状态空间模型的输出 和 Solve 得到的结果不一致

Question

Remove["Global`*"] // Quiet;
\[DoubleStruckCapitalM] = {{m1, 0, 0}, {0, m2, 0}, {0, 0,  m3}}; 
\[DoubleStruckCapitalK] = {{k11, k12, k13}, {k21, k22, k23}, {k31, k32, k33}}; 
\[DoubleStruckCapitalC] = {{c11, c12, c13}, {c21, c22, c23}, {c31, c32, c33}}; 
\[DoubleStruckCapitalB]\[DoubleStruckS] = {{1, -1, 0}, {0, 1, -1}, {0, 0, 1}}; 
\[DoubleStruckX] = {{x1[t]}, {x2[t]}, {x3[t]}}; 
uvec = {{u1[t]}, {u2[t]}, {u3[t]}}; 
\[CapitalLambda] = {{1}, {1}, {1}};
lhs = \[DoubleStruckCapitalM].D[\[DoubleStruckX], {t,2}] + \[DoubleStruckCapitalC].D[\[DoubleStruckX],t] + \[DoubleStruckCapitalK].\[DoubleStruckX] // Flatten;
rhs = \[DoubleStruckCapitalB]\[DoubleStruckS].uvec - \[DoubleStruckCapitalM].\[CapitalLambda] ddxg[t] // Flatten;
eq = lhs == rhs // Thread;

\[DoubleStruckZ] = {x1[t], x2[t], x3[t], x1'[t], x2'[t], x3'[t]};
\[DoubleStruckY] = {x1''[t], x2''[t], x3''[t]};
\[DoubleStruckU] = {u1[t], u2[t], u3[t], ddxg[t]};
ss = StateSpaceModel[eq, \[DoubleStruckZ], \[DoubleStruckU], \[DoubleStruckY], t];
{AA, BB, CC, DD} = Normal[ss];
ddxSS = CC.\[DoubleStruckZ] + DD.\[DoubleStruckU] // Flatten;
ddxEQ = {x1''[t], x2''[t], x3''[t]} /. Solve[eq, {x1''[t], x2''[t], x3''[t]}] // Flatten;
ddxSS - ddxEQ // Simplify(*this should be {0,0,0}*)

Comments

“状态空间模型的输出”，是指啥呀。

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非常感谢您的关注。状态模型的输出就是指CC.\[DoubleStruckZ] + DD.\[DoubleStruckU]。我在stackexchange上也问了相同的问题（希望这样不违反规定:)），已经得到了正确的答案（https://mathematica.stackexchange.com/questions/208869/strange-output-of-a-simple-statespace-model）。问题在于我的模型已经是一个描述性的状态空间模型了，状态发生了改变。

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哦，状态空间这方面我没有学过。
不过你得到想要的答案了就好。