Jean-Philippe Lucking Bigué

Soft Two-Degree-of-Freedom Dielectric Elastomer Position Sensor Exhibiting Linear Behavior

Soft robots could bring robotic systems to new horizons, by enabling safe human-machine interaction. For precise control, these soft structures require high-level position feedback that is not easily achieved through conventional one-degree-of-freedom (DOF) sensing apparatus. In this paper, a soft two-DOF dielectric elastomer (DE) sensor is specifically designed to provide accurate position feedback for a soft polymer robotic manipulator. The technology is exemplified on a soft robot intended for MRI-guided prostate interventions. DEs are chosen for their major advantages of softness, high strains, low cost, and embedded multiple-DOF sensing capability, providing excellent system integration. A geometrical model of the proposed DE sensor is developed and compared to experimental results in order to understand sensor mechanics. Using a differential measurement approach, a handmade prototype provided linear sensory behavior and 0.2 mm accuracy on two-DOF. This correlates to a 0.7% error over the sensors 30 mm × 30 mm planar range, demonstrating the outstanding potential of DE technology for accurate multiDOF position sensing.

Experimental Study of Dielectric Elastomer Actuator Energy Conversion Efficiency

Dielectric elastomer actuators (DEAs) have raised interest in the field of mobile robotics. In such a field, actuator design requires a fundamental understanding of DEA energy conversion performance. To provide insight into DEA mechanical work, energy consumption, and efficiency, this paper proposes a simple thermodynamic description completed by experimental loss factors obtained over a broad range of operating conditions and modes. Extensive data gathered on cone actuators show practical efficiency limits of ~ 26% for acrylic actuators (VHB 4905) operating under constant charge mode and ~ 18% for silicone actuators operating under constant voltage mode. While charge recovery could raise these limits to ~ 60%, the study of a DEA rotary motor shows significant efficiency degradation caused by unconstrained electrode boundaries.

Understanding the super-strong behavior of magnetorheological fluid in simultaneous squeeze-shear with the Péclet number

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.

Projected PID controller for Tendon-Driven Manipulators actuated by magneto-rheological clutches

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 controllers 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.

Design of a MRI-compatible dielectric elastomer powered jet valve

Binary Pneumatic Air Muscles (PAM) arranged in an elastically-averaged configuration can form a cost effective solution for Magnetic Resonance Imaging (MRI) guided robotic interventions like prostate cancer biopsies and brachytherapies. Such binary pneumatic manipulators require about 10 to 20 MRI-compatible valves to control the pressure state of each PAM. In this perspective, this paper presents the design of a novel dielectric elastomer actuator (DEA) driven jet-valve to control the states of the PAMs. DEAs are MRI compatible actuators that are well suited to the simplicity and cost-effectiveness of the binary manipulation approach. The key feature of the proposed valve design is its 2 stages configuration in which the pilot stage is moved with minimal mechanical friction by a rotary antagonistic DEA made with acrylic polymer films. The prismatic geometry also integrates the jet nozzle within the DEA volume to provide a compact embodiment with a reduced number of parts. The low actuation stretches enabled by the rotary configuration minimize viscoelastic losses, and thus, maximize the frequency response of the actuator while maximizing its reliability potential. The design space of the proposed jet valve is studied using an Ogden hyperelastic model and the valve dynamics is predicted with a 1D Bergstrom-Boyce viscoelastic model. Altogether, the low friction of the pilot stage and optimized DEA dynamics provide an experimental shifting time of the complete assembly in the 200-300ms range. Results from this work suggest that the DEA driven jet valve has great potential for switching a large number of pneumatic circuits in a MRI environment with a compact, low cost and simple embodiment.

Squeeze-strengthening of magnetorheological fluids (part 2): Simultaneous squeeze–shear at high speeds:

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).

Squeeze-strengthening of magnetorheological fluids (part 1): Effect of geometry and fluid composition

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.

Design of an Antagonistic Bistable Dielectric Elastomer Actuator Using the Bergstrom-Boyce Constitutive Viscoelastic Model

Dielectric Elastomer Actuators (DEA) has great potential for low cost, high performance robotic and mechatronic devices. However, the reliability of these actuators remains an important issue when used in continuous strain applications. To improve actuators reliability, DEAs can be used in a binary or bistable manner where actuators flip between two stable positions, thus maintaining one of two equilibrium states without any electrical energy input. This paper presents an antagonistic bistable DEA concept using a single, planar polymer film that can lead to compact high force multilayered actuators. The system is made bistable by the addition of carbon fiber leaf springs designed to maximize actuator strain output. The strong viscoelastic nature of the chosen polymer film significantly affects the system’s output force and is accounted for in the Bergstrom-Boyce material model. The model shows good agreement with experimental stress relaxation curves and is used to set the leaf springs’ force curve. Experimental results have shown that the acrylic polymer film’s (VHB 4905) strong viscoelastic nature limits the actuator speed at ∼ 0.9 mm/s; at higher speeds, the leaf springs cannot be matched with the proposed concept. The study also demonstrates that the proposed antagonistic actuator configuration is an interesting solution to provide reliable bistable actuation for compact structures and that developing polymer films with low viscoelasticity is key for optimal performance.Copyright

Design of a Binary Needle Manipulator Using Elastically Averaged Air Muscles for Prostate Cancer Treatments

Affecting 1 out of 8 subjects in the U.S., prostate cancer is the most common form of cancer in men. Current medical procedures could be improved by the development of an MRI compatible (Magnetic Resonance Imaging) needle manipulator system, to precisely reach small tumors (<5 mm) inside the prostate. This paper presents and analyzes the potential of such a needle manipulator concept, based on hyper-redundant binary air muscles, all controlled by MRI compatible valves (e.g. piezoelectric or dielectric elastomer actuators). The proposed manipulator uses 12 polymer air muscles, each driven by 2 different actuation pressures, offering a total of 4096 (212 ) discrete needle positions. Based on a hyperelastic continuum mechanics air muscle model, a theoretical manipulator design is used to evaluate clinically-relevant design metrics, such as size, stiffness, workspace, accuracy and sensitivity. In this model, the manipulator’s equilibrium configuration (for a given set of input pressures and applied forces) is found by minimizing the system’s potential energy. The model capability is verified experimentally by a one degree of freedom (DOF) prototype. Simulation results show that the proposed elastically averaged air muscle concept can meet all design requirements. In particular, the needle workspace of about 70 mm by 80 mm entirely covers the prostate area, where targets are accurately reachable within 0.7 mm. Also, the pneumatic actuators can generate high forces leading to a system stiffness of ∼4.6 N/mm at the needle tip. Such stiffness can adequately sustain the needle during insertion with minimal deflection to guaranty accurate positioning.Copyright