MATLAB: Pzmap(G+G) produces incorrect plot

MATLABpole-zeropzmapsum of lti systems

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?

Best Answer

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)
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