Kok-Meng Lee

Kinematic analysis of a three degrees of freedom in-parallel actuated manipulator

The dynamic analysis of a three-degrees-of-freedom in-parallel actuated manipulator is presented. The equations of motion have been formulated in joint-space using the Lagrangian approach. The analysis provides the solution to predict the forces required to actuate the links so that the manipulator follows a predetermined trajectory. A dynamic simulation program illustrates the influence of the link dynamics on the actuating force required. An example of tracing a helical path is chosen to illustrate the dynamic simulation and to show that the Cartesian position of the moving platform may be controlled at a sacrifice of orientation freedoms. The dynamic analysis provides a basis for future theoretical research to develop the control scheme, for experimental research to estimate the inertia parameters, and for design optimization of the prototype manipulator. >

Design concept development of a spherical stepper for robotic applications

The design concept of a spherical stepper motor capable of three-degrees-of-freedom (DOF) motion in a single joint is presented. The ball-joint-like motor has no singularities except at the boundary of the workspace and can perform isotropic manipulation in all three directions. Due to its relatively simple ball-like structure, undesired cross-coupling and centrifugal components of wrist rotor dynamics can be effectively minimized or eliminated. The spherical stepper motor has potential in robotic applications as a three-DOF shoulder or an eyeball, as well as a wrist actuator. In particular, the systematic conceptualization of a spherical stepper is presented, and the feasibility of constructing the spherical stepper is examined. Along with the experimental data, an analytical approach based on the permeance formula was used to predict the driving forces generated by a neodymium-iron permanent magnet. The force-displacement curves provide useful information for rational spherical motor design and control. >

A three-degrees-of-freedom micromotion in-parallel actuated manipulator

The development of a three-degree-of-freedom (DOF) micromotion in-parallel actuated manipulator is discussed. The micromotion manipulator, which has one translation and two orientation freedoms, is actuated by piezoelectric effect. A closed-form solution and an experimental verification of the forward kinematics are presented. In addition, the dynamic model of the piezoelectric actuated link was determined experimentally, providing a rational basis for the design and prismatic joint force control of the high-speed micromotion manipulator. A special configuration that approaches an optimal design, in terms of working range, rigidity, and bandwidth, is highlighted. >

Analytical and experimental investigation on the magnetic field and torque of a permanent magnet spherical actuator

This paper presents the torque model of a ball-joint-like three-degree-of-freedom (3-DOF) permanent magnet (PM) spherical actuator. This actuator features a ball-shaped rotor with multiple PM poles and a spherical stator with circumferential air-core coils. An analytical expression of the magnetic field of the rotor is obtained based on Laplaces equation. Based on this expression and properties of air-core stator coils, Lorentz force law is employed for the study of the relationship between the rotor torque and coil input currents. By using linear superposition, the expression of the actuator torque in terms of current input to the stator coils can be obtained in a matrix form. The linear expression of the actuator torque will facilitate real-time motion control of the actuator as a servo system. Experimental works are carried out to measure the actual magnetic field distribution of the PM rotor in three-dimensional (3-D) space as well as to measure the actual 3-D motor torque generated by the actuator coils. The measurement results were coincident with analytical study on the rotor magnetic field distribution and actuator torque expressions. The linearity and superposition of the actuator torque were also verified through the experiments

Development of a novel intelligent robotic manipulator

This paper describes the design features of a new robotic manipulator incorporating a novel spherical motor capable of three degrees of motion in a single joint for purposes of dexterous actuation, a loadable device at the end of the wrist actuator as the end effector with tactile and proximity sensing capabilities, and appropriate conventional and intelligent planning and control algorithms to support the execution of a series of complex tasks in an uncertain or hostile environment. The spherical wrist actuator is developed through analytic studies and the design of position and torque control instrumentation. The end effector is a micromanipulator based on the principle of in-parallel mechanisms. Heuristics, manifested in fuzzy logic, are employed to incorporate artificial intelligence for decision making and control in the robotic manipulator. The intent is to provide an overview of the significant design and algorithmic features of the manipulator deferring a detailed treatment of the proposed approaches to forthcoming publications.

Distributed Multipole Model for Design of Permanent-Magnet-Based Actuators

This paper presents a general method for deriving a closed-form solution for precise calculation of the magnetic field around a permanent magnet (PM) or an electromagnet (EM). The method, referred here as distributed multipole (DMP) modeling, inherits many advantages of the dipole model originally conceptualized in the context of physics, but provides an effective means to account for the shape and magnetization of the physical magnet. Three examples are given to illustrate the procedure of developing a DMP model, which derives an appropriate set of distributed dipoles from a limited set of known field points, for a general cylindrical PM, a customized PM, and a multilayer coil. The DMP modeling method has been validated by comparing simulated fields and calculated forces against data obtained experimentally and numerically; the comparisons show excellent agreement. Finally, we illustrate how the closed-form DMP models can offer an inexpensive means to visualize the effect of the EM fields on the leakage and unexpected flux paths, which have significant influences on the magnetic torque of a spherical motor.

A real-time optical sensor for simultaneous measurement of three-DOF motions

The need for simultaneous measurement of multiple degree-of-freedom (DOF) motions can be found in numerous applications such as robotic assembly, precision machining, optical tracking, wrist actuators, and active joysticks. Conventional single-axis encoders, though capable of providing high-resolution (linear or angular) measurements, rely on mechanical linkages (that often introduce frictions, backlashes, and singularities) to constrain the device so that the three-DOF (3-DOF) motion can be deduced from the individual orthogonal measurements. We present here a noncontact optical sensor for 3-DOF planar and spherical orientation measurements. We begin with the operational principle of a microscopic-surface-based optical sensor. The design concept and theory of a dual-sensor system capable of measuring 3-DOF planar and spherical motions in real time are then presented. Along with a detailed analysis, the concept feasibility of two prototype 3-DOF dual-sensor systems for measuring the instantaneous center of rotation and the angular displacement of a moving surface is demonstrated experimentally. It is expected that the analysis will serve as a basis for optimizing key design parameters that could significantly influence the sensor performance.

Effects of the torque model on the control of a VR spherical motor

This paper presents the effects of the torque model on the control of a variable reluctance spherical motor (VRSM) that offers several attractive features by combining multi-DOF motions in a single joint. A general form of the torque model for a VRSM is derived using the principle of energy conversion. The torque models for two specific design configurations developed at Georgia Tech are compared. The first has been based on an existing design characterized by a torque model in quadratic form. For feedback control of the spherical motor, the quadratic form of the torque model requires the use of nonlinear optimization schemes for computing the stator coil current inputs. The second design incorporating high coercive permanent magnets has a linear torque-current relationship and thus allows a closed form solution for both forward and inverse torque models. The effects of the torque model on a PD-controlled VRSM prototype has been studied both numerically and experimentally. Experimental results agree well with the computation derived analytically.

Open-Loop Controller Design and Dynamic Characteristics of a Spherical Wheel Motor

This paper presents a control system design for a particular form of variable-reluctance spherical motors, referred to here as a spherical wheel motor (SWM). The method decoupling the spin from the inclination offers a means to control, in open loop (OL), the inclination of a continuously rotating shaft. Specifically, the OL controller presented in this paper combines a multispeed switching control law for controlling the spin motion and a dynamic model-based control law for regulating the rotor inclination of an SWM. The concept feasibility of the OL-controlled SWM (consisting of permanent magnets in a rotor and electromagnets in a stator) has been experimentally demonstrated. The experimental study not only demonstrates the design procedure but also provides intuitive insights into the effects of key operation parameters on the SWM dynamics. The results presented here will serve as a basis for developing feedback controllers for increasing accuracy and robustness for disturbance rejection.

Distributed Multipole Models for Design and Control of PM Actuators and Sensors

Design and control of multi-degree-of-freedom (DOF) electromagnetic actuators require a good understanding of the magnetic fields, and involve real-time calculation of magnetic forces. This paper presents a method to derive distributed multipole (DMP) models for characterizing the magnetic field and torque of permanent magnet (PM) based devices. The DMP method, which offers magnetic-field solutions in closed form, inherits many advantages of the dipole model originally conceptualized in the context of physics, but provides an effective means to account for the shape and magnetization of the physical magnet. Three practical applications are given to demonstrate the DMP models for design of PM-based actuators and sensing systems. The magnetic fields and forces calculated using DMP models have been validated by comparing against numerical and experimental results which show excellent agreement.