Ian W. Hunter

Artificial muscle technology: physical principles and naval prospects

The increasing understanding of the advantages offered by fish and insect-like locomotion is creating a demand for muscle-like materials capable of mimicking natures mechanisms. Actuator materials that employ voltage, field, light, or temperature driven dimensional changes to produce forces and displacements are suggesting new approaches to propulsion and maneuverability. Fundamental properties of these new materials are presented, and examples of potential undersea applications are examined in order to assist those involved in device design and in actuator research to evaluate the current status and the developing potential of these artificial muscle technologies. Technologies described are based on newly explored materials developed over the past decade, and also on older materials whose properties are not widely known. The materials are dielectric elastomers, ferroelectric polymers, liquid crystal elastomers, thermal and ferroelectric shape memory alloys, ionic polymer/metal composites, conducting polymers, and carbon nanotubes. Relative merits and challenges associated with the artificial muscle technologies are elucidated in two case studies. A summary table provides a quick guide to all technologies that are discussed.

Time-varying stiffness of human elbow joint during cyclic voluntary movement

SummaryThe objective of this study was to determine the extent to which subjects modulate their elbow joint mechanical properties during ongoing arm movement. Small pseudo-random force disturbances were applied to the wrist with an airjet actuator while subjects executed large (1 rad) elbow joint movements. Using a lumped parameter model of the muscle, tendom and proprioceptive feedback dynamics, a time-varying system identification technique was developed to analyze the phasic changes in the elbow joints mechanical response. The mechanical properties were found to be time-varying, and well approximated by a quasi-linear second-order model. The stiffness of the arm was found to drop during movement. The arm was always underdamped, with the damping ratio changing during movement. Inertia estimates were constant and consistent with previous measurements. Overall, the moving arm was found to be very compliant, with a peak stiffness value less than the lowest value measured during posture, and a natural frequency of less than 3 Hz. Changing the speed of movement, or the load from gravity, changed the stiffness measured, but not in strict proportion to the change in net muscle torque.

A perceptual analysis of stiffness

SummaryThe perception of stiffness was studied in ten human subjects using two servo-controlled electromagnetic linear motors with computercontrolled stiffness, one motor coupled to each wrist of the subject. Using the contralateral limbmatching procedure in which subjects adjusted the stiffness of the motor connected to one (matching) arm until it was perceived to be the same as that connected to the other (reference) arm, a psychophysical function for stiffness was calculated. Eight different stiffness intensities were matched by subjects with five repetitions at each stimulus amplitude. The relation between the stiffness of the reference and matching motors was linear, and the accuracy with which subjects could match stiffness paralleled that reported previously for force and displacement. The Weber fraction for stiffness was 0.23 which is three times that reported for elbow flexion forces and forearm displacement. These findings were interpreted as indicating that subjects can perceive changes in the stiffness of mechanical devices used to effect action in the environment and that these perceptions are based on sensory signals conveying force and movement information.

Fast contracting polypyrrole actuators

Conducting polymer-based actuators are capable of producing at least 10 times more force for a given cross-sectional area active . stress than skeletal muscle, and potentially 1000 times more, with strains typically between 1% and 10%. Low operating voltages make them particularly attractive for use in micro-electromechanical systems, in place of electrostatic and piezoelectric actuators. A drawback of conducting polymer actuators is their relatively slow speed, and hence low power-to-mass ratio. In this paper, shaped voltage pulses are applied to generate strain rates of up to 3% s y1 , with peak power to mass ratios of 39 W kg y1 of polymer, nearly matching mammalian skeletal muscle. Results are obtained from polypyrrole linear and bilayer actuators and employ both liquid and gel electrolytes. q 2000 Elsevier Science S.A. All rights reserved.

The identification of nonlinear biological systems: LNL cascade models

Systems that can be represented by a cascade of a dynamic linear (L), a static nonlinear (N) and a dynamic linear (L) subsystem are considered. Various identification schemes that have been proposed for these LNL systems are critically reviewed with reference to the special problems that arise in the identification of nonlinear biological systems. A simulated LNL system is identified from limited duration input-output data using an iterative identification scheme.

A teleoperated microsurgical robot and associated virtual environment for eye surgery

We have developed a prototype teleoperated microsurgical robot (MSR-1) and associated virtual environment for eye surgery. Bidirectional pathways relay visual, auditory, and mechanical information between the MSR-1 master and slave. The surgeon wears a helmet (visual master) that is used to control the orientation of a stereo camera system (visual slave) observing the surgery. Images from the stereo camera system are relayed back to the helmet (or adjacent screen) where they are viewed by the surgeon. In each hand the surgeon holds a pseudotool (a shaft shaped like a microsurgical scalpel) that projects from the left and right limbs of a force reflecting interface (mechanical master). Movements of the left and right pseudotools cause corresponding movements (scaled down by 1 to 100 times) in the microsurgical tools held by the left and right limbs of the micromotion robot (mechanical slave) that performs the surgery. Forces exerted on the left and right limbs of the slave microsurgical robot via the microtools are reflected back (after being scaled up by 1 to 100 times) to the pseudotools and hence surgeon via actuators in the left and right limbs of the mechanical master. This system enables tissue cutting forces to be felt including those that would normally be imperceptible if they were transmitted directly to the surgeons hands. The master and slave subsystems (visual, auditory, and mechanical) communicate through a computer system which serves to enhance and augment images, filter hand tremor, perform coordinate transformations, and perform safety checks. The computer system consists of master and slave computers that communicate via an optical fiber connection. As a result, the MSR-1 master and slave may be located at different sites, which permits remote robotic microsurgery to become a reality. MSR-1 is being used as an experimental testbed for studying the effects of feedforward and feedback delays on remote surgery and is used in research on enhancing the accuracy and dexterity of microsurgeons by creating mechanical and visual telepresence.

System identification of human triceps surae stretch reflex dynamics

SummaryThe interpretation of stretch-evoked reflex responses is complicated by the fact that the pattern of response will depend upon both the underlying reflex mechanisms and the time course of the stretch used to evoke the response. The objective of the present study was to use engineering systems analysis techniques to identify the dynamics of the human triceps surae (TS) stretch reflex in terms of its impulse response by deconvolving the position input from the observed response.Five normal subjects were instructed to maintain a tonic contraction of (TS) while subjected to repeated, computer-generated, stochastic perturbations of ankle position. Position, torque and smoothed, rectified surface EMGs were recorded and ensemble averaged over 25 stimulus presentations.Linear impulse response functions describing the dynamic relation between ankle velocity and TS EMG were found to account for a significant amount of the observed EMG variance (mean 60%). However, the impulse responses were noisy and the predicted EMG was systematically smaller than the observed EMG during the dorsiflexing phases of displacement. These findings suggested that a direction dependent nonlinearity might be present. Consequently, impulse responses relating half-wave rectified velocity to TS EMG were computed and found to be less noisy and to account for significantly more variance (mean 74%) than the purely linear model.The undirectional, velocity-sensitive impulse response functions were dominated by a large peak at about 40 ms followed by a smaller period of reduced activity. This is consistent with its mediation by primary spindle afferents. Although the shape of the impulse response remained unchanged, its amplitude, which provides a measure of relative gain, varied systematically with the level of contraction and the displacement amplitude. Multiple regression analysis demonstrated that most of the variation in the impulse response amplitude could be attributed to proportional increases with level of contraction (measured by average EMG) and proportional decreases with displacement amplitude.

Effect of fatigue on force sensation

Recent experiments have suggested that a sense of effort can be separated from a sense of developed force or tension in muscular contractions. The evidence for this distinction was examined during submaximal fatiguing contractions. Subjects were required to maintain until maximal endurance a constant isometric force with their right, reference arm, and at 15-s intervals they estimated the magnitude of this force with a matching contraction of the contralateral arm. The matching force produced by the unfatigued limb was the measure of force sensation. Both force and the brachial biceps and triceps EMG were recorded from each arm. During the fatiguing contractions the matching force increased linearly as did the biceps EMG of the fatiguing muscle. The rate of increase was dependent on the level of force exerted. A linear relation between the reference arm EMG and the perceived force was observed, which suggested that the over-estimation of force was due to the increase in the excitatory input to the fatiguing muscle. These results provide support for a centrally mediated theory of force perception, and indicate that during fatigue subjects are unable to estimate accurately the force of contraction. Furthermore, they suggest that under those conditions a sense of tension is not distinguishable from a sense of effort.

Identification of time-varying biological systems from ensemble data (joint dynamics application)

The theory underlying a new method for the identification of time-varying systems is described. The method uses singular value decomposition to obtain least-squares estimates of time-varying impulse response functions from an ensemble of input-output realizations. No a priori assumptions regarding the system structure or form of the time-variation are required and there are few restrictions on the input signal. Simulation studies, using a model of time-varying joint dynamics, show that the method can track rapid changes in system dynamics accurately and is robust in the presence of output noise. An application of the method is demonstrated by using it to track dynamic ankle stiffness during a rapid, voluntary, isometric contraction. During the transient phase of the contraction, low-frequency ankle stiffness gain decreased in a manner which could not be described with the second-order model of joint dynamics often used under stationary conditions.<<ETX>>

Fast reversible NiTi fibers for use in microrobotics

The authors report the experimentally determined characteristics of NiTi fibers which have been modified using a preparation procedure in which the fibers were subjected to brief very large current pulses during forced stretching. The modified fibers contract and relax fast enough to be of use in microrobotics. The modified fibers generate a maximum extrapolated stress of 230 MN/m/sup 2/ and yield a peak measured power/mass approaching 50 kW/kg. The theory of a micro-actuator incorporating the modified fibers is presented.<<ETX>>