PLC: Central vs Distributed

Hello again.

Yes, yes I am still going on about PLCs. This time, I am looking at them from a system architecture point of view. Take a look at the picture below of the typical parts of an MBE system. Can you guess what goes in the big black box? Stop and take a guess, there is no wrong answer.

Ok if you said “a cat” you are not wrong, however your mind works very differently to mine. Moving on…

The first and perhaps most obvious answer is “the deposition chamber”. This makes sense since the deposition chamber (and by extension effusion cells, ion gauges, shutter and LN2 shroud) is connected to all these parts. The deposition chamber is the “outward facing” connecting part. But what is the “inward facing” part?

The next most obvious answer is the PC. It is indeed common to have the MBE software that resides on the PC connected to most of these parts. Perhaps most people would not consider connecting the pumps, water chiller and RHEED gun controller to the PC since these parts do not need “regular interaction”. However at the same time these parts are not “regularly monitored”. In the interest of preventing MBE being Mostly Broken Equipment, a viable solution is to connect ALL hardware to the PC and run a “Watchdog” program that acts as a “soft-PLC“.

The actual answer kind of given away in the title is the PLC controller. In this configuration the PLC is at the heart of all the hardware and the actual PC is just another device connected to the PLC.

The advantage of this centralised configuration is that the PLC is at the centre of knowledge. It can directly interact with all the hardware and prioritise communication. The PLC holds the complete state of the whole system. The PC is simply makes read and write requests to the PLC and the PLC passes them onto the corresponding hardware.

In the case where the PLC is NOT in the centre the system becomes distributed and complicated. No one, not even the PC, has the complete picture of the system and if the PC is part of any ensuing problem there can be no solution. In this case the configuration requires either (1) a Watchdog running on the PC or (2) basic error states passing to the PLC via digital IOs from each piece of hardware. The problem being that not all the hardware reports “error states” via digital out.

This ultimately means that, when considering system architecture the PLC need to be at the centre. The PC based control software should ideally be designed to work with the PLC and the PLC alone. The PLC carries the heavy burden of needing to work with all the other hardware from all different manufacturers using all sorts of COMs interfaces and protocols.

To sum up, the solutions in order of preference are:

(1) Centralised PLC designed to work with PC software

(2) Distributed PLC overriding software via digital IOs

(3) A PC based watchdog integrated into the PC control software

Thanks for reading. Have a great day everyone.

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