Basics of a Control Loop

Control loop consists of all the key elements necessary to move the final control elements. We have different types of final control elements like the Valves, Motors, Fans etc. The final control elements makes it possible to keep controlled variable on target.

Let's Consider the following diagram:

The Process measuring elements sends a signal to the controller. In this case the level transmitter sends the signal of the measured process variable to the level controller.  At the same time the already determined set point is compared with the signal coming from the measuring device. 

The error detector makes a comparison between the signal from the process and the set point signal. If there is a difference between this, an error signal is sent to the control logic that changes the position of final control device (In our case; it is the control valve). The change in the Final control device will cause a change in the process. This process will continue until the set point is maintained.

Key elements of a control loop includes:

• The measuring device/Process measurement instrument
• Controller
• Final Control device, in this case, the control valve

You can also read: 

• The Basics of Industrial Instrumentation Systems
• Closed Loop Control System

Common terms used:

• Process Variable
• Manipulated Variable
• Controller Error
• Set point

Explaining the terms in more details:

Measuring device: This is the instrument that measures the physical quantity. This can be the temperature, level, flow rate, Pressure etc. It then converts this into a signal that can be easily picked up by the controller.

Process Variable: This is the process value or the process parameter i.e. the current measured value of a particular part of a process which is being monitored or controlled. The current level is the process variable while the desired level is known as the set point.

Process Control: The automatic control of certain process variables to hold them within given limits.

Set point: The reference value for a controlled variable in a process control loop.

Controller: The element in a process control loop that evaluates any error of the measured variable and initiates corrective action by changing the manipulated variable.

Controller Error: The difference in value between a measured signal and a set point.

Final Control Device (Actuator): A device that performs an action on one of the input variables of a process according to a signal received from the controller.

Manipulated Variable: The variable controlled by an actuator to correct for changes in the measured variable. 

Related:  The Transition from Relay Control Systems to PLC Systems

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Key Elements that make up a Control Loop

A process is a sequence of events designed to control the flow of materials through a number of steps in a plant to produce a final product. To get a better understanding of a process control system, we can have a look at the following diagram showing elements in a continuous control process with a feedback loop.

Note that, the process can be a simple one, with a few steps, or a complex sequence of events with a large number of interrelated variables. The examples in the above diagram show single steps that may occur in a process.

Measurement is the determination of the physical amplitude of a parameter of a material. The measurement value must be consistent and repeatable.

Sensors are used for the measurement of the physical parameters. Sensors are devices that convert the physical parameters repeatedly and reliably into a form that can be used in the control system.  For example, you will find sensors being used to convert Pressure, Temperature, Flow, Level etc. into an electrical signal that is well understood by the system.

Error Detection is the determination of the difference between the amplitude of the measured variable and a desired set reference point or normally called set point. Any difference between the two is an error signal, which is amplified and conditioned to drive a control element. The reference point is usually stored in the memory of the controller.

The Controller is a microprocessor-based system that can determine the next step to be taken in a sequential process, or evaluate the error signal in a continuous process control system to determine what action is to be taken.

The controller can condition the signal, i.e. correct the signal for temperature effects or non-linearity in the sensor. The controller also has the parameters of the process input control element, and conditions the error sign to drive the final element.

The controller can monitor several input signals that are sometimes interrelated, and can drive several control elements simultaneously.

You can also read: 

• The Basics of Industrial Instrumentation and Control Systems
• How Smart Sensors are used in Industrial Control

The control element is the device that controls the incoming material to the process. Typically the element is a flow control element, and can have On/Off characteristics or can provide linear control with drive.  The control element is used to adjust the input to the process, bringing the output variable to the value of the set point.

In our figure above, the measuring element consists of a sensor to measure the physical property of a variable, a transducer to convert the sensor signal into an electrical signal, and a transmitter to amplify the electrical signal, so that it can be transmitted without loss.

The control element has an actuator which changes the electrical signal from the controller into a signal to operate the valve, and also control the valve. In a closed loop control system or a feedback loop, the controller has memory and a summing circuit to compare the set point to the sensed signal, so that it can generate an error signal.

The controller then uses the error signal to generate a correctional signal to control the valve via actuator and the input variable.

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