Definition of a sensor• Def. 1. (Oxford dictionary)

– A device giving a signal for the detection or measurement of a physical property to which it responds.

• Def. 2. – A sensor is a device that receives a signal or stimulus and

response with an electrical signal.

Classification of sensors

Attributes which can be used to classify sensors:. stimulus. working principle. properties (attributes of the characteristic). application

Detectable PhenomenonStimulus Quantity

Acoustic Wave (amplitude, phase, polarization), Spectrum, Wave Velocity

Biological & Chemical Fluid Concentrations (Gas or Liquid)

Electric Charge, Voltage, Current, Electric Field (amplitude, phase,

polarization), Conductivity, Permittivity

Magnetic Magnetic Field (amplitude, phase, polarization), Flux, Permeability

Optical Refractive Index, Reflectivity, Absorption

Thermal Temperature, Flux, Specific Heat, Thermal Conductivity

Mechanical Position, Velocity, Acceleration, Force, Strain, Stress, Pressure, Torque

Physical Principles• Amperes’s Law

– A current carrying conductor in a magnetic field experiences a force (e.g. galvanometer)

• Curie-Weiss Law– There is a transition temperature at which ferromagnetic materials

exhibit paramagnetic behavior

• Faraday’s Law of Induction– A coil resist a change in magnetic field by generating an opposing

voltage/current (e.g. transformer)

• Photoconductive Effect– When light strikes certain semiconductor materials, the resistance of the

material decreases (e.g. photoresistor)

Need for Sensors

• Sensors are omnipresent. They embedded in our bodies, automobiles, airplanes, cellular telephones, radios, chemical plants, industrial plants and countless other applications.

• Without the use of sensors, there would be no automation !!

Choosing a Sensor

MeasurementsHeisenberg (1927): ”The momentum and position of a particle can not both be precisely determined at the same time.”Measuring activity disturbs the physical process (loading effect).Measurement error:That is the difference between the measured value and the true value.error = measured value - true valueDeterministic errors:They are repeated at every measurement, e.g. reading offset or bias. Sucherrors can be corrected by calibration.Random errors:They are caused by several parameters and change in time in anunpredictable fashion. They can be quantified by mean errors, standarddeviation.Precision:Measurements with small deviationAccuracy:Measurements with small errors, i.e. small bias and high precision.

Sensor properties

A sensor should represent a physical variable as fast and as accurately as possible.A sensor is represented by its characteristic.Ideally, the sensor characteristic is a straight line

input

output

ideal

factual

SENSOR CHARACTERISTIC

• Full scale input (input span)– A range of stimuli that can be converted by

one sensor.• Full scale output (output span)

– Full scale output is the algebraic difference between the output signals measured with maximum input stimulus and with minimum input stimulus applied.

SENSOR CHARACTERISTICAccuracy : Error measurementSensitivity: change in output for unit change in

inputResolution: the smallest change in the signal that

can be detected and accurately indicated by a sensor.

Linearity: the closeness of the calibration curve to a straight line.

Drift: the deviation from the null reading of the sensor when the value is kept constant for a long time.

SENSOR CHARACTERISTICHysteresis: the indicated value depends on

direction of the test (increasing and decreasing)Repeatability (precision): the maximum deviation

from the average of repeated measurements of the same static variable.

Dynamic Characteristics: A sensor may have some transient characteristic. The sensor can be tested by a step response where the sensor output is recorded for a sudden change of the physical variable.

The rise time, delay time, peak time, settling time, percentage overshoot should be as small as possible.

Typical specification