Search found 107 matches

by karankhosla
Mon Nov 08, 2010 6:19 am
Forum: Vensim
Topic: Prevent negative stock
Replies: 4
Views: 8882

Hi Veasna, Suppose for a moment that you have a simple stock of "Plankton with an outflow "death rate"; if the rate of outflow "death rate" has no feedback about the level of the stock of "plantkon" it is possible, over time, for the stock to become negative, which...
by karankhosla
Tue Oct 05, 2010 6:22 am
Forum: Vensim
Topic: Unit Error: Mantissa of ^ must be dimensionless
Replies: 3
Views: 3222

I would introduce the following "unit factors" (see unit description below): Food Production = Total Food Productivity Factor*(Unit Population*Unit Tools) * (Population/Unit Population) ^ Food Elasticity of Labour * (Tools/Unit Tools) ^ Food Elasticity of Capital Food Production --> Food/Y...
by karankhosla
Thu Sep 23, 2010 7:30 pm
Forum: Vensim
Topic: Simple Optimization
Replies: 6
Views: 4530

Simple Optimization

Dear Giovanni, Attached is my model... on the same lines of both what JJ and Tom were saying. I created a penalty function that measures the square of the difference between desired and actual inventory. We then cumulate this difference and normalize it in the form of an objective function to give y...
by karankhosla
Tue Sep 21, 2010 4:34 pm
Forum: Vensim
Topic: Discrete Processes Stella Vs Vensim
Replies: 0
Views: 3172

Discrete Processes Stella Vs Vensim

Recently, there have been many posts about discrete functions and also references to the numerous books/examples on management written using Stella's discrete processes. A few years ago, I created a Vensim model (Based on the "grocery model" Chapter 4 in Bruce Hannon's: Modeling Dynamic Ec...
by karankhosla
Thu Sep 16, 2010 5:50 pm
Forum: Vensim
Topic: How to allocate without allocation functions?
Replies: 4
Views: 2843

There was a question in the thread some time ago:

http://www.ventanasystems.co.uk/forum/v ... php?t=3997

and

http://www.ventanasystems.co.uk/forum/v ... php?t=4020

Best Wishes,

Karan
by karankhosla
Fri Sep 03, 2010 8:51 am
Forum: Vensim
Topic: Error on Symultaneous equation
Replies: 7
Views: 5780

Hi Giovanni, A lot of the unit errors can be easily corrected For example you have defined: "cash in" {$/year} = "Credit Collection Rate" {$/year} "Credit Collection Rate" = DELAY3("Revenue", "Receivable Collection Delay") "Revenue" is in {...
by karankhosla
Sat Jul 31, 2010 6:03 am
Forum: Vensim
Topic: S-shaped growth with maximum and minimum absolute points
Replies: 2
Views: 2255

Dear Geamaro, Please take a look at Business Dynamics (Sterman) page (333): If I understand you correctly, you want a min value for the stock of "Adopters" when the model run begins and at some point in time you want it to saturate at a max value. 1) If your stock of "Adopters" s...
by karankhosla
Mon Jul 19, 2010 6:58 pm
Forum: Vensim
Topic: TREND function and Vensim PLE
Replies: 7
Views: 5296

Macros

Dear Lukas, As for the macros, you have to go into the text editor for Vensim (under view "As Text") and insert the macro code in the equation set. For example, here is a macro which demonstrates (one possible) way to find the first derivative of a variable: :MACRO: firstderiv(input,dt) fi...
by karankhosla
Mon Jul 19, 2010 6:35 pm
Forum: Vensim
Topic: TREND function and Vensim PLE
Replies: 7
Views: 5296

Business Dynamics (John Sterman, McGraw Hill) gives a fantastic explanation for the TREND function (with equations) in Chapter 16 (pages 634-635)

Best Wishes,

Karan
by karankhosla
Thu Apr 29, 2010 5:46 am
Forum: Vensim
Topic: Optimization error
Replies: 4
Views: 3385

Could you please post your model?
by karankhosla
Wed Mar 10, 2010 5:31 pm
Forum: Vensim
Topic: Optimize for particular value at a specific time
Replies: 15
Views: 7897

Thanks Tom.
by karankhosla
Mon Mar 08, 2010 8:06 am
Forum: Vensim
Topic: data storage
Replies: 8
Views: 4915

Here is a macro to find the second derivative (you can modify it for the first derivative):

:MACRO: seconderiv(input,dt)
seconderiv = (firstderiv-SMOOTH(firstderiv, dt))/dt~input/dt/dt~|
firstderiv = (((input-SMOOTH(input,dt))/dt)) ~input/dt~|
:END OF MACRO:
by karankhosla
Thu Mar 04, 2010 10:02 am
Forum: System Dynamics Discussion
Topic: Eigenvalue and Eigenvector Analysis of Dynamic Systems
Replies: 7
Views: 6261

That's an interesting critique JJ!
by karankhosla
Thu Mar 04, 2010 6:12 am
Forum: System Dynamics Discussion
Topic: Eigenvalue and Eigenvector Analysis of Dynamic Systems
Replies: 7
Views: 6261

Hi JJ, I'm glad you asked! I have an organization called EarthSafe with which I try to promote systems thinking in sustainable development practice. To be honest, the only reason I started EarthSafe was as an excuse to devote all my time to learning everything I possibly can about the science of sys...
by karankhosla
Wed Mar 03, 2010 2:10 pm
Forum: System Dynamics Discussion
Topic: Eigenvalue and Eigenvector Analysis of Dynamic Systems
Replies: 7
Views: 6261

hmm...apparently the forum won't upload my .zip files (745kb in size). You can download them from the following link: http://www.earthsafeonline.com/Papers_Research/DomLoops.zip JJ - my work is research oriented, in particular trying to model various issues of socio-economic relevance in developing ...
by karankhosla
Wed Mar 03, 2010 10:27 am
Forum: Vensim
Topic: Optimize for particular value at a specific time
Replies: 15
Views: 7897

JJ That works very well too. Thank you.
by karankhosla
Wed Mar 03, 2010 7:43 am
Forum: System Dynamics Discussion
Topic: Eigenvalue and Eigenvector Analysis of Dynamic Systems
Replies: 7
Views: 6261

The articles!

[Edited on 3-3-2010 by karankhosla]
by karankhosla
Wed Mar 03, 2010 7:42 am
Forum: Vensim
Topic: Optimize for particular value at a specific time
Replies: 15
Views: 7897

Thanks Tom! I'm going to try it out. I presume the logic behind it is that, as the weight is multiplied to the payoff at each time step, the pulse function forces the value of the payoff to be "x" at a particular time and zero elsewhere, which forces the optimizer to (if so desired) to min...
by karankhosla
Wed Mar 03, 2010 7:38 am
Forum: System Dynamics Discussion
Topic: Eigenvalue and Eigenvector Analysis of Dynamic Systems
Replies: 7
Views: 6261

Eigenvalue and Eigenvector Analysis of Dynamic Systems

Lately, I have had a little time to catch up on some SD reading! It seems there are two interesting methods out there for calculating dominant feedback loops in a system; David N. Ford's is the first, in which changes in the behavior of a loop's "atomic" pattern (calculated by observing th...
by karankhosla
Mon Mar 01, 2010 11:07 am
Forum: Vensim
Topic: Optimize for particular value at a specific time
Replies: 15
Views: 7897

Aha! JJ - Thank you!
by karankhosla
Mon Mar 01, 2010 10:17 am
Forum: Vensim
Topic: Optimize for particular value at a specific time
Replies: 15
Views: 7897

Thanks a lot JJ, yes it was just a trial to see if I could get fulfill a time constraint as well. I was just reviewing Geoff Coyle's book (the section on optimizing) he defines the objective function as a level of a penalty, e.g. pen.k = pen.j +dt*(Desired Population - Population)^2: pen0 = 0. Then ...
by karankhosla
Mon Mar 01, 2010 7:44 am
Forum: Vensim
Topic: Optimize for particular value at a specific time
Replies: 15
Views: 7897

Thanks JJ!
by karankhosla
Sun Feb 28, 2010 8:47 pm
Forum: Vensim
Topic: Optimize for particular value at a specific time
Replies: 15
Views: 7897

I presume one way would be too end the simulation at the time of interest and values would be optimized accordingly.
by karankhosla
Sun Feb 28, 2010 6:36 pm
Forum: Vensim
Topic: Optimize for particular value at a specific time
Replies: 15
Views: 7897

Optimize for particular value at a specific time

Greetings!

Please see the sample model I made; how can I optimize parameters to fulfill a value constraint and also a time constraint? I would like the population to be near 300 by time 20 (months).

Thanks!

Karan
by karankhosla
Tue Feb 16, 2010 7:13 am
Forum: Vensim
Topic: Optimize Lookup
Replies: 7
Views: 4594

Thanks Tom!