PLC advantages compared to electro mechanical relays
1.Ability to interface / communicate with computers
2.Simple programming
3.High reliability
4.Easy maintenance
5.Rugged construction - can operate in extremely harsh conditions
6.Small size
7.Easy expandability
8.Economical in long term
the relay logic is what is coded as PLC program in most of the cases with PLC having additional features of PID timers, etc. Also the PLC downtime is very very less compared to traditional relay logics.
I would like to add that relay logic is a physically hardwired logic where you can’t edit/modify easily. You need to shut down the whole process. Moreover, your design should be error free. In PLC, you can check for the logic before applying.
Let me give you an example of the downside of a PLC vs relay logic:
An example - a level bubbler panel that uses two field adjustable time delay relays for the pneumatic purge circuit and a digital panel meter (for level indication) with a discrete input (initiated by a momentary NO push button switch) to hold the process variable during purge.
The disadvantage of a PLC in that application is that the end user needs software to make a change to the purge timing. Purge timing v varies from site to site depending on the piping.
The expected life span of a level bubbler control panel is 15 -20 years. Any PLC software will change with Windows operating systems 5-6 times in 20 years and past experience shows that the original software will not likely load or operate a couple Windows versions down the road.
PLC software might or might not be ‘free’. The software might be licensed, increasing the cost to the customer. Any PLC software requires a much steeper learning curve than the pot/knob/pushbutton/display adjustable timer relays. PLC software requires some sort of communications cable, which might or might not be readily available or readily documented. PLC software might or might not be available 10 or more years from now. A replacement DIN rail mounted on-delay and off-delay timer will be available, even if not the same make and model. The PC on which software is installed is not likely to be available any time down the road beyond 3 years. The task of locating and installing software on an updated Windows box is cost-of-ownership cost to the end customer. Most customers are not good at maintaining an inventory of seldom used vendor software (speaking from 20 years of experience).
In conclusion, there is a market place for PLC’s and there is a different, continuing market place for relay logic.