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Nov 26

It’s time to replace that old PLC

The following PLC systems are now considered obsolete: Siemens S5, Modicon 884 & 984 and Allen-Bradley PLC2, PLC3, SLC 100 & 150.

Some Allen-Bradley PLC 5 & SLC500 components are ‘silver series’ meaning that at some stage in the future (typically in the next 5 years) they will no longer be available, for example; from the start of 2013 you will no longer be able to buy a new PLC5 chassis and some components are only available as ‘remanufactured’ items on an exchange and repair basis with Rockwell.  Rockwell are discouraging the sale of ‘silver series’ hardware for new systems and are recommending that plant owners with ‘silver series’ hardware should plan for migration.

PLC manufacturers are reluctant to withdraw hardware from sale as there is a large installed base, however they eventually run out of the components required to build this hardware.

If you have critical plant controlled by an obsolete PLC system then you will need to hold an extensive range of spares to ensure a system failure does not result in significant plant downtime.  However even holding spares is no guarantee that you won’t face major problems as electrical components break down overtime. There is no guarantee that a module that has been sitting in your store room for over 10 years will work when required.

Your best option is to plan to replace your obsolete, or about to be obsolete, legacy PLC system by migrating to a new control system.

So what options do you have if you have old PLC’s running important parts of your factory?  PLC manufacturers have a vested interesting in keeping you as a customer so they provide various options for partial or total replacement of your legacy control system that minimizes the cost and disruption of the upgrade.

PLC systems essentially consist of two main components, the processor and the IO (Inputs & Outputs).  As a partial, staged upgrade the processor can be replaced with the original IO (and field wiring) retained.  The new processor will sit in a new rack with the original IO connected via a communications network.  The original IO rack will require an ‘adapter module’ that replaces the original PLC, and acts as a gateway between the new processor and original IO.

This type of partial upgrade allows for a quick migration between the old and new processor and therefore a limited amount of plant downtime and disruption.

The new processor will generally not run the old PLC software.  To overcome this issue most PLC manufactures offer a conversion tool or a conversion service to convert the old code.

However there are a lot of good reasons to not simply convert the code in this way.  Modern PLC systems are a lot more powerful and flexible than earlier versions and a code rewrite offers the opportunity to fix a lot of problems the old machine will undoubtedly have.  Generally old control systems have very limited alarming and diagnostics, they are generally not very robust when minor issues occur like product jams or when restarting from unplanned stops.  Old control systems are also fairly inflexible when handling different products from those the system was originally designed for.  These are all issues that can be addressed when upgrading the control system and rewriting the PLC software.  It may also be possible to reduce bottle necks or delays in the operation of the machine and therefore increase productivity.

A new HMI (operator display panel) could be added to the control system to display alarms and equipment status as well as allow operator control over various parts of the machine.  Recipes or different operation modes can be added as well as monitoring and display of production information and maintenance data.

Replacing the legacy PLC IO can be carried out as part of the control system upgrade or as a separate stage 2 project.  This is largely an exercise in re-terminating the field wiring to the new IO modules.  It is also a good opportunity to update the electrical drawings or generate a new set of electrical drawings if the originals have been lost.  An up to date set of electrical drawings will greatly assist electrical maintenance staff in diagnosing electrical problems.

Upgrading the controls on an existing machine can significantly extend the useful life of a machine while making it more reliable, robust and flexible and can be  significantly cheaper than replacing the machine with a totally new machine.

If you would like to know more about upgrading your control system then don’t hesitate to contact us.

3 comments

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