Tachometer | Photoelectric tachometer | Contactless type tachometer | Inductive and Capacitive Pick up

By | February 5, 2019

Tachometer | Photoelectric tachometer | Contactless type tachometer | Inductive pick up | Capacitive Pick up | Advantages | Disadvantages

Tachometer :

In this article we learn 2 types of tachometer with details.

A) Photoelectric tachometer :

1) Fig. 5.5.5 shows the photoelectric tachometer.
2) This method of measuring speed of rotation consists of mounting an opaque disc on the rotating shaft.
3) The disc has a number of equidistant holes on its periphery.
4) At one side of the disc a light source is fixed and at the other side of the disc and in line with light source, a light sensor such as phototube is placed.


Fig. Photoelectric tachometer.

5) When the, opaque portion of the disc is between the light source and the light sensor, the latter is un-illuminated and produces no output.
6) But when a hole appears between the two the light falling upon the sensor produces an output pulse.
7) The frequency at which these pulses are produced depends upon the number of holes in the disc and its speed of rotation.
8) As the number of holes is fixed, the pulse rate is function of speed of rotation.
9) The pulse rate can be measured by an electronic counter which can be directly calibrated in terms of speed in rpm.
10) Following are the advantages of photoelectric tachometer.


1) Simple in construction.
2) The output is in digital form.
3) Easy to operate and understand.
4) Used to measure very high speed.
5) Less maintenance.
6) It is contactless method.

B) Contactless type tachometer :

1) This type of tachometer produces pulses from a rotating shaft without being Mechanically connected to it.
2) As the energy produced by these devices is not sufficient to actuate an indicator directly, amplifiers of sufficient sensitivity are used.
3) The different types of contactless tachometer are based on the following types :
1) Electromagnetic pick up.
2) Capacitive pick up.
3) Optical pick up.

a) Inductive Pick up :

1) This type of pick up consists of a metallic toothed rotor mounted on the shaft whose speed is to be measured. Fig. 5.5.6 shows the inductive pick up.


Fig. Schematic diagram of inductive pick up.

2) A magnetic pick up is placed near the toothed rotor.
3) The magnetic pick up consists of a housing containing a permanent magnet with a coil wound round it.
4) When the rotor rotates, the reluctance of the air gap between pick up and the toothed rotor changes giving rise to an induced emf in the pick up coil.

5) This output is in the form of pulses, with a variety of wave shape.
6) The frequency of the pulses of induced voltage will depend upon the number of teeth of the rotor and its speed of rotation.
7) As the number of teeth is known, the speed of rotation can be determined measuring the frequency of pulses with an electronic counter. .
8) Consider the rotor has T teeth,-the speed of rotation is r.p.m. and number of pulses per second is P.

9) A typical rotor consists of 60 teeth.
10) Thus if the counter counts the pulse in one second, the counter will directly display the speed in r.p.m.
11) These types of tachometers are generally used for low as well as high speed about 7000 r.p.m. with very high accuracy.

Following are some advantages of inductive pick up:

1) It is simple and rugged in construction.
(2) It is maintenance free.
(3) It is easy to calibrate.
(4) The information from this device can be easily transmitted.
(5) No direct physical contact is required with shaft to whose speed is to be measured.

b) Capacitive Pick up :

1) For measurement of rotational speed, vane attached to one end of the shaft whose speed is to be measured.
2) Two capacitor plates are placed on either side of vane forming a capacitor, where one plate is fixed and other is movable.
3) When shaft, rotates between the fixed capacitive plates, change in capacitance take place
4) Fig. shows the principle of working of capacitive pick up.


Fig. Capacitive pick up tachometer :

Advantages of capacitive pick up :

(1) High sensitivity
(2) It has very small loading effect.

Limitations / Disadvantages of capacitive pick up :

(1) Electronic circuit is complex.
(2) to measure small displacements.

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