Comment on “Optimal tuning of sigmoid PID controller using nonlinear sine cosine algorithm for the automatic voltage regulator system”

In this short note, the relative stability results (based on root locus and Bode analyses) reported in “Optimal tuning of sigmoid PID controller using nonlinear sine cosine algorithm for the automatic voltage regulator system” (Suid and Ahmad, 2022) are analyzed. It is shown that the use of closed-loop transfer functions (CLTFs) should be avoided in relative stability analysis since they may lead to less accurate stability results. The present study affirms that the results of relative stability analysis obtained using loop transfer functions (LTFs) are more reliable and accurate.