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Symbolic derivative of function using diff()

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Oscar Sandoval
Oscar Sandoval el 23 de Mayo de 2014
Editada: Walter Roberson el 13 de Jun. de 2015
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.

Respuestas (2)

Mahdi
Mahdi el 23 de Mayo de 2014
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
  6 comentarios
Oscar Sandoval
Oscar Sandoval el 23 de Mayo de 2014
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
Mahdi el 26 de Mayo de 2014
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.

Iniciar sesión para comentar.


Mischa Kim
Mischa Kim el 13 de Jun. de 2015
Check out the Euler-Lagrange tool on File Exchange.

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