JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech II CONTROL THEORY Previous Question Paper (July/August - 2021 - R16)

In an open-loop traffic control system, the traffic signals operate based on a pre-set timer regardless of the actual traffic density on the roads. The controller executes a fixed sequence of green, yellow, and red lights based on a clock. Because there is no feedback mechanism to monitor the number of vehicles, the system cannot adjust to unexpected changes in traffic flow, leading to potential congestion or inefficient signal timing.

In a closed-loop traffic control system, sensors such as inductive loops, cameras, or infrared detectors are installed at intersections to monitor real-time traffic volume. This data is fed back to the controller, which compares the actual traffic density with the desired reference value. The controller then dynamically adjusts the signal timing (e.g., extending the green light duration) to minimize waiting times and optimize traffic flow, effectively creating a self-regulating system.

Negative feedback is preferred in control systems because it provides stability and robustness against disturbances. By subtracting the output from the input, the system generates an error signal that drives the output toward the desired reference, effectively reducing the sensitivity of the system to parameter variations and external noise.

Key advantages include:

• Stability: It allows for the design of systems that converge to a steady state rather than diverging.
• Disturbance Rejection: It automatically compensates for external influences by adjusting the control effort.
• Bandwidth Improvement: It increases the operating frequency range of the system.
• Sensitivity Reduction: The overall gain becomes less dependent on the variations in the internal components of the plant.

Conversely, positive feedback tends to amplify signals, which often leads to instability and uncontrolled oscillations, making it unsuitable for most practical control applications.