Variable Impedance Controller for Improved Physical Human-Robot Interaction


Understanding of physical human-robot interaction (pHRI) has benefited many fields including industry, military, and rehabilitation. The human user’s safety is the main consideration of any human-robot system, and hence, guaranteeing the system’s stability is critical. One design approach that ensures safety and stability is impedance control which adds constant positive damping to the system at the interaction point. However, the addition of positive damping to the system can reduce the agility of the user and require additional user effort to overcome the damping. As a result, improving the agility/stability trade-off is an active research area within the field of pHRI. 

Invention Description
Researchers at Arizona State University have developed a variable impedance controller which dynamically modulates both the damping and stiffness components of a robotic impedance controller. This improves the agility/stability trade-off in coupled human-robot systems while also reducing the human user’s effort. 

The controller applies a range of robotic damping from negative to positive values based on the user’s intent of motion. In addition to modulating the robotic damping, the controller also determines the user’s directional intent, and then applies variable stiffness from zero to positive values along the calculated trajectory. 

To evaluate the controller, a study was designed for human subjects to perform a 2D target-reaching task while coupled with a wearable ankle robot. A positively damped condition was used as a control with which to compare the variable impedance condition. The position, speed, and muscle activation responses were then used to quantify the user’s stability, agility, and effort, respectively. Stability was quantified spatially and temporally, with both overshoot and time to stabilize showing no statistically significant difference between the two experimental conditions. Agility was quantified using mean and maximum speed, with both speed metrics increasing from the positive damping to variable impedance conditions by 29.8% and 59.9%, respectively. Effort was quantified by the overall and maximum muscle activation data, both of which showed about a 10% reduction in effort for all movement directions.

Potential Applications
•  Human-robot systems
•  Medical and rehabilitation
•  Military

Benefits and Advantages
•  Improves human agility while maintaining overall system stability
•  Reduces human effort 

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James Arnold Hyunglae Lee

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