Reference to non-existent field 'aD42'? Please help.

I am trying to calculate the acceleration of a mechanism and I am stuck with this error:

Reference to non-existent field 'aD42'.

Error in xyz12 (line 285) aDpds = eval(solaD.aD42);

here are the code lines that describe the acceleration and the aD42 variable:

aE=[0 0 0];
ep5z = sym('ep5z','real'); 
ep5 = [ 0 0 ep5z ];

aD42=sym('aD42','real');

aD4D2 = [ aD42*cos(phi4) aD42*sin(phi4) 0 ];

aD5=aE+cross(ep5,rD-rE)- dot(Omega5,Omega5)*(rD-rE);
aD4 = aD5;
aD4D2cor = 2*cross(omega2,VD42);
eqaD = aD4 - ( aB2 + aD4D2 + aD4D2cor );
eqaDx = eqaD(1); eqaDy = eqaD(2);
solaD = solve(eqaDx,eqaDy);
ep5zs = eval(solaD.ep5z);

aDpds = eval(solaD.aD42);

Ep5 = [0 0 ep5zs];
AD42 = aDpds*[cos(phi4) sin(phi4) 0];
AD5=aE+cross(Ep5,rD-rE)-...
dot(Omega5,Omega5)*(rD-rE);

[Azzi Abdelmalek]

Please post all your data rD,rE,... to allow testing your code

[Walter Roberson]

Put a breakpoint after the solve() and see what fields were returned for solaD

Note: you should not eval() a symbolic variable. symbolic variables are not MATLAB expressions, they are just very similar to MATLAB expressions. You should subs() or double() the symbolic variable.

Answer 1

[Matt Fig]

Use this line:

solaD = solve(eqaDx,eqaDy,ep5z,aD42)

Instead of the one you use for solaD. The problem is you have three unknowns and two equations, so SOLVE chose to leave aD42 as the independent variable in the solutions.

Also, when you call FPRINTF with a symbolic variable you need to convert in with CHAR first.

fprintf('aD4D2cor = [%g, %g, %d] (m/sˆ2)\n', char(aD4D2cor))

and so for the rest.

[gheorghe]

Thank you so very much!

Answer 2

[gheorghe]

    clear all % sterge toate variabilele
    clc % sterge fereastra command si duce cursorul in prima pozitie
    close all % inchide toate ferestrele grafice
    AB=0.25; % marimea segmentului AB in m
    BC=223.94*0.0025;
    phi=130*(pi/180); % definirea unghiului phi1

    %pozitia cuplei A
    xA=0; yA=0;
    %pozitia cuplei B
    xB=AB*cos(phi);
    yB=AB*sin(phi);
    %pozitia cuplei C
    yC=0;
    eqnD1='(xB-xCsol)^2+(yB-yC)^2=BC^2'; % distanta intre 2 pct
    solC=solve(eqnD1,'xCsol'); %rezolvarea ec pt xC
    xC1=eval(solC(1));
    xC2=eval(solC(2));

    if xC1 > xB xC = xC1; 
    else xC = xC2; end %conditia ca cupla sa se afle in cadranul 1

    phi2 = atan((yB-yC)/(xB-xC));%unghiul care il face bara BC cu axa xx

    % vectori pozitie in coordonate spatiale x y z
    rA=[xA yA 0]; 
    rB=[xB yB 0];
    rC=[xC yC 0];

    AE=0.3;
    BD=90.42*0.0025;
    ED=149.76*0.0025;
    CF=357.73*0.0025; % din C pana in capatul barei
    DF=43.37*0.0025;
    phi0=pi;
    xE=0;
    yE=0.3;

    % punctele B C D sunt pe aceeasi linie au acceasi panta
    % m = (y2 -y1) / (x2 -x1)

    eqnD1='(xB-xDsol)^2+(yB-yDsol)^2=BD^2'; % distanta intre 2 puncte
    eqnD2=' (yB-yC) / (xB -xC)= (yDsol - yB) / (xDsol -xB)';
    solD = solve(eqnD1, eqnD2, 'xDsol, yDsol');
    xDpositions = eval(solD.xDsol);
    yFpositions = eval(solD.yDsol);
    % first component of the vector xDpositions
    xD1 = xDpositions(1);
    % second component of the vector xDpositions
    xD2 = xDpositions(2);
    % first component of the vector yDpositions
    yD1 = yFpositions(1);
    % second component of the vector yDpositions
    yD2 = yFpositions(2);

    if xD1 <= xC
    xD = xD1; yD=yD1;
    else
    xD = xD2; yD=yD2;
    end

    rD = [xD yD 0]; % vector pozitie D

    eqnF1='(xD-xFsol)^2+(yD-yFsol)^2=DF^2'; % distanta intre 2 puncte
    eqnF2=' (yB-yC) / (xB -xC)= (yFsol - yB) / (xFsol -xB)';
    solF = solve(eqnF1, eqnF2, 'xFsol, yFsol');
    xFpositions = eval(solF.xFsol);
    yFpositions = eval(solF.yFsol);
    % first component of the vector xDpositions
    xF1 = xFpositions(1);
    % second component of the vector xDpositions
    xF2 = xFpositions(2);
    % first component of the vector yDpositions
    yF1 = yFpositions(1);
    % second component of the vector yDpositions
    yF2 = yFpositions(2);

    if xF1 <= xC
    xF = xF1; yF=yF1;
    else
    xF = xF2; yF=yF2;
    end
    %sau mai simplu xF = xD + DF*cos(phi2+pi) ; yF = yD + DF*sin(phi2+pi) ;

    rF = [xF yF 0]; % pozitie vector F

    %trasarea graficului
    plot([xA,xB],[yA,yB],'r-o',[xB,xC],[yB,yC],'b-o',[xB,xD],[yB,yD],'g-o',...
    [xE,xD],[yE,yD],'y-o',[xD,xF],[yD,yF],'k-o') % trasarea mecanismului 
    %cu proprietaile axis([xMIN xMAX yMIN yMAX]); r = red; - = solid line; 
    %o = cerculet in fiecare punct; b = blue; g = green; c = cyan; m = magenta
    xlabel('x (m)') % definirea axelor
    ylabel('y (m)')
    title('pozitie mecanism pentru \phi = 130 (deg)') % titlu grafic

    text(xA,yA,' A'),... % virgula si cele 3 puncte reprezinta asteptarea
    % urmatoarei comenzi fara a se executa instructiunea
    text(xB,yB,' B'),...
    text(xC,yC,' C'),...
    text(xD,yD,' D'),...
    text(xE,yE,' E'),...
    text(xE,yE,' E'),...
    axis([-0.6 0.6 -0.2 0.5]),... % marimea axelor
    grid
    % mai avem comenzi ca hold on - retine graficul
    % ’LineWidth’,1.5 grosime linie
    % grid on
    % text(xA,yA,'\leftarrow A = ground',...'HorizontalAlignment','left'),...

    phi4 = phi2;
    phi5 = atan((yD-yE)/(xD-xE));
    rE = [xE yE 0]; % pozitie vector E

    %afisarea valorilor in fereastra command
    fprintf('xA = %g (m) \n', xA)
    fprintf('yA = %g (m) \n', yA)
    fprintf('xB = %g (m) \n', xB)
    fprintf('yB = %g (m) \n', yB)
    fprintf('xC = %g (m) \n', xC)
    fprintf('yC = %g (m) \n', yC)
    fprintf('xD = %g (m) \n', xD)
    fprintf('yD = %g (m) \n', yD)
    fprintf('xE = %g (m) \n', xE)
    fprintf('yE = %g (m) \n', yE)
    fprintf('xF = %g (m) \n', xF)
    fprintf('yF = %g (m) \n', yF)
    fprintf('phi0 = %g (degrees) \n', phi0*180/pi)
    fprintf('phi1 = %g (degrees) \n', phi*180/pi)
    fprintf('phi2 = %g (degrees) \n', phi2*180/pi)
    fprintf('phi4 = %g (degrees) \n', phi4*180/pi)
    fprintf('phi5 = %g (degrees) \n', phi5*180/pi)
    fprintf('rA = [ %g, %g, %g ] (m) \n', rA)
    fprintf('rB = [ %g, %g, %g ] (m) \n', rB)
    fprintf('rC = [ %g, %g, %g ] (m) \n', rC)
    fprintf('rD = [ %g, %g, %g ] (m) \n', rD)
    fprintf('rE = [ %g, %g, %g ] (m) \n', rE)
    fprintf('rF = [ %g, %g, %g ] (m) \n', rF)

    % vB = vB1 = vA+vBA = vA+?1×rAB = ?1×rB
    omega1=[0 0 20]; % rad/s constant
    vA=[0 0 0]; %punctul A se afla pe batiu
    vB1=vA+cross(omega1,rB); % viteza punctului B1
    vB2=vB1;
    vB = norm(vB1); % norm() is the vector norm
    fprintf('omega1 = [ %g, %g, %g ] (rad/s)\n', omega1)
    fprintf('vB=vB1=vB2 = [ %g, %g, %g ] (m/s)\n', vB1)
    fprintf('|vB|= %g (m/s)\n', vB)

    % dot(u,v) produs scalar dintre u si v
    % cross(u,v) produs vectorial dintre u si v

    % vC = vC2 = vB+vCB = vB2 +?2×rBC = vB+?2×(rC ?rB)
    omega2z = sym('omega2z','real');% creare variabile simbolice sym
    vCx = sym('vCx','real');
    omega2 = [ 0 0 omega2z ];
    vC = [ vCx 0 0 ];
    eqvC = vC - (vB2 + cross(omega2,rC-rB));
    eqvCx = eqvC(1); % equation component on x-axis
    eqvCy = eqvC(2); % equation component on y-axis
    solvC = solve(eqvCx,eqvCy);
    % solutiile
    omega2zs = eval(solvC.omega2z);
    vCxs = eval(solvC.vCx);
    % rezulta
    Omega2 = [0 0 omega2zs];
    VC = [vCxs 0 0];
    vCB = cross(Omega2,rC-rB); %viteza lui C in functie de B

    % pentru a arata corect expresiile ecuatiilor eqvCx si y
    qvCx = vpa(eqvCx,6);
    fprintf('x-axis: %s = 0 \n', char(qvCx))
    qvCy = vpa(eqvCy,6);
    fprintf('y-axis: %s = 0 \n', char(qvCy))
    %vpa(S,D) compute each element of S to D decimal digits of accuracy
    % char() creates a character array (string)
    % fprintf('|vC|= %g (m/s)\n', norm(VC))
    fprintf('=>\n')
    fprintf('omega2z = %g (rad/s)\n', omega2zs)
    fprintf('vCx = %g (m/s)\n', vCxs)
    fprintf('\n')
    fprintf('omega2 = [ %g, %g, %g ] (rad/s)\n', Omega2)
    fprintf('vC = [ %g, %g, %g ] (m/s)\n', VC)
    fprintf('vCB = [ %g, %g, %d ] (m/s)\n', vCB)
    fprintf('\n')

    % Acceleratie punct B
    aA = [0 0 0 ]; % (m/sˆ2)
    ep1=[0 0 0]; % rad/s^2 omega1 este constant
    aB1 = aA + cross(ep1,rB) - dot(omega1,omega1)*rB;
    aB2 = aB1;
    aBn = - dot(omega1,omega1)*rB;
    aBt = cross(ep1,rB);
    fprintf('|aB1|=|aB2|= %g (m/s^2)\n', norm(aB1))
    fprintf('|aBn|= %g (m/s^2)\n', norm(aBn))
    fprintf('|aBt|= %g (m/s^2)\n', norm(aBt))

    ep2z=sym('ep2z','real');
    aCx=sym('aCx','real');
    ep2 = [ 0 0 ep2z ]; 
    aC = [aCx 0 0 ];
    eqaC=aC-(aB1+cross(ep2,rC-rB)-...
    dot(Omega2,Omega2)*(rC-rB));
    eqaCx = eqaC(1); % equation component on x-axis
    eqaCy = eqaC(2); % equation component on y-axis
    solaC = solve(eqaCx,eqaCy);
    ep2zs=eval(solaC.ep2z);
    aCxs=eval(solaC.aCx);
    Ep2 = [0 0 ep2zs];
    aCs = [aCxs 0 0];
    aCB=cross(Ep2,rC-rB)-dot(Omega2,Omega2)*(rC-rB);
    aCBn=-dot(Omega2,Omega2)*(rC-rB);
    aCBt=cross(Ep2,rC-rB);

    fprintf('aC = aB + omega2 x rBC - (omega2.omega2)rBC =>\n')
    qaCx=vpa(eqaCx,6);
    fprintf('x-axis: %s = 0 \n', char(qaCx))
    qaCy=vpa(eqaCy,6);
    fprintf('y-axis: %s = 0 \n', char(qaCy))
    fprintf('=>\n')
    fprintf('ep2z = %g (rad/sˆ2)\n', ep2zs)
    fprintf('aCx = %g (m/sˆ2)\n', aCxs)
    fprintf('\n')
    fprintf('alpha2 = [%g, %g, %g] (rad/sˆ2)\n', Ep2)
    fprintf('aC = [ %g, %g, %g ] (m/sˆ2)\n', aCs)
    fprintf('aCB = [ %g, %g, %d ] (m/sˆ2)\n', aCB)
    fprintf('aCBn = [ %g, %g, %d ] (m/sˆ2)\n', aCBn)
    fprintf('|aCBn| = %g (m/sˆ2)\n', norm(aCBn))
    fprintf('aCBt = [ %g, %g, %d ] (m/sˆ2)\n', aCBt)
    fprintf('|aCBt| = %g (m/sˆ2)\n', norm(aCBt))
    fprintf('\n')

    omega5z = sym('omega5z','real'); 
    omega5 = [ 0 0 omega5z ];
    vD42 = sym('vD42','real'); 
    vD4D2 = [ vD42*cos(phi4) vD42*sin(phi4) 0 ];
    vE = [0 0 0 ];

    vD5 = vE + cross(omega5,rD-rE);
    vD4 = vD5;

    eqvD = vD4 - ( vB2 + vD4D2);
    eqvDx = eqvD(1); eqvDy = eqvD(2);
    solvD = solve(eqvDx,eqvDy);
    omega5zs = eval(solvD.omega5z);
    vD42s = eval(solvD.vD42);
    Omega5 = [0 0 omega5zs];
    VD42 = vD42s*[cos(phi4) sin(phi4) 0];
    VD5 = vE + cross(Omega5,rD-rE);

    fprintf('vD4 = vD2 + vD4D2 => \n')
    qvDx=vpa(eqvDx,6);
    fprintf('x-axis:\n')
    fprintf('%s=0\n',char(qvDx))
    qvDy=vpa(eqvDy,6);
    fprintf('y-axis:\n')
    fprintf('%s=0\n',char(qvDy))
    fprintf('=>\n')
    fprintf('omega5z = %g (rad/s)\n', omega5zs)
    fprintf('vD42 = %g (m/s)\n\n', vD42s)
    fprintf('omega5 = [%g, %g, %g](rad/s)\n', Omega5)
    fprintf('vD4D2 = [ %g, %g, %d ] (m/s)\n', VD42)
    fprintf('\n')
    fprintf('vD5 = [ %g, %g, %d ] (m/s)\n', VD5)

    aE=[0 0 0];
    ep5z = sym('ep5z','real'); 
    ep5 = [ 0 0 ep5z ];
    aD42=sym('aD42','real');
    %  parallel to the sliding direction
    aD4D2 = [ aD42*cos(phi4) aD42*sin(phi4) 0 ];
    aD5=aE+cross(ep5,rD-rE)- dot(Omega5,Omega5)*(rD-rE);
    aD4 = aD5;
    aD4D2cor = 2*cross(omega2,VD42);
    eqaD = aD4 - ( aB2 + aD4D2 + aD4D2cor );
    eqaDx = eqaD(1); eqaDy = eqaD(2);
    solaD = solve(eqaDx,eqaDy);
    ep5zs = eval(solaD.ep5z);
    aDpds = eval(solaD.aD42);
    Ep5 = [0 0 ep5zs];
    AD42 = aDpds*[cos(phi4) sin(phi4) 0];
    AD5=aE+cross(Ep5,rD-rE)-...
    dot(Omega5,Omega5)*(rD-rE);
    % print the equations for calculating
    % alpha3z and aB21
    fprintf...
    ('aD4D2cor = [%g, %g, %d] (m/sˆ2)\n', aD4D2cor)
    fprintf('\n')
    fprintf...
    ('aD4 = aD2 + aD4D2 + aD4D2cor => \n')
    qaDx=vpa(eqaDx,6);
    fprintf('x-axis:\n')
    fprintf('%s=0\n',char(qaDx))
    qaDy=vpa(eqaDy,6);
    fprintf('y-axis:\n')
    fprintf('%s=0\n',char(qaDy))
    fprintf('=>\n')
    fprintf('ep5z = %g (rad/sˆ2)\n', ep5zs)
    fprintf('aD42 = %g (m/sˆ2)\n', aDpds)
    fprintf('\n')
    fprintf...
    ('ep5 = [ %g, %g, %g ] (rad/sˆ2)\n', Ep5)
    fprintf('aD4D2 = [ %g, %g, %d ] (m/sˆ2)\n', AD42)
    fprintf('aD5 = [ %g, %g, %d ] (m/sˆ2)\n', AD5)
    fprintf('\n')
    omega45 = omega4 - Omega5;
    ep45 = ep4 - Ep5;
    fprintf...
    ('omega45 = [ %g, %g, %g ] (rad/s)\n', omega45)
    fprintf...
    ('ep45 = [ %g, %g, %g ] (rad/sˆ2)\n', ep45)
    % end of program