How do I call "frd" for discrete time?

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?!?

Answer 1

[Sebastian Castro]

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

[Mark]

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.