JNTU B.Tech III PROCESS CONTROL INSTRUMENTATION Previous Question Paper (February/March - 2022 - R15)

A simple level process consists of a tank with an inlet flow rate Q_i and an outlet flow rate Q_o through a resistance (valve). The level h is determined by the mass balance of the fluid.

The mass balance equation is: Rate of accumulation = Input flow - Output flow. Assuming constant density and cross-sectional area A, this is expressed as:

The outlet flow Q_o through the resistance R is given by Q_o = h / R. Substituting this into the balance equation:

Taking the Laplace transform assuming zero initial conditions:

Rearranging for the transfer function H(s) / Q_i(s):

Where K = R is the process gain and τ = AR is the time constant of the level process.

In a servo operation, the controller is designed to track changes in the setpoint. The process variable is expected to follow the setpoint as closely as possible. An example is a robotic arm position control where the input command changes frequently to move the arm to new coordinates.

In a regulator operation, the setpoint is kept constant, and the controller is designed to reject disturbances. The objective is to maintain the process variable at the desired value despite external load changes. An example is a temperature control system in a chemical reactor where the setpoint remains fixed, but fluctuations in ambient temperature or feed concentration act as disturbances that the controller must counteract.

The fundamental difference lies in the objective: servo control focuses on command tracking (transient response to setpoint changes), while regulator control focuses on disturbance rejection (stability and steady-state accuracy against load changes).