Modeling of Agricultural Systems [AGS721]

LAB 5


Introduction to AUXILIARY Variable and the PD Plus build-in functions: SWITCH and PULSE

Auxiliary Variable

Unlike constant variable, auxiliary variable is the system component that changes its value during the course of simulation. Generally auxiliary variables are often useful in assisting the formulation of the complex rate equations. Hence, if the formulation of rate equations involve more than just a constant variable that has its value fixed during the course of simulation, the auxiliary variables would then be the variables used in the flow diagram.

The symbol used for representing the auxiliary variable in the flow diagram is a circle. The relationship or link between auxiliary variable and the other variables is connected through the information flow.

Exercise:

Bank balance model

1. Assuming that you are opening a bank account with the initial deposit of ß1,000 and you do not intend to withdraw money from this account for the next 50 years. Given that the bank offers a fixed interest rate of 10.5%. Construct flow diagram and the model using PD Plus to simulate the behavior of dynamic of money in the account.

2. Suppose that you want to withdraw your money 30% (widthdrawal fraction = 0.30) of the total amount of money in the account whenever the money in the account reach ß5,000. Construct flow diagram and the model using FD Plus. In order to construct the model according to this condition, you need to use PD Plus's build-in functin namely CLIP or FIFGE.

3. Suppose that the withdrawal fraction is no longer constant i.e. 30% of the total amount of money in the account, however the widthdrawal fraction the linear relationship with the amount of money in the account which could be expressed as follow;

Widthdrawal fraction = 0.006+0.000022*Balance

Construct flowdiagram and formulate the model.

Reference:

WebSites:

System Dynamic Info


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