M.Sc Thesis

M.Sc StudentGilad Fursht
SubjectNon-Linear Control Algorithms for Low Cost Seeker Heads
DepartmentDepartment of Aerospace Engineering
Supervisor Professor Idan Moshe
Full Thesis textFull thesis text - English Version


LOS control systems are widely used both in military and civilian applications. Military applications, for example, are surveillance, reconnaissance, laser designation, missile guidance, gunnery systems, etc. Astronomical observations, aerial photography, communication, solar cells, and film industry are some of the civilian applications in use. Generally, the LOS control system is incorporated in every automated pan and tilt mechanism. In the current research, control algorithms for low-cost missile imaging seeker head are investigated.

Low-cost seeker head LOS control design presents a great challenge due to system uncertainty and internal disturbances caused by the simple mechanical and electromechanical system components. One of the most severe phenomena that are difficult to model and hence hard to compensate for is friction.

Conventional controllers, such as the commonly used PID controller cannot guarantee that the required performance characteristics are met over the entire seeker field-of-regard, which is nowadays, quite large in seeker heads designs. Therefore it is suggested to utilize nonlinear robust control methods, such as Sliding-Mode-Control (SMC), which may introduce enhanced capabilities helping to deal with system uncertainties and disturbances.

In this research, two such nonlinear controllers are proposed to address the LOS control task: a conventional SMC and high order Super-Twisting SMC. The higher order sliding-mode controller was introduced in order to better deal with the chattering phenomenon accompanied with the conventional SMC design. Due to the strong nonlinearities and the multi-input-multi-output (MIMO) coupled structure of the seeker head setup, MIMO SMC design methods were used.

The performance of the proposed controllers is compared to a commonly used PID controller. The comparison is based on numerical simulation and laboratory tests carried out with a real low-cost seeker head. The controllers were tested while performing command tracking tasks. In addition, axes decoupling and disturbance rejection were evaluated. The numerical simulation is based on a simplified seeker head mathematical model that was used to tune the compared controllers and access their performance bounds.

In the numerical simulation study, performed without severe modeling errors and disturbances, all controllers exhibit similar performance. Performance differences between the controllers were evident in the laboratory tests that clearly demonstrated the advantage of the robust nonlinear controllers in meeting system performance characteristics, while properly compensating for system nonlinearities, unmodeled dynamics, coupling and disturbances. In addition, the performance of the high order super-twisting controller both in command following and in coupling/disturbance rejection, was superior to both the PID and conventional SMC controllers.