INTRODUCTION TO SENSORS, TRANSDUCERS ......Highly sophisticated relays are utilized to protect...

INTRODUCTION TO SENSORS, TRANSDUCERS & ACTUATORS

© Rohan Desai- Automobile Dept- New Polytechnic, Kolhapur Page 1

INTRODUCTION

Transducers play a major role in mechatronics engineering & technology.

These are the basic elements that convert or transform one form of energy to

another form. Let us change the word ‘energy’ into the word ‘signal’ which is more

convenient to use. Thus transducers convert one form of signal to another form.

Transducers are mainly used to quantify the physical, electrical, fluidic and

mechanical variables such as temperature, pressure, magnetic field, voltage, flow

and vibration. Such variables or parameters are also called as measurands.

Q. What is the difference between transducer and sensor?

Transducers are the physical element which is an active part of a sensor. In another

word, a sensor is a sophisticated transducer which contains signal conditioning

circuits. This signal conditioning circuit is used for amplifying and refining the weak

signal that is available at the output of the transducer. Some of the commonly used

signal conditioning circuits are amplifiers, filters and Analog to Digital Converters

(ADC). The input signal is referred to as measurands. The output of the transducer

is referred to as equivalence.

Q. List the sensors or transducers.

1. Limit switches

2. Proximity switches

3. Electric sensors

4. Light sensors- optical encoders

Q. Write the working and applications of limit switches.

Limit switches are normally not operator accessible. Generally they are activated by

moving parts on the machine. They are mechanically or magnetically operated. They

are sometimes called cam switches because many are operated by a camming action

when a moving part pushes the switch.



Functions of limit switches:

Detecting presence/ absence

Counting

Detecting range of movement

Detecting positioning and travel limit

Breaking a live circuit when unsafe conditions arise.

Applications:

Material handling

Food packaging

Manufacturing- Automotive/heavy equipments, machining, marine/aviation

Metal forming

Q. What do you mean by proximity switches? List and give their

applications.

A proximity sensor consist of an element that changes either its state or an analog

signal when it is close to, but often not actually touching, an object. Magnetic,

electrical capacitance, inductance and eddy current methods are particularly suited

to the design of a proximity sensor.

Type:

Inductive type

Capacitive and

Optical sensors

Reed switches

Functions:

Detecting presence/ absence

Counting

Applications:

Material handling

Food packaging

In AGVs (Automated Guided vehicles)

Q. Write the working of inductive sensor.

Inductive sensors use currents induced by magnetic fields to detect nearby metal

objects. The inductive sensor uses a coil (an inductor) to generate a high frequency

magnetic field. If there is a metal object near the changing magnetic field, current

will flow in the object. This resulting current flow sets up a new magnetic field that



opposes the original magnetic field. The net effect is that it changes the inductance

of the coil in the inductive sensor. By measuring the inductance the sensor can

determine when a metal have been brought nearby.

Q. Write the working of capacitive sensor.

Inside the sensor is a circuit that uses the supplied DC power to generate AC, to

measure the current in the internal AC circuit, and to switch the output circuit when

the amount of AC current changes. Remember that capacitors can hold a charge

because, when one plate is charged positively, negative charges are attracted into

the other plate, thus allowing even more positive charges to be introduced into the

first plate. Unless both plates are present and close to each other, it is very difficult

to cause either plate to take on very much charge. Only one of the required two

capacitor plates is actually built into the capacitive sensor! The AC can move current

into and out of this plate only if there is another plate nearby that can hold the

opposite charge. The target being sensed acts as the other plate. If this object is

near enough to the face of the capacitive sensor to be affected by the charge in the



sensor's internal capacitor plate, it will respond by becoming oppositely charged near

the sensor, and the sensor will then be able to move significant current into and out

of its internal plate.

Q. Write the working of Optical sensor.

These sensors are more commonly known as light beam sensors of the thru-beam

type or of the retro reflective type. Both sensor types are shown below.

The light beam is generated on the left, focused through a lens. At the detector side

the beam is focused on the detector with a second lens. If the beam is broken the

detector will indicate an object is present. The oscillating light wave is used so that

the sensor can filter out normal light in the room. The light from the emitter is

turned on or off at a set frequency. When the detector receives the light it checks to

make sure that it is at the same frequency. If light is being received at the right

frequency then the beam is not broken.

Retro-reflective type light sensors have the transmitter and receiver in the same

package. They detect targets that reflect light back to the sensor. Retro-reflective

sensors that are focused to recognize targets within only a limited distance range

are also available.



Q. Write the working of Reed switch sensor.

It consists of two magnetic switch contacts sealed in a glass tube. When a magnet is

brought close to the switch, the magnetic reeds are attracted to each other and

close the switch contacts. It is widely used for checking the closure of doors.

Q. Write the construction & applications of thumb wheel switch sensor.

Thumb wheel adjustment with 10 positions is available. This switch consists

of a glass epoxy stator with tin plates copper tracks for ways.

The switches are sealed to protect it from dust. The pole is spring loaded

brass contact. The operating lever and assembly is plastic coated.

Applications: computers and measuring instruments

Q. Write the working and applications of optical encoders.

Optical encoders are used to detect the position, velocity, acceleration and direction

of movement of rotors, shafts, pistons of rotary machines and translational systems.

The optical encoder provides encoded pulsed signals in response to the movement.

Continuous optical signals are coded by the use of a specially designed rotating disk

containing code patterns called ‘track’. The pattern or track on the disk consists of

alternate appearance of opaque and transparent segment and the pattern is circular.

Basically, two types of encoders, such as incremental encoder and absolute encoder

exist.

Incremental encoder: this device consists of a circular disk that contains

alternative evenly spaced opaque and transparent segments over a circle. A light

source is located on one side of the disk and a photo detecting device (photo

detector) is placed on the other side of the disk. Light emitting diode (LED) is used

as the light source that provides continuous light signal. A photo diode or photo

transistor is commonly used as the light detector. Light signals can be received if the

transparent segment of the pattern is in between the light source and photo

detector. In the real application the disc is rigidly fixed with the rotor or shaft of the

rotating element. The evenly spaced transparent radial lines on its surface, rotates

past the light source. The output is taken from the photo detector. The number of



pulses determines the position of the disk and the number of pulses per second

measures the velocity of the disk.

Absolute encoder: It also provides angular position and velocity value, which is

derived from the pattern of the coded disc. They are more capable than incremental

encoders. They provide a unique output for every position. The coded disk consists

of a number of concentric patterns of opaque and transparent segments. The

concentric patterns are called tracks. The number of tracks determines the

resolution of the encoder. Each track has its own photo detector. The length of the

transparent segments of the track towards the center of the disk increases in a

specific order to satisfy the binary coding technique.



Q. What is actuator? List them.

Actuation is the process of conversion of energy to mechanical form. Actuator is a

device that accomplishes this conversion. The microcontroller provides command

signal to the actuator for actuation. They are responsible for transmission of power

and motion from one place to another by the use of mechanical components.

Solenoids

Relays

DC and AC motors.

Q. Write the working and applications of solenoid.

Solenoids are the most common actuator components. There is a moving ferrous

core (piston) that will move inside wire coil. Normally the piston is held outside the

coil by a spring. When a voltage is applied to the coil and current flows, the coil

builds up a magnetic field that attracts the piston and pulls it into the center of the

coil. The piston can be used to supply a linear force.

Applications: pneumatic valves and car door openers.

Q. Write short note on relays.

Relays are electrically operated actuators or switches in which changing a current in

one electrical circuit switches a current on or off in another circuit. Relays are like

remote control switches and are

used in many applications

because of their relative

simplicity, long life, and proven

high reliability. Relays are used

in a wide variety of applications

throughout industry, such as in

telephone exchanges, digital

computers and automation

systems. Highly sophisticated relays are utilized to protect electric power systems

against trouble and power blackouts as well as to regulate and control the



generation and distribution of power. In the home, relays are used in refrigerators,

washing machines and dishwashers, and heating and air-conditioning controls.

Q. what are ON-OFF and latching applications in relays?

ON-OFF Applications:

In figure (a) the relay is off. The metal arm is at its rest position and so there is

contact between the Normally Closed (N.C.) switch contact and the 'common' switch

contact.

If a current is passed through the coil, the resulting magnetic field attracts the metal

arm and there is now contact between the Normally Open (N.O.) switch contact and

the common switch contact, as shown in figure (b).

Latching relay circuit:

If a relay is connected as shown in figure, it will become 'latched' on when the coil is

energized by pressing the Trigger button. The only way to turn the relay off will

then be to cut the power supply by pressing the Reset button (which must be a

push-to-break type).



Q. Write short on solid state relays.

These active semiconductor devices use light instead of magnetism to actuate a

switch. The light comes from an LED, or light emitting diode. When control power is

applied to the device’s output, the light General Purpose Relay is turned on and

shines across an open space. On the load side of this space, a part of the device

senses the presence of the light, and triggers a solid state switch that either opens

or closes the circuit under control. Often, solid state relays are used where the

circuit under control must be protected from the introduction of electrical noises.

Advantages of Solid State Relays include long life, no moving parts, no contact

bounce, and fast response. The drawback to using a solid state relay is that it can

only accomplish single pole switching.

Q. write short note on motors.

Motors or electromechanical actuators convert electrical energy into mechanical

energy. The fundamental principle of operation of such actuators comes from the

fact that when an electric current is passed through a group of wire loops placed in a

magnetic field, the loop rotates, and the rotating motion is transmitted to a shaft,

providing useful mechanical work. They are used in robotics, machine tools,

petrochemicals, paper mills. Besides industrial control applications, the

electromagnetic actuators are used in many consumer products such as, fans,

vacuum cleaner, mixer, hair dryer and CD player. There are three types of motors

such as AC, DC and stepper motors.

Q. How DC motor works? And explain their types.

DC motor is the most versatile actuator and sometimes called rotating machine. The

DC motor has two parts, stator and rotor. The stator is the outer part of the motor

which contains evenly spaced magnetic poles. The stator is fixed and rotor rotates

inside the stator. Rotor consists of an electromagnet with poles facing toward the

stator poles.



The electromagnet is formed by current carrying conductors (copper wires), which

are wound around the iron core (rotor).

The wound wire is called armature coil or armature winding. When a DC current is

applied to the armature windings, the rotor is immediately turned in to a group of

electromagnets. The current that passes through the rotor winding is called

armature current. When this current carrying rotor is placed in magnetic field which

is established by use of magnet of stator, a force is exerted that makes it possible to

rotate a rotor. The magnetic field, armature current and force are mutually

perpendicular to each other. The direction of current, the direction of magnetic field

and the stator-rotor assembly are such, based on the law that the force exerted on

the coil makes it possible the rotor to rotate. The force exerted is given by,

F= I L B

Where, I is armature current, B is magnetic field and L is the length of the

conductor coil. DC motors are categorized as brushed, brushless and coreless type.

Q. Write the construction and working of brushed DC motor.

Brush and commutator are two important parts of the DC motors. When current is

passed through the armature winding, it develops a group of electromagnets whose

poles are attracted by the opposite poles of stator poles. Among the group, consider

a single electromagnet. Assume that only one field magnet (two poles) is present.

The poles of this electromagnet will be attracted by the opposite poles of the stator

poles (Fig a) and by virtue of which the rotor electromagnet would make a motion

and then stop in the position as shown in fig b.



That is, once the magnetic lines of forces of the two electromagnets (stator and

rotor) become parallel and opposite shown in fig b, no further motion of the rotor

would occur. However, key to a DC motor is that the moment the rotor

electromagnet aligns, its field flips, i.e., the direction of flow of current through the

coil is deliberately altered (Fig c). The alteration of current causes the poles of the

rotor electromagnet to alter. Notice that prior to the flipping the pole of the rotor

electromagnet and the pole of stator electromagnet were not similar. In this

particular example they were North Pole and South Pole respectively, and just after

flipping the poles of the two electromagnets become similar. The alteration allows

the rotor electromagnet to continue its motion since the same pole will now repel

each other. If the field of the rotor electromagnet flipped at just the right moment at

the end of each alignment, the rotor would rotate freely.



Q. Write working of brushless and coreless DC motors.

Brushless motors

A brushless motor operates much in the same way as a brushed motor. In a brushed

motor, the current is supplied to the rotating coil by a mechanical commutator (split-

ring) and brush arrangement. In a brushless motor, on the other hand the

commutation is achieved electronically. The electronic commutator can reverse the

external connections at a high rate. The commutator and brushes are in contact and

during rotation both the elements rub against each other. The brushless motor is a

better construction because the commutation is achieved using electronic circuit that

overcomes the problem of wearing arising out of physical contact. A brushless motor

is often used when high reliability, long life and high speeds are required.

Coreless motors

Iron core construction has several major disadvantages. The core has relatively high

inertia that limits acceleration. They also own electrical inductances and core losses,

which stay even with laminated structure. To overcome these drawbacks many

motors are designed without a core. These types of motors are called coreless

motors. Coreless motors depend upon the winding itself to provide structural

integrity for the armature.

Q. Write the working of AC motors.

AC motors

AC motors consume alternating electrical power to produce mechanical actuation in

terms of angular movement. The principle of operation of all AC motors relies on the

interaction of a revolving magnetic field created in the stator by AC current, with an

opposing magnetic field at the rotor. The opposing magnetic field is originated by

virtue of induction or by supplying an armature current by a separate the DC current

source. The principle of operation of AC motor in relation to the origin of opposing

field differs. Accordingly, AC motors are of two types

induction motor

synchronous motor

AC motors are either single phase or multiphase, depending upon the input signal

requirement and internal construction.

Induction motors

The interaction of magnetic fields of the rotor and the stator makes the induction

motor to rotate. The stator windings are connected to the power supply, which could



be a one or multiphase type. For example, a single phase induction motor is

connected to the single phase power line and three-phase induction motors to the

three-phase power line, respectively.

When an alternating voltage across the stator windings is applied, a radial rotating

magnetic field is produced. The rotor has conductive loop along its periphery. The

rotating magnetic fields produced by the stator induce a current into the conductive

loops of the rotor. Once that occurs, the magnetic field causes forces to act on the

current carrying conductors, which results in a torque on the rotor.

Synchronous motors

The synchronous motors are a 3-phase system in which the magnets are mounted

on the rotor and are excited by the DC current. The stator winding is divided into

three parts, which are fed with three-phase AC current respectively. Because of the

nature of the connection, the magnetic field rotates at a constant speed that is

determined by the frequency of the current in the AC signal. The variation of the

three waves of current input to the stator winding causes a varying magnetic

interaction with the poles of the magnets of the rotor. This, in turn, causes the rotor

to rotate. Typical characteristics of synchronous motors can be attributable to as

follows. They:

run at constant speed fixed by frequency of the input powers signal

require DC current for excitation

have low starting torque

Q. Write the working of stepper motor.

Many applications require precise positioning control. Traditionally this has been

achieved by the use of small DC motors. However, small DC motors have very

gradual acceleration and deceleration providing low stabilisation property. Gearing

mechanism can help to reduce this problem to some extent. But overshoot still

persists deviating the reachability, which means that desired position control cannot

be achieved. The only way to effectively use a DC motor for precise positioning is to



apply servo mechanism, a control realisation principle based on feedback rule

furthermore; DC motors are not efficient for producing high torque at relatively low

speeds.

The stepper motor also consists of a rotor and stator. As the name suggests, the

stepper motor steps a bit at a time. The motor can be controlled using a

microcontroller as it can responds to digital pulse trains. The rotor of the motor

rotates a specified number of degrees by each pulse the motor receives from its

controller. The motion caused by one pulse is called one step.

There are two forms of stepper motor

Variable reluctance motor

Figure shows the basic form of the variable reluctance stepper motor. With this

form the rotor is made of soft steel (having less reluctance than air) and is

cylindrical with four poles, i.e. fewer poles than on the stator. When an opposite

pair of windings has current switched to them, a magnetic field is produced with

lines of force which pass from the stator poles through the nearest set of poles on

the rotor. Since lines of force can be considered to be rather like elastic thread and

always trying to shorten themselves, the rotor will move until the rotor and state

poles line up. This is termed the position of minimum reluctance. This form of

stepper motor generally gives step angles of 7.5° or 15°.

Permanent magnet stepper

Figure shows the basic form of the permanent magnet motor. The motor shown has

a stator with four poles. Each pole is wound with a field winding, the coils on

opposite pairs of poles being in series. Current is supplied from a DC source to the

windings through switches.



The rotor is a permanent magnet and thus when a pair of stator poles has a current

switched to it, the rotor will move to line up with it. Thus for the currents giving the

situation shown in the figure the rotor moves to the 45° position. If the current is

then switched so that the polarities are reversed, the rotor will move further 40° in

order to line up again. Thus by switching the currents through the coils the rotor

rotates in 45° steps. With this type of motor, step angles are commonly 1.8°, 7.5°,

15°, 30°, 34° or 90°.

Date post:	15-Mar-2020
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INTRODUCTION TO SENSORS, TRANSDUCERS ......Highly sophisticated relays are utilized to protect...

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