@article {679888,
	title = {Variable Damping Control for pHRI: Considering Stability, Agility, and Human Effort in Controlling Human Interactive Robots},
	journal = {IEEE Transactions on Human-Machine Systems (THMS). ( * : equally contributed 1st authors.)},
	year = {2021},
	abstract = {This article presents a multi-degree-of-freedom variable damping controller to manage the trade-off between stability and agility and to reduce user effort in physical human-robot interaction. The controller accounts for the human body{\textquoteright}s inherent impedance properties and applies a range of robotic damping from negative (energy injection) to positive (energy dissipation) values based on the user{\textquoteright}s intent of motion. To evaluate the effectiveness of the proposed controller in balancing the trade-off between stability/agility and reducing user effort, two studies are performed on both the human upper-extremity and lower-extremity to represent both industrial and rehabilitation applications of the proposed controller. These studies required subjects to perform a series of multidimensional target reaching tasks while the human user interacted with either the end-effector of a robotic arm for the upper-extremity study or a wearable ankle robot for the lower-extremity study. Stability, agility, and user effort are quantified by a variety of performance metrics. Stability is quantified by both overshoot and stabilization time. Mean and maximum speed are used to quantify agility. To quantify the user effort, both overall and maximum muscle activation, and mean and maximum root-mean-squared interaction force are calculated. The results of both the upper- and lower-extremity studies demonstrate that the controller is able to reduce user effort while increasing agility at a negligible cost to stability.},
	url = {https://doi.org/10.1109/THMS.2021.3090064},
	author = {Fatemeh Zahedi* and James Arnold* and Connor Phillips and Hyunglae Lee}
}
@article {672368,
	title = {Variable Impedance Control for pHRI: Impact on Stability, Agility, and Human Effort in Controlling a Wearable Ankle Robot},
	journal = {IEEE Robotics and Automation Letters},
	volume = {6},
	number = {2},
	year = {2021},
	pages = {2429-2436},
	abstract = {This letter introduces a variable impedance controller which dynamically modulates both its damping and stiffness to improve the trade-off between stability and agility in coupled human-robot systems and reduce the human user{\textquoteright}s effort. The controller applies a range of robotic damping from negative to positive values to either inject or dissipate energy based on the user{\textquoteright}s intent of motion. The controller also estimates the user{\textquoteright}s intent of direction and applies a variable stiffness torque to stabilize the user towards an estimated ideal trajectory. To evaluate the controller{\textquoteright}s ability to improve the stability/agility trade-off and reduce human effort, a study was designed for human subjects to perform a 2D target reaching task while coupled with a wearable ankle robot. A constant impedance condition was selected as a control with which to compare the variable impedance condition. The position, speed, and muscle activation responses were used to quantify the user{\textquoteright}s stability, agility, and effort, respectively. Stability was quantified spatially and temporally, with both overshoot and stabilization time showing no statistically significant difference between the two experimental conditions. Agility was quantified using mean and maximumspeed,with both increasing fromthe constant impedance to variable impedance condition by 29.8\% and 59.9\%, respectively. Effort was quantified by the overall and maximum muscle activation data, both of which showed a \~{}10\% reduction in effort. Overall, the study demonstrated the effectiveness of the variable impedance controller.},
	url = {https://doi.org/10.1109/LRA.2021.3062015},
	author = {James Arnold and Hyunglae Lee}
}
@proceedings {672370,
	title = {Effects of Variable Damping-Defined Environments on Mediolateral Ankle Stability and Agility},
	journal = {The 27th Congress of the International Society of Biomechanics (ISB 2019) / The 43rd Annual Meeting of the American Society of Biomechanics (ASB 2019)},
	year = {2019},
	address = {Calgary, Canada},
	author = {James Arnold and Eric Slovak and Hyunglae Lee}
}
@conference {672369,
	title = {Variable Damping Control of the Robotic Ankle Joint to Improve Trade-off between Performance and Stability},
	booktitle = {IEEE International Conference on Robotics and Automation},
	year = {2019},
	pages = {1699-1704},
	address = {Montreal, QC, Canada},
	abstract = {This paper presents a variable damping control strategy to improve trade-off between agility/performance and stability in the control of the ankle exoskeleton robot. Depending on the user{\textquoteright}s intent of movement, the proposed variable damping controller determines the robotic ankle damping from negative to positive damping values. The range of damping values is determined by incorporating the knowledge of human ankle damping in order to always secure stability of the ankle joint of the coupled human-robot system. To evaluate the effectiveness of the proposed controller, we performed a set of human experiments with three different robotic damping conditions: fixed positive damping, fixed negative damping, and variable damping. Comparison of the two fixed damping conditions confirmed that there exists a clear trade-off between ankle agility and stability. Further, analysis of the variable damping condition demonstrated that humans could get benefits of not only positive damping to stabilize the ankle but also negative damping to enhance the agility of ankle movement as necessary during dynamic ankle movement. On average, the variable damping condition improved the agility of ankle movement by 76\% and stability by 37\% compared to the constant positive damping condition and the constant negative damping condition, respectively. Outcomes of this study would allow us to design a robotic controller that significantly improves agility/performance of the human-robot system without compromising its coupled stability.},
	url = {https://doi.org/10.1109/ICRA.2019.8793869},
	author = {James Arnold and Harrison Hanzlick and Hyunglae Lee}
}
@proceedings {672371,
	title = {Effects of fixed and variable damping environments on ankle agility and stability},
	journal = {The 42nd Annual Meeting of the American Society of Biomechanics (ASB 2018)},
	year = {2018},
	note = {Finalist for the Undergraduate Student Poster Competition},
	address = {Rochester, Minnesota},
	author = {Harrison Hanzlick and James Arnold and Hyunglae Lee}
}
