Unable to perform assignment because the indices on the left side are not compatible with the size of the right side.

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AOF 2020-6-20

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https://ww2.mathworks.cn/matlabcentral/answers/551989-unable-to-perform-assignment-because-the-indices-on-the-left-side-are-not-compatible-with-the-size-o

回答： Devineni Aslesha 2020-7-24

% DH parameters using symbolic tool box 
clear 
clc
%% 
syms theta1 theta2 theta3
R1 = [ cos(theta1),      0,  sin(theta1), 0;
       sin(theta1),      0, -cos(theta1), 0;
                 0,      1,           0, 0;
                 0,       0,          0, 1];
R2 =[ cos(theta2), -sin(theta2),      0, 0.1*cos(theta2);
      sin(theta2),  cos(theta2),      0, 0.1*sin(theta2);
               0,             0,      1,             0;
               0,             0,      0,             1];
 
R3 =[ cos(theta3),      0,  sin(theta3), 0;
      sin(theta3),      0, -cos(theta3), 0;
                0,      1,            0, 0;
                0,      0,            0, 1];
% Total Forward Matrix 
A_03 = simplify(R1 * R2 * R3);
A_13 = R2 * R3;
%% inverse kinematic 
Px = -0.135;
Py = -0.000;
Pz = -0.128;
A_03_ = A_03;
A_03_(1:3,4) = [Px Py Pz];
A_13_ = simplify(R1 \ A_03_);
A_23_ = simplify(R2 \ A_13_);
I = simplify(R3 \ A_23_);
%%
eq(1) = Px == A_03(1,4);
eq(2) = Py == A_03(2,4);
eq(3) = Pz == A_03(3,4);
eq(4) = A_13_(1,4) - R1(1,4) == 0 ;
eq(5) = A_13_(2,4) - R1(2,4) == 0 ;
eq(6) = A_13_(3,4) - R1(3,4) == 0 ;
eq(7) = A_23_(1,4) - R2(1,4) == 0 ;
eq(8) = A_23_(2,4) - R2(2,4) == 0 ;
eq(9) = A_23_(3,4) - R2(3,4) == 0 ;
eq(10) = I(1,4) == 0;
eq(11) = I(2,4) == 0;
eq(12) = I(3,4) == 0;
sol = solve(eq);
q(1:4,1) = double(sol.theta1);
q(1:4,2) = double(sol.theta2);
q(1:4,3) = double(sol.theta3);

I dont know why it keep sending me an error

Error in Inverse_kinematic (line 58)
q(1:4,1) = double(sol.theta1);

How should i change this:

sol = 
  struct with fields:
    theta1: [0×1 sym]
    theta2: [0×1 sym]
    theta3: [0×1 sym]

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Star Strider 2020-6-20

在 MATLAB Online 中打开

The reason is that it cannot solve your system:

sol = 
  struct with fields:
    theta1: [0×1 sym]
    theta2: [0×1 sym]
    theta3: [0×1 sym]

AOF 2020-6-20

编辑：AOF 2020-6-20

how should i change it ?

because in the work space it show me 1x1 sym

i am really confused

How to solve this problem ?

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Answer 1

Devineni Aslesha 2020-7-24

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链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/551989-unable-to-perform-assignment-because-the-indices-on-the-left-side-are-not-compatible-with-the-size-o#answer_470395

The reason for showing 1x1 sym for theta1 is that they are declared as symbolic variables using syms theta1. However, 0x1 sym is for struct sol fields theta1,theta2 and theta3 as it cannot solve your system.

For more information, rfer the following link

https://www.mathworks.com/help/symbolic/equation-solving.html

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