Mechanical Thermal Electromagnetic Optical Chemical

Transducers & Sensors Flashcards by Catherina Facal

A device that converts a primary form of energy into corresponding signal with a different energy form. It is also defined as a device which provides usable output response to a specific input measured which may be a physical quantity.

Transducer

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A Transducer is capable of converting the ______ into a proportional quantity such as voltage or current. And when actuated by energy in one system, it is a device that supplies energy in the same form or in another form to a second system.

physical quantity

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Primary energy forms

Mechanical
Thermal
Electromagnetic
Optical
Chemical

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Transducers contains two parts that are closely related to each other:

Sensing element
Transduction element

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It is the sensing element or a device that produces measurable response to change in physical conditions.

Sensor

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It convert the sensor output to suitable electrical form

TRANSDUCTION ELEMENT

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Sensor Output block diagram

Parameter > Sensing Element > TRANSDUCTION ELEMENT > Electric signal

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ADVTANTAGES OF ELECTRICAL TRANSDUCERS

Power requirement is very low for controlling the electrical or electronic system
An amplifier may be used to amplify the electrical signal according to requirement
Friction effect is minimized
Mass-inertia is also minimized, because in case of electrical or electronics signals the intertia effect is due to the mass of electrons, which can be negligible
Output can be indicated and recorded remotely from the sensing element

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CHARACTERISTICS OF TRANSDUCERS

Ruggedness
Linearity
Repeatability
Accuracy
High stability and reliability
Speed of response
Sensitivity
Small size

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TRANSDUCERS SELECTION FACTORS

Operating Principle
Sensitivity
Operating Range
Accuracy
Cross Sensitivity
Errors
Transient and Frequency Response
Loading effects
Environmental compatibility
Insensitivity to unwanted signals

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The transducer are many times selected on the basis of ________ used by them. The _________ used may be resistive, inductive, capacitive, optoelectronic, piezo electric, and etc.

OPERATING PRINCIPLE

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The transducer must be sensitive enough to produce detectable output

SENSITIVITY

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The transducer should maintain the range requirement and have a good resolution over the entire range

OPERATING RANGE

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High _____ is assured

ACCURACY

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It has to be taken into account when measuring mechanical quantities. There are situation where the actual quantity is being measured is in one plane and the transducer is subjected to variation in another plan

CROSS SENSITIVITY

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The transducer should maintain the expected input-output relationship as described by the transfer function so as to avoid errors.

ERRORS

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The transducer should meet the desired time domain specification like peak overshoot, rise time, setting time and small dynamic error.

TRANSIENT AND FREQUENCY RESPONSE

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The transducer should have a high input impedance and low output impedance to avoid ______

LOADING EFFECTS

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It should be assured that the transducer selected to work under specified environmental conditions maintains its input-output relationship and does not break down.

ENVIRONMENTAL COMPATIBILITY

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The transducer should be minimally sensitive to unwanted signals and highly sensitive to desired signals.

INSENSITIVITY TO UNWANTED SIGNALS

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TRANSDUCERS CAN BE CLASSIFIED AS:

1.Active and passive transducers
2.Analog and digital transducers
3.On the basis of transduction principle used
* Capacitive
* Inductive
* Resistive
4. Primary and secondary transducer
5. Transducers and inverse transducer

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These transducers do not need any external source of power for their operation. Therefore they are also called as self generating type of transducers.

ACTIVE TRANSDUCERS

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Types of Active Transducers

Photovoltaic
Thermoelectric
Electromagnetic
Piezo electric
Chemical

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I. These transducers need external source of power for their operation. So they are not self generating type transducers.
II. A DC power supply or an audio frequency generator is used as an external power source.
III. These transducers produce the output signal in the form of variation in resistance, capacitance, inductance or some other electrical parameter in response to the quantity to be measured.

PASSIVE TRANSDUCER

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Some transducers contain the mechanical as well as electrical device. The mechanical device converts the physical quantity to be measured into a mechanical signal. Such mechanical device are called as the ______ _________, because they deal with the physical quantity to be measured.

primary transducers

The electrical device then convert this mechanical signal into a corresponding electrical signal. Such electrical device are known as ______ ________

secondary transducers

Transduction principle

* Capacitive Transduction * Electromagnetic Transduction * Inductive Transduction * Piezo Electric Transduction * Photovoltaic Transduction * Photoconductive Transduction

The measurand is converted to a change in the capacitance.

CAPACITIVE TRANSDUCER

* A typical capacitor is comprised of two parallel plates of conducting material separated by an electrical insulating material called a _______. The plates and the ________may be either flattened or rolled. * The purpose of the ________ is to help the two parallel plates maintain their stored electrical charges.

Dielectric

The relationship between the capacitance and the size of capacitor plate, amount of plate separation, and the dielectric is given by

C = (ε0*εr*A) / d where: d is the separation distance of plates (m) C is the capacitance (F, Farad) ε0 : absolute permittivity of vacuum εr : relative permittivity A is the effective (overlapping) area of capacitor plates (m2)

The measurand is converted to voltage induced in conductor by change in the magnetic flux, in absence of excitation.

electromagnetic transduction

The electromagnetic transducer are ________

self generating active transducers

The ______ between a piece of magnet and an electromagnet is responsible for the change in flux

motion

The measurand is converted into a change in the self inductance of a single coil. It is achieved by displacing the core of the coil that is attached to a mechanical sensing element

inductive transduction

The measurand is converted into a change in electrostatic charge q or voltage V generated by crystals when mechanically it is stressed as shown in fig.

piezoelectric induction

The measurand is converted to voltage generated when the junction between dissimilar material is illuminated as shown in fig.

photovoltaic transduction

the measurand is converted to change in resistance of semiconductor material by the change in light incident on the material.

photoconductive transduction

_______ convert non electrical quantity to electrical quantity. ________ convert electrical quantity to a non electrical quantity

TRANSDUCER INVERSE TRANSDUCER

are those transducers in which the resistance change due to the change in some physical phenomenon.

Resistive transducers

The resistance of a metal conductor is expressed by a simple equation.

R = ρL/A Where R = resistance of conductor in Ω L = length of conductor in m A = cross sectional area of conductor in m2 ρ = resistivity of conductor material in Ω-m.

There are 4 type of resistive transducers.

1. Potentiometers (POT) 2. Strain gauge 3. Thermistors 4. Resistance thermometer

The _______ are used for voltage division. They consist of a resistive element provided with a sliding contact. The sliding contact is called as ____.

potentiometer wiper

The contact motion may be linear or rotational or combination of the two. The combinational potentiometer have their resistive element in helix form and are called ________.

helipots

The _____ ______ is a passive, resistive transducer which converts the mechanical elongation and compression into a resistance change. This change in resistance takes place due to variation in length and cross sectional area of the gauge wire, when an external force acts on it.

strain gauge

The type of strain gauge are as

1. Wire gauge >Unbonded >Bonded >Foil type 2. Semiconductor gauge

This gauge consist of a wire stretched between two point in an insulating medium such as air. The wires may be made of various copper, nickel, crome nickle or nickle iron alloys.

Unbonded strain gauge

The bonded metal wire strain gauge are used for both stress analysis and for construction of transducer.

Bonded Strain Gauge

Bonded Metal foil strain gauge parts

1. Base (carrier) Materials 2. Adhesive 3. Leads

several types of _____ _____ are used to support the wires. Impregnated paper is used for room temp. applications.

Base (carrier) Materials:

The _____ acts as bonding materials. Like other bonding operation, successful starain gauge bonding depends upon careful surface preparation and use of the correct bonding agent.

Adhesive

The _____ should be of such materials which have low and stable resistivity and also a low resistance temperature coefficent

Leads

______ _____ are used in application where a high gauge factor is desired. A high gauge factor means relatively higher change in resistance that can be measured with good accuracy.

Semiconductor gauge

________ of metal increase with increases in temperature. Therefore metals are said to have a positive temperature coefficient of resistivity.

Resistance

_______ are temperature dependent resistors. They are made of semiconductor material which have negative temperature coefficient of resistivity i.e. their resistance decreases with increase of temperature.

Thermistor

________ is a contraction of a term “thermal resistor”.

Thermistor

Thermistor composition

* magnese * nickle * cobalt * copper * iron * uranium

Thermistor forms

1. beads 2. rods 3. discs

When a pair of dissimilar metals are joined at one end, and there is a temperature difference between the joined ends and the open ends, thermal emf is generated, which can be measured in the open ends.

Thermocouples

An ______ ______ is a transducer which converts the physical motion into the change in inductance. _______ ______ are mainly used for displacement measurement.

inductive electromechanical transducer

The variable inductance transducers work on the following principles.

1. Variation in self inductance 2. Variation in mutual inductance

It is a device that detects a change in a physical stimulus and turns it into a signal which can be measured or recorded. Acquires information from the real world.

SENSOR

General Sensor System

1. Sensor/Transducer 2. Signal Conditioning

converts the sensed signal into an analog or digital electrical value

Signal Conditioning

sense real world parameter and converted into a suitable signal

Sensor/Transducer

DESIRABLE FEATURES OF SENSORS ARE:

* RANGE/SPAN * ERRORS AND ACCURACY * NONLINEARITY * HYSTERESIS * DEAD BAND AND SATURATION * OUTPUT IMPEDANCE * REPEATABILITY * RELIABILITY * SENSITIVITY * RESOLUTION * FREQUENCY RESPONSE * RESPONSE TIME * CALIBRATION

lowest and highest values of the stimulus

RANGE

the arithmetic difference between the highest and lowest values of the input that being sensed

SPAN

is equal to SPAN

INPUT FULL SCALE (IFS)

difference between the upper and lower ranges of the output of the sensor

OUTPUT FULL SCALE (OFS)

ratio between the upper and lower limits and is usually expressed in db

DYNAMIC RANGE

is the difference between the result of the measurement and the true value of the quantity being measured

ERRORS Error = measured value – true value

is the extent to which the measured value might be wrong and normally expressed in percentage

ACCURACY

is the deviation of the sensor’s output at any given point when approached from two different directions

HYSTERESIS

is defined as the maximum deviation from the ideal linear transfer function

NONLINEARITY

Nonlinearity must be deduced from the actual transfer function or from the calibration curve A few methods to do so are:

* By use of the range of the sensor. Pass a straight line between the range points (line 1) * Use a linear best fit (least square) through the points of curve (line 2) * Use tangent to the curve at some point on the curve. Take a point in the middle of the range of interest. Draw the tangent and extend to the range of the curve (line 3)

the lack of response or insensitivity of a device over a specific range of input

DEADBAND

ratio of the rated output voltage and short circuit current of the port

OUTPUT IMPEDANCE

failure of the sensor to represent the same value under identical conditions when measured at different times

REPEATABILITY/REPRODUCIBILITY Repeatability = (max-min values given/full range) * 100

a statistical measure quality of a device which indicates the ability of the device to perform its stated function, under normal operating conditions without failure for a stated period of time or number of cycles

RELIABILITY

is defined as the change in output for a given change in input, usually a unit change in input

SENSITIVITY

the minimum increment in stimulus to which the sensor can respond. It is the magnitude of the input change which results in the smallest observable output.

RESOLUTION

the ability of the device to respond to a harmonic (sinusoidal) input

FREQUENCY RESPONSE

indicates the time needed for the output to reach steady state for a step change in input

RESPONSE TIME

the experimental determination of the transfer function of a sensor or actuator

CALIBRATION

Calibration method

1. Known Transfer Function 2. Unknown Transfer Function

METHOD 1: KNOWN TRANSFER FUNCTION

1. Determine the slope and crossing point (line function) from two known stimuli (say two temperatures) if the transfer function is linear 2. Measure the output 3. Calculate the slop and crossing point in V=aT+b 4. If the function is more complex, need more points: V=aT+bT2+cT3+d 5. 4 measurements to calculate a, b, c, and d 6. Must choose points effectively. If not, use equally spaced points or points around the locaitions of highest curvature

METHOD 2: UNKNOWN TRANSFER FUNCTION

1. Measure the output Ri, as many input values Ti as is practical 2. Use the entire span 3. Calculate the best linear fit (least squares for example) 4. If the curve is not linear, use polynomial fit 5. May use piecewise linear segments if the number of points is large

Are concerned with the measurement of amount by which some object has moved.

DISPLACEMENT SENSORS

Are concerned with the determination of the position of some object with reference to some reference point

POSITION SENSORS

are a form of position sensors. They are used to determine when an object has moved to within some particular critical distance of the sensor

PROXIMITY SENSORS

When selecting sensors, consider the following:

1. The size of displacement 2. The required resolution and accuracy 3. Nature of displacement 4. The material of the measured object

The movement of the sensor element is used to cause a change in electrical voltage, resistance, capacitance or mutual inductance

CONTACT SENSORS

The presence in the vicinity of the measured object cause change in air pressure or change in inductance or capacitance

NON-CONTACT SENSORS

It consist of a constant resistance per unit length with sliding contact which can be moved over the length of the element. It can be used for linear or rotary displacements

POTENTIOMETER

The output voltage is proportional to the angle through which the slider has rotated, hence an angular displacement can be converted into a potential difference

ROTARY POTENTIOMETER

is a metal wire, metal foil or a strip of semiconductor material. These elements can be stuck onto surfaces like a postage stamp. When subjected to strain, its resistance R changes. The fractional change in resistance being proportional to the strain.

STRAIN GAUGE ELEMENT

is the ration of change in length/ original length

Strain

Transducers & Sensors Flashcards

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