Angular velocity measurement


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Angular velocity measurements for the subject mechanical measurements and metrology

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Angular velocity measurement

  2. 2. Overview • Introduction • Tachometers • Tachogenerators • Stroboscope and Stroboscopic Methods
  3. 3. INTRODUCTION • Velocity is the first derivative of displacement. • It is mainly categorized as linear and angular velocity. • There are various methods to measure linear velocity and they are less complicated. • We will learn the methods of measuring angular velocity in different work environments using appropriate Tachometers.
  4. 4. NEED TO MEASURE ANGULAR VELOCITY • As the mechanical industry has been growing since the invention of the steam engine in 1781 by James Watt, there has been a need to measure the speed of the engine’s crankshaft. • This need was fulfilled by Dietrich Uhlhorn in 1817 by inventing the Tachometer. Since 1840 it is significantly used for angular velocity measurement till date.
  5. 5. TACHOMETER Tachometer is – • An instrument used for measurement of angular velocity, as of shaft ,either by registering the total number of revolutions during the period of contact, or by indicating directly the number of revolutions per minute. • An instrument which either continuously indicates the value of rotary speed or displays a reading of average speed over rapidly operated short intervals of time.
  6. 6. CLASSIFICATION OF TACHOMETER Tachometers can be classified  On the basis of data acquisition • • Contact Non contact  On the basis of the measurement technique • • Time based Frequency based  On the basis of method of display • Digital • Analog  On the basis of working principle • Electrical • Mechanical
  8. 8. ELECTRICAL TACHOMETERS • Electrical tachometers mainly depend upon an electrical signal generated in proportion to the rotational speed of the shaft. Depending upon the type of transducer used there is a variety of different designs.
  9. 9. DRAG CUP TACHOMETER Construction • Drag cup tachometer basically consists of stator and a rotor. The stator has two windings mounted at 900 to each other known as » Reference Winding » Quadrature winding • A low reluctance path is provided by a ferromagnetic core.
  10. 10. Reference winding Ferromagnetic rotor 1 2 2 1 Quadrature winding Shaft Drag Cup rotor
  11. 11. Rotor Stator
  12. 12. Construction • Rotor is made up of thin aluminum cylinder which is called drag cup. This rotor is highly conducting and acts as short- circuited secondary winding. • An A.C voltage is applied to the AC winding, while the output is taken from the quadrature winding
  13. 13. Ѳ0 ῳi Ѳ0 Conducting but non-magnetic cup N S ῳi
  14. 14. Working • The driving shaft rotates the permanent magnet and this induces eddy current in a drag cup held closed to the magnet. • The eddy current produce a torque which rotates the cup against the torque of spring. • The cup turns in the direction of the rotating magnetic field until the torque developed equals that of the spring which results into turning of the pointer
  15. 15. Advantages 1. With the help of phase sensitive demodulator, the tachometer can show the difference in the direction of applied speed. 2. A linear relationship can be derived between output voltage and speed by carrier frequency excitation. 3. They are rugged and inexpensive. 4. Need less maintenance. 5. Ripple free output.
  16. 16. Disadvantages 1. Calibration is difficult as the input voltage should be maintained absolutely constant. 2. At high speed there is a non linear relationship between output voltage and input speed. Hence we need to excite the reference winding with higher frequency
  17. 17. COMMUTATED TYPE TACHOMETER • Construction • Tachometer head containing a reversible switch, operated by a spindle which reverses two times with one revolution. • Indicating unit, voltage source, capacitor, a millimeter and a calibrated circuit
  19. 19. Working • It’s operation is based on alternatively charging and discharging capacitance. • When the switch is closed in one direction, the capacitor gets charged from D.C supply and the current starts flowing through the ammeter.
  20. 20. Working • When the spindle operates the reversing switch to close it in opposite direction, capacitor discharges through the ammeter with the current flow direction remaining the same. • The indications proportional to the rate of reversal of contacts which in turn are proportional to the speed of the shaft and reflected on the scale accordingly.
  21. 21. TACHOGENERATORS • They employ small magnet type DC or AC generator which convert rotational speed into DC or AC voltage signals. • Magnitude of the voltage generated by relative perpendicular motion between the magnetic field and a conductor is a direct function of the strength of the magnetic field and the speed of the conductor.
  22. 22. Dc tachogenerators
  23. 23. AC tachogenerator
  24. 24. DC vs. AC TACHOGENERATORS DC TACHOGENERATORS AC TACHOGENERATORS • The construction consists of a horse shoe type permanent magnet. • Speed is measured with a moving coil voltmeter. • It consists of a stator and a rotor arrangement or a squirrel cage setup. • Speed is measured with a moving coil instrument either a permanent magnet or an electromagnet.
  25. 25. S N To output Dc tachogenerator
  26. 26. n s Permanent magnet coil To output AC Tachogenerators
  27. 27. DIFFERENCE BETWEEN AC & DC TACHOGENERATORS DC TACHOGENERATORS • Reversal of rotation causes the voltmeter to show a negative reading , hence keeping the pointer on the mid scale speeds in both the directions can be displayed. AC TACHOGENERATORS • Reversal of rotation causes the same action except there is a 180 degree phase shift.
  28. 28. DC TACHOGENERATORS AC TACHOGENERATORS • Direction of rotation is directly indicated by polarity of the output voltage. • Conventional voltmeters can be used as indicators. • Change in the direction of the rotation causes phase shift. • Output voltage is rectified with a permanent magnet moving coil instrument.
  29. 29. Advantages • DC type – A conventional setup reduces the installation cost – The direction of voltage is directly indicated by the polarity of the output voltage • AC type – Increased output at same speed as compared to DC tachogenerators. – Accurate to ± 1 % – Installable in inaccessible region.
  30. 30. Disadvantages • DC type – Brushes produce maintenance cost and labor. – The assembly requires maintenance – If the field of the permanent magnet is distorted it gives rise to non linearity. • AC type – Frequency of output voltage is low at low speed. – At high frequencies the impedance of the coils increases.
  31. 31. MECHANICAL TACHOMETERS • Mechanical Tachometers work on the basic principle of physical displacement of indicating parts either linearly or non-linearly. • This displacement is used as a measurand which is proportional to the driver shaft speed. • They employ only mechanical parts and mechanical movements for measurement of speed.
  32. 32. REVOLUTION COUNTER AND TIMER Construction • It consists of a worm gear coaxially attached to the driving shaft which provides the speed source. • A spur gear is connected with its rotating axis perpendicular to the axis of the worm gear. • Pointer indicates number of revolutions. • The dial of the pointer is attached on the frame.
  33. 33. Working • The worm gear is driven by the speed source. • The rotating worm gear drives the spur gear which in turn actuates the pointer on the dial. • Hence indicating number of revolution in certain span of time. • This function requires a separate timer. • The revolution counter thus, gives an average rotational speed rather than instantaneous rotational speed.
  34. 34. Advantages • It gives average rotational speed with respect to time. • Ideal for measuring speed of engines having low operating speed. • Assembly is sturdy and the mechanism lasts for longer duration without maintenance. • Ideal for heavy duty machinery.
  35. 35. Disadvantages • Can not give instantaneous velocity of driving shaft. • Impose a load on the shaft on which they are connected • As a result they absorb power • The counter and timer cannot be started simultaneously. • Not ideal for high speed operations and low power engines as it may show errors.
  36. 36. TACHOSCOPE Construction • This tachometer is a contact type tachometer and has the basic working principle of revolution counter and timer. • The difficulty of the timer and the counter not being able to start simultaneously has been conquered. • The construction incorporates a revolution counter with a built-in timer integrally mounted.
  37. 37. Working • The revolution counter and timer start simultaneously on contact with the driving shaft. • The entire assembly functions so long as the contact is maintained. • The rotational speed is calculated from the individual readings of the counter and timer.
  38. 38. Advantages • The counter and the timer being integrally mounted, can be started simultaneously hassle-free unlike the previous type. • It can measure high rotating speeds upto 5000 rpm.
  39. 39. Disadvantages • Need periodic maintenance. • Due to the inertia of the moving parts, they are less responsive to speed variation. • Complex assembly. • Synchronization of the counter and the dial needs to be rectified accordingly.
  40. 40. STROBOSCOPIC METHODS • This method measures the periodic or rotary motions by a device called a STROBOSCOPE. • This instrument is a simple and manually operated device. • The speed is measured by adjusting the receptor frequency so that the moving section is visible at a particular time interval.
  41. 41. Working principle • The receptor circuit is based upon variable frequency oscillator which controls the flashing frequency. • A strong light is flashed on a moving object , at the time each flash occurs , in an instantaneous position , the object will appear to be stationary.
  42. 42. • A strobotron is the high frequency source of light whose frequency can be varied and controlled. • For measuring the speed of shaft , a mark is made on the disc attached to the shaft. • The flashing frequency is adjusted until the mark appears stationary. • The flashing rate is reduced gradually and the flashing frequencies are noted for all single line images. n = Fm . F1 (m-1) (Fm – F1)
  43. 43. Video
  44. 44. Advantages • Imposes no load on the shaft hence no power loss. • Non contact type hence, no attachments needed. • Convenient to use for spot checks on machinery speeds and laboratory work.
  45. 45. Disadvantages • The variable frequency oscillator circuit cannot be stabilized to give a fixed frequency hence less accurate than digital meters. • Cannot be used where ambient light is above a certain level. • Requires well defined lighting conditions for efficient operations • Errors are caused due to slight variation in the frequency.
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