Inertial Line-Of-Sight Stabilization Using a 3-DOF Spherical Parallel Manipulator with Coaxial Input Shafts (2312.02641v3)

Abstract: This article dives into the use of a 3-RRR Spherical Parallel Manipulator (SPM) for the purpose of inertial Line Of Sight (LOS) stabilization. Such a parallel robot provides three Degrees of Freedom (DOF) in orientation and is studied from the kinematic point of view. In particular, one guarantees that the singular loci (with the resulting numerical instabilities and inappropriate behavior of the mechanism) are far away from the prescribed workspace. Once the kinematics of the device is certified, a control strategy needs to be implemented in order to stabilize the LOS through the upper platform of the mechanism. Such a work is done with MATLAB Simulink using a SimMechanics model of our robot.

• The paper introduces a novel 3-DOF SPM design with coaxial input shafts that enable unlimited bearing rotation for inertial LOS stabilization.
• The study employs detailed kinematic analysis and MATLAB Simulink modeling to design a robust control strategy against platform disturbances.
• Simulation results validate the system’s fast, stable response on maritime platforms, highlighting its practical applications in LOS stabilization.

Introduction to Spherical Parallel Manipulators

Parallel manipulators, often used in industry and robotics, are closed-loop kinematic chains providing several degrees of freedom (DOF). Among them, the Spherical Parallel Manipulator (SPM) stands out for its compactness and ability to rotate around a fixed point, offering three DOFs in orientation: bearing, elevation, and bank. This paper examines a 3-DOF SPM designed for inertial Line Of Sight (LOS) stabilization on moving platforms, such as ships affected by waves.

Kinematics and Design

The SPM's kinematics are thoroughly analyzed with a focus on ensuring that singular configurations—positions where control becomes problematic—are avoided within its prescribed workspace. Similarly, ensuring that these singularities are absent reassures that the manipulator will not face sudden losses of controllability. The SPM discussed in this article is distinguished by having coaxial input shafts, which allow unlimited bearing rotation. The SPM detailed includes three actuated links at the base with passive joints elsewhere, and its movement derives from the angular motion of these actuators. An overview of the design parameters and a mathematical representation of the configured workspace and kinematic relationships are provided.

Control Strategy

Stabilizing the LOS involves counteracting disturbances caused by the carrier platform's movement—achieved through an inertial measurement unit (IMU). Utilizing MATLAB Simulink with a SimMechanics™ model, the paper describes implementing a control strategy designed to stabilize the LOS. The control loop has three components: the system itself, the sensor measuring inertial operational speed, and the controller responsible for calculating the necessary commands for correction. A digital model of the SPM is also used for simulating its dynamics and determining torque requirements for stabilization tasks.

Results and Discussion

Simulation results show the controller's success in maintaining the orientation of the LOS against perturbations, attesting to the robustness of the control strategy. The findings demonstrate a fast and stable response to disturbances, which translates well to practical applications such as stabilizing cameras or sensors on maritime vessels. Furthermore, the manipulator's actuators move within reasonable bounds, substantiating the system's capability to operate effectively without imposing strain on its components.

Conclusion

The paper concludes with an affirmation of the 3-DOF SPM's capability for inertial LOS stabilization. However, it also acknowledges that while the kinematics are certified for the specified workspace, the controller's robustness, particularly under uncertainties or outside of the safe operational zone, has not been fully established. Future developments are suggested to enhance the control system's resilience and manage physical limits to maintain operations within a certified safe zone. Additionally, exploring the SPM's dynamics is proposed as an extension of this kinematic paper.