Physical Human-Robot Interaction (pHRI)

Variable Damping Control of a Robotic Arm to Improve Trade-off between Agility and Stability and Reduce User Effort

This study presents a variable damping controller to improve the trade-off between agility and stability in physical human-robot interaction (pHRI), while reducing user effort. Variable robotic damping, defined as a dual-sided logistic function, was determined in real time throughout a range of negative to positive values based on the user's intent of movement. To evaluate the effectiveness of the proposed controller, we performed a set of human experiments with subjects interacting with the end-effector of a 7 degree-of-freedom robot. Twelve subjects completed target reaching tasks under three robotic damping conditions: fixed positive, fixed negative, and variable damping. On average, the variable damping controller significantly shortened the rise time by 22.4% compared to the fixed positive damping. It is also important to note that the rise time in the variable damping condition was as fast as that in the fixed negative damping condition and there was no statistical difference between the two conditions. The variable damping controller significantly decreased the percentage overshoot by 49.6% and shortened the settling time by 29.0% compared to the fixed negative damping. Both the maximum and mean root-mean-squared (RMS) interaction forces were significantly lower in the variable damping condition than the other two fixed damping conditions, i.e., the variable damping controller reduced user effort. The maximum and mean RMS interaction forces were at least 17.3% and 20.3% lower than any of the fixed damping conditions, respectively. The results of this study demonstrate that humans can extract the benefits of the variable damping controller in the context of pHRI, as it significantly improves the trade-off between agility and stability and reduces user effort in comparison to fixed damping controllers.

• T. Bitz, F. Zahedi, and H. Lee, IEEE International Conference on Robotics and Automation (ICRA 2020)

2D Variable Damping Control of the Robotic Ankle Joint to Improve Trade-off between Agility and Stability

This study introduces a two-dimensional (2D) variable damping controller for implementation in wearable robots. The controller applies a range of robotic damping between negative and positive to the coupled human-robot system. A wearable ankle robot was used to test this controller, and human experiments were performed to understand and quantify the effects of the controller. Within the study, human subjects performed target reaching tasks while the wearable ankle robot provided the system with constant positive, constant negative, or variable damping. These three damping conditions were then compared to analyze the performance of the system in terms of stability and agility. Stability was quantified both spatially and temporally. All metrics found an improvement in stability of variable damping as compared with negative damping, with the normal and tangential overshoot being reduced by 19.5% and 49.2%, respectively, and time to stabilize after reaching a target was 12.8% faster. Agility was quantified by evaluating the mean and maximum speeds achieved for each target reaching trial, with mean and maximum speeds found to be 22.2% and 67.9% higher, respectively, in variable damping than in positive damping. Overall, the study demonstrated that a variable damping controller can balance the trade-off between agility and stability in human-robot interactions and therefore has many practical implications.

Variable Damping Control of the Robotic Ankle Joint to Improve Trade-off between Performance and Stability

This study 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’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.

• J. Arnold, H. Hanzlick, and H. Lee, IEEE International Conference on Robotics and Automation (ICRA 2019)

Regulation of 2D Arm Stability against Unstable, Damping-Defined Environments in Physical Human-Robot Interaction

This paper presents an experimental study to investigate how humans interact with a robotic arm simulating primarily unstable, damping-defined, mechanical environments, and to quantify lower bounds of robotic damping that humans can stably interact with. Human subjects performed posture maintenance tasks while the robotic arm simulated a range of negative damping-defined environments and transiently perturbed the human arm to challenge postural stability. Analysis of 2-dimensional kinematic responses in both the time domain and phase space allowed us to evaluate stability of the coupled human-robot system in both anterior-posterior (AP) and medial-lateral (ML) directions, and to determine the lower bounds of robotic damping for stable physical human-robot interaction (pHRI). All subjects demonstrated higher capacity to stabilize their arm against negative damping-defined environments in the AP direction than the ML direction, evidenced by all 3 stability measures used in this study. Further, the lower bound of robotic damping for stable pHRI was more than 3.5 times lower in the AP direction than the ML direction: -30.0 Ns/m and -8.2 Ns/m in the AP and ML directions, respectively. Sensitivity analysis confirmed that the results in this study were relatively insensitive to varying experimental conditions. Outcomes of this study would allow us to design a less conservative robotic impedance controller that utilizes a wide range of robotic damping, including negative damping, and achieves more transparent and agile operations without compromising coupled stability and safety of the human-robot system, and thus improves the overall performance of pHRI.

• F. Zahedi, T. Bitz, C. Phillips, and H. Lee, IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS 2020)

Stability of the Human Ankle in Relation to Environmental Mechanics

This study presents quantification of multi-dimensional ankle stability in relation to mechanical environments having different levels of stability. This study, for the first time, explores the range of stiffness-defined haptic environments over which young healthy individuals can maintain stability despite aggressive perturbation. Ankle stability was quantified in 2 degree-of-freedom (DOF) of the ankle, in both the sagittal and frontal planes. Importantly, the magnitude of negative environmental stiffness that the subjects could maintain stability is 4 times as great in the sagittal plane as in the frontal plane. In addition to managing a wider range of unstable environments in the sagittal plane, subjects were also more efficient at regaining stability after perturbation and less sensitive to changes in the environmental stiffness. Outcomes of this study would be beneficial to the design and control of robots physically interacting with human lower extremities, such as lower-limb exoskeletons and powered ankle-foot orthoses.

• H. Hanzlick, H. Murphy, and H. Lee, IEEE International Conference on Robotics and Automation (ICRA 2017)