Mechanical sensors 2


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Mechanical sensors 2

  2. 2. MECHANICAL SENSORS A sensor is a device that detects the state of the environment such as energy, heat, light, magnet, supersonic, etc. and convert them to electric signals.
  3. 3. SENSOR , TRANSDUCER & ACTUATOR  Transducer  a device that converts a primary form of energy into a corresponding signal with a different energy form  Primary Energy Forms: mechanical, thermal, electromagnetic, optical, chemical , etc.  take form of a sensor or an actuator  Sensor (e.g., thermometer)  a device that detects/measures a signal or stimulus  acquires information from the “real world”  Actuator (e.g., heater)  a device that generates a signal or stimulus
  4. 4. SENSOR , TRANSDUCER & ACTUATOR  Transducer: a device that converts energy from one form to another  Sensor: converts a physical parameter to an electrical output (a type of transducer, e.g. a microphone)  Actuator: converts an electrical signal to a physical output (opposite of a sensor, e.g. a speaker)
  5. 5. TYPES OF SENSORS • Displacement Sensors:  Resistance, inductance, capacitance, Edd y current sensors, LVDT • Proximity sensors  Pneumatic proximity sensors • Motion Sensors:  Optical Encoder  Incremental , Absolute Encoder
  6. 6. Displacement Measurements  Used to measure directly and indirectly the size, shape, and position of the object.  Displacement measurements can be made using sensors designed to exhibit a resistive, inductive, capacitive or piezoelectric change as a function of changes in position
  7. 7. DISPLACETMENT SENSORS [POTENTIOMETERS]  Measure linear and angular position  Resolution a function of the wire construction  Measure velocity and acceleration
  8. 8. INDUCTIVE SENSORS  Ampere’s Law: flow of electric current will create a magnetic field  Faraday’s Law: a magnetic field passing through an electric circuit will create a voltage i v v1 v2 + - + - 2 2 1 1 v N N vN1 N2 dt d Nv
  9. 9. INDUCTIVE SENSORS dt di Lv GnL 2 n = number of turns of coil G = geometric form factor m = effective magnetic permeability of the medium
  10. 10. Eddy Current Sensor Eddy current: caused when a conductor is exposed to a changing magnetic field due to relative motion of the field source and conductor; or due to variations of the field with time. The eddy current generates a opposite magnet field, which superimposes with the exciting magnet field. As consequence, the impedance Z of the sensor coil changes.
  11. 11. CAPACITIVE SENSOR (PRINCIPLE)  If two metal plates are placed with a gap between them and a voltage is applied to one of the plates, an electric field will exist between the plates. This electric field is the result of the difference between electric charges that are stored on the surfaces of the plates. Capacitance refers to the “capacity” of the two plates to hold this charge. A large capacitance has the capacity to hold more charge than a small capacitance. The amount of existing charge determines how much current must be used to change the voltage on the plate.
  12. 12. CAPACITIVE SENSOR (PRINCIPLE) Size of the plates: capacitance increases as the plate size increases Gap Size: capacitance decreases as the gap increases Material between the plates (the dielectric): Dielectric material will cause the capacitance to increase or decrease depending on the material
  13. 13. CAPACITIVE SENSORS x A C r0 0 = dielectric constant of free space r = relative dielectric constant of the insulator A = area of each plate x = distance between plates Change output by changing r (substance flowing between plates), A (slide plates relative to each other), or x.
  14. 14. CAPACITIVE SENSORS When the capacitor is stationary xo the voltage v1=E. A change in position x = x1 -xo produces a voltage vo = v1 – E. dt dv ci c 20 x A x C r
  15. 15. LVDT (LINEAR VARIABLE DIFFERENTIAL TRANSFORMER) Inductive transducer Translates liner motion into electrical signals + - + - a) As x moves through the null position, the phase changes 180 , while the magnitude of vo is proportional to the magnitude of x. a phase-sensitive demodulator is required.
  16. 16. LVDT  Variable inductance sensors for linear displacement measurement  Three symmetrically spaced coils wound  Series opposing circuit  A single primary winding  Two secondary windings wound on a former – equal turns – placed on either side of primary winding  Primary winding – connected to AC  Movable soft iron core is placed inside the former
  17. 17. A single primary winding Two secondary windings wound on a former – equal turns – placed on either side of primary winding Primary winding – connected to AC Movable soft iron core is placed inside the former
  18. 18. WORKING  Core Made of high permeability, annealed nickel hydrogen – gives high sensitivity, low null voltage-  slotted longitudinally – reduces eddy I losses  Entire assembly – in stainless steel housing  At normal (NULL) position, flux in both coils equal- so E =0  If core is moved left E1 > E2 ( depends on flux linkage). E in phase with primary Voltage  If core is moved right E1 < E2 , E –out of phase
  19. 19. LVDT( ADVANTAGES & DISADAVANTAGES)  High range (from 1.25mm to 250 mm)  Low power consumption (<1 w)  High sensitivity  Frictionless device  Tolerant to shocks & vibrations  Immunized to external effects  Large displacement required for small o/p  Sensitive to stray magnet  Performance affected by temp  Limited dynamic response ADVANTAGES DISADVANTAGES
  20. 20. APPLICATIONS OF LVDT  LVDT is used to measure displacement ranging from fraction millimeter to centimeter.  Acting as a secondary transducer, LVDT can be used as a device to measure force,weight and pressure, etc..
  21. 21. ENCODER  What is an encoder? An encoder is a sensor for converting rotary motion or position to a series of electronic pulses
  22. 22. ENCODERS  A rotary encoder, also called a shaft encoder, is an electro-mechanical device that converts the angular position or motion of a shaft or axle to an analog or digital code.  The output of absolute encoders indicates the current position of the shaft, making them angle transducers.  The output of incremental encoders provides information about the motion of the shaft, which is typically further processed elsewhere into information such as speed, distance, and position.
  23. 23. OPTICAL ENCODER  An encoder is a sensor for converting rotary motion or position to a series of electronic pulses  Incremental Optical Encoders : Optical incremental encoders are linear/angular position sensors that use light and optics to sense motion.
  24. 24. ABSOLUTE ENCODERS •Absolute encoders have a unique code that can be detected for each angular position •Absolute encoders are much more complex and expensive than incremental encoders
  25. 25. ABSOLUTE ENCODERS  Every position of an absolute encoder is unique. Unlike an incremental encoder, where position is determined by counting pulses from a zero mark or home base, the absolute encoder reads a system of coded tracks to establish position information. No two positions are alike..  Since each position is unique, true position verification is available as soon as power is up. It is not necessary to initialize the system by returning to home base.
  26. 26. ABSOLUTE ENCODERS  An absolute encoder disk features several concentric tracks, each consisting of a pattern of transparent and opaque segments. These independent tracks provide a unique combination of absolute values for each position. The coded format is a variation of Binary code called Gray code.
  27. 27. INCREMENTAL ENCODER  Measures instantaneous angular position to some arbitrary datum but unable to give any indication about the absolute position of shaft  Pulses from LEDS are counted to provide rotary position. Two detectors are used to determine direction (quadrature. Index pulse used to denote start point Otherwise pulses are not unique
  31. 31. APPLICATIONS Encoders are wildly used in industry  machine tools  textile machinery  printing presses  wood working machines  handling technology  conveying and storage technology  robotics
  32. 32. APPLICATIONS  While a lead screw or rack-and-pinion converts rotary motion to linear motion, an encoder converts the same motion into electronic pulses. The pulses typically are used as input signals for counters, PLCs, or numerical- control equipment  Roll or sheet materials need to be measured during transport through converting or cut-to-length machinery. An encoder, when combined with a measuring wheel or coupled to a roller, will produce electronic pulses equal to units of length. Since fractional units may be measured, very precise operation is possible