# Process Control

Cascade In cascade control system two controllers are cascaded  to each other as shown in the diagram below.  Primary controllers output is fed to secondary controller as a Set point. Both the controllers has their own sensing elements but only secondary controller gives output to the control valve. Primary controller receives set point from …

## Ratio Control

Ratio Control In Ratio Control, the controlled variable is the ratio of two measured variables. Control is effected by adjusting one of the variables –  the controlled variable, to follow in proportion to a second – the wild variable. The proportionality constant is the ratio. Ratio relays are nothing more than manually adjustable gain devices …

## Common Control Loops

Common Control Loop  1.Flow   Liquid flow processes are fast,  typically 0.5 sec response or less. Gas flow is slightly slower because of the compressibility of the gas. Control components – transmitter, valve and transmission lines –  are the main dynamic elements.  The most common measuring means is the orifice plate and differential pressure …

## Derivative Or Rate Control

Derivative Control Derivative control (also called rate control) responds to the rate of change of the controlled variable. For example, if the temperature of a process is changing rapidly, the controller must respond quickly to keep the temperature within acceptable limits. In effect, derivative control action uses the rate of change in a process to …

## Process Control – Integral Control

Integral (or Reset) Control     Integral control (also called Reset control) can be used when unacceptable amounts of offset occur in a process with proportional control. Integral control is used to return the value of the controlled variable to the set point. This mode of control is most often used along with proportional control. …

## Process Control – Proportional Control

Proportional Control  In some processes, the fluctuations around the set point (called oscillation) may be too severe to achieve the desired results. Many processes require a more stable form of control. Proportional control is one way to achieve that stability  When proportional control is used in a system, the final control element may be in …

## Instruments Used In Control Loop

Instruments  I hope you have a general understanding what a feedback control loop is, if not please refer the following article – Feedback Control Loop  Sensors You have read  that a sensor monitors and measures what is happening in the process. Examples of sensors include: thermocouples, floats, limit switches, pressure switches, pressure switches, moisture meters, and …

## Basics of Feedback Control – Elements of Feedback control

Feedback Control Feed-back control is gathering information on the controlled variable and comparing it to the set point; making a decision based on that comparison; and taking action to eliminate any differences between the controlled variable and the and the set point In feedback control, there is a constant flow of information in a closed …

## Elements of Process Control

In most processes, there are six basic elements.Process control elements are        1. Controlled variable What you want to control (temperature pressure, level, flow rat, dimensions, position, etc.)   2. Measured variable What you observe in order to determine the actual condition of the controlled variable  In most cases, you measure the controlled …

## Components of Pneumatic Control System

Pneumatic Control System  What is pneumatic ? Pneumatic is the technology that makes use of gas or pressurized air. Components of Pneumatic control system 1.Directional control Valves   These are functional valves used in logic implementation. Divided according to the number of ports / passages available and number of valve positions 2.Two Way Valves …

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