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1. Stability and Rendering Limitations of High-Performance Admittance Based Haptic Interfaces

   https://doi.org/10.1109/HAPTICS52432.2022.9765573  

   Dills, Patrick ; Gabardi, Kaitlyn ; Zinn, Michael ( March 2022 , 2022 IEEE Haptics Symposium (HAPTICS))

   This work investigates the stability and rendering limitations of admittance-type haptic devices. We investigated a wider range of impedances than had previously been considered, including stiffness, damping, mass, and combinations thereof. The coupled human driving impedance, actuator position control bandwidth, and loop delay are identified as major factors affecting the range of stable impedances. Finally, the theoretical results are experimentally verified using a custom one degree of freedom admittance type haptic device. 

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2. Transparent, High-Force, and High-Stiffness Control of Haptic Actuators with Backlash

   https://doi.org/10.1109/WHC56415.2023.10224471  

   Dills, Patrick ; Zinn, Michael ( July 2023 , IEEE)

   Haptic actuators employing speed reductions display desirable increased force capability but have difficulty producing feelings of free space motion due to friction and inertia magnification implicit to actuator dynamics. This work describes a control topology that enables geared haptic actuators to produce highly transparent free space motion when combined with backlash nonlinearities. While the presence of backlash enables the proposed free space motion control, it is also a source of instability, limit cycles, and to some extent rendering distortion. We introduce a smoothed gain scheduling function to mitigate limit cycling and expand the range of stable impedances that can be rendered. The introduction of a design metric called the free space envelope provides a framework to evaluate the effectiveness of the free space controller. Together these two control approaches enable transparent free space, high-force, and stable haptic interactions in systems with backlash, a characteristic common in many speed reducers. 

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3. A Hybrid Active-Passive Actuation and Control Approach for Kinesthetic Handheld Haptics

   https://doi.org/10.1109/HAPTICS45997.2020.ras.HAP20.12.af578b0a  

   Dills, Patrick ; Colonnese, Nick ; Agarwal, Priyanshu ; Zinn, Michael ( March 2020 , 2020 IEEE Haptics Symposium (HAPTICS))

   Handheld haptic devices are often limited in rendering capability, as compared to traditional grounded devices. Strenuous design criteria on weight, size, power consumption, and the ungrounded nature of handheld devices, can drive designers to prioritize actuator force or torque production over other components of dynamic range like bandwidth, transparency, and the range of stable impedances. Hybrid actuation, the use of passive and active actuators together, has the potential to increase the dynamic range of handheld haptic devices due to the large passive torque capability, the stabilizing effects of passive actuators, the high bandwidth of conventional DC servomotors, and the synergy between actuators. However, to date the use of hybrid actuation has been limited due to the highly nonlinear torque characteristics of available passive actuators that result in poor rendering accuracy. This paper describes a hybrid actuation approach and novel control topology which aims to solve actuation challenges associated with nonlinear passive actuators in hybrid and handheld haptic devices. The performance of the device is assessed experimentally, and the approach is compared to existing handheld devices. 

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4. Admittance Control of a Teleoperated Motorized Functional Electrical Stimulation Rehabilitation Cycle

   K. J. Stubbs, B. C. ( December 2020 , IFAC Conference on Cyber-Physical Human-Systems)

   null (Ed.)

   A bilateral teleoperated rehabilitation cycling system is developed for people with movement impairments due to various neurological disorders. A master hand-cycling device is used by the operator to set the desired position and cadence of a lower-body functional electrical stimulation (FES) controlled and motor assisted recumbent cycle. The master device also uses kinematic haptic feedback to reflect the lower-body cycle's dynamic response to the operator. To accommodate for the unknown nonlinear dynamics inherent to physical human machine interaction (pHMI), admittance controllers were developed to indirectly track desired interaction torques for both the haptic feedback device and the lower-body cycle. A robust position and cadence controller, which is only active within the regions of the crank cycle where FES produces sufficient torque values, was used to determine the FES intensity. A Lyapunov analysis is used to prove the robust FES controller yields global exponential tracking to the desired position and cadence set by the master device within FES stimulation regions. Outside of the FES regions, the admittance controllers at the hands and legs work in conjunction to produce desired performance. Both admittance controllers were analyzed for the entire crank cycle, and found to be input/output strictly passive and globally exponentially stable in the absence of human effort, despite the uncertain nonlinear dynamics. 

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5. Stability and Rendering Limitations of a Parallel Hybrid Active-Passive Haptic Interface

   https://doi.org/10.1109/HAPTICS52432.2022.9765621  

   Dills, Patrick ; Dawson-Elli, Alexander ; Gruben, Kreg ; Adamczyk, Peter ; Zinn, Michael ( March 2022 , IEEE Haptics Symposium 2022 (HAPTICS))

   Impedance based kinesthetic haptic devices have been a focus of study for many years. Factors such as delay and the dynamics of the device itself affect the stable rendering range of traditional active kinesthetic devices. A parallel hybrid actuation approach, which combines active energy supplying actuators and passive energy absorbing actuators into a single actuator, has recently been experimentally shown to increase the range of stable virtual stiffness a haptic device can achieve when compared to the active component of the actuator alone. This work presents both a stability and rendering range analysis that aims to identify the mechanisms and limitations by which parallel hybrid actuation increases the stable rendering range of virtual stiffness. Increases in actuator stability are analytically and experimentally shown to be linked to the stiffness of the passive actuator. 

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