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This paper describes a semi-powered ankle prosthesis and corresponding unified controller that provides biomimetic behavior for level and sloped walking without requiring identification of ground slope or modulation of control parameters. The controller is based on the observation that healthy individuals maintain an invariant external quasi-stiffness (spring like behavior between the shank and ground) when walking on level and sloped terrain. Emulating an invariant external quasi-stiffness requires an ankle that can vary the set-point (i.e., equilibrium angle) of the ankle stiffness. A semi-powered ankle prosthesis that incorporates a novel constant-volume power-asymmetric actuator was developed to provide this behavior, and the unified controller was implemented on it. The device and unified controller were assessed on three subjects with transtibial amputations while walking on inclines, level ground, and declines. Experimental results suggest that the prosthesis and accompanying controller can provide a consistent external quasi-stiffness similar to healthy subjects across all tested ground slopes.

This article describes the design of a new type of knee prosthesis called a stance-control swing-assist (SCSA) knee prosthesis. The device is motivated by the recognition that energetically passive stance-controlled microprocessor-controlled knees (SCMPKs) offer many desirable characteristics, such as quiet operation, low weight, high-impedance stance support, and an inertially driven swing-phase motion. Due to the latter, however, SCMPKs are also highly susceptible to swing-phase perturbations, which can increase the likelihood of falling. The SCSA prosthesis supplements the behavior of an SCMPK with a small motor that maintains the low output impedance of the SCMPK swing state, while adding a supplemental closed-loop controller around it. This article elaborates upon the motivation for the SCSA prosthesis, describes the design of a prosthesis prototype, and provides human-subject testing data that demonstrate potential device benefits relative to an SCMPK during both nonperturbed and perturbed walking.

This paper describes the design, control, and preliminary assessment of a novel microprocessor-controlled multifunction ankle prosthesis that provides three microprocessor-controlled behaviors—a selectable stiffness equilibrium angle, lockable conformal damping, and swing-phase repositioning. Following a description of the motivation for providing these behaviors, the authors provide a detailed description of the device and walking controller design. This device utilizes a power-asymmetric linear actuator to provide the desired functionality in a compact and lightweight package through a combination of both hydraulic and electromechanical actuation approaches. The device is controlled for level ground walking via a finite-state machine. The functionality of the prosthesis is demonstrated by a set of benchtop experiments that characterize the ability of the prosthesis to provide the three desired behaviors and by an experiment in which the prosthesis was worn by a transtibial amputee during walking. Both sets of experiments indicate that the prosthesis provides the functionality for which it was designed.

This paper presents a control approach for an overground lower limb exoskeleton that is intended to provide guidance and assistance to poorly ambulatory individuals during walking without unduly interfering with their ability to maintain balance. The control approach achieves these objectives by emulating a viscous flow field acting on the lower limb joints. The extent to which the control approach achieves the objectives was assessed in experiments, conducted on five healthy subjects, comparing guidance and disturbance characteristics of the velocity-based controller to a potential-field-based controller. Results show that the flow controller provides a combination of lower guidance error and lower disturbance to the user, relative to the potential-field-based controller. The paper also discusses various potentially beneficial characteristics of the flow controller, such as first-order homogeneous behavior, implicitly combined guidance and assistance behaviors, and improved directionality in error correction relative to a potential-field-based controller.

This paper presents a control system for a powered transtibial prosthesis that provides stair ascent and descent capability, as well as ability for user-controlled transitions between walking, standing, stair ascent, and stair descent. The control system was implemented on a powered prosthesis and evaluated on a single unilateral transtibial amputee subject. The ability of the prosthesis to provide appropriate functionality during stair ascent and descent was assessed by comparing gait kinematics and kinetics of the prosthesis to those of a passive dynamic elastic response prosthesis and those of a set of non-amputee subjects. Data from the assessment indicates that the powered prosthesis is able to provide some desirable stair ascent and stair descent characteristics, relative to the passive prosthesis.

This paper describes the development of a controller for a powered ankle prosthesis that is intended to provide appropriate biomechanical behavior for walking on both even and uneven terrain without having to explicitly detect local slope to do so. In order to inform development of the controller, the authors conducted a small study of five healthy subjects walking on even and uneven terrain. Data from the healthy subject study were used to formulate behavioral models for the healthy ankle, which were then implemented as controller behaviors in the powered prosthesis prototype and comparatively assessed on an amputee subject.

This paper describes a lower limb exoskeleton control approach that facilitates a desired movement coordination between the hip and knee joints during the swing phase of gait. An important feature of the proposed controller is that it provides movement guidance while allowing a user to control step time and step length. Specifically, control of step time is enabled by the use of time-invariant movement constraints, while control of step length is enabled by a real-time path-planning feature. As such, the controller provides movement coordination, but still enables a user to retain the step-to-step variability required to maintain balance during walking. The controller was implemented on a lower limb exoskeleton and tested on five healthy subjects. The subjects walked overground in the exoskeleton without a stability aid under two conditions: with the proposed coordination controller (i.e., with sagittal plane movement constraints) and with no control implemented (i.e., without sagittal plane movement constraints). Data from these assessments indicate that the controller provided substantial movement coordination while still allowing subjects substantive control of step time and length across a range of walking speeds.

This paper describes a transhumeral prosthesis prototype intended for the purpose of experimentally investigating design features and control strategies for the control of transhumeral prostheses. This paper specifically focuses on the design and performance characterization of a powered wrist rotator and powered elbow joint, in addition to the embedded system that controls them. In addition to outlining design objectives associated with the wrist and elbow joints, this paper describes the design of both joints, and the embedded system that provides control of them and the arm system. Experimental data are presented that characterizes the performance characteristics of both joints, including data associated with electrical power consumption and audible noise. The arm prosthesis described here is intended to be used with a multigrasp hand prosthesis, previously published by the authors.

This article describes a method to estimate the displacement of a pneumatic actuator based solely on pressure sensing at the control valve, which can enable low cost and remote sensing and fault detection in pneumatic actuation applications, such as in manufacturing. Pressure sensors located at the outlet ports of a pneumatic five-port two-way directional control valve are used in a generalized system model to estimate mass flow to the actuator. The approach enables observation of piston and rod displacement in real time and enables detection of motion faults without the use of displacement or proximity sensors on the actuator, which is typically located remotely from the control apparatus. The method was validated experimentally on two commercially available five-way pilot-assisted solenoid spool valves; each on two different double-acting, single rod, pneumatic linear actuators; and under different load and movement obstacle conditions. In the aggregate, the method provided a real-time estimate of actuator displacement within 7.1% error (relative to full stroke), and also provided a real-time mass flow estimate within 6.3% error. These real-time estimates are well within the accuracies suited to real-time fault detection in binary positioning systems, such as those typically used in manufacturing.

This paper presents the design of an anthropomorphic prosthetic hand that incorporates four motor units in a unique configuration to explicitly provide both precision and conformal grasp capability. The paper describes the design of the hand prosthesis, and additionally describes the design of an embedded control system located in the palm of the hand that enables self-contained control of hand movement. Following the design description, the paper provides experimental characterizations of hand performance, including digit force capability, bandwidth of digit movement, physical properties such as size and mass, and electrical power measurements during activities of daily living.

This paper presents a control approach for a lower-limb exoskeleton intended to facilitate recovery of walking in individuals with lower-extremity hemiparesis after stroke. The authors hypothesize that such recovery is facilitated by allowing the patient rather than the exoskeleton to provide movement coordination. As such, an assistive controller that provides walking assistance without dictating the spatiotemporal nature of joint movement is described here. Following a description of the control laws and finite state structure of the controller, the authors present the results of an experimental implementation and preliminary validation of the control approach, in which the control architecture was implemented on a lower limb exoskeleton, and the exoskeleton implemented in an experimental protocol on three subjects with hemiparesis following stroke. In a series of sessions in which each patient used the exoskeleton, all patients showed substantial single-session improvements in all measured gait outcomes, presumably as a result of using the assistive controller and exoskeleton.

This paper presents a running control architecture for a powered knee and ankle prosthesis that enables a transfemoral amputee to run with a biomechanically appropriate running gait and to intentionally transition between a walking and running gait. The control architecture consists firstly of a coordination level controller, which provides gait biomechanics representative of healthy running, and secondly of a gait selection controller that enables the user to intentionally transition between a running and walking gait. The running control architecture was implemented on a transfemoral prosthesis with powered knee and ankle joints, and the efficacy of the controller was assessed in a series of running trials with a transfemoral amputee subject. Specifically, treadmill trials were conducted to assess the extent to which the coordination controller provided a biomechanically appropriate running gait. Separate trials were conducted to assess the ability of the user to consistently and reliably transition between walking and running gaits.

This article describes the design and control of a powered knee and ankle prosthesis for transfemoral amputees. Following a description of the design hardware, a hybrid control approach that provides coordination for level walking is described. The hybrid control approach combines a piecewise-passive impedance-based component during the stance phase of gait with a high impedance trajectory-tracking component during the terminal stance and swing. To validate the design, the controller was implemented on the powered prosthesis prototype, and its ability to provide level walking functionality was evaluated on three transfemoral amputee subjects. The data presented from these experimental trials indicate that the prosthesis and control approach reproduce knee and ankle joint kinematic and kinetic features that are highly representative of corresponding healthy joint biomechanics.

This paper describes the mechanical design and control of a pneumatic quadrupedal robot. Additionally, the authors propose a method of joint control that combines stance/swing gain scheduling with open-loop damping, the combination of which provides stable joint level control, without the oscillatory behavior associated with pneumatically actuated walking robots. A set of joint trajectories are described to provide stable walking. The joint trajectories and joint-level controllers are implemented on the pneumatic quadruped and experimentally shown to provide stable walking without significant (unwanted) oscillations of the body or legs. Further, the robot's normalized speed and payload capacity are experimentally characterized. The robot's performance in these metrics is shown to be highly competitive within the published literature.

This paper presents a finite state-based control system for a powered transfemoral prosthesis that provides stair ascent and descent capability. The control system was implemented on a powered prosthesis and evaluated by a unilateral, transfemoral amputee subject. The ability of the powered prosthesis to provide stair ascent and descent capability was assessed by comparing the gait kinematics, as recorded by a motion capture system, with the kinematics provided by a passive prosthesis, in addition to those recorded from a set of healthy subjects. The results indicate that the powered prosthesis provides gait kinematics that are considerably more representative of healthy gait, relative to the passive prosthesis, for both stair ascent and descent.

The authors have previously described a method for enabling fully actuated biped walking without prescribing joint angle trajectories or imposing kinematic constraints between joints. This method was hypothesized to offer a more natural-looking bipedal gait and a higher locomotive efficiency relative to methods requiring accurate joint trajectory tracking. In this paper, the authors present experimental evidence to support both hypotheses. Specifically, the authors describe the design of a seven- link bipedal robot appropriate for the previously proposed control method; present the implementation of the "nonkinematic" control approach on the biped robot; demonstrate (with data, photo- graphic sequences, and video) the "relaxed" style of walking resulting from the control method; and experimentally characterize the locomotive efficiency of the biped in terms of the mechanical cost of transport. The latter results are compared to corresponding measures reported elsewhere in the literature.

This paper presents the design and preliminary experimental validation of a multigrasp myoelectric controller. The described method enables direct and proportional control of multigrasp prosthetic hand motion among nine characteristic postures using two surface electromyography electrodes. To assess the efficacy of the control method, five nonamputee subjects utilized the multigrasp myoelectric controller to command the motion of a virtual prosthesis between random sequences of target hand postures in a series of experimental trials. For comparison, the same subjects also utilized a data glove, worn on their native hand, to command the motion of the virtual prosthesis for similar sequences of target postures during each trial. The time required to transition from posture to posture and the percentage of correctly completed transitions were evaluated to characterize the ability to control the virtual prosthesis using each method. The average overall transition times across all subjects were found to be 1.49 and 0.81 s for the multigrasp myoelectric controller and the native hand, respectively. The average transition completion rates for both were found to be the same (99.2%). Supplemental videos demonstrate the virtual prosthesis experiments, as well as a preliminary hardware implementation.

This paper presents the design of an electrically actuated, proportional brake that provides a significantly greater torque-to-weight ratio than a magnetic particle brake (MPB) (considered a benchmark of the state of the art) without sacrificing other characteristics, such as dynamic range, bandwidth, or electrical power consumption. The multidisc brake provides resistive torque through a stack of friction discs, which are compressed by a dc-motor-driven ball screw. Unlike nearly all other proportional brakes, which operate in a normally unlocked mode, the brake presented here is designed such that it may be configured in either a normally unlocked or normally locked mode. The latter enables lower electrical energy consumption and added safety in the event of electrical power failure in certain applications. Following the device description, experimental data are presented to characterize the performance of the brake. The performance characteristics are subsequently compared to those of a commercially available MPB of comparable size.

This paper extends a previously developed level- ground walking control methodology to enable an above knee amputee to walk up slopes using a powered knee and ankle prosthesis. Experimental results corresponding to walking on level ground and two different slope angles (5 (°) and 10 (°)) with the powered prosthesis using the control method are compared to walking under the same conditions with a passive prosthesis. The data indicate that the powered prosthesis with the upslope walking controller is able to reproduce several kinematic characteristics of healthy upslope walking that the passive prosthesis does not (such as knee flexion after heel strike and a powered ankle plantarflexion during push-off). Finally, results are shown that demonstrate the ability of the prosthesis to generate a slope estimate, which is in turn utilized to adapt the underlying control parameters to the corresponding slope.

This paper presents a method for providing volitional control of a powered knee prosthesis during nonweight-bearing activity such as sitting. The method utilizes an impedance framework, such that the joint can be programmed with a given stiffness and damping that reflects the nominal impedance properties of an intact joint. Volitional movement of the knee joint is commanded via the stiffness set-point angle of the joint impedance, which is commanded by the user as a function of the measured surface electromyogram (EMG) from the hamstring and quadriceps muscles of the residual limb. Rather than using the respective EMG measurements from these muscles to directly command the flexion or extension set point of the knee, the presented approach utilizes a combination of quadratic discriminant analysis and principal component analysis to align the user's intent to flex or extend the knee joint with the pattern of measured EMG. The approach was implemented on three transfemoral amputees, and their ability to control knee movement was characterized by a set of knee joint trajectory tracking tasks. Each amputee subject also performed the same set of trajectory tracking tasks with his sound side (intact) knee joint. The average root mean square trajectory tracking errors of the prosthetic knee employing the EMG-based volitional control and the intact knee of the three subjects were 6.2° and 5.2°, respectively.

This paper presents an approach for the closed-loop control of a fully actuated biped robot that leverages its natural dynamics when walking. Rather than prescribing kinematic trajectories, the approach proposes a set of state-dependent torques, each of which can be constructed from a combination of low-gain spring-damper couples. Accordingly, the limb motion is determined by interaction of the passive control elements and the natural dynamics of the biped, rather than being dictated by a reference trajectory. In order to implement the proposed approach, the authors develop a model-based transformation from the control torques that are defined in a mixed reference frame to the actuator joint torques. The proposed approach is implemented in simulation on an anthropomorphic biped. The simulated biped is shown to converge to a stable, natural-looking walk from a variety of initial configurations. Based on these simulations, the mechanical cost of transport is computed and shown to be significantly lower than that of trajectory-tracking approaches to biped control, thus validating the ability of the proposed idea to provide efficient dynamic walking. Simulations further demonstrate walking at varying speeds and on varying ground slopes. Finally, controller robustness is demonstrated with respect to forward and backward push-type disturbances and with respect to uncertainty in model parameters.

This paper presents the design of a gas-actuated anthropomorphic arm prosthesis with 21 degrees of freedom and nine independent actuators. The prosthesis utilizes the monopropellant hydrogen peroxide as a gas generator to power nine pneumatic type actuators. Of the nine independent actuators, one provides direct- drive actuation of the elbow, three provide direct-drive actuation for the wrist, and the remaining five actuate an underactuated 17 degree of freedom hand. This paper describes the design of the prosthesis, including the design of small-scale high-performance servovalves, which enable the implementation of the monopropellant concept in a transhumeral prosthesis. Experimental results are given characterizing both the servovalve performance and the force and/or motion control of various joints under closed-loop control.

This paper describes a direct-injection configuration of a monopropellant-powered actuator that is intended to provide high-energy-density actuation for a self-powered position- or force-controlled human-scale robot. The proposed actuator is pressurized by a pair of solenoid injection valves (each of which control the flow of a monopropellant through a catalyst pack and directly into the respective side of a pneumatic-type cylinder), and depressurized via a three-way hot-gas proportional exhaust valve. A controller is described that coordinates the control of the two solenoid propellant injection valves, together with the control of the proportional hot-gas exhaust valve, in order to provide actuator force tracking. Experimental results are presented that validate the effectiveness of the force-control approach. Finally, energetic performance of the proposed actuator is experimentally assessed and shown to provide an energetic figure of merit, an order of magnitude greater than that of a battery-powered servomotor approach

The stable simulation of high-stiffness surfaces remains a challenge in impedance-type haptic simulations of mechanical environments. In this paper, the authors propose an approach to achieving a stable, high-stiffness surface in a haptic interface by leveraging the open-loop properties of pneumatic actuators. By using the open-loop component of the actuator stiffness as a primary component of stiffness simulation in a haptic interface, the system requires a comparatively small component of simulated stiffness from the closed-loop control of the actuator. A passivity analysis is presented describing how the presence of an open-loop stiffness enhances the range of passivity of haptically simulated high-stiffness surfaces. Experimental results both with and without a human operator are presented that demonstrate the effectiveness of the approach and its enhanced passivity relative to motor-actuated devices.

This paper describes the design and energetic characterization of an actuator designed to provide enhanced system energy and power density for self-powered robots. The proposed actuator is similar to a typical compressible gas fluid-powered actuator, but pressurizes the respective cylinder chambers via a pair of proportional injector valves, which control the flow of a liquid monopropellant through a pair of catalyst packs and into the respective sides of the double-acting cylinder. This paper describes the design of the proportional injection valves and describes the structure of a force controller for the actuator. Finally, an energetic characterization of the actuator shows improvement relative to prior configurations and marked improvement relative to state-of-the-art batteries and motors.

This paper presents and experimentally demonstrates a control methodology that provides transparency and stability robustness in bilateral telemanipulator systems. The approach is based upon a previously published method that structures the human-manipulators-environment system in a manner that enables the application of frequency-domain loop-shaping methods. This paper reformulates the human-manipulator interaction described in the previously published work, and experimentally demonstrates the approach on a single degree-of-freedom telemanipulation system. Experimental measurements indicate significant improvements offered by the method in both the stability robustness and transparency of the human-manipulators-environment system. Finally, experimental results are presented that demonstrate the robustness in the transparency to significant changes in the environment dynamics.

This brief presents a quantitative study of the effects of virtual surface stiffness in a simulated haptic environment on the haptic perception of detail. Specifically, the haptic perception of detail is characterized by identification, detection, and discrimination of round and square cross section ridges. Test results indicate that performance, measured as a percent correct score in the perception experiments, improves in a nonlinear fashion as the maximum level of virtual surface stiffness in the simulation increases. Further, test subjects appeared to reach a limit in their perception capabilities at maximum stiffness levels of 300 to 400 N/m, while the hardware was capable of 1000 N/m of maximum virtual surface stiffness. These results indicate that haptic interface hardware may be able to convey sufficient perceptual information to the user with relatively low levels of virtual surface stiffness.

This paper describes the design of a power supply and actuation system appropriate for position or force controlled human-scale robots. The proposed approach utilizes a liquid monopropellant to generate hot gas, which is utilized to power a pneumatic-type actuation system. A prototype of the actuation system is described, and closed-loop tracking data are shown, which demonstrate good motion control. Experiments to characterize the energetic performance of a six-degree-of-freedom actuation system indicate that the proposed system with a diluted propellant offers an energetic figure of merit five times greater than battery-powered DC motors. Projections based on these experiments indicate that the same system powered by undiluted propellant would offer an energetic figure of merit in an order of magnitude greater than a comparable battery-powered DC motor actuated system.

This paper presents a quantitative study of the effects of maximum capable force magnitude of a haptic interface on the haptic perception of detail. Specifically, the haptic perception of detail is characterized by identification, detection, and discrimination of round and square cross-section ridges, in addition to corner detection tests. Test results indicate that performance, measured as a percent correct score in the perception experiments, improves in a nonlinear fashion as the maximum allowable level of force in the simulation increases. Further, all test subjects appeared to reach a limit in their perception capabilities at maximum-force output levels of 3-4 N, while the hardware was capable of 10 N of maximum continuous force output. These results indicate that haptic interface hardware may be able to convey sufficient perceptual information to the user with relatively low levels of force feedback. The data is compiled to aid those who wish to design a stylus-type haptic interface to meet certain requirements for the display of physical detail within a haptic simulation.

Presents the design of a piezoelectric-ceramic (PZT)-stack-actuated brake that is similar to a magnetic particle brake in dynamic range, size, weight, and cost, while providing a significantly larger bandwidth and requiring significantly less electrical power for a given continuous torque output. The device is essentially a single-pad drum brake that is actuated with a PZT-stack actuator. A significant component of the design is the compliant-mechanism-based transmission utilized to transmit the PZT-stack actuation into brake pad motion. Following the device description, experimental data is presented to characterize the performance of the brake. The performance characteristics are subsequently compared to those of a commercially available magnetic particle brake of comparable size and weight.

A nonlinear lumped-parameter model of a piezoelectric stack actuator has been developed to describe actuator behavior for purposes of control system analysis and design, and, in particular, for microrobotic applications requiring accurate position and/or force control. In formulating this model, the authors propose a generalized Maxwell resistive capacitor as a lumped-parameter causal representation of rate-independent hysteresis. Model formulation is validated by comparing results of numerical simulations to experimental data. Validation is followed by a discussion of model implications for purposes of actuator control.