sensors i.ppt

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EN 2850 APLLIED ELECTRONICSLECTURE 02 Sensors(1)

Level 2/Semester 1

22/09/2009

Kosala Jayasundara


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OUTLINE

Defining sensors and transducers

Classifying sensors Some important sensor characteristics Some important physical principles used in sensing. Advantages of electrical transducers. (sensors)


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Sensors and transducers

Sensor is a device that receives a stimulusand outputs an electrical signal

Upon a stimulus (the quantity, property orthe condition that is being sensed), sensors

generate electrical output. Transducer is device that converts one form

of energy to another form of energy. All sensors are transducers but all the

transducers are not necessarily sensors. We want the stimulus to be converted to

electrical domain rather than any otherdomain. (mechanical)


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Sensors and transducers (Contd)

A sensor can comprises of several transducers

Example A chemical sensor can comprise of a transducer that converts thechemical energy to heat and another transducer that converts heat toelectrical signal.

Stimulus Chemical reaction, Output electrical signal

The direct sensor is a common part in every sensor and it make use

of different physical principals like hall effect, photoeffect etc.


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Sensors and transducers (Contd)

Thus a sensor can be a direct sensor (one element,single transducer) or a complex sensor (severalelements, several transducers)

The stimulus for sensors can be any conceivablephysical or chemical characteristic such as

Light flux

Temperature

Pressure

Humidity

Vibration

Position

Displacement


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Classifying sensors

Different sensor classification criteria are available.

Sensors

Activesensors

Passivesensors

Generates the output by the applied stimulus itself. Noneed additional energy source.

In addition to the stimulus there is a power sourceapplied to the sensor. The stimulus is modulated in theexcitation signal (external energy) and given as theoutput.

E.g. Thermocouple, piezo electric sensors

E.g. Temperature sensitive resistors (Thermistors),


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Classifying sensors (Contd)

Depending on the reference selected

Sensors

Relativesensors

Absolutesensors

Sense the stimulus with reference to an absolutephysical scale which is independent from themeasurement conditions

Produce the output to the stimulus with respect to aspecial case of the stimulus. Not directly connectedwith an absolute physical scale.

E.g. Thermistor K scale

E.g. Thermocouple


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Sensor characteristics

We will look at the sensor as a black box andconsider some of its characteristics that characterizethe stimulus, output and the transition.

These sensor characteristics are significant when

selecting a sensor to a particular application. Some may not be applicable for all type of sensors.

Frequency response and bandwidth

The range of frequencies within which the

sensor can be effectively used is called thebandwidth. Usually defined as 3dB bandwidth.That is the frequency range within which theoutput lies between the (half of the maximumoutput) and the (maximum output).


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Sensor characteristics - Sensitivity

Sensitivity refers to the ability of the sensor toproduce a change in output for a unit change inthe stimulus.

Defined as Change of output/Change of input

Unit depends on the actual sensor which isunder interest.

Static error The difference between themeasured and the actual values.

%100ueActual_Val

ErrorentageError_Perc


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Sensor characteristics (Contd)

Full Scale Input (Span) The dynamic rangeof stimuli that the sensor can sense is calledthe span. It gives the highest possible inputthat can be sensed with an acceptable level

of accuracy. Sometimes the span is given bydecibels. (dB) Eg: Thermistor from -900C to 1300C

Thermocouple from -1850C to 17000C

Capacitive Proximity Sensor - from 1mmto 5cm


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Full Scale Output (FSO) This is thealgebraic difference between theelectrical output signals obtained with

the highest possible input and thelowest possible input respectively.

Eg: 0V 5V for temperature sensor14-20mA for temperature sensor2

0-24V for load cell1


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AccuracyTells how inaccurate the sensor is !!!

Measured as the highest deviation of a valueindicated by the sensor, from the actual (true) valueat that input.

The accuracy (inaccuracy) is expressed as, Directly in terms of the measured output value

As a percentage of the input full scale (span)

In terms of the output signal

E.g. A piezo-resistive pressure sensor having 100 kPa span and10 full scale output, has an accuracy of either +-0.5%, +-500Pa or +-0.05


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Hysteresis Output of some sensors not only depend on thepresent stimuli but also depend on the past inputs. (history).

Because of this the output can be differentfor the same value of input when themeasurement is taken for increasing inputand decreasing input separately.

Physical and structural changes inmaterials and friction can cause thehysteresis


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SaturationAt some levels of the input stimuli theoutput will no longer be responsive to the input.Then further increase in stimuli will not produce

considerable output.

After the linear span the sensorexhibits span end nonlinearity orsaturation.


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RepeatabilityAbility of the sensor to produce the sameoutput in different measurement trials under the same

conditions.

Repeatability error is the inability to do so. Repeatability

error can be caused by build up charges, material plasticity,thermal noise.

Dead bandTells the insensitivity of a sensor over a givenrange of input signal.


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Resolution The smallest change in the stimuli thatis capable of resulting a measurable outputdifference.

Output impedance Plays an important role when

interfacing the sensors with other circuitry.

Sensor having voltage output Sensor having current output


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Reliability Ability of a sensor to perform its functionunder stated conditions for a given period of time.

Usually this figure is expressed as a probability thatthe sensor will function properly with out a failureover a given period of time.

But not commonly specified by the sensormanufacturers.

Can be expressed as a MTBF (Mean Time BetweenFailure) figure.


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Some physical principles used for sensing

Capacitance Inductance/Magnetism Resistance

Temperature sensitivity Strain sensitivity Moisture sensitivity

Piezoelectric effect

Pyroelectric effect Hall effect - Seebeck effect Sound waves - Heat transfer -


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Advantages of electrical transducers(sensors)

Electrical amplification and attenuation is possible

Effect of friction is minimized.

Mass inertia effects are minimized.

Output can be indicated and recorded remotely.

Output can be modified or conditioned in order tomeet the requirement imposed by the other controlelements.

The electrical/electronics based systems can becontrolled with a small power.

Electrical signal can be easily transmitted for furthermanipulation.

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