If you go back long enough, before programmable logic controllers (PLCs) were around, we had relays, timers and electric loop controllers. Process control systems were highly distributed and hardwired into the factory. Monitoring these systems was time-consuming and would often require technicians walking around the factory all day. As you can imagine, this made fault finding very difficult. This was clearly a serious problem for high-value businesses, where downtime could cost millions of dollars. This spurred a growing demand for integrated control. As a result, engineers began looking for a reliable alternative to relay logic. Out of this emerged PLCs.
PLCs used to be so big that businesses would install them in central control rooms because of their size and also environmental requirements. They were remotely wired to field devices using large amounts of cable. Even with all the additional expense, PLCs made a huge impact. Faults were easier to find, downtime reduced, throughput increased and safety improved. In the end, it came down to making businesses more profitable, and the PLC achieved that.
Thanks to advances in technology, PLCs have steadily shrunk in size and cost and are capable of operating in harsher environments. This has presented new opportunities to increase the level of control. Now we can connect all our machines together, in multiple factories, in any part of the world. Those machines can run a master program fed from a smaller central controller to individual PLCs. For less complicated processes, a simple outboard PLC on a device removes the need for complex and costly electromechanical options.
More businesses will benefit from the Industrial Internet of Things (IIoT) as technology becomes more affordable. For instance, certain low-cost AC drives now come with an onboard PLC at no extra cost.
Some of the benefits of using a decentralized PLC include:
- Large PLCs can be expensive. One option is to remove the central controller and spread the work across smaller PLCs. Costs can be reduced further by using an AC drive with onboard PLC.
- When upgrading or making a change to a system, extra coding might be required. On a centralized control system, this could mean shutting down a whole production line. The other option is to have a local controller that can be isolated, leaving the rest of the line to keep going.
- In some cases, processes run too fast for a central controller to respond immediately. By having a decentralized controller, it’s possible to reduce lag, benefiting in more uptime.
- It’s easy to connect when the unit is local to the operator, allowing for capabilities such as trending analysis, alarming, batching or printing.
- It can be difficult to troubleshoot when using one large system, as opposed to working on smaller ones. Distributed control software is easier to maintain because there is less information to analyze.
- Field-based distributed controllers do not wholly rely on the central controller. This means that if the central controller goes into fault, the entire process won’t fail, allowing users to continue running their process.
- It’s also easier to partition decentralized systems. For example, you can have enhanced safety using zoned interlocks or light guards.
In defense of the centralized system
Despite these benefits, there’s a reason why many people still use centralized systems. To begin with, they are proven over many years to be an effective way of managing factory-wide communications. Plus, changing an existing system would be expensive and counterproductive; why introduce needless downtime when something already works well?
For new equipment, decentralization might add extra complexity that is not needed. For example, in industries where airborne particles or high temperatures can cause malfunction, a simple setup with a conditioned centralized PLC is the better option.
Most importantly, you need a master in your system. That means one PLC that sends the master communications to the rest of your system. Usually, these are powerful processors that spur off messages to smaller PLCs. What’s great about this approach is that you can keep control centralized and gain from the autonomy of individual parts of the system.
Until recently, size, communication and software standards all impacted our choices. They forced our hand when building equipment. But now, the possibilities of IIoT are about to be realized. Before long, new businesses will start to appear, offering the same products but producing more intelligent products that increase uptime.
With that pending reality, the question becomes: Are you ready for the next wave of innovation and the coming of age of IIoT?
For more information, visit Control Techniques at acim.nidec.com/en-us/drives/control-techniques.
