Today’s PLCs (programmable logic controllers) are cheaper and more flexible than ever before.
Still, some OEMs and system builders prefer relay logic for their electrical controls – and in some cases, this may be justified.
Let’s take a look at some of the advantages and disadvantages of PLC controls versus relay logic.
Flexibility and ease of service
Changing functionality or adding features to a relay logic board will generally always require wiring changes. A PLC system, on the other hand, can be easily reprogrammed without needing to physically change any wiring at all.
Programming a PLC however, is a specialised task that many OEMs aren’t comfortable with. But today’s remote access solutions can remove that hassle by allowing a programmer to securely update and debug the program remotely over the Internet.
In terms of fault-finding, it’s about a tie – tracing wires around a relay logic board can be just as maddening as troubleshooting a PLC panel without the programming software.
Cost
For small and simple control systems, relay logic can be ideal – it’s low cost and easy to service.
But as process complexity grows, a point is quickly reached where both cost and wiring complexity of relay logic overtakes a PLC solution.
Add a few analog instruments and a variable speed drive (VSD) or two, and a relay logic system rapidly becomes not only more expensive but technically impractical.
Space advantages
A small PLC can be roughly the size of a house brick. On the other hand, a row of 10-20 relays can be twice the length, and require twice the amount of wiring (i.e. larger cable ducts).
Where space is at a premium, a PLC is often a better option.
Communication and interoperability
It’s becoming far more common that OEM packages are required to be integrated into existing on-site control and SCADA systems. Relay logic panels fall back on simple hard-wired signals – one pair of wires per output – for this.
PLC control systems on the other hand, can be designed to easily integrate and communicate with other systems using standard protocols such as Ethernet/IP or Profibus, over a single Ethernet or serial cable.
To find out more about how you could improve your control systems with PLCs, contact our engineers.