1. First
Challenge – Positive or Negative Feedback Loop?
I face the first challenge before
I even start building the model in iThink while I read through the
EcoNowMics
pages.
I look at the Milk Glass Video Game model. I see a
Target so I think that it is a First Order Negative
Feedback system.
Note that
"First Order" denotes a system with one Level.
I know from reading
Road Maps
that filling a Level to a Target implies a Goal and a Gap, two of the
basic components that make up a First Order Negative Feedback loop. I take another look at the system
structure. There is a milk flow Rate
into the Level. The milk flow Rate is increasing the Level. This seems like a positive feedback loop. I start to feel some
confusion.
Throughout my studies I do not recall a First Order
Negative Feedback system
structure with a Rate flowing into the Level.
I am under the impression that First Order Negative Feedback systems contain
Outflow Rates from the Level only.
I carry on. I experiment with
filling the glass with milk on Ed’s Milk Glass Video Game on the
EcoNowMics
pages.
As I fill the glass to the target line I notice that I act to minimize the discrepancy between
the Level and the target line. This seems
like negative feedback  however I am still not clear on how I can have negative
feedback while the Level is increasing. I
recall that positive feedback has exponential growth in which the Inflow Rate
increases the Level.
I find that I do not really know the difference between First Order Positive
Feedback and First Order Negative Feedback. I decide
to do some research to resolve this question.
The
Milk Glass Video Game System Structure
I research the Road Maps papers and find a negative feedback model with a
Rate that flows into the Level. I read
through the paper carefully in order to better understand how this system structure
behaves. The model I find (the Widgets
Model, below) has a Rate that flows into the Level and exhibits asymptotic growth as
it approaches the Target. I have my
first insight. A system with an Inflow Rate, a Goal and a Gap can also be a First Order Negative
Feedback system.
The
Widgets Model
Clip:
Road
Maps, Beginner Modeling Exercises, Section 3,
Mental Simulation of Simple Negative Feedback, Pg 15
I also notice in Chapter 3, ‘Negative Feedback Structure’ of Study Notes
in System Dynamics by Michael R. Goodman, another system structure with an Inflow Rate into the Level that looks similar to the Milk Glass Video Game model structure. The chapter outlines the basic elements of
First Order Negative Feedback systems. These are:

the Target (Goal);

the Discrepancy (Gap);

the Action (in this particular case, the Inflow Rate); and

the system
state (Level).
I feel that I am
getting more understanding on First Order Negative Feedback systems. I also notice that I am
still not clear on the difference between First Order Positive and First Order Negative
Feedback systems.
First Order Negative
Feedback Structure
Ed senses my confusion about the difference between First Order Positive and
First Order Negative Feedback loops. We have a
discussion about the difference between the two. I describe the First Order Negative Feedback system to Ed as
a system that has a Goal and a Gap in its system structure. I also mention that the behavior we might
observe in First Order Negative Feedback systems, either, as asymptotic growth or
exponential decay.
I
describe a First Order Positive Feedback system structure as a structure that consists of a
Level, an Inflow Rate and a Constant. I also mention that First Order Positive Feedback systems exhibit exponential
growth.
Positive
Feedback Structure
Study Notes in System Dynamics,
Michael R. Goodman, Chapter 3, Pg 40
Ed tells me that both First Order Positive and Negative Feedback loops have a
Level,
a Reference Level (Target, Goal) and a Delta (Gap or Discrepancy) that results from comparing
the Level and the Reference Level. This
is new to me.
System
Structure – shows the Policy (see section on Policy below)
Note: other names for
Delta is Gap or Discrepancy; and
for the Reference Level is
Target or Goal.
I do not recall thinking about First Order Positive and
Negative Feedback systems in
this way before. Ed tells me that the
main difference between First Order Positive and First Order Negative Feedback systems is that
Negative
Feedback systems act to decrease the Delta while positive feedback systems act
to increase the Delta.