Clark C. Davis

Antagonistic control of a tendon driven manipulator

Two antagonistic control algorithms are described. These algorithms are used to control manipulator links which are antagonistically driven by two actuators via tendons. They have been simulated and experimentally shown to produce better active and passive performance for an electric test system than control algorithms developed earlier. The algorithms use both positive and negative (push and pull) commands to be given to the actuators. Previous systems generated only pull commands, ensuring that tendons would not go slack and give rise to backlash and other problems. The proposed controllers allow push commands to the actuators but still do not allow tendons to go slack. Each actuator, in addition to being fed back its respective tendon force, is fed back both positive and negative manipulator joint torques. This feature allows both actuators to simultaneously respond to torque errors.<<ETX>>

Control strategies for tendon-driven manipulators

Two antagonist control algorithms are presented. These algorithms are used to control manipulator links antagonistically driven by two actuators via tendons. They have been simulated and experimentally shown to produce better active and passive performance for an electric test system than control algorithms developed earlier. There are two fundamental differences between the control algorithms and earlier ones. First, the new algorithms allow both positive and negative (push and pull) commands to be given to the actuators. Previous systems generated only pull commands, ensuring that tendons would not go slack and give rise to backlash and other problems. The new controllers allow push commands to the actuators, but still do not allow tendons to go slack. Second, each actuator, in addition to being fed back its respective tendon force, is fed back both positive nd negative manipulator joint torques. This feature allows both actuators to respond simultaneously to torque errors.<<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>>

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>>

An investigation of hydraulic actuator performance trade-offs using a generic model

The effect of the variation of parameters of system elements on the overall performance of a generic model of a hydraulic actuation system for a robot is investigated. Specifically, an examination is made of the effects on actuator performance of two issues: intrinsic compliance i.e. the physical compliance within the actuator itself, and independent control of the actuator valve areas (e.g. supply and return areas for hydraulic fluid chamber) versus control of actuator valves with fixed area relationships. Increasing intrinsic compliance in the actuator degrades response to controller commands but improves the ability of the actuator to tolerate disturbances. Independent control of valve areas provides better response to commands and better rejection of disturbances than control with valves that have fixed area relationships. The performance information provided by the model permits behavior-based design of hydraulic actuation systems.<<ETX>>

High stiffness and low slew drag in an antagonistic control system

An antagonistic controller which is to be used in tendon-driven master/slave end-effectors is described. High stiffness was achieved by developing a control law which eliminates one local force loop in the flexion/extension control loops. One force loop maintains tendon cocontraction, while the other force loop is eliminated when the tendon tension is greater than cocontraction. In order to minimize the input impedance or slew drag caused by inertia and intrinsic damping of the system, force compensations were implemented according to the characteristics of the actuators. These techniques have been implemented experimentally on both electromechanical and electrohydraulic systems. A computer analysis of an electromechanical antagonistic control system is included. Servostiffness equations are derived from the transfer function of the electromechanical system.<<ETX>>