Peter B. Shull

Rotational Skin Stretch Feedback: A Wearable Haptic Display for Motion

We present a wearable haptic feedback device that imparts rotational skin stretch to the hairy skin, along with the results of psychophysical tests to determine its resolution and accuracy for motion display. Tracking experiments with visual markers reveal the pattern of skin motion and strain imparted by the device, confirming subjective impressions that the design represents a trade-off between perception at low stimulus levels and comfort at maximum stimulus levels. In an isolated environment, users were able to discriminate between different rotational displacements of stretch within two to five degrees, depending on the reference stimulus. In a more realistic setting, subjects were able to use feedback from the device to control the positioning of a virtual object within six degrees or ±6.5 degrees of the total range of motion. When subjects were passive and exposed to arbitrary rotations of the device, the accuracy was poorer, although it improved with training. The results suggest that wearable skin stretch devices can be an effective means of providing feedback about a users controlled joint or limb motions for motion training and similar applications.

Haptic wearables as sensory replacement, sensory augmentation and trainer – a review

Sensory impairments decrease quality of life and can slow or hinder rehabilitation. Small, computationally powerful electronics have enabled the recent development of wearable systems aimed to improve function for individuals with sensory impairments. The purpose of this review is to synthesize current haptic wearable research for clinical applications involving sensory impairments. We define haptic wearables as untethered, ungrounded body worn devices that interact with skin directly or through clothing and can be used in natural environments outside a laboratory. Results of this review are categorized by degree of sensory impairment. Total impairment, such as in an amputee, blind, or deaf individual, involves haptics acting as sensory replacement; partial impairment, as is common in rehabilitation, involves haptics as sensory augmentation; and no impairment involves haptics as trainer. This review found that wearable haptic devices improved function for a variety of clinical applications including: rehabilitation, prosthetics, vestibular loss, osteoarthritis, vision loss and hearing loss. Future haptic wearables development should focus on clinical needs, intuitive and multimodal haptic displays, low energy demands, and biomechanical compliance for long-term usage.

Haptic gait retraining for knee osteoarthritis treatment

In this paper we introduce haptic gait retraining as a new method for treating early stage medial compartment knee osteoarthritis and for reducing risk of the disease in individuals who may be susceptible. The hardware and software for implementation are presented including rotational skin stretch and vibration haptic devices used to inform subjects of alterations in gait movements. We also present a method based on real-time motion analysis for predicting new subject-specific gaits tailored to change knee joint loading. This approach uses correlation data between gait parameters and knee loading as well as a localized linearization technique to compute a final combined-parameter gait with minimum change from the subjects original, unaltered gait. Finally, we validate the haptic gait retraining system with a user experiment and show that, for the duration of the experiment, the user is able to positively change knee joint loading to approximately the same degree as HTO surgery.

Validity of FitBit, Jawbone UP, Nike+ and other wearable devices for level and stair walking

BACKGROUND Increased physical activity can provide numerous health benefits. The relationship between physical activity and health assumes reliable activity measurements including step count and distance traveled. This study assessed step count and distance accuracy for Nike+ FuelBand, Jawbone UP 24, Fitbit One, Fitbit Flex, Fitbit Zip, Garmin Vivofit, Yamax CW-701, and Omron HJ-321 during level, upstairs, and downstairs walking in healthy adults. METHODS Forty subjects walked on flat ground (400m), upstairs (176 steps), and downstairs (176 steps), and a subset of 10 subjects performed treadmill walking trials to assess the influence of walking speed on accuracy. Activity monitor measured step count and distance values were compared with actual step count (determined from video recordings) and distance to determine accuracy. RESULTS For level walking, step count errors in Yamax CW-701, Fitbit Zip, Fitbit One, Omron HJ-321, and Jawbone UP 24 were within 1% and distance errors in Fitbit Zip and Yamax CW-701 were within 5%. Garmin Vivofit and Omron HJ-321 were the most accurate in estimating step count for stairs with errors less than 4%. An important finding is that all activity monitors overestimated distance for stair walking by at least 45%. CONCLUSION In general, there were not accuracy differences among activity monitors for stair walking. Accuracy did not change between moderate and fast walking speeds, though slow walking increased errors for some activity monitors. Nike+ FuelBand was the least accurate step count estimator during all walking tasks. Caution should be taken when interpreting step count and distance estimates for activities involving stairs.

Informing haptic feedback design for gait retraining

Gait retraining, a promising treatment for knee osteoarthritis, requires the modification of three separate joint motions. In this paper we present the results of three studies to inform the design of a wearable haptic feedback system for this application. The first study motivates our choice of feedback modality for each of the motions. The latter two studies explore how to present haptic feedback to train three different motions concurrently. When feedback is presented simultaneously, subjects have poor perception of three or more haptic cues, tend to focus on only one motion at a time, and require several steps to modify all three motions. These findings suggest that vibrational feedback should be presented one joint at a time for haptic gait retraining.

Novel Foot Progression Angle Algorithm Estimation via Foot-Worn, Magneto-Inertial Sensing

Objective: The foot progression angle (FPA) is an important clinical measurement but currently can only be computed while walking in a laboratory with a marker-based motion capture system. This paper proposes a novel FPA estimation algorithm based on a single integrated sensor unit, consisting of an accelerometer, gyroscope, and magnetometer, worn on the foot. Methods: The algorithm introduces a real-time heading vector with a complementary filter and utilizes a gradient descent method and zero-velocity update correction. Validation testing was performed by comparing FPA estimation from the wearable sensor with the standard FPAs computed from a marker-based motion capture system. Subjects performed nine walking trials of 2.5 min each on a treadmill. During each trial, subjects walked at one speed out of three options (1.0, 1.2, and 1.4 m/s) and walked with one gait pattern out of three options (normal, toe-in, and toe-out). Results: The algorithm estimated FPA to within 0.2° of error or less for each walking conditions. Conclusion: A novel FPA algorithm has been introduced and described based on a single foot-worn sensor unit, and validation testing showed that FPA estimation was accurate for different walking speeds and foot angles. Significance: This study enables future wearable systems gait research to assess or train walking patterns outside a laboratory setting in natural walking environments.

Somatotopical feedback versus non-somatotopical feedback for phantom digit sensation on amputees using electrotactile stimulation

BackgroundTranscutaneous electrical stimulation can provide amputees with tactile feedback for better manipulating an advanced prosthesis. In general, there are two ways to transfer the stimulus to the skin: somatotopical feedback (SF) that stimulates the phantom digit somatotopy on the stump and non-somatotopical feedback (NF) that stimulates other positions on the human body.MethodsTo investigate the difference between SF and NF, electrotactile experiments were conducted on seven amputees. Electrical stimulation was applied via a complete phantom map to the residual limb (SF) and to the upper arm (NF) separately. The behavior results of discrimination accuracy and response time were used to examine: 1) performance differences between SF and NF for discriminating position, type and strength of tactile feedback; 2) performance differences between SF and NF for one channel (1C), three channels (3C), and five channels (5C). NASA-TLX standardized testing was used to determine differences in mental workload between SF and NF.ResultsThe grand-averaged discrimination accuracy for SF was 6% higher than NF, and the average response time for SF was 600 ms faster than NF. SF is better than NF for position, type, strength, and the overall modality regarding both accuracy and response time except for 1C modality (p<0.001). Among the six modalities of stimulation channels, performance of 1C/SF was the best, which was similar to that of 1C/NF and 3C/SF; performance of 3C/NF was similar to that of 5C/SF; performance of 5C/NF was the worst. NASA-TLX scores indicated that mental workload increased as the number of stimulation channels increased.ConclusionsWe quantified the difference between SF and NF, and the influence of different number of stimulation channels. SF was better than NF in general, but the practical issues such as the limited area of stumps could constrain the use of SF. We found that more channels increased the amount and richness of information to the amputee while fewer channels resulted in higher performance, and thus the 3C/SF modality was a good compromise. Based on this study, we provide possible solutions to the practical problems involving the implementation of tactile feedback for amputees. These results are expected to promote the application of SF and NF tactile feedback for amputees in the future.

Skin nonlinearities and their effect on user perception for rotational skin stretch

Given that skin is a nonlinear, anisotropic and viscoelastic material, we are interested in exploring the relationship between skin mechanical properties and skin stretch perception, for the case of rotational displacements applied to the skin on a users limbs. Studies were conducted with 10 naive subjects first to characterize the nonlinear stiffness of the skin with respect to applied rotations and subsequently to characterize the skin viscoelastic response and hysteresis. Despite substantial subject-to-subject variability, when results are normalized by each subjects maximum torque, the torque/displacement results are fairly consistent across subjects, at low and high speeds, and can be fit with a third order polynomial. For roughly one half of the subjects, a similar nonlinearity is discernible in the perceived versus actual rotation; other subjects produced nearly linear results across the range of positive and negative rotations. Viscoelastic and hysteresis effects in the skin were also evident. However, while subjects can clearly distinguish between slow and rapid movements, the speed of the applied motion does not significantly affect their perception of rotation.

Evaluation of a laboratory model of human head impact biomechanics

This work describes methodology for evaluating laboratory models of head impact biomechanics. Using this methodology, we investigated: how closely does twin-wire drop testing model head rotation in American football impacts? Head rotation is believed to cause mild traumatic brain injury (mTBI) but helmet safety standards only model head translations believed to cause severe TBI. It is unknown whether laboratory head impact models in safety standards, like twin-wire drop testing, reproduce six degree-of-freedom (6DOF) head impact biomechanics that may cause mTBI. We compared 6DOF measurements of 421 American football head impacts to twin-wire drop tests at impact sites and velocities weighted to represent typical field exposure. The highest rotational velocities produced by drop testing were the 74th percentile of non-injury field impacts. For a given translational acceleration level, drop testing underestimated field rotational acceleration by 46% and rotational velocity by 72%. Primary rotational acceleration frequencies were much larger in drop tests (~100 Hz) than field impacts (~10 Hz). Drop testing was physically unable to produce acceleration directions common in field impacts. Initial conditions of a single field impact were highly resolved in stereo high-speed video and reconstructed in a drop test. Reconstruction results reflected aggregate trends of lower amplitude rotational velocity and higher frequency rotational acceleration in drop testing, apparently due to twin-wire constraints and the absence of a neck. These results suggest twin-wire drop testing is limited in modeling head rotation during impact, and motivate continued evaluation of head impact models to ensure helmets are tested under conditions that may cause mTBI.

Presenting spatial tactile messages with a hand-held device

This paper introduces a multi-actuator tactile device designed for remote touch communication. While closely-spaced high-frequency vibrotactile actuators can be difficult to distinguish, our system utilized four linear DC motors for presenting spatial tactile messages through low-frequency actuation. An experiment was conducted to determine accuracy for recognizing stimuli presented on the palm of the hand. Participants were asked to identify 10 predefined stimulus patterns created from the four linear actuators positioned in either a diamond or square configuration. Results showed that positional, linear, and circular stimuli were recognized with mean response accuracies of 98.8, 96.5, and 90.2 %, respectively. No statistically significant differences were found between the actuator configurations. These findings can be utilized in developing a remote communication channel that supports the transfer of spatial aspects of touch such as mapping the location of finger touch of one user to tactile sensation on the palm of another user.