The power and hence cost of the PLC or DCS controlling the plant is a function of the control requirements, and not the cost of the plant itself. Don't assume that because the process plant costs millions, so must the processor. Its entirely possible to have a multi-million pound plant controlled by an SLC. Similarly, a small process plant might need a top end processor such as a PLC 5/80 with dozens of analogue I/O.
The HMI for a process plant will either be part of a DCS system, or a package separate from the PLC. One or more operator stations consisting of standard computers running packages such as Intellution's iFix or Allen Bradley's RSview communicate with the PLC over Ethernet links. Despite the fact that these operator stations provide a window to the plant's operation, there is often a significant software requirement within the PLC to organise and obtain data.
Software controlling a process plant will normally be split into three sections: Start Up, Steady State and Shut Down.
The Start Up routines will take the plant from rest to the normal operating conditions. Alarms and warnings have to be modified to allow for the non-equilibrium state of the plant. Typically, more operator intervention is required at this stage than the normal running conditions.
The Steady State program will operate the plant by infusing raw ingredients, typically through PID loops, to produce the desired product. Alarm conditions will be monitored and statistics gathered. Operator input will be less than Start Up, since this part of the program is ideally an auto pilot.
The Shut Down routines take the plant from Steady State to the Rest State over a fixed period of time, possibly days, by reducing the supply of raw ingredients in a controlled way. Shut Down is normally invoked by the operator. Serious alarm conditions normally shut the plant down much more rapidly, in a uncontrolled way.
Greysoft Ltd has experience in providing both the controlling software for the PLC and the HMI configuration. Operator training is part of the development process and this can also be arranged.