Feedback Flashcards
What is feedback? (4)
- Feedback is one of the fundamental elements in system dynamics
- This can be seen through causal loop diagrams
- Almost any system with meaningful complexity has some sort of feedback
- Occurs when output influences the input
2 broad groups of feedback.
- Negative or balancing feedback
2. Positive or reinforcing feedback
Negative Feedback: (3)
- Goal seeking as it causes the system to move towards some state irrespective of where the system currently is
- Output feeds back into the system to regulate
- Negative does not necessarily mean something BAD
Simplified Version of a Negative Feedback Loop:
Linear Proportion Control: (4)
+ Form of control
+ Linear relationship between the input and output of the system
+ Important considerations
- The input value
- Set-point or reference value
- Feedback drives the difference between them to 0
+ The output will be proportional to the difference between input and setpoint
To illustrate: thermoregulation
total body heat: energy in the body (J)
Heat inflow Rate =
Heat Outflow Rate =
- Mechanisms to generate heat —–> basal metabolic rate
- Assume constant with no control
- Mechanisms to lose heat —–> conduction, convection, radiation and perspiration etc.
heat outflow
—–> mechanisms to lose heat: (4)
= conduction
= convection
= radiation
= perspiration .. etc
Example 1:
Temperature __ Set-point temperature
Temperature difference __ 0
Heat Outflow —-> ______
> positive
Example 2:
Temperature ___ Set-point temperature
Temperature difference ___ 0
Heat Outflow —-> _____.
<
<
negative
Example 3:
Temperature = ___-____ temperature
Temperature difference = 0 (IDEAL)
Set-point
What happens in terms of feedback when you are sick?
When you are sick, you have a fever and therefore the set point would increase. The temperature would still need to increase in order to reach set-point until the fever breaks and then the set-point would have to return to normal and temperature would then decrease. In extreme conditions, the feedback mechanisms would break —-> hypo/hyperthermia
Positive Feedback: (2)
—-> Unstable process – output feeds back into the system to cause growth or decay in the
same direction
—-> Positive does not mean GOOD and positive feedback is NOT desired (biological
sciences)
Causal Loop Diagram Positive Feedback:
Level Rate Diagram Positive Feedback:
Positive and Negative Feedback Loops: (3)
- —> In reality positive feedback loops are accompanied by negative feedback loops
- —-> Look for dominant feedback in the system as this will tells you how the system responds
- —-> Dynamics may change depending on the relationship between the positive and negative feedback loops
This is not actually representative of what happens in reality and in order to resolve this, a
dependency needs to be drawn from the number of bacteria to the death rate constant.
How does one fix this?
Now the death rate constant will increase as the population or amount of bacteria increases. The system will now increase until such a stage that it is equal to the growth rate constant which will result in the S-Shaped Growth Curve.
—–> The system is brought to a state of equilibrium
