Speed measurement can be done by analog and digital way. In general, the measurement of speed is divided into two ways, namely: angular way and way of translation. To measure the translational velocity can be derived from the angular measurement method. What is meant by angular measurement is the measurement of rotational speed (rotation), while the measurement of translational velocity is irregular straight-line velocity and irregular straight-line speed.
The most commonly used sensor for angular velocity sensors is the tacho generator. Tacho generator is a small generator that generates DC voltage or AC voltage. In terms of excitation tacho generator can be generated with excitation from the outside or electromagnetic impact of permanent magnet.
Tacho DC generator can generate DC voltage which can directly generate speed information, tacho sensitivity of DC generator is good enough especially at high speed area. Tacho high-quality DC generators have many magnetic poles that can generate DC voltages with high frequency wavelengths that are easy to trim. The main advantage of this tacho generator is the information obtained from the direction of rotation.
While the weaknesses are:
1. Brush the commutator easily run out
2. If used at high temperatures, the permanent magnet will experience fatigue, for this case, tacho generator is often calibrated.
3. Sensitive to dust and corrosion
Tacho AC generator is a singkron generator, permanent magnet placed in the center that serves as a rotor. While the stator shaped soft iron coil. When the rotor rotates the induced voltage is generated in the stator part. Another type of AC tacho generator is induction type, rotor is made serrated, stator in the form of iron-core wire reels. The permanent magnetic field is installed simultaneously in the stator. When the rotor rotates, there is a change of magnetic field in the tooth which then induces into the stator windings.
The main advantages of the AC tacho generator are relatively resistant to corrosion and dust, while the disadvantage is not to provide direction information.
Figure 3.45 Tacho Generator DC Construction
Figure 3.46 Tacho Generator AC Construction
Figure 3.47 Construction of Tacho Generator AC with toothed rotor