By means of phase plane analysis, theoretical study has been made on the step responses of discontinuous damping servomechanism devised by the author, nonlinear damping servomechanisms, Flügge-Lotz's nonlinear control system, Tsuchiya's control system with two constant controllers and positive-negative feedback control system. The conclusions obtained from the study may be summarized as follows:
1) Discontinuous damping servomechanism and Flügge-Lotz's control system have step responses favourable for any initial condition.
2) In the first and third quadrants where e and e are of the same sign, linear system has better responses than in nonlinear damping servo-systems.
3) Tsuchiya's control system has the excellent step responses when initial values of output and output derivative are zero, but it is not always so if initial values of output derivative are not zero.
4) In the P-N control system, which has the excellent step responses for all initial conditions, nonlinear controller is complex.