Understanding Control Systems

Learn Logix Part 1.0. This article gives a high-level overview of the components that make up programmable control systems and the way information flows through a control system.

Learn Logix is a series of articles designed to teach you everything you need to know to work with Rockwell Automation’s Studio 5000 Logix Designer and the Logix 5000 series of PLCs.

If you’re new to the series, you can find all of the previous parts here.

A programmable control system is a collection of components, often connected over a network, that can be programmed to automatically control processes, machines, and devices.

Programmable control systems are preferred to hardwired control systems because they can be reprogrammed to respond to changes in production requirements. In a hardwired control system, the components in the control system would have to be re-wired which is a tedious, laborious, and error-prone task.

In this article, you will learn about the components that make up a programmable control system and how information flows through a programmable control system.

Components of a Control System

A Simple Control System

The picture above shows a simple programmable control system. It is made up of controllers, input devices, and output devices.

A controller is an industrial device that can be programmed to automatically control processes, machines, and other devices. Controllers have programmable memory that is used to implement functions like logic, timing, communication, and data manipulation.

The controller is the brain of a programmable control system — it reads and evaluates the status of input devices, makes decisions, and modifies the state of output devices to control a machine or process.

You may hear controllers being referred to as processors, CPUs, Programmable Logic Controllers (PLCs), or Programmable Automation Controllers (PACs)

In a simple programmable control system, the controller is connected directly to input and output devices.

Input devices are pieces of hardware that convert real-world data such as distance to an object or amount of light being received into electrical signals that the controller can understand. Examples of input devices include photoelectric sensors and pushbuttons.

Output devices are pieces of hardware that receive signals from the controller and actuate hardware devices to create behavior in the real world. Examples of output devices include signal lights and motors.

Information Flow in a Control System

Information Flow in a Control System

Now that we understand what components make up a control system, let’s look at how information flows through a control system using a simple example.

In a typical control system, the controller monitors the input devices to detect changes. In the system above, the controller is monitoring the input of the pushbuttons to determine when a button is pressed.

When the controller detects that the Start button is pressed, it evaluates this event against a set of user-defined rules in the program. In the example above, the controller determines that when the Start button is pressed, the motor should run.

To run the motor, the controller sends a command, in the form of a digital output, to the motor and it begins to run.

Conclusion

In this part of the series, we went through a high-level overview of what components make up a control system and how information flows through a control system.

In the next part of the series, we will take a deeper dive into the specific components that make up Rockwell Automation’s Logix 5000 platform. If you haven't already, sign up to the mailing list below to be notified when that article is available.

Update: You can find the next part of the Learn Logix series, The Logix 5000 Systems, here. This article gives an overview of the systems that make up the Logix 5000 platform and the use cases for each system.

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