Runaway or uncontrolled processes or "open-loop unstable": thankfully, there are very few of them in the industry

A classic example of a process runaway An inverted pendulum (also known as an open-loop unstable pendulum) is an inverted pendulum: a vertical wooden arm balanced at the end, moving from side to side. An inverted pendulum is typically constructed in a laboratory setting to fix the arm on an axle supported by a frame. The frame is then fitted with wheels and a reversible motor to provide lateral control.

A sensor (usually a potentiometer) detects the angle of the arm with respect to the vertical axis, reporting the angle to the controller as the process variable. The motor would then be the final control element.

The defining characteristic of a runaway process is its tendency to accelerate rapidly away from its stable condition without any correction. We can observe this behavior in the following process trend:

A synonym for “runaway”"It is negative self-regulation or negative lag, because the process variable with respect to time resembles the inverse of the curve of a self-regulating system with a time lag.".

What makes a process “runaway”"Having that behavior is your internal positive feedback. It's easy to demonstrate for yourself how difficult it is to control runaway processes. Let's simply try balancing a large wooden arm vertically on the palm of our hands. Understand that the only way to maintain stability is to react quickly to or against changes in the angle of the vertical arm, especially by applying healthy doses of derivative control action to counteract any movement away from the vertical.".

Fortunately, runaway (open-loop unstable) processes are the least common of industrial processes. I say "fortunately" because these processes are notoriously more difficult to control, and it is usually assumed that a certain level of stability is required. ELDERLY This is a risk associated with self-regulating processes. Many runaway processes are also non-linear, making their behavior less intuitive for operators.

Nuclear fission reactors generally exhibit a behavior“runaway”Like the Chernobyl reactor (Ukraine, Russia) which was a design where its power output could “run out of control” under certain conditions, and that is exactly what happened on April 26, 1986. The reactor used solid graphite blocks as its main moderating and neutralizing substance, and since its circulating cold water did not produce enough negative feedback to stabilize it, it began to “run out of control” until it caused the disaster.

Summary

• Runaway processes are characterized or graphed with an exponential ramp of the process variable in response to a step change in the final control or load element.
• This “loss of control” or “runaway” occurs as a result of some form of positive feedback within it.
• Runaway processes cannot be controlled solely with proportional and integral control action, and will always require derivative action to stabilize them.
• A moderate amount of integral action is needed in runaway processes to compensate for load changes.
• A runaway process will become self-regulating if the necessary internal negative feedback is naturally introduced, as is the case with water-moderated fission reactors.

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