MULTI-DOMAIN MODELING OF PAN TILT PLATFORM WITH MECHANICAL IMPERFECTIONS

Abstract

Pan Tilt Platform (PTP) is a complex electromechanical system whose dynamics is characterized by coupled nonlinear behavior. The system is typically used as mounting platform for surveillance and tracking equipment. Mounted equipment as payload requires accurate position and velocity control from PTP. Velocity control is required in surveillance mode whereas position control is critical in tracking mode. Conventional controller design techniques rely primarily on making more robust controllers by ignoring modeling details of physical system. In demanding applications, it’s hard to design controllers for variable operational needs. The problem becomes difficult with the presence of mechanical imperfections in the physical system. These mechanical imperfections include mass imbalance in actuated bodies; friction and backlash at constraint joints of these actuated bodies. Practically, these mechanical imperfections are always present in the physical system since they are difficult to remove. NUST Man Portable Ground Surveillance Radar System is one such equipment which has Pan Tilt Platform as its mounting and actuating platform. Current study is aimed to model Pan Tilt Platform of the said system. The system requires position control in tilt axis; whereas position and velocity control in pan axis. The study aims at building multiple PTP models having different abstraction levels and fidelity. It starts with a multi-bond graph model of an ideal PTP using Object Oriented Modeling(OOM) approach. OOM approach capture dynamics of individual components of multi-body system and allows system modeling directly from system topology, thereby providing modularity in the model. Ideal PTP model is made more realistic by adding details of actuation mechanisms and mechanical imperfections in step wise manner. At first, a payload as a mechanical imperfection of mass imbalance is introduced as an additional rigid body in tilt axis. Then actuation details of DC servomechanism in pan joint and linear actuator mechanism in tilt joint are added. Finally, mechanical imperfections of ii friction and backlash are added in pan joint. Friction is modeled using a general CSVS friction model whereas backlash is modeled using classical dead zone model. Experiments are conducted on PTP of NUST Ground Surveillance Radar to quantify backlash gap and to estimate friction parameters and Inertia. These parameters are estimated using MATLAB Optimization Toolbox. For comparing individual models, their equivalent models are build using MSc ADAMS, Pro/Engineer and MATLAB/Simscape. In the end, pan joint with classical friction and backlash models, is compared with experimental results.