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MathGroup Archive 2005

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Re: Representation and Simulation of Dynamic Systems

  • To: mathgroup at smc.vnet.net
  • Subject: [mg56957] Re: [mg56928] Representation and Simulation of Dynamic Systems
  • From: Chris Chiasson <chris.chiasson at gmail.com>
  • Date: Wed, 11 May 2005 05:24:15 -0400 (EDT)
  • References: <200505100742.DAA08370@smc.vnet.net>
  • Reply-to: Chris Chiasson <chris.chiasson at gmail.com>
  • Sender: owner-wri-mathgroup at wolfram.com

It seems like you are solving systems with constant coefficients.

The trickiest parts would be (a) designing an interface into which one
could place a block diagram and (b) having Mathematica interpret that
diagram.

If you could do that, control system professional can probably do
whatever else you might need ...

http://library.wolfram.com/examples/CSPExtending/

Does anyone know if there is a good bond graph or block diagram
"package" available for Mathematica?

I would really like to see the equivalent of
that-simulation-link-addition-which-must-not-be-named on
that-mathematics-laboratory-program-which-must-not-be-named for
Mathematica.

Regards,

On 5/10/05, Caffa Vittorio Dr. <Caffa at iabg.de> wrote:
> The behavior of (time-continuous, non-linear) dynamic systems can be
> numerically investigated with NDSolve. One can first sketch a block
> diagram of the system and then convert it into equations. Here is a toy
> example after the conversion:
> 
>    pos'[t] = vel[t]
>    vel'[t] = -k pos[t] + force[t] / m
> 
> This works fine if the variables are all states, as in the example
> above. But often, in order to describe a given dynamic system you want
> or you have to introduce some auxiliary variables (i.e. variables which
> are not states). This is in fact the case if you want to describe a
> generic dynamic system. Here are the standard equations:
> 
>    x'[t] = f[x[t], u[t], t]    (state equations)
>    y[t] = g[x[t], u[t], t]     (output equations)
> 
> where: x = state vector, u = input vector, y = output vector, t = time.
> In this case the components of the output vector are the "auxiliary"
> variables.
> 
> I'm considering here a scheme for representing dynamic systems (possibly
> using a block diagram as a starting point) which allows the usage of
> auxiliary variables. This representation can be transformed into
> equations for NDSolve automatically. After having solved the equations
> it is possible to inspect not only the state variables but also the
> auxiliary variables.
> 
> Comments or alternative solutions to the problem I'm considering are
> welcome!
> 
> Procedure
> 
> o) Sketch the system on a piece of paper. Here is a toy example:
> 
>                           ---------- [ -k ] ---------
>                          |                           |
>                          V                           |
> force[t] --> [ 1/m ] --> + --> [ 1/s ] ---> [ 1/s ] --->  pos[t]
>                             |           |
>                             |            -------------->  vel[t]
>                             |
>                             --------------------------->  acc[t]
> 
> Note: [ 1/s ] is an integrator block
>       [  k  ] is a gain block
> 
> o) Convert the sketch into a system description:
> 
> In[1]:= sys = {pos'[t] -> vel[t],
>         vel'[t] -> acc[t],
>         acc[t] -> -k pos[t] + force[t] / m};
> 
> Note: the arrow points to the source of the signal.
> 
> o) Make a list of the state variables:
> 
> In[2]:= states = {pos[t], vel[t]};
> 
> o) Form the differential equations (the following steps could be
> performed by a function):
> 
> In[3]:= lhs = D[states, t]
> 
> Out[3]= {pos'[t], vel'[t]}
> 
> In[4]:= rhs = D[states, t] //. sys
> 
>                  force[t]
> Out[4]= {vel[t], -------- - k pos[t]}
>                     m
> 
> In[5]:= eqns = Join[Thread[lhs == rhs], {pos[0] == pos0, vel[0] ==
> vel0}]
> 
>                                        force[t]
> Out[5]= {pos'[t] == vel[t], vel'[t] == -------- - k pos[t], pos[0] ==
> pos0,
>                                           m
> vel[0] == vel0}
> 
> o) Specify the parameters:
> 
> In[6]:= params = {m -> 10, k -> 2, pos0 -> 0, vel0 -> 0, force[t] ->
> Sin[t]};
> 
> o) Solve the differential equations:
> 
> In[7]:= sol = First[NDSolve[eqns /. params, states, {t, 0, 10}]]
> 
> Out[7]= {pos[t] -> InterpolatingFunction[{{0., 10.}}, <>][t],
> 
>          vel[t] -> InterpolatingFunction[{{0., 10.}}, <>][t]}
> 
> o) Inspect the results (including auxiliary variables)
> 
> In[8]:= Plot[pos[t] /. sol, {t, 0, 10}]
> 
> Out[8]= -Graphics-
> 
> In[9]:= Plot[acc[t] //. sys /. params /. sol, {t, 0, 10}]
> 
> Out[9]= -Graphics-
> 
> Cheers, Vittorio
> 
> --------------------------------------------
> Dr.-Ing. Vittorio G. Caffa
> IABG mbH
> Abt. VG 32
> Einsteinstr. 20
> 85521 Ottobrunn / Germany
> 
> Tel. (089) 6088 2054
> Fax: (089) 6088 3990
> E-mail: caffa at iabg.de
> Website : www.iabg.de
> --------------------------------------------
> 
> 


-- 
Chris Chiasson
http://chrischiasson.com/
1 (810) 265-3161


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