#WishIknew – What is process control?

Control System

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I wish I knew what process control is !

Process Automation stands on three pillars: measurement, control, and action. Here we’ll discuss the second pillar, control.

To automate a process, you must measure the variables of said process. By variables we mean any values important for a good quality output. The temperature of a mixture, pH of a fluid, pressure in a certain point, level of a storage tank – all of these could affect your product.

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Field devices such as flow meters and pressure transmitters measure these variables and create the first pillar of Process Automation. However, they can only measure and maybe store this data. If your variables don’t meet the needs of your process, then you need a controller.

The controller works as the brain of the process. It reads and analyzes the data from the field devices. If all values meet their setpoints, the process continues with no change. However, if a device sends a different value than the one expected, the controller reads that as an error. It will then apply a control algorithm to make a decision and send a signal to the actuators (the last pillar of automation). The actuators can change portions of the process to bring the variable to the expected value.


Example of Process control

Let’s say you have a mixing tank with two chemicals fed to it, one acid and one basic. You want the final mixture to have a pH of 6 and a temperature of 32 degrees Celsius (° C). So the controller must maintain the mixture at these values.

Now suppose your field devices measure your process and send readings of 4.5 pH and 26° C to your controller. When it receives this data, it calculates an error of -1.5 pH and -6° C. After running its algorithm, it sends a signal to the heating system, turning it on, and to the pump, feeding more acid to the mixture until it reaches the proper values.

This example represents a closed-loop control system. This process control system gets feedback from the sensors to automate the process, correcting errors all the time. You may hear also the term open-loop control. This type of system has no feedback loop and thus no automation.

Your microwave runs on an open-loop system. It doesn’t stop when your food reaches the desired temperature, but when the timer runs out. It has a controller, but it only keeps motors running for a certain amount of time. Then they stop the process. This system doesn’t count as an automated process.

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