Design and construction of microprocessor based remote speed measurement, monitoring and control of d. C motor

2.2 Types of Tachometers
1). Mechanical Tachometers: This is a contact type tachometer. As seen in the diagram below this type of tachometer is nothing but a linkage of shafts, gears and rotating weights. When the input shaft which is seen horizontal rotates the vertical shaft it also rotates the weights attached to it which are hinged and free to move inward and outwards. The movement of these flyweights rotates a pointer which is calibrated to give the speed in desired units such as RPM.

Figure 2.1: Mechanical Tachometer
Two main drawbacks of this are that the mechanical weights have inertia and hence not very accurate and secondly it does not give an indication of the direction of rotation.
2). Electro-Mechanical Tachometers: These are contact tachometers also known as voltage based tachometers that use voltage to determine speed. The basis of their operation is simple: The amount of voltage produced relies solely on how fast they turn. These types of tachometers are also capable of providing the rotation direction along with the speed, which is essential to provide the necessary feedback signals. They are of two types namely: Electric generator tachometer (DC & AC generator tachometer) and eddy current tachometer.
3). Digital Tachometer
Digital Tachometers are used for precise measurement and monitoring of all time related quantities, which can be converted into a proportional frequency using appropriate sensor.
Digital tachometers mostly used optical sensors which are more suitable especially the reflective optical sensors. It makes use of the reflective IR sensor, which is used to convert the angular velocity of the rotating body into corresponding frequency. Once the Frequency corresponding to angular velocity is known it can be used to calculate the RPM using any one of the following methods.

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  Direct pulse counting,
  
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  Single pulse time measurement,
  
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  Constant elapsed time (CET),
  
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  Pulse time measurement using a variable number of counted pulses.