Analog Sensors Intro and Explanation ppt
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![Page 1: Analog Sensors Intro and Explanation ppt](https://reader036.fdocuments.net/reader036/viewer/2022062511/5501821f4a795971028b46c3/html5/thumbnails/1.jpg)
Chap 3 Analog sensors
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3.1 Introduction
● to prevent error ⇒ error measurement sensor are used (type and
characteristics have to be found) ● size (length) measurement is important in precision machines sensors measuring length error are mostly studied (in the course)
● sensor limitations sometimes machine structure can be changed
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3.1 Introduction
● sensor is a device that responds to or detects a physical quantity and transmits the resulting signal to a controller
● transducer: transforms energy types
▶ piezo material: electric E ⇔ mechanical E (sensor or actuator)▶ shape memory alloy: thermal E ⇒ mechanical E (transform: change to original shape)
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Definitions
● absolute: output is always relative to a fixed reference regardless of the initial condition● incremental: output is a series of binary pulses ● analog: continuous output (proportional to physical quantity being measured)● digital: discrete output ▶ series of binary pulses▶ each pulse represents sensor’s resolution
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Sensor performance
ex) accuracy, resolution, repeatability ● average outputaverage value of sensor output from many data ⇒ As N increases, random error decreases at the ratio of (N)1/2 (noise level decrease) ⇒ resolution increases
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sampling rate
● v: max. slew rate (related to measurand change)● increase resolution() by a factor of N (resolution δ is limited by random noise) ● total sampling time: ttotal sampling = δ/Nv(during this time, measurand will not change by more than 1/N times the resolution)● in order to increase resolution by averaging random noise ⇒ N2 data points have to be taken⇒⇒ minimum required sampling period
tsample = /N3v
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frequency response
● effect on the output of sensor of the physical quantity being measured
frequency()
1
Sensor outputphys quan
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Definitions● hysteresis: maximum difference in sensor output between measurements made from 0 to 100% full scale output and 100 to 0%● linearity: variation in proportionality constant (between output signal and measured physical quantity)● mapping: measuring sensor response to a known input under known conditions and storing results in a look-up table or fitting mathematical expression to the data⇒ nonlinearity, hysteresis, and temperature effects can be compensated
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Definitions ● noise: magnitude of any part of the sensor output that is not directly related to the physical quantity being measured ● noise input margin: maximum noise input level (ex, deviation in supply voltage) that can be tolerated without affecting desired sensor performance ● resolution: smallest detectable change in the measured physical quantity
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Definitions
● sensitivity: variation in sensor output caused by a variation of physical quantity
● slew rate error: how the accuracy of sensor changes with the rate of change of measured physical quantity ● standoff distance: distance between sensor and target
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Definitions● step response: time-varying change in sensor output given a step change in the measured physical quantity
Delay timeRise time
Storage time
Fall time
Time
Sen
sor
oup
ut
10%
90%
100%
Ideal response to a step input
Actual response to a step input
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3.1.2 System dynamics
● sensor frequency response ⇒ system’s ability to respond to changes in the measurand(fast process ⇒ inaccurate measurement) ⇒ frequency that a sensor’s output tends to decrease because it can no longer accurately detect changes in a rapidly changing measurand ⇒ determined experimentally or analytically How quickly system accuracy can be degraded with increasing frequency
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Mathematical modeling of a Dynamic system
● sensor system (complicated in machine tools)
2
'' ' ( )
( )( )
mx bx kx u t
U sG s
ms bs k
( )( )
( )
log ( ) log ( ) log ( )
i
j
i j
N sG s
D s
G s N s D s
log-log ⇒ graphical addition
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Mathematical modeling of a Dynamic system
● Most sensor frequency response is given in terms of 3 dB response pt (1st order system)- = n
- response = 0.707 of the response at zero frequency input(dc) (30% error)- Operates well before its -3dB frequency
● response (dB) = 20 /log10G
frequency()
1
센서출력물리량
0.7
n frequency()n
phaselag
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3.2 Nonoptical sensor systems
● generating analog signals or digital pulses in response to a physical process by other than optical means
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3.2.1 capacitance sensors
● determines the distance (gap) between probe and target surface ⇒ measures capacitance formed by two parallel plates● non-contact● performance (accuracy, linearity) determined primarily by probe capacitance
▶ large probe: 10∼100 pF ▶ small probe: 0.01∼0.1 pF
target
sensor
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3.2.1 capacitance sensors ● target material: metal (conductor), dielectric, semiconductor ⇒ affects the sensor output ▶ metal : affects sensor output equallyex) calibrated over STS measurement over brass and aluminum▶ dielectric material : different calibrations
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Applications of capacitance sensors
● motion of rotating parts (spindles, bearings)● mapping flatness of delicate objects (lens, silicon wafers)● thickness (two probes are used)
- metal thicknessl1 l2
t
1 2( )t l l l
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Applications of capacitance sensors
● thickness of dielectric plateone probe is used in conjunction with a grounded metal surface dielectric material is introduced into the gap resultant change in capacitance thickness of material is determined
metal metal대상
부도체
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Applications of capacitance sensors
● pressure measurementrigid frame + diaphragmThe diaphragm deforms due to pressure ⇒ capacitance change between diaphragm and sensor
- proximity (presence/no presence) of nonmetal and liquid level (cf, cheap inductive proximity sensor for metal)
diaphragm
p
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probe design for capacitance sensors
● depends on application and electronic circuit configuration- matching probe shape to target surface ⇒ performance improve ● stray capacitance (between probe sensor and outer body) – removed or reduced by using guard electrode (10-14∼10-16 F) ▶ operate guard and electrode by same voltage waveform ⇒ reduce stray capacitance effect▶ collimate electric field line between sensor and target ⇒⇒ capacitance between parallel plates
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probe design for capacitance sensors
body guard
CGB
target
CGT
CSGCST
sensor
bodyguard
sensor
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Dielectric constant
● dielectric constant, ε ⇒ how easily electromagnetic waves can travel through a medium
ε = f(temp, pressure, humidity, media type)
/C A d
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probe design for capacitance sensors
● temperature, humidity ⇒ easy to control● pressure ⇒ difficult to control ⇒ reference gage (against a fixed gap) is used to compensate pressure change● cutting oil, oil shower (affecting ε) should not affect guard can be used ● A (area) ⇒ affecting accuracy▶ depending on temperature (temperature affects machine and sensor)▶ As A/d increases, accuracy, resolution better ▶ A’s surface finish large average effect
small topography effect
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3.2.2 Hall effect sensors
● Lorentz's law: charged particle moves in a magnetic field ⇒ force acting ⇒ motion trajectory of the particle changes ⇒ Lorentz force ● Hall effect: result of Lorentz force acting on electrons flowing through a semiconductor ⇒ potential produced in direction orthogonal to the excitation current and magnetic fieldOutput voltage ∼ order of millivolts ⇒ electronics is necessary to amplify voltage⇒ combination of (Hall element + amplifier electronics) into one: Hall effect transducer
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3.2.2 Hall effect sensors
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3.2.2 Hall effect sensors
Design Hall effect sensors to be sensitive to magnetic pole (triggering)
V=0
V=VHall
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3.2.2 Hall effect sensors
bipolar digital on/off Hall effect sensor ● combined with transistor⇒ ‘+’ (south pole) maximum trigger pt, ‘-’ (north pole) release pt ● with a magnet, Hall effect sensor produces an analog voltage proportional to the magnetic field strength⇒ distance measuring sensor (voltage is proportional to magnetic strength, and the magnetic strength is related to the distance)
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Characteristics
● magnet is needed (attached to the target)- strength changes depending on time ⇒ inadequate to ultra high precision (resolution > 5 m)- other magnets and electric field magnetic strength changes● inexpensive compared with capacitance sensor by 2 orders of magnitude● accuracy depends on the accuracy of power supply (supply voltage to semiconductor: 5V±0.001V)
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Typical applications
● unipolar head-on modeone magnet triggers Hall effect sensor (moving parallel to the direction of magnetic field)
G1
G2
D1 D2
S
B
Gau
ss
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Typical applications
● unipolar slide-by modeone magnet triggers Hall effect sensor (perpendicular, large motion)
S
B
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Typical applications
● bipolar slide-by mode
Have directionality- analog : voltage = f(distance)- digital : trigger, release
Gau
ss
Distance
B
NS
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Typical applications
● bipolar slide-by mode
B
NN
S
B
S
S
NN
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Typical applications
● Hall effect sensor can be used as a digital on/off proximity sensor
- Monitors position of objects hidden from view as long as the barrier does not block magnetic field lines⇒ sensing through dielectric and nonferrous metals
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3.2.3 Inclinometers
● electromechanical levelPrecisely measures small angle of a body (wrt horizontal or vertical reference)
▶ movement data during construction▶ motion of local geologic formation▶ machine platform stability measurement▶ machine orientation, slope measurement
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Types
● precision pendulums are used
● mercury bubble wets a linear resistor (the more device tilts, the more resistor wet and the greater the change in the output voltage)▶ surface tension low resolution▶ inexpensive
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inclinometer● inclinometer tilts ⇒ position sensor generates an electric signal (amplified and fed back to galvanometer ⇒ galvanometer produces torque ⇒ mass to original position ⇒ current applied to galvanometer to generate balancing torque is proportional to sinθ(current x resistance = voltage) ● natural frequency depends on (g/l)1/2
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3.2.4 Inductive digital on/off proximity sensors
● consists of wire-wound ferrite (iron) core, oscillator, detector, solid-state switch
● oscillatorproduces (high-frequency) electromagnetic field centered on core
Metal target
Coil and core
Oscillator
Detector
Output
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3.2.4 Inductive digital on/off proximity sensors
Target motion
Housing
Ferrite core
Shield
Target motion
Housing
Ferrite core
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3.2.4 Inductive digital on/off proximity sensors
● metal object moves inside electromagnetic field ⇒ eddy current inside object ⇒ receives energy from field ⇒ amplitude of oscillation decreases ⇒ if amplitude change > a value, transistor in sensor is triggered on- Object is removed⇒ transistor is back to original state (off) ● sensor response time depends on effective inductance of circuit (L), resistance(R) time delay occurs (analogous to mechanical mkb system)
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3.2.4 Inductive digital on/off proximity sensors
● other trigger, release pt ⇒ the difference is embeddied in the sensor- Small sensitive to noise- normally 2∼15% of full-scale range (decided by manufacturer) ● sensor diameter – proportional to allowable standoff distance (larger longer distance)
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head on sensor
● shield to make field in front of sensor sharp⇒ count parts
Targer motion
Housing
Ferrite core
Shield
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slide-by sensor
● wide field without shield (oscillating field of sensor recovered after target passes ⇒ should be released before next object triggers)
Targer motion
Housing
Ferrite core
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inductive digital on/off proximity sensor
● no magnet (cf, Hall effect sensor) ● metal powder should not be attached● non-contact (cheap)● large standoff distance(∼25mm) is possible● conductive targets are needed● no moving parts ⇒ low failure possibility(replacing mechanical switch)
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3.2.5 Inductive distance measuring sensors
Target
referencecoil
active coil
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3.2.5 Inductive distance measuring sensors
● apply AC current to reference coil⇒ electromagnetic field occurs⇒ current is inducted in conductor⇒ inductive current generates magnetic field and reduces intensity of original field⇒ changes effective impedance of active coil⇒ impedance change is detected⇒ analog voltage (related to distance between sensor and target) is produced
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3.2.5 Inductive distance measuring sensors
● performance depends on material properties- Homogeneous electric properties- Good conductivity, small magnetic permeability (good for aluminum, copper, brass) requiredFerrous metal is not good ⇒ thin target material is plated or bonded (epoxy)
- 0.5mm (gold, silver, copper, aluminum)
- 1.3mm (magnesium, brass, bronze, lead)
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Characteristics ● output affected by conductive material - dirt (if nonmetal) has no problem (in optical, big problem)● thickness, diameter measurement, concentricity, 2 axis alignment● range ↑ resolution ↑
Sheet thickness
Orbit tracing and shaft error motions diameter
Shaft and dynamic displacement
ConcentricityTwo axis alignment
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Precision measurement
differential mode sensor is used for precision measurement (2 sensors)
● one against fixed object only environmental effect occurs
● difference between outputs of two sensors error compensated
● two sensors should have similar characteristics (environmental effect to be similar)
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3.2.6 Inductosyns
● copper sheet on metal plate by insulating adhesive (0.07mm), square wave ● inductive coupling is used between two coils (many overlapping windings used ⇒ error averaging) ● linear Inductosyn is a linear motion transducer having scale and slider
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3.2.6 Inductosyns
Slider
Scale
Slider
Two windings 90 out of phase
Scale
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scale
● thin STS plate is covered with insulating adhesive
● wire strip is attached onto insulating surface by printed circuit technology⇒ continuous square wave (pitch = 0.1", 0.2", 2mm)
● fixed onto machine axis
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slider
● attached onto carriage (table)
● moving above scale surface (0.1mm)(required straightness of slider = 10∼20μm)
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Slider output ● scale is excited by 5∼10kHz signal (A sint) ⇒ slider output
B, X determined from two outputsB: amplitudeX: linear displacementS: waveform period – known
13
24
sin sin(2 / )
sin cos(2 / )
S B t X S
S B t X S
13 24/ tan(2 / )S S X S
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● precision, resolution (∼ 0.12 μm) depends on number of waveforms per unit length
● overlapping of many coils in slider, scale averaging effect noise removed, high precision ● coarse/fine position sensing system
- waveform: coarse position information- sine wave interpolation: fine position resolution
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Characteristics ● most simple (almost no problem)
● performance is affected only when dirt particle breaks circuit(dirt is fatal in optical sensors)
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Tyes of commercial linear Inductosyn
● standard (∼ 250mm)
● tape-type- One end attached to machine element and the other mounted for tension control- Tension adjustment when installed ⇒ precision is fine adjustable
● adjustable InductosynClamp and tension adjusting screw at each 3”, and adjustable independently
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rotary Inductosyn ● stator (slider)- Two separated square waveforms (sine track, cosine track are repeated) ⇒ two tracks cover entire stator
● rotor (scale)- square waveform covers entire circumference (overlapping occurs) extremely good random noise reduction (∵averaging effect)- Periodic error does not decrease ⇒ compensated by mapping
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rotary Inductosyn ● many sine, cosine waveforms
- averaging effect error reduction- High resolution (0.1 μrad)- high repeatability (0.5 μrad) ⇒ used in precision rotary table
● if too many ⇒ sine, cosine waveforms are crowded ⇒ coupling between two occurs ⇒ resolution↑, accuracy ↓
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3.2.7 Linear and rotary variable differential transformers (LVDT,
RVDT) ● electromagnetic induction principle is used- linear motion detection (<10∼20 cm)- rotary motion detection (1 rotation)
Vout
Vreference
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3.2.7 Linear and rotary variable differential transformers (LVDT,
RVDT)
core
e1 e2
e1- e2
Linear operating region
X motion
Output (e1- e2)
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LVDT components
● armature (or core) – made of ferritic (magnetic) alloy
● stem - nonmagnetic alloy- fix core to object
● transformer- Consists of a primary ac excited coil and 2 secondary coils
![Page 63: Analog Sensors Intro and Explanation ppt](https://reader036.fdocuments.net/reader036/viewer/2022062511/5501821f4a795971028b46c3/html5/thumbnails/63.jpg)
LVDT components
● armature moves in coil (noncontact) ● if primary is excited by AC power⇒ armature position affects output voltages of two secondary coil(one +, the other - ⇒ directionality determined) ● relative distance between two objects ⇒ core fixed on one, and transformer fixed on the other
![Page 64: Analog Sensors Intro and Explanation ppt](https://reader036.fdocuments.net/reader036/viewer/2022062511/5501821f4a795971028b46c3/html5/thumbnails/64.jpg)
Characteristics of operation
● non-contact between winding and armature ⇒ no friction, wear, hysteresis ⇒ theoretically infinite life, extremely high reliability
● if core is properly supported, no stick/slip ⇒ theoretically infinite resolution(accuracy, resolution depends on signal conditioning electronics and A/D converter)
● stable AC excitation source is needed
![Page 65: Analog Sensors Intro and Explanation ppt](https://reader036.fdocuments.net/reader036/viewer/2022062511/5501821f4a795971028b46c3/html5/thumbnails/65.jpg)
Characteristics of operation
● signal conditioner converts AC voltage in secondary coil to DC
● high output ⇒ simple circuit
● measurement length can be easily increased
● less sensitive to core radial motion
● simple, shock resistant ⇒ theoretically no maintenance is necessary
![Page 66: Analog Sensors Intro and Explanation ppt](https://reader036.fdocuments.net/reader036/viewer/2022062511/5501821f4a795971028b46c3/html5/thumbnails/66.jpg)
RVDT
● similar to small electric motor ⇒ generates output voltage varying linearly depending on shaft angle
● rotation is analogous to linear motion in LVDT
● generates output voltage in secondary coil depending on shaft angle
![Page 67: Analog Sensors Intro and Explanation ppt](https://reader036.fdocuments.net/reader036/viewer/2022062511/5501821f4a795971028b46c3/html5/thumbnails/67.jpg)
3.2.8 Magnetic scales
● sliding sensing head is used ⇒ detects magnetic field strength from magnetically recorded scale (sine and cosine outputs) ● scale: wire (many north, south pole pairs) wire imprinted scale
sliding sensing head
N S S N N S
![Page 68: Analog Sensors Intro and Explanation ppt](https://reader036.fdocuments.net/reader036/viewer/2022062511/5501821f4a795971028b46c3/html5/thumbnails/68.jpg)
3.2.8 Magnetic scales
![Page 69: Analog Sensors Intro and Explanation ppt](https://reader036.fdocuments.net/reader036/viewer/2022062511/5501821f4a795971028b46c3/html5/thumbnails/69.jpg)
3.2.8 Magnetic scales
● merit (compared with optical sensors) – less affected by dirt, fluid contamination ● incremental⇒ slide produces two waves ⇒ electronics interprets moving distance and produces digital signal ● magnitude scale: thin wire ⇒ scale minor misalignment (0.1mm wrt moving axis) causes measurement error, irrelevant to wear
![Page 70: Analog Sensors Intro and Explanation ppt](https://reader036.fdocuments.net/reader036/viewer/2022062511/5501821f4a795971028b46c3/html5/thumbnails/70.jpg)
3.2.9 Magnetostrictive sensors
● "magnetostriction" - length change of ferromagnetic material in magnetic field- magnetization change under mechanical stress
● non-contact torsion sensormeasures torsional stress of a rotating shaft ⇒ power measurement (to prevent excessive stress in rotating shaft)
● strain gage was used (attached on shaft)(expensive, electric noise, unreliable due to wear)
![Page 71: Analog Sensors Intro and Explanation ppt](https://reader036.fdocuments.net/reader036/viewer/2022062511/5501821f4a795971028b46c3/html5/thumbnails/71.jpg)
Villari effect
● magnetic field change in the direction of mechanical strain (shaft twiated ⇒ strain in 45°) ● several Villari differential torque transformers along circumference ⇒ torsional stress can be precisly measured ● initial calibration is necessary (ferrous material behaves differently under similar conditions) ● sensor mounted close to the shaft ⇒ large output signal (∼ mV)
![Page 72: Analog Sensors Intro and Explanation ppt](https://reader036.fdocuments.net/reader036/viewer/2022062511/5501821f4a795971028b46c3/html5/thumbnails/72.jpg)
Villari effect
![Page 73: Analog Sensors Intro and Explanation ppt](https://reader036.fdocuments.net/reader036/viewer/2022062511/5501821f4a795971028b46c3/html5/thumbnails/73.jpg)
Guillemen effect
● magnetic material is in magnetic field size changes ● diameter of a long small rod changes locally
v
L
2L=vt
![Page 74: Analog Sensors Intro and Explanation ppt](https://reader036.fdocuments.net/reader036/viewer/2022062511/5501821f4a795971028b46c3/html5/thumbnails/74.jpg)
Guillemen effect
⇒ diameter change stress wave reflecting point
⇒ ultrasonic transducer sends stress wave and measures time (wave reflected)
⇒ time is related to distance (magnetic field and ultrasonic transducer)
![Page 75: Analog Sensors Intro and Explanation ppt](https://reader036.fdocuments.net/reader036/viewer/2022062511/5501821f4a795971028b46c3/html5/thumbnails/75.jpg)
3.2.11 Piezo material bonded sensors
● force is applied to crystalline structure oscillation ⇒ high-energy electrons emits (current flow) ⇒ stress is predicted by measuring current ● examples) accelerometer, precision loadcell, ultrasnic transducer
![Page 76: Analog Sensors Intro and Explanation ppt](https://reader036.fdocuments.net/reader036/viewer/2022062511/5501821f4a795971028b46c3/html5/thumbnails/76.jpg)
piezoelectric accelerometer
● speed control, vibration sensing, position measurement
● resolution (1 μg)
● frequency response ∼ 100 kHz (good response)
● small size
![Page 77: Analog Sensors Intro and Explanation ppt](https://reader036.fdocuments.net/reader036/viewer/2022062511/5501821f4a795971028b46c3/html5/thumbnails/77.jpg)
precision load cells
● thin film crystalline piezo material is used ⇒ measures nanostrains
● more sensitive than metal strain gage (2∼3 orders of magnitude)
● load range limit (maximum strain level ~ μ strain)
![Page 78: Analog Sensors Intro and Explanation ppt](https://reader036.fdocuments.net/reader036/viewer/2022062511/5501821f4a795971028b46c3/html5/thumbnails/78.jpg)
ultrasonic piezoelectric sensors
● piezoelectric, magnetostrictive and electrostatic are possible
● stress wave generating/receiving sends pressure wave and receives echo pulse measure time determines distance to impurities or shape change
![Page 79: Analog Sensors Intro and Explanation ppt](https://reader036.fdocuments.net/reader036/viewer/2022062511/5501821f4a795971028b46c3/html5/thumbnails/79.jpg)
3.2.12 Potentiometers
● change in electric resistance resulting from change in physical process ⇒ definition of “potentiometer” ● consists of coil or high-resistance film and wiper (used mostly)
![Page 80: Analog Sensors Intro and Explanation ppt](https://reader036.fdocuments.net/reader036/viewer/2022062511/5501821f4a795971028b46c3/html5/thumbnails/80.jpg)
3.2.12 Potentiometers
● wiper position target motion
● DC voltage is applied to entire length of coil ⇒ wiper picks off intermediate voltage (variable resistance) ⇒ determine wiper position ● plastic film (large resistance)Film is continuous, potentiometers are analog devices ⇒submicron resolution is possible, but relies on DC power supply and D/A converter
![Page 81: Analog Sensors Intro and Explanation ppt](https://reader036.fdocuments.net/reader036/viewer/2022062511/5501821f4a795971028b46c3/html5/thumbnails/81.jpg)
potentiometer
● most expensive
● high output voltage (no amplification necessary)
● mechanical contact between wiper and film ⇒ contaminated by dust or oil ⇒ resistance change, error ⇒ preventing contamination by using various seals
● friction and wear exist
● very small overhead compared with size
![Page 82: Analog Sensors Intro and Explanation ppt](https://reader036.fdocuments.net/reader036/viewer/2022062511/5501821f4a795971028b46c3/html5/thumbnails/82.jpg)
3.2.13 Synchros and resolvers
● electromagnetic induction ⇒ (rotary) transformer (between 1st and 2nd coil)
● used in shaft positioning
● can measure infinite revolutions
![Page 83: Analog Sensors Intro and Explanation ppt](https://reader036.fdocuments.net/reader036/viewer/2022062511/5501821f4a795971028b46c3/html5/thumbnails/83.jpg)
Characteristics
● high precision, inexpensive, insensitive to contamination (compared with potentiometer or optical sensor)
● disadvantage: analog device (∵ mostly used with digital control system)
![Page 84: Analog Sensors Intro and Explanation ppt](https://reader036.fdocuments.net/reader036/viewer/2022062511/5501821f4a795971028b46c3/html5/thumbnails/84.jpg)
synchro
● variable transformer (output voltage is varying):The magnitude of the electromagnetic coupling between primary and secondary coils (which determines output voltage) varies with the relative angular position of the coils ● 2 modes- control synchro: electric signal depending on shaft rotation- torque synchro: shaft rotation depending on electric signal
![Page 85: Analog Sensors Intro and Explanation ppt](https://reader036.fdocuments.net/reader036/viewer/2022062511/5501821f4a795971028b46c3/html5/thumbnails/85.jpg)
control synchro
● transmitters, differentials, controltransformers, receivers
● used in servo control axes
![Page 86: Analog Sensors Intro and Explanation ppt](https://reader036.fdocuments.net/reader036/viewer/2022062511/5501821f4a795971028b46c3/html5/thumbnails/86.jpg)
torque synchro
● transmitters, differentials, receivers
● dial of instrument is rotated by transferring rotating angle from transmitter
![Page 87: Analog Sensors Intro and Explanation ppt](https://reader036.fdocuments.net/reader036/viewer/2022062511/5501821f4a795971028b46c3/html5/thumbnails/87.jpg)
synchro applications
● long distance transmission from transmitter to receiver (up to 4km)
● used in remote devices
ex) remote steering in ships, opening/closing water gates in a power plant
![Page 88: Analog Sensors Intro and Explanation ppt](https://reader036.fdocuments.net/reader036/viewer/2022062511/5501821f4a795971028b46c3/html5/thumbnails/88.jpg)
transmitters and receivers
● single-phase rotorpower supply via a slip ring (brush)● three-phase stator (electromagnetically coupled with rotor )● rotor winding is excited by AC (60∼400Hz) ⇒ voltage is inducted in stator winding (proportional to cone of angle between rotor coil and stator coil)
rotor
statorS1
S2
S3
R1
R2
![Page 89: Analog Sensors Intro and Explanation ppt](https://reader036.fdocuments.net/reader036/viewer/2022062511/5501821f4a795971028b46c3/html5/thumbnails/89.jpg)
transmitters and receivers
● accuracy, repeatability and linearity depends on winding quality● exciting voltage at the rotor
2 1
2 1
2 1
2 1
1 3
2 1
3 2
max
sin
sin
sin( / 3)
sin( / 3)
/out
rotor o
rotor
rotor
rotor
in
V V t
V kV
V kV
V kV
k V V
![Page 90: Analog Sensors Intro and Explanation ppt](https://reader036.fdocuments.net/reader036/viewer/2022062511/5501821f4a795971028b46c3/html5/thumbnails/90.jpg)
transmitters and receivers
● transmitter sends information
● receiver receives information- analog signal- converted to digital signal ⇒ information decoding device
synchro-to-digital converter (SDC)
![Page 91: Analog Sensors Intro and Explanation ppt](https://reader036.fdocuments.net/reader036/viewer/2022062511/5501821f4a795971028b46c3/html5/thumbnails/91.jpg)
torque synchro
● torque transmitter and receiver are the same● angle information is transferred electrically (not mechanically)● a gage dial is rotated by the angle
Vin
S2
S1
S3
S2
S1
S3R1
R2
R1
R2
CG CR
![Page 92: Analog Sensors Intro and Explanation ppt](https://reader036.fdocuments.net/reader036/viewer/2022062511/5501821f4a795971028b46c3/html5/thumbnails/92.jpg)
control synchro
● control transmitter, control transformer, control differential transmitter are used ⇒ signals are combined electrically and amplified ⇒ rotates motor(because of being amplified, control transmitter winding does not have to be as good as torque synchro)
● synchro performance- positioning accuracy: ±10 arcminutes- Maximum torque: 3 g-mm per receiver angle
![Page 93: Analog Sensors Intro and Explanation ppt](https://reader036.fdocuments.net/reader036/viewer/2022062511/5501821f4a795971028b46c3/html5/thumbnails/93.jpg)
differential
● used with transmitter and receiver● three-phase stator, three-phase rotor● via slip rings (brushes) current is transferred to rotor winding
rotorstator
S2
S3 R1
R2
S1 R3
![Page 94: Analog Sensors Intro and Explanation ppt](https://reader036.fdocuments.net/reader036/viewer/2022062511/5501821f4a795971028b46c3/html5/thumbnails/94.jpg)
control transformer
● stator, rotor coil structures are the same as in transmitter
● transmitter sends position information through 3 wires to control transformer transformed to voltage and amplified to drive the motor
● angle of motor axis is mechanically coupled to angular motion of the control transformer
● if the angles of motor and transmitter are different the shaft rotates because voltage is applied
![Page 95: Analog Sensors Intro and Explanation ppt](https://reader036.fdocuments.net/reader036/viewer/2022062511/5501821f4a795971028b46c3/html5/thumbnails/95.jpg)
control transformer
Vin
S2
S1
S3
S2
S1
S3R1
R2
R1
R2
CG CR motorAmp
![Page 96: Analog Sensors Intro and Explanation ppt](https://reader036.fdocuments.net/reader036/viewer/2022062511/5501821f4a795971028b46c3/html5/thumbnails/96.jpg)
resolver
● special form of synchro● better accuracy, resolution than synchro● resolver transmitter: ends of rotor windings connected ⇒ one frequency excitation is needed
rotor
R1
R2
S2
S1
S4
S3
![Page 97: Analog Sensors Intro and Explanation ppt](https://reader036.fdocuments.net/reader036/viewer/2022062511/5501821f4a795971028b46c3/html5/thumbnails/97.jpg)
resolver
● windings added ⇒ resolution ↑
13
24
13
24
sin
sin sin
sin cos
tan
V A t
S kA t
S kA t
S
S
![Page 98: Analog Sensors Intro and Explanation ppt](https://reader036.fdocuments.net/reader036/viewer/2022062511/5501821f4a795971028b46c3/html5/thumbnails/98.jpg)
synchro, resolver-to-digital converters
● device to transform signal to digital (in order to be used in digital control) ● SDC, RDC 16 bit resolution ⇒ total rangeis divided into 216
⇒ resolution = total range/216
![Page 99: Analog Sensors Intro and Explanation ppt](https://reader036.fdocuments.net/reader036/viewer/2022062511/5501821f4a795971028b46c3/html5/thumbnails/99.jpg)
3.2.14 ultrasonic sensors
● stress wave generating/ receiving ⇒ pressure wave to medium ⇒ measures amplitude and return time of the echo
Time is related to the distance to impurities or shape change in material ● piezoelectric transducers
- used in NDT (∼100 kHz) - measurement of thickness and surface
roughness (resolution: 10 to 12 bits)
![Page 100: Analog Sensors Intro and Explanation ppt](https://reader036.fdocuments.net/reader036/viewer/2022062511/5501821f4a795971028b46c3/html5/thumbnails/100.jpg)
3.2.14 ultrasonic sensors
● magnetostriction transducers ● electrostatic transducers- Use metal film, metallic backing dish- oscillating applied to film ⇒ inductive current in dish due to electric field ⇒ current in field generates force ⇒ attracting and pusjing forces between dish and metalized film act repeatedly ⇒ ultrasonic wave occurs
- dish, film are small mass ⇒ fast response
- high frequency used ⇒ high precision, high resolution in small range
![Page 101: Analog Sensors Intro and Explanation ppt](https://reader036.fdocuments.net/reader036/viewer/2022062511/5501821f4a795971028b46c3/html5/thumbnails/101.jpg)
3.2.15 Velocity sensors
● velocity feedback is needed for position, velocity control ● position signal differentiated ⇒ noise spikes● relative motion between magnet and (moving) coil induces voltage- Proportional to relative velocity between coil and magnet and rate of magnetic flux change (due to moving magnet)
![Page 102: Analog Sensors Intro and Explanation ppt](https://reader036.fdocuments.net/reader036/viewer/2022062511/5501821f4a795971028b46c3/html5/thumbnails/102.jpg)
Linear velocity transducers (LVT)
● measures linear velocity
● similar to LVDT, but does not use excitation voltage ● uses two coils ⇒ voltage difference represents speed, and sign of voltage represents direction
![Page 103: Analog Sensors Intro and Explanation ppt](https://reader036.fdocuments.net/reader036/viewer/2022062511/5501821f4a795971028b46c3/html5/thumbnails/103.jpg)
Linear velocity transducers (LVT)
● modeled as pure inductance, resistor ⇒ behaves as 1st order system
= 2 L/R (time constant: ~0.001 sec) ● speed ↑ ⇒ time lag ↓⇒ linearity ↓
N S
V1 V2
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Tachometer
● velocity transducer
● types: - permanent magnet stator dc tachometers- drag-torque tachometers- capacitor tachometers- digital tachometers- dc brushless tachometers
![Page 105: Analog Sensors Intro and Explanation ppt](https://reader036.fdocuments.net/reader036/viewer/2022062511/5501821f4a795971028b46c3/html5/thumbnails/105.jpg)
permanent-magnet stator dc tachometers
● consists of rotor winding, commutator assembly● armature (rotor winding) rotates ⇒ windings passes stator magnetic field ⇒ voltage is induced in windings proportional to rotational speed● reverse of dc motor● voltage is transferred by brush, slip-ring● number of armature coils is discrete ⇒ ripple occurs ⇒To reduce ripple, filtering techniques and many coils are used● other error source: commutator interface contamination, temperature change, magnetic field
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permanent-magnet stator dc tachometers
![Page 107: Analog Sensors Intro and Explanation ppt](https://reader036.fdocuments.net/reader036/viewer/2022062511/5501821f4a795971028b46c3/html5/thumbnails/107.jpg)
brushless tachometers
● magnetic rotor, wound stator ⇒ rotor and stator in brushed tachometer are switched
● no problem of brush wear
● to produce homogeneous DC signal proportional to speed, switching to other windings is required ⇒ expensive, complicated, noise reduction, no brush wear