Stable Haptic Interaction: Time Domain Passivity Approach

Time domain passivity approach

Stable interaction with high stiffness virtual environments (VEs) still remains a challenging issue for kinesthetic haptic devices with impedance causality. In particular, it has been recognized that the maximum achievable impedance with the traditional digital control loop is limited by the lack of information to the controller caused by time discretization, position quantization, related to the use of encoder as a position sensor, and zeroth-order hold (ZOH) of force command during each servo cycle. These lead to energy leak and eventually instability if not dissipated through the intrinsic friction of the device, controller, or damping from the user’s grasp.

BioRobotics Lab has performed several studies to guarantee stable haptic interaction with high stiffness VEs. As an initial effort, BioRobotics Lab has proposed an energy-based approach, the so-called time-domain passivity approach (TDPA), which injects an adaptive virtual damping to satisfy the time-domain passivity constraint.

Stable Haptic Interaction: Memory-based Passivity Approach

Memory-based approach

Inspired from the graphical interpretation of a passivity in position vs. force x-y graph, we propose a new concept of passivation method to increase the dynamic range of impedance in which a haptic interface can passively interact. The position vs. force graph represents the energy behavior in a one-port haptic interface while the haptic interface is traveling in (pressed) or out of (released) a Virtual Environment (VE). It interprets that to make the one-port system passive, it is sufficient to bind the releasing path below the pressing path in the position vs. force graph. To realize the bound, the computed force output from the VE is saved into a designated memory, addressed by current position, while the haptic interface is pressed. Thereafter, the saved force can be reused to upper-bound the releasing path below the saved pressing path while the haptic interface is released.

Stable Haptic Interaction: Less Conservative Approach Is Coming

Passivity has been a major criterion on designing a stable haptic interface due to many advantages. However, passivity has been suffering from its intrinsic conservatism since it only represents a small set of the whole stable region. Therefore, there was always limitation to increase the performance due to the small design margin from the passivity criterion. We are currently working on less conservative approach for stable haptic interaction.

Master Device for Intuitive Teleoperation

Intuitive master device

In teleoperation, a master device should not only allow human operator to easily give command to a slave robot with an intuitive manner, but also improve the operators’ perception of the target slave and its environment by enabling richer interaction feel. Our lab is interested in developing nonconventional master devices for different teleoperation areas. one of the early works from our lab is developing a novel tele driving interface for wheeled and tracked vehicles. Currently, we are developing a novel master device for underwater vehicles and manipulators.

Mobile robot teleoperation

Mobile robot teleoperation

Mobile robot teleoperation system with obstacle based force feedback is different from classical master-slave manipulator bilateral teleoperation system in a sense that a slave robot interacting with environment is directly affected by environmental forces. The second difference of the mobile robot teleoperation system from classical teleoperation system is the rate mode control which is not often used in manipulator teleoperation, while the rate mode control is essential for mobile robot teleoperation due to its kinematic properties.

Our lab proposed several methodologies to improve the performance in mobile robot teleoperation such as position-velocity switching command strategy and velocity dependent adaptive feedback gain scheduling method.

Improving Teleoperation Performance: Surgical Robot Teleoperation

Surgical robot

As MIS (Minimal Invasive Surgery) growing rapidly, new and more complicated surgical instruments are developed to make the tool reliable and decrease the incision area as small as possible. However, this increase the surgeon’s mental burden for properly tele-operating the complex surgical robot systems. Our lab is developing a teleoperation method to reduce the surgeon’s mental workload considering ergonomics.

*Courtesy of da Vinci Surgery.