% Nise, N.S.
% Control Systems Engineering, 6th ed.
% John Wiley & Sons, Hoboken, NJ, 07030
% Control Systems Engineering Toolbox Version 6.0
% Copyright 2011 by John Wiley & Sons, Inc.
% ch2sp4 (Example 2.10) MATLAB's Symbolic Math Toolbox may be used to simplify
% the solution of simultaneous equations by using Cramer's rule. A system of simultaneous
% equations can be represented in matrix form by Ax = B, where A is the matrix formed
% from the coefficients of the unknowns in the simultaneous equations, x is a vector
% containing the unknowns, and B is a vector containing the inputs. Cramer's rule states
% that xk, the kth element of the solution vector, x, is found using xk = det(Ak)/det(A),
% where Ak is the matrix formed by replacing the kth column of matrix A with the input
% vector, B. In the text we refer to det(A) as "delta". In MATLAB matrices are written with a
% space or comma separating the elements of each row. The next row is indicated with a
% semicolon or carriage return. The entire matrix is then enclosed in a pair of square
% brackets. Applying the above to the solution of Example 2.10:
% A=[(R1+L*s) -L*s;-L*s (L*s+R2+(1/(c*s)))] and Ak=[(R1+L*s) V;-L*s 0]. The function
% det(matrix) evaluates the determinant of the square matrix argument. Let us now find
% the transfer function G(s) = I2(s)/V(s), asked for in Example 2.10. The command
% simple(S), where S is a symbolic function, is introduced in the solution. Simple(S)
% simplifies the solution by shortening the length of S. The use of simple(I2) shortens
% the solution by combining like powers of the Laplace variable, s.
'(ch2sp4) Example 2.10' % Display label.
syms s R1 R2 L c V % Construct symbolic objects for frequency
% variable 's', and 'R1', 'R2', 'L', 'c', and 'V'.
% Note: Use lower-case "c" in declaration for
A2=[(R1+L*s) V;-L*s 0] % Form Ak = A2.
A=[(R1+L*s) -L*s;-L*s (L*s+R2+(1/(c*s)))]
% Form A.
I2=det(A2)/det(A); % Use Cramer's rule to solve for I2(s).
I2=simple(I2); % Reduce complexity of I2(s).
G=I2/V; % Form transfer function, G(s) = I2(s)/V(s).
'G(s)' % Display label.
pretty(G) % Pretty print G(s).