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Highlights from
MATLAB in Dynamics

MATLAB in Dynamics

by

 

22 Nov 2004 (Updated )

Companion Software for "MATLAB with Applications in Engineering Research"

dtx
                     function dtx
% ***************************************************************
%                    P r o g r a m   DTX
% ***************************************************************
%
%  PURPOSE:
%     Resolve numerically differential equation of rectilinear motion
%     of a particle
%                      m*d2x/dt2 = Fx(t,x,v)
%     and plots graphics of coordinate, velocity and phase plane.
%     If possible, the program could solve the problem analytically.
%
%  INPUT DATA:
%     m    - mass of the particle;
%     Fx   - projection of forces on axis x - Fx = Fx(t,x,v);
%     x0   - initial value of the coordinate ;
%     v0   - initial value of the velocity ;
%     Tend - upper bound of the integration ;
%     eps  - precision of the integration ;
%     np   - number of parameters .
%     P{1}, P{2}, ..., P{np} - names of the parameters (array of cells);
%
%  NOTES:
%   1. The coordinate is designed by the symbol 'x' and velocity by 'v';
%   2. The physical names of the parameters are assigned to the
%      cells of the array P like this: P{1}='m', P{2}='c',...;
%   3. For analytical solution the values of Tend, eps, np and P are not
%      needed;
%   4. Initial values x0, v0 have to be entered as strings, even though
%      they represent numbers!
%   5. All the data can be input from file or in interactive mode.
% 
%  EXAMPLE of DATA FILE:
%   % Data for problem ...
%     m    = 'm';  ( or m = '5.3';)
%     Fx   = '-k*v - c*x'; % k, c - parameters
%     x0   = 'x0'; ( or x0 = '0.1';)
%     v0   = 'v0'; ( or v0 = '10';)
%     Tend = 20;
%     eps  = 1.e-8;
%     np   = 3;
%     P{1} = 'm';
%     P{2} = 'k';
%     P{3} = 'c';

% ---------------------------------------------------------
%                DATA INPUT OF THE PROBLEM
% =========================================================

clear
 disp(' ');
 disp(' How will you input the data ?    ');
 disp('     1. From a data file;         ');
 disp('     2. In interactive mode.      ');
 ans = input(' Number of Your choice : '  );
 flag = 0;
 if ans == 1
    while 1
       disp(' ');
       indat = input(' Input the name of the data file :', 's');
       if exist([cd,'\',indat]) % Search only in current directory
          eval(indat);
          flag = 1; break  % Successful
        else               % Unsuccessful
          disp(' ');
          disp([' File ',indat,' not exist!'])
          disp(' You have to:')
          disp(' 1. Enter another DATA file name, or')
          disp(' 2. Input the DATA interactively !')
          ans2 = input(' Your choice, please: ');
          if ans2 == 2, break , end
       end
    end
 end
if flag == 0  
    %Input of the data in-line mode
    disp(' ');
    m    = input(' Mass of the particle m: '              );
    Fx   = input(' Expression of the projection Fx : ','s');
    x0   = input(' Initial coordinate x0 : '              );
    v0   = input(' Initial velocity v0 : '                );
    Tend = input(' Upper bound of the integration Tend : ');
    eps  = input(' Precision of the calculations eps : '  );
    np   = input(' Number of parameters  np : '           );
    % Asigning names of the parameters
    if np > 0
       disp(' ');
       disp(' Enter names of the parameters:')
       for i = 1:np
           ii = num2str(i);
           P{i} = input([' Name of parameter P',ii,': '],'s');
       end
    end 
 end
 
% ---------------------------------------------------------
%             Differential Equation of Motion
% =========================================================

                  syms x v D2x 
                  Fx = subs(Fx, 'v', 'Dx');
                  deq = m*D2x - Fx;

% ---------------------------------------------------------
%                  ANALYTICAL SOLUTION
% =========================================================

 disp(' ');
 ans = input(' Would you like analytical solution? (Y/N): ','s');
 if ans =='Y' | ans == 'y'
    if ~isstr(x0), x0 = num2str(x0); end % Repairing user 
    if ~isstr(v0), v0 = num2str(v0); end % input errors!
    syms x
    inicond = ['x(0)=',x0,',Dx(0)=',v0];
    x = dsolve(char(deq), inicond, 't');
    if ~isempty(x)
        disp(' ');
        disp('  Low of Motion');
        disp('*****************');
        disp(' ');
        disp('x = '); pretty(x)
        disp(' ');
        fname = input(' Name of file to write solution: ','s');
        save(fname, 'x');
    end
    disp(' ');
    ans = input(' Would you like numerical solution? (Y/N): ','s');
    if ans == 'N' | ans == 'n', return, end 
    x = 'x'; % Clear contents of x
 end

% ---------------------------------------------------------
%                  NUMERICAL SOLUTION
% ========================================================= 

% Input the name of the file-function
disp(' ');
fname = input(' Name of the File-function to be generated: ','s');
flag1 = 'Y';
if exist([cd,'\',fname]) % Search only in current directory!
    disp(' ');
    disp([' A file-function with name ',fname,' already exist !'])
    flag1 = input(' Overwrite it ? (Y/N): ', 's');
end

% ---------------------------------------------------------
%              GENERATING THE FILE-FUNCTION
% ---------------------------------------------------------

if ( flag1 == 'Y' | flag1 == 'y' )
   Fx = subs(Fx,{'x','Dx'},{'y(1)','y(2)'});
% Opening the file for writing file-function
   [Fid,mes] = fopen([fname,'.m'],'wt');
% Generating the string with physical parameters: m, c ...
   strpar = '';
   for j = 1:np
      strpar = [strpar,',',P{j}];
   end
   disp(' ');
   titl = input(' Denomination of the Problem: ','s');
%       Writing the headline of the File-function
   fprintf(Fid,['function yt = ',fname,'(t,y',strpar,')\n']);
   fprintf(Fid,['%% ',titl]);
%       Writing the first derivatives
   fprintf(Fid,'\n%% The first derivatives\n');
   fprintf(Fid, '  yt(1) = y(2); \n');
   fprintf(Fid,['  yt(2) = ',char(Fx/m),'; \n']);
   fprintf(Fid,'  yt = yt'';\n');
   fprintf(Fid,['%% *** End of File-function ',fname,' ***']);
   fclose(Fid);
   edit(fname)
end

% ---------------------------------------------------------
%        INTEGRATION AND VISUALIZATION OF THE REZULTS
% ---------------------------------------------------------

flag2 = 0;
% Initial entering values of the parameters and generating
% the string with parameters 'P{1}, P{2}, ..., P{np}' to be
% passed to the File-function as actual arguments 
if np > 0
   PP = P; % Saving the physical names of the parameters in PP 
   parameters = ' ';
   disp(' ');
   disp(' Input the numerical values of the parameters: ')
   for i = 1:np
       i = num2str(i);
       eval(['P{',i,'}=input([''   '',P{',i,'},'' = '']);']);
       parameters = [parameters,',P{',i,'}'];
   end
else
   parameters = []; 
end
% Check-up type of x0 and v0 and correct
% it if needed
if ischar(x0)
    x0 = str2num(x0);
    if isempty(x0), x0 = input(' x0 = '); end
end
if ischar(v0)
    v0 = str2num(v0);
    if isempty(v0), v0 = input(' v0 = '); end
end
while 1  
    if flag2 == 1
        disp(' ');
        eps  = input(' Precision of the computations eps: '  );
        Tend = input(' Upper bound of the integration Tend: ');
        x0   = input(' Initial coordinate x0: '              );
        v0   = input(' Initial velocity v0: '                );
        if np > 0
          P = PP; % Restoring the names of the parameters !
          disp(' ');
          disp(' Input the numerical values of the parameters: ')
          for i = 1:np
              i = num2str(i);
              eval(['P{',i,'}=input([''   '',P{',i,'},'' = '']);']);
          end 
        end
    end
    y0 = [x0 v0]; % initial conditions
    options = odeset('AbsTol',eps,'RelTol',100*eps);
    % Choosing of the Solver
    disp('                                        ');
    disp('      Choose the proper Solver:         ');
    disp('  -------------------------------       ');
    disp(' A. Non stiff differential equations    ');
    disp('   1. ode45   - middle precision;       ');
    disp('   2. ode23   - low precision;          ');
    disp('   3. ode113  - from low to upper.      ');
    disp('                                        ');
    disp(' B. Stiff differential equations        ');
    disp('   1. ode15s  - from low to upper;      ');
    disp('   2. ode23s  - low precision;          ');
    disp('   3. ode23t  - middle precision;       ');
    disp('   4. ode23tb - low precision.          ');
    disp('                                        ');
    solver = input(' The name of the Solver: ','s');
   
    % Integration of the Differential Equations
        
    eval(['[t,y] = feval(solver,eval([''@'',fname]),',...
                  '[0 Tend],y0,options',parameters,');']);
    % Plotting graphs
    tmin  = min(t); 
    tmax  = max(t);
    y1min = min(y(:,1)); 
    y1max = max(y(:,1));
    y2min = min(y(:,2)); 
    y2max = max(y(:,2));
    dy1   = y1max - y1min;
    dy2   = y2max - y2min;
    xmin  = y1min - 0.1*dy1;
    xmax  = y1max + 0.1*dy1;
    ymin  = y2min - 0.1*dy2;
    ymax  = y2max + 0.1*dy2;
    % Coordinate x
    figure % 1
    comet(t,y(:,1))
    plot(t,y(:,1),[tmin tmax],[0 0],'k'), grid on
    axis([tmin, tmax, xmin, xmax]);
    set(gca,'FontName','Arial Cyr','FontSize',12);
    title('Low of motion {\itx} = {\itx}({\itt})')
    xlabel('{\itt}'); ylabel('{\itx}'); pause
    % Velocity v 
    figure % 2
    comet(t,y(:,2))
    plot(t,y(:,2),[tmin tmax],[0 0],'k'), grid on
    axis([tmin, tmax, ymin, ymax]);
    set(gca,'FontName','Arial','FontSize',12);
    title('Velocity {\itv} = {\itv}({\itt})')
    xlabel('{\itt}'); ylabel('{\itv}'); pause
    % Coordinate and Velocity
    figure % 3
    subplot(2,1,1), plot(t,y(:,1),[tmin tmax],[0 0],'k'),grid on
    axis([tmin, tmax, xmin, xmax]);
    set(gca,'FontName','Arial','FontSize',12);
    title('Low of motion {\itx} = {\itx}({\itt})')
    subplot(2,1,2), plot(t,y(:,2),[tmin tmax],[0 0],'k'),grid on
    axis([tmin, tmax, ymin, ymax]);
    set(gca,'FontName','Arial','FontSize',12);
    title('Velocity {\itv} = {\itv}({\itt})')
    pause
    % Phase Plane
    figure % 4 
    subplot(1,1,1)
    comet(y(:,1),y(:,2))
    plot(y(:,1),y(:,2), [xmin,xmax],[0 0],'k',...
                  [0 0],[ymin,ymax],'k'), grid on
    axis([xmin, xmax, ymin, ymax]);         
    set(gca,'FontName','Arial Cyr','FontSize',12);
    title(' Phase Plane {\itv} = {\itv}({\itx})')
    xlabel('{\itx}'), ylabel('{\itv}'), pause
    flag2 = 1;
    close all
    disp(' ');
    ans = input(' Would you like to continue? (Y/N): ','s');
    if ans == 'n' | ans == 'N', break, end
end

%  ****************** End of Program DTX *******************

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