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.

PLC: Hard vs Soft

Hello again.

Yes I am continuing with the theme of Programmable Logic Controller, that much overlooked but so critical feature. This time around I am going to explore the age old question of whether a software or hardware based PLC controller is better. (Spoiler alert it is hardware).

Many people would argue that a software based solution is easier. Most MBE systems already have a basic software package that communicates with most of the system hardware in order to run recipes and grow samples. It is therefore fairly intuitive to add some kind of a “Watchdog” feature to that software. Intuitive and fairly easy.

Easy does not equate to better. Imagine if Michelangelo had told Pope Julius II that despite his idea to pain the ceiling of the Sistine Chapel, it would be must easier in fact to paint the floor, and in crayon. (Note I recently visited the Rome and it was amazing, hence the reference).

My point is that easy does not equate to better. A software watchdog is better than no watchdog at all, but bear in mind that Windows is inherently equally flexible and unstable. The all seeing watchtower monitoring for constant threats should not reside in the land of BSOD (that is Blue Screen of Death) and OIASWYITMOSINTR (that is Oops I am sorry were you in the middle of something I needed to restart).

The fact is PLCs are hardware based for a reason: A dedicated microprocessor with a single task monitoring and dealing with faults. Some of the more complex PLCs are very rugged computers, but for MBE with a few dozen inputs and outputs, a single microprocessor is plenty.

That is not to say a PLC cannot have a sleek, elegant user interface. I have to admit I do love a sweet UI. A PC based UI that can add, edit and delete “rules” for the PLC can only be a good thing. The actual critical task of monitoring and reacting should however be performed on a dedicated little blinking box that does not get distracted by services and kernel threads and spyware and viruses and… and… and…

Thanks for reading. See you all tomorrow.