I am often asked “Why are distributed control systems necessary” and I’ve often found myself going into layers of deep technical explanations and customer requirements in seeking a response.
In reality the answer lies in every day things we consume as well as the history of computerization of the automation Industry.
Where does milk fit into this equation? Like many things we consume in its raw state milk comes from a cow with a varying levels of sub components made up as water, fat solids and non fat solids. These levels vary from cow to cow, from farm to farm and through out the year. I however have a preference for 2% milk on my cereal in the morning and no amount of convincing will teach a cow to precisely produce that for me.
Amongst all of the activity that goes on in a Dairy factory one key activity is called standardization, which is a process of separating milk into its components and then re-blending to just the right formulation. It then gets heat treated. These processes form the basis for modulating (or regulating control). This is where we regulate analog values (pressure, flow rates, temperature, tank levels) to manufacture on a continuous basis. This is the key talent and one of defining attributes of a distributed control system.
The conversion of a raw fluid to a refined product, in this case milk is similar concept to produce gasoline from crude oil, or water to make steam that generates electricity or get gas from the ground ready for consumption.
Contrast this with the manufacture of the box that the milk comes in. Sheets of cardboard are laminated to keep the milk safe from sunlight and germs, and are folded and glued to make a convenient package into which the milk is put so that it can be delivered to us and stored conveniently in the refrigerator. This is a mechanical manufacturing process that largely involves logic or sequential control to control the machine that makes the boxes.
Before the days of computers based automation these two different processes were conducted by pneumatic based control devices (for modulating control), and relays, timers and drum sequencers (for logic control). Both were vastly different disciplines often handled by different people and as such gave birth to two prevailing computing platforms for automation today…the DCS (Distributed control System) and the PLC (Programmable logic controller).
Please enjoy your cornflakes.