Tachometer

Measurement & Control

Speed MeasurementTachometer

Mr.Ashwin JoshiM.Tech.[CAD / CAM]

Device for indicating the angular (rotary) speed of a rotating shaft. Either by registering the total no. of revolution during the period of contact or by indicating the number of revolution per min.

Tachometers are classified as follows.1)Mechanical tachometer2)Electrical tachometer

Tachometer

What Are the Different Types of Tachometers?

Tachometers can be classified 1.On the basis of data acquisition

Contact Non contact types

2. Classified as data type Analog Digital

3. On the basis of power .MechanicalElectrical

Aliasgar Kutiyanawala

Comparison

Contact Type

1) The tachometer has to be in physical contact with the rotating shaft.

2) Preferred where the tachometer is generally fixed to the machine.

3) Generally, optical encoder / magnetic sensor is attached to shaft of tachometer.

Non Contact Type

1) The tachometer does not need to be in physical contact with the rotating shaft.

2) Preferred where the tachometer needs to be mobile.

3) Generally, laser is used or an optical disk id attached to rotating shaft and read by a IR beam or laser.

Aliasgar Kutiyanawala

How To Choose a Tachometer?

1) Accuracy2) Precision3) Range4) Acquisition Time5) Contact type / Non Contact type6) Portable / Fixed7) Digital / Analog8) Cost

1)Mechanical tachometer

Mechanical tachometers utilize the fact that the centrifugal force on a rotating mass depends on the speed of rotation and can be used to stretch or compress a mechanical spring.

These employ only mechanical parts and mechanical movement.

1.Revolution Counters.2.Centrifugal Force Tachometer.

Limitations

1) Mechanical tachometers imposes load on rotating shaft which they are connected therefore they absorb the power. Not applicable for shafts involving small power .

2) More maintenance require.3) Faulty parts doesn’t give accurate reading.4) It is normally use an arrangement where in

a counter is used for counting the numbers of revolution and a stop what is used for measurement of time.

1.Revolution counter

1.Revolution counter

Revolution counter is used to measure an average of rational speed instead of instantaneous rotational speed.

It consists of a worm gear that is usually attached to a spindle. It has two dials, an inner one and an outer one.

The inner dials represent one revolution of the outer dials and the outer dials represent on revolution of the spindle.

The tachometer has a stopwatch attached to the revolution counter and is used to indicated time.

Advantages1. Simple to operate.2. No attachment require to shaft.

Disadvantages1.Limited to low speed.2.Chances of operational error.

2.Centrifugal Force Tachometer

A sliding collar is mounted on a shaft. The coupling has hinged arms carrying mass that spread apart when the shaft rotates, moving the sliding coupling along the shaft against a counterbalancing spring.

The position of the coupling on the shaft depends on the speed of rotation and is transmitted by an arm mechanism to an indicator pointer; the indicator dial is calibrated in revolutions per minute. The tachometer shaft may be driven directly, by the controlled mechanism, or indirectly, by a flexible shaft.

2)Electrical tachometer

Electrical tachometers are of several types. The eddy-current or drag type is widely used

in automobile speedometers; a magnet rotated with the shaft being measured produces eddy currents that are proportional to angular speed.

Electric-generator tachometers work by generating either an alternating or a direct current.

The stroboscope, an instrument that illuminates rotating objects so that they appear to have stopped moving, can be used as a tachometer.

a) Eddy current or drag type tachometer.b) Electric generator tachometer.c) Contactless type tachometer.d) Frequency type tachometer.e) Ignition type tachometer.f) Stroboscopic tachometer.

Eddy Current tachometer

(1) permanent magnet, (2) rotor, (3) shaft with pointer, (4) spring

It consist of a permanent magnet as stator and a low resistance solid metallic cylinder as rotor, which drives the pointer and scale arrangement. the rotor is also connected to shaft which has a provision to get contact with the rotating machines, whose rpm has to be measured. when it get contact to a rotating machine the shaft rotates which eventually rotates the metallic cylinder in strong magnetic field , its known that when a low resistant metal rotated in a strong magnetic field eddy current gets induced in the low resistance metal opposing the cause of action which held the pointer in equilibrium position showing some reading which will be calibrated for the corresponding speeds hence it shows the correct rpm in which the machine rotates

Tachogenerators provide a voltage value which is proportional to the speed and may be a.c. or d.c. instruments. The d.c. tachogenerator is a small d.c, generator with a permanent field. The output voltage is proportional to speed and may be measured on a voltmeter calibrated in units of speed. The a.c. tachogenerator is a small brushless alternator with a rotating multi-pole permanent magnet. The output voltage is again measured by a voltmeter although the varying frequency will affect the accuracy of this instrument.

An eddy-current tachometer (Figure 2) uses the interaction of the magnetic fields generated by a permanent magnet and a rotor, whose speed of rotation is proportional to the eddy currents generated. The currents tend to deflect a disk, which is mounted on the shaft and restrained by a spring, through a certain angle. The deflection of the disk, which is rigidly connected to a pointer, is indicated on a dial.

Electric tachometers may be of the generator or impulse type. In tachometer generators the electromotive force of a DC or AC generator is proportional to the angular velocity, from which the shaft speed can be determined; the readings are transmitted to a remote measuring instrument. The operation of impulse tachometers is based on conversion of pulses generated in the primary circuit of an ignition system by the opening of interrupter contacts into a current that is fed to a permanent-magnet indicator. The frequency of pulses in the primary circuit is proportional to the speed of rotation of the engine shaft.

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 e.m.f. 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.

In case of variable reluctance sensors, we have coil wounded on permanent magnet not on iron core, this configuration enable us to measure rotational speed of the systems. - See more at:

In the construction of variable reluctance sensor, we use ferromagnetic gearwheel. As the gearwheel rotates, change in magnetic flux take place in the pickup coil which further induces voltage. This change in magnitude is proportional to the voltage induced in the sensor. - See more at:

http://www.ibiblio.org/kuphaldt/electricCircuits/AC/AC_13.html#xtocid174388