In the next tutorial we will look at output devices called Actuators.

Actuators convert an electrical signal into a

corresponding physical quantity such as movement, force, or sound. One such commonly used output device is
the Electromagnetic Relay.

Electrical Relays
Thus far we have seen a selection of Input devices that can be used to detect or "sense" a variety of physical
variables and signals and are therefore called Sensors. But there are also a variety of devices which are classed
as Output devices used to control or operate some external physical process. These output devices are
commonly called Actuators.
Actuators convert an electrical signal into a corresponding physical quantity such as movement, force, sound etc.
An actuator is also a transducer because it changes one type of physical quantity into another and is usually
activated or operated by a low voltage command signal. Actuators can be classed as either binary or continuous
devices based upon the number of stable states their output has.
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 360 o motion. The most
common types of actuators or output devices are Electrical Relays, Lights,Motors and Loudspeakers and 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 Relay generally refers to a device that provides an electrical connection between two or 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 to be manually (physically) turned "ON" or "OFF". Also,
they are relatively large, slow and only switch small electrical currents.
Electrical Relays however, 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 being called "contactors".
In this tutorial about electrical relays we are just concerned with the fundamental operating principles 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 load currents are normally fractions of an ampere up to 20+ amperes.
As their name implies, electromechanical relays are electro-magnetic devices that convert a magnetic flux
generated by the application of a low voltage electrical control signal either AC or DC 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.
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 freely move 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 terms Normally Open, Normally Closed orMake and Break
Contacts refer 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.

The relays contacts are electrically conductive pieces of metal which touch together completing a circuit and
allow the circuit current to flow, just like a switch. When the contacts are open the resistance between the
contacts is very high in the Mega-Ohms, producing an open circuit condition and no circuit current flows.
When the contacts are closed the contact resistance should be zero, a short circuit, but this is not always the
case. All relay contacts have a certain amount of "contact resistance" when they are closed and this is called the
"On-Resistance", similar to FET's.
With a new relay and contacts this ON-resistance will be very small, generally less than 0.2's because the tips
are new and clean, but over time the tip resistance will increase.
For example: If the contacts are passing a load current of say 10A, then the voltage drop across the contacts
using Ohms Law is 0.2 x 10 = 2 volts, which if the supply voltage is say 12 volts then the load voltage will be
only 10 volts (12 - 2). As the contact tips begin to wear, and if they are not properly protected from high inductive
or capacitive loads, they will start to show signs of arcing damage as the circuit current still wants to flow as the
contacts begin to open when the relay coil is de-energized.
This arcing or sparking across the contacts will cause the contact resistance of the tips to increase further as the
contact tips become damaged. If allowed to continue the contact tips may become so burnt and damaged to the
point were they are physically closed but do not pass any or very little current.
If this arcing damage becomes to severe the contacts will eventually "weld" together producing a short circuit
condition and possible damage to the circuit they are controlling. If now the contact resistance has increased due
to arcing to say 1's the volt drop across the contacts for the same load current increases to 1 x 10 = 10 volts dc.
This high voltage drop across the contacts may be unacceptable for the load circuit especially if operating at 12
or even 24 volts, then the faulty relay will have to be replaced.
To reduce the effects of contact arcing and high "On-resistances", modern contact tips are made of, or coated
with, a variety of silver based alloys to extend their life span as given in the following table.

Contact Tip Materials

Contact
Material
Ag
(fine silver)

Tip

Characteristics
Electrical and thermal conductivity are the highest of all metals, 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
Expensive.

platinum,
gold
silver alloys

and

contact

wear,

greater

hardness.

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 capacitive loads 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 is achieved by
connecting a Resistor-Capacitor network called an RC Snubber Network electrically in parallel with the contact
tips. The voltage peak, which occurs at the instant 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

Relay Contact Types.

As well as the standard descriptions of Normally Open, (NO) and Normally Closed, (NC) used to describe
how the relays contacts are connected, relay contact arrangements can also be classed by their actions.

Electrical relays can be made up of one or more individual switch contacts with each "contact" being referred to
as a "pole". Each one of these contacts or poles can be connected or "thrown" together by energizing the relays
coil and this gives rise to the description of the contact types as being:

SPST
Single
SPDT
Single
DPST
Double
DPDT - Double Pole Double Throw

Pole
Pole
Pole

Single
Double
Single

Throw
Throw
Throw

with the action of the contacts being described as "Make" (M) or "Break" (B). Then a simple relay with one set of
contacts
as
shown
above
can
have
a
contact
description
of:

"Single Pole Double Throw - (Break before Make)", or SPDT - (B-M) .

Examples of just some of the more common contact types for relays in circuit or schematic diagrams is given
below but there are many more possible configurations.

Relay Contact Configurations

Where:

C is the Common terminal

NO is the Normally Open contact

NC is the Normally Closed contact

One final point to remember, it is not advisable to connect relay contacts in parallel to handle higher 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 same instant 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 voltages through 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 (RL Series 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 induced reverse 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.

One way of preventing damage to the transistor or any switching semiconductor device, is to connect a reverse
biased diode across the relay coil.
When the current flowing through the coil is switched "OFF", an induced back emf is generated as the magnetic
flux collapses in the coil.
This reverse voltage forward biases the diode which conducts and dissipates the stored energy preventing any
damage to the semiconductor transistor.
When used in this type of application the diode is generally known as a Flywheel Diode, Free-wheeling
Diode and even Fly-back Diode, but they all mean the same thing. Other types of inductive loads which require
a flywheel diode for protection are solenoids, motors and inductive coils.
As well as using flywheel Diodes for protection of semiconductor components, other devices used for protection
include RC Snubber Networks, Metal Oxide Varistors or MOV and Zener Diodes.

The Solid State Relay.

While the electromechanical relay (EMR) are inexpensive, easy to use and allow the switching of a load circuit
controlled by a low power, electrically isolated input signal, one of the main disadvantages of an
electromechanical relay is that it is a "mechanical device", that is it has moving parts so their switching speed
(response time) due to physically movement of the metal contacts using a magnetic field is slow.
Over a period of time these moving parts will wear out and fail, or that the contact resistance through the
constant arcing and erosion may make the relay unusable and shortens its life. Also, they are electrically noisy
with the contacts suffering from contact bounce which may affect any electronic circuits to which they are
connected.
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 type Light Sensor.

The Solid State Relay provides 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 electromechanical relay. 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 the inherent 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 the relay itself reducing the need for additional external components.
Non-zero crossing detection switching (instant "ON") type SSR's are also available for phase controlled
applications such as the dimming or fading of lights at concerts, shows, disco lighting etc, or for motor speed
control type applications.
As the output switching device of a solid state relay is a semiconductor device (Transistor for DC switching
applications, or a Triac/Thyristor combination for AC switching), the voltage drop across the output terminals of
an SSR when "ON" is much higher than that of the electromechanical relay, typically 1.5 - 2.0 volts. If switching
large currents for long periods of time an additional heat sink will be required.

Input/Output Interface Modules.

Input/Output Interface Modules, (I/O Modules) are another type of solid state relay designed specifically to
interface computers, micro-controller or PIC's to "real world" loads and switches. There are four basic types of I/O
modules available, AC or DC Input voltage to TTL or CMOS logic level output, and TTL or CMOS logic input to an
AC or DC Output voltage with each module containing all the necessary circuitry to provide a complete interface
and isolation within one small device. They are available as individual solid state modules or integrated into 4, 8
or 16 channel devices.

Modular Input/Output Interface System.

The main disadvantages of solid state relays (SSR's) compared to that of an equivalent wattage
electromechanical relay is their higher costs, the fact that only single pole single throw (SPST) types are
available, "OFF"-state leakage currents flow through the switching device, high "ON"-state voltage drop and
power dissipation resulting in additional heat sinking requirements. Also they can not switch very small load
currents or high frequency signals such as audio or video signals although special Solid State Switches are
available for this type of application.
In this tutorial about Electrical Relays, we have looked at both the electromechanical relay and the solid state
relay which can be used as an output device (actuator) to control a physical process. In the next tutorial we will
continue our look at output devices called Actuators and especially one that converts a small electrical signal
into a corresponding physical movement using electromagnetism.

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 the electromechanical relay (EMR) seen in
the previous tutorial and like relays, they can also be controlled by transistors or MOSFET. A Linear Solenoid is
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.Solenoids are available in a variety of
formats with the more common types being the linear solenoid also 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 a latching 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 of the 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

When an electrical current is passed through the coils windings, it behaves like an electromagnet and 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 the plunger 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 Solenoid due 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" that act 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

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 angular movement is very
small with the angle of rotation being the angle moved from the start to the end position. Commonly available
rotary solenoids have movements of 25, 35, 45, 60 and 90 o as 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-0 o, 3-position self
restoring, for example 0o to +45o or 0o to -45o as well as 2-position latching.
Rotary solenoids produce a rotational movement when either energised, de-energised, or a change 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 the electromagnetic 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 with special high speed,
low power or variable positioning solenoids with high force or torque are available such as those used in dot
matrix printers, typewriters, automatic machines or automotive applications etc.

Solenoid Switching
Generally solenoids either linear or rotary operate with the application of a DC voltage, but they can also be used
with AC sinusoidal voltages by using full wave bridge rectifiers to rectify the supply which then can be used to
switch
the
DC
solenoid.
Small
DC
type
solenoids
can
be
easily
controlled
usingTransistor or MOSFET switches and are ideal for use in robotic applications, but again as we saw with
relays, solenoids are "inductive" devices so some form of electrical protection is required across the solenoid coil
to prevent high back emf voltages from damaging the semiconductor switching device. In this case the standard
"Flywheel Diode" is used.

Switching Solenoids using a Transistor

Reducing Energy Consumption

One of the main disadvantages of solenoids and especially the linear solenoid is that they are "inductive
devices" which convert some of the electrical current into "HEAT", in other words they get hot!, and the longer the
time that the power is applied to a solenoid coil, the hotter the coil will become. Also as the coil heats up, its
electrical resistance also changes allowing more current to flow.
With a continuous voltage input applied to the coil, the solenoids coil does not have the opportunity to cool down
because the input power is always on. In order to reduce this self generated heating effect it is necessary to
reduce either the amount of time the coil is energised or reduce the amount of current flowing through it.
One method of consuming less current is to apply a suitable high enough voltage to the solenoid coil so as to
provide the necessary electromagnetic field to operate and seat the plunger but then once activated to reduce
the coils supply voltage to a level sufficient to maintain the plunger in its seated or latched position. One way of
achieving this is to connect a suitable "holding" resistor in series with the solenoids coil, for example:

Reducing Solenoid Energy Consumption

Here, the switch contacts are closed shorting out the resistance and passing the full supply current directly to the
solenoid coils windings. Once energised the contacts which can be mechanically connected to the solenoids
plunger action open connecting the holding resistor, RH in series with the solenoids coil. This effectively connects
the resistor in series with the coil.
By using this method, the solenoid can be connected to its voltage supply indefinitely (continuous duty cycle) as
the power consumed by the coil and the heat generated is greatly reduced, which can be up to 85 to 90% using a
suitable power resistor. However, the power consumed by the resistor will also generate a certain amount of
heat, I2R (Ohm's Law) and this also needs to be taken into account.

Duty Cycle
Another more practical way of reducing the heat generated by the solenoids coil is to use an "intermittent duty
cycle". An intermittent duty cycle means that the coil is repeatedly switched "ON" and "OFF" at a suitable
frequency so as to activate the plunger mechanism but not allow it to de-energise during the OFF period of the
waveform. Intermittent duty cycle switching is a very effective way to reduce the total power consumed by the
coil.
The Duty Cycle (%ED) of a solenoid is the portion of the "ON" time that a solenoid is energised and is the ratio of
the "ON" time to the total "ON" and "OFF" time for one complete cycle of operation. In other words, the cycle time
equals the switched-ON time plus the switched-OFF time. Duty cycle is expressed as a percentage, for example:

Then if a solenoid is switched "ON" or energised for 30 seconds and then switched "OFF" for 90 seconds before
being re-energised again, one complete cycle, the total "ON/OFF" cycle time would be 120 seconds, (30+90) so
the solenoids duty cycle would be calculated as 30/120 secs or 25%. This means that you can determine the
solenoids maximum switch-ON time if you know the values of duty cycle and switch-OFF time.
For example, the switch-OFF time equals 15 secs, duty cycle equals 40%, therefore switch-ON time equals 10
secs. A solenoid with a rated Duty Cycle of 100% means that it has a continuous voltage rating and can therefore
be left "ON" or continuously energised without overheating or damage.
In this tutorial about solenoids, we have looked at both the Linear Solenoid and the Rotary Solenoid as an
electromechanical actuator that can be used as an output device to control a physical process. In the next tutorial
we will continue our look at output devices called Actuators, and one that converts a electrical signal into a
corresponding rotational movement again using electromagnetism. The type of output device we will look at in
the next tutorial is the DC Motor.

Electrical Motors
Electrical Motors are continuous actuators that convert electrical energy into mechanical energy in the form of a
continuous angular rotation that can be used to rotate pumps, fans, compressors, wheels, etc. As well as rotary
motors, linear motors are also available. There are basically three types of conventional electrical motor
available: AC type Motors, DC type Motors and Stepper Motors.

A Typical Small DC Motor

AC Motors are 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 Motors and Stepper Motors which are used in many electronics, positional control,
microprocessor, PIC and robotic circuits.

The DC Motor
The DC Motor or 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 be controlled, 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 three types of DC Motor available.

Brushed Motor - This type of motor produces a magnetic field in a wound rotor (the part that rotates)
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 a wound 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 smaller but 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 positional
feedback control connected to the rotor shaft. They are connected to and controlled by a PWM type 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 being determined 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 them suitable 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 Stator and the inner part which rotates producing the movement called the Rotor or "Armature" for DC
machines.
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 whose stator 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
in series with the armature. Likewise, a shunt wound DC motor has its stator field windings connected
in parallel with 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 causing the 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 the rotational speed of the motor can also be varied.

Conventional (Brushed) DC Motor

Permanent magnet (PMDC) brushed motors are generally much smaller and cheaper than their equivalent
wound stator type DC motor cousins as they have no field winding. In permanent magnet DC (PMDC) motors
these field coils are replaced with strong rare earth (i.e. Samarium Cobolt, or Neodymium Iron Boron) type
magnets which have very high magnetic energy fields. This gives them a much better linear speed/torque
characteristic than the equivalent wound motors because of the permanent and sometimes very strong magnetic
field, making them more suitable for use in models, robotics and servos.
Although DC brushed motors are very efficient and cheap, problems associated with the brushed DC motor is
that sparking occurs under heavy load conditions between the two surfaces of the commutator and carbon
brushes resulting in self generating heat, short life span and electrical noise due to sparking, which can damage
any semiconductor switching device such as a MOSFET or transistor. To overcome these
disadvantages, Brushless DC Motors were developed.

The "Brushless" DC Motor

The brushless DC motor (BDCM) is very similar to a permanent magnet DC motor, but does not have any
brushes to replace or wear out due to commutator sparking. Therefore, little heat is generated in the rotor
increasing the motors life. The design of the brushless motor eliminates the need for brushes by using a more
complex drive circuit were the rotor magnetic field is a permanent magnet which is always in synchronisation with
the stator field allows for a more precise speed and torque control. Then the construction of a brushless DC
motor is very similar to the AC motor making it a true synchronous motor but one disadvantage is that it is more
expensive than an equivalent "brushed" motor design.
The control of the brushless DC motors is very different from the normal brushed DC motor, in that it this type of
motor incorporates some means to detect the rotors angular position (or magnetic poles) required to produce the
feedback signals required to control the semiconductor switching devices. The most common position/pole
sensor is the "Hall Effect Sensor", but some motors also use optical sensors.

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 permanent magnet rotor and an internal electromagnet stator or
an internal permanent magnet rotor and an external electromagnet stator.
Advantages of the Brushless DC Motor compared 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 they are more expensive and more complicated to
control.

The DC Servo Motor

DC Servo motors are 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 include Resolvers, 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 capable of 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 input signal or reference signal applied to
the device.

RC Servo Motor
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 it down. 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 servos are 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, Ground and 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 of circuit uses a bipolar Transistor as a
Switch (A Darlington transistor may also be used were a higher 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 is delivered 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 direction only) 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.
A flywheel diode is connected across the motor terminals to protect the switching transistor or MOSFET from any
back emf generated by the motor when the transistor turns the supply "OFF".
As well as the basic "ON/OFF" control the same circuit can also be used to control the motors rotational speed.
By repeatedly switching the motor current "ON" and "OFF" at a high enough frequency, the speed of the motor
can be varied between stand still (0 rpm) and full speed (100%). This is achieved by varying the proportion of

"ON" time (tON) to the "OFF" time (tOFF) and this can be achieved using a process known as Pulse Width
Modulation.

Pulse Width Speed Control

The rotational speed of a DC motor is directly proportional to the mean (average) value of its supply voltage and
the higher this value, up to maximum allowed motor volts, the faster the motor will rotate. In other words more
voltage more speed. By varying the ratio between the "ON" (t ON) time and the "OFF" (tOFF) time durations, called
the "Duty Ratio", "Mark/Space Ratio" or "Duty Cycle", the average value of the motor voltage and hence its
rotational speed can be varied. For simple unipolar drives the duty ratio is given as:

and the mean DC output voltage fed to the motor is given as: Vmean = x Vsupply. Then by varying the width
of pulse a, the motor voltage and hence the power applied to the motor can be controlled and this type of control
is called Pulse Width Modulation or PWM.
Another way of controlling the rotational speed of the motor is to vary the frequency (and hence the time period of
the controlling voltage) while the "ON" and "OFF" duty ratio times are kept constant. This type of control is
called Pulse Frequency Modulation or PFM. With pulse frequency modulation, the motor voltage is controlled
by applying pulses of variable frequency for example, at a low frequency or with very few pulses the average
voltage applied to the motor is low, and therefore the motor speed is slow. At a higher frequency or with many
pulses, the average motor terminal voltage is increased and the motor speed will also increase.
Then, Transistors can be used to control the amount of power applied to a DC motor with the mode of
operation being either "Linear" (varying motor voltage), "Pulse Width Modulation" (varying the width of the pulse)
or "Pulse Frequency Modulation" (varying the frequency of the pulse).

H-bridge Motor Control

While controlling the speed of a DC motor with a single transistor has many advantages it also has one main
disadvantage, the direction of rotation is always the same, its a "Uni-directional" circuit. In many applications we
need to operate the motor in both directions forward and back. One very good way of achieving this is to connect
the motor into a Transistor H-bridge circuit arrangement and this type of circuit will give us "Bi-directional" DC
motor control as shown below.

Basic Bi-directional H-bridge Circuit

The H-bridge circuit above, is so named because the basic configuration of the four switches, either electromechanical relays or transistors resembles that of the letter "H" with the motor positioned on the centre bar.
The Transistor or MOSFET H-bridge is probably one of the most commonly used type of bi-directional DC
motor control circuits. It uses "complementary transistor pairs" both NPN and PNP in each branch with the
transistors being switched together in pairs to control the motor.
Control input A operates the motor in one direction ie, Forward rotation while input B operates the motor in the
other direction ie, Reverse rotation. Then by switching the transistors "ON" or "OFF" in their "diagonal pairs"
results in directional control of the motor.
For example, when transistor TR1 is "ON" and transistor TR2 is "OFF", point A is connected to the supply
voltage (+Vcc) and if transistor TR3 is "OFF" and transistor TR4 is "ON" point B is connected to 0 volts (GND).
Then the motor will rotate in one direction corresponding to motor terminal A being positive and motor
terminal B being negative. If the switching states are reversed so that TR1 is "OFF", TR2 is "ON", TR3 is "ON"
and TR4 is "OFF", the motor current will now flow in the opposite direction causing the motor to rotate in the
opposite direction.
Then, by applying opposite logic levels "1" or "0" to the inputs A and B the motors rotational direction can be
controlled as follows.

H-bridge Truth Table

Input A

Input B

Motor Function

TR1 and TR4

TR2 and TR3

Motor Stopped (OFF)

Motor Rotates Forward

Motor Rotates Reverse

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, TR1 and TR2 switched "ON" at the same time, (fuse = bang!).
As with uni-directional DC motor control as seen above, the rotational speed of the motor can also 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 IC or 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 Motors are also electromechanical actuators that convert a pulsed 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 of many, 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 conventional DC 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 a rotation 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 (360o in exactly 100 steps. Then the step angle for the motor is given as 360 degrees/100 steps = 3.6
degrees per step. This value is commonly known as the stepper motors Step Angle.
There are three basic types of stepper motor, Variable Reluctance,Permanent Magnet and Hybrid (a sort of
combination of both). A Stepper Motor is particularly well suited to applications that require accurate positioning
and repeatability with a fast response to starting, stopping, reversing and speed control and another key feature
of the stepper motor, is its ability to hold the load steady once the require position is achieved.
Generally, stepper motors have an internal rotor with a large number of permanent magnet "teeth" with a number
of electromagnet "teeth" mounted on to the stator. The stators electromagnets are polarized and depolarized
sequentially, causing the rotor to rotate one "step" at a time.
Modern multi-pole, multi-teeth stepper motors are capable of accuracies of less than 0.9 degs per step (400
Pulses per Revolution) and are mainly used for highly accurate positioning systems like those used for magnetic-

heads in floppy/hard disc drives, printers/plotters or robotic applications. The most commonly used stepper motor
being the 200 step per revolution stepper motor. It has a 50 teeth rotor, 4-phase stator and a step angle of 1.8
degrees (360 degs/(50x4)).

Stepper Motor Construction and Control

In our simple example of a variable reluctance stepper motor above, the motor consists of a central rotor
surrounded by four electromagnetic field coils labelled A, B, C and D. All the coils with the same letter are
connected together so that energising, say coils marked A will cause the 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 rotary motion.
The stepper motor driver controls both the step angle and speed of the motor by energising the field 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 degree intervals. 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 = 15o.
Obviously, the more rotor teeth and or stator coils would result in more control and a finer step angle. 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 must be 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 delay between 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 additional feedback 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 position would only need a total
of: 216 degrees/(3.6 degs/step) = 80 pulses applied to the stator coils.
There are many stepper motor controller IC's available which can control the step speed, speed of rotation 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 selected direction, 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

In this tutorial about Rotational Actuators, we have looked at the brushed and brushless DC Motor, theDC
Servo Motor and the Stepper Motor as an electromechanical actuator that can be 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. The type of output device we will look at in the next tutorial is the Loudspeaker.

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 mechanical vibrations produced by a Sound Transducer used 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.