High speed measurements are possible with digital tachometers which work in conjunction with digital frequency meters. Digital methods doesnot require direct physical contact with the shaft whose speed is to be measured. Therefore, no load is imposed upon the shaft by measuring device. Digital tachometers are of two types: i.) Photo electric Type ii.) Inductive Type

Photoelectric Tachometer:

This method of measuring speed of rotation consists of mounting an opaque disc on the rotating shaft as shown in fig. The disc has a number of equidistant holes on its periphery. At one side of the disc a light source is fixed and at the other side of the disc, and in line with the light source, a light sensor such as photo tube or same photo sensitive semi conducting device is placed. When the opaque portion of the disc is between the light source and the light sensor, the latter is illuminated and produces no output. But when a hole appears between the two, the light falling upon the sensor produces an output pulse. The frequency at which these pulses are produced depends upon the number of holes in the disc and its speed of rotation. Since, the number of holes is fixed; the pulse rate is a function of speed of rotation. The pulse rate can be measured by an electromagnetic counter which can be directly calibrated in terms of speed in r.p.m.

Advantages:

1. The output is digital, no analog to digital conversion is necessary.
2. The pulse amplitudes are constant. This simplifies the electronic circuitry.

Disadvantages:

1. The light source must be replaced from time to time. A typical life time for light source is 50,000 hours.

2. The digital meters measure frequency by counting the number of input pulses which occur in short period of time called gating period. If this period is too small serious errors may be caused.