Jawaharlal Nehru Technological University Hyderabad B.Tech III INDUSTRIAL INSTRUMENTATION Previous Question Paper (March - 2021 - R16)

One common technique for angular measurement is the use of a Synchro. A synchro is an electromagnetic transducer that converts the angular position of a shaft into an electrical signal. It consists of a stator with three-phase windings (usually Y-connected) and a rotor with a single-phase winding.

The rotor is connected to the shaft whose angular position is to be measured. When an AC excitation voltage is applied to the rotor, it induces voltages in the stator windings. The magnitude and phase of these induced voltages are functions of the rotor's angular position. By measuring the ratio of these output voltages, the precise angular displacement of the shaft can be determined.

Sketch description: The device is represented by a cylindrical housing containing a rotor (connected to the shaft) and three stator coils spaced 120 degrees apart. The rotor receives an AC input, and the stator terminals provide the output signals that vary according to the rotor's rotation.

A Photoelectric Tachometer is a non-contact speed measurement device that uses light interruption to count rotations. It consists of a light source (LED), a rotating disc with slots or reflective markings attached to the shaft, and a photodetector (photodiode or phototransistor).

As the shaft rotates, the slots on the disc periodically interrupt the light beam directed at the photodetector. Each time light hits the detector, it generates an electrical pulse. The frequency of these pulses is directly proportional to the rotational speed of the shaft. The pulses are then processed by a frequency-to-voltage converter or a digital counter to display the RPM.

Diagram description: A light source is positioned on one side of a slotted disc mounted on the rotating shaft, with a photodetector placed directly opposite. The output of the detector is connected to a pulse counting circuit. This method is highly accurate and avoids mechanical loading on the measured shaft.