Comments on "An improved approximation for settling time of second-order linear systems"

The paper is based on the assertion that the formula for the settling time of linear underdamped second-order systems, using the exponential envelope of the response, is unconservative. This is incorrect: the envelope formula yields an upper bound on the settling time. We also show how the envelope formula can be approximated by a truncated McLaurin series, yielding a practical design formula that is consistent with current formulas, and more general than the one proposed by Bert.