How do I call "frd" for discrete time?

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Mark
Mark 2014-11-20
评论： Mark 2014-11-21
Following along this example for mu analysis: Robust Performance Analysis
I have M, and blk from
[M,NDelta,blk] = lftdata(ClosedLoop);
Making a frequency vector and defining my discrete time step as:
freq=logspace(-1,2,100);
dt = 0.05;
I now want to call frd as:
M_g = frd(M,freq,dt);
but get the error: "The frequency units must be specified as one of the strings 'rad/TimeUnit', 'cycles/TimeUnit', 'rad/s', 'Hz', 'kHz', 'MHz', 'GHz', or 'rpm'."
I have no idea how to specify these units though... Help?!?

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回答（1 个）

Sebastian Castro
Sebastian Castro 2014-11-20
编辑：Sebastian Castro 2014-11-20
Hi Mark,
I think you may need to discretize the system M_g first using c2d before using the frd command.
The following commands worked for me:
M_disc = c2d(M,0.05);
M_g = frd(M_disc,freq,'rad/s');
After displaying M_g, at the very end I get the following:
Sample time: 0.05 seconds
Discrete-time frequency response.
Best,
Sebastian
  1 个评论
Mark
Mark 2014-11-21
Well, my system is already in discrete time with a sample time of 0.05 before I call lftdata, so the use of c2d is unnecessary. Unfortunately, calling
M_g = frd(M_disc,freq,dt)
yields the same error. I believe the third argument is required for discrete time. Calling
M_g = frd(M_disc,freq,dt,'rad/s')
also returns an invalid syntax error.

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