Advanced Process Control _ Single, Interactive, Independent and Compound Variable Process Control

Control System Strategies

The purpose of a process control system is to maintain some process variable at the setpoint. The configuration and complexity of process control system depend on the characteristics of process under control.

Single Variable Process Control:

It is the most elementary type process control. In this strategy there is only a single variable. The variable is measured directly and maintained at the setpoint regardless of any other process variables. It is the most simplest and cheapest process control strategy.

In the above example, there is process tank. Water is fed through an inlet tube. Water is taken out through an outlet tube. There is a flow controller to maintain constant flow through the inlet tube. The water inside the tank is heated. There is a temperature control system to maintain the tank temperature.

Consider the flow control system. The flow is measured directly and maintained at the setpoint with the help of a flow controller. The flow is not affected by any other control loops.

Independent Single Variable Control:

In certain processes, the control of one variable may be effected by the manipulation of some other variable. In this case the controlled variable is called the dependent variable and the manipulated variable is also known as the independent variable.  The controlled variable depends on the independent variable.

In the example, there is a process tank in which water is collected. Water is fed in to the tank through an inlet tube. There is an outlet tube to carry water out of the tank. There is a level sensor in the tank. The level controller maintains the water level by manipulating the inlet flow. Here the water level in the tank depends on the inlet flow rate.

Interactive variable control system:

In certain processes, there may be more than one control loops. In certain cases, the action of a control loop may affect the other loops. The loops may interact. In this case the controller design becomes complicated. The corrective action of one loop disturbs the other loop

In the above example, there is process tank. Water is fed through an inlet tube. Water is taken out through an outlet tube. There is a flow controller to maintain constant flow through the inlet tube. The water inside the tank is heated. There is a temperature control system to maintain the tank temperature.

Under nominal conditions, the flow in to the tank is held constant and the temperature is also held constant. If the setpoint of the flow control system changes, the flow control loop adopts a new flow rate, which will appear as a load change to the temperature control system.

Compound Variable Control System:

The word compound means combination of more than one variable. In compound variable control system, more than one process variables are measured. The variables are combined and used to maintain a single variable under control. Ratio control system is an example of compound control system.

In this example the control system maintains the ratio of two reactants  A and B. One of the flow rates is measured but allowed to float (that is, not regulated), and the other is both measured and adjusted to provide the specified constant ratio. The flow rate of reactant A is measured and added, with appropriate scaling, to the measurement of flow rate B. The controller reacts to the resulting input signal by adjustment of the control valve in the reactant B input line.

Ref: Curtis Johnson- Process Control Instrumentation Technology ( pages 558 to 560)

This is what I want to give as an introduction to my first class

A BRIEF HISTORY of ELECTRICITY

Long before any knowledge of electricity existed, people were aware of shocks from electric fish. Patients suffering from ailments such as gout or headache were directed to touch electric fish in the hope that the powerful jolt might cure them. Ancient cultures around the Mediterranean knew that certain objects, such as rods of amber, could be rubbed with cat's fur to attract light objects like feathers. The Mesopotamians may have had knowledge of electroplating, based on the 1936 discovery of the Baghdad Battery, which resembles a galvanic cell, though it is uncertain whether the artifact was electrical in nature. English scientist William Gilbert in 1600 AD coined the New Latin word electricus to refer to the property of attracting small objects after being rubbed. In the 18th century, Benjamin Franklin conducted extensive research in electricity, selling his possessions to fund his work. In June 1752 he is reputed to have attached a metal key to the bottom of a dampened kite string and flown the kite in a storm-threatened sky.^[11] A succession of sparks jumping from the key to the back of his hand showed that lightning was indeed electrical in nature and understood that electricity consisting of both positive and negative charges.  Alessandro Volta's battery provided scientists with a more reliable source of electrical energy. Christian Orsted, Andre Ampere and Michel Faraday conducted experiments in relationship between electricity and magnetism which lead to all present day developments in the field. Later it was invented that electric charge shows entirely different effects when it is stationary and in motion. 

Coming centuries witnessed immense growth in electricity production and distribution methods.