Virtual Stiffness Research Articles

Tactile Exploration Strategies With Natural Compliant Objects Elicit Virtual Stiffness Cues.

When interacting with deformable objects, tactile cues at the finger pad help inform our perception of material compliance. Nearly all prior studies have relied on highly homogenous, engineered materials such as silicone-elastomers and foams. In contrast, we employ soft plum fruit varying in ripeness; ecological substances associated with tasks of everyday life. In this article, we investigate volitional exploratory strategies and contact interactions, for comparison to engineered materials. New measurement techniques are introduced, including an ink-based method to capture finger pad to fruit contact interactions, and instrumented force and optical sensors to capture imposed force and displacement. Human-subjects experiments are conducted for both single finger touch and two finger grasp. The results indicate that terminal contact areas between soft and hard plums are indistinguishable, but the newly defined metric of virtual stiffness can differentiate between the fruits' ripeness, amidst their local variations in geometry, stiffness, and viscoelasticity. Moreover, it affords discrimination independent of one's touch force. This metric illustrates the tie between the deployment of active, exploratory strategies and the elicitation of optimal cues for perceptual discrimination. Compared to single finger touch, perceptual discrimination improves further in pinch grasp, which is indeed a more natural gesture for judging ripeness.

Developing a virtual stiffness-damping system for airfoil aeroelasticity testing

In this research a two-degrees-of-freedom (2-DOF) virtual stiffness-damping system (VSDS) is developed to facilitate industrial and laboratory testing of airfoil aeroelasticity instability. Other existing test-beds in this field rely on elastic elements or structures to set airfoil elasticity in tests, which can be costly and inconvenient in cases of frequent stiffness adjustment across a wide range. A possible alternative is the VSDS that utilizes electric drives to simulate structural elasticity and damping, as seen in marine and bio-mechanical engineering, which however, cannot be directly applied to airfoil aeroelasticity testing (AAT) due to operation requirements and conditions being different. Therefore, in this study a new VSDS is developed specifically for AAT. Firstly, the concept of 1-DOF VSDS is extended to 2 DOFs, with the dynamics coupling between each DOF addressed at the stage of operation principle determination, by the proposed direct force/torque regulation with force/torque feedback. Secondly, resolution loss in position/velocity measurement is identified as a main problem associated with the non-reduction transmission required, and is solved by a modified extended-state observer (MESO) proposed for fast position/velocity estimation. Thirdly, system identification and calibration procedures involved in developing the new VSDS are reduced to minimum through a robust force/torque tracking controller design, with detailed numerical study on parametric analysis given. As validated in wind-tunnel experiments the new VSDS can closely track the desired force/torque and provide satisfactory virtual stiffness and damping in AAT.

Non-Linear Regression Method for Instability Problems in Haptic Rendering System

This paper addresses instability problems in 6-DoFs haptic rendering system due do the virtual stiffness technology in haptic device by using non-linear polynomial regression analysis method for haptic feedback value. The method is based on the locally optimized generalized penetration computation algorithm which computes the minimum translational and rotational motion to separate two overlapping objects. The essence of penalty-based haptic rendering method is the stability problem which is the prospect in virtual manipulation and physical experiment simulation. The algorithm which employs the virtual coupling method in order to optimize the haptic feedback value in haptic rendering system. Combining with the non-linear regression method, we get a well-distributed output and stable force feedback. The constructed rendering algorithm can process the complexity polygon-soup models and make no assumption about the underlying geometry topology. The experiment result shows the non-linear regression analysis method can make the haptic system stable and get realistic force feedback.

Evolution and Analysis of Hapkit: An Open-Source Haptic Device for Educational Applications.

We present the design, evolution and analysis of "Hapkit," a low-cost, open-source kinesthetic haptic device for use in educational applications. Hapkit was developed in 2013 based on the design of the Stanford Haptic Paddle, with the goal of decreasing cost and increasing accessibility for educational applications, including online teaching, K-12 school use, and college dynamic systems and control courses. In order to develop Hapkit for these purposes, we tested a variety of transmission, actuation, and structural materials. Hapkit 3.0, the latest version, uses a capstan drive, inexpensive DC motor, and 3-D printed structural materials. A frequency-domain system identification method was used to characterize Hapkit dynamics across the various designs. This method was validated using a first principles parameter measurement and a transient response analysis. This characterization shows that Hapkit 3.0 has lower damping and Coulomb friction than previous designs. We also performed a user study demonstrating that Hapkit 3.0 improves discrimination of virtual stiffness compared to previous designs. The design evolution of Hapkit resulted in a low-cost, high-performance device appropriate for open-source dissemination and educational applications.

A novel active balance assistive control strategy based on virtual stiffness model of XCoM

PurposeThis paper aims to propose a novel balance-assistive control strategy for hip exoskeleton robot.Design/methodology/approachA hierarchical balance assistive controller based on the virtual stiffness model of extrapolated center of mass (XCoM) is proposed and tested by exoskeleton balance assistive control experiments.FindingsExperiment results show that the proposed controller can accelerate the swing foot chasing XCoM and enlarge the margin of stability.Originality/valueAs a proof of concept, this paper shows the potential for exoskeleton to actively assist human regain balance in sagittal plane when human suffers from a forward or backward disturbing force.

Application of a Redundant Haptic Interface in Enhancing Soft-Tissue Stiffness Discrimination

Haptic-enabled teleoperated surgical systems have the potential to enhance the accuracy and performance of surgical interventions. The user interface of such a system can provide haptic feedback to the surgeon to more intuitively perform surgical tasks. In this letter, we study the added benefits of redundant manipulators as haptic interfaces for teleoperated surgical systems. First, we introduce the intrinsic benefits of employing a redundant haptic interface, namely, reduced apparent inertia and increased manipulability (one result of which is reduced friction forces). Next, we demonstrate that the haptic interface redundancy can further reduce its apparent inertia and friction via appropriately manipulating the extra degrees of freedom of the interface. This will consequently enhance the haptic feedback resolution (sensitivity) for the user. Finally, a psychophysical experiment is performed to validate the improved force perception for the user in a virtual soft-tissue palpation task. We conduct a set of perceptual experiments to evaluate how a redundant and non-redundant user interface affects the perception of the virtual stiffness. Experimental results demonstrate that the redundancy in the haptic user interface helps to enhance tissue stiffness discrimination ability of the user by reducing the distortions caused by the kinematics and dynamics of the user interface.

Haptic Performance Using Voltage-Mode Motor Control

Haptic devices driven by dc motors are usually controlled in current mode due to the direct relationship between current and torque. This paper analyzes the performance of voltage-mode controllers whose main drawback is that the torque of the actuator depends on its electrical dynamics. However, the electrical dynamics of the motor add the viscosity generated by the back electromotive force. Since the motor damping seen from the handle of the interface is increased by the square of the transmission ratio, the physical damping of the mechanism can be very high, maintaining low inertia. As a result, very high performance in terms of critical stiffness can be achieved, even using cost-effective electronics. There is a tradeoff between the achievable virtual stiffness during the haptic interaction and the backdrivability in free movement, if the damping is set too high. To investigate the benefits of this motor control strategy, CEIT's haptic gearshift is used as a testbed. The experimental results confirm that a very high critical stiffness can be achieved using this strategy.

Safety design and performance analysis of humanoid rehabilitation robot with compliant joint

Human safety represents the key requirement in physical human-robot interaction (pHRI). However, the majority of current research works focus on the safety evaluation. There is no definite way to design a robot that not only meets the performance but also has inherent safety. A humanoid robot for human joint movement rehabilitation is developed, to fulfill the requirement of both safety and its performance. A nonlinear model of human-robot collision with effective mass and stiffness of robot’s end-effecter (EE) is proposed. An important parameter involved in the model is the joint stiffness of the robot, which has an inherent direct effect on the safety. The influence of joint compliance on the modal frequencies is analyzed, and the kinematic performance of the robot is estimated roughly by the lowest order modal frequency. The design method of compliant joint stiffness is put forward, which can balance the safety and kinematics performance requirements. When the compliance is utilized intentionally in joints to improve safety, the question whether or not rigid performance of compliant joint can be achieved or approached by control is arose naturally. Such a challenge is addressed by the cascade control, where outer position loop with link-side position feedback is constructed. Here, restoring torque in the elastic transmission is taken as virtual control that is, in turn, set as the reference command of the inner torque loop. Intuitively, once the virtual control is reproduced as rapidly as possible in the inner torque loop, quasi-rigid performance is exhibited in the outer position loop as if there were no series elasticity in compliant joint. The stability criterion is derived, and the virtual stiffness and dynamic performance are examined. Finally, experiments are performed to validate effectiveness of the suggested control scheme.

FW-Touch: A Finger Wearable Haptic Interface With an MR Foam Actuator for Displaying Surface Material Properties on a Touch Screen.

The haptic interface plays an increasingly important role in enhancing the realism and immersion of the user's interaction with the touch screen. Inspired by the wearable haptic system, this paper proposes a finger wearable device called FW-Touch for touch screen interaction. The device provides normal force, lateral force, and vibrotactile feedback for the interaction of the finger and the touch screen through three internally integrated actuators. By displaying the hardness, friction, and roughness of a virtual surface, the device is capable of simulating the active exploration and sensing process of the finger on a real surface. This paper describes the design and specifications of the FW-Touch, and details the design process of a magnetorheological (MR) foam actuator that uses a Hall sensor to correct the output force. Through physical measurements and psychophysical experiments, we comprehensively evaluated the force feedback performance of the FW-Touch and its ability in displaying the stiffness and friction of the virtual surface. The results show that improving the accuracy of force feedback is necessary for virtual stiffness display, and the accuracy and effectiveness of the FW-Touch in displaying virtual surface features can be confirmed from the measured stiffness and friction Weber fractions.

Weaver: Hexapod robot for autonomous navigation on unstructured terrain

AbstractLegged robots are an efficient alternative for navigation in challenging terrain. In this paper we describe Weaver, a six‐legged robot that is designed to perform autonomous navigation in unstructured terrain. It uses stereo vision and proprioceptive sensing based terrain perception for adaptive control while using visual‐inertial odometry for autonomous waypoint‐based navigation. Terrain perception generates a minimal representation of the traversed environment in terms of roughness and step height. This reduces the complexity of the terrain model significantly, enabling the robot to feed back information about the environment into its controller. Furthermore, we combine exteroceptive and proprioceptive sensing to enhance the terrain perception capabilities, especially in situations in which the stereo camera is not able to generate an accurate representation of the environment. The adaptation approach described also exploits the unique properties of legged robots by adapting the virtual stiffness, stride frequency, and stride height. Weaver's unique leg design with five joints per leg improves locomotion on high gradient slopes, and this novel configuration is further analyzed. Using these approaches, we present an experimental evaluation of this fully self‐contained hexapod performing autonomous navigation on a multiterrain testbed and in outdoor terrain.

Optimal passivity design of a virtual coupling including FIR-type fractional derivatives for a haptic interface

This paper describes a haptic interface with a virtual coupling, including fractional derivatives. A haptic interface is a force feedback technology in virtual reality that takes advantage of the human sense of touch. In a haptic interface, virtual impedance, that is called virtual coupling (VC), is commonly used between the virtual and real objects to calculate reaction force. VC generally consists of a virtual stiffness and damper, but the stiffness has to be set low in a system with long sampling periods. In order to increase the virtual stiffness, this paper considers a VC including fractional derivatives approximated by an FIR approximation. First, we theoretically analyse effects of a single fractional derivative term by using the passivity analysis, but the result shows that its effects depend on the occurred frequency. This paper, therefore, proposes a method to combine multiple fractional derivative terms in a VC and a method to optimize parameters in each fractional derivative term. Finally, experiments are performed to measure the maximum value of the stiffness to illustrate the effects of the proposed method.

Effect of Dual-rate Sampling on the Stability of a Haptic Interface

Conventional controllers in impedance-based haptic interfaces are sampled data systems that utilize position and velocity information for the necessary force feedback. In a canonical virtual wall simulation, this feedback force is generated based on interaction with the wall simulating certain stiffness and damping. Increasing the sampling rate of the controller increases the stable range of virtual wall stiffness. However, an increased sampling rate exacerbates the velocity information, decreasing the stable range of virtual wall damping. In this work, the authors propose a dual-rate sampling scheme in which the position and velocity loops of a haptics controller are decoupled and sampled at different rates. The scheme enables independent sampling of the position data at higher rates, while simultaneously sampling the velocity data at appropriate rates. In this paper, the authors provide experimental and theoretical implications of the effect of dual-rate sampling on the stability of a haptic interface. Experiments with a single degree-of-freedom (DOF) haptic interface reveals an enhanced region of stable virtual wall stiffness for a particular range of virtual wall damping, compared to the values with conventional uniform-rate scheme. Virtual wall stiffness ranging from 150 - 360 Nm/rad was stably implemented over a range of 0 - 1 Nms/rad of virtual wall damping using the proposed scheme at position loop sampling rate of 20kHz and velocity loop sampling rate of 2kHz. Whereas in the conventional scheme, the stable range of virtual stiffness dropped considerably (∼ 0 Nm/rad) for the virtual wall damping above 0.1 Nms/rad when the uniform rate sampling of 20kHz was used for both the position and velocity sampling loops. Theoretical stability analyzes using classical control tools and simulations justified the effectiveness of the proposed scheme. The scheme is easy to implement and extensible to multi-DOF haptic interfaces as well.

A real-time impedance-based singularity and joint-limits avoidance approach for manual guidance of industrial robots

This paper studies real-time manual guidance considering singularity and joint-limits avoidance using impedance control in an industrial scenario. The operator is responsible for keeping the end-effector (EE) away from the robot’s singularity and joint-limits. The proposed approach detects the singularity and joint-limits in real-time. Then, virtual stiffness and damping are added to target stiffness and damping as the robot is getting close to the singularity or joint-limit. A criterion is presented for detection of singularity by combining manipulability ellipsoid and condition number. Also a new joint to Cartesian space transformation is formulated in order to convert joint stiffness and damping to Cartesian stiffness and damping for joint-limits avoidance method. The presented approach is applied on a SCARA robot. An experiment is performed in this paper to investigate singularity and joint-limits avoidance separately as well as together. Increase in stiffness and damping warn the operator of the possibility of singularity or joint-limits allowing the operator to changes the EE path. The proposed approach, eliminates the need for a robotics expert by allowing any operator with no knowledge about robot singularity and joint-limits to interact and teach the robot in a safe, real-time and time-saving manner.

중력 보상 팔 기능 지지대의 설계 및 제어 성능 평가

This paper presents the design and the control performance of a novel dynamic compliant-arm support with parallel elastic actuators that was developed to assist with the daily living activities of those whose arms are compromised by muscular disease or the aging process. The parallel elastic-arm support consists of a compliant mechanism with combined passive and active components for human interaction and to reach the user’s desired positions. The achievement of these tasks requires impedance control, which can change the virtual stiffness, damping coefficients, and equilibrium points of the system; however, the desired-position tracking by the impedance control is limited when the end-effector weight varies according to the equipping of diverse objects. A prompt algorithm regarding weight calibration and friction compensation is adopted to overcome this problem. A result comparison shows that, by accurately assessing the desired workspace, the proposed algorithm is more effective for the accomplishment of the desired activities.

Alternative Admissible Functions for Natural Frequencies and Modeshapes of a Beam with Lumped Attachments

In this paper, the use of a combination of trigonometric and low order polynomials as admissible functions to be used with the method of assumed modes is investigated for the calculation of the natural frequencies and modeshapes of a beam with lumped attachments. Since the admissible functions do not satisfy the boundary conditions, penalty terms are used to replace the constraints of the boundary conditions of the beam, with virtual stiffness elements of appropriate values representing the boundary conditions. By comparison with previously obtained results, the proposed method using the assumed modes approach with admissible functions and penalty terms is evaluated for accuracy and computational effectiveness. It is shown that the proposed method is accurate and shows no ill-conditioning for the problem of an Euler-Bernoulli beam with lumped attached elements.

Augmented virtual stiffness rendering of a cable-driven SEA for human-robot interaction

Human-robot interaction U+0028 HRI U+0029 is fundamental for human-centered robotics, and has been attracting intensive research for more than a decade. The series elastic actuator U+0028 SEA U+0029 provides inherent compliance, safety and further benefits for HRI, but the introduced elastic element also brings control difficulties. In this paper, we address the stiffness rendering problem for a cable-driven SEA system, to achieve either low stiffness for good transparency or high stiffness bigger than the physical spring constant, and to assess the rendering accuracy with quantified metrics. By taking a velocity-sourced model of the motor, a cascaded velocity-torque-impedance control structure is established. To achieve high fidelity torque control, the 2-DOF U+0028 degree of freedom U+0029 stabilizing control method together with a compensator has been used to handle the competing requirements on tracking performance, noise and disturbance rejection, and energy optimization in the cable-driven SEA system. The conventional passivity requirement for HRI usually leads to a conservative design of the impedance controller, and the rendered stiffness cannot go higher than the physical spring constant. By adding a phase-lead compensator into the impedance controller, the stiffness rendering capability was augmented with guaranteed relaxed passivity. Extensive simulations and experiments have been performed, and the virtual stiffness has been rendered in the extended range of 0.1 to 2.0 times of the physical spring constant with guaranteed relaxed passivity for physical humanrobot interaction below 5 Hz. Quantified metrics also verified good rendering accuracy.

Kinesthetic Feedback During 2DOF Wrist Movements via a Novel MR-Compatible Robot

We demonstrate the interaction control capabilities of the MR-SoftWrist, a novel MR-compatible robot capable of applying accurate kinesthetic feedback to wrist pointing movements executed during fMRI. The MR-SoftWrist, based on a novel design that combines parallel piezoelectric actuation with compliant force feedback, is capable of delivering 1.5 N [Formula: see text] of torque to the wrist of an interacting subject about the flexion/extension and radial/ulnar deviation axes. The robot workspace, defined by admissible wrist rotation angles, fully includes a circle with a 20 deg radius. Via dynamic characterization, we demonstrate capability for transparent operation with low (10% of maximum torque output) backdrivability torques at nominal speeds. Moreover, we demonstrate a 5.5 Hz stiffness control bandwidth for a 14 dB range of virtual stiffness values, corresponding to 25%-125% of the device's physical reflected stiffness in the nominal configuration. We finally validate the possibility of operation during fMRI via a case study involving one healthy subject. Our validation experiment demonstrates the capability of the device to apply kinesthetic feedback to elicit distinguishable kinetic and neural responses without significant degradation of image quality or task-induced head movements. With this study, we demonstrate the feasibility of MR-compatible devices like the MR-SoftWrist to be used in support of motor control experiments investigating wrist pointing under robot-applied force fields. Such future studies may elucidate fundamental neural mechanisms enabling robot-assisted motor skill learning, which is crucial for robot-aided neurorehabilitation.

Haptic Perception of Edge Sharpness in Real and Virtual Environments.

We investigate the accuracy with which the haptic sharpness perception of a virtual edge is matched to that of a real edge and the effect of the virtual surface stiffness on the match. The perceived sharpness of virtual edges was estimated in terms of the point of subjective equality (PSE) when participants matched the sharpness of virtual edges to that of real edges with a radius of 0.5, 2.5, and 12.5 mm over a virtual stiffness range of 0.6 to 3.0 N/mm. The perceived sharpness of a real and a virtual edge of the same radius was significantly different under all but one of the experimental conditions and there was a significant effect of virtual surface stiffness on the accuracy of the match. The results suggest that the latter is presumably due to a constant penetration force employed by the participants that influenced the penetration depth and perceived sharpness of virtual edges at different surface stiffness levels. Our findings provide quantitative relations for appropriately offsetting the radii of virtual edges in order to achieve the desired perceived sharpness of virtual edges.

Data analysis on virtual stiffness in 6DoFs haptic rendering system

We present an optimization analysis method for virtual stiffness in 6-DoFs haptic rendering system. The method is based on the locally optimized generalized penetration computation algorithm which computes the minimum translational and rotational motion to separate two overlapping objects. The essence of penalty-based haptic rendering method is computing an amount of penetration depth, and the output force magnitude is following the Hooke׳s law. We use the virtual coupling method to calculate the output force and analysis the damping and stiffness coefficient in order to get a rendering force which optimizes the haptic feedback value in haptic rendering system. We mapped the virtual contact results to the feedback force and torque, and successfully integrated the algorithm into the off-the-shelf 6Dof haptic device. Our rendering algorithm can handle highly complexity polygon models and make no assumption about the underlying geometry topology. The experiment result shows the optimization analysis method can generate stable and realistic haptic feedback.

Study on effects of effects of surface properties in haptic perception of virtual curvature

Stiffness and friction are regarded as important haptic cues for designers of haptic virtual environments. One virtual concave curvature discrimination test based on randomised block design with three factors, radius, stiffness and friction, has been designed and performed to understand human perceptual performance. Randomised block design with three factors can separately distinguish the subject random effect from main effects. In the experiment, subjects compare two virtual sequential concave surfaces and identify the surface having the higher curvature. The statistical results based on Analysis of Variance ANOVA, regression tree and logistic regression indicate that variations in curvature discrimination may be slightly attributable to varying virtual haptic stiffness between 0.3 N/mm and 0.7 N/mm. Also, a clear interaction in radius and friction coefficient between 0 and 0.2 is found. While the experimental results are limited by some factors, the authors believe that the resulting effects of haptic surface properties may serve to improve the fidelity of medical virtual reality simulators.