Jean-Philippe Lucking Bigué

In the last decade, the development of simple and easy-to-use robotics has provided a significant step towards the democratization of collaborative robots, which can now work closely alongside humans. However, state-of-the-art cobots still provide safety at the cost of work-speed, which is purposely limited to avoid high-energy collisions. In order to increase safety and work speed simultaneously, force/impedance-controlled actuators with high dynamic performance, backdrivability and low-reflected inertia are of interest. In this letter, a novel robotic actuator architecture with two independent magnetorheological (MR) power chains is proposed, built and experimentally characterized. The modular 30 Nm robotic joint is shown to have similar size (85 X 85 X 124 mm), weight (1.1 kg) and torque density (27 Nm/kg) as the Universal Robot (UR) size 1 robotic joint, which relies on a Harmonic Drive (HD) gearbox, but with 150 times less inertia, high backdrivability (0.7% of max torque), high torque bandwidth (>70 Hz) and inherent torque-limiting that makes it immune to impacts. The dual MR power chain allows ultra-fast transparent interactions with bluezero backlash over half its torque range and peak torque when motor reversal is allowed. To illustrate the force/impedance control performance, a 3-DOF robotic arm is built and used to simulate virtual objects in both open and closed loop.

Aging of magnetorheological fluids (MRFs) during the operation of mechanical devices is accompanied by degradation of both the particles and the fluid components. This study has shown that collisions among micron-size magnetic iron particles under shear can result in nanoscale changes to surface topography. Moreover, it has shown that shear-induced collisions can lead to redistributions among different oxidation state species in surface and near sub-surface regions of the particles. The redistributions among oxidation states and chemical species is the result of interfacial reactions that can involve adventitious oxygen, catalysis and other energetic processes that can act on either or both, the host fluid medium and additives introduced to stabilize the MRF. The interfacial chemistry is complex, inviting studies that couple macroscale processes (shear events in a clutch mechanism) with nanoscale inquiry into the effects of such processes. This study examined the chemical and morphological states of MRF particles subjected to continual shear loading in a rotary clutch device over well-defined periods of time for both hydrocarbon and fluorocarbon-based host fluids. New insights into chemical transformations, on and beneath, particle surfaces provide reference for better durability assessment, MRF formulation, and device design in the future. X-ray photoelectron spectroscopy iron, fluorine, oxygen and carbon depth profiles provided insight into the evolving oxidation states of the MRF particles. Over time, fluorine and oxygen insinuate themselves into the particle where they give rise to a host of different oxidized iron species. Simultaneously, the iron content of the surface diminishes. Scanning electron microscopy revealed how the particles transform from nascent nearly spherical shape prior to shear to irregular morphology in the clutch mechanisms. Transmission electron microscopy revealed carbon-rich residues with embedded nanoparticle fragments in aged MRF particle samples, suggesting that MRF particle surfaces may have served as reaction interfaces.

Lower-limb exoskeletons have shown increasing potential to augment human performance in many locomotion tasks. However, most lower-limb exoskeletons use highly-geared, non-back-drivable actuators with limited power and force bandwidth in order to be light enough to be carried without metabolic penalty. Moreover, they rely on controllers that depend on past motion history to assist the user, which limits the multifunctional capabilities of exoskeletons. Here we study the potential of delocalized magnetorheological (MR) clutches to provide transparent but yet powerful multifunctional exoskeleton assistance. A single high-speed, lightweight motor is coupled with two MR clutches that modulate the plantar-flexion torque at each ankle. The exoskeleton is controlled by a state map controller that can assist users in real time while walking, jumping and landing. Results confirm the potential of the magnetorheological actuation approach by demonstrating instantaneous adaptation to transient walking and by producing a maximal torque of 90 Nm per ankle with a total power of 1.4 kW when jumping. The system also actively braked landing impact and achieved multifunctional assistance in a sequence of walking, jumping and landing. With a total mass of 6.2 kg including 0.9 kg on each leg, the system reduces metabolic cost of walking by 5.6% on average with tethered electronics and power supply.

The durability of magnetorheological (MR) fluid is a fundamental aspect of any MR device, as MR fluid is submitted to shear stresses that cause its degradation. While various studies have identified MR-fluid degradation behaviours and proposed improvements, there exists no generalized failure theory to help understand the in-use degradation of MR fluids. In this regard, this study suggests that a relation exists between the Mason number and the maximum achievable lifetime dissipated energy (LDE) of magnetorheological (MR) fluids. To validate this hypothesis, custom (perfluoropolyether) and commercial (hydrocarbon) MR fluids are aged in various Mason number conditions using a clutch-type durability device. Experimental results show that under a critical Mason number of ~ 1, lifetime dissipated energy of the MR fluids can be related to the Mason Number using an analogy to metal fatigue failure. Above this critical Mason number, the fundamental degradation mechanism appears to change from a dominant mechanical wear of the particles (i.e. particle-limited) to another mechanism (i.e. fluid-limited) which is not yet fully understood. Since MR generally operate well below the critical Mason number of ~1, the relation between the Mason number and the particle-limited regime durability of MR fluids can be very useful to design novel MR fluids or MR devices, as it provides a way to estimate the impact of various design strategies on the total expected life of the device.

To perform untethered operations, soft robots require mesoscale power units (10–1000 W) with high energy densities. In this perspective, air-breathing combustion offers an interesting alternative to battery-powered systems, provided sufficient overall energy conversion efficiency can be reached. Implementing efficient air-breathing combustion in mesoscale soft robots is notoriously difficult, however, as it requires optimization of very small combustion actuators and simultaneous minimization of fluidic (e.g., hydraulic) losses, which are both inversely impacted by actuations speeds. To overcome such challenges, this article proposes and evaluates the potential of hydrogen-powered, hydraulic free-piston pump architecture. Experimental data, taken from two combustion-driven prototypes, reveal (1) the fundamental role of using hydrogen as the source of fuel to reduce heat losses, (2) the significant impact of compression ratio, equivalence ratio, and surface-to-volume ratio on energy conversion efficiency, and (3) the importance of load matching between combustion and fluidic transmission. In this work, a small-bore combustion actuator demonstrated a 20% efficiency and a net mean output power of 26 W, while a big-bore combustion actuator reached a substantially higher efficiency of 35% and a net mean output power of 197 W. Using the small-bore combustion actuator, the hydrogen-powered, hydraulic free-piston pump provided a 4.6% overall efficiency for a 2.34 W net mean output power, thus underlying the potential of the approach for mesoscale soft robotic applications.

The first part of this study demonstrated that magnetorheological fluid undergoes squeeze-strengthening in pure squeeze conditions that promote filtration. This behavior, however, is greatly reduced when shear deformation is superimposed onto the squeezing motion. In order to understand this phenomenon and achieve high yield stresses at high rotational velocities, the second part of this study conducts a thorough experimental characterization of magnetorheological fluid behavior under combined squeeze–shear. After demonstrating that a von Mises yield criterion is applicable to magnetorheological fluid, this criterion is included in the Péclet number, derived in the first part of this study, and used to predict filtration in magnetorheological fluid submitted to simultaneous squeeze–shear. Results show squeeze-strengthening is well predicted in squeeze-dominant flows but gradually delayed in shear-dominant flows. In such conditions, a better prediction is provided by a modified Péclet number, which also takes into account the evolution of the magnetorheological fluid microstructure through the squeeze-to-shear-rate ratio. This ratio is also found to dictate the linear relation between shear stress and compressive force when squeeze-strengthening is observed. Based on the provided understanding, high yield stresses (>1000 kPa) are obtained at high rotational velocities (200 r/min) by maximizing the filtration phenomenon in order to achieve squeeze-strengthening at high compression velocities (5 mm/s).

Recent research has shown that magnetorheological fluid can undergo squeeze-strengthening when flow conditions promote filtration. While a Péclet number has been used to predict filtration in non-magnetic two-phase fluids submitted to slow compression, the approach has yet to be adapted to magnetorheological fluid behavior in order to predict the conditions leading to squeeze-strengthening behavior of magnetorheological fluid. In this article, a Péclet number is derived and adapted to the Bingham rheological model. This Péclet number is then compared to the experimental occurrence of squeeze-strengthening behavior obtained from several squeeze geometries and magnetorheological fluid compositions submitted to pure-squeeze conditions. Results show that the Péclet number well predicts the occurrence of squeeze-strengthening behavior in high-concentration magnetorheological fluid made from various particle sizes and using various squeeze geometries. Moreover, it is shown that squeeze-strengthening occurrence is increased when using annulus geometries or by increasing average particle radius. While lowering concentration increases filtration, tested conditions only led to squeeze-strengthening behavior after concentration had increased close to packing limit. Altogether, results suggest that the Péclet number derived in this study can be used to predict the occurrence of squeeze-strengthening for various magnetorheological fluids and squeeze geometries using the well-known rheological properties of magnetorheological fluids.

Tendon-Driven Manipulators (TDM) have been used in various applications requiring low inertia robots having multiple degrees-of-freedom (DOF) and redundancy. TDMs are generally actuated by gear electric motors placed at the base of the robot, and consequently require complex cable tension feedback from force sensors or dynamic modelling to maintain cables under tension. This paper presents a high performance TDM actuated by magneto-rheological clutches along with a specialized motion control algorithm, termed Projected PID, that requires no tension feedback. A 2-DOF proof-of-concept TDM powered by magneto-rheological clutches is used to demonstrate overall controller performance, and a reconfigurable 2-DOF TDM powered by direct-drive electric motors is used to demonstrate the controller's ability to compensate for configuration variations and actuator failure. Experimental results also demonstrate the ability of Projected PID to control magneto-rheological cable-driven TDMs with high accuracy.

Under squeezing flow, magnetorheological fluid can undergo a strengthening phenomenon which results in a drastic increase of its yield stress. This behavior, also known as the super-strong effect, could be used to significantly increase the performance (e.g. torque-to-weight) of rotary magnetorheological fluid devices (e.g. brakes, clutches), but has yet to be exploited due to limited predictability of the phenomenon. To better understand the occurrence of the super-strong effect, a novel test bench capable of small amplitude oscillatory shear is designed to study the behavior of high-concentration magnetorheological fluid submitted to different simultaneous squeeze-shear conditions and magnetic field strengths. Experimental results, obtained up to 5 mm/s compression speed, show that the squeeze-strengthening effect can be correlated to the Péclet number when squeeze flow is dominant, suggesting that the super-strong behavior is governed by solid–liquid phase separation. This super-strong effect, however, is found greatly reduced when the super-imposed shear-rate approaches the squeeze-rate order of magnitude.

This paper presents the design and experimental characterization of a binary jet valve, specifically developed to control an all-polymer needle manipulator during intramagnetic resonance imaging (MRI) prostate interventions (biopsies and brachytherapies). The key feature of the MRI-compatible valve is its compact dual-stage configuration. The first stage is composed of a low-friction jet nozzle, driven by a small rotary dielectric elastomer actuator (DEA). The second stage provides sufficient air flow and stability for the binary robotic application through an independent air supply, activated by a bistable spool. A hyperelastic stress-strain model is used to optimize the geometrical dimensions of the DEA jet assembly. Fully functional valve prototypes, made with 3M's VHB 4905 films, are monitored with a high-speed camera in order to quantify the system's shifting dynamics. The impact of nozzle clearance, dielectric elastomer film viscoelasticity, mechanical friction, and actuator torque generation on overall dynamic behavior of two different valve setups is discussed. Results show an overall shifting time of 200–300 ms when the friction of the nozzle and DEA actuation stretches are minimized. Low shifting time combined with compactness, simplicity, and low cost suggest that the low friction DEA-driven jet valves have great potential for switching a large number of pneumatic circuits in an MRI environment as well as in traditional pneumatic applications.