Measurement of Angular Velocity.    

The measurement of angular velocity is more prominent than that of linear velocity. In many cases, the only way for measuring linear velocity is by its conversion into an angular velocity. The main problem with linear velocity measurement is in use of a fixed reference and in detection in case of moving body travelling over a long distance.

The various devices used in measurement of angular veloc­ity are eddy current tachometer, dc generator tachometer, ac generator tachometer, drag cup rotor ac generator tachom­eter, toothed rotor or variable reluctance tachometer, and pho­ to electric pickup tachometer.

A typical permanent magnet dc generator tachometer has 11 coils, sensitivity of 5 V/1,000 rpm, range ± 6,000 rpm, non­ linearity of ± 0.01%, ripples less than 5% of dc output, internal resistance of the order of 300 ohms, and of size approximately 70 mm long and 30 mm diameter. The dc generator tachometers are widely used for measurement of shaft speeds.

Drag cup rotor ac generator tachometers are rugged in construction, cheaper in cost, require little maintenance, give ripple-free output, and give a linear relationship between the output voltage and rotational speed if the excitation winding is energized with a high-frequency supply (say 400 Hz).

The drawbacks of such transducers are 

(i) necessity of maintaining input or excitation voltage absolutely constant 

(ii) difficulties in calibration and 

(iii) nonlinear relationship between the output voltage and rotational speed at very high speeds.

A typical drag cup rotor ac generator tachometer has an excitation supply of 110 V, 400 Hz, the sensitivity of 2.8 V/1,000 rpm, range of 0 - 3,600 rpm, the nonlinearity of 0.05%.

Toothed rotor tachometers are simple and rugged in construction, easy to calibrate, do not need any maintenance, and give information that can be easily transmitted.

The main drawback of such tachometers is that these can not measure small speeds because at low speed the magnitude of induced voltage pulses may not be sufficient to trigger the counter. Toothed rotor tachometer probably is the most popular tachometer in use nowadays.

The advantages of the photoelectric pickup tachometer are that it gives pulses of constant amplitude and so needs simple electronic circuitry and gives output in digital format and so does not require any A/D converter when used in a digital instrumental system.