I am reading a paper that presents the following system, represented by a second-order transfer function:
$G(s) = \frac{K\times(1+0.036s)}{(1+0.0018s)(1+as)}$,
where the gain $K$ is a known constant. They explicitly declare $a<0.0018$, but I don't understand why. What particular property or behavior needs this condition?
I do not come from a solid background with skills of control or differential equations, so please bear with me if this is too simple. I'm not even sure if math.stackexchange is the correct stackexchange place for this question.
I have tried to analyse this by reading a lot about this second-order systems and the only thing that seems interesting is that the damping factor $\zeta >1$ for all values of $a$ except $a=0.0018$, where the damping factor is 1.