Process Control Fundamentals

 

What is Process Control?

Process control is the deliberate alteration of a process in response to process monitoring data. The process owner makes an adjustment to bring the process back under control once the process monitoring tools have identified an out-of-control scenario.

Although various techniques are employed to manage these processes, this monitoring and management are collectively referred to as process control.

Control Types

The type of control necessary to maintain the process depends on the type of process involved. Each process has a unique set of traits depending on the kind of process it is and the physical components that make it up.

Self-regulated processes and non-self-regulated processes are the two main categories into which processes may be categorized.

Self-Regulating Process

Self-regulating processes are ones that have this ability by nature. Self-regulated systems are prone to self-regulation because of inherent feedback features.

Time Constants

Every self-regulated process is coupled with a temporal constant. The process's time constant is the length of time it takes to modify 63.2% of its ultimate value.

Process Gain

The process' capacity and reluctance to change have an impact on the time constant. The time constant increases with processing capacity and decreases with the number of resistive elements (small pipes, penetrations, valves, etc.) in the process.

Dead Time

Dead time is defined as "the time interval between a change in a process and the moment at which the change is discovered." Dead time is a component in the control loop that must be taken into consideration while turning the loop since it occurs in all operations.

Non-Self-Regulating Process

A process that does not have a tendency toward self-regulation is said to be non-self-regulating. These processes lack any self-regulating feedback qualities and have the propensity to become unstable without external regulation.

Process Control Terminology

The figure below shows the numerous parts that make up a control system. Each of these elements has distinct properties, and altering any one of them can alter the control loop's overall reaction.

Controlled Variable 

A process parameter is regulated by the controlled variable. The controlled variable in this example is the water level in the tank, but you may choose to control any process instead.

Measured Variable 

The electrical or pneumatic representation of a controlled variable's value is referred to as a measured variable. Usually, a transmitter is used to measure the controlled variable and provide an output that is a representation of it.

Setpoint 

The measured variable is forwarded to the loop's controller, where it is contrasted with the setpoint, a target value. A comparison is performed between the setpoint and the measured variable to provide an error signal. Operators frequently manually enter the setpoint, but it can also be automatically retrieved from other systems. With the letter "r," setpoint is denoted.

Error Signal 

The discrepancy between the measured level and the setpoint is the error signal. It may have a positive or negative value. The controller output is then produced by adding the error signal to the controller's base signal level.

Controller Output 

The controller output is just the sum of the controller's outputs. The output is determined by the controller when it is set to automatic. The output may be manually changed to any desired location if the controller is set to manual. The letter "p" stands for controller output.

Manipulated Variable 

The parameter that is changed to return the process to the intended setpoint is known as the manipulated variable. In this scenario, the water entering the tank is the controlled variable. As the output requirement varies, the water flow is adjusted to maintain the level. A variable that has been changed is denoted by the letter 'm.'