How a Process Control Loop Works in Automatic Control Systems

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In today’s modern plants, processes are controlled to achieve desired objectives. The term control means methods or means used to force parameters in the environment to have specific or desired values. To achieve control, several elements are coordinated together to achieve the control objective. All the elements necessary to accomplish the control objectives, including the instrumentation systems, are usually described by the term control system.

Control can either be manual or automatic. Manual control involves human intervention and it often entails forcing a given parameter to specific value with the human factor actually carrying out the control function. For example, suppose we want to control the level of water in an open tank which has an inlet valve through which water passes, all we simply do is to station an operator who basically uses his judgement to close the valve when the tank has become full or open the valve when the tank is almost empty.

In automatic control, no human intervention is required rather sensors, controllers, actuators and other control elements are used to automatically control a system to force the system parameters to desired levels.

Elements of an Automatic Control Loop:

An automatic control loop is made up of the following elements:

The process:

In general, a process can consist of a complex assembly of phenomena that relates to some manufacturing sequence or any system we wish to control. Many variables may be involved in such a process, and it may be desirable to control all these variables at the same time. There are single-variable processes, in which only one variable is to be controlled, as well as multi-variable processes, in which many variables, perhaps interrelated, may require regulation.

Measurement:

To achieve the control of a variable in a process, we must have information on the variable itself. Such information is found by measuring the variable. In general, a measurement refers to the conversion of the variable into some corresponding analog signal of the variable, such as a pneumatic pressure, an electrical voltage, or current. The result of the measurement is a conversion of the variable into some proportional information in a useful form required by the other elements in the process control operation.

Sensors/Transducers:

A sensor is a device that performs the initial measurement and energy conversion of a variable into analogous electrical or pneumatic information. Sometimes further transformation or signal conditioning may be required to complete the measurement function. The sensor used for measurement may also be called a transducer. The word sensor is preferred for the initial measurement device, while a "transducer" represents a device that converts any signal from one form to another. Thus, for example, a device that converts a voltage into a proportional current would be a transducer. In other words, all sensors are transducers, but not all transducers are sensors.

Error Detector:

This is the device that determines whether the variable we desire to control, often called the process variable is above or below the desired level called the Setpoint or reference value. If the process variable is above or below the setpoint, an error signal proportional to the error is generated. This error signal is then used by the controller to generate a control action. So before any control action takes place, an error signal must be generated. It is worthy to note that the error detector is often an integral part of the controller device; however it is important to keep a clear distinction between the two.

Controller:

The device that acts on the error signal generated to determine what control action, if any, to be taken is called a controller. The evaluation performed to determine control action can be done by electronic signal processing, by pneumatic signal processing, or by a computer. Computer use is growing rapidly in the field of process control because computers are easily adapted to the decision-making operations and because of their inherent capacity to handle control of multivariable systems. The controller requires an input of both a measured indication of the controlled variable and a representation of the reference value of the variable, expressed in the same terms as the measured value. The reference value of the variable, you will recall, is referred to as the setpoint. Evaluation consists of determining action required to bring the controlled variable to the setpoint value.

Control Element:

The final element in the control loop is a control element that exerts a direct influence on the process; it is the device that provides those required changes in the controlled variable to bring it to the setpoint. This element accepts an input from the controller, which is then transformed into some proportional operation performed on the process. In most process control loops, the final control element is a valve which is often referred to as the final control element.

The Control Loop:

In a control loop, the signal flow forms a complete circuit from the process through measurement, error detector, controller, and final control element. This is called a loop, and in general we speak of a process control loop. In most cases this is called a feedback loop, because we determine an error and feed back a correction to the process. We also have open loop. In open loop , there is no feedback from the process. Here, the control action does not depend on changes in the process variable. Control action is usually based on changes in the input to the process.

Shown below is the block diagram of a typical process control loop with feedback control:

How a Process Control Loop Works:

To understand how a process control loop works, I took sometime to explain what control is and what a control system does. I also explored the various elements that make up a control loop. Having now understood these basic concepts, how then does a process control loop works? To understand how it works, we refer to the above block diagram of a feedback control loop.

The first point of interest for any process control endeavor is the process variable, PV. It is the variable we have chosen to control or maintain at a given reference value or setpoint. So as shown in the process control loop above, the process variable, PV, is measured with the aid of sensors/transducers. This measured signal is then fed into a controller incorporating an error detector device. Here, the Process variable, PV is compared with the desired value of the process variable or the setpoint, SP and an error signal with a specific magnitude and polarity is generated and further processed within the controller. Based on the processed error signal, the controller initiates a control action with the aid of the control element or final control element as it is often known. The final control element, initiates a change in the process by changing the manipulated variable, MV, which then alters the process until it settles at the setpoint. In this way, the process variable is taken back to its desired value or setpoint. This is essentially how a process control loop works. Most complex process plants are operated with this simple underlying principle of process control.