solving state space using ode45

2021 5 12
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Hello Everybody
I am trying to solve a state space equation using ODE45 so that I can plot my states. My equation is of the form idot=Ai +Bv, the dimensions are [4,1]=[4;4]*[4,1]+[4,4][4,1]
When I run the code, I get " errors in 'sys' " but I dont know how to fix them. I have attached a picture of my code. Please kindly assist, thank you in advance.

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Walter Roberson
Walter Roberson 2021-5-12

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You are trying to run sys() without passing in values for t or i . sys() then fails when it needs to use one of the input variables.
You might possibly be pressing the green Run button. When you use the Run button, unless you take special steps, MATLAB will run the function without passing anything in to it.
When you run a function that has input arguments, MATLAB never pokes around looking in calling environments to see whether it can find a variable with a matching name. You must pass values to the function explicitly if you want the function to process the inputs.

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Basetsana Sebolao
Basetsana Sebolao 2021-5-12
Thank you, I realise my mistake. I tried passing in the arguments for t and i in the command window however I get an error saying that " sys must return a column vector". I have attached an image of the code
Walter Roberson
Walter Roberson 2021-5-12
Your current code tries to calculate a 4 x 4 matrix, which implies that you expect your boundary inputs, i to be 4 x 4. But your boundary is going to be received as a column vector. So early on in the function you should
i = reshape(i, 4, 4);
and then at the end of the function,
di = di(:);
Basetsana Sebolao
Basetsana Sebolao 2021-5-12
This is my first time trying to solve a state space equation on Matlab, I am a bit lost, please bear with me. I was expecting to get a 4 x 1 matrix, my input matrix, v is also a 4 x 1 matrix however in your response you said that my i matrix is actually a 4 x 4 matrix, may i ask why that is so?
Walter Roberson
Walter Roberson 2021-5-12
Your A is 4 x 4 and you use .* which is element-by-element multiplication, so you get a 4 x 4 output.
If i is 4 x 1 then you should use A*i to get a 4 x 1 result, and likewise the other multiplication in the expression would have to be * instead of .*
Basetsana Sebolao
Basetsana Sebolao 2021-5-12
ohh I see, thank you very much!

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