John E. Wood

Quantitation of human shoulder anatomy for prosthetic arm control—II. Anatomy matrices

Part I presented mathematically continuous surfaces and origin-to-insertion centroidal trajectory data for the muscles of the human shoulder. Part II presents linear trajectory data for the same muscles, in addition to kinematic descriptions of the joints. Anatomy matrices for musculature, which convert muscle forces (as estimated by cutaneously monitored EMG signals) to moments, within a prosthetic arm controller, are developed for both the linear and non-linear (centroidal) data, and then compared. Graphical analyses of the muscle functions are also presented via computer-generated circle diagrams.

Quantitation of human shoulder anatomy for prosthetic arm control-I. Surface modelling

Anatomical data and models for the human shoulder musculo-skeletal system are developed with the intent of quantifying physiological subcomponents of a model-based multi-axis prosthetic limb control scheme which has heretofore been implemented empirically. Part I presents the controller formulation, the surface descriptions of the muscles (and bones), and the centroidal trajectory data of the muscles. The data partially quantify the muscle modelling components of the controller, and set the stage for the analysis of the force-to-moment anatomical conversion factors of Part II.

A design overview of an eccentric-motion electrostatic microactuator (the wobble motor)

Abstract A description is given of the design, fabrication and analysis of an electrostatically-driven micro-actuator in which a planetary-motion rotor rolls inside a cylindrically shaped stator cavity. The design has four primary advantages: (1) the motor geometry and the rolling motion enable very small gaps to be achieved, which are accurate and stable, and across which electrostatic forces act, leading to high forces on the rotor; (2) relative motion is achieved by rolling rather than sliding, thus obviating the concern over internal friction; (3) higher output torques can be traded for lower rotor speeds, due to immediate planetary reduction; and (4) the power output should be higher than for systems constructed using two-dimensional silicon fabrication approaches, since woble motor lengths are not limited by such fabrication methods. The stator segment recruitment logic can range from simple, open-loop stepping to full servo-controlled commutation using rotor position sensors. Two-dimensional analytical and finite-element simulations that estimate motor torque generated by electrostatic fields have been used to determine the influece of: (1) rotor and stator radii; (2) stator segment angular width and position with respect to the contact point; and (3) dielectric properties and dimensions (e.g., insulator thickness on rotor) of motor materials. Dynamic modelling is being used in the comparison of predicted and observed motor behavior, and for the study of the effects of stator segment recruitment logic. A number of eccentric-motion micromotors constructed via different fabrication techniques, have been operated. Electro-discharge machining (EDM) is the fabrication method of choice for the prototypes presently used for experimental studies. Typical rotor diameters for the EDM motor are about 500 μm, with lengths of 500 μm. Motor operation has been achieved with commutation rates in excess of 120 000 r.p.m.

The wobble motor: an electrostatic, planetary-armature, microactuator

A variety of micromotor concepts has been evaluated, with the wobble motor (WM) approach being one of those selected for extensive study. Various WM configurations have been analytically evaluated using finite-element methods and closed-form solutions. Important performance characteristics have been estimated such as stall torque and free speed, and alternate strategies for motor control have been examined. Five motor configurations and a variety of silicon-based and non-silicon-based fabrication techniques have been examined in detail. In exploratory exercises, motors have been fabricated using direct mechanical assembly, electro-discharge machining (EDM), cylindrical photolithographic etching, and coextrusion of metal and plastic. The EDM approach was selected as the motor alternative which could function as an experimental testbed. Fifteen EDM WMs have been constructed and utilized for experimental purposes. Experiments aimed at generating simple preliminary data have been conducted, and results compare reasonably to analytical studies.<<ETX>>

Fabrication of micro-structures using non-planar lithography (NPL)

The authors present specific nonplanar lithographic (NPL) techniques for use in fabricating both monolithic micromachines and microcomponents for use in larger systems. The emphasis is on the use of numerically controlled E-beam-based lithography, with the resist exposed over nonplanar surfaces. Previously, nonplanar, optical-mask-based approaches have been used to fabricate devices such as wobble motor rotors, but with less success than the NPL techniques due to depth-of-field problems. The specific focus is on etching cylindrically shaped metal structures which are either (1) homogeneous or (2) layered by successive deposition, masking, and etching. Structures on the order of 80 to 500 microns in diameter have been constructed of either solid metals or sputtered thin metallic layers on quartz shafts. A number of either deep or shallow patterns have been fabricated on and through the structures, with promising results. Examples include helices, longitudinal lines, holes, notches, flexures, barbs, alphanumeric characters, and electrostatic field emitting patterns for use in wobble motors. Efforts are now proceeding toward generating complete systems, including transducers and actuators for industrial and medical applications.<<ETX>>

The wobble motor: design, fabrication and testing of an eccentric-motion electrostatic microactuator

The basic characteristics and advantages of microelectromechanical systems, machines constructed of small moving subelements which have characteristic dimensions in the range of about 0.5 to 500 mu m, are reviewed; design problems are considered; and particular attention is given to the wobble motor (WM) developed by S.C. Jacobsen et al. (1989) using electrodischarge machining (EDM). Fifteen EDM WMs have been constructed and utilized for experimental purposes. WM explorations have demonstrated that micro systems can be quite easily constructed by means other than etching silicon. Experiments aimed at generating simple preliminary data have been conducted, and results compare reasonably to analytical studies. Results reported include stall torque measurements, speed measurements, torque-speed measurements, and wear measurements.<<ETX>>

Modelling considerations for electrostatic forces in electrostatic microactuators

Abstract Electrostatic force generation may offer distinct advantages over more familiar magnetostatics at size scales approaching microns. The fabrication of very small electrostatic actuators is becoming technologically feasible, but is extremely difficult, so that mathematical modelling of actuator designs is likely to be very important in the advancement of this technology. Modelling involves difficulties not only in finding solution (typically numerical) to a mathematical problem, but more important, it requires that the mathematical problem be well formulated. This in turn requires an understanding of, and intuition for, what electrostatic effects are likely to be revelant, as well as an appreciation for the behavior of materials in electrostatic interactions and for the impact on other machine components (bearing, loads, etc). The well-established lore of magnetostatics is not of much use as a guide in this task for several reasons: magnetic materials tend to be either highly permeable (i.e. ferromagnetic) or to have no magnetic effect. By contrast, there are no electrostatically inert materials; the relative dielectric constant e of any solid (of normal density) is of order two or greater, and thus any solid element of an electrostatic configuration has a significant influence on the field. Also, the sources of magnetic fields, currents or magnetization, can be specified with some confidence, while the sources of the electrostatic field, electric charge and polarization, are much more elusive and subject to change. It is the purpose of this paper to point out some of the effects that must be taken into account if a mathematical model is to give an adequate representation of the behavior of an actualy system. To do this we sketch a brief list of the types of electrostatic elements and interactions (conductors, dielectrics, compensated and uncompensated electrets, ferroelectrics, image forces, dielectrophretic forces, etc.) and use this list as background for discussing some electrostatic effects that may be important in the design or modelling of microactuators. For some of these effects, applicable results are reported from experimental investigations carried out with both a small (several hundred micron scale) electrostatically actuated device (‘SCOFSS’) built to study aspects of microelectromechanical design and of control via electrostatic actuation, and the ‘Wobble Motor’, a successful electrostatic microactuator.

Advanced intelligent mechanical sensors (AIMS)

A system of integrated, intelligent mechanical transducers, based on the microelectromechanical systems (MEMS) approach, is being developed. The FET-based detectors coexist in the silicon chip with other circuit elements which can amplify, digitize, and multiplex signals produced by detector/emitter interactions. Two transducer types being developed are used as examples: the rotary displacement transducer (RDT) and the uniaxial strain transducer (UAST). By appropriate arrangement of emitter and detector arrays, transducers are being designed to achieve desired tradeoffs between resolution, dynamic range, bandwidth, sensor population size, and other features. With detector and associated circuitry on the same chip, transducers are being designed to implement local intelligence necessary for functions such as self-calibration, correction for subsystems failure, and interaction with adjacent sensors.<<ETX>>

Model-Based, Multi-Muscle EMG Control of Upper-Extremity Prostheses

Mathematical modelling of natural limb motion and actuation can greatly facilitate understanding of the biomechanics and control of a human limb, and can be used in the design of controllers for multi-axis prosthetic arms or the design of functional neuro-stimulators (FNS) for paralyzed limbs. The motion of a human limb involves the simultaneous control of each muscle of the limb. This simultaneous control provides stability, linkage stiffness, and force balance of the entire limb system, if not the entire body, in addition to the primary action of the limb. This idea of synergy of the entire system suggests that a prosthetic limb or a neuro-stimulated paralyzed limb should not be considered as an autonomous system but rather as an integral, dynamically coupled part of the person. Such a control scheme should free functionally sound parts of the body from controlling or actuating prosthetic or stimulated limbs, thus minimizing the conscience effort on the part of the person.

Design, Analysis, And Experimental Results For The Wobble Motor: An Eccentric-Motion Electrostatic Microactuator

A description is given of the design, fabrication and analysis of an electrostatically-driven microactuator in which a planetary-motion rotor rolls inside a cylindrically shaped stator cavity. The design has four primary advantages: (1) the motor geometry and the rolling motion enable very small gaps which are accurate and stable, and across which electrostatic forces act, leading to high forces on the rotor, (2) relative motion is achieved by rolling rather than sliding, thus obviating the concern over internal friction, (3) higher output torques can be traded for lower rotor speeds, due to immediate planetary reduction, and (4) the power output should be higher than for systems constructed using two-dimensional silicon fabrication approaches, since wobble motor lengths are not limited by such fabrication methods. The stator segment recruitment logic can range from simple, open-loop stepping to full servo-controlled commutation using rotor position sensors. Two-dimensional analytical and finite-element simulations which estimate motor torque generated by electrostatic fields have been used to determine the influence of: (1) rotor and stator radii, (2) stator segment angular width and position with respect to the contact point, (3) stator segment voltage(s), and (4) dielectric properties and dimensions (e.g., insulator thickness on rotor) of motor materials. Dynamic models of motor behavior are also under development. A number of eccentric-motion micro motors, constructed via different fabrication techniques, have been operated. Electro-discharge machining (EDM) is the fabrication method of choice for the prototypes presently used for experimental studies. Typical rotor diameters for the EDM motor are about 500 microns, with lengths of 5,000 microns. Motor operation has been achieved with commutation rates in excess of 120,000 RPM.