Digital Transducers Group 3

1
Digital Transducers Group 3

• Souvik Bhattacharya
• Noah Berhanu
• Binnu Mammen
• Vishal Raj Karunala

2
Introduction

• Any transducer that presents information as
  discrete samples and that does not introduce an
  quantization error when the reading is
  represented in digital form may be classified as
  a digital transducer.
• Digital transducers generate discrete output
  signals such as pulse trains or encoded data that
  can be directly read by a control processor.
• When the output is a pulse signal, a counter is
  used.the count is first represented as a digital
  word,then it is read by a data acquisition and
  control computer.If the output is automatically
  available in a coded form,it can be directly read
  by a computer.

3
Shaft Encoders

• Any transducer that generates a coded reading of
  a measurement can be termed as an encoder.
• Used for measurement of angular displacements and
  angular velocities.
• Advantages of shaft encoders over their analog
  counterparts are
• High Resolution
• High Accuracy
• Relative ease of adoption in digital control
  systems
• Reduction in system cost
• Improvement in system reliability

4
Encoder Types

• SHAFT ENCODERS can be classified into two
  categories
• 1.Incremental Encoders
• 2. Absolute Encoders

5
Absolute Encoders

• It has many pulse tracks on its transducer
  disk.when the disk rotates, several pulse trains
  equal to the number of tracks on the disk, are
  generated simultaneously.
• At a given instant, the magnitude of each pulse
  signal will have one of two signal levels (I.e
  binary state), as determined by level detector.
  This signal level corresponds to binary 1 or 0.
• Thus the set of pulse trains gives an encoded
  binary number.
• The pulse windows on the tracks can be organized
  into some pattern (code) so that each of these
  binary numbers corresponds to the angular
  position of the encoder disk at the time when
  the particular binary number is detected.
• It has several rows of tracks,equal in number to
  the bit size of the output data word.
• It needs at least as many signal pick off sensors
  as there are tracks.
• Commonly used to measure fractions of a
  revolution.

6
Incremental encoder

• An incremental encoder disk requires only one
  primary track that has equally spaced and
  identical window areas. the window area is equal
  to the area of inter-window gap.

7
Signal Generation

• There are four techniques of transducer signal
  generation
• Optical (photo sensor) Method
• Sliding Contact (electrical conducting) Method
• Magnetic Saturation (reluctance) Method
• Proximity Sensor Method
• For a given type of encoder ,the method of signal
  interpretation is identical for all the four
  types of signal generation

8
Optical Encoder

• Optical Encoder uses an opaque disk (code disk)
  that has one or more circular tracks,with some
  arrangement of identical transparent windows
  (slit) in each track.
• A beam of light is projected to all the tracks
  from one side of the disk.
• The transmitted light is then picked off using a
  bank of photo sensors on the other side of the
  disk that typically has one sensor on each track.
• Since the light from the source is interrupted by
  the opaque areas of the track, the output signal
  from the probe is a series of voltage pulses.
• This signal is interpreted to obtain the angular
  position and angular velocity of the disk.

9
Representation of an Optical Encoder
10
Sliding Contact Encoder

• The transducer disk is made up of electrically
  insulating material.
• Circular tracks on the disk are formed by
  implanting a pattern of conducting areas.these
  conducting regions correspond to the windows on
  an optical encoder disk.All conducting areas are
  connected to a common slip ring on the encoder
  shaft.
• A constant voltage Vref is applied to the slip
  ring using a brush mechanism.
• A sliding contact such as a brush touches each
  track,and as the disk rotates,a voltage pulse
  signal is picked off by it.
• The signal interpretation is done as it is for
  optical encoders.

11

• Advantages
• Simplicity of construction (low cost)
• High sensitivity (depending upon the supply
  voltage)
• Disadvantages
• It includes the familiar drawbacks related to
  contacting and communicating devices like
  friction, wear, brush bounce due to vibration,
  signal glitches and metal oxidation due to
  electrical arcing.

12
Schematic representation of sliding contact
encoder
13
Magnetic Encoders

• Magnetic encoder have high strength magnetic
  areas imprinted on the encode disk using
  techniques such as etching ,stamping or
  recording.
• These magnetic areas correspond to the
  transparent windows on an optical encoder disk.
• The signal pick off device is a micro transformer
  that has primary and secondary windings on a
  circular ferromagnetic core.
• A high frequency (typically 100khz) primary
  voltage induces a voltage in the secondary
  winding of the sensing element operating at the
  same frequency ,operating as a transformer.

14
Contd.

• A magnetic field of sufficient strength can
  saturate the core ,however, thereby increasing
  the reluctance and dropping the induced voltage.
• By demodulating the induced voltage ,a pulse
  signal is obtained. This signal is interpreted in
  the usual manner.
• Advantage They have non- contacting pick off
  sensors.
• Disadvantage they are more costly than the
  contacting devices.

15
Schematic representation of Magnetic Encoder
16
Proximity Sensor Encoder

• These use proximity sensor as a signal pick off
  element.
• Two types

Magnetic induction probe
Eddy current probe
17
Magnetic Induction Probe

• In this type, the disk is made of ferromagnetic
  material.
• The encoder tracks have raised spots of the same
  material, serving a purpose analogous to that of
  the windows on an optical disk encoder.
• As a raised spot approaches a the probe, the flux
  linkage increases as a result of the associated
  decrease in reluctance ,thereby raising the
  induced voltage level.
• The output voltage is a pulse-modulated signal
  at the frequency of the supply voltage of the
  proximity sensor.
• This is then demodulated, and the resulting pulse
  signal is interpreted.

18
Eddy current probe

• In this type, the pulse areas in the track are
  plated with a conducting material.
• A flat plate may be used in this case as the
  non-conducting areas in the disk do not generate
  eddy currents.

19
(No Transcript)
20
Incremental Optical Encoders

• Circular motion measurement
• Available in various sizes and resolutions
• Used in computer mouse rollers
• Components
• Circular disk with slots
• Diode and photo sensor
• Digital circuitry to process pulses

21
Working mechanism
Light source, diode

• 1 full pinch is equivalent to one full cycle made
  by the transparent and opaque windows

photo sensor
22
contd.
diode
Logical 0
Logical 1
Photo sensor

• Rotating disk blocking light
• Photo sensor has a low logic output

• Rotating disk allowing light through
• Photo sensor has a high logic output

• Continuous train of pulses formed as the photo
  sensor traverses between logic high and logic low
  in response to the rotation of the disk

23
Types

• Two types
• Offset sensor
• Single disk with opaque and transparent windows
  of same size
• Two photodiode sensors placed a quarter pinch
  apart and on the same side
• Offset track
• Two identical disks, each with windows and opaque
  region placed a quarter pinch apart facing each
  other
• Additional reference pick up photo sensor near
  the center of the disk to initiate counting

24
Problems and solutions

• Voltage supply may be unstable
• Sensor and environment noises may degrade the
  voltage level from sensors
• Solution
• An identical fixed disk, i.e. two disks one
  rotating another stationary are used
• Two photo sensors are used, both behind the
  stationary disk
• The output from the sensor aligned with the non
  rotating disk is always low logic, the output
  from the other disk depends on the windows
  orientation.
• Both outputs are fed to a differential amplifier

25
Direction detection

• Two photo sensor/diode pairs, a quarter pinch
  apart are used
• A high frequency clock is used. Counting starts
  at rising edge of pulse from sensor 1

Voltage output from sensor 1
Voltage output from sensor 2
Anti Clock wise
Clock wise
26
..contd.

• we define two counts
• n1 number of clock cycles until second pair
  starts to rise
• n2 number of clock cycles until first pair
  starts to rise again
• n1 gt (n2 n1) for one direction (clock wise)
• n1 lt (n2 n1) for opposite direction
  (anti clock wise)

Voltage output from sensor 1
Voltage output from sensor 2
n1
27
Angular Position ? and resolution

• To determine ? we need the count of the pulse, n
• maximum count M, corresponding to highest
  displacement ?max
• ? n ?max / M
• If we use a digital counter of resolution r bits
• M 2 r-1, including 0
• M 2 r-1 1, excluding 0.

28
contd.

• Resolution depends on N, number of windows and r,
  number of bits used to represent the counts
• With two sensor/diode pairs and with a rising
  edge circuitry detection
• ??p 3600/ 4N . Purely mechanical
• ??d 3600 / 2r . Purely digital
• Largest of the two governs the resolution

29
Angular velocity ? and resolution

• Pulse counting
• Number of pulses, n over the sampling period , T
  of the digital processor ( n has higher
  frequency)
• tav T /n,
• trevl NT/n, for N windows
• ? ? / t 2pn / NT
• Not suitable for low speed measurement
• Resolution
• ??c 2p/NT
• High N and T increases resolution

Controller sampling pulse (clock)
T
n
tavT/n
Pulse train
30
contd.

• Pulse timing
• m clock cycles for 1 pitch or for 1 count
• tsingle mt m / f , where f is frequency of
  the clock (time for 1 pulse)
• trevl Nm / f
• ? ? / t 2pf / N
• Not suitable for high speed measurement
• Resolution
• ??c N?2/2pf
• High N, and ? reduces resolution
• High clock frequency increases resolution

Pulse train
tsingle mt
t
m
clock
31
Conclusion

• Optical incremental encoder is used to measure
• Position
• Velocity
• Direction of rotation
• No need for other A/D circuits
• Outputs can be directly fed to microprocessor/micr
  ocontroller
• References
• www.renco.com
• www.opticalencoder.com
• www.usdigital.com

32
Absolute Optical Encoder

• Absolute Optical Encoder
• Gray Coding
• Encoder Error
• Digital Resolvers

33
Absolute Optical Encoder

• Absolute Encoder directly generates Coded Data to
  Represent Angular positions using sequence of
  pulses.
• No pulse coding is involved in this case.
• Code pattern on an Absolute Encoder disk is
  direct binary code.

34
Absolute Optical Encoder
35
Absolute Optical Encoder
working

• A disc or a plate containing opaque and
  transparent segments passes between a light
  source (such an LED) and detector to interrupt a
  light beam.
• The electronic signals that are generated are
  then fed into the controller where position and
  velocity information is calculated based upon the
  signals received

36
Absolute Optical Encoder working

• The number of tracks is n in this case.
• The disk is divided into 2n Sectors.
• Each partioned area of the matrix correspond to a
  bit of data. Transparent
  area1,Opaque area0
• Each track has a pick off sensors arranged on
  radial line facing track on one side of the disk
  it is illuminated by a light source from the
  other side of the disk.
• As Disk rotates bank of pick up sensor generates
  a set of pulses.
• At a given instant coded data word will determine
  the position of the disk and the resolution angle
  

37
Absolute Optical Encoder

• In the fig word size of data is 4bits.
• Outermost Element is LSB.
• Innermost Element is MSB.
• The Angular position is given as360/24.
• The direct binary representation of the disk
  sector shown in the table

38
Absolute Optical Encoder using gray coding
39
Why we use Gray Coding?

• There is a data interpretation problem associated
  with using binary codes in absolute encoder.
• For E.g.-The transition from 0011 to 0100
  requires three bit switching. Here rotation was
  from angle 2 to angle 3 actually rotation was
  from angle 3 to angle 4 such errors can be
  avoided using gray coding.
• If the sensors are not aligned properly
  manufacturing errors and printing on the disk
  have result large number of errors.

40
Encoder Error

• Quantization error due to digital word size
  limitation.
• Assembly error (eccentricity)
• Coupling error (gear backash,belt slipage)
• Structural limitations (disk deformation)
• Manufacturing tolerances (error due to
  inaccurately imprinted code)
• Ambient effects (Vibration, temperature)

41
Eccentricity Error Eccentricity Error

• Eccentricity is (denoted by e) of an encoder is
  distance between the center of rotation C of the
  code disk and geometric Center G
• The Primary contribution to eccentricity is
• Shaft eccentricity
• Assembly eccentricity
• Track eccentricity
• Radial play

42
Eccentricity Error Eccentricity Error

• Shaft eccentricity results if the rotating shaft
  on which the code disk is mounted is imperfect.
• Assembly eccentricity is caused if the code disk
  is improperly mounted on the shaft so that the
  center of code disk doesnt fall on the shaft
• Track eccentricity comes from irregularities in
  the code track imprinting process.
• Radial play is caused by any looseness in the
  assembly in the radial direction.

43
Digital Resolvers

• Digital Resolvers or Mutual Encoders. Somewhat
  act like analog Resolvers.
• A digital Resolvers has two disk facing each
  other one the stator and other the rotor

44
Digital Resolvers

• The rotor has a fine electric conductor imprinted
  on it.
• The stator disk has two printed patterns which
  was identical to each other.
• The primary voltages in the rotor circuit induces
  voltage in the secondary at same freq
• When the foil pulse coincide the induced voltage
  is maximum
• When rotor foil has a half pitch offset from the
  stator foil then the induced voltage cancel each
  other

45
Digital Resolvers

• If the Speed of rotation is constant the output
  voltages v1 and v2carrier frequency modulated by
  periodic and sinusoidal signals with a phase
  shift of 900
• When the speed is not constant Pulse width will
  vary with time
• Resolution up to 0.00050 can be obtained with
  this transducers

46
Digital Tachometer

• A device that employs a toothed wheel to measure
  angular velocities

47

• Magnetic induction tachometer of
  variable-reluctance type.
• Teeth -gt Ferromagnetic material.
• The 2 magnetic induction proximity probes are
  placed facing the teeth radially, a quarter-pitch
  apart.
• When the toothed wheel rotates, the 2 probes
  generate output signals that are 90 degrees out
  of phase.
• One signal leads the other in one direction of
  rotation lags the other in the opposite
  direction. Thus, directional readings are
  obtained.
• Speed is computed either by counting the pulses
  over a sampling period or by timing the pulse
  width.
• Eddy current Tachometer The teeth of pulsing
  wheel are made of or plated with
  electricity-conducting material the probe emits
  a radio-frequency magnetic field.
• ADVANTAGES OF DIGITAL (PULSE) TACHOMETERS OVER
  OPTICAL ENCODERS Simplicity, robustness low
  cost.
• DISADVANTAGES Poor resolution (determined by the
  number of teeth, the speed of rotation, and the
  word size used for data transmission) and
  mechanical errors due to loading, hysteresis and
  manufacturing irregularities.

48

• HALL EFFECT SENSORS (HES)
• Used to detect the proximity, presence or absence
  of a magnetic object using a critical distance.
• They function via an electrical potential that is
  developed across an axis transverse to an applied
  current flow in the presence of a magnetic field
  applied to the conductor.
• HALL EFFECT
• Refers to the potential difference (Hall voltage)
  on opposite sides of a thin sheet of conducting
  or semiconducting material in the form of a 'Hall
  bar' or an element through which an electric
  current is flowing, created by a magnetic field
  applied perpendicular to the Hall element.
• The ratio of the voltage created to the amount of
  current is known as the Hall resistance, and is a
  characteristic of the material in the element.

49
Hall effect diagram, showing electron flow

• 1. Electrons
• 2. Hall element, or
• Hall sensor
• 3. Magnets
• 4. Magnetic field
• 5. Power source

50

• In drawing "A", the Hall element takes on a
  negative charge at the top edge (symbolised by
  the blue color) and positive at the lower edge
  (red color).
• In "B" and "C", either the electric current or
  the magnetic field is reversed, causing the
  polarization to reverse.
• Reversing both current and magnetic field
  (drawing "D") causes the Hall element to again
  assume a negative charge at the upper edge.
• The Hall effect comes about due to the nature of
  the current flow in the conductor. Current
  consists of many small charge-carrying
  "particles" (typically electrons) which
  experience a force due to the magnetic field.
• Some of these charge elements end up forced to
  the sides of the conductors, where they create a
  pool of net charge.
• This is only notable in larger conductors where
  the separation between the two sides is large
  enough.
• One very important feature of the Hall effect is
  that it differentiates between positive charges
  moving in one direction and negative charges
  moving in the opposite.

51

• By measuring the Hall voltage across the
  element, one can determine the strength of the
  magnetic field applied. This can be expressed as
  
• 
  
• where
• VH is the voltage across the width of the plate
• I is the current across the plate length
• B is the magnetic field
• d is the depth of the plate
• e is the electron charge
• n is the bulk density of the carrier electrons.

52
(No Transcript)
53

• Consider a semiconductor element subject to a DC
  voltage V-ref. If a magnetic field is applied
  perpendicular to the direction of this voltage, a
  voltage V-o will be generated in the third
  orthogonal direction within the semiconductor
  element. This is known as Hall Effect.
• Can be used for motion sensing in many ways. Ex.
  As an analog proximity sensor, a digital limit
  switch or a digital shaft encoder.
• PROXIMITY AND HALL EFFECT
• Since O/P voltage increases as the distance from
  the magnetic source to the semiconductor element
  decreases, O/P signal Vo can be used as a measure
  of proximity.
• Alternatively, a certain threshold level of
  output voltage Vo can be used to activate a
  digital switch or to create a digital output
  hence forming a digital limit switch.

54
MODERN DAY EXAMPLES
HALL EFFECT CURRENT SENSOR WITH INTERNAL
INTEGRATED CIRCUIT AMPLIFIER
HALL EFFECT SPEED SENSOR
55

• Applications include Sensors Reed switch
  electrical motors, valve position, level
  detection, process control, machine control,
  security, etc. are some of them.
• ADVANTAGES Hall Effect devices when
  appropriately packaged are immune to dust, dirt,
  mud, and water. These characteristics make Hall
  effect devices better for position sensing than
  alternative means such as optical and
  electromechanical sensing.

56
HALL EFFECT SHAFT ENCODER
57

• The semiconductor element and the magnetic source
  are fixed relative to one another in a single
  package.
• By moving the ferromagnetic member into the air
  gap between the magnetic source and the
  semiconductor element, the flux linkage can be
  altered. This changes Vo.
• Suitable both as an analog proximity sensor and
  as a limit switch.
• The relationship between the output voltage Vo
  and the distance of a Hall effect sensor measured
  from the moving member is non linear. Linear Hall
  effect sensors use calibration to linearize their
  outputs.

DIGITAL TACHOMETER
58
WHERE USED
59
DIGITAL TACHOMETER AS A MEASUREMENT DEVICE
60

• MEASUREMENT OF TRANSLATORY MOTIONS
• Sensors are used to determine the rectilinear
  motions which are produced from a rotary motion.
  The sensors convert rectilinear motion to rotary
  motion within the sensor itself.
• 1. Cable Extension Sensors.
• 2. Moiré Fringe Displacement Sensors.
• CABLE EXTENSION SENSORS

CABLE EXTENSION SENSOR
61

• Suitable for measuring motions that have large
  excursions.
• Uses an angular motion sensor with a spool
  rigidly coupled to the rotating part of the
  sensor and a cable that wraps around the spool.
• Other end of cable is attached to object whose
  rectilinear motion is to be sensed.
• Housing of rotary sensor is firmly mounted on a
  stationary platform so that the cable can extend
  in the direction of motion.
• When the object moves, cable extends causing
  spool to rotate. This angular motion is measured
  by the rotary sensors.
• A spring is used to maintain tension and to
  re-wind the spool.
• When the spool is connected to a potentiometer,
  the output is an analog signal representing the
  absolute position of the spool.
• Proper calibration.
• These sensors provide an indication of motion by
  sensing light, a magnetic field, or simply the
  presence of an object.
• Disadvantages Mechanical loading of the moving
  object, time delay in measurements, errors caused
  by the cable including irregularities, slack
  tensile deformation.

62
MODERN DAY EXAMPLE
63

• MOIRE FRINGE DISPLACEMENT SENSORS

FIGURE REPRESENTING A MOIRE FRINGE DISPLACEMENT
SENSOR
64
Mask Plate (fixed)
Moving Plate
Light Source
Photo Sensor
WORKING PRINCIPLE OF A MOIRE FRINGE DISPLACEMENT
SENSOR
65
BLACK CLOUD
MOON
66
MOIRE FRINGE EFFECT

• Both of the strips are transparent (or
  reflective), with black lines at measured
  intervals.
• The spacing of the lines determines the accuracy
  of the position measurements.
• The stationary strip is offset at an angle so
  that the strips interfere to give irregular
  patterns.
• As the moving strip travels by a stationary
  strip the patterns will move up, or down,
  depending upon the speed and direction of motion.
• The initial application of the moiré technique in
  metrological applications was for measurement of
  in-plane deformations and strains.

67

• MASK PLATE Stationary transparent plate with
  opaque lines arranged in parallel in transverse
  (crosswise) direction.
• MOVING PLATE Second moving transparent plate
  with identical set of ruled lines.
• Lines are evenly spaced and line width is equal
  to the spacing between adjacent lines.
• Light source placed on moving plate side and
  light transmitted through the common area of the
  2 plates is detected on the other side by using
  one or more photo sensors.
• When the lines on 2 plates coincide, max. light
  will pass through the common area of the 2
  plates.
• When lines on one plate fall on transparent
  spaces of other plate, virtually no light passes
  though the plates.
• As one plate moves relative to the other, a pulse
  train is generated by the photo sensor it can
  be used to determine the rectilinear displacement
  velocity.
• Moiré fringes are the shadow patterns formed in
  this manner.
• Very small resolutions of ex. 0.0002 in can be
  realized. Provides improved sensitivity over a
  basic optical encoder provides increased
  resolution.
• Also used to detect deformations of one body with
  respect to the other.

68

• LIMIT SWITHCES
• Limit Switch is a mechanical device that requires
  the physical contact of an object with the
  switchs actuator to make the contacts change
  state.
• OR
• A limit switch is a mechanical device which can
  be used to determine the physical position of
  equipment.

LIMIT SWITCH
69
WORKING PRINCIPLE
FIGURE 2
FIGURE 1
FIGURE 3
FIGURE 4
70

• Actuator Mechanism within the limit switches to
  operate the contacts.
• Overtravel The movement of the actuator beyond
  the contact trip position without damage
  occurring to the switch.
• Pre-travel The distance or angle through which
  the actuator moves before reaching the point at
  which the contacts are tripped.
• The actuator is at its initial position. The
  limit switch contacts are in their normal
  untriggered position.
• Contact is made with the target object and the
  actuator moves its Pre-travel distance. Contacts
  are still in their normal untriggered position.
• The actuator reaches its operating point where
  the contacts change from their normal
  untriggered position to their triggered
  position. In the case of a lever actuator, there
  is some Overtravel allowing the lever to move
  beyond the operating point.
• On plunger actuators, the Overtravel distance is
  a safety margin for the sensor to avoid breakage.

71

• The actuator begins the return to its initial
  position. The contacts return to their normal
  untriggered position as the actuator reaches
  its release point and resets the contacts.
• The differential is the difference between the
  operating and release point.
• Differential is engineered into the switch to
  guard against the effects of vibration and rapid
  on/off oscillations of the switch right at the
  operating point.
• Only 2 states are used On/off, present/absent,
  go/no-go etc.
• Can be represented by 1 bit thats why it is
  considered as Digital Transducer.
• Additional logic needed for direction of contact.
  LS available for both rectilinear and angular
  motion.

72

• STRENGTHS AND WEAKNESSES

73

• APPLICATIONS
• Limit switches can be used to turn off a washing
  machine if the load becomes unbalanced. In
  automobiles, they turn on lights when the door is
  opened.
• In industry, limit switches are used to limit the
  travel of machine parts, sequence operations or
  to detect moving items on a conveyor system.
• Industrial Refrigerators
• Military Equipment
• Assembly Equipment
• Medical Equipment
• Transportation Equipment
• Fitness Equipment
• Farming Equipment

74

• MICROSWITCHES
• Snap Action Switches, also called Micro switches,
  are switch devices that can open and/or close an
  electrical circuit at a rapid speed.
• These snap action Micro switches are
  characterized by small and closely definable
  movements, good repeat accuracy and long
  mechanical life.
• These Micro switches are called snap action
  switches because of the rapid movement of
  spring-assisted moving contacts from one stable
  position to another, the speed being basically
  independent of the actuator speed.

FIGURE 1
FIGURE 2
TYPES OF MICROSWITCH
75

• Typical Applications areas include
• Security
• Medical
• Process Controls
• Business machines
• HVAC
• Material Handling
• Vending
• Gaming
• Circuit Breakers
• Household Appliances

76

• .

THANK YOU Questions?