Nowadays large rigid relay based control systems are being replaced with PLCs.
PLCs have a lot more functions that just use like relay contacts, though they were first made to replace relays. The ladder logic that is used to program the PLC made in form of traditional ladder logic using NO/NC contact of I/Os. PLCs provide a grater flexibility than the relays and are easy to upgrade.
But before you start replacing your relays with PLC you should consider the following:-
- Cost effectiveness- As the PLC have a large I/O capability therefore do insure that atleast 50% of I/O handling capability are used.
- Response time- PLCs have a response time of 50 - 500 msec at max. However relays have a response time less than 10 msec. Do consider that as some safety application response time is critical.
- Voltage level- As relays are used to isolate the various voltage levels, and relays will be used even if you use PLCs as they typical operation at 24Vdc.
Despite these PLC add a lot more like HMI integration, Soft signal transfer, better diagnostics, Historian, trends etc and definitely better in ease of use and maintenance.