ANGULAR VELOCITY MEASUREMENT

Overview Introduction Tachometers Stroboscope and Stroboscopic Methods

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.

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.

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.

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

TYPES OF TACHOMETERS MECHANICAL REVOLUTION COUNTER & TIMER TACHOSCOPEELECTRICALDRAG CUP COMMUTATED CAPACITOR

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.

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.

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.

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.

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.

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.

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.

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.

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.

HAND SPEED INDICATOR

The indicator has an integral stop watch and counter with automatic disconnect. The spindle operates when brought in contact with the shaft, but the counter does not function until the start and wind button is pressed to start the watch and engage the automatic clutch. Depressing of the starting button also serves to wind the starting watch. After a fixed time-interval (usually 3 or 6 seconds), the revolution counter automatically gets disengaged. The instrument indicates the average speed over the short interval, and the dial is designed to indicate the rotational speed directly in rpm. These speed measuring units have an accuracy of about 1% of the full scale and have been used for speeds within the range 20,000 to 30,000 rpm.

SLIPPING CLUTCH TACHOMETER

The rotating shaft drives an indicating shaft through at slipping clutch. A pointer attached to the indicator shaft moves over a calibrated scale against the torque of a spring. The pointer position gives a measure of the shaft speed.

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.

DRAG CUP TACHOMETER Construction Drag cup tachometer basically consists of stator and a rotor. The stator has two windings mounted at 90 0 to each other known as » Reference Winding » Quadrature winding A low reluctance path is provided by a ferromagnetic core.

Reference winding Ferromagnetic rotor Quadrature winding Drag Cup rotor Shaft

RotorStator

Drag cup Tachometer

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

N S ῳiῳi Conducting but non-magnetic cup Ѳ0Ѳ0 Ѳ0Ѳ0 ῳiῳi

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

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.

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

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

TACHOMETER HEAD (REVERSING SWITCH ONLY) SPINDLE CAPACITOR RPM INDICATOR A MILLIAMETER BATTERY

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.

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.

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.

Dc tachogenerators

AC tachogenerator

DC vs. AC TACHOGENERATORS DC TACHOGENERATORS The construction consists of a horse shoe type permanent magnet. Speed is measured with a moving coil voltmeter. AC TACHOGENERATORS 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.

N S To output Dc tachogenerator

AC Tachogenerators To output n s Permanent magnet coil

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.

DC TACHOGENERATORS Direction of rotation is directly indicated by polarity of the output voltage. Conventional voltmeters can be used as indicators. AC TACHOGENERATORS Change in the direction of the rotation causes phase shift. Output voltage is rectified with a permanent magnet moving coil instrument.

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.

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.

PICK UP TACHOMETERS

Various pick-up devices can be used in conjunction with a digital counter to give a direct reading of speed. An inductive pick-up tachometer is shown in Figure (a). As the individual teeth pass the coil they induce an emf. pulse which is appropriately modified and then fed to a digital counter. A capacitive pick- up tachometer is shown in Figure (b). As the rotating vane passes between the plates a capacitance change occurs in the form of a pulse. This is modified and then fed to the digital counter.

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.

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.

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)

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.

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.

THANK YOU