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Digital Transducers

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Published in: Instrumentation
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this presentation talks about various types of digital transducers and their application.

Jandeep / Chandigarh

2 years of teaching experience

Qualification: B.Tech.

Teaches: All Subjects, Economics, Geography, History, Mathematics

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  1. BY Jandeep Kaur
  2. Topics Shaft Encoders Incremental Optical Encoder Absolute Optical Encoder Encoder Error Digital Resolvers Digital Tachometers Hall Effect Sensors Measurement of Translatory Motion Limit Switches
  3. INTRODUCTION What is a Digital Transducer ? Any transducer that presents information as discrete samples and that does not introduce a quantization error when the reading is represented in the digital form may be classified as a digital transducer What is an encoder ? Any transducer that generates a coded of a measurement can be termed an encoder SHAFT ENCODERS— They are Digital Transducers that are used for measuring ANGULAR DISPLACEMENTS and ANGULAR VELOCITIES.
  4. Applications Of Shaft Encoders 1 .Control of robotics manipulators 2.Machine tools 3.Digital tape-transport mechanisms 4.Servo plotters 5 .Printers 6.Satellite mirror positioning system
  5. Encoder Types Shaft encoders can be classified into three categories 1. Incremental Encoders 2. Incremental Optical Encoders 3. Absolute Optical Encoders Incremental Encoders 1.0ptical (photosensor) method 2.Sliding contact (Electrical conducting) method 3.Magnetic saturation (Reluctance) method
  6. Optical Encoder Method Output LED Light Source itocell, Phototransistor, or Photodiode (Light Sensor) (1) t Encoder Track Transparent Windows Opaque Disk
  7. Optical Encoder (1) The optical encoder uses an opaque disk that has one or more circular tracks, with some arrangement of identical transparent windows. A parallel beam of light is projected to all tracks from one side of the disk The light sensor could be a silicon photodiode, a phototransistor, or a photovoltaic cell. 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
  8. Sliding Contact Method (2) Sliding Contact Brush Commutator Brush Conducting (Metal) Areas Output Signal Code Disk
  9. Sliding Contact (2) The transducer disk is made of an electrically insulating material The conducting regions correspond to the transparent windows on an optical encoder disk All conducting areas are connected to a common slip ring on a encoder shaft A constant voltage Vref is applied to the slip ring using a brush mechanism
  10. N/A
  11. Incremental Optical Encoders Pick-Off Pick-Off 2 00 1 R eference Window (2) Code Disk Reference Pulse Pick-Off 0
  12. Incremental Optical Encoder (2) The disk has a single circular track with identical and equally spaced transparent windows. The area of the opaque region between adjacent windows is equal to the window area. Two photodiode sensors (pick offs 1 and 2) are positioned facing the track a quarter-pitch
  13. 900 900 Lags by 900 Time Leads by 900 Tirn e T i r-ne Time (c)
  14. N/A
  15. Absolute Optical Encoders(3) Absolute rotary en coder with corrventional code disk. Photaoetectar Stationary mask Rotating Encoder Disk LEO Light scurce
  16. Absolute Optical Encoders(3) The disk has a circular track with identical and equally spaced transparent windows. In absolute optical encoders photo sensors are not used. The output can be binary, gray code, natural binary code.
  17. Single-Turn Shaft/Hub Shatt/Hollow Shaft Shaft Resolution Diameter Size Interface Suppty Voltage Output Code Formats RS20 (CoreTech) cpr 2.0 in SSI. Push-pull. Open collector, TTL 10...30 v, 8-24 v, Gray, Gray Excess, Natural Binary, Binary Coded Decimal RS 25 (CoreTech) 2..32,768 cpr 2.5 in SSI, Push-pull. Open collector, TTL 10-.30 v, 8...24 v, Gray, Gray Excess, Natural Binary, Binary Coded Decimal RS 60 L(CoreTech) 2...32,768 cpr 60 mm SSI, Push-pull 10-32 v Gray, Gray Excess, Natural Binary, Binary Coded Decimal Multi-Turn Hollow TM 90-A I(SSI) 13 bits per turn x 8,192 tums (24 bit max), programmable 93 mm SSI 10-.32 v Gray or Natural Binary TM 90-P (Proflbus) 13 bits per tum x 8,192 turns (26 bit max), programmable 93 mm Frofile for encoders (07hex), Class 2 10...32 v
  18. Encoder Error Any transducer that generates a coded reading of a measurement is known as Encoder. The primary sources of errors in shaft encoder are: 1) Quantization error 2) Assembly error 3) Coupling error 4) Structural limitations 5) Manufacturing tolerances 6) Ambient effects
  19. One form of error in an encoder reading is the hysteresis. For a given position of the moving object, if the encoder reading depend on the direction of motion, the measurement has a hysteresis error. The causes of hysteresis includes mechanical deformation in the code disk and shaft, delays in electronic circuitry, loose fits, backlash in gear couplings, and noisy pulse signals.
  20. Eccentricity Error: Eccentricity (e) of an encoder is defined as the distance between the center of the rotation C of the code disk and the geometric center G of the circular code track. Nonzero eccentricity causes a measurement error known as the eccentricity error. Primary contributions to eccentricity are: 1)Shaft eccentricity (es) 2)Assembly eccentricity (ea) 3)Track eccentricity (et) 4) Radial play (ep)
  21. The mean value of the overall eccentricity is given by 2 2 2 2 And assuming that the individual eccentricities are independent variables, the standard deviation of the overall eccentricity is given by 2
  22. Digital Resolvers Digital resolvers or mutual induction encoders operate using the principle of mutual induction. They are commercially known as Inductosyns. A digital resolver has two disks namely stator and rotor which is coupled to the rotating object. The rotor has fine electric conductor foil imprinted which is connected to a high frequency AC supply. The stator has two separate printed patterns identical to the rotor pattern but are shifted by a quarter-pitch from one another.
  23. Schematic Diagram of Digital Resolver Rotor Disk Commutator with Bruå 0 Supply AC 'ref Stator Disk Primary Output Output
  24. Digital Tachometers As Shaft encoders are also used for measuring angular velocities, they can be considered as Tachometers. A Magnetic induction tachometer of variable- reluctance type is shown in figure. Teeth on the wheel are made of ferromagnetic material. Two magnetic induction proximity probes are placed facing the teeth radially, a quarter —pitch apart. Speed is computed either by counting pulses over a sampling period or by timing the pulse width.
  25. Schematic Diagram of Pulse Tachometer Ferromagnetic Teeth raf Probe 1 (Magnatic Induction) Outputs Probe 2 •ref
  26. Alternative types of digital tachometers use eddy current proximity probes or capacitive proximity probes. Disadvantages of digital tachometers over optical encoders are poor resolution, mechanical errors due to loading, hysteresis, and manufacturing irregularities. Advantages of these Digital tachometers are Simplicity, robustness and low cost.
  27. HALL EFFECT An electromotive force developed as a result of interaction when a steady state current flows in a steady state magnetic field; the direction of the emf is at right angles to both the direction of the current and the magnetic field vector, and the magnitude of the emf is proportional to the product of current intensity, magnetic force, and sine of the angle between current direction and magnetic field vector.
  28. N/A
  29. Magnetic So u rce Supply ref
  30. vo = I B/ ( n e d) Where Vo = Voltage across the width of the plate = Magnetic flux density B = Current across the plate I — Charge of electron e d = Depth of the plate — Bulk density of the carrier electrons n
  31. HALL EFFECT SENSORS A Hall effect sensor is an electronic device that varies its output voltage in response to changes in magnetic field density. Hall sensors are used for proximity switching, positioning, speed detection and current sensing applications
  32. Hall Effect Shaft Encoder or Digital Tachometer Fixed Magnetic Source + OutpL F i xed Semiconductor Element Toothed Wheel Ferromagnetic)
  33. Measurement of Translatory Motion CABLE EXTENSION SENSORS MOIRE FRINGE DISPLACEMET SENSOR
  34. CABLE EXTENSION SENSORS Spool and cable Encoder or Potentiometer Auto-tensioner
  35. MOIRE FRINGE DISPLACEMENT SENSORS
  36. LIMIT SWITCHES A limit switch is a mechanical hall effect sensor used to stop the motion of a machine slide or element once it reaches a fixed point The limit of a movement can be detected by a simple contact mechanism to close a circuit or trigger a pulse
  37. Reference http://en.wikipedia.org/wiki/Rotary encoder Analog sensors and Actuators — clarence w.De Silva
  38. Questions: What are different types of Encoders? Explain any one of them? What is Eccentricity? How can u find out the Mean value and Standard deviation of overall eccentricity? What is meant by hall effect? Describe the methods of measuring the rotatory motion of an object?

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