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Spherical motors capable of omnidirectional rotations in a ball joint provide a highly dexterous actuator system yet also present great challenges in developing a high-performance regulation and tracking controller due to the complex rotor dynamics. This article presents a complementary H2-H∞(C-H2-H∞) control method for precisely controlling the multidegree of freedom (DOF) orientation of spherical motors in presence of external disturbances as well as model uncertainties and inaccuracies. In order to deal with the tradeoff between the robustness and control performance resulted from conventional control methods, the proposed controller featured with a 2-DOF structure presents an H2 controller for a nominal plant and then complements it with an additional regulator designed in H∞sense to assure robustness. Unlike traditional H∞ or mixed H2and H∞ controller, the H∞regulation is conducted online in accordance with the monitored modeling mismatch in a way that does not adversely degrade the performance delivered by H2 control, hence, improving the conservativeness of the total control performance and leading to significant improvements of tracking accuracy and response time at same time. Both numerical simulations and experiments on a spherical motor testbed are conducted to validate the superior performance of the proposed controller versus conventional control methods. [ABSTRACT FROM AUTHOR]

This article presents a back-drivable spherical wrist with smart compliance for rendering human wrist-like capabilities in robotic applications. In order to synthetically impose both motion accuracy and active compliance for dedicated wrist manipulations, a robust hybrid control method is proposed that can switch between motion control (at free motion) and impedance control (at interactions with the environment). A sliding manifold is formulated so that when the control law drives the sliding manifold to zero, the motion/impedance errors will automatically converge to bounded regions despite system uncertainties/inaccuracies. Due to the direct-drive configuration of the proposed spherical wrist, the contact torque can be precisely estimated with an observer, providing an intuitive criterion of contact based on which the control modes can switch autonomously without external force/torque sensors and predefined geometric boundaries that are usually required in conventional hybrid force/position control. The concept is validated on a spherical wrist test-bed in the context of a throat swabbing process. The performance of the hybrid motion-impedance control evaluated against conventional impedance and admittance control methods demonstrate that the proposed spherical wrist enables a robotic system with both fine manipulations and smooth compliant interactions in wrist space. [ABSTRACT FROM AUTHOR]

This paper presents a new method, referred to here as back-EMF method, for sensing the multi-DOF motion of a permanent magnet spherical motor (PMSM). With an explicit model characterizing the magnetic flux in the PMSM, a relationship between the electromotive force (EMF) induced in the winding of electro-magnets and the motion of the rotor permanent magnets is derived; and closed-form solutions that solves for multi-DOF Euler angles and angular velocities of the rotor are presented. This method allows for simultaneous estimation of both quantities in real-time by only measuring the voltages across the electro-magnets. Requiring no installation of additional sensors or fixtures on the rotor, the back-EMF method retains the structural simplicity of the PMSM. This method has been experimentally investigated on a prototype PMSM; the estimated Euler angles and angular velocities compare favorably with measurements from a commercialized gyroscope. As an immediate application, the motion states acquired using the back-EMF method are used for parameter estimation of the PMSM. [ABSTRACT FROM AUTHOR]

This paper presents an alternative method utilizing real-time measurements of existing rotor magnetic field in the feedback loop for multi degree-of-freedom (DOF) orientation control of a permanent-magnet spherical motor (PMSM) characterized with redundant inputs. As the direct field-feedback control (DFC) system requires only measured magnetic fields, it eliminates the need of an external orientation sensing system and its major components can operate independently and permit parallel processing. The DFC method greatly reduces accumulated errors and time delay due to serial computations commonly encountered in existing methods that rely on orientation-dependent models for feedback control of a multi-DOF PMSM. In this paper, the method for determining the bijective relationship between the rotor orientation and measured magnetic field is presented, which enables the replacement of the orientation error by the magnetic field error in the control law. Using analytical magnetic field and torque models, the DFC design method is illustrated with two examples: a one-DOF motion system and a three-DOF PMSM; the latter has been experimentally implemented, for which an embedded multisensor system with connected bijective domains is designed. Excellent trajectory control results were obtained validating the concept feasibility of the DFC method for the PMSM. [ABSTRACT FROM PUBLISHER]

This paper presents an efficient method to adaptively determine the locations for taking measurements for automated calibration of electromagnetic systems using reconstructed magnetic fields. This coupled measurement- computation method solves the Laplace's equation with measured boundary conditions. Along with the formulation of two selection criteria (chord-height and data-spacing), an adaptive scanning algorithm has been developed, which bases four local measurements to determine the next measurement point. This adaptive method, which relaxes the assumption of approximately known structure, has been illustrated (with experimental verification) with three practical applications; electromagnetic velocity probe, electromagnetic flow-meter (EMF) and spherical motor. Comparisons against published data demonstrate that the adaptive measurement algorithm greatly reduces the number of measurements and shortens the scanning route length of all three applications without sacrificing the accuracy of the computed results. Dry calibration results of an EMF were also experimentally verified against test data obtained from a standard flow-rig confirming that a relative error of 0.2% can be achieved. This finding makes the cost-effective dry calibration a practical alternative to the conventional flow rig calibration. As demonstrated on a spherical motor, the flexibility to include a least-square curve fit offers a practical means to filter measurement noise. [ABSTRACT FROM AUTHOR]

Design and control of many electromagnetic actuators involve solving three dimensional (3D) magnetic fields of permanent magnets and/or electromagnets in the presence of magnetic conducting surfaces. This paper extends the distributed multipole (DMP) method, which offers compact but precise analysis for three dimensional magnetic fields in closed form, to account for the effects of magnetic conducting boundaries using the image method on the torque generated by electromagnetic actuators. We validate the proposed method referred to here as DMP-Image method by comparing the calculated torques against results computed by a finite element method (FEM). While two methods agree to within 5% in maximum torque, the DMP-image method takes less than 1% of the FEM computation time. Finally, we demonstrate the DMP-Image method to design a spherical motor in a class of multi-DOF actuators. While developed in the context of the multi-DOF actuators, the modeling methods presented in this paper are applicable to design of other PM-based actuators. [ABSTRACT FROM AUTHOR]

Abstract: A ball-joint-like spherical motor capable of offering smooth, continuous multi-DOF motion is presented as an alternative design for haptic applications. With a two-mode configuration, this device can be operated as a joystick manipulating a target in six degrees-of-freedom (DOF), and provides realistic force/torque feedback in real-time. Utilizing the magnetic field measurements, the orientation and the torque-to-current coefficients can be computed in parallel; this novel scheme greatly improves the sampling rate as well as reduces error accumulation commonly found in multi-DOF robotic devices. Of particular interest here is to explore this two-mode design in a computer-aided virtual environment. As an intuitive illustration, the disassembly process of a snap-fit (consisting of a typical cantilever hook and a wedge-shaped end) is simulated, where the two-mode permanent magnet spherical motor haptic device is incorporated as an interfacing device that receives motion commands from a virtual design environment and delivers torque feedback to the designer/user. [Copyright &y& Elsevier]

In order to facilitate a ball-joint-like spherical motor to operate with positioning systems to offer cascaded 6-DOF motion, a control method for precisely manipulating the spherical motor orientation in moving frame is proposed. By characterizing the cascaded motion with a pendulum-cart model, the rotor dynamics operated in moving frame is formulated. The properties of the dynamic model and the system uncertainties due to parameter inaccuracies of the cascaded system are analyzed. A terminal-sliding-mode control system is developed which incorporates a feedforward term to compensate for the effects of external motion inputs on the rotor orientation. The effectiveness of the proposed control method is experimentally validated on a cascaded platform consisting of a spherical motor and a XY-stage. The capabilities of chatter suppression and error convergence of the proposed control law are investigated, leading to a fine-tuning algorithm to find optimal control parameters based on a histogram analysis of tracking errors. The cascaded system with proposed control system is demonstrated in the context of a conformal printing task for curved electronics. [ABSTRACT FROM AUTHOR]