Virtual Stiffness Research Articles

Diagnostic efficiency of intravoxel incoherent motion-based virtual magnetic resonance elastography in pulmonary neoplasms

BackgroundThe aim of the study were as below. (1) To investigate the feasibility of intravoxel incoherent motion (IVIM)-based virtual magnetic resonance elastography (vMRE) to provide quantitative estimates of tissue stiffness in pulmonary neoplasms. (2) To verify the diagnostic performance of shifted apparent diffusion coefficient (sADC) and reconstructed virtual stiffness values in distinguishing neoplasm nature.MethodsThis study enrolled 59 patients (37 males, 22 females) with one pulmonary neoplasm who underwent computed tomography-guided percutaneous transthoracic needle biopsy (PTNB) with pathological diagnosis (26 adenocarcinoma, 10 squamous cell carcinoma, 3 small cell carcinoma, 4 tuberculosis and 16 non-specific benign; mean age, 60.81 ± 9.80 years). IVIM was performed on a 3 T magnetic resonance imaging scanner before biopsy. sADC and virtual shear stiffness maps reflecting lesion stiffness were reconstructed. sADC and virtual stiffness values of neoplasm were extracted, and the diagnostic performance of vMRE in distinguishing benign and malignant and detailed pathological type were explored.ResultsCompared to benign neoplasms, malignant ones had a significantly lower sADC and a higher virtual stiffness value (P < 0.001). Subsequent subtype analyses showed that the sADC values of adenocarcinoma and squamous cell carcinoma groups were significantly lower than non-specific benign group (P = 0.013 and 0.001, respectively). Additionally, virtual stiffness values of the adenocarcinoma and squamous cell carcinoma subtypes were significantly higher than non-specific benign group (P = 0.008 and 0.001, respectively). However, no significant correlation was found among other subtype groups.ConclusionsNon-invasive vMRE demonstrated diagnostic efficiency in differentiating the nature of pulmonary neoplasm. vMRE is promising as a new method for clinical diagnosis.

Elevating haptic interfaces: Dual‐rate sampling and field programmable gate array implementation for multi‐degree‐of‐freedom performance enhancement

AbstractIn this work, our primary focus centered on exploring the adaptability of the dual‐rate sampling scheme proposed earlier to enhance the performance of multi‐degree‐of‐freedom (multi‐DOF) impedance‐based haptic interfaces. The scheme employed independent sampling rates in a haptics controller, effectively mitigating the issue of reduced Z‐width at higher sampling rates. A key aspect of our investigation was the intricate implementation of the dual‐rate sampling scheme on a field programmable gate array (FPGA). This implementation on a logic hardware FPGA was challenging and led to the effective comparison of the uniform‐rate and dual‐rate sampling schemes of the multi‐DOF haptic controller. We used an in‐house developed two‐DOF pantograph as the haptic interface and an FPGA for implementing the controller strategy. FPGA‐based implementation presented challenges that were vital in testing controller performances at higher sampling rates. Virtual wall experiments were conducted to determine the stable and unstable interactions with the virtual wall. To complement the experimental results, we simulated the haptics force law for multi‐DOF system on Simulink/MATLAB. Notably, the dual‐rate sampling approach maintained the Z‐width of the two‐DOF haptic interface, even at higher controller sampling rates, distinguishing it from the conventional two‐DOF uniform‐rate control scheme. For example, employing a dual‐rate sampling combination of 20–2 kHz consistently ensured the stable rendering of a maximum virtual stiffness of approximately 700 N/mm and maintained a reliable virtual damping range spanning from 0 to 5 Ns/mm. In contrast, the 20 kHz uniform‐rate sampling approach failed to ensure interface stability in the presence of virtual damping, ultimately resulting in the unsuccessful implementation of any virtual stiffness at higher sampling rates. This work, therefore, establishes the potential of dual‐rate sampling in the realm of haptic technology, with practical applications in multi‐DOF systems.

Development and Adaptive Assistance Control of the Robotic Hip Exoskeleton to Improve Gait Symmetry and Restore Normal Gait

Neurological disorders and aging induce abnormal gait kinematics such as asymmetric gait, reduced stride length, and walking speeds. Gait asymmetry increases musculoskeletal pain and joint impairment in the affected leg, increases the risk of patients falling, and induces higher energy expenditure. This paper presents a novel assistive control framework for the series elastic actuator-driven robotic hip exoskeleton that improves gait symmetry and restores normal gait. The proposed assistive control depends on virtual stiffness compensation with a series elastic actuator (SEA)-driven hip exoskeleton to assist without needing estimation, prediction of the gait phase, or human intent. In addition, adaptive adjustment of the virtual stiffness based on the approximate dynamic programming method is used to improve gait symmetry. The proposed control strategy is successfully implemented, and experiments are validated with human subjects’ gait assistance using a SEA-driven wearable hip exoskeleton. <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Note to Practitioners</i> —This paper was motivated by the problem of hip exoskeleton assistive control. The goal of hip exoskeleton assistance is to improve gait symmetry and restore the normal gait of the patient with neurological disorders and the elderly. Existing approaches to fixturing such parts generally need to estimate or predict the gait phase or human intent. And the existing approaches generally adopted fixed control parameters. The hip exoskeleton’s assistance cannot adapt to various walking conditions and human gait differences. The proposed assistive control is based on virtual stiffness compensation with a SEA-driven hip exoskeleton to assist with walking under various walking conditions without estimating or predicting the gait phase or human intent. And the proposed method is online adjusted control parameters with human-in-the-loop to improve the gait symmetry. The human walking experiment results showed that the hip exoskeleton assistive controller not only reduced muscle activation and human effort but also improved the interjoint coordination of the impaired joint online. We also showed how the assistance controller changes the control parameters according to the gait symmetry with a human-in-the-loop strategy.

Analog Position Estimation for Enhanced Stability and Fidelity of Haptic Systems.

In this paper, we propose three methods to compute low-latency analog position where two of them fuse encoder and rate gyro signals. While one method is based on gyro with bias correction using encoder information, the other one is encoder-referenced combined with a resettable integrator to minimize the staircase form of encoder signals. Experiments on a one degree-of-freedom haptic simulation system have shown that a low-latency analog position with an accuracy over 98% compared to the sampled encoder signal can be obtained. The analog position signals are then utilized to produce analog viscoelastic virtual environments to assess and benchmark the proposed methods through uncoupled stability and perceived fidelity tests. The results have shown that a virtual stiffness range larger than 400% can be obtained with enhanced fidelity compared to common digital implementations.

Interaction force modeling and analysis of the human–machine kinematic chain based on the human–machine deviation

A mechanical model for a human–machine interaction force based on the man–machine kinematic chain is established. This is combined with screw theory and a virtual rigid body model for the human–machine interaction force is proposed. This model interprets the basic principle model of the human–machine contact force. The deviation of the human–machine kinematic chain is calculated using the virtual model. In order to carry out the calibration simulation for the model, a 6-sps parallel mechanism is taken as an example to illustrate the construction principle of the human–machine interaction virtual rigid body model. This is calibrated by introducing finite element software. Finally, using the knee exoskeleton as an example, a numerical simulation is introduced. This illustrates the relationship between the driving force of the exoskeleton, the human–machine deviation as well as the virtual stiffness. The modeling method of this paper provides theoretical reference for controller design of human–machine interaction forces in the future.

Exploring the Just Noticeable Interaction Stiffness Differences of an Impedance-Controlled Series Elastic Actuator

The integration of a passive elastic element in series between a motor and its load is popular in many human–robot interaction scenarios. By virtually imposing elastic behavior on the motor, an impedance control can act as a second stiffness to such an actuator. In this study, we investigated how participants perceived the different stiffness settings in a series elastic actuator by measuring the Just Noticeable Difference (JND) of the real stiffness of the elastic element and the virtual stiffness introduced by impedance control. We conducted a user study during which participants interacted with an impedance-controlled Series Elastic Actuator through a lever. During the user study, we varied the real stiffness of the elastic element and the virtual stiffness. We found that participants seem to perceive both the virtual stiffness and the real stiffness in the same way and in accordance to Weber’s law, which states that the stiffness JND is always equal to a fraction of the initial stiffness. Following these findings, we concluded that the impedance controller can implement an effective virtual stiffness with a behavior comparable to a real torsional spring. Therefore, a system combining real and virtual stiffness can simulate a single combined stiffness for a user interacting with it.

A Fundamental Limitation of Passive Spring Rendering With Series Elastic Actuation.

We establish a fundamental limitation of stiffness rendering with series elastic actuation by rigorously proving that no causal controller can passively render virtual stiffness levels that are higher than the stiffness of the physical series elastic element. To relax this bound, we propose the addition of a viscous damping term parallel to the elastic element to form series damped elastic actuation (SDEA). We show that there exist LTI controllers for SDEA that can relax the passivity bound on the virtual stiffness, and derive the necessary and sufficient conditions for the passive stiffness rendering with SDEA under such a controller. We experimentally verify the relaxed passive stiffness rendering bounds for SDEA, by studying the coupled stability of interactions with most destabilizing environments.

Virtual Physical Coupling of Two Lower-Limb Exoskeletons.

Physical interaction between individuals plays an important role in human motor learning and performance during shared tasks. Using robotic devices, researchers have studied the effects of dyadic haptic interaction mostly focusing on the upper-limb. Developing infrastructure that enables physical interactions between multiple individuals' lower limbs can extend the previous work and facilitate investigation of new dyadic lower-limb rehabilitation schemes. We designed a system to render haptic interactions between two users while they walk in multi-joint lower-limb exoskeletons. Specifically, we developed an infrastructure where desired interaction torques are commanded to the individual lower-limb exoskeletons based on the users' kinematics and the properties of the virtual coupling. In this pilot study, we demonstrated the capacity of the platform to render different haptic properties (e.g., soft and hard), different haptic connection types (e.g., bidirectional and unidirectional), and connections expressed in joint space and in task space. With haptic connection, dyads generated synchronized movement, and the difference between joint angles decreased as the virtual stiffness increased. This is the first study where multi-joint dyadic haptic interactions are created between lower-limb exoskeletons. This platform will be used to investigate effects of haptic interaction on motor learning and task performance during walking, a complex and meaningful task for gait rehabilitation.

Numerical realization of a semi-active virtual acoustic black hole effect

Noise mitigation by means of the acoustic black hole (ABH) effect is a well-known engineering solution. However, the conventional method of applying ABH effect which requires modification of the structure geometry has various limitations which encourage the research of virtual ABH concept. In this study, the effect of ABH was applied through introducing virtual stiffness by a shunt circuit. According to the force-voltage electric analogy, stiffness has an inverse relationship with capacitance. So that the ABH effect can be virtually realized by following a power law profile using an array of independent capacitive shunts. The concept is studied through finite element simulation developing a macro code in ANSYS Parametric Design Language (APDL). To evaluate the influence of capacitance profile on the acoustic radiated power, parametric studies are conducted. Based on the results of the parametric studies, the capacitance profile is tuned for minimum radiated power. It is revealed that the virtual acoustic black hole (ABH) effect can offer 10.29%, 6.37%, and 7.47% reduction in the radiated power from the first to the last targeted mode, respectively. The virtual ABH effect introduced in this study can be used for semi-active structural noise isolation without any weight or manufacturing penalty.

Virtual Stiffness: A Novel Biomechanical Approach to Estimate Limb Stiffness of a Multi-Muscle and Multi-Joint System

In recent years, different groups have developed algorithms to control the stiffness of a robotic device through the electromyographic activity collected from a human operator. However, the approaches proposed so far require an initial calibration, have a complex subject-specific muscle model, or consider the activity of only a few pairs of antagonist muscles. This study described and tested an approach based on a biomechanical model to estimate the limb stiffness of a multi-joint, multi-muscle system from muscle activations. The "virtual stiffness" method approximates the generated stiffness as the stiffness due to the component of the muscle-activation vector that does not generate any endpoint force. Such a component is calculated by projecting the vector of muscle activations, estimated from the electromyographic signals, onto the null space of the linear mapping of muscle activations onto the endpoint force. The proposed method was tested by using an upper-limb model made of two joints and six Hill-type muscles and data collected during an isometric force-generation task performed with the upper limb. The null-space projection of the muscle-activation vector approximated the major axis of the stiffness ellipse or ellipsoid. The model provides a good approximation of the voluntary stiffening performed by participants that could be directly implemented in wearable myoelectric controlled devices that estimate, in real-time, the endpoint forces, or endpoint movement, from the mapping between muscle activation and force, without any additional calibrations.

A Force-Controlled Parallel Robot for Large-Range Stiffness Rendering in Three Dimensions

A haptic device is used to transmit impedance to a human user to mimic the impedance of a virtual or real environment. Existing haptic devices use serial or parallel robots to deliver impedance in multiple dimensions. These robots usually have nonconstant Jacobian matrices that result in poor dynamic properties and low impedance stability limits in certain regions within the workspace. To account for these regions, the range of stiffness rendering is limited. This letter presents a three-degrees-of-freedom (DoFs) translational parallel robot with a constant Jacobian matrix in the entire workspace. The consistent dynamic parameters allow a large-range virtual stiffness to be rendered. To provide the accurate and large output force required for high-stiffness rendering, series elastic actuators (SEAs) are used as the input for the parallel robot. SEAs can be used to minimize the geartrain friction and effective inertia to control the output force and impedance more accurately. Design, modeling, and three-dimensional impedance control of the haptic device are presented in this work. Multi-dimensional impedance and virtual-wall control experiments are illustrated to demonstrate the accuracy and rendering range of the haptic device. Since the stable range of virtual stiffness is much larger than existing ones, it is expected that this novel device can be used to render accurate stiffness for both soft and stiff environments.

Human-in-the-Loop Stability Analysis of Haptic Rendering With Time Delay by Tracking the Roots of the Characteristic Quasi-Polynomial: The Effect of arm Impedance

The uncoupled stability of haptic rendering has been analyzed extensively, predominantly using passivity considerations. Yet, the role of the operator in improving the stability of haptic systems has received less attention. Here, towards a human-in-the-loop stability analysis of haptic rendering with delay, we study the effect of the operator impedance on the stability boundaries. We employ a method that tracks the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$j\omega$</tex-math></inline-formula> crossings of the roots of a characteristic second order equation of a coupled system that includes the impedance of the operator, the haptic device, the rendered virtual stiffness and damping, and the time delay. We found that the consideration of a realistic operator impedance leads to significantly less conservative stable time delay margins. Moreover, for certain arm impedance parameters, the coupled system may allow for: (1) assuring delay-independent stability, and (2) being stabilized by increasing the delay. These results indicate that a human-centered approach can push the boundaries of safe haptic feedback, especially in applications where the delays are substantial and unavoidable, such as in teleoperation.

Diffusion-Based Virtual MR Elastography of the Liver: Can It Be Extended beyond Liver Fibrosis?

Background: Strong correlation has been reported between tissue water diffusivity and tissue elasticity in the liver. The purpose of this study is to explore the capability of diffusion–based virtual MR elastography (VMRE) in the characterization of liver tumors by extending beyond liver fibrosis assessments. Methods: Fifty-four patients (56 liver tumors: hepatocellular carcinoma (HCC), 31; metastases, 25) who underwent MRE, diffusion-weighted imaging (DWI) (b: 0, 800 s/mm2), and VMRE (b: 200, 1500 s/mm2) were enrolled. The MRE shear modulus (µMRE), apparent diffusion coefficient (ADC), and shifted ADC (sADC) were obtained. Virtual stiffness (µdiff) was estimated from the relationship between µMRE and sADC. A linear discriminant analysis combining VMRE and MRE to classify HCC and metastases was performed in a training cohort (thirty-two patients) to estimate a classifier (C), and evaluate its accuracy in a testing cohort (twenty-two patients). Pearson’s correlations between µMRE, sADC, and ADC were evaluated. In addition to the discriminant analysis, a receiver operating characteristic (ROC) curve was used to assess the discrimination capability between HCC and metastases. Results: The correlations between µMRE and sADC were significant for liver, HCC, and metastases (r = 0.91, 0.68, 0.71; all p < 0.05). Those between µMRE and ADC were weaker and significant only for metastases (r = 0.17, 0.20, 0.55). µdiff values were not significantly different between HCC and metastases (p = 0.56). Areas under the curves (AUC) to differentiate HCC from metastases were as follows: VMRE, 0.46; MRE alone, 0.89; MRE + VMRE, 0.96. The classifier C also provided better performance than MRE alone, in terms of sensitivity (100 vs. 93.5%, respectively) and specificity (92 vs. 76%, respectively, p = 0.046). Conclusions: The correlation between sADC and µMRE was strong both in the liver and in tumors. However, VMRE alone could not classify HCC and metastases. The combination of MRE and VMRE, however, allowed discriminant performance between HCC and metastases.

Usability of Haptic Volumetric Assistance for Surgical Navigation Tasks

Abstract Manual control of surgical instruments represents a sensorimotor control task with at least 3-6 degrees of freedom (DoF). The impact of haptic guidance on volumetric navigation tasks, such as milling of planned volumes for prosthesis fits or preserving sensitive tissues, is investigated. Interaction centered studies are performed to evaluate the usability of the assistance modes for navigation within a volume, along the surface of a volume and around forbidden regions. Results show that haptic assistance can reduce the number of constraint violations, if the virtual stiffness is high enough. However, haptic assistance also can increase error rates when counterforces are close to the absolute perception threshold, as a false sense of security can arise. For navigation along complex surfaces bilateral haptic constraints should be preferred, while unilateral constraints are sufficient for simple geometries. This study complements previous publications as a basis for a flexible rule-based selection or adaptation of modular haptic assistance systems.

Experimental Structural Stiffness Analysis of a Surgical Haptic Master Device Manipulator

Abstract This paper deals with haptic devices for master–slave telesurgical applications. Namely, a stiffness model fitting methodology and its fine-tuning are proposed based on experimental results. In particular, the proposed procedure is based on virtual joint structural stiffness modeling to be applied in time-efficient compliance compensation strategies. A specific case study is discussed by referring to the HISS haptic device that has been developed and built at Izmir Institute of Technology. Two different experimental setups are designed for stiffness evaluation tests. Experimental results are discussed to demonstrate their implementation in the proposed methodology for the fine-tuning of stiffness model.

A Hybrid Impedance Controller for Series Elastic Actuators to Render a Wide Range of Stable Stiffness in Uncertain Environments

Accurate and wide-range stiffness control is important for safe human-robot interaction. Accurate stiffness control can be better achieved using series elastic actuators (SEAs) than conventional rigid actuators. However, the stable range of virtual stiffness rendered by SEAs is limited by the stiffness of the actual spring, which cannot be too high in order to ensure good force control accuracy. Adding a virtual damper or derivative gain can increase the stable range of virtual stiffness, but the stable range would highly depend on the environmental stiffness. To relax the stiffness limitation in uncertain environments and explore more merits of SEAs, this letter proposes a hybrid impedance controller. This new controller linearly combines the spring force feedback and inertia force feedback. The stable range of virtual stiffness can be easily increased to ten times the actual spring stiffness with minimum effect on the force control accuracy. Unlike typical impedance controllers, the environmental stiffness can be used to raise the stable range of stiffness and hence the robustness of the controller can be ensured. Experiments will be provided to verify the hybrid impedance controller. We expect that the hybrid impedance controller can be used for SEAs in unstructured environments to provide a wide range of virtual stiffness.

Applying Hip Stiffness With an Exoskeleton to Compensate Gait Kinematics.

Neurological disorders and aging induce impaired gait kinematics. Despite recent advances, effective methods using lower-limb exoskeleton robots to restore gait kinematics are as yet limited. In this study, applying virtual stiffness using a hip exoskeleton was investigated as a possible method to guide users to change their gait kinematics. With a view to applications in locomotor rehabilitation, either to provide assistance or promote recovery, this study assessed whether imposed stiffness induced changes in the gait pattern during walking; and whether any changes persisted upon removal of the intervention, which would indicate changes in central neuro-motor control. Both positive and negative stiffness induced immediate and persistent changes of gait kinematics. However, the results showed little behavioral evidence of persistent changes in neuro-motor control, not even short-lived aftereffects. In addition, stride duration was little affected, suggesting that at least two dissociable layers exist in the neuro-motor control of human walking. The lack of neuro-motor adaptation suggests that, within broad limits, the central nervous system is surprisingly indifferent to the details of lower limb kinematics. The lack of neuro-motor adaptation also suggests that alternative methods may be required to implement a therapeutic technology to promote recovery. However, the immediate, significant, and reproducible changes in kinematics suggest that applying hip stiffness with an exoskeleton may be an effective assistive technology for compensation.

Precise slave-side force control for security enhancement of bilateral motion control during application of excessive force by operator

The high probability of a future environment featuring human–robot coexistence is motivating studies on man–machine interface safety. Bilateral control that enables contact including haptic sensations to distant locations through master–slave actuators is an emerging technology in the fields of human tele support, care, and assistance. This paper proposes a safe operating technique for bilaterally controlled robots based on environmental properties. This technique protects the remote environment from excess force applied by the master operator. We have described the operation as the normal mode and safe mode. The system follows four-channel acceleration-based bilateral control throughout normal mode operation. The safe mode operation corresponds to virtual environmental sensation of the master operator together with force-position limitation on the slave side. In this work, a conventional virtual stiffness controller was modified to overcome the loss of environmental sensation from the remote slave side. Precise simultaneous force-position control is achieved on the slave side by focusing on the object safe force limit. The modified method is applied to the bilateral controller preserving the common mode force servoing property. The proposed method is validated using experiments.

Crashworthiness Design for Trapezoid Origami Crash Structure Numerical

Corrugations can be considered to be one of the ways to improve the mechanical properties of thin-walled structure in terms of manipulation of surface area. However, this theory requires further validation through experimentation of different materials. Although many research works have been done towards the corrugated shell structures, the flexibility of corrugated sheets of thermoset composite material remains unknown. This study focused on the effects of surface area manipulation by using trapezoid origami structure which is trapezoidal folded lobe shape on the absorbed energy and mechanical properties of Epoxy reinforced with S-type fibreglass. Then the trapezoidal folded lobe shape design was drawn by using AutoCAD which consist of the design of the corrugated composite sheets and the design of trapezoidal folded lobe shape mould. Moreover, the fabrication of the Aluminum mould was done by using a CNC milling machine according to the drawing. So, a compression moulding machine will be used to fabricate the composite structure. Therefore, the vibration and compression tests were carried out to perform a study on the behaviour of the trapezoidal folded lobe thermoset samples and to investigate their deformation behaviour respectively. Based on those tests, the results are shown that the trapezoidal origami samples have higher virtual stiffness than the flat samples, and the trapezoidal origami crash thin wall absorbs 40 % more energy in Y-axis direction compared to in X-axis direction.

Variable Admittance Control With Virtual Stiffness Guidance for Human–Robot Collaboration

Human-robot collaboration (HRC) is a promising solution for expanding the use of robotic systems in unstructured environments and complex processes in various industries. In this paper, a novel variable admittance control (VAC) with virtual stiffness guidance (VSG) is proposed to improve the performance of HRC. The proposed VAC prevents unnecessary changes of the damping parameter by classifying the human intentions in the low-velocity region, which results in smooth movement. Additionally, the VAC with VSG makes the robot actively assist an operator using a virtual spring. Under the proposed VSG scheme, the equilibrium position of the virtual spring can be adjusted by the operator during a task. The proposed control strategies are implemented in a four-degree-of-freedom hydraulic manipulator referred to as HydCobot. Two experimental tasks for evaluating the accuracy, effort, and elapsed time are conducted to validate the effectiveness of the proposed methods. The results indicate that the proposed methods effectively enhance the performance of HRC.