

need for humans to be involved. Here is a table showing the advantages and

disadvantages of computer monitoring compared with people taking readings.

Advantages of computer monitoring Disadvantages of computer monitoring

Humans are unable to take readings at very frequent intervals as

they need to make a note of each reading. This takes time, during

which they cannot take another reading. Computers are able to

take readings at more frequent intervals and are capable of reading

more than one variable simultaneously. Humans can only do one

thing at a time, so take longer.

It is very difficult for humans to keep taking readings for sustained

periods, whereas computers can be left on to take readings at

any time, day or night. The readings are always taken at regular

intervals unlike with a human who might forget to take them.

Computers can be expensive to buy, whereas humans

would be expected to do the monitoring as part of

their job. Computers are also expensive to maintain.

It takes time for people to draw accurate graphs, whereas

computers can produce them automatically after processing the

data.

It is quite difficult to program computers to interpret

the results, but humans can interpret results and are

also needed to program the computers in the first place.

Results are produced automatically after the readings are

received by the microprocessor or computer, unlike a human

who would take a lot of time to write them down.

Readings taken by computers tend to be more accurate than those

by humans as computers are not subject to ‘human error’.

Sensors can deteriorate after a period of time,

whereas humans will tend to be more consistent.

V Table 3.2 Advantages and disadvantages of monitoring technologies

100

120

Sensor

Key

Standard

V Figure 3.2 Multipoint calibration

3.2 Control technologies

A control system is one that uses microprocessors or computers to control

certain physical variables. Computers can do this by maintaining certain physical

conditions at the same level for a period of time or by controlling certain devices

which cause the variables to change. Physical variables that can be controlled by

computers and microprocessors include temperature, pressure, humidity, light,

and moisture.

Control systems use real-time processing, which was described in Chapter1.

They make use of actuators to control devices, although some devices are

actuators in their own right, such as a motor. Unlike in monitoring systems, in

control systems the output affects the input. For example, think about a room’s

temperature as being controlled by a microprocessor connected to a temperature

sensor and a heater. The temperature is input by the sensor to the microprocessor.

If the temperature is below a certain value, the microprocessor sends a signal

to the heater to switch on, which is the output. The heater being on causes the

temperature to rise which means the input value has now changed, so the output

has obviously affected the input. Control systems involve continuous processes.

Sensors

All of the sensors described in Section 3.1 on monitoring technologies can also be

used in control systems, but there are some sensors that are more likely to be found

in control systems than monitoring systems. Table 3.3 shows some ofthese.

Type of sensor Description and use

Electromagnetic

field sensor

Often referred to simply as a magnetic field sensor, this sensor measures the change in the Earth’s

natural magnetic field caused by the presence of a ferromagnetic object. When a vehicle, for

example, is above the sensor, the metal in the body of the vehicle distorts the Earth’s magnetic

field and so its presence is detected. The sensor is small, so its installation and maintenance are

easier and cheaper than an induction loop which performs more or less the same function. Magnetic

field sensors and induction loops are used at the entrances to car parks to control barriers but

magnetic field sensors can be used to detect the number of spaces available. They are also used in

some automated car parking systems to help drivers park their car, in a similar way to ultrasonic

sensors (see below).

Ultrasonic sensors An ultrasonic sensor is actually made up of a device that sends out sound waves with a frequency

greater than that of the human audible range (so that a human cannot hear it) and a sensor that

receives the sound waves which are reflected back. It can be used to measure how far away an object

is. It measures the amount of time taken for the sound to be sent and received which, combined with

knowledge of the speed of these sound waves, can be used by the microprocessor to calculate the

distance. It is used in automated car parking systems which let the driver know when they are close to

another vehicle or other object so they can park their car without hitting that obstacle.

Proximity sensor A proximity sensor can be a mixture of sensors but usually comprises a device that sends out

a signal and a sensor which receives the reflected signal back. This can be an infrared beam,

ultrasound or a magnetic field. One use is in smartphones to switch off the screen display when

the phone is held near to the ear.

Touch sensors One type of touch sensor is used for measuring fluid levels. A capacitive touch sensor

measures the capacitance between two conductors separated by an insulating plate. One of

the conductors will be the fluid whose level is being measured. When the fluid is touching the

sensor, it detects that it is at that level. This type of sensor is often used in detection devices

used to measure fluid levels such as the cooling water level in nuclear power plants to ensure

that there is sufficient water to cool the reactors.

V Table3.3Usesofsensorsincontrolsystems

3 MONITORING AND CONTROL

Actuators

Just as sensors provide the input to a control system, so actuators provide the

output. An actuator controls a device, such as the valve which allows water to

flow through heaters or sprinklers in a greenhouse. It can actually be a motor

which controls the opening or shutting of windows in a greenhouse or a switch

which turns a heater on. It is essentially a device that turns an electrical signal

from a microprocessor into movement. A common actuator is the linear solenoid

actuator, which is basically an electrical coil wound around a cylindrical tube

encasing a plunger, which can move in and out of the coil. They can be used to

open doors, open or close valves and activate electrical switches.

Plunger

Case

Coil

V Figure 3.3 A linear solenoid actuator

Uses of control-technology systems

All of the following examples are microprocessor-controlled. The term

‘computer’ can be substituted for microprocessor, so all these examples could be

called computer-controlled.

Greenhouses (glasshouses)

Greenhouses (or glasshouses, as they are

sometimes called) are used in countries with

cooler weather so that people can grow plants

that are normally grown in warmer countries. The

sensors needed in a greenhouse are:

» a temperature sensor to measure the air

temperature in the greenhouse

» a moisture sensor to measure the water content

of the soil

» a light sensor to measure the light level inside

the greenhouse.

Maintaining the required temperature

At the start of the process, the user inputs the

required temperature (pre-set value) using a

keypad, number pad or touchscreen.

The computer receives data about the temperature of the greenhouse from the

temperature sensor. It needs an ADC to change the analogue temperature data

Temperature,

moisture and

light sensors

Light source

Keypad

Heater

Sprinkler

Window motor

V Figure3.4Agreenhousewithsensors

3.2 Control technologies

to a digital value the computer can understand. The computer compares the

sensor data to the pre-set value input to the system earlier by the user. If the

temperature is above the pre-set value, it sends a signal to the window motor

to open or leave it open if it already is. If the temperature is below or equal to

the pre-set value, the computer sends a signal to the motor to close the window

or leave it closed if it already is closed. It also sends a signal to an actuator

to activate the heater or leave it on if it already is on. This whole process is

continuous as long as the system is switched on.

The layout of algorithmic constructs will be explained in detail in Chapter4.

The algorithms shown below are not as detailed as they might be. In Chapter4

you will be asked to refine them and full answers will be provided. Here

is a possible algorithm which describes the processing carried out by the

microprocessor (computer), assuming the pre-set (required) temperature has

already been input to the system:

1 WHILE system switched on

2 INPUT temperature

3 IF temperature > pre-set

4 THEN

5 send signal to window motor to open window or leave it open

send signal to actuator to switch off heater or leave it off

7 ELSE

send signal to window motor to close window or leave it closed

9 send signal to actuator to switch on heater or leave it on

10 ENDIF

11 ENDWHILE

Notice that this is just a representation of how the microprocessor (computer)

deals with the data. It does not contain all the steps which are involved in

this system. The input of a pre-set value needs to be done beforehand and the

analogue to digital conversion has not been mentioned.

Maintaining the required soil moisture

At the start of the process, the user inputs the required moisture level (pre-set

value) using a keypad, number pad or touchscreen.

The microprocessor (computer) receives data about the amount of moisture in

the soil in the greenhouse from the moisture sensor. It needs an ADC to change

the analogue moisture data to a digital value the computer can understand. The

computer compares the sensor data to the pre-set value input earlier by the user.

If the level of moisture is below the pre-set value, it sends a signal to an actuator

which activates the sprinkler valve to open or leave it open if it already is open.

If the moisture level is above or equal to the pre-set value, the computer sends

a signal to the actuator to close the sprinkler valve or leave it closed if it already

is closed. This whole process is continuous as long as the system is switched

on. Some greenhouses have sensors which measure the humidity of the air as

well, because some plants require high humidity as well as warmth. These have

separate watering systems, one to spray water into the air and another to put

more moisture into the soil. The principle for these systems is much the same as

for the light one described later.

3 MONITORING AND CONTROL

Here is an algorithm which describes the processing carried out by the

microprocessor, assuming the pre-set (required) moisture level has already been

input to the system:

1 WHILE system switched on

2INPUT moisture

3 IF moisture < pre-set

4 THEN

5 send signal to actuator to open sprinkler valve or remain

open

6 ELSE

7 send signal to actuator to close sprinkler valve or remain

closed

8 ENDIF

9ENDWHILE

Maintaining the required level of light

With regard to the level of light in the greenhouse, things are a little more

complicated. When the light is not very good, a light source needs to come on

so that the plants have good growing conditions. A pre-set value still needs to be

entered. If the light value is less than the pre-set value, the light source needs to

be activated. However, when night comes, the light source needs to be switched

off. A second pre-set value therefore needs to be entered, so that when the

amount of light falls below that value, as at night, the light source is switched off.

The steps required in this process are similar to the two outlined above, except

that two pre-set values need to be input. A light sensor is needed to measure the

amount of light coming into the greenhouse and the analogue to digital conversion

is still required. Here is an algorithm which could describe the processing carried

out by the microprocessor, assuming the pre-set (required) light levels have already

been input to the system. Note that pre-set1 is the light level below which it is dull

during the day, and pre-set2 is the light level as it gets dark at night time.

1 WHILE system switched on

2INPUT light_level

3 IF light _ level < pre-set1

4 THEN

5 IF light _ level < pre-set2

6 THEN

7 send signal to actuator to switch off light source

or leave off

8 ELSE

9 send signal to actuator to switch on light source

or leave on

10 EN DIF

11 ELSE

12 send signal to actuator to switch off light source or leave off

13 ENDIF

14 ENDWHILE

3.2 Control technologies

Central-heating systems

Central-heating systems and microprocessor control were discussed in Chapter 1

in the section on real-time processing. The steps involved are outlined below.

1 User enters required temperature using keypad/

touchscreen

2 Microprocessor stores required temperature as a

pre-set value

3 Microprocessor receives temperature from sensor

4 Microprocessor compares temperature from sensor to

pre-set value

5 If temperature from the sensor is lower than the

pre-set value, the microprocessor sends a signal to

an actuator to open the gas valves

6 If temperature from the sensor is lower than the

pre-set value, the microprocessor sends a signal

toan actuator to switch the pump on

7 If temperature is higher than or equal to the pre-

set value the microprocessor sends a signal to

switch the pump off and close the valves

8 This sequence is repeated until the system is

switched off

Start

Stop

system switched

on?

temperature <

pre-set?

INPUT

temperature

Yes

Send signal

to open valves

Send signal to

close valves

Send signal to

switch pump off

Send signal to

switch pump on

V Figure3.5Flowchartdescribingtheprocessingcarriedoutbythemicroprocessorina

central heating system

Notice that the flowchart has been drawn assuming the pre-set (required)

temperature has already been input to the system. The flowchart does not take

into account whether the valves are already open/closed or whether the pump is

already on/off. This will be addressed in Chapter 4.

3 MONITORING AND CONTROL

Air-conditioning systems

Like central-heating systems, air-conditioning systems and microprocessor

control were discussed in Chapter 1 in the section on real-time processing.

The steps involved were outlined as shown below.

1 User enters required temperature using keypad/

touchscreen

2 Microprocessor stores required temperature as a

pre-set value

3 Microprocessor receives temperature from sensor

4 Microprocessor compares temperature from sensor to

pre-set value

5 If temperature from the sensor is higher than the

pre-set value, the microprocessor sends a signal to

an actuator to switch on the fans

6 If temperature is lower than, or equal to, the pre-

set value, the microprocessor sends a signal to

switch the fans off

This sequence is repeated until the system is switched off

It is important to be aware that the algorithms which have been written up to this

stage are fairly limited. We have not taken into account that when a signal is sent to

switch on a device such as a heater, light source, valve or fan, the device may already be

switched on. Similarly, we have not considered the fact that if a signal is sent to switch

off a device, it may already be switched off. This will be dealt with in Chapter 4.

Activity 3b

Draw a flowchart for an air-conditioning system which could describe the

processing carried out by the microprocessor, assuming the pre-set (required)

temperature has already been input to the system.

Burglar alarms

Microprocessor-controlled burglar alarm systems are used in many houses to

protect against intruders. The sensors needed in such a system are:

» infrared sensors to detect movement of human bodies, which emit heat

» sound sensors to detect the level of sound an intruder might make

» pressure sensors that are placed under a carpet or rug to detect an increase in

weight caused by a burglar treading on it.

The microprocessor is programmed to have certain acceptable levels and it only

acts if the sensor readings are greater than these. An ADC is required so that

data from the sensors can be understood by the microprocessor. In the event of

detecting an intruder, the burglar alarm sounds an alarm and causes lights to

flash and probably also sends a signal to the police to alert them to the presence

of a possible intruder.

Of course, at the start of the process the user would need to switch the system on!

Activity 3c

Write out all the steps in the operation of a burglar alarm system, from the

moment that the system is activated.

3.2 Control technologies

Control of traffic/pedestrian flow

Apart from traffic lights, the main use of computer control regarding managing

traffic flow is found on motorways (known as freeways or expressways in some

countries). Many countries around the world are introducing smart motorways,

but this is particularly the case in the UK, Australia and New Zealand. A smart

motorway is a section of motorway that uses

active traffic management (ATM)

techniques. The system involves the use of variable speed limits and/or being

able to drive on the hard shoulder (‘shoulder’ in the USA and elsewhere; this is

the area at the side of a motorway or other road where you are allowed to stop if

your car breaks down) at certain times. Computers constantly monitor the road

and can change the speed limit or open the hard shoulder to traffic. Normally,

traffic is not allowed on the hard shoulder and a red cross is displayed above it (see

Figure 3.6). If the traffic becomes congested, the computer opens up the hard

shoulder by removing the red cross (Figure 3.7).

V Figure3.6Aredcrossshowsthat

the hard shoulder should not be used

V Figure3.7Theredcrossisremovedandsotrafficcan

use the hard shoulder

There are two main types of devices computers use to monitor the volume of

traffic. One is rather like an induction loop (which will be explained in the next

section) and is positioned just beneath the road surface. There are several of

these sited at intervals of about 500 metres. The other uses a ‘side-fire radar’,

which involves the firing of a radar beam at an angle to the roadside. The device

measures the speed of the vehicles it is pointed at by detecting a change in

frequency of the radar signal when it is reflected back off an oncoming vehicle.

This has advantages over the induction loop system because it is not susceptible

to damage caused by potholes or erosion in the road.

All this data is constantly fed back to computers, which then process the data

and decide what action, if any, needs to be taken; if the road becomes very busy,

the system can automatically lower the speed limit or open the hard shoulder to

traffic.

The computer can reduce the speed limit from 70 miles per hour to, for

example, 50 mph. This slows traffic down and reduces the chance of traffic

congestion. The decision made by the computer will often remain free of human

intervention unless the decision appears to be unusual. However, computers are

not programmed to react to a crash and are not allowed to close lanes. Figure 3.8

shows the result of action taken by people in a control room which makes use of

CCTV to monitor traffic.

3 MONITORING AND CONTROL

V Figure 3.8 Drivers are alerted to an accident on the motorway ahead

Traffic lights have been used to manage pedestrian flow for many years. The use

of computers to control traffic lights has been the case since the 1990s. Their

use will be explained in much more detail in the section on traffic lights.

Car-park barriers

One of the most common ways to detect vehicles in a microprocessor-controlled

car-park barrier system is by using an induction (sometimes called inductive) loop

buried just below the surface of the road in front of the barrier. As a vehicle passes

over the loop, it causes a change in inductance which is detected by the loop. The

metal in the vehicle causes a change in the magnetic field. This in turn causes a

current to flow. The loop sends back data which is converted to digital and if the

computer detects any change compared to a pre-set value, it sends a signal to the

actuator. In this case, the actuator is the motor which, when activated, causes the

barrier to rise. There is usually a second sensor, often a light sensor, which is used

to detect when the vehicle has passed beyond the barrier. A light beam passes

across the space occupied by the vehicle. If the vehicle prevents the light beam

from reaching the sensor, then the microprocessor will keep the barrier raised.

When the vehicle is clear of the barrier, the microprocessor detects that the light

beam has resumed and so can send a signal to the motor to retract and allow the

barrier to lower. This makes sure the barrier stays up until the vehicle has passed

through the beam.

Barrier arm

Barrier rest

Induction

loop

Barrier housing

Barrier

arm

ous

ing

Light sensor

Light beam

source

V Figure3.9Car-parkbarrierusinganinductionloopsystem

3.2 Control technologies

Activity 3d

Write out the steps in the operation of a car-park barrier system, putting each

distinct step on a separate line.

Some car-park barrier systems issue a ticket on entry which contains a barcode.

When the driver wishes to leave the car park, he or she enters the ticket into a

machine which has a barcode reader, and then pays for the time parked. The

barcode reader feeds the information to a computer and registers that the

barcode belongs to a driver that has paid. As the vehicle leaves the car park, the

driver inserts the ticket into a machine at the barrier which contains another

barcode reader. The computer checks that the barcode matches the list of those

barcodes belonging to drivers that have paid and sends a signal to the motor to

lift the barrier.

Some systems use a magnetic stripe instead of a barcode, but the process is the

same. Some systems use automatic number plate recognition (ANPR) instead of

a ticket with a barcode. The customer types in details of their car’s number plate

at the ticket machine before paying. As the car approaches the exit barrier, a

camera sends the details of the number plate to the computer which uses optical

character recognition (OCR) to read the number plate. If the number plate

matches a plate which is recorded as having paid, it sends a signal to the motor

to lift the barrier.

Ultrasonic sensors are sometimes used in car-park barrier systems instead of

light sensors to prevent the barrier descending if the car has not moved beyond

the barrier.

Traffic lights

There are two types of traffic light systems. One is called a fixed-time traffic

light control system, which is controlled mechanically and turns the traffic

lights green or red after a certain amount of time. This is usually about one

minute, though some systems have less time and others have more. With this

system, drivers often get very annoyed when they see that there is no traffic

going across them, yet their light stays red. A system which can respond to the

volume of traffic using the road is much more preferable and, to this end, many

traffic lights at road intersections are now controlled by computers.

The computer is programmed to react to different volumes of traffic during the

day and often uses the same method as in car-park barriers (induction loops)

to detect these. The computer receives data from the induction loops by way

of the ADC and counts the number of vehicles travelling in each direction.

These counts are then compared with pre-set values and the computer changes

the traffic light timings/sequences as required by sending signals back to the

control box in the traffic lights, which then operates the new sequence or

timings. The whole process is continuous.

So, for example if a line of cars is coming from one direction and none from

the other, the computer will decide to keep the light on red for the road

which has no traffic. When a sufficient number of vehicles have stopped at

the red light, the computer will cause the red light to turn green and the

other one red.

3 MONITORING AND CONTROL

V Figure3.10Acomputer-controlledtrafficlightjunction

These systems sometimes need to allow for pedestrians crossing at the junction.

If this is the case, the pedestrian presses a button at the side of the road. The

computer registers this input and after a predefined delay sends a signal to the

actuator to change the traffic lights from green to red, in addition to lighting up

the sign telling the pedestrians they can cross (usually a green man). There can

be a reasonable delay if the lights have only recently changed, but most modern

systems, using induction loops, can tell if traffic is light and thereby reduce the

time delay.

Wireless sensor and actuator networks

A wireless sensor and actuator network (WSAN), as the name suggests,

is a networked group of sensors and actuators that communicate wirelessly.

They are sometimes called ‘wireless sensor and actor networks’, because the

actuators might be grouped together to perform a collaborative action and

are thus referred to as an actor. For example, a robot is classified as an actor

because it contains several actuators in one unit. An actor works as if it has a

microprocessor included within it because it is capable of making decisions.

Other examples of WSANs are smart parking systems, which alert drivers to

where there are car parking spaces available, and combined heating, ventilation

and air-conditioning (HVAC) systems. Networks such as wireless sensor

networks (WSN) are slightly different, in that they take no action but merely

monitor the environment. Examples are wearable computer devices used to

monitor an athlete’s performance, and air and water pollution monitoring where

these are performed remotely from the computer without the use of cabled

connections.

Smart homes

A smart home is a home in which devices and appliances are connected so that

they can communicate with each other and can be controlled using commands

by anyone living in that home. A smart home makes use of the home computer

network and router and is an example of a WSAN. The commands can be given

by voice, remote control, tablet or smartphone. The most common devices

controlled in this way are televisions, music centres, lights, burglar alarms,

central heating and air-conditioning units. Smart homes have developed due

to the increase in the use of smartphones and tablet computers. These are

3.2 Control technologies

continuously connected to the internet and can thus be used to control any

number of online devices.

The ability to control such devices remotely – it is possible to switch on the

heating while the user is in an office miles away, for example – is called the

Internet of Things (IoT). IoT is a term used to describe the remote control of

appliances and devices that are interconnected through digital networks and

includes refrigerators, cookers and others. One of the benefits of a smart home

is preventing the possible panic which can occur when a homeowner is on their

way to work. Did they lock the front door? Was the cooker switched off? Was

the burglar alarm switched on? A smart home or IoT allows the homeowner

to do all these things and more, remotely. For example, when still in their

office at work, the homeowner can switch the oven on so that dinner will be

ready as soon as they walk in the door; the central heating or air-conditioning

unit can be switched on and the temperatureset so that the house will be nice

and comfortable for when they return home. However, there is at least one

drawback to having a smart home and that is being vulnerable to hackers, who

could access a home network and turn off the burglar alarm, making it easy for

someone to break in, or they couldjust cause a nuisance by turning lights off,

changing channels on the television, and so on.

Advantages and disadvantages of using control technology

The use of computer-controlled car-park barriers instead of human parking

attendants, for example, has increased unemployment but, on the other hand, IT

technicians are needed to maintain the computers, and programmers are required

to program the systems, so increasing employment. However, the number of new

jobs created tends to be far fewer than the number of old jobs lost.

Microprocessor-controlled burglar alarms can give people a greater sense of

security as they feel free from the risk of being burgled.

Smart homes can reduce the amount of energy needed to heat and provide

light within a home. But they can lead to people becoming lazy since they can

become over-reliant on microprocessor-controlled devices in the home. They

have also caused a loss of manual household skills and prevent people from

performing simple exercise such as walking around or using their hands and

arms as much as they used to.

The use of computer-controlled traffic lights has led to there being fewer traffic

jams than when they are mechanically or time controlled.

Air-conditioning units in shopping malls tend to make shopping a more

comfortable experience when the weather is warm but can lead to increased

costs for shops, which in turn leads to an increase in prices for the consumer.

Most control systems, generally, can help people with disabilities who may

find it difficult to get around and use devices in the home. The use of smart-

home technology can also lead to savings in household costs such as electricity

as it tends to ensure that more economic use is made of power, switching off

appliances automatically when not required. However, it may cost a lot of money

to buy the system in the first place. Smart devices used in a smart home are

much more expensive than non-smart devices.

Computer-controlled systems process data more quickly than a human could

which, in turn, leads to almost immediate reactions to changes in the inputs to

the system. A computer-controlled system will not be able to function if there

is a problem with the computer or there is a power outage without backup

power supplies.

3 MONITORING AND CONTROL

The use of such systems does leave people free to do other things such as

pursuing leisure activities since they do not have to be in the house to do the

cooking or washing clothes or dishes.

With a computer-controlled greenhouse it is possible that a human could forget

to take readings or be so busy that they are unable to take readings, whereas a

computer can take readings at regular intervals. This means it can take action,

almost immediately, whereas human action might be delayed if they forget

to take a reading. Computers can monitor more than one variable at a time,

whereas a human would have to spend more time taking the readings since

they would have to do them one at a time. The readings taken by computers

are more reliable and more accurate than human readings as humans can make

mistakes. Computers can carry out readings more frequently so in a greenhouse,

any lack of water in the soil, for example, can be dealt with far sooner than if a

human was taking readings every couple of hours, say. In addition, readings can

be taken and control can be carried out at any time, such as at night or when

people go on holiday.

Examination-style questions

1 A school has a computerised weather station which monitors a number of

atmospheric conditions or variables. Describe how data is collected and

processed by the computer. [6]

Give three advantages of using computers rather than humans to monitor

soil quality.

[3]

Give three disadvantages of using computers in a smart home. [3]

Draw a flowchart to represent the computer control of a car-park barrier. [6]