The defining property of this system is that no significant logical decisions are made by the hardware, either on the display side or in the control channels In a system like that shown in Fig 8 1, the feedback controllers provide only process control at a given static operating level All control actions are taken by the operator and are based on the information he obtains by observing the process variables displayed at the consoles and his interpre­tation, through experience and training, of this information in terms of needed process changes

8- 3.2 Monitored Operator Control

Hardware logic may be placed in the path from the operator to the process to prevent a control action at the wrong time or under adverse conditions These may range from simple electrical interlocks to a complicated set of prerequisites in a plant start-up sequence Such systems are now seldom seen in the absence of other automatic control features They are mentioned here only to illustrate one aspect of automation easing the operator’s decision-making burden

8- 3.3 Operator Guidance

In a large plant the amount of data generated cannot be assimilated by one operator or even a staff of operators In many installations digital computers are used only to get process information to the operator quickly and in mean­ingful form This involves data acquisition, on-line analysis, and information display, and it often results in a highly complex instrumentation system made practical only through the use of a digital computer The essence of the operator guidance system is that it comprises nearly all the hardware elements needed for fully automatic control, the only missing item being the part of the computer—process interface which sends control signals back to the process

The operator guidance system appears to produce a low degree of automation, nevertheless, it provides all the aspects of automatic control except direct feedback action No data analysis or decision making is required of the operator In practice, however, the operator is seldom reduced to a robot On the contrary his effectiveness is enhanced by his possession of a continuous and up-to-date knowledge of the process This includes energy balances, anticipatory alarms based on predicted plant behavior, or instructions for optimizing the process, all requiring that data be processed faster and more accurately than is possible by humans

8- 3.4 Automatic Control

When under a given set of conditions a process is run efficiently without human direction, its operation represents the highest degree of automation Input data are analyzed, decisions are made, and control actions are taken entirely under the guidance of the control equipment

Automating a facility to a high degree may allow a significant reduction in the size of the control-room staff, although resulting operating cost savings may be partly offset by the need for computer programmers and more highly skilled maintenance specialists More importantly the operators are relieved of trivial and routine tasks requiring continual alertness and are allowed to perform more complex functions, such as general surveillance and emer­gency intervention, which are beyond the capability of a control system of reasonable cost 8