Comparison of MRI-Compatible Mechatronic Systems With Hydrodynamic and Pneumatic Actuation

The strong magnetic fields and limited space make it challenging to design the actuation for mechatronic systems intended to work in MRI environments. Hydraulic and pneumatic actuators can be made MRI-compatible and are promising solutions to drive robotic devices inside MRI environments. In this paper, two comparable haptic interface devices, one with hydrodynamic and another with pneumatic actuation, were developed to control one-degree-of-freedom translational movements of a user performing functional MRI (fMRI) tasks. The cylinders were made of MRI-compatible materials. Pressure sensors and control valves were placed far away from the end-effector in the scanner, connected via long transmission lines. It has been demonstrated that both manipulandum systems were MRI-compatible and yielded no artifacts to fMRI images in a 3-T scanner. Position and impedance controllers achieved passive as well as active subject movements. With the hydrodynamic system we have achieved smoother movements, higher position control accuracy, and improved robustness against force disturbances than with the pneumatic system. In contrast, the pneumatic system was back-drivable, showed faster dynamics with relatively low pressure, and allowed force control. Furthermore, it is easier to maintain and does not cause hygienic problems after leakages. In general, pneumatic actuation is more favorable for fast or force-controlled MRI-compatible applications, whereas hydrodynamic actuation is recommended for applications that require higher position accuracy, or slow and smooth movements.     

Force display using a hybrid haptic device composed of motors and brakes

In the virtual environment, force feedback to the human operator makes virtual experiences more realistic. However, the force feedback using active actuators such as motors can make the system active and sometimes unstable. To ensure the safe operation and enhance the haptic feeling, system stability should be guaranteed. Both active actuators such as motors and passive ones such as brakes are commonly used for haptic devices. Motors can generate a torque in any direction, but they can make the system active and thus, sometimes unstable during operation. On the other hand, brakes can generate a torque only against their rotation, but they dissipate energy during operation and this dissipation makes the system intrinsically stable. Consequently, motors and brakes are complementary to each other. In this research, a two degree-of-freedom (DOF) haptic device equipped with motors and brakes is designed, in which each DOF is actuated by a pair of motor and brake. Simultaneous operation of motors and brakes is analyzed. Models for some environments, virtual wall contact and frictional effect, are proposed. The results for the hybrid haptic system are compared with those for the active haptic system and the passivity based control system. The experimental results show that the hybrid haptic device is more suited to some applications than the other haptic systems.     

Stepped-Impedance Transformers and Filter Prototypes

Quarter-wave transformers are widely used to obtain an impedance match within a specified tolerance between two lines of different characteristic impedances over a specified frequency band. This paper gives design formulas and extensive tables of designs, most of which were especially derived so that an integrated account could be presented for the first time. Numerous examples are given. Only homogeneous, synchronous transformers and filters are included in this paper, but a short bibliography on related topics is appended. The theory is also applied to band-pass filters, by showing how to convert quarter-wave transformers into half-wave filter prototypes. The theoretical and numerical results presented are applicable to the design of impedance transformers, direct-coupled cavity filters, short-line low-pass filters, optical antireflection coatings and interference filters, acoustical transformers, branch-guide directional couplers, TEM-mode coupled-transmission-line directional couplers, and other circuits. These applications have been or will be dealt with in separate papers; this paper gives the basic theory and some of the numerical data required for these applications.     

Development and implementation of an FPGA based fractional order controller for a DC motor

Fractional calculus has been gaining more and more popularity in control engineering in numerous fields, including mechatronic applications. One of the most common applications in all mechatronic domains is the control of DC motors. Several control algorithms have been proposed for such motors, ranging from traditional PID algorithms, to the more sophisticated advanced methods, including fractional order controllers. Nevertheless, very little information regarding the implementation problems of such fractional algorithms exists today. The paper proposes a simple approach for designing a fractional order PI controller for controlling the speed of a DC motor. The resulting controller is implemented on an FPGA target and its performance is compared to other possible benchmarks. The experimental results show the efficiency of the designed fractional order PI controller. Beside the initial DC motor, two other different DC motors are also used in the experiments to demonstrate the robustness of the controller.     

Direct yaw moment control actuated through electric drivetrains and friction brakes: Theoretical design and experimental assessment

A significant challenge in electric vehicles with multiple motors is how to control the individual drivetrains in order to achieve measurable benefits in terms of vehicle cornering response, compared to conventional stability control systems actuating the friction brakes. This paper presents a direct yaw moment controller based on the combination of feedforward and feedback contributions for continuous yaw rate control. When the estimated sideslip exceeds a pre-defined threshold, a sideslip-based yaw moment contribution is activated. All yaw moment contributions are entirely tunable through model-based approaches, for reduced vehicle testing time. The purpose of the controller is to continuously modify the vehicle understeer characteristic in quasi-static conditions and increase yaw and sideslip damping during transients. Skid-pad, step-steer and sweep steer tests are carried out with a front-wheel-drive fully electric vehicle demonstrator with two independent drivetrains. The experimental test results of the electric motor-based actuation of the direct yaw moment controller are compared with those deriving from the friction brake-based actuation of the same algorithm, which is a major contribution of this paper. The novel results show that continuous direct yaw moment control allows significant “on-demand” changes of the vehicle response in cornering conditions and to enhance active vehicle safety during extreme driving maneuvers.     

Printing Reconfigurable Bundles of Dielectric Elastomer Fibers

Active soft materials that change shape on demand are of interest for a myriad of applications, including soft robotics, biomedical devices, and adaptive systems. Despite recent advances, the ability to rapidly design and fabricate active matter in complex, reconfigurable layouts remains challenging. Here, the 3D printing of core-sheath-shell dielectric elastomer fibers (DEF) and fiber bundles with programmable actuation is reported. Complex shape morphing responses are achieved by printing individually addressable fibers within 3D architectures, including vertical coils and fiber bundles. These DEF devices exhibit resonance frequencies up to 700 Hz and lifetimes exceeding 2.6 million cycles. The multimaterial, multicore-shell 3D printing method opens new avenues for creating active soft matter with fast programable actuation.     

Fabrication and Electromechanical Characterization of a Piezoelectric Structural Fiber for Multifunctional Composites

The use of piezoceramic materials for structural sensing and actuation is a fairly well developed practice that has found use in a wide variety of applications. However, just as advanced composites offer numerous benefits over traditional engineering materials for structural design, actuators that utilize the active properties of piezoelectric fibers can improve upon many of the limitations encountered when using monolithic piezoceramic devices. Several new piezoelectric fiber composites have been developed; however, almost all studies have implemented these devices such that they are surface‐bonded patches used for sensing or actuation. This paper will introduce a novel active piezoelectric structural fiber that can be laid up in a composite material to perform sensing and actuation, in addition to providing load bearing functionality. The sensing and actuation aspects of this multifunctional material will allow composites to be designed with numerous embedded functions, including structural health monitoring, power generation, vibration sensing and control, damping, and shape control through anisotropic actuation. This effort has developed a set of manufacturing techniques to fabricate the multifunctional fiber using a SiC fiber core and a BaTiO₃ piezoelectric shell. The electromechanical coupling of the fiber is characterized using an atomic force microscope for various aspect ratios and is compared to predictions made using finite element modeling in ABAQUS. The results show good agreement between the finite element analysis model and indicate that the fibers could have coupling values as high as 68% of the active constituent used.     

Microstructured Actuation of Liquid Crystal Polymer Networks

Smart microstructured materials enable functions such as actuation, detection, transportation, and sensing with potential applications ranging from robotics and photonics to biomedical devices. Of the many materials systems, liquid crystal polymer networks (LCN) are fascinating owing to their ability to exhibit reversible macroscopic deformation driven by a molecular order–disorder phase transition. LCN have been increasingly explored for their utility in the design and fabrication of smart actuating devices capable of complex shape changes or motions upon external stimulation of humidity, heat, light, and other stimuli, and recent studies in this field show that their actuation complexity can be enriched and actuation performance enhanced by having some sort of microstructures. Herein, the recent progress in microstructured actuation of LCN materials with substructures in scale ranging from micrometer to millimeter is reported, placing the emphasis on the main approaches to generating a microstructure in LCN, which include patterned LC director fields, patterned chain crosslinking in LCN with uniaxial orientation of mesogens, 3D/4D printing, and replica molding. The potential applications in microstructured 3D actuators and devices as well as functional LCN surfaces are also highlighted, with an outlook on important issues and future trends in smart microstructured LCN materials and actuators.     

Development of two-axis arm motion generator using active impedance

In this research a two-dimensional arm motion generator, composed of two linear motors, was developed. The inertia, damping and/or stiffness characteristics of the motion generator can be changed on the real-time basis by properly regulating the force generated by the linear motors. That is, active impedance is implemented without actual change in the physical structure of the motion generator. Control of the motor force is carried out by regulating the input currents supplied to the linear motors by using a common voltage-driven driver. In the control system, the time delay due to A/D conversion of the current output has an adverse effect on the stability of the system. Furthermore, disturbances caused by characteristics of the motion generator also exist. To cope with these difficulties, a 2-DOF controller combined with LQ-servo and H-infinity controllers was used. The gains of the controller are selected so that disturbance rejection, stability guarantee and tracking performance may be achieved. This motion generator can be used to measure kinesthetic sense associated with the human arm and thus leads to developing the products for which the kinesthetic sense is taken into account.     

Test Strategies for Electrode Degradation in Bio-Fluidic Microsystems

Electrode technology is fundamental to numerous actuation and sensing functions in bio-fluidic microsystems that target portable bio-analytical instruments. Within these systems high levels of reliability and robustness are crucial and normally complemented by requirements for extremely low probabilities of false positives or negatives being generated. New methods of validating functionality and integrity of the reading are hence required. Embedded test and condition monitoring are crucial technologies for delivering these capabilities. This paper presents two solutions for detecting degradation in electrodes that interface to fluidic or biological systems. In the first solution, a low frequency, impedance based method for identifying degraded structures within an array is proposed. This method depends on measuring and comparing the impedance of each sensing electrode. This research is backed up by physical measurements from an electrode array for drug testing on cardiac and neuron tissue. In the second solution, a mid-frequency oscillation test technique is proposed that is sensitive to degradation in the bio-fluidic interface capacitance, to contamination and to fouling.     

Experimental Evaluation of Dynamic Redundancy Resolution in a Nonholonomic Wheeled Mobile Manipulator

Mobile manipulators derive significant novel capabilities for enhanced interactions with the world by merging mobility with manipulation. However, a careful resolution of the redundancy and active control of the reconfigurability, created by the surplus articulated DOFs and actuation, are the keys to unlocking this potential. Nonholonomic wheeled mobile manipulators, formed by mounting manipulator arms on disc-wheeled mobile bases, are a small but important subclass of mobile manipulators. The primary control challenges arise due to the dynamic-level coupling of the nonholonomy of the wheeled mobile bases with the inherent kinematic and actuation redundancy within the articulated chain. The solution approach in this paper builds upon a dynamically consistent and decoupled partitioning of the articulated system dynamics between the external (task) space and internal (null) space. The independent controllers, developed within each decoupled space, facilitate active internal reconfiguration, in addition to resolving redundancy at the dynamic level. Specifically, two variants of null-space controllers are implemented to improve disturbance rejection and active reconfiguration during performance of end-effector tasks by a primary end-effector impedance mode controller. These algorithms are evaluated within an implementation framework that emphasizes both virtual prototyping and hardware-in-the-loop testing with representative case studies.     

Coplanar Waveguide: A Surface Strip Transmission Line Suitable for Nonreciprocal Gyromagnetic Device Applications

A coplanar waveguide consists of a strip of thin metallic film on the surface of a dielectric slab with two ground electrodes running adjacent and parallel to the strip. This novel transmission line readily lends itself to nonreciprocal magnetic device applications because of the built-in circularly polarized magnetic vector at the air-dielectric boundary between the conductors. Practical applications of the coplanar waveguide have been experimentally demonstrated by measurements on resonant isolators and differential phase shifters fabricated on low-loss dielectric substrates with high dielectric constants. Calculations have been made for the characteristic impedance, phase velocity, and ripper bound of attenuation of a transmission line whose electrodes are all on one side of a dielectric substrate. These calculations are in good agreement with preliminary experimental results. The coplanar configuration of the transmission system not only permits easy shunt connection of external elements in hybrid integrated circuits, but also adapts well to the fabrication of monolithic integrated systems. Low-loss dielectric substrates with high dielectric constants may be employed to reduce the longitudinal dimension of the integrated circuits because the characteristic impedance of the coplanar waveguide is relatively independent of the substrate thickness; this may be of vital importance for Iow-frequency integrated microwave systems.     

Loss-free complex impedance network elements

The concept of loss-free complex impedance network elements (i.e., elements with active and reactive impedance components and yet loss-free) is introduced. Synthesis of such elements by means of switched-mode power converters with appropriate control has been demonstrated to be possible. Some possible power processing-related applications of these elements are indicated, such as improved matching between ac sources and loads and VAR compensation. It has further been demonstrated that loss-free complex impedance elements are suitable for modeling many power processing systems for the purpose of analysis as well as for design purposes.     

Two-Photon Polymerization of Sugar Responsive 4D Microstructures

Stimuli-responsive hydrogels have attracted much attention owing to the versatility of their programmed response in offering intelligent solutions for biomimicry applications, such as soft robotics, tissue engineering, and drug delivery. To achieve the complexity of biomimetic structures, two photon polymerization (2PP) has provided a means of fabricating intricate 3D structures from stimuli-responsive hydrogels. Rapid swelling hydrogel microstructures are advantageous for osmotically driven stimuli-response, where actuation speed, that is reliant on the diffusion of analytes or bioanalytes, can be optimized. Herein, the flexibility of 2PP is exploited to showcase a novel sugar-responsive, phenylboronic acid-based photoresist. This offers a remarkable solution for achieving fast response hydrogel systems that have been often hindered by the volume-dependent diffusion times of analytes to receptor sites. A phenylboronic acid-based photoresist compatible with 2PP is presented to fabricate stimuli-responsive microstructures with accelerated response times. Moreover, microstructures with programmable actuation (i.e., bending and opening) are fabricated using the same photoresist within a one-step fabrication process. By combining the flexibility of 2PP with an easily adaptable photoresist, an accessible fabrication method is showcased for sophisticated and chemo-responsive 3D hydrogel actuators.     

How to reduce the impedance for pHRI: Admittance control or underactuation?

Many applications of physical human–robot interaction (pHRI) seek to minimize the impedance felt by the operator. For large-scale operations, such as industrial material handling, two architectures have been proposed. The first uses a rigid robotic manipulator with force sensing and an admittance controller, such as the class of Intelligent Assist Devices (IADs). The second utilizes an underactuated macro-mini manipulator system, such as the uMan Assist Device. Given an application with a large payload, which of these two systems can offer the lowest inertia interaction? This work analyzes the effective inertia experienced by the operator in each of the systems. It compares the conditions under which each provides the lightest manipulation, conditions that depend on the payload as well as the frequency of manipulation. The results are validated on a large overhead robotic system configured alternatively in each of the configurations.     

Linear induction machines I. History and theory of operation

The history of linear induction motors extends as far back as the 19th century. Although these machines have been practically forgotten for the last 30 or 40 years, there appears to be a genuine revival of interest in them. The fascinating history of these ``unrolled' motors and their theory of operation is the subject of Part I of this article. Part II will deal with the unique advantages that such high-performance systems have for modern-day applications.     

High-power industrial drives

After the classification and a short review of the evolution, the different drive systems which became important in industrial applications are described. It is shown how motor, converter, and controls form a drive system. The specific characteristics of each system are emphasized in order to show which system is best suited for which kind of applications. Based on the development potential the paper also gives a projection for the future. The systems discussed include synchronous motors fed by externally commutated current source converters, induction motors fed by current source converters, cycloconverter fed synchronous motors, voltage source inverters, and slip power controlled drives     

Stable and High-Strain Dielectric Elastomer Actuators Based on a Carbon Nanotube-Polymer Bilayer Electrode

Dielectric elastomer actuators (DEAs) have shown promises in numerous applications such as bio-inspired robotics, tactile displays, tunable optics, and microfluidics, owing to their unique combination of large actuation strain, high energy density, and light weight. However, the practical applications of the DEAs have been hindered partly due to their poor reliability and durability under high-strain actuation. A major failure mechanism is from the localized electrical breakdown. Compliant electrodes with self-clearing capability have been studied to prevent premature failures. Here, an interpenetrating bilayer compliant electrode comprising a thin layer of a water-based polyurethane (WPU) overcoated on an ultrathin single-walled carbon nanotube (SWNT) layer is reported. The thin polyurethane layer serves as the dielectric barrier to suppress corona discharges of the nanotubes in air. The SWNT+WPU bilayer electrode has the capability to self-clear at the breakdown sites, enhancing the fault tolerance and mendability of the DEA at a large-strain actuation. Stable actuation at 150% area strain for 1000 cycles under square-wave voltage and 5.5-h continuous actuation at a constant voltage have been achieved for acrylic elastomer-based DEAs.     

Quantifying information transfer through a head-attached vibrotactile display: principles for design and control

Vibrotactile displays can extend the perception capabilities of visually impaired persons. Placing such devices on the head promises easy attachment and detachment without reducing other interaction abilities. However, the effectiveness of head-attached vibrotactile displays has never been thoroughly tested. This paper presents the results obtained from experiments with 22?subjects equipped with a display containing 12 coin-type motors equally spaced in a horizontal plane around the upper head region. Our display allowed single- as well as multimotor activation with up to six simultaneously active motors. We identified the minimum and comfort strength of vibrotactile stimulation, and measured the precision in perceiving the accurate number of active motors as well as the precision in localizing the stimuli on the head. While subjects identified the correct number of active motors in 94% of the cases when presented with only one active motor, this precision dropped to 40% for two and down to 5% for five simultaneously active motors. This strongly suggests to avoid multipoint stimulation even though the precision of localizing a position of a stimulus on the head is barely affected by the number of simultaneously active motors. Localization precision, however, varied significantly with the region of the head suggesting that the most front and back regions of the head should be avoided if high precision is required.     

Optimized binary modular reconfigurable robotic devices

Binary robotic devices with large degrees of freedom have been proposed by a number of researchers. However, experimental implementations of these concepts have been built with conventional components. These physical systems are heavy, complex and far from being practical devices. In this paper, a lightweight, compliant mechanism driven by optimized magnet-coil actuators is proposed and developed as an element for modular hyper-redundant degrees of freedom robotic systems. Such elements could be used in a number of applications and would replace conventional, complex, and heavy components. The device has a parallel kinematic structure. Its binary actuation simplifies its control architecture.