JNTU B.Tech III INDUSTRIAL INSTRUMENTATION Previous Question Paper (September - 2021 - R16)

Rotational velocity measurement using photoelectric pulse counting is a non-contact method that converts mechanical rotation into a series of electrical pulses. The frequency of these pulses is directly proportional to the rotational speed of the shaft.

The system consists of three primary components:

• Light Source: Typically an LED or an incandescent lamp that provides a steady beam of light.
• Rotating Member: A disc or wheel mounted on the rotating shaft, featuring a series of evenly spaced slots or reflective/non-reflective segments around its periphery.
• Photoelectric Sensor: A photodiode or phototransistor placed on the opposite side of the disc (for transmission) or on the same side (for reflection) to detect the light interruptions.

The operation follows these steps:

1. As the shaft rotates, the slots on the disc periodically interrupt the light beam falling on the sensor.
2. Each time a slot passes the sensor, the light beam is either blocked or transmitted, causing the sensor to generate an electrical pulse.
3. If the disc has N slots and the shaft rotates at a speed of n revolutions per second, the sensor produces N × n pulses per second.
4. These pulses are fed into an electronic counter or a frequency-to-voltage converter. The frequency of the pulse train is measured over a fixed time interval.
5. The rotational velocity in revolutions per minute (RPM) is calculated using the formula:

This method offers high precision, low mechanical loading on the shaft, and immunity to electromagnetic interference. Accuracy can be increased by increasing the number of slots on the disc, which provides a higher pulse count per revolution, allowing for better resolution at low speeds.

A gyroscope is a device used for measuring or maintaining orientation and angular velocity. It is based on the principle of conservation of angular momentum. The device consists of a spinning rotor mounted inside a set of gimbals, which allow the rotor to maintain its orientation in space regardless of the movement of the base or frame.

The working principle is governed by two fundamental properties:

• Rigidity in Space: A spinning rotor tends to maintain its axis of rotation fixed in space unless acted upon by an external torque. This allows the gyroscope to serve as a stable reference platform.
• Precession: When an external torque is applied to the spinning rotor, the axis of rotation shifts in a direction perpendicular to the applied force. The rate of precession is directly proportional to the applied torque and inversely proportional to the angular momentum of the rotor.

In a typical mechanical gyroscope, the rotor is mounted in two or three gimbals. As the outer frame rotates, the gimbals isolate the rotor, allowing it to remain stationary relative to inertial space. By measuring the displacement or the torque required to prevent precession, the angular rate or orientation of the system can be determined.

Applications of the gyroscope include:

• Navigation Systems: Essential for inertial navigation systems (INS) in aircraft, ships, and submarines to determine position without external references.
• Aerospace Stabilization: Used in autopilots and flight control systems to stabilize aircraft and missiles.
• Consumer Electronics: Integrated into smartphones and tablets for screen rotation, gaming motion control, and augmented reality.
• Robotics: Provides orientation feedback for balance and precise movement in autonomous robots and drones.
• Space Exploration: Used for attitude control and pointing of satellites and space telescopes.
• Industrial Instrumentation: Employed in borehole surveying and platform stabilization for heavy machinery.