Tachometer: Rotation measurement Techniques-Magnetic and Optical method

Measuring the speed of rotation is an important parameter in production industries. Tachometers are the instruments used to measure the rotational speed. There are two types of tachometer based on the working principle; Magnetic and Optical

Magnetic Tachometer:

One of the most common transducers for measuring the angular speed of a rotating member (gear, shaft, pulley, etc.) is the magnetic pickup. It consists of a permanent magnet with a coil of wire wrapped around and the end of it is attached to connector pins.

A coil of wire is magnetically coupled to a permanent magnet around which it is wrapped. When a ferrous material disturbs the magnetic field, the coil senses a change of flux and a small-induced voltage appears at the coil output wires. The rate of change of the flux (the speed of the rotating member), the “strength” of the magnet, and the size of the air gap will affect the output amplitude. The frequency of the output signal will be directly proportional to the rotational speed of the member. Typically, the air gap is on the order of a few millimetres. The shell of the magnetic pickup is often threaded so that it can be mounted and adjusted as needed.

Advantages:

  • A linear relationship can be derived between the output voltage and speed carrier frequency excitation
  • They are rugged and inexpensive
  • Needless for maintenance
  • Ripple free output

Optical Tachometer:

With the use of light emitting diodes (LEDs) and photodiodes, non-contact optical speed sensing transducers can be constructed. Light reflective tape or paint can be applied to a portion of a rotating item.

A LED light source is fixed at a position from which light can be aimed to the rotating surface. A reflecting strip is placed in the rotating surface, a reverse-biased photodiode is aimed at the same section, then each time the reflective portion of the rotating item is aligned with the light source and the photodiode, the photodiode will be forward biased. This change in bias can be detected by measuring the voltage drop across a resistor placed in series with the photodiode. Then the number of rotation is the count of light detection.

Advantages: 

  • Imposes no load on the shaft hence no power loss
  • Non-contact type hence no attachment needed
  • Convenient to use for spot checks on machinery speeds and laboratory work
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