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Human And Automated Controllers

Note: The figures that follow were adapted from Nancy Leveson‘s “Engineering A Safer World“.

In the good ole days, before the integration of fast (but dumbass) computers into controlled-process systems, humans had no choice but to exercise direct control over processes that produced some kind of needed/wanted results. During operation, one or more human controllers would keep the “controlled process” on track via the following monitor-decide-execute cycle:

  • monitor the values of key state variables (via gauges, meters, speakers, etc)
  • decide what actions, if any, to take to maintain the system in a productive state
  • execute those actions (open/close valves, turn cranks, press buttons, flip switches, etc)

As the figure below shows, in order to generate effective control actions, the human controller had to maintain an understanding of the process goals and operation in a mental model stored in his/her head.

With the advent of computers, the complexity of systems that could be, were, and continue to be built has skyrocketed. Because of the rise in the cognitive burden imposed on humans to effectively control these newfangled systems, computers were inserted into the control loop to: (supposedly) reduce cognitive demands on the human controller, increase the speed of taking action, and reduce errors in control judgment.

The figure below shows the insertion of a computer into the control loop. Notice that the human is now one step removed from the value producing process.

Also note that the human overseer must now cognitively maintain two mental models of operation in his/her head: one for the physical process and one for the (supposedly) subservient automated controller:

Assuming that the automated controller unburdens the human controller from many mundane and high speed monitoring/control functions, then the reduction in overall complexity of the human’s mental process model may more than offset the addition of the requirement to maintain and understand the second mental model of how the automated controller works.

Since computers are nothing more than fast idiots with fixed control algorithms designed by fallible human experts (who nonetheless often think they’re infallible in their domain), they can’t issue effective control actions in disturbance situations that were unforeseen during design. Also, due to design flaws in the hardware or software, automated controllers may present an inaccurate picture of the process state, or fail outright while the controlled process keeps merrily chugging along producing results.

To compensate for these potentially dangerous shortfalls, the safest system designs provide backup state monitoring sensors and control actuators that give the human controller the option to override the “fast idiot“. The human controller relies primarily on the interface provided by the computer for monitoring/control, and secondarily on the direct interface couplings to the process.

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