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Chapter 8: SIMULINK and GUIs
multiplication by a constant, and one Sum block, used for addition. These are all chosen from the “Math” library (fourth from the right in Figure 8-1, or fourth from the top in Figure 8-2). Hooking them up the same way we did with the Integrator blocks gives a model window that looks something like this:
1 s
1 s
1
u’
u
Gain
1 Gain1
1 Gain2
We need to go back and edit the properties of the Gain blocks, to change the constants by which they multiply from the default of 1 to 5 (in “Gain”), −1 (in “Gain1”), and 2 (in “Gain2”). To do this, double-click on each Gain block in turn. A Block Parameters box will open in which you can change the Gain parameter to whatever you need. Next, we need to send u , the output of the first Integrator block, to the input port of block “Gain2”. This presents a problem, since an Integrator block only has one output port and it’s already connected to the next Integrator block. So we need to introduce a branch line. Position the mouse in the middle of the arrow connecting the two Integrators, hold down the CTRL key with one hand, simultaneously push down the left mouse button with the other hand, and drag the mouse around to the input port of the block entitled “Gain2”. At this point we’re almost done; we just need a block for viewing the output. Open up the “Sinks” library and drag a copy of the Scope block into the model window. Hook this up with a branch line (again using the CTRL key) to the line connecting the second Integrator and the Gain block. At this point you might want to relabel some more of the blocks (by editing the text under each block), and also label some of the arrows (by double-clicking on the arrow shaft to open a little box in which you can type a label). We end up with the model shown in Figure 8-4. Now we’re ready to run our simulation. First, it might be a good idea to save the model, using Save as... from the File menu. One might choose to give it the name li e OD . (MATLAB automatically adds the file extension . l.) To see what is happening during the simulation, double-click on the Scope block to open an “oscilloscope” that will plot u as a function of t. Of course one needs to set initial conditions also; this can be done by double-clicking on