Solving symbolic Matlab equation in term of two variables (x/Va)

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Mitch Ott
Mitch Ott 2017-10-8
评论： Mitch Ott 2017-10-8
Guys/Girls any input will be great. I'm so stuck been spinning my wheels for hours. so.. here is what I got
All i want to do is solve this equation symbolically, for x/va. (the transfer function) so I want matlab to spit out the answer x/Va = blah blah blah. but I'll I get is weird matrices or errors. Honestly any help is appreciated. Thanks again.

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Walter Roberson
Walter Roberson 2017-10-8
编辑：Walter Roberson 2017-10-8
syms s J B x K kt Va ke Ra La
La = 0;
eqn = (J*(s^2)*x)+ (B*s*x)+ (K*x)== kt*((Va)-(ke*s*x)/(Ra+(La*s)));
syms F
eqn1 = subs(eqn, x, F*Va);
x/Va == simplify( solve(eqn1, F) )
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Walter Roberson
Walter Roberson 2017-10-8
You want F = x/Va . Multiply through by Va, to get F*Va = x. Substitute that F*Va for x in the formula, which gets rid of all of the x in the formula and puts in the new variable F instead, where we know that F = x/Va because that is how we built F. Solve for the variable F, to get something that is implicitly of the form F = SomeSolution. Now expand back F on the left side because that is what you expect to see, so now you have x/Va = SomeSolution
MATLAB cannot solve() for expressions, only for variables, so this is a very common mathematical trick, to take an existing variable that is involved with the formula you want to solve with respect to, and substitute for that variable an expression in a new variable that is the inverse. It is a "change of variables"
Mitch Ott
Mitch Ott 2017-10-8
Walter, John, thank you so much. I appreciate you taking the time to help.

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