Velocity, Torque, Voltage, Light

Working principle of velocity transducer with frequency output (photodetector)

• shaft with reflective strip on non-reflective band rotates

• detector detect light source when reflective part faces light source

2 types of angular velocity transducer

revolution counter- measure rate of rotation

tachogenerators- generate output proportional to angular velocity due to electromagnetic induction

working principle of a variable reluctance tachometer

pick-up coil surrounds permanent magnet that is placed close to ferromagnetic gear wheel mounted on sensing shaft

reluctance of magnetic circuit varies due to changes in distance of pole to gear tooth

reluctance: resistance to change in magnetic flux (high reluctance at greater distance)

magnetic flux in coil changes which induce electromotive force as they are proportional

working principle of tachogenerator

• transducer use energy of rotation of sensing shaft to generate electric output via electromagnetic induction

• when conductor is moving perpendicular to field, emf is maximum as theta=90 and voltage=Blv

Working principle of strain gauge torque measurement+ equation of torque in terms of shear strain angle

• measured by detecting angular displacement of sensing shaft

• strain gauge detects strain on surface of shaft

• to maximise strain, place gauge in 45 degree angle

• place dummy gauge perpendicular to measuring gauge for temperature compensation

Definition and formula of shear sensitivity

strain and shear strain angle relationship + gauge sensitivity when gauge is placed 45 degrees to axis of shaft

• assume unit square area is small so that displacement=phi/2

Working principle of analogue voltmeter/ammeter

• use of electromotive forces

working principle of digital voltmeter

1. unknown amplified voltage applied to input terminal

2. passed to a successive approximation analog-to-digital converter(ADC)

3. convert continuous analog voltage signal to discrete digital code

4. code sent to microprocessor which determines actual voltage and convert to inputted scale and unit

5. microprocessor sent to digital display

working principle of successive approximation ADC

• compare input to reference voltage → if reference voltage<input, reference voltage increase by half and repeated

• reference voltage adjusted at smaller increments until it matches input voltage

Advantages of DVM over analogue voltmeter

• eliminate observational error by operator

• no parallax or approximation error

• quick reading

• output can be stored on memory device

• low power

• compact and cheap and accurate

working principle of photodiode

• when light of sufficient energy strike photodiode, it creates electron hole pair in semiconductor called photoelectric effect

• current is produced

working principle of LDR

• light energy excite electron from valence band to conductive band (E=hv)

• reduces resistivity

• semiconductor require lower excitation energy than metals