Recently, students in an important learning system dynamics, and it will take a few days to learn a bit today to find a simple piece of paper, spent half a day with Vensim built a lower mold, the feeling of this software and system dynamics the beauty, the feeling is quite interesting. This paper describes the song as a lift, et al paper "Modeling and Analysis based in Harbin private car travel characteristics of the system dynamics," and with Vensim reproduce a bit. Because I was the first time you use this software, so some view the article may not be correct, some practices may be improved, please give me a message to see the big brothers, certainly correct, thank you!
1, the article outlines
This article is vensim software for private cars to travel in Harbin features with analysis of the evolution of advancing time, first article describes the shape under the influence of factors on car travel characteristics, which lay to build a causal loop diagram a base, and then to cause the circuit diagram drawn as follows:
which can then further analyzed above cause some of the circuit of FIG. how quantitative index quantified. This paper gives specific causal loop diagrams and flow stock chart looks like this:
With this map you can use to model Vensim, but not over, because the model is not enough light there, but also this equation and assignment a crucial step.
The article gives some key structural formulas and initial values:
in theory, then we can reuse existing content of the article.
2, Vensim Modeling
About Vensim software you can watch a video to learn B station, the teacher speaks very very carefully, you can see it again operated. Portal.
2.1 causal loop diagrams and flow chart to build stocks
I use Vensim software modeling results are as follows:
the model and the paper model is not the same thing by some amount, mainly because I added some "transit variables" and "coordination dimension" and so on, because the units check can not pass , so the addition of a number of auxiliary variables to adjust dimension. Follow-up will continue to be a dimension of the problem description.
2.2结构等式输入和赋值（量纲不匹配问题说明及解决方法）
论文里给出了一些结构等式，但是还有很多变量没有给出初始值和结构等式，于是我就按照定性的理解自己大致给了些等式，这些等式往往是经验公式，因此量纲是不一定会完全统一的。 也就是说经验性公式可能无法通过unit check，那是不是说软件就不好呢？那也不是，因为实际上经验性公式也是需要去对齐量纲的（使得等式两边的量纲一致），怎么做呢？就是引入一些系数，借助这些系数来调整量纲，在本例子中，我懒得去一个个命名这些系数了，于是就叫做“调整量纲”、“中转变量”之类的名字了，其实它们便是经验公式的系数了。 此外，我还发现了一个这个软件的漏洞，比如我通过计算会得出下面的一个单位“亿元/万人”，其实这个单位就是“万元/人”，但是由于有些变量我的单位取为“亿元”，有些变量我的单位取为“万人”，因此如果想要通过unit check就只能取为“亿元/万人”这样的单位了，这个我没想到更好的解决办法。此外，我还有一个想法，其实为了简单起见，我们可以把所有的变量都设为dmnl，也就是无量纲（其中有几个需要设为dmnl.time unit），这样就一定会通过unit check了。当然这种做法是不推荐，也是毫无意义的，因为unit check本来就是一种检验正确性的机制，逃避这种机制就等于增加了错误几率，而且其实unit check即使不通过也是可以进行simulink的，而且对结果是没有影响的，但是也不推荐，因为量纲不正确，往往意味着存在一些bug。不过，有时候，存在上面提到的问题，也就是对于一个经验性公式，我们懒得定义系数了，这时如果确保公式是没问题的，也可以忽略这个unit does not match的问题，有时候，还有下面的问题，我们使用了Y = IN(X)，而X是有量纲的，这时unit check也会报错的，说是IN函数不允许有量纲的值作为输入，但是我们也懒得去定义一个系数去除X的量纲了，那么这时也可以忽略这个unit does not match的问题，总之还是具体问题具体分析吧。
2.3仿真结果展示
Figure 4 below is 4 level variable simulation results, the unit may have some problems, because less paper in the initial explanation many variables and equations, and then himself in time to do a bit anxious, so many units will be removed the order by unit check. But in general should be no problem, graphical trend should be said is consistent with the feedback of graphics. But the results of the article and there are some discrepancies. For the conclusion of the article to be honest I was doubtful, especially private cars total, from the flow chart, we can know the total amount of private cars "- the amount of private car growth" - growth of private cars "- per capita GDP" -GDP and the main city population, per capita GDP in general is certainly increasing, decreasing impossible, or else the economy will decline, the growth rate of private cars is certainly increasing, and thus increase the amount of private cars is certainly increasing, and finally private cars the total is certainly increasing, that is my simulation out of the map, after the first increased and then decreased thesis I doubt it, unless the article made some amendments in private cars per capita GDP growth rates and at this function, but the authors did not given this function, so it is not known.
Thesis results.
3, Vensim acquisition model
This paper reproduction of the papers and get Vensim model I built.
References
Song to move, Xue Dawei, Zhang Peng. Modeling analysis [J]. Harbin private car travel characteristics of the system dynamics. Heilongjiang Institute of Technology. 2019, No. 3
