JNTU B.Tech IV TELEMETRY AND TELECONTROL Previous Question Paper (December - 2020 - R16)

A position telemetry system is designed to measure and transmit the angular or linear displacement of a mechanical component to a remote location. The most common implementation is the Synchro-based position telemetry system, which utilizes electromagnetic coupling to transmit shaft position information.

The system consists of two main units: a Synchro Transmitter (TX) at the measurement site and a Synchro Receiver (RX) at the monitoring station. Both units are constructed similarly to a three-phase alternator, featuring a stator with three windings displaced by 120 degrees and a rotor with a single winding.

The operation follows these steps:

• The rotor of the transmitter is mechanically coupled to the shaft whose position is to be measured.
• When the reference AC voltage is applied to the transmitter rotor, it induces voltages in the three stator windings. These voltages are functions of the rotor's angular position.
• The three stator terminals of the transmitter are connected to the corresponding stator terminals of the receiver.
• The receiver's stator windings produce a magnetic field identical to the transmitter's field. This field interacts with the receiver's rotor, which is also energized by the same AC reference voltage.
• The interaction creates a torque that forces the receiver rotor to align itself with the angular position of the transmitter rotor.
• The receiver rotor shaft provides the remote indication of the measured position.

This system provides high accuracy and reliability for short-to-medium distance transmission. For longer distances, the signal is often converted into a DC voltage or a digital format (such as Gray code) using an Analog-to-Digital Converter (ADC) before transmission via wire or radio link, requiring a Digital-to-Analog Converter (DAC) at the receiving end to drive a display or control mechanism.

Telemetry systems are categorized based on the transmission medium used to convey information from a remote sensor to a central station. Each type offers distinct trade-offs regarding distance, bandwidth, and environmental suitability.

Electrical Telemetry Systems

These systems use electrical signals (voltage or current) transmitted over copper wires. Advantages: They are simple to install, highly compatible with standard electronic instrumentation, and allow for easy signal processing and amplification. Disadvantages: They are highly susceptible to electromagnetic interference (EMI) and radio frequency interference (RFI). They also present fire hazards in explosive environments and are limited by significant signal attenuation over long distances.

Pneumatic Telemetry Systems

These systems utilize compressed air (typically 3-15 psi) to transmit process variables. Advantages: They are inherently explosion-proof, making them ideal for hazardous environments like oil refineries. They are robust, reliable, and do not require electrical power at the field transmitter. Disadvantages: They have very slow response times due to the compressibility of air, are limited to short transmission distances, and require a constant supply of clean, dry, compressed air. They are also difficult to integrate with modern digital control systems.

Optical Telemetry Systems

These systems use light signals transmitted through fiber optic cables. Advantages: They offer extremely high bandwidth and immunity to all forms of electromagnetic and radio frequency interference. They provide excellent electrical isolation between the transmitter and receiver, preventing ground loops. They are lightweight, secure against signal tapping, and capable of transmitting data over very long distances without repeaters. Disadvantages: The initial installation cost of fiber optic infrastructure is high. They require specialized equipment for splicing and maintenance, and the fiber cables are fragile and susceptible to physical damage if not properly protected.