pzmap(G+G) produces incorrect plot

Given a system with 2 zeros and 5 poles:

s = tf('s')

G = 8.75*(4*s^2+0.4*s+1)/((s/0.01+1)*(s^2+0.24*s+1)*(s^2/100+2*0.02*s/10+1))

pzmap(G+G) produces a pole zero map in which all the poles are cancelled by zeros, which is clearly incorrect. It is also different to the result of pzmap(2*G), which would be expected to be the same.

Can anyone explain this behaviour?

Answer 1

[Star Strider]

The ‘+’ operator connects the two ‘G’ models in parallel. They do appear to have pole-zero cancellation as the result:

s = tf('s');
G = 8.75*(4*s^2+0.4*s+1)/((s/0.01+1)*(s^2+0.24*s+1)*(s^2/100+2*0.02*s/10+1))

GG = G+G
figure
pzmap(GG)

Calculating the minimum realisation solves the problem:

GGmr = minreal(GG)
figure
pzmap(GGmr)

[Geraint Bevan]

Thank you - that explains it!

[Star Strider]

As always, my pleasure!

The ‘*’ operator would connect them in series.