TRANSDUCERS

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MECHANICAL MEASUREMENTS
Prof. Dr. Ing. Andrei Szuder Tel.
40.2.1.4112604 Fax. [phone] www.labsmn.pub.
ro szuder_at_labsmn.pub.ro
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TRANSDUCERS
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What is a transducer?

• Converts (transduces) variations in measurand to
  variation in
• voltage
• current
• resistance
• position of a pointer
• height of a liquid column
• fluid pressure
• Also known as
• probe
• gauge
• sensor

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Why do we need additional sensors?

• Our sensors require contact, or at best operate
  at short range
• Examination of the sensory homunculus indicates
  that we rely mostly on touch (lips, fingers
  tongue)
• Our eyes are sensitive to a very small band in
  the EM spectrum
• Our ears are sensitive to a small range of
  vibrations
• We use additional sensors to extend both the
  frequency sensitivity and dynamic range of our
  existing senses

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Why our Eyes are Sensitive between 400 and 700nm
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Definition of a Sensor and a Transducer

• A TRANSDUCER IS A DEVICE THAT CONVERTS INPUT
  ENERGY INTO OUTPUT ENERGY, THE LATTER USUALLY
  DIFFERING IN KIND BUT BEARING A KNOWN
  RELATIONSHIP TO THE INPUT.
• A SENSOR IS A TRANSDUCER THAT RECEIVES AN INPUT
  STIMULUS AND RESPONDS WITH AN ELECTRICAL SIGNAL
  BEARING A KNOWN RELATIONSHIP TO THE INPUT.
• Many measuring and sensing devices including
  loudspeakers, thermocouples, microphones and
  phonograph pickups, may be termed transducers.

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Using the Electromagnetic Spectrum
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Some EM Applications
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Using the Acoustic Spectrum
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Automobile Electronics
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Automotive Sensors
Oxygen Sensor
Accelerometer
Airflow Sensor
Oil Pressure
Water Temperature
CO Sensor
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Transducers

• Transducers convert energy or information from
  one form to another.
• They are important in measurement systems because
  a better measurement of a quantity (e.g.,
  temperature, strain, or light intensity) can be
  made if it can be converted to another form that
  is more easily or accurately displayed.

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Transducer

• A device that transforms one physical effect into
  another.

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Sensors

• Transducers that are used in measurement systems
  are often called sensors.
• Sensors that convert measurands (e.g.,
  temperature, strain, or light intensity) into
  electrical signals make up the vast majority of
  sensors used today.

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Selection Criteria

• What is to be measured
• Magnitude, range, dynamics of measured quantity
• Required resolution, accuracy
• Cost
• Environment
• Interface Requirements
• Output quantity (voltage, current, resistance,)
• Sensitivity
• Signal conditioning
• A/D requirements (bits, data rate)

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Basic Concepts

• Accuracy the deviation of the instruments
  reading from a known input, usually expressed as
  a percentage of full scale
• Precision (repeatability) the ability to
  reproduce a certain reading with a given accuracy
• Resolution (least count) smallest detectable
  difference in measured quantity

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Performance Descriptors

• Range, Span min to max values of input, span
  max-min
• Error actual value measured value
• Accuracy extent the measured value might be
  wrong
• ex. 2C or 2 of full scale
• Sensitivity (gain) linear output/unit input
  ex. 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
• 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

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Static / Dynamic

• Dynamic input changes with time. Static does
  not.

Input
Dynamic
Static
Time
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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

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Frequency Response

• Overall behavior of system to the frequency of a
  dynamic input.

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Frequency
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Linear Freq. Response

• The ratio of output to input amplitudes remains
  the same over input frequency range.

1
Frequency
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Natural Frequency

• The frequency where the output to input amplitude
  ratio increases greatly if system is under
  damped.

Natural Frequency
1
wn
Frequency
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Phase Shift

• When the output is delayed in time from the
  input.

Phase Shift
Time
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Linear Amplitude Response

• The ratio of output to input amplitudes remains
  the same over input amplitude range.

AI
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Rise Time or Delay

• Time for the output amplitude to rise to the
  input level after a change in input level.

Input signal
Input
Output response
Rise Time
Time
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Slew Rate

• The rate of change of the output amplitude.

Input signal
Input
Output response
Slew rate
Time
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Time Constant

• The time required for the output to change 63.2
  of its total change.

Input signal
Output response
Input
63.2
Time
Time constant
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Transducers
Analogic
Transducers
Numeric
Parametric
Piezo electrics
Generators
Photo electrics
Thermo electrics
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Parametric transducers
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Parametric transducers

• Resistive
• Inductive
• Capacitive

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Resistive transducers

• The starting resistance of the strain gage is R.

• As the strain gage is strained (stretched or
  compressed), the resistance changes by an amount
  DR. DR can be or - .
• The strain is given by
• Where S is a property of the strain gage called
  the gage factor.

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Beam Elongation with Tensile Load
Strain
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Electrical Properties of the Resistance Gage
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Strain Gage

• Most strain gages consist of thin film metal
  wires bonded to a plastic backing.
• This complete chip is usually glued to the
  structure whose strain you want to measure.

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Strain Gage

• The gauge length limits the spatial resolution of
  the sensor.
• Connection to the bridge is made at the solder
  tabs.
• The backing material needs to be made of
  something that can
• Withstand the temperatures encountered
• Transmit strain but electrically insulate
• Accept the bonding adhesive

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Gauge Factor
For most strain gauges, n 0.3 and GF 2.
Strain gauges are calibrated by their
manufacturer in a biaxial strain field generated
by the bending of a standard beam. Therefore,
the GF value includes some sensitivity to lateral
strains.
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Gauge Factor Errors
If we use the gauge in a strain field with a
different lateral strain, we will get an error
described by
Where ea, eL axial and lateral strains npo
Poissons ratio of member used for calibration,
usually 0.285 eL error as percent of axial
strain Kt lateral sensitivity of the strain
gauge
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Strain Gage Sets
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Resistive transducers
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Resistive transducers
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Semiconductor Strain Gauges
The semiconductors gauges are dominated by the
piezoresistive component of the change in
resistance and have several advantages and
disadvantages

• Pros
• Very high gauge factors (up to 200)
• Higher resistance
• Longer fatigue life
• Lower Hysteresis
• Smaller
• High frequency response

• Cons
• Temperature sensitivity
• Nonlinear output
• More limited on maximum strain

Mostly used for construction of transducers
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Inductive Transducers
A current I will produce a magnetic field in
the loop represented by B with a unit of tesla
or gauss. I T 1 newton/ampere-meter 104 gauss.

The magnetic flux F produced by current I in a
loop with area A BA
Inductance is defined as the change in the
magnetic flux per unit change in current in a
loop.
For one turn coil ,
For N turn coil ,
From Faradays Law of Induction,
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Inductive Transducers
For an N turn coil of length x and diameter d,
the magnetic flux density B is given by
where µ is the permeability in air 4px10-7
if xgtgtd
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Inductive Transducers
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LVDT - Linear Variable Differential Transformer
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LVDT - Linear Variable Differential Transformer
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LVDT - Linear Variable Differential Transformer
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LVDT - Linear Variable Differential Transformer
LVDT - Linear Variable Differential Transformer
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LVDT - Linear Variable Differential Transformer

• A sensing shaft is attached to an iron core
• The shaft moves within a cylinder
• The cylinder has one primary coil, two secondary
  coils
• An AC voltage is applied to the primary coil
• The secondary coils are connected in a specific
  way

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LVDT - Linear Variable Differential Transformer

• When the sensing shaft is at the centre, no emf
  is induced in the secondaries
• Away from centre, an emf is induced
• The amplitude of the induced emf is related to
  the displacement
• The phase of the induced emf depends on the
  direction of motion
• Produces an output voltage which is proportional
  to the sensing shaft position
• Have to measure amplitude and phase of induced
  voltage
• Also have rotary version (RVDT)
• Can be purchased with signal conditioning - DC
  input, output proportional to position

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Proximity inductive transducer
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Capacitive Transducers
where Q is charge, e relative permitivity e0
8.8x10-12 faraday/m
Either A, d or e can be varied.
Differential capacitor system linear and more
accurate
C1
C2
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Capacitive Transducers
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Energetic (Active) transducers
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Passive Sensors

• Passive sensors directly generate an electric
  signal in response to a stimulus.
• They do not emit radiation.
• Cannot be detected (covert)
• Rely on a locally generated or natural source of
  radiation (sunlight) or a field (gravity).
• Can operate from ELF (lt3x103 Hz) to gamma rays
  (gt3x1019 Hz).
• Prone to feature ambiguity and errors of scale
• Availability is not guaranteed (contrast, light
  levels etc.)
• Good reliability due to simplicity

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Passive Sensor Collage
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Active Sensors

• Active sensors require the application of
  external power for their operation. This
  excitation signal is modified by the sensor to
  produce an output.
• Often matched to the target characteristics -
  efficient
• Restricted to frequencies that can be generated
  and radiated fairly easily. This excludes part of
  the far IR, the UV and gamma ray spectra.
• Ambiguity constrained by range and angle
• Easy to detect because they radiate (not covert)
• Long range operation possible
• More complex than passive sensors so less reliable

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Active Sensor Collage
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Photovoltaic Detectors

• Photovoltaic effect consists of the generation of
  a potential difference as a consequence of the
  absorption of radiation
• The primary effect is photo-ionisation, or the
  production of hole-electron pairs that can
  migrate to a region where charge separation can
  occur.
• This charge separation usually occurs at a
  potential barrier between two layers of solid
  material. These can include semiconductor PN
  junctions and metal-semiconductor interfaces
• For a material with a conversion efficiency ?,
  the average current (amps) produced by a light
  beam with optical power P is as follows
• A
• As the output current is proportional to the
  input power, this is a square law detector

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Heating Detectors

• Micro Bolometers operate by lattice absorption
  resulting in increased vibrational energy and
  hence changes in resistance
• Metal types have a ve temperature coefficient of
  resistance
• Semiconductor (thermistor) types generally have a
  ve temperature coefficient of resistance
• Pyroelectric sensors produce a change in
  electrical polarisation with changes in
  temperature

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Heating Detectors

• Golay Cells rely on the expansion of a gas when
  heated to measure thermal radiation intensity
• Crookes radiometer relies on thermally induced
  motion of gas molecules to measure radiation
  intensity
• Thermocouple operation relies on the temperature
  dependent potential difference that exists
  between dissimilar metals in contact
• Thermal detectors measure the rate at which
  energy is absorbed and are therefore insensitive
  to frequency over a wide range

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Thermistors

• Thermistors change their resistance with changes
  in temperature in a rather exaggerated way.
• Two types positive temperature coefficient (ptc)
  and negative temperature coefficient (ntc).
• ptc thermistors the resistance increases with
  increasing temperature (as it does for a pure
  metal), however, the response is usually
  extremely nonlinear
• ntc thermistors, the resistance decreases with
  increasing temperature.

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IMAGING INFRARED

• Electro-optical thermal imagers include the
  following
• Forward looking Infrared (FLIR)
• Thermal imaging systems (TIS)
• Infrared search and tracking (IRST)
• Generally use the temperature gradient across an
  object to produce TV like images
• Should not be confused with image intensifiers,
  though the boundaries between the two
  technologies are becoming blurred.

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Example of a Thermal Imager Some Images made
using an Uncooled Sensor
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Thermal Infrared Detectors

• PHOTOCONDUCTIVE DETECTORS
• Absorb photons to elevate an electron from the
  valence band to the conduction band of the
  material, and so change the conductivity of the
  detector. To detect far IR (8-12?m) radiation
  they must be cooled to eliminate the noise
  generated by thermally generated carriers
• PHOTOVOLTAIC DETECTORS
• Absorb photons to create an electron-hole pair
  across a PN junction to produce a small electric
  current or potential difference
• MICRO BOLOMETERS
• Absorb thermal energy over all wavelengths, heat
  up slightly and change their resistance. Do not
  require cooling.

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The Thermocouple principles

• Operation based on Seebeck effect

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• Which results in the thermoelectric emf if the
  circuit is cut in half

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How do we measure thermoelectric emf?

• Any DVM that we might use has copper (possibly
  tin-plated or nickel-plated) therminals!

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• Which is equivalent to

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Thermocouples
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Accelerometers

• All work with spring-mass-damper arrangement
• Mass moves relative to case in proportion to the
  acceleration
• A variety of transduction methods are used to
  measure this movement

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Piezoelectric transducers

• When pressure is applied to a crystal, it is
  elastically deformed. This deformation results in
  a flow of electric charge (which lasts for a
  period of a few seconds). The resulting electric
  signal can be measured as an indication of the
  pressure which was applied to the crystal.
• These sensors ca not detect static pressures, but
  are used to measure ra idly changing pressures
  resulting from blasts, explosions, pressure
  pulsations (in rocket motors, engines,
  compressors) or other sources of shock or
  vibration.

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Piezoelectric transducers
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Photoelectric Sensor

• A photoelectric sensor is an electrical device
  that responds to a change in the intensity of the
  light falling upon it.

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Biomedical
Ultrasound Transducer
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Radiation detectors phototube,
photodiodes, phototransistors
Phototube, photomultiplier tube
Photocathode is made of photoemissive materials
like antimony (Sb) or cesium (Cs) that emit
electrons when struck by light photons. The
electrons then are accelerated toward the anode
to gain more energy.
-

e-
See Fig. 2-18 on p74 for spectral characteristics
of a few materials
e-
e-
e-
Phototube
Anode
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Photodiodes reverse biased p-n junction
-

p
n
Ir
The reverse current Ir is proportional to the
incident light intensity
Light intensity 0
I
Light intensity gt 0
Si is most sensitive in the the infrared region
V
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Displacement, proximity, position, level

• Potentiometer (rotary and linear)
• Strain Gage
• Proximity switch (mechanical)
• Piezo-ceramic, piezo-resistive
• LVDT

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Non-Contact Sensors

• Ultrasonic
• Optical
• Magnetic (Inductive, Reed, Hall Effect)
• Laser vibrometer, interferometer
• Capacitive
• Eddy current

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Motion/Velocity

• Accelerometer (piezo-electric)
• Optical Encoder absolute or incremental
  position, direction
• Tachometer shaft velocity, typically a PM DC
  motor

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Force/Torque

• Strain gage
• Piezo-electric (AC coupled)
• Piezo-resistive, piezo-ceramic

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Pressure

• Microphone
• Diaphragm
• Tube, Bellows
• Manometer

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Flow measurement

• Orifice plate, venturi
• Turbine meter
• Float
• Rotameter
• Hot-wire anemometer
• Laser interferometer
• Pitot tube
• Positive displacement meter (rotary vane)

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Temperature

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

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Optical

• Photo-voltaic cell
• CdS sensor (R output)
• Phototransistor

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Position transducers

• Mechanical
• Electrical
• Optical
• Other

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Mechanical position transducers

• Rule
• Caliper/
• Vernier caliper
• Micrometer
• Dial gauge
• Float
• Precision comparators

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Optical position transducers

• Encoders
• Gratings
• Interferometer
• Triangulation
• Imaging
• Laser Radar
• Holographic

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Other position transducers

• Ultrasonic
• Radio waves
• Radioactivity

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Potentiometric transducers

• A resistive wire is wound round a core
• A pointer is attached to a sensing shaft
• The pointer contacts the wire
• The resistance between A C varies as the shaft
  moves
• Vac varies with the sensing shaft position
• The variation is reasonably linear over most of
  the range

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Potentiometric transducers
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Potentiometric transducers
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Potentiometric transducers

• resolution spacing of turns
• accuracy -gt manufacture
• Also have rotary versions
• Simple operation- cheap

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Questions

• 1. Estimate likely values for range
• 2. Estimate likely values for resolution
• 3. Estimate dynamic response
• 4. What sort of problems occur in operation?

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Digital position transducers

• The position is indicated by a series of pulses
• Each pulse represents one unit of movement
• May be linear or rotary (more common)

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Methods of producing pulses in of digital
position transducers

• Conducting
• Inductive
• Optical

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Conducting

• As the shaft moves along, the voltage turns on
  and off

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Inductive

• Induced EMF changes with position
• Used on machine tools

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Optical

• Uses a glass or plastic disc or strip
• Light source and light detector
• The patterns are deposited photographically
• Can use two gratings to reduce errors
• Question
• What are the relative advantages and
  disadvantages of resistive/ inductive/ optical
  digital position transducers?

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Different types of digital position transducer

• Tachometer encoder
• Incremental encoder
• Absolute encoder

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Tachometer encoder (digital position transducer)

• The pulses are counted to indicate displacement
• It is not possible to tell the direction of
  motion
• No indication of the origin is given - it
  measures relative motion only

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Incremental encoder

• Two or three sets of tracks and detectors
• One track is 1/4 of a cycle behind the other
• Indicates direction as well as magnitude of the
  motion
• A 3rd track may count cycles
• Indicates direction of motion but not origin - no
  absolute measurement

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Incremental encoder
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Absolute encoder - (shaft encoder)

• Several tracks
• Each track has a light source and detector
• Each position has a unique binary code
• The absolute position is determined from the
  combination of values in each track
• Used in machine tools, CMMs, robots etc.

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Absolute encoder - (shaft encoder)
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SAQ 18

• An angular encoder has 16 tracks with a total
  angular range of 360o. Its angular resolution is
• a) 5o
• b) 0.5o
• c) 0.05o
• d) 0.005o

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SAQ 19

• The accuracy with which an encoder cam measure
  angle is determined by
• a) the number of tracks
• b) the size of the tracks
• c) the accuracy of manufacture of the tracks
• d) the separation of the tracks

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Non-contact position and displacement measuring
systems

• Vision systems
• Laser scanning
• Optical triangulation
• Laser interferometer
• Moire methods
• Radar techniques

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Vision systems.

• An image is formed by a video camera.
• It is input to a computer
• Image processing software is used
• It can identify components automatically (for
  automatic assembly systems, robot handling)
• It can detect faults
• It can make measurements

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Optical triangulation

• The position of the surface changes in the
  z-direction
• The position of the laser spot image on the
  detector moves
• Used with video cameras
• Measures z-position of surface
• Used to monitor thickness of gypsum board as it
  is extruded.
• Also used in CMMs (co-ordinate measuring
  machines) to measure profile rapidly

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Laser interferometry

• The motion of one mirror gives a changing
  intensity at the detector.
• Used to calibrate machine tools
• Used to measure straightness and flatness
• Also used as a position transducer in a surface
  finish measuring instrument

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Radar techniques.

• Send out a pulse
• Measure time taken for it to return.
  
• Can use
• Laser
• Ultrasonics.
• Radio waves
• Particularly useful for robotic sensors,
  automatic guided vehicles

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MEASUREMENT OF STRAIN.

• Strain change in length per unit length.
• Important for
• testing structures
• use in other transducers

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Sensing methods.

• Change of electrical resistance, i.e. strain
  gauging.
• Brittle lacquer
• Photoelasticity
• Holographic and Moire methods.

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Resistive strain gauge

• The resistance, R, of a piece of wire is
  inversely proportional to its length
• If the wire is stretched
• l increases
• R increases

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Gauge factor

• We define the gauge factor ? by
• 6 for metal gauges
• May be up to 150 for semi-conductor gauges
• Question
• What is significance of the gauge factor?

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Measuring strain using strain gauges

• Metal wire gauges most commonly used.
• Why is a long thin wire required?
• A thin wire is attached in a zigzag or circular
  pattern to a substrate.
• Produced by photochemical etching.
• Multi-element gauges (rosettes) are used to
  measure strain in more than one direction.

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Bonding of strain gauges.

• Requires care and expertise
• Require maximum mechanical coupling
• Specialised adhesives available - discuss with
  manufacturer.

122
Temperature dependence of strain gauges.

• Resistance varies with temperature
• Why does this affect strain gauges?
• Must be taken into account in strain gauging.
• Use dummy gauge
• Use two or four active gauges

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Other points

• Dynamic response of strain gauges can be up to
  100kHz.
• Strain gauges may operate down to 7oK and up to
  1500oC - special techniques required for
  adhesion.
• Used in difficult environments, e.g. monitoring
  movement of undersea oil rigs.

124
Other strain measuring methods.

• Brittle lacquer
• Holographic interferometry
• Speckle interferometry
• Moire methods
• Photoelasticity

125
Strain gauges in position sensors

• Usually use four on a moving member
• Two expand, two contract
• Use a bridge
• The output voltage is related to the displacement

126
Force transducers

• Some sensing mechanisms
• balancing the force against the gravitational
  force of a known mass
• Applying the force to an elastic member and
  measuring the deformation
• Measuring the change in frequency of a wire
  tensioned by the force

127
Force transducers

• Using force balance - schematic diagram

128
Force transducers

• Use an elastic member
• May use position transducer to measure total
  displacement
• May use strain gauges to measure change in form

129
Temperature transducers

• Sensing methods
• thermal expansion
• change in shape or size with temperature
• thermocouples
• two wires of different materials formed into a
  loop produce a current when one junction changes
  temperature
• resistive devices
• electrical resistance changes with temperature

130
Temperature transducers

• Thermal expansion - bi-metallic elements
• Two metals with different thermal expansion
• Structure deforms as temperature changes

131
Pressure transducers

• Sensing methods
• Manometers - varying liquid level in a tube
• Elastic devices (similar to force transducers)

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Pressure transducers

• Various elastic devices
• Shape changes in response to change in pressure
• Change in shape may be detected by a position
  transducer or strain gauges

133
Other transducers

• Velocity
• Flow
• Torque
• Level
• Humidity
• Sound level
• Light level
• and many others

134
Mutual Inductance

I1
V1
Coil 1
Core

I2
Coil 2
V2
Example Non-contact monitoring of respiratory
motion
I1
Coil 1
I2
Coil 2
135
Inductive Displacement Transducer linear
variable differential transformer (LVDT) which is
a 3-coil system consisting a primary coil and two
secondary coils with high permeability alloy
slug. Its advantage is higher sensitivity and
better linearity. Its disadvantage is that the
output is phase sensitive.
Inductive Transducers are most ideal for
radiotelemetry i.e. non-contact transmission of
radio signals.
Excitation voltage 3 10 v at 60 Hz to 20 kHz,
sensitivity 0.2 5 mv/0.001 in/v, displacement
0.005 to 1 in