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When a stock either changes exponentially or stabilizes, there is a control mechanism at work. Such a mechanism operates through a feedback loop.
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A feedback loop is a closed chain of causal connections from a stock, through a set of decisions or rules or physical laws or actions that are dependent on the level of stock and back again through a flow to change a stock.
- Balancing / Negative feedback loops are equilibrating or goal-seeking structures in systems and are sources of stability and resistance to change.
- Reinforcing / Positive feedback loops are self-enhancing leading to exponential growth or runaway collapse. They are found whenever a stock has the capacity to reinforce or reproduce itself.
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Feedback loops may not necessarily be effective due to delays or due to the actions triggering them not being strong enough.
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When one feedback loop dominates another, it has a stronger impact on behavior. Complex behavior of systems often arise as the relative strengths of feedback loops shift, causing a dominance shift.
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Feedback loops are everywhere. Part of systems thinking is to be able to recognize how feedback loops can occur.
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A system can have many interrelated feedback loop that influence the same stocks.
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The information delivered by a feedback loop — even nonphysical feedback, can only affect future behavior. It can’t deliver a signal fast enough to correct behavior that drove the current feedback. Even nonphysical information takes time to feedback into the system.
- There will always be a delay in responding.
- A flow will not react instantly to a flow, it will react only to a change in stock and only after a delay in registering the incoming information.
- Changes in the delay may (or may not depending on its type and the relative lengths of other delays) make a large change in the behavior of the system.
- Delays that are too short cause overreaction and amplified oscillations.
- Delays that are too long cause damped, sustained or exploding oscillations. This can destabilize the system and cause it to collapse.
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A stock maintaining-balancing feedback loop must have its goal set appropriately to compensate for draining or inflowing processes that affect the stock. Otherwise, the feedback process will fall short of or exceed the target for the stock.
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A delay in a balancing feedback loop makes the system likely to oscillate.
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In physical, exponentially growing systems, there must be at least one reinforcing loop driving the growth and at least one balancing loop constraining the growth because no physical system can grow forever in a finite environment.
- A quantity growing exponentially toward a constraint or limit reaches that limit in a surprisingly short time.
- If the physical constraint is higher, the growth loop will elude the constraint loop and the convergence to the limit will be slower.
Control
- Feedback loops are the basis of control for systems.
- Open Loop Control involves a controller simply issuing a command to a process and the process carries out the command. This type of control has no feedback loop
- Closed Loop Control involves the presence of a feedback loop where flows are monitored and adjusted based on desired performance.
- A form of feedback control involves maintaining homeostasis — maintaining a stable, healthy state for the system.