I am trying to design a digital phase lead compensator using the c2d function in the Octave 4.0.0 control package.
The output of the b & a coefficients I fed in looks like this:
7.474 z^2 - 14.29 z + 6.829
z^2 - 1.52 z + 0.5772
I noticed a couple things about this output:
1) The powers of the 'a' coefficients start positive. Does that not make this system non-causal?
2) Why is this discrete time LTI model represented as "y1" and not "H(z)"? I fed in the coefficients of H(s) by running c2d this way:
comp_tf = tf(gain*numc,denc); %create the s domain LTI transfer function
comp_tf_d = c2d(comp_tf,0.001); % sample at 1kHz
By dividing both the numerator and denominator by z^2 you'll get an equivalent expression with non-positive powers of z.