Sensors(part2)

Instructor: Shuvra DasMechanical Engineering Dept.

University of Detroit Mercy

Flowchart of Mechatronic Systems

Some Sensors: Strain gages

• Bonded to surface of object.• Resistance varies as a function of the

surface strain experienced by the object• Hence subjected to same strain as the object

when the object elongates or compresses• Can be used to measure stress, force,

torque, pressure, etc.

Some Sensors: Strain gages

• R = (L)/A = ( ) , = resistivity• Stretching/compression changes Resistance

– G= (R/R0)/(L/L) = (R/R0)/

• G is called gage factor

L

Some Sensors: Strain gages

• G = gage factor ~ 2 for metal foil strain gages• Maximum strain foil can measure = 0.005=0.5%• R= R0G R0~120R=120 X 2 X 0.005 =

1.2 , for a strain of 0.005 • Strain gage is placed in a wheatstone bridge

circuit and the change in resistance is measured

Some Sensors: Strain gages

• Wheatstone bridge is used • Voltage change is measured • Voltage change is related to R • Example:

– cantilever beam– 4 strain gages, 2 on top, 2 bottom– R1&R4 are in tension– R2&R3 are in compression

Strain gages

• Can be used in quarter bridge mode, half bridge mode and full bridge mode.

• All resistances are chosen as equal initially.

• The bridge is used to measure the change in resistance and that leads to strain.

Strain gages

• Full bridge mode: R1 and R4 stretches• R2 and R3 compresses• R1=R4 = R0+ R;

• R2 = R3 = R0 - R;

• V0 = va-vb = iaR1-ibR3• V0=VsR1/(R1+R2) - VsR3/(R3+R4)• V0=Vs(R0+ R)/(R0+ R+ R0-R) -

Vs(R0-R)/(R0-R+ R0+ R)

Strain gages

• V0=Vs(R0+ R)/(R0+ R+ R0-R) - Vs(R0-R)/(R0-R+ R0+ R)

• V0 = Vs (2R)/2R0=Vs R/R0

• V0 =Vs R/R0=VsG

• V0 =VsGVsGf(F)

• relation will change for 1/4 bridge or half bridge

Some sensors: Strain gages

• Strain gages are typically mounted in the cantilever mode or on surfaces of tubes to form load cells. Load cells are used to measure forces/loads

Some sensors: Thermocouples

• A thermocouple is formed by the junction of two dissimilar metal

• The junction is called a thermoelectric junction and produces a voltage proportional to the temperature of the junction

• the above effect is called SEEBECK effect (1821).

Metal A Metal B

Some sensors: Thermocouples

• To form an electrical loop the thermocouple junctions should be in pairs. The voltage difference between the two junctions is related to the temperature difference.

V

Thermocouples: examples

Some Sensors:Thermocouples

• As the previous table shows the temperature is non-linearly related to the measured voltage difference and can be expressed as

• in the previous table V is measured in VOLTS

in

iiVaT

0

Thermocouples

• Correspondingly the voltage can be represented in terms of the temperature as the third table shows and in this case:

• In the table on the following page the voltage is shown in millivolts

in

iiTcV

0

Thermocouples

• As the previous table indicates voltage has a non-linear dependence on temperature.

• This dependence is obtained by polynomial curve fitting of empirical data

• All the data in these tables are with reference to 0 degrees

• if the reference is a non-zero value one needs to convert the voltage reading (see a little later)

Thermocouple: connections

• To measure voltage in a thermocouple circuit additional leads may be introduced which work as additional dissimilar-metal junctions.

• Special connections are needed to handle these situations.

Thermocouple: connections

• Need to know these junction temperatures

• another option is to use a cold-junction compensation (this works as a reference)

Thermocouples: Cold junction compensation

• Measured voltage now depends on T1-Tref

• Isothermal block keeps external junctions at the same temperature

Thermocouples: laws

• Law of intermediate temperatures: the voltage due to two junctions in a circuit is only dependent on the temperatures of the junctions and is independent of any intermediate temperature.

• E t/0 = E t/i+E i/0

• the tabulated emf values are all E t/0

t i i 0

Thermocouples: Laws

• Law of intermediate metals: A third metal in a circuit has no effect on the resulting voltage if the new junctions created are all at the same temperatures. This is why copper connectors used for measurements has no effect on measured temp.

V

T3 T3

Thermocouples:Example

• A standard two-junction thermocouple configurations being used to measure temperatures in a wind tunnel. The reference junction is held at a constant temp of 10C. We have only standard thermocouple tables that are referenced to 0C. We want to determine the output voltage when the measuring junction is exposed to an air temp. of 100C.

Thermocouple: ExampleJunction temp(C ) Output Voltage (mV)0 010 0.50720 1.01930 1.53640 2.05850 2.58560 3.11570 3.64980 4.18690 4.725100 5.268

6

Thermocouple: Example

• E 100/0 = E 100/10+E 10/0

• 5.268 = E 100/10 +0.507

• E 100/10 = 5.268 - 0.507=4.761 mV

Some Sensors: Capacitive sensor

• Also used to measure displacements• Parallel plate capacitor with a fixed end• One plate fixed and the other moves in

harmony• Displacements => change in

capacitance=>change in voltage in a circuit• e.g. condenser microphone

Ultrasonics

• Sonar was developed in WWII• Two designs:• * Passive sonar: Detects the

noise emitted from submarines• * Active sonar: Transmits a

signal and listens to the reflected signal. Distance can be determined from the time it takes the pulse to travel back and forth.

• Typical transmission and receiving pattern for a sonar onboard a ship

To find distance

• To determine the range of an object that has reflected a signal back to the sonar’s receiver, we can use:

• d=0.5vt, Where– d = distance (m)– v = propagation velocity of sound in the water (m/s)– t = time (s)

Three types

• Magnetorestrictive (high power)• piezoelectric (high frequency, <600 khz)• electrostatic (low frequency, 20-50 khz)

Ultrasonic transducer

• Piezoelectric element resonates at a specific frequency.

• Pulses supplied through the connector and the impedance matcher.

• Damping chamber absorbs residual energy to prevent “ringing”.

• Acoustic lens focuses acoustic energy.

The Electrostatic Microphonea.k.a Capacitor Microphone

• Diaphragm and the fixed plate, together, form a parallel plate capacitor.

• As the diaphragm vibrates, due to sound pressure, the distance between the two capacitor plates varies, producing a variable capacitance output.

• The variable output impedance can be used to measure the acoustic pressure.

• With an oblique surface the reflection may not go back towards the sensor.

• A diffuse surface will scatter the sound pulse instead of reflecting it back to the sensor. On the Polaroid sensor, the surface cannot be more than 25 degrees off perpendicular with the sensor.

Implementation Issues

Implementation Issues

• With a wide beam sensor it has more of a chance of hitting something normal but directionality is lost. With a narrow beam, the object must reflect the pulse close to normal but location measurement is very accurate.

Applications

• distance measurement• density measurement• crack/failure detection• flow rate measurement (doppler)• fluid level sensing• speed measurements (doppler)

Active Noise Control (ANC)Systems

• Active Noise Control (concept): use a microphone as a sensor and a loudspeaker as an actuator to minimize the effect of a primary noise source at a specific location.

Outside noise signal processing resulting wave

ULTRASONIC DETECTION

• Detection of sonic energy that is beyond 20 KHz (up to 5 MHz).

• Used extensively in medicine for diagnostic purposes. In some cases replacing x-rays.

• Used in industry and in consumer electronics for the measurement of distance, velocity, and detection of small objects.

• Very high frequencies are used (as opposed to sonar detection), since shorter wavelengths bounce off smaller surface areas far more efficiently.

Detection of flaws in workpiece• Because of the known

geometry of the workpiece the echo pattern is known ahead of time.

• Any flaws in the workpiece will add additional echos to this pattern. Also referred to as pulse echo ultrasonic method.

Application: Measuring Velocity

• A continuous signal of a specific frequency is transmitted.• Frequency of the echo signal is compared with that of the

original signal.• the change in frequency indicates the velocity of the target

(Doppler Effect).

Application (Acoustic Imaging)• The precise distance measuring

capabilities of ultrasonic transducers is utilized to generate an acoustical image of an object.

• The target is scanned by a highly collimated ultrasonic beam. An accurate representation of the target’s relief features can then be developed.

• The resultant image is produced on a screen

Mems sensors: Airbag

• Almost all the collisions occur within 0.125 seconds after car crash according to investigation.

• The airbag is designed to inflate in less than 0.04 second. • In a collision, the airbag begins to fill within 0.03 second.• By 0.06 second, the airbag is fully inflated and cushions the

occupant from impact. • The airbag then deflates (de powers) 0.12 second after

absorbing the forward force. • The entire event, takes about 55 milliseconds --- about half

the time to blink an eye

Mems sensors

Capacitive Sensing & Actuation

• A large number of MEMS devices operate based on capacitive sensing and actuation techniques.

• Actuation: Applying a voltage exerts force on parallel plates.

• Sensing: Movement of plates, changes the size of the gap, and hence the voltage sensed in the capacitor.

Airbag sensor element

• Upper flat plate of capacitor is asymmetrically shaped and is balanced by the torsion bar.

• An acceleration produces moment about the torsion bar that makes the plate rotate about torsion bar, thus altering the capacitance.

• Typical dimensions are 1000 X 600 X 5 microns

Displacement, proximity, position, (linear and angular)

• Proximity/limit switch (mechanical)– Binary output

• Potentiometer (rotary or linear)– Analog output

• LVDT: Linear Variable differential Transformer

LVDT Joystick (2 pots)

Microswitch™ Limit Switch

Differential Transformer

Non-Contact Sensors

• Ultrasonic• Optical • Magnetic (Inductive, Reed, Hall Effect)

• Laser vibrometer• Capacitive or Eddy current

– measuring vibration of rotating shafts

UltrasonicFluid Level Meter

Rotational Position/Velocity

• Optical Encoder –absolute or incremental angle, direction

500 pulses/rev Incremental, Quadrature50 pulses/rev Incremental4 Bit Absolute Encoder

Rotational Position/Velocity

•Tachometer – voltage shaft velocity, •typically a small PM DC motor used as a generator

•Toothed wheel (gear) + magnetic pickup + counter

Vibration

• Accelerometer – Piezo-electric (AC)

– IC, Strain gage (DC)

Force/Torque

• Strain gageResistance Strain

• Piezo-electric (AC coupled)

• Piezo-resistive, piezo-ceramic

FORCE

Pressure

• Microphone(AC coupled)

• Diaphragm(for static measurement)

• Tube, Bellows

• Manometer

Flow measurement• Orifice plate, venturi• Turbine meter• Float• Rotameter• Hot-wire anemometer

Flow measurement

•Laser interferometer•Pitot tube•Positive displacement meter (rotary vane)

Temperature

• Thermometer• Thermocouple• Thermistor• RTD (platinum)• Solid state sensor (thermodiodes and transistors)• Pyro-electric sensor• Bimetallic strip• Optical pyrometer

Optical

• Photo-voltaic cell

• CdS sensor (R output) • Phototransistor

Selection Criteria

• What is to be measured?• Magnitude, range, dynamics of measured quantity (fast,

slow?)• Required resolution, accuracy• Cost • Environment (hot, dirty?)• Interface Requirements:

– Output quantity (voltage, current, resistance,…)– Sensitivity – Signal conditioning– A/D requirements (#bits, data rate)

Sensor Specifications

• Accuracy/Error: Conformity of the measurement to the true value

• Precision: Ability to reproduce the reading repeatedly

• Resolution: Smallest measurable increment• Span: Linear operating range; min to max

value• Range: Range of measurable values

Other related terms

• Error = actual value – measured value• Accuracy = extent the measured value might be

wrongex. ± 2°C or ± 2% of full scale

• Sensitivity = (gain) linear output/unit input e.g. 5 mv/psi, 0.5/ °C

• Hysteresis error – output value depends on whether input is rising or falling

• Non-linearity error – error resulting when assuming that the output is linearly related to input

Other related terms

• Repeatability/reproducibility – same output for repeated same input?

• Stability – drift of output over time for constant input• Dead band/time- range of input for no measurable

output• Resolution (least count)– output steps, smallest

measurable change in input• Output impedance – how sensor output is effected by

the electrical characteristics of what it is connected to

Static and Dynamic Characteristics

• Response time – time to 95% of final value for step input

• Time constant – time to 63.2% (1-e-1) of final value

• Rise time – time to rise some specified percentage of s.s. output

• Settling time – time to get to within 2% of the s.s. value

Sensor Specification and meaning

• Strain gage pressure sensor specifications:• Range: 70 to 1000 kPa• supply voltage: 10V dc or ac rms• full range output: 40mV• non-linearity and hysterisis: +/- 0.5% full range output• Temperature range: -54C to 120C when operating• Thermal zero shift: 0.030% full range output/ degree C

Sensor Specification and meaning

• Range: Can measure 70 to 1000 kPa• supply voltage: requires supply voltage of 10V dc or ac rms• full range output: 40mV will be output for 1000kPa

pressure• non-linearity and hysterisis will lead to an error of +/- 0.5%

(I.e. +/- 5kPa).• Can be used between -54C to 120C • when temp changes by 1C the output for zero input will

change by 0.030% of 1000 = 0.3kPa.