The paper is dedicated to extending a root locus theory to systems with interval parameters, which are described with a characteristic polynomial with interval coefficients. On a basis of angular equation of a root locus a set of interval inequalities, including departure angles of root locus edge branches, determining oscillability degree of a control system, was derived. Solving the system of inequalities for low-order control systems resulted in a set of vertices of a characteristic polynomial coefficients polytope, projections of which on a complex plane determine an oscillability of such systems. Examples of application are provided.

In the paper, a characteristic polynomial of an interval control system, whose coefficients are unknown or may vary within certain ranges of values, is considered. Parametric variations cause migration of interval characteristic polynomial roots within their allocation areas, whose borders determine robust stability degree of the interval control system. To estimate a robust stability degree, a projection of a polytope of interval characteristic polynomial coefficients on a complex plane must be examined. However, in order to find a robust stability degree it is enough to examine some vertices of a coefficient polytope and not the whole polytope. To find these vertices, which fully determine a robust stability degree, it is proposed to use a basic phase equation of a root locus method. Considering the requirements to placing allocation areas of system poles an interval extension of expressions for angles included to the phase equation. The set of statements, allowing to find a sum of pole angles intervals in the case of degree of oscillating robust stability, were formulated and proved. From these statements, a set of double interval angular inequalities was derived. The inequalities determine ranges of angles of all root locus edge branches departure from every pole. Considered research resulted in a procedure of finding coordinates of verifying vertices of a coefficients polytope and vertex polynomials according to these vertices. Such polynomials were found for oscillating robust stability degree analysis of interval control systems of the second, the third and the forth order. Also, similar statements were derived for aperiodical robust stability degree analysis. Numerical examples of vertex analysis of oscillating and aperiodical robust stability degree were provided for interval control systems of the second, the third and the fourth order. Obtained results were proved by examining root allocation areas of interval characteristic polynomials examined in application examples of proposed methods.

Modern underwater unmanned vehicle must operate in unpredictable conditions of water environment. This leads to a problem of motion control systems with uncertain parameters development. The paper is dedicated to development of robust motion control system for an underwater remotely operated vehicle. Several issues of developing control systems with parametric uncertainty were considered: deriving a mathematical model of an underwater remotely operated vehicle with interval parametric uncertainty, estimation of interval parameters values, developing a structure of motion control system and synthesizing a set of robust controllers, providing desired control quality in the system. 

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