Programmable Logic Controllers (PLCs) 1: selection of a specific PLC for a given industrial application

Introduction
Programmable Logic Controllers (PLCs) were invented by the American Richard (‘Dick') Morley in 1969, to be used in the manufacture of cars. Prior to that date production lines had been controlled by a mass of hard-wired relays. Using programmable devices in their place meant that changes in production could be implemented much faster without the need to rewire control circuits.

The aim of this unit is to further develop students' skills in the use of PLCs and their specific applications within engineering and manufacturing. Among the topics included in this unit are device interface methods, PLC signal processing and communications with other devices, PLC programming methodology and alternative programmable control devices.

On successful completion of this unit students will be able to research the design, selection and use of PLCs as part of a larger system, programme a PLC to solve an industrial process problem for a given application and illustrate the alternative strategies for using other available types of programmable control devices.

Task 1

As a junior engineer you have been asked to improve the automation in a factory. The factory has been using a lot of outdated motor starters and as a result the company is experiencing a lot of down time due to machines breakdowns. With your knowledge of PLCs advantages and efficiency you want to do a presentation to your manager and other engineers about the applications of PLCs.
The following are common industrial application of programmable logic controllers
• Process control and Automation
• Process condition monitoring
• Data acquisition
• Machine control

Investigate the key characteristics of the four given industrial application explaining to your colleagues why PLCs are ideal for those applications.

The manager wants to have a detailed report on the alternatives PLCs vendors available. Compare the operational features and characteristics of PLCs manufactured by Siemens, Mitsubishi, Allen Bradley, Schneider and Omron. Consider the followingPLC operational characteristics:
• speed,
• current,
• voltages,
• memory

You have also been asked to specify the control system of the furnace. Which brand would you use for an application to control the temperature of a furnace between two limits? Your choice should put into consideration the PLC selection criteria under the following headings.
• System requirements
• Application requirements
• Electrical Requirements
• Speed of operation
• Communication Method
• Operator Interface
• Physical environment

Task 2

In most cases SCADA, PLCs and HMIs work together. SCADA communicates with the PLC, which is how the system functions and communicates within the operational environment and HMIs in the form of a screen are there for interaction with operators. SCADA systems are intended to,
• Automate processes and procedures
• Track and analyse data and errors
• Perform remote site monitoring
• Increase productivity and guarantee same-quality outputs
A water utility company is installing a SCADA system which will be used to monitor and collect data. Using systematic PLC selection criteria determine the suitable PLC to use in this system. Explain with reasons which PLC would be suitable for this utility water industrial application?

Task 3

When engineers are choosing the right PLC for an application, they consider a lot of specifications including
• Compatibility with the current system
• Size of a PLC including its I/Os
• The speed of the CPU
• Operator interface SCADA/HMIs
• The programming language
• Financial resources
• Vendors support available
• Project life cycle
• Instrument integrated to the interface and protocol

As a senior industrial engineer,you have been tasked to design a control system for a box sorting station. Sorting out boxes based on height using pusher. The station must be able to sort out three sizes of boxes using their height. The PLC should have enough inputs and outputs to accommodate start and stop buttons, emergency stop, and sensors. The assessment and explanation of your selection should show or prove your choice to be reasonable from an engineering perspective.