sensor tutorial

7/22/2019 Sensor Tutorial

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Tutorial: 2 of 9

Position Sensors

In this tutorial we will look at a variety of devices which are classed as Input Devicesand are

therefore called "Sensors" and in particular those sensors which are Positionalin nature which

means that they are referenced either to or from some fixed point or position. As their nameimplies, these types of sensors provide a "position" feedback.

One method of determining a position, is to use either "distance", which could be the distance

between two points such as the distance travelled or moved away from some fixed point, or by

"rotation" (angular movement). For example, the rotation of a robots wheel to determine itsdistance travelled along the ground. Either way, Position Sensorscan detect the movement of an

object in a straight line usingLinear Sensorsor by its angular movement using RotationalSensors.

The Potentiometer.

The most commonly used of all the "Position Sensors", is thepotentiometer because it is an

inexpensive and easy to use position sensor. It has a wiper contact linked to a mechanical shaft

that can be either angular (rotational) or linear (slider type) in its movement, and which causesthe resistance value between the wiper/slider and the two end connections to change giving an

electrical signal output that has a proportional relationship between the actual wiper position on

the resistive track and its resistance value. In other words, resistance is proportional to position.

Potentiometer

Potentiometers come in a wide range of designs and sizes such as the commonly available round

rotational type or the longer and flat linear slider types. When used as a positional sensor themoveable object is connected directly to the shaft or slider of the potentiometer and a DC

reference voltage is applied across the two outer fixed connections forming the resistive element.

The output voltage signal is taken from the wiper terminal of the sliding contact as shown below.

this configuration produces a potential or voltage divider type circuit output which is

proportional to the shaft position. Then for example, if you apply a voltage of say 10v across theresistive element of the potentiometer the maximum output voltage would be equal to the supply

voltage at 10 volts, with the minimum output voltage equal to 0 volts. Then the potentiometerwiper will vary the output signal from 0 to 10 volts, with 5 volts indicating that the wiper orslider is at its half-way or centre position.

Potentiometer Construction


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The output signal (Vout) from the potentiometer is taken from the centre wiper connection as itmoves along the resistive track, and is proportional to the angular position of the shaft.

Example of a simple Positional Sensing Circuit

While resistive potentiometer position sensors have many advantages: low cost, low tech, easy to

use etc, as a position sensor they also have many disadvantages: wear due to moving parts, lowaccuracy, low repeatability, and limited frequency response.

But there is one main disadvantage of using the potentiometer as a positional sensor. The rangeof movement of its wiper or slider (and hence the output signal obtained) is limited to thephysical size of the potentiometer being used. For example a single turn rotational potentiometer

generally only has a fixed electrical rotation between about 240 to 330ohowever, multi-turn pots

of up to 3600oof electrical rotation are also available. Most types of potentiometers use carbon

film for their resistive track, but these types are electrically noisy (the crackle on a radio volume

control), and also have a short mechanical life.

Wire-wound pots also known as rheostats, in the form of either a straight wire or wound coil

resistive wire can also be used, but wire wound pots suffer from resolution problems as their

wiper jumps from one wire segment to the next producing a logarithmic (LOG) output resultingin errors in the output signal. These too suffer from electrical noise.


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The number of transparent and dark segments or slots on the disk determines the resolution of

the device and increasing the number of lines in the pattern increases the resolution per degree of

rotation. Typical encoded discs have a resolution of up to 256 pulses or 8-bits per rotation.

The simplest incremental encoder is called a tachometer. It has one single square wave output

and is often used in unidirectional applications where basic position or speed information only isrequired. The "Quadrature" or "Sine wave" encoder is the more common and has two output

square waves commonly called channel Aand channel B. This device uses two photo detectors,

slightly offset from each other by 90o thereby producing two separate sine and cosine output

signals.

Simple Incremental Encoder

By using theArc Tangentmathematical function the angle of the shaft in radians can be

calculated. Generally, the optical disk used in rotary position encoders is circular, then theresolution of the output will be given as: = 360/n, where nequals the number of segments oncoded disk. Then for example, the number of segments required to give an incremental encoder a

resolution of 1o will be: 1

o= 360/n, therefore, n = 360 windows, etc. Also the direction of

rotation is determined by noting which channel produces an output first, either channel A orchannel B giving two directions of rotation, A leads B or B leads A. This arrangement is shown

below.

Incremental Encoder Output


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One main disadvantage of incremental encoders when used as a position sensor, is that they

require external counters to determine the absolute angle of the shaft within a given rotation. Ifthe power is momentarily shut off, or if the encoder misses a pulse due to noise or a dirty disc,the resulting angular information will produce an error. One way of overcoming this

disadvantage is to use absolute position encoders.

Absolute Position Encoder

Absolute Position Encodersare more complex than quadrature encoders. They provide a

unique output code for every single position of rotation indicating both position and direction.

Their coded disk consists of multiple concentric "tracks" of light and dark segments. Each track

is independent with its own photo detector to simultaneously read a unique coded position value

for each angle of movement. The number of tracks on the disk corresponds to the binary "bit"-resolution of the encoder so a 12-bit absolute encoder would have 12 tracks and the same coded

value only appears once per revolution.


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4-bit Binary Coded Disc

One main advantage of an absolute encoder is its non-volatile memory which retains the

exact position of the encoder without the need to return to a "home" position if the power fails.Most rotary encoders are defined as "single-turn" devices, but absolute multi-turn devices are

available, which obtain feedback over several revolutions by adding extra code disks.

Typical application of absolute position encoders are in computer hard drives and CD/DVD

drives were the absolute position of the drives read/write heads are monitored or inprinters/plotters to accurately position the printing heads over the paper.

In this tutorial about Position Sensors, we have looked at several examples of sensors that can

be used to measure the position or presence of objects. In the next tutorial we will look at sensorsthat are used to measure temperature such as thermistors, thermostats and thermocouples.

Tutorial: 3 of 9

Temperature Sensor Types

The most commonly used type of all the sensors are those which detect Temperatureor heat.These types of temperature sensor vary from simple ON/OFF thermostatic devices which controla domestic hot water system to highly sensitive semiconductor types that can control complex

process control plants.

We remember from our school science classes that the movement of molecules and atoms

produces heat (kinetic energy) and the greater the movement, the more heat that is

generated. Temperature Sensorsmeasure the amount of heat energy or even coldness that isgenerated by an object or system, allowing us to "sense" or detect any physical change to thattemperature producing either an analogue or digital output.

There are many different types of Temperature Sensoravailable and all have different

characteristics depending upon their actual application. Temperature sensors consist of two basic

physical types:

Contact Temperature Sensor Types - These types of temperature sensor are required to be in

physical contact with the object being sensed and use conduction to monitor changes in

temperature. They can be used to detect solids, liquids or gases over a wide range of

temperatures.

Non-contact Temperature Sensor Types- These types of temperature sensor use convection andradiation to monitor changes in temperature. They can be used to detect liquids and gases that

emit radiant energy as heat rises and cold settles to the bottom in convection currents or detect

the radiant energy being transmitted from an object in the form of infra-red radiation (the sun).


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Thermocouple Sensor Colour Codes

Extension and Compensating Leads

Code

TypeConductors (+/-) Sensitivity

British

BS 1843:1952

E Nickel Chromium /Constantan

-200 to 900oC

J Iron / Constantan 0 to 750oC

KNickel Chromium /

Nickel Aluminium-200 to 1250

oC

N Nicrosil / Nisil 0 to 1250oC

T Copper / Constantan -200 to 350oC

U

Copper / Copper Nickel

Compensating for

"S" and "R"

0 to 1450oC

The three most common thermocouple materials used above for general temperature

measurement areIron-Constantan(Type J), Copper-Constantan(Type T), andNickel-

Chromium(Type K). The output voltage from a thermocouple is very small, only a fewmillivolts (mV) for a 10

oC change in temperature difference and because of this small voltage

output some form of amplification is generally required.

Thermocouple Amplification

The type of amplifier, either discrete or in the form of anOperational Amplifierneeds to becarefully selected, because good drift stability is required to prevent recalibration of the
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thermocouple at frequent intervals. This makes the chopper and instrumentation type of amplifier

preferable for most temperature sensing applications.

Other types of Temperature Sensornot mentioned here include, Semiconductor Junction

Sensors, Infra-red and Thermal Radiation Sensors, Medical type Thermometers, Indicators and

Colour Changing Inks or Dyes.

In this tutorial about Temperature Sensor Types, we have looked at several examples of

sensors that can be used to measure changes in temperature. In the next tutorial we will look atsensors that are used to measure light quantity, such as Photodiodes, Phototransistors,

Photovoltaic Cells and the Light Dependant Resistor.

Light SensorsTutorial: 4 of 9

A Light Sensor generates an output signal indicating the intensity of light by measuring theradiant energy that exists in a very narrow range of frequencies basically called "light", and

which ranges in frequency from "Infrared" to "Visible" up to "Ultraviolet" light spectrum. Thelight sensor is a passive devices that convert this "light energy" whether visible or in the infrared

parts of the spectrum into an electrical signal output. Light sensors are more commonly known as

"Photoelectric Devices" or "Photo Sensors" because the convert light energy (photons) intoelectricity (electrons).

Photoelectric devices can be grouped into two main categories, those which generate electricity

when illuminated, such asPhoto-voltaicsorPhoto-emissives etc, and those which change theirelectrical properties in some way such asPhoto-resistorsorPhoto-conductors. This leads to the

following classification of devices.

Photo-emissive Cells- These are photodevices which release free electrons from a light

sensitive material such as caesium when struck by a photon of sufficient energy. The amount ofenergy the photons have depends on the frequency of the light and the higher the frequency, the

more energy the photons have converting light energy into electrical energy.

Photo-conductive Cells- These photodevices vary their electrical resistance when subjected tolight. Photoconductivity results from light hitting a semiconductor material which controls the

current flow through it. Thus, more light increase the current for a given applied voltage. The

most common photoconductive material is Cadmium Sulphide used in LDR photocells.

Photo-voltaic Cells - These photodevices generate an emf in proportion to the radiant light

energy received and is similar in effect to photoconductivity. Light energy falls on to two

semiconductor materials sandwiched together creating a voltage of approximately 0.5V. Themost common photovoltaic material is Selenium used in solar cells.

Photo-junction Devices - These photodevices are mainly true semiconductor devices such asthe photodiode or phototransistor which use light to control the flow of electrons and holes


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The most commonly used photoresistive light sensor is the ORP12Cadmium Sulphide

photoconductive cell. This light dependent resistor has a spectral response of about 610nm in the

yellow to orange region of light. The resistance of the cell when unilluminated (dark resistance)is very high at about 10M's which falls to about 100's when fully illuminated (lit resistance).

To increase the dark resistance and therefore reduce the dark current, the resistive path forms azigzag pattern across the ceramic substrate. The CdS photocell is a very low cost device often

used in auto dimming, darkness or twilight detection for turning the street lights "ON" and"OFF", and for photographic exposure meter type applications.

Connecting a light dependant resistor in series with a standard resistor like this across a single

DC supply voltage has one major advantage, a different voltage will appear at their junction for

different levels of light.


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In this basic dark sensing circuit, the light dependent resistor LDR1and the

potentiometer VR1form one adjustable arm of a simple resistance bridge network, also known

commonly as a Wheatstone bridge, while the two fixed resistors R1and R2form the other arm.

Both sides of the bridge form potential divider networks across the supply voltage whoseoutputs V1and V2are connected to the non-inverting and inverting voltage inputs respectively

of the operational amplifier.

The operational amplifier is configured as aDifferential Amplifieralso known as a voltage

comparator with feedback whose output voltage condition is determined by the differencebetween the two input signals or voltages, V1and V2. The resistor combination R1and R2form

a fixed voltage reference at input V2, set by the ratio of the two resistors. The LDR -

VR1combination provides a variable voltage input V1proportional to the light level being

detected by the photoresistor.

As with the previous circuit the output from the operational amplifier is used to control a relay,which is protected by a free wheel diode, D1. When the light level sensed by the LDR and its

output voltage falls below the reference voltage set at V2the output from the op-amp changes

state activating the relay and switching the connected load. Likewise as the light level increases

the output will switch back turning "OFF" the relay. The hysteresis of the two switching points isset by the feedback resistor Rfcan be chosen to give any suitable voltage gain of the amplifier.

The operation of this type of light sensor circuit can also be reversed to switch the relay "ON"when the light level exceeds the reference voltage level and vice versa by reversing the positions

of the light sensor LDRand the potentiometer VR1. The potentiometer can be used to "pre-set"

the switching point of the differential amplifier to any particular light level making it ideal as a

simple light sensor project circuit.

Photojunction Devices

Photojunction Devicesare basicallyPN-Junctionlight sensors or detectors made from silicon

semiconductor PN-junctions which are sensitive to light and which can detect both visible lightand infrared light levels. Photo-junction devices are specifically made for sensing light and this

class of photoelectric light sensors include thePhotodiodeand thePhototransistor.
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Photo darlington devices consist of a normal phototransistor whose emitter output is coupled to

the base of a larger bipolar NPN transistor. Because a darlington transistor configuration gives a

current gain equal to a product of the current gains of two individual transistors, aphotodarlington device produces a very sensitive detector.

Typical applications of Phototransistorslight sensors are in opto-isolators, slotted optoswitches, light beam sensors, fibre optics and TV type remote controls, etc. Infrared filters are

sometimes required when detecting visible light.

Another type of photojunction semiconductor light sensor worth a mention is the Photo-

thyristor. This is a light activated thyristor or Silicon Controlled Rectifier, SCR that can be

used as a light activated switch in AC applications. However their sensitivity is usually very lowcompared to equivalent photodiodes or phototransistors. To increase their sensitivity to light

photo-thyristors are made thinner around the gate junction. The downside to this process is that it

limits the amount of anode current that they can switch. Then for higher current AC applications

they are used as pilot devices in opto-couplers to switch larger more conventional thyristors.

Photovoltaic Cells.

The most common type of photovoltaic light sensor is the Solar Cell. Solar cells convert light

energy directly into DC electrical energy in the form of a voltage or current to a resistive loadsuch as a light, battery or motor. Then photovoltaic cells are similar to a battery because theysupply DC power. Unlike the other photo devices above which use light intensity even from a

torch to operate, photvoltaic cells work best using the suns radiant energy.

Solar cells are used in many different types of applications to offer an alternative power source

from conventional batteries, such as in calculators, satellites and now in homes offering a form

of renewable power.

Photovoltaic Cell

Photovoltaic cellsare made from single crystal silicon PN junctions, the same as photodiodes

with a very large light sensitive region but are used without the reverse bias. They have the same

characteristics as a very large photodiode when in the dark. When illuminated the light energy

causes electrons to flow through the PN junction and an individual solar cell can generate an

open circuit voltage of about 0.58v (580mV). Solar cells have a "Positive" and a "Negative" sidejust like a battery.

Individual solar cells can be connected together in series to form solar panels which increases the

output voltage or connected together in parallel to increase the available current. Commercially

available solar panels are rated in Watts, which is the product of the output voltage and current(Volts times Amps) when fully lit.


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For example, a relay is a binary actuator as it has two stable states, either energised and latched

or de-energised and unlatched, while a motor is a continuous actuator because it can rotate

through a full 360omotion. The most common types of actuators or output devices are Electrical

Relays, Lights,Motorsand Loudspeakersand in this tutorial we will look at electrical relays,

also called electromechanical relays and solid state relays or SSR's.

The Electromechanical Relay

The term Relaygenerally refers to a device that provides an electrical connection between twoor more points in response to the application of a control signal. The most common and widely

used type of electrical relay is the electromechanical relay or EMR.

Electrical Relay

The most fundamental control of any equipment is the ability to turn it "ON" and "OFF". The

easiest way to do this is using switches to interrupt the electrical supply. Although switches can

be used to control something, they have their disadvantages. The biggest one is that they have tobe manually (physically) turned "ON" or "OFF". Also, they are relatively large, slow and only

switch small electrical currents.

Electrical Relayshowever, are basically electrically operated switches that come in many

shapes, sizes and power ratings suitable for all types of applications. Relays can also have single

or multiple contacts with the larger power relays used for high voltage or current switching beingcalled "contactors".

In this tutorial about electrical relays we are just concerned with the fundamental operatingprinciples of "light duty" electromechanical relays we can use in motor control or robotic

circuits. Such relays are used in general electrical and electronic control or switching circuits

either mounted directly onto PCB boards or connected free standing and in which the loadcurrents are normally fractions of an ampere up to 20+ amperes.

As their name implies, electromechanical relays are electro-magneticdevices that convert a

magnetic flux generated by the application of a low voltage electrical control signal either AC orDC across the relay terminals, into a pulling mechanical force which operates the electrical

contacts within the relay. The most common form of electromechanical relay consist of an

energizing coil called the "primary circuit" wound around a permeable iron core.


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This iron core has both a fixed portion called the yoke, and a moveable spring loaded part called

the armature, that completes the magnetic field circuit by closing the air gap between the fixed

electrical coil and the moveable armature. The armature is hinged or pivoted allowing it to freelymove within the generated magnetic field closing the electrical contacts that are attached to it.

Connected between the yoke and armature is normally a spring (or springs) for the return stroke

to "reset" the contacts back to their initial rest position when the relay coil is in the "de-energized" condition, ie. turned "OFF".

Electromechanical Relay Construction

In our simple relay above, we have two sets of electrically conductive contacts. Relays may be

"Normally Open", or "Normally Closed". One pair of contacts are classed as Normally Open,

(NO)or make contacts and another set which are classed as Normally Closed, (NC)or break

contacts. In the normally open position, the contacts are closed only when the field current is

"ON" and the switch contacts are pulled towards the inductive coil.

In the normally closed position, the contacts are permanently closed when the field current is

"OFF" as the switch contacts return to their normal position. These termsNormally Open,Normally ClosedorMake and Break Contactsrefer to the state of the electrical contacts when the

relay coil is "de-energized", i.e, no supply voltage connected to the inductive coil. An example of

this arrangement is given below.


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Contact Tip Materials

Contact Tip

MaterialCharacteristics

Ag

(fine silver)

Electrical and thermal conductivity are the highest of allmetals, exhibits low contact resistance, is inexpensive and

widely used.

Contacts tarnish through sulphur influence.

AgCu

(silver copper)

"Hard silver", better wear resistance and less tendency to

weld, but slightly higher contact resistance.

AgCdO

(silver cadmium oxide)

Very little tendency to weld, good wear resistance and arc

extinguishing properties.

AgW(silver tungsten)

Hardness and melting point are high, arc resistance is

excellent.

Not a precious metal.High contact pressure is required.

Contact resistance is relatively high, and resistance to

corrosion is poor.

AgNi

(silver nickel)

Equals the electrical conductivity of silver, excellent arc

resistance.

AgPd

(silver palladium)

Low contact wear, greater hardness.

Expensive.

platinum, gold and

silver alloys

Excellent corrosion resistance, used mainly for low-current

circuits.

Relay manufacturers data sheets give maximum contact ratings for resistive DC loads only and

this rating is greatly reduced for either AC loads or highly inductive or capacitive loads. In order

to achieve long life and high reliability when switching AC currents with inductive or capacitiveloads some form of arc suppression or filtering is required across the relay contacts.

Extending the life of relay tips by reducing the amount of arcing generated as they open isachieved by connecting a Resistor-Capacitor network called an RC Snubber

Network electrically in parallel with the contact tips. The voltage peak, which occurs at theinstant the contacts open, will be safely short circuited by the RC network, thus suppressing any

arc generated at the contact tips. For example.

Relay Snubber Circuit


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Where:

Cis the Common terminal

NOis the Normally Open contact

NCis the Normally Closed contact

One final point to remember, it is not advisable to connect relay contacts in parallel to handlehigher load currents. For example, never attempt to supply a 10A load with two relays in parallel

that have 5A contact ratings each as the relay contacts never close or open at exactly the sameinstant of time, so one relay contact is always overloaded.

While relays can be used to allow low power electronic or computer type circuits to switch a

relatively high currents or voltages both "ON" or "OFF". Never mix different load voltagesthrough adjacent contacts within the same relay such as for example, high voltage AC (240v) and

low voltage DC (12v), always use separate relays for safety.

One of the more important parts of any relay is the coil. This converts electrical current into an

electromagnetic flux which is used to operate the relays contacts. The main problem with relay

coils is that they are "highly inductive loads" as they are made from coils of wire. Any coil of

wire has an impedance value made up of resistance ( R ) and inductance ( L) in series (RLSeries Circuit).

As the current flows through the coil a self induced magnetic field is generated around it. When

the current in the coil is turned "OFF", a large back emf (electromotive force) voltage is

produced as the magnetic flux collapses within the coil (transformer theory). This inducedreverse voltage value may be very high in comparison to the switching voltage, and may damage

any semiconductor device such as a transistor, FET or microcontroller used to operate the relay

coil.
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To overcome these disadvantages of the electrical relay, another type of relay called a Solid

State Relayor (SSR) for short was developed which is a solid state contactless, pure electronic

relay. It has no moving parts with the contacts being replaced by transistors, thyristors or triacs.The electrical separation between the input control signal and the output load voltage is

accomplished with the aid of an opto-coupler typeLight Sensor.

The Solid State Relayprovides a high degree of reliability, long life and reduced

electromagnetic interference (EMI), (no arcing contacts or magnetic fields), together with a

much faster almost instant response time, as compared to the conventional electromechanicalrelay. Also the input control power requirements of the solid state relay are generally low enough

to make them compatible with most IC logic families without the need for additional buffers,

drivers or amplifiers. However, being a semiconductor device they must be mounted onto

suitable heatsinks to prevent the output switching semiconductor device from over heating.

Solid State Relay

The AC type Solid State Relay turns "ON" at the zero crossing point of the AC sinusoidal

waveform, prevents high inrush currents when switching inductive or capacitive loads while theinherent turn "OFF" feature of Thyristors and Triacs provides an improvement over the arcing

contacts of the electromechanical relays.

Like the electromechanical relays, a Resistor-Capacitor (RC) snubber network is generally

required across the output terminals of the SSR to protect the semiconductor output switching

device from noise and voltage transient spikes when used to switch highly inductive or

capacitive loads. In most modern SSR's this RC snubber network is built as standard into therelay itself reducing the need for additional external components.

Non-zero crossing detection switching (instant "ON") type SSR's are also available for phasecontrolled applications such as the dimming or fading of lights at concerts, shows, disco lighting

etc, or for motor speed control type applications.
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especially one that converts a small electrical signal into a corresponding physical movement

using electromagnetism.

Tutorial: 6 of 9

The Linear Solenoid

Another type of electromagnetic actuator that converts an electrical signal into a magnetic field is

called a Solenoid. The linear solenoid works on the same basic principal as theelectromechanical relay (EMR) seen in the previous tutorial and like relays, they can also be

controlled by transistors or MOSFET. A Linear Solenoidis an electromagnetic device that

converts electrical energy into a mechanical pushing or pulling force or motion.

Linear Solenoid

Solenoids basically consist of an electrical coil wound around a cylindrical tube with a ferro-magnetic actuator or "plunger" that is free to move or slide "IN" and "OUT" of the coils

body.Solenoidsare available in a variety of formats with the more common types being

the linear solenoidalso known as the linear electromechanical actuator (LEMA) and the rotary

solenoid.

Both types, linear and rotational are available as either a holding (continuously energised) or as alatching type (ON-OFF pulse) with the latching types being used in either energised or power-off

applications. Linear solenoids can also be designed for proportional motion control were the

plunger position is proportional to the power input.

When electrical current flows through a conductor it generates a magnetic field, and the direction

of this magnetic field with regards to its North and South Poles is determined by the direction ofthe current flow within the wire. This coil of wire becomes an "Electromagnet" with its own

north and south poles exactly the same as that for a permanent type magnet. The strength of this

magnetic field can be increased or decreased by either controlling the amount of current flowing

through the coil or by changing the number of turns or loops that the coil has. An example of an"Electromagnet" is given below.

Magnetic Field produced by a Coil


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When an electrical current is passed through the coils windings, it behaves like an electromagnetand the plunger, which is located inside the coil, is attracted towards the centre of the coil by the

magnetic flux setup within the coils body, which inturn compresses a small spring attached to

one end of the plunger. The force and speed of the plungers movement is determined by the

strength of the magnetic flux generated within the coil.

When the supply current is turned "OFF" (de-energised) the electromagnetic field generated

previously by the coil collapses and the energy stored in the compressed spring forces theplunger back out to its original rest position. This back and forth movement of the plunger is

known as the solenoids "Stroke", in other words the maximum distance the plunger can travel in

either an "IN" or an "OUT" direction, for example, 0 - 30mm.

Linear Solenoids

This type of solenoid is generally called a Linear Solenoiddue to the linear directional

movement of the plunger. Linear solenoids are available in two basic configurations called a

"Pull-type" as it pulls the connected load towards itself when energised, and the "Push-type" thatact in the opposite direction pushing it away from itself when energised. Both push and pull

types are generally constructed the same with the difference being in the location of the return

spring and design of the plunger.

Pull-type Linear Solenoid Construction


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Linear solenoids are useful in many applications that require an open or closed (in or out) type

motion such as electronically activated door locks, pneumatic or hydraulic control valves,robotics, automotive engine management, irrigation valves to water the garden and even the

"Ding-Dong" door bell has one. They are available as open frame, closed frame or sealed tubular

types.

Rotary Solenoids

Most electromagnetic solenoids are linear devices producing a linear back and forth force or

motion. However, rotational solenoids are also available which produce an angular or rotary

motion from a neutral position in either clockwise, anti-clockwise or in both directions (bi-directional).

Rotary Solenoid

Rotary solenoids can be used to replace small DC motors or stepper motors were the angularmovement is very small with the angle of rotation being the angle moved from the start to theend position. Commonly available rotary solenoids have movements of 25, 35, 45, 60 and 90

oas

well as multiple movements to and from a certain angle such as a 2-position self restoring or

return to zero rotation, for example 0-to-90-to-0o, 3-position self restoring, for example 0oto+45

oor 0

oto -45

oas well as 2-position latching.

Rotary solenoids produce a rotational movement when either energised, de-energised, or achange in the polarity of an electromagnetic field alters the position of a permanent magnet rotor.

Their construction consists of an electrical coil wound around a steel frame with a magnetic disk

connected to an output shaft positioned above the coil. When the coil is energised theelectromagnetic field generates multiple north and south poles which repel the adjacent

permanent magnetic poles of the disk causing it to rotate at an angle determined by the

mechanical construction of the rotary solenoid.

Rotary solenoids are used in vending or gaming machines, valve control, camera shutter withspecial high speed, low power or variable positioning solenoids with high force or torque areavailable such as those used in dot matrix printers, typewriters, automatic machines orautomotive applications etc.

Solenoid Switching


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AC Motorsare generally used in high power single or multi-phase industrial applications were a

constant rotational torque and speed is required to control large loads. In this tutorial on motors

we will look only at simple light duty DC Motorsand Stepper Motorswhich are used in manyelectronics, positional control, microprocessor, PIC and robotic circuits.

The DC Motor

The DC Motoror Direct Current Motor to give it its full title, is the most commonly used

actuator for producing continuous movement and whose speed of rotation can easily becontrolled, making them ideal for use in applications were speed control, servo type control,

and/or positioning is required. A DC motor consists of two parts, a "Stator" which is the

stationary part and a "Rotor" which is the rotating part. The result is that there are basically threetypes of DC Motor available.

Brushed Motor - This type of motor produces a magnetic field in a wound rotor (the part thatrotates) by passing an electrical current through a commutator and carbon brush assembly, hence

the term "Brushed". The stators (the stationary part) magnetic field is produced by using either awound stator field winding or by permanent magnets. Generally brushed DC motors are cheap,

small and easily controlled.

Brushless Motor- This type of motor produce a magnetic field in the rotor by using permanent

magnets attached to it and commutation is achieved electronically. They are generally smallerbut more expensive than conventional brushed type DC motors because they use "Hall effect"

switches in the stator to produce the required stator field rotational sequence but they have better

torque/speed characteristics, are more efficient and have a longer operating life than equivalent

brushed types.

Servo Motor- This type of motor is basically a brushed DC motor with some form of positionalfeedback control connected to the rotor shaft. They are connected to and controlled by a PWMtype controller and are mainly used in positional control systems and radio controlled models.

Normal DC motors have almost linear characteristics with their speed of rotation beingdetermined by the applied DC voltage and their output torque being determined by the current

flowing through the motor windings. The speed of rotation of any DC motor can be varied from

a few revolutions per minute (rpm) to many thousands of revolutions per minute making themsuitable for electronic, automotive or robotic applications. By connecting them to gearboxes or

gear-trains their output speed can be decreased while at the same time increasing the torque

output of the motor at a high speed.

The "Brushed" DC Motor

A conventional brushed DC Motor consist basically of two parts, the stationary body of the

motor called the Statorand the inner part which rotates producing the movement called

the Rotoror "Armature"for DC machines.


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The motors wound stator is an electromagnet circuit which consists of electrical coils connected

together in a circular configuration to produce the required North-pole then a South-pole then a

North-pole etc, type stationary magnetic field system for rotation, unlike AC machines whosestator field continually rotates with the applied frequency. The current which flows within these

field coils is known as the motor field current.

These electromagnetic coils which form the stator field can be electrically connected in series,

parallel or both together (compound) with the motors armature. A series wound DC motor has its

stator field windings connected inserieswith the armature. Likewise, a shunt wound DC motorhas its stator field windings connected inparallelwith the armature as shown.

Series and Shunt Connected DC Motor

The rotor or armature of a DC machine consists of current carrying conductors connected

together at one end to electrically isolated copper segments called the commutator. The

commutator allows an electrical connection to be made via carbon brushes (hence the name"Brushed" motor) to an external power supply as the armature rotates.

The magnetic field setup by the rotor tries to align itself with the stationary stator field causingthe rotor to rotate on its axis, but can not align itself due to commutation delays. The rotational

speed of the motor is dependent on the strength of the rotors magnetic field and the more voltage

that is applied to the motor the faster the rotor will rotate. By varying this applied DC voltage therotational speed of the motor can also be varied.

Conventional (Brushed) DC Motor


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Using Hall effect sensors, the polarity of the electromagnets is switched by the motor control

drive circuitry. Then the motor can be easily synchronized to a digital clock signal, providing

precise speed control. Brushless DC motors can be constructed to have, an external permanentmagnet rotor and an internal electromagnet stator or an internal permanent magnet rotor and an

external electromagnet stator.

Advantages of the Brushless DC Motorcompared to its "brushed" cousin is higher efficiencies,

high reliability, low electrical noise, good speed control and more importantly, no brushes or

commutator to wear out producing a much higher speed. However their disadvantage is that theyare more expensive and more complicated to control.

The DC Servo Motor

DC Servo motorsare used in closed loop type applications were the position of the output motor

shaft is fed back to the motor control circuit. Typical positional "Feedback" devices includeResolvers, Encoders and Potentiometers as used in radio control models such as airplanes and

boats etc. A servo motor generally includes a built-in gearbox for speed reduction and is capableof delivering high torques directly. The output shaft of a servo motor does not rotate freely as do

the shafts of DC motors because of the gearbox and feedback devices attached.

DC Servo Motor Block Diagram

A servo motor consists of a DC motor, reduction gearbox, positional feedback device and some

form of error correction. The speed or position is controlled in relation to a positional inputsignal or reference signal applied to the device.

RC Servo Motor


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The error detection amplifier looks at this input signal and compares it with the feedback signal

from the motors output shaft and determines if the motor output shaft is in an error condition

and, if so, the controller makes appropriate corrections either speeding up the motor or slowing itdown. This response to the positional feedback device means that the servo motor operates

within a "Closed Loop System".

As well as large industrial applications, servo motors are also used in small remote control

models and robotics, with most servo motors being able to rotate up to about 180 degrees in both

directions making them ideal for accurate angular positioning. However, these RC type servosare unable to continually rotate at high speed like conventional DC motors unless specially

modified.

A servo motor consist of several devices in one package, the motor, gearbox, feedback device

and error correction for controlling position, direction or speed. They are widley used in robotics

and models as they are easily controlled using just three wires,Power, Groundand Signal

Control.

DC Motor Switching and Control

Small DC motors can be switched "On" or "Off" by means of switches, relays, transistors or

mosfet circuits with the simplest form of motor control being "Linear" control. This type ofcircuit uses a bipolarTransistor as a Switch(A Darlington transistor may also be used were ahigher current rating is required) to control the motor from a single power supply.

By varying the amount of base current flowing into the transistor the speed of the motor can be

controlled for example, if the transistor is turned on "half way", then only half of the supply

voltage goes to the motor. If the transistor is turned "fully ON" (saturated), then all of the supply

voltage goes to the motor and it rotates faster. Then for this linear type of control, power isdelivered constantly to the motor as shown below.

Unipolar Transistor Switch

The simple switching circuit on the left, shows the circuit for a Uni-directional(one directiononly) motor control circuit. A continuous logic "1" or logic "0" is applied to the input of the

circuit to turn the motor "ON" (saturation) or "OFF" (cut-off) respectively.
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Input A Input B Motor Function

TR1 and TR4 TR2 and TR3

0 0 Motor Stopped (OFF)

1 0 Motor Rotates Forward

0 1 Motor Rotates Reverse

1 1 NOT ALLOWED

It is important that no other combination of inputs are allowed as this may cause the power

supply to be shorted out, ie both transistors, TR1and TR2switched "ON" at the same time, (fuse= bang!).

As with uni-directional DC motor control as seen above, the rotational speed of the motor canalso be controlled using Pulse Width Modulation or PWM. Then by combining H-bridge

switching with PWM control, both the direction and the speed of the motor can be accurately

controlled. Commercial off the shelf decoder IC's such as the SN754410 Quad Half H-Bridge ICor the L298N which has 2 H-Bridges are available with all the necessary control and safety logic

built in are specially designed for H-bridge bi-directional motor control circuits.

The Stepper Motor

Like the DC motor above, Stepper Motorsare also electromechanical actuators that convert apulsed digital input signal into a discrete (incremental) mechanical movement are used widely in

industrial control applications. A stepper motor is a type of synchronous brushless motor in that

it does not have an armature with a commutator and carbon brushes but has a rotor made up ofmany, some types have hundreds of permanent magnetic teeth and a stator with individual

windings.

Stepper Motor

As it name implies, a stepper motor does not rotate in a continuous fashion like a conventionalDC motor but moves in discrete "Steps" or "Increments", with the angle of each rotational

movement or step dependant upon the number of stator poles and rotor teeth the stepper motor

has.

Because of their discrete step operation, stepper motors can easily be rotated a finite fraction of arotation at a time, such as 1.8, 3.6, 7.5 degrees etc. So for example, lets assume that a stepper

motor completes one full revolution (360oin exactly 100 steps. Then the step angle for the motor


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magnetic rotor to align itself with that set of coils. By applying power to each set of coils in turn

the rotor can be made to rotate or "step" from one position to the next by an angle determined by

its step angle construction, and by energising the coils in sequence the rotor will produce a rotarymotion.

The stepper motor driver controls both the step angle and speed of the motor by energising thefield coils in a set sequence for example, "ADCB, ADCB, ADCB, A..." etc, the rotor will rotate

in one direction (forward) and by reversing the pulse sequence to "ABCD, ABCD, ABCD, A..."

etc, the rotor will rotate in the opposite direction (reverse).

So in our simple example above, the stepper motor has four coils, making it a 4-phase motor,

with the number of poles on the stator being eight (2 x 4) which are spaced at 45 degreeintervals. The number of teeth on the rotor is six which are spaced 60 degrees apart. Then there

are 24 (6 teeth x 4 coils) possible positions or "steps" for the rotor to complete one full

revolution. Therefore, the step angle above is given as: 360o/24 = 15

o.

Obviously, the more rotor teeth and or stator coils would result in more control and a finer stepangle. Also by connecting the electrical coils of the motor in different configurations, Full, Half

and micro-step angles are possible. However, to achieve micro-stepping, the stepper motor mustbe driven by a (quasi) sinusoidal current that is expensive to implement.

It is also possible to control the speed of rotation of a stepper motor by altering the time delaybetween the digital pulses applied to the coils (the frequency), the longer the delay the slower the

speed for one complete revolution. By applying a fixed number of pulses to the motor, the motor

shaft will rotate through a given angle and so there would be no need for any form of additionalfeedback because by counting the number of pulses given to the motor the final position of the

rotor will be exactly known. This response to a set number of digital input pulses allows the

stepper motor to operate in an "Open Loop System" making it both easier and cheaper to control.

For example, lets assume that our stepper motor above has a step angle of 3.6 degs per step. To

rotate the motor through an angle of say 216 degrees and then stop again at the require positionwould only need a total of: 216 degrees/(3.6 degs/step) = 80 pulsesapplied to the stator coils.

There are many stepper motor controller IC's available which can control the step speed, speed ofrotation and motors direction. One such controller IC is the SAA1027 which has all the

necessary counter and code conversion built-in, and can automatically drive the 4 fully

controlled bridge outputs to the motor in the correct sequence. The direction of rotation can also

be selected along with single step mode or continuous (stepless) rotation in the selecteddirection, but this puts some burden on the controller. When using an 8-bit digital controller, 256

microsteps per step are also possible

SAA1027 Stepper Motor Control Chip


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In this tutorial about Rotational Actuators, we have looked at the brushed and brushless DC

Motor, theDC Servo Motorand the Stepper Motoras an electromechanical actuator that canbe used as an output device for positional or speed control. In the next tutorial about

Input/Output devices we will continue our look at output devices called Actuators, and one in

particular that converts a electrical signal into sound waves again using electromagnetism. Thetype of output device we will look at in the next tutorial is theLoudspeaker.

Tutorial: 8 of 9

The Sound Transducer

Sound is the general name given to "acoustic waves" that have frequencies ranging from just

1Hz up to many tens of thousands of Hertz with the upper limit of human hearing being around

the 20 kHz, (20,000Hz) range. The sound that we hear is basically made up from mechanicalvibrations produced by a Sound Transducerused to generate the acoustic waves, and for sound

to be "heard" it requires a medium for transmission either through the air, a liquid, or a solid.

Piezo Sound Transducer

Also, sound need not be a continuous frequency sound wave such as a single tone or a musicalnote, but may be an acoustic wave made from a mechanical vibration, noise or even a single

pulse of sound such as a "bang".

Sound Transducersinclude both sensors, that convert sound into and electrical signal such as a

microphone, and actuators that convert the electrical signals back into sound such as a
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The most common forms of sensors are those that detect Position, Temperature, Light, Pressure

and Velocity.

The simplest of all input devices is the switch or pushbutton.

Some sensors called "Self-generating" sensors generate output voltages or currents relative to

the quantity being measured, such as thermocouples and photo-voltaic solar cells and their

output bandwidth equals that of the quantity being measured.

Some sensors called "Modulating" sensors change their physical properties, such as inductanceor resistance relative to the quantity being measured such as inductive sensors, LDR's and

potentiometers and need to be biased to provide an output voltage or current.

Not all sensors produce a straight linear output and linearization circuitry may be required.

Signal conditioning may also be required to provide compatibility between the sensors low

output signal and the detection or amplification circuitry.

Some form of amplification is generally required in order to produce a suitable electrical signal

which is capable of being measured.

Instrumentation type Operational Amplifiers are ideal for signal processing and conditioning of

a sensors output signal.

Output Devices or Actuators

"Output" devices are commonly called Actuatorsand the simplest of all actuators is the lamp.

Relays provide good separation of the low voltage electronic control signals and the high

power load circuits.

Relays provide separation of DC and AC circuits (i.e. switching an AC current path via a DC

control signal or vice versa).

Solid state relays have fast response, long life, no moving parts with no contact arcing or

bounce but require heatsinking.

Solenoids are electromagnetic devices that are used mainly to open or close pneumatic valves,security doors and robot type applications. They are inductive loads so a flywheel diode is

required.

Permanent magnet DC motors are cheaper and smaller than equivalent wound motors as they

have no field winding.

Transistor switches can be used as simple ON/OFF unipolar controllers and pulse width speed

control is obtained by varying the duty cycle of the control signal.

Bi-directional motor control can be achieved by connecting the motor inside a transistor H-

bridge.

Stepper motors can be controlled directly using transistor switching techniques.

The speed and position of a stepper motor can be accurately controlled using pulses so can

operate in an Open-loop mode.

Microphones are input sound transducers that can detect acoustic waves either in the Infra

sound, Audible sound or Ultrasound range generated by a mechanical vibration.

Loudspeakers, buzzers, horns and sounders are output devices and are used to produce an

output sound, note or alarm.


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sensor tutorial

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