Abstract :
The paper tries to point out the essentials of a direct digital control system. If the plant is to run 24 hours every day, those parts of the program which operate control valves can never be stopped. A clear distinction between what is fundamentally d.d.c. and what is really ´supervisory´ is therefore necessary. A simple form of multiprogramming is described, in which it is possible to change ´supervisory´ programs without losing control of the plant. The fundamental d.d.c. programs cannot be changed without stopping the machine, but, fortunately, this should rarely be necessary. Most of the complaints about systems being late in delivery and then suffering for months from `teething troubles¿ have arisen from trying to write a complete d.d.c. and supervisory program before the machine went on line, and then spending a lot of time patching in machine code or recompiling from an ´assembler´. The author´s approach commissions the system in minimal form, and specifically allows for the supervisory programs to be built up steadily over several months. The system works as well with machine code and a small store as it does with a high-level language and a backing store. The second half of the paper deals with methods of valve operation, an area where there is still room for improvement. The importance of taking a measurement of actual valve position and using it in the control loop is stressed. Finally, there is a section on operator´s panels, and in the Appendix is a typical software specification for a complete d.d.c. installation.
