CHAPTER 6 UNIMAPUNIMAP SENSORS AND TRANDUCERS

6.1 INTRODUCTION  To introduce the basic concepts in measurement systems  To define sensor terminology  To identify sensor applications  To present the need for microsensors UNIMAPUNIMAP OBJECTIVES

UNIMAPUNIMAP 6.2 MEASUREMENT SYSTEMS Input Signal = Measurand Sensor (Input Tranducer) Chemical Quantity Eg. displacement, presure Eg. Gas concentration Physical Quantity Sensor : a device that converts a non-electrical physical or chemical quantity into an electrical signal.

UNIMAPUNIMAP DetectModify Display Record Transmit System Boundary Input Signal Output Signal Fig. 6-1: Functional block diagram of a measurement 6.2 MEASUREMENT SYSTEMS (cont……)

UNIMAPUNIMAP 6.2 MEASUREMENT SYSTEMS (cont…) SensorProcessorActuator (input transducer) (output transducer) System Boundary Input Signal Output Signal Fig. 6-2: Basic components of a measurement or information-processing system Processor : a device that modifies the electrical signal coming from the sensor without changing the form of the energy that describes the signal. Actuator or output transducer : a device that converts an electrical signal into a physical or chemical quantity.

UNIMAPUNIMAP 6.3 CLASSIFICATION OF SENSING DEVICES Form of SignalMeasurands Thermal Radiation Mechanical Magnetic Chemical Temperature, heat, heat flow, entropy, heat capacity. Gamma rays, X-rays, ultra-violet, visible, infra red, micro-waves, radio waves. Displacement, velocity, acceleration, force, torque, pressure, mass, flow, acoustic wavelength and amplitude. Magnetic field, flux, magnetic moment, magnetisati- on, magnetic permeability. Humidity, pH level and ions, concentration of gases, vapours and odours, toxic and flammable materials, pollutants. Table 6-1: Classification of sensors by signal form.

UNIMAPUNIMAP Form of SignalMeasurands Biological Electrical Sugars, proteins, hormones, antigens. Charge, current, voltage, resistance, conductance, capacitance, inductance, dielectric permittivity, polarisation, frequency. Table 6-1: Classification of sensors by signal form. 6.3 CLASSIFICATION OF SENSING DEVICES (cont……)

UNIMAPUNIMAP Table 6-2: Classification of the human senses. Human Sense SignalMeasurandSensing Device Analogue Device Sight Hearing Smell Radiant Mechanical Chemical Intensity and wavelength of light Intensity and frequency of sound Odorants Rods and cones in retina Cochlea in inner ear Olfactory receptor cells in nose Photographic film, photodiode, Phototransistor Microphone Electronic nose 6.3 CLASSIFICATION OF SENSING DEVICES (cont……)

UNIMAPUNIMAP Table 6-2: Classification of the human senses. Human Sense SignalMeasurandSensing Device Analogue Device Touch Taste Mechanical Biological Pressure, force Proteins Nerves Taste buds in tongue Potentiometers and LVDTs (simple touch), optical gauging and tactical arrays (complex touch) 6.3 CLASSIFICATION OF SENSING DEVICES (cont……)

UNIMAPUNIMAP Table 6-3: Classification of some common actuators. FunctionActuatorSignalPrinciple Display Transmit Light emitting diode Visual display unit Liquid crystal display Loudspeaker Aerial Electric motor Radiant Mechanical Radiant Mechanical Current generation of photons Fluorescent screen Transmittance of polarised molecular Crystals Generation of sound Generation of radio wave Generation of motion 6.3 CLASSIFICATION OF SENSING DEVICES (cont……)

UNIMAPUNIMAP Table 6-3: Classification of some common actuators. FunctionActuatorSignalPrinciple RecordThermal printer Magnetic recording head Laser Thermal Magnetic Radiant Ink is melted Magnetisation of thin films on computer disc Ablation of material on optical disc 6.3 CLASSIFICATION OF SENSING DEVICES (cont……)

UNIMAPUNIMAP 6.4 Ideal Sensor Characteristics and Practical Limitations Input SENSOR Output x (t)y (t) System SENSOR Output x (t)y (t) + y d External drive xdxd Input (a) Self-exciting(b) Modulating Fig. 6-3: Basic representation of self-exciting and modulating sensor systems A sensor in its simplest form may be regarded as a system with an input x (t) and output y (t).

UNIMAPUNIMAP 6.4 Ideal Sensor Characteristics and Practical Limitations (cont……) A self-exciting sensor has its output energy supplied entirely by the input signal x (t). The general equation that describes a self-exciting sensor system is where F(x(t)) is the characteristic relationship that describes the behavior of a self-exciting sensor. (6.1)

UNIMAPUNIMAP Eg : A thermocouple → input signal = the difference in junction temperatures ΔT(t) and the output = e.m.f Φ(t) in volts. In the case of the modulating sensor, the system equation can be written more explicitly as where the external supply signal x d (t) should ideally be stationary and noise free. 6.4 Ideal Sensor Characteristics and Practical Limitations (cont……) (6.2)

UNIMAPUNIMAP Fig. 6-4: Ideal input-output relationship of self-exciting and modulating sensors. Slope, S ymym x m -x d ydyd Sensor output Sensor input 0 x d ≠ 0 x d = 0 The ideal sensor not only has a linear output signal y(t) but it should instantaneously follow the input signal x(t), whence The slope S is usually referred to as the sensitivity. 6.4 Ideal Sensor Characteristics and Practical Limitations (cont……) (6.3)

UNIMAPUNIMAP General properties of a good sensor are: 1) Optimum measurement accuracy 2) Good durability 3) Ease of calibration and reconditioning 4) High sensitivity 5) Good reproducibility 6) Long term stability 7) Fast response 8) Continuous operation 9) Insensitivity to electrical and other environmental interference 10) Low fabrication, operation and maintenance cost