MECHANICAL MEASUREMENTS

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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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Displacement sensors
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Motion Sensors

• Measure kinematic variables
• displacement
• velocity
• acceleration
• These are all derivatives or integrals of each
  other!
• Very commonly used in a wide range of applications

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Displacement Sensors

• Resistive
• Potentiometers
• Inductive
• Linear Variable Differential Transformers (LVDTs)
• Resolvers, Synchros
• Electro-optical
• optical encoders
• Moire fringe (interferometric) devices
• Capacitive, piezo-electric, ultrasonic,
  magnetostrictive, etc.

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Displacement sensors
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Inductive (eddy current) displacementsensor

• Inductive displacement sensors utilize a
  high-frequency magnetic field which is generated
  by passing a high frequency current through the
  sensor head coil.
• When a metal target is present in the magnetic
  field, electromagnetic induction causes an eddy
  current perpendicular to the magnetic flux
  passage to flow on the surface of the target.
  This changes the impedance of the sensor head
  coil.

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Inductive (eddy current) displacementsensor

• Inductive displacement sensors measure the
  distance between the sensor head and target,
  based on this change in oscillation status.
  increases and oscillation amplitude becomes
  smaller.
• The oscillation amplitude is rectified and
  amplitude variations are converted to DC voltage
  variations.

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Inductive displacementsensor
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Inductive displacementsensor

• As the target comes closer to the sensor head,
  the oscillation amplitude becomes smaller and the
  phase difference from the reference waveform
  becomes larger.
• By detecting changes in the amplitude and phase,
  the sensor can obtain a value approximately
  proportional to the change in the distance
  between the sensor head and target.

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Linearization of analog output voltage

• As the target approaches the sensor, the eddy
  current increases and oscillation amplitude
  becomes smaller.
• The oscillation amplitude is rectified and
  amplitude variations are converted to DC voltage
  variations. variations.
• With its linearization circuit, the sensors
  corrects the output voltage-distance
  characteristic in order to optimize linearity.

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• Semiconductor laser displacement sensors and
  meters comprise a light-emitting element and a
  position sensitive detector (PSD) and detect
  targets using triangulation.
• A semiconductor laser is used as the light
  emitting element. A lens focuses the beam on the
  target.
• The target reflects the beam back through the
  lens where it is focused on the
  position-sensitive detector (PSD), forming a beam
  spot.
• The beam spot moves as the target moves.
  Displacement can be determined by detecting the
  movement of the beam spot.

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Laser confocal displacement meters

• When the lens accurately focuses the laser beam
  on the target (Fig. A), the reflected beam
  converges precisely at the pinhole over the
  light-receiving element. At this lens position,
  the maximum quantity of light is directed to the
  light-receiving element.
• As the lens moves closer to or farther from the
  target, however, the reflected beam is diffused
  (Figs. B and C). As a result, the uantity of
  light passing through the pinhole to the
  light-receiving element decreases greatly. Fig. D
  shows the relationship between the lens position
  and the quantity of light received.
• A detection signal is generated only when the
  lens is precisely positioned for maximum light
  reception (peak light quantity). The LT then
  calculates the lens position and outputs a
  measured value.

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Resolution
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Linearity
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Response frequency for analog output
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Zero adjustment(range)
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Span adjustment
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Capacity proximity sensors

• Capacitive proximity sensors have an oscillating
  electric field, sensitive to all materials
• dielectric materialssuch as glass, rubber and
  oil and conductive materialsmetals,
• salty fluids, moist wood, etc.
• Capacitance is a function of the size of the
  electrodes, the
• distance between them, and the dielectric
  constant (D) of the material between the
  electrodes.

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Capacity proximity sensors

• The top electrode is the face of the sensor.
• A seal ring, the target, passes between it and
  the ground electrode (a metal conveyor belt).
• The sensor housing insulates the electrode from
  galvanic coupling to ground. The rubber seal ring
  has a dielectric constant (D) of 4.0.
• When it enters the electric field, the
  capacitance increases.
• The sensor detects the change in capacitance and
  provides an output signal.

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Capacity proximity sensors

• Figure illustrates a metal target, or some other
  conductive material, entering the electric field.
  
• The resulting increase in capacitance is detected
  and converted to an output signal. If the
  effective distance between electrodes is
  reduced (by the factor t), the result is an
  increase in capacitance

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Capacity proximity sensors

• The level of conductive fluid pouring into a
  glass bottle is below the sensor.
• With no change in capacitance, there is no
  output.
• When the fluid reached the level of the sensor,
  providing the ground electrode. This happens even
  though the fluid and the metal table are
  separated by the glass of the bottle.
• The three materials form a capacitor. The
  alternating current provides a path to ground.
  With the ground electrode now in place, the
  circuit closes and a signal results.

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Capacity proximity sensors
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Environmental performance

• Figure shows a shielded sensor with two sensing
  fields its own, and the compensation field
  which the electrode creates.
• When contaminants lie directly on the sensor
  face, both fields are affected, and the
  capacitance increases by the same ratio.
• The sensor does not see this as a change in
  capacitance, and an output is not produced.

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• PHOTOELECTRIC SENSORS

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Detection configurations
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Detection configurations
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Detection configurations
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Detection glossary
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Detection glossary
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Detection glossary
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Light sources
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Fiber Photoelectric Sensors

• The optical fiber consists of the core and the
  cladding, which have different efractive indexes.
  
• The light beam travels through the core by
  repeatedly bouncing off the wall of the cladding.
  
• The light beam, having passed through the fiber
  without any loss in light quantity, is dispersed
  within an angle of approximately 60 and emitted
  to the target.

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Fiber Photoelectric Sensors

• Plastic-fiber
• The core of the plastic-fiber consists of one or
  more acrylic-resin fibers 0.25 to 1 mm 0.01" to
  0.04" in diameter, encased in a polyethylene
  sheath. Plastic fibers are light, cost-effective,
  and flexible and are used for the majority of
  optical fiber photoelectric sensors.
• Glass-fiber
• The glass-fiber consists of 10 to 100 µm 0.39 to
  3.90 Mil diameter glass fibers encased in
  stainless steel tubing, allowing it to be used at
  high operating temperatures (350C max.).

• Features
• Versatile installation
• A flexible optical fiber is employed for easy
  installation in areas such as the small spaces
  between machines.
• Detection of small objects
• The light-emitting surface of the sensor head is
  extremely compact for stable detection of small
  objects.
• Stable operation in harsh environment
• The optical fiber unit may be used even in an
  explosive environment, as no electric current
  flows through it. In addition, optical fibers are
  unaffected by electrical noise.
• Heat-resistant
• The heat-resistant fiber unit allows detection
  in a high temperature environment.

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Fiber Photoelectric Sensors Shape

• The optical fiber sensors are broadly divided
  into two categories thrubeam and reflective.
• The thrubeam type comprises a transmitter and a
  receiver.
• The reflective type, which is a single unit, is
  available in 3 types parallel, coaxial, and
  separate, according to the shape of the
  cross-section of the optical fiber.

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Liquid level detection fiber units

• Liquid immersion type
• When the fiber unit tip is present in the air,
  the emitted light is entirely reflected by the
  fiber units Teflon sheath and returns back to
  the receiver because the difference in refraction
  factor between the Teflon sheath and air is
  large. On the other hand, when the fiber unit tip
  is immersed in liquid, most of the emitted light
  is radiated into the liquid and does not return
  back to the receiver
• Because the difference in refraction factor
  between the Teflon sheath and liquid is small.
  The fiber unit detects presence or absence of
  liquid by using the above characteristics.

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Liquid presence detection fiber units

• Tube-mountable type
• When the tube to which the fiber unit is mounted
  contains no liquid, the emitted light is
  reflected by the inside wall of the tube and
  returns back to the receiver because the
  difference in refraction factor between the tube
  and air is large. On the other hand, when the
  tube contains liquid, most of the emitted light
  is radiated into the liquid and does not return
  back to the receiver because the difference in
  refraction factor between the tube and liquid is
  small.

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Color differentiation charts

• For color differentiation, choose a light source
  producing a distinct difference in the
  reflectance of the 2 colors to be differentiated
  (i.e. select a light source that allows the
  sensitivity adjustment trimmer setting positions
  corresponding to the 2 colors being discriminated
  to be as far apart as possible).
• The charts shown here give reference data for
  color differentiation. Detection is, however,
  affected by the surface condition and luminosity
  of the target.
• Confirm the sensitivity difference of the colors
  to be differentiated using the actual target.
• The received light level is a value which
  numerically expresses the light quantity received
  by the sensor.
• The above are sample colors. Note that they
  may differ slightly from those used in obtaining
  the data due to print quality.

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Color differentiation charts
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• PROXIMITY SENSORS

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PROXIMITY SENSORS

• A proximity sensor can detect metal targets
  approaching the sensor, without physical contact
  with the target.
• Proximity sensors are roughly classified into the
  following three types according to the operating
  principle
• the high-frequency oscillation type using
  electromagnetic induction,
• the magnetic type using a magnet,
• the capacitance type using the change of
  capacitance.

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PROXIMITY SENSORS

• Features
• Non-contact detection, eliminating damage to
  sensor head and target.
• Non-contact output, ensuring long service life.
• Stable detection even in harsh environments
  exposed to water or oil splash.
• High response speed.
• Compact sensor head for installation
  flexibility.

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PROXIMITY SENSORS
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High-frequency Oscillation Type Proximity
Sensor-General sensor

• A high-frequency magnetic field is generated by
  coil L in the oscillation circuit. When a target
  approaches the magnetic field, an induction
  current (eddy current) flows in the target due
  to electromagnetic induction. As the target
  approaches the sensor, the induction current flow
  increases, which causes the load on the
  oscillation circuit to increase. Then,
  oscillation attenuates or stops.
• The sensor detects this change in the oscillation
  status with the amplitude detecting circuit, and
  outputs a detection signal.

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High-frequency Oscillation Type Proximity
Sensor-General sensor
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High-frequency Oscillation Type Proximity Sensor
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High-frequency Oscillation Type Proximity Sensor
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High-frequency Oscillation Type Proximity Sensor
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High-frequency Oscillation Type Proximity Sensor
- all-metal sensor

• The all-metal type is basically included in the
  high frequency oscillation type. The all-metal
  type incorporates an oscillation circuit in which
  energy loss caused by the induction current
  flowing in the target affects the approaches the
  sensor, the oscillation frequency increases
  regardless of the target metal type. The sensor
  detects this change and outputs a detection
  signal.

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High-frequency Oscillation Type Proximity Sensor
nonferros-metal sensor

• The nonferrous-metal type is basically included
  in the high-frequency oscillation type.
• The nonferrous-metal type incorporates an
  oscillation circuit in which energy loss caused
  by the induction current flowing in the target
  affects the change of the oscillation frequency.
• When a nonferrous-metal target such as aluminum
  or copper approaches the sensor, the oscillation
  frequency increases.
• On the other hand, when a ferrous-metal target
  such as iron approaches the sensor, the
  oscillation frequency decreases.
• When the oscillation frequency becomes higher
  than the reference frequency, the sensor outputs
  a detection signal.

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High-frequency Oscillation Type Proximity Sensor
nonferros-metal sensor
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• SENSING

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Photoelectric sensors

• A photoelectric sensor is an electrical device
  that responds to a change in the intensity of the
  light falling upon it. The first photoelectric
  devices used for industrial presence and absence
  sensing applications took the shape of small
  metal barrels, with a collimating lens on one end
  and a cable exiting the opposite end. The cable
  connected a photoresistive device to an external
  vacuum tube type amplifier. A small incandescent
  bulb, protected inside a matching metal barrel,
  was the opposing light source.
• These small, rugged ncandescent sensors were the
  forerunners of todays industrial photoelectric
  sensors.

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The light spectrum.
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LED (Light Emitting Diode)

• An LED is a solid-state semiconductor, similar
  electrically to a diode, except that it emits a
  small amount of light when current flows through
  it in the forward direction.
• LEDs can be built to emit green, blue,
  blue-green, yellow, red, or infrared light.
  (Infrared light is invisible to the human eye)
• In applications which sense color contrasts, the
  choice of LED color can be important.

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LED (Light Emitting Diode)

• LEDs are solid-state, they will last for the
  entire useful life of a sensor.
• LED sensors can be totally encapsulated and
  sealed, making them smaller yet more reliable
  than their incandescent counterparts.
• Unlike incandescent light sources, LEDs are not
  easily damaged by vibration and shock, and worry
  about filament sag is also eliminated.
• In general, LEDs produce only a small percentage
  of the light generated by an incandescent bulb of
  the same size.
• Laser diodes are a recent exception to this. New
  sensor designs that incorporate laser diodes can
  produce many times the light intensity (and
  sensing range) of ordinary LEDs.
• Infrared types are the most efficient LED light
  generators, and were the only type of LED offered
  in photoelectric sensors until 1975.

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Sensing Modes

• The optical system of any photoelectric sensor is
  designed for one of three basic sensing modes
• opposed,
• retroreflective,
• proximity.
• The photoelectric proximity mode is further
  divided into four submodes
• diffuse proximity,
• divergent-beam
• proximity,
• convergent-beam proximity,
• fixed-field
• adjustable-field proximity..

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Opposed mode

• Opposed mode sensing is often referred to as
  "direct scanning", and is sometimes called the
  "beam-break" mode.
• In the opposed mode, the emitter and receiver
  are positioned opposite each other so that the
  sensing energy from the emitter is aimed directly
  at the receiver. An object is detected when it
  interrupts the sensing path established between
  the two sensing components.

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Alignment

• Alignment of a sensor means positioning the
  sensor(s) so that the maximum amount of emitted
  energy reaches the receive sensing element. In
  opposed sensing, this means that the emitter and
  the receiver are positioned relative to each
  other so that the radiated energy from the
  emitter is centered on the field of view of the
  receiver.

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Sensing range

• Sensing range is specified for all sensors. For
  opposed mode sensors, range is the maximum
  operating distance between the emitter and the
  receiver.

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Effective beam

• A sensor's effective beam is the "working" part
  of the beam it is the portion of the beam that
  must be completely interrupted in order for an
  object to be reliably sensed.
• The effective beam of an opposed mode sensor pair
  may be pictured as a rod that connects the
  emitter lens (or ultrasonic transducer) to the
  receiver lens (or transducer).
• This rod will be tapered if the two lenses (or
  transducers) are of different sizes.

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Effective beam
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Retroreflective mode

• The photoelectric retroreflective sensing mode is
  also called the "reflex" mode, or simply the
  "retro" mode).
• A retroreflective sensor contains both emitter
  and receiver circuitry. A light beam is
  established between the emitter, the
  retroreflective target, and the receiver. Just as
  in opposed mode sensing, an object is sensed when
  it interrupts the beam.
• Retroreflective range is defined as the distance
  from the sensor to its retroreflective target.
  The effective beam is usually coneshaped and
  connects the periphery of the retro sensor lens
  (or lens pair) to that of the retroreflective
  target..

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Retroreflective mode
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retroreflectors

• Retroreflective targets are also called
  "retroreflectors" or "retro targets". Most
  retroreflective targets are made up of many small
  corner-cube prisms, each of which has three
  mutually perpendicular surfaces and a hypotenuse
  face. A light beam that enters a cornercube prism
  through its hypotenuse face is reflected from the
  three surfaces and emerges back through the
  hypotenuse face parallel to the entering beam In
  this way, the retroreflective target returns the
  light beam to its source.

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retroreflectors
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Specular reflection

• A single mirrored surface may also be used with a
  retroreflective sensor. Light striking a flat
  mirror surface, however, is reflected at an angle
  that is equal and opposite to the angle of
  incidence.
• This is called specular reflection. In order for
  a retroreflective sensor to "see" its light
  reflected from a flat mirrored surface, it must
  be positioned so that its emitted beam strikes
  the mirror exactly perpendicular to its surface.

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Skew angle

• If a shiny object has flat sides and passes
  through a retroreflective beam with a predictable
  orientation, the cure for proxing is to orient
  the beam so that the objects specular surface
  reflects the beam away from the sensor.
• This is called scanning at a skew angle to the
  objects surface The skew angle usually need be
  only 10 to 15 degrees (or more) to be effective.
• This solution to proxing may, however, be
  complicated if the shiny object has a rounded
  (radiused) surface or if the object presents
  itself to the beam at an unpredictable angle.
• In these cases, the best mounting scheme,
  although less convenient, has the beam striking
  the object at both a vertical and a horizontal
  skew angle

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Skew angle
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Skew angle
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Polarizing filters

• Polarizing filters are readily available for use
  with visible emitters. When used on visible
  retroreflective sensors, polarizing filters
  (sometimes called anti-glare filters) can
  significantly reduce the potential for proxing. A
  polarizing filter is placed in front of both the
  emitter lens and the receiver lens. The two
  filters are oriented so that the planes of
  olarization are at 90 degrees to one another.
  When the light is emitted, it is polarized
  "vertically" (
• When the light reflects from a corner-cube retro
  target, its plane of polarization is rotated 90
  degrees, and only the polarized target-reflected
  light is allowed to pass through the polarized
  receiver filter and into the receiver. When the
  polarized emitted light strikes the shiny surface
  of the object being detected, its plane of
  polarization is not rotated, and the returned
  non-polarized beam is blocked from entering the
  receiver.

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Polarizing filters
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Proximity mode

• Proximity mode sensing involves detecting an
  object that is directly in front of a sensor by
  detecting the sensors own transmitted energy
  reflected back from the objects surface. For
  example, an object is sensed when its surface
  reflects a sound wave back to an ultrasonic
  proximity sensor. Both the emitter and receiver
  are on the same side of the object, usually
  together in the same housing. In proximity
  sensing modes, an object, when present, actually
  "makes" (establishes) a beam, rather than
  interrupts the beam.
• Photoelectric proximity sensors have several
  different optical arrangements. They are
  described under the following headings diffuse,
  divergent, convergent beam, fixed-field, and
  adjustable field.

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Diffuse mode

• Diffuse mode sensors are the most commonly used
  type of photoelectric proximity ensor. In the
  diffuse sensing mode, the emitted light strikes
  the surface of an object at some arbitrary angle.
  The light is then diffused from that surface at
  many angles. The receiver can be at some other
  arbitrary angle, and some small portion of the
  diffused light will reach it.

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Diffuse mode

• Most diffuse sensors can guarantee a return
  lightsignal only if the shiny surface of the
  material presents itself perfectly parallel to
  the sensor lens This is usually not possible with
  radiused parts like bottles or shiny cans. It is
  also a concern when detecting webs of metal foil
  or poly film where there is any amount of web
  "flutter".

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Divergent mode

• To avoid the effects of signal loss from shiny
  objects, special shortrange, unlensed divergent
  mode sensors should be considered. By eliminating
  collimating lenses, the sensing range is
  shortened, but the sensor is also made much less
  dependent upon the angle of incidence of its
  light to a shiny surface that falls within its
  range.

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Divergent mode

• The range of any proximity mode sensor also may
  be affected by the size and profile of the object
  to be detected. A large object that fills the
  sensors beam will return more energy to the
  receiver than a small object that only partially
  fills the beam.
• A divergent sensor responds better to objects
  within about one inch of its sensing elements
  than does a diffuse mode sensor. As a result,
  divergent mode sensors can successfully sense
  objects with very small profiles, like yarn or
  wire.

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Convergent Beam

• A proximity mode that is effective for sensing
  small objects is the convergent beam mode. Most
  convergent beam sensors use a lens system that
  focuses the emitted light to an exact point in
  front of the sensor, and focuses the receiver
  element at the same point. This design produces a
  small, intense, and well-defined sensing area at
  a fixed distance from the sensor lens

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Convergent Beam

• This is a very efficient use of reflective
  sensing energy. Objects with small profiles are
  reliably sensed. Also, materials of very low
  reflectivity that cannot be sensed with diffuse
  or divergent mode sensors can often be sensed
  reliably using the convergent beam mode.

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Mechanical convergent beam sensors

• Mechanical convergent beam sensors direct a
  lensed emitter and a separate lensed receiver
  toward a common point ahead of the sensor.
• It is particularly useful for detecting the
  presence of materials that do not offer enough
  height differential from their background to be
  recognized by a convergent beam or fixed-field
  sensor.

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Fixed-field sensors

• Fixed-field sensors compare the mount of
  reflected light that is seen by two
  differently-aimed receiver ptoelements. A target
  is recognized as long as the amount of light
  reaching eceiver R2 is equal to or greater than
  the amount "seen" by R1. The sensors output is
  cancelled as soon as the amount of light at R1
  becomes greater than the amount of light at R2.
• Fixed-field sensors have a definite limit to
  their sensing range they ignore objects that lie
  beyond their sensing range, regardless of object
  surface reflectivity.

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Fixed-field sensors
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Adjustable field

• The receiver element of an adjustable field
  sensor produces two currents I1 and I2. In
  adjustable field sensing, the ratio of the two
  currents changes as the received light signal
  moves along the length of the receiver element.
• The sensing cutoff distance relates directly to
  this ratio, which is made adjustable via a
  potentiometer. Even highly reflective objects
  lying beyond the cutoff distance

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Adjustable field
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Ultrasonic proximity

• Ultrasonic transducers vibrate with the
  application of ac voltage.
• This vibration alternately compresses and expands
  air molecules to send "waves" of ultrasonic sound
  outward from the face of the transducer.
• The transducer of an ultrasonic proximity sensor
  also receives "echoes" of ultrasonic waves that
  are located within its response pattern.

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Ultrasonic proximity

• Ultrasonic sensors are categorized by transducer
  type in
• Electrostatic types fill requirements for very
  long range proximity detection.. These long-range
  sensors are the solution to applications that
  require level monitoring in large bins or tanks.
• Piezoelectric types usually have a somewhat
  shorter proximity range typically up to 10 feet,
  but can be sealed for protection againstharsher
  operating conditions.

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Ultrasonic proximity
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• The basic effect is change of resistance (MRs) or
  output voltage (Hall elements, depending on the
  influence of magnetic fields. With suitable
  set-up these effects can ideally be used for
• Position sensors
• Current sensors
• Angle encoders
• Rotational sensors
• Applying magnetic semiconductor sensors provides
  some major advantages for many applications
• Contactless operation
• No wear and tear
• No degradation effects measurable
• with InSb-MRs and GaAs Hall devices

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Linear Hall Elements and Magneto Resistors
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Linear Hall Elements and Magneto Resistors