Question

Explain the parts of a control loop include how components, signals from each component, types of loops such as location, what they measure, and modes of loops.

Answer 1

Components of a Control Loop
A controller seeks to maintain the measured process variable (PV) at set point (SP) in spite of unmeasured disturbances (D). The major components of a control system include a sensor, a controller and a final control element. To design and implement a controller, we must

1. have identified a process variable we seek to regulate, be able to measure it (or something directly related to it) with a sensor, and be able to transmit that measurement as an electrical signal back to our controller
2. have a final control element (FCE) that can receive the controller output (CO) signal, react in some fashion to impact the process (e.g., a valve moves), and as a result cause the process variable to respond in a consistent and predictable fashion.

Some common examples of process control loops:

• Home Temperature Control

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As shown above the home heating control system described in this article can be organized as a traditional control loop block diagram. Block diagrams help us visualize the components of a loop and see how the pieces are connected.A home heating system is simple on/off control with many of the components contained in a small box mounted on our wall. Nevertheless, we introduce the idea of control loop diagrams by presenting a home heating system in the same way we would a more sophisticated commercial control application

As the energy output of the furnace rises or falls, the temperature of our house increases or decreases and a feedback loop is complete. The important elements of a home heating control system can be organized like any commercial application:
Control Objective: maintain house temperature at SP in spite of disturbances
Process Variable: house temperature
Measurement Sensor: thermistor; or bimetallic strip coil on analog models
Measured Process Variable (PV) Signal: signal transmitted from the thermistor
Set Point (SP): desired house temperature
Controller Output (CO): signal to fuel valve actuator and furnace burner
Final Control Element (FCE): solenoid valve for fuel flow to furnace
Manipulated Variable: fuel flow rate to furnace
Disturbances (D): heat loss from doors, walls and windows; changing outdoor temperature; sunrise and sunset; rain

2. Example of Modeling a Stirred Tank Heater

Consider the stirred tank heater in the Fig as shown below. The question is what would change in case a change is occurred in the input condition (either in the manipulated variable or the disturbance). It is evident that inlet flow rate and its temperature are the input condition which can undergo a change and in such situation the mass and energy content (state variables) of the tank would show a progression. In normal situation, flow rate or temperature of an inlet flow does not have a potential to displace the tank physically from its normal position. Hence, there is no scope of progression of momentum of the tank. In other words, one need not carry out momentum balance operation on this process, rather mass and energy balance operation would suffice.

Let us now apply the material balance and energy balance operation on this process that would yield the following two equations:

Material balance

Rate of accumulation of water = rate of water inlet - rate of water outlet

Where A is the cross sectional area of the tank. If we assume the density of water to be constant then the material balance equation would take the final form as

For a free flow system,


Where c is a constant. Hence,

(1)

Energy balance

Rate of accumulation of heat = rate of heat in -  rate of heat out +  rate of heat supplied

If we assume the density and specific heat of water to be constant and the reference temperature to be zero, then the energy balance equation would take the form as

	(2)

Equation (1) and (2) represent the mathematical model of the stirred tank heater.