Symbolic derivative of function using diff()

Question

Oscar Sandoval 2014-5-23

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编辑： Walter Roberson 2015-6-13

For those familiar, I am writing a function to find the EOMs using the lagrangian.

I have defined all variables as syms: x(t), y(t), z(t)...

Afterward, I define eqns KE and PE as functions of x,y, and z; and L = KE - PE.

When I call diff(L,x(t)) matlab returns the correct answer, but when attempting to take a second derivative with respect to time [diff(diff(L,x(t)),t)], Matlab calls the incorrect form of diff() and yields: "Error using sym/diff, [secondary] arguments must not be symbolic." Is there a way to force this command such that it yields a result? Happy to provide specific code on request, thank you.

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Answer 1

Mahdi 2014-5-23

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https://ww2.mathworks.cn/matlabcentral/answers/130832-symbolic-derivative-of-function-using-diff#answer_138011

在 MATLAB Online 中打开

Why don't you just tell diff to take the second derivative immediately? Look under the Higher-Order Derivatives subsection for diff.

In this case, you can do:

diff(L,x(t),2) % Second derivative

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Oscar Sandoval 2014-5-23

Mahdi,

x(t) and y(t) are two of six coordinates defining position and orientation of the Center of Mass of a dynamic system. These variables are measured inputs of the system. I am attempting to solve the function symbolically to arrive at the EoMs of the system, which will then be evaluated at the measured values of x and y, amongst other inputs, in order to design a real-time controller. Are you familiar with the Euler-Lagrange equations?

Mahdi 2014-5-26

Sorry for the late reply,

I am afraid that I am not. I would suggest posting the question again and adding a bit more detail. Try searching the forums because I have run into this problem into the past and found a solution online.

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