Haptic displays can greatly facilitate manual control tasks. Their capacity of allowing the operator to perceive the desired dynamics is an important design parameter. However, attempts to evaluate haptic displays on the basis of what dynamics humans actually perceive are scarce. This paper proposes a two-step framework which incorporates the characteristics of human haptic perception into the evaluation of haptic displays. The first step is to evaluate the haptic display based on a recently developed model of the threshold for changes in the perception of system dynamics. It allows us to know the frequency spectrum in which a haptic device alters the operator's perception of the system dynamics. The second step is then to understand how the perceived dynamic distortions affect the operator's characterization of the system dynamics. Findings from recent psychophysical studies allow us to relate the changes in perception caused by the haptic device to changes in the perceived mechanical properties of the system. A numerical example illustrates how haptic displays can be evaluated using the proposed framework.@en

Abstract—Just notable difference (JND) thresholds for the perception of manipulator dynamic properties are relevant for tele-operation and simulation of vehicles. Manipulator dynamic properties are characterized by multiple variables (describing mass, spring and damping for a linear manipulator) and the JND threshold for any of these variables is affected by variation in the remaining variables. In previous work, we demonstrated and modeled the coupling of the stiffness JND and the mass JND, and investigated the effects of stiffness and mass properties on the damping JND. In this work we investigate how changes in the damping parameter affect the JND in perceiving stiffness and mass. In an experiment our subjects were instructed to discriminate between different levels of manipulator’s stiffness or mass, while tracking a prescribed sinusoidal manipulator movement. Results show that the JND in spring force and the JND in inertia force are identical, and increase for higher damping levels. The JND model developed in our previous work can successfully describe the experimental observations, thereby providing an extension of Weber’s law. The impedance of the manipulator is considered as the reference stimulus in the frequency domain, so that a single ratio describes the JND thresholds for all three properties. Index Terms—Just noticeable difference, Mass-spring-damper systems, Frequency response function, Weber’s law, haptics @en

At present, the rapid development of automation technologies allows robots remarkable precision and endurance, as well as the strength in accomplishing repetitive tasks. Despite this, manual control is still indispensable in many domains where robots and humans play complementary roles, as humans demonstrate superior competence in improvisation and flexibility, as well as the excellent ability to take on tasks where things cannot be fully specified. Haptic interfaces provide a prime example which combines the strengths of these two elements, allowing them to interact and merge into a highly integrated control loop. A haptic interface is usually created by providing force feedback related to the task on a control device. The haptic feedback makes performing manual control more intuitive, allowing the operator to physically act upon what (s)he feels, rather than generating the control activity through only interpreting other sensory inputs, such as visual and auditory cues. Over the last few decades, haptic interfaces have gained popularity as being powerful tools to facilitate manual control. By analogy with a visual interface, one can interpret a haptic interface as the display that presents information to and accepts commands from a human operator. While giving input through the interface, the neuromuscular system of the operator also acts as the eye that perceives the information being presented by a display. This highly interactive nature underlines the importance of orienting the development of all haptic systems towards humans, particularly towards what humans feel and how they need to act. To facilitate future development of haptic interfaces, this thesis focuses on two of the main challenges that have not been adequately addressed from such a human centric perspective: (i) among various possibilities, how can we select the one that works more effectively with humans, i.e., using understanding of human control behavior (how humans act) to guide the development of the philosophy of the design?, and (ii) how can we know whether a device ensures a transparent haptic interaction, i.e., incorporating the characteristics of human haptic perception (what humans feel) into the evaluation of the quality of the display? ...@en

Time delays in haptic teleoperation affect the ability of human operators to assess mechanical properties (damping, mass, and stiffness) of the remote environment. To address this, we propose a unified framework for human haptic perception of the mechanical properties of environments with delayed force feedback. In a first experiment, we found that the delay in the force feedback led our subjects to underestimate all the three mechanical properties. Moreover, subjects perceived additional damping or stiffness properties that the environment did not possess. It was found that the extents of these changes in the perception depend on both time-delay magnitude and the frequency of the movement with which subjects interacted with the environment. This was due to the fact that subjects were not able to distinguish the delay-caused phase shift in the movement-force relation from changes in the three mechanical properties. Based on this, we proposed a framework that allowed for a prediction of the change associated with delayed force in perception of mass-spring-damper environments. The framework was corroborated by a second experiment, in which a combined mass-damper environment was tested. Our hypotheses that the delay would cause subjects to underestimate the mass but overestimate the damping and that the extents of the under- A nd overestimation would differ between individual subjects due to the difference in the interaction frequency were confirmed.@en

Limitations of a haptic device can cause distortions of the force feedback it presents. Just-noticeable difference (JND) in system dynamics is important for creating transparent haptic interaction. Based on the previous work, this article presents a unified model that extends the existing JND rule. Our approach projects the JNDs in the mechanical properties of a second-order mass-spring-damper system onto the real and imaginary components of the system's frequency response function (FRF). We discuss the results of two experiments and show that the JNDs obtained for both the real and imaginary components can be expressed as the same fraction of, and thus are proportional to, the magnitude of the total system's FRF. Furthermore, the findings are generalized to cases where the system's dynamics order is different than two. What results is a unified model that accurately describes the threshold for changes in human perception of any linear system dynamics with only two dimensions: the real and imaginary axes in the complex plane.@en

Wave-variable transformation is a means to maintain stability of haptic teleoperation in the presence of communication time delays. Its drawback is that it affects haptic perception of remote properties and thereby degrades transparency. This paper studies the effect of wave-variable transformation on human haptic perception. Based on a framework of haptic perception developed in previous work, we systematically investigated how the wave variable affects human perception of damping, mass and stiffness properties of an arbitrary linear environment. Both the original wave-variable approach and the generalized wave-variable approach are investigated. Results show how both approaches change human perception of all three mechanical properties of the environment, and how these changes vary with both excitation frequency and time delay. The generalized wave-variable approach on the whole outperforms the original in terms of rendering mass and stiffness, but not always for rendering damping. Results also show that human perception of the dynamics rendered by both approaches is similar to that of the original environment only when time delays are small. As the time delay increases, evaluating the mechanical properties can become very difficult for a human operator if the interaction with the environment is not static.@en

A large variation of the haptic Just Noticeable difference (JND) in stiffness is found in literature. But no underlying model that explains this variation was found, limiting the practical use of the stiffness JND in the evaluation work of control loading system (CLS). To this end, we investigated the cause of this variation from humans' strategy for stiffness discrimination, by two experiments in which a configurable manipulator was used to generate an elastic force proportional to its angular displacement (deflection). In a first experiment, the stiffness JND was measured for three stiffness levels, and an invariant Weber fraction was obtained. We found that for stiffness discrimination, subjects reproduced the same amount of the manipulator deflection and used the difference in the terminal forces as the indication of the stiffness difference. We demonstrated that the stiffness Weber fraction and the force Weber fraction could be related by a systematic bias in the deflection reproduction, which was caused by the difference in the manipulator stiffness. A second experiment with two conditions was done to verify this model. In one condition, we measured the stiffness JND while asking subjects to move the manipulator to a target angular displacement. Thus the bias in the deflection reproduction was eliminated, and this resulted a stiffness Weber fraction that equaled the force Weber fraction. In the other condition, the stiffness JND was measured without the deflection target, and a bias in deflection reproduction was again observed. This bias related the measurements for the two conditions by the formulation obtained from the first experiment. This suggests that the accuracy of reproducing the manipulator position for stiffness discrimination, which may be susceptible to experimental setting, can be used to explain the variation of stiffness JND in literature. Suggestions are given for CLS evaluation and applications requiring precise manipulator motion control.@en

We discuss an extension of the basic principles underlying the human haptic just noticeable difference (JND) in perceiving a manipulator's mechanical properties from force feedback. Two cases are studied: 1) the JND in perceiving the stiffness of manipulators with various masses; 2) the JND in perceiving the damping of a combined mass-spring-damper system with varying stiffness and mass. The extended JND laws are obtained through mapping psychophysical findings to JND formulations based on frequency response functions. We first present two human-factor experiments in which subjects discriminated between different levels of manipulator stiffness/damping while moving the manipulator with a prescribed sinusoidal deflection. For the two testing cases both JNDs violate Weber's law: Due to the increases in mass, the normalized stiffness JND (the Weber fraction) decreases as the reference stiffness level increases; The damping JND for a constant reference damping increases with higher combined responses of stiffness and mass. On the basis of weighting the frequency response magnitude of mechanical properties, we performed model identification that fit the experimental observations, and extended the JND laws for the two testing cases. Our extended JND laws indicate that: 1) stiffness and mass affect the stiffness JND in the same way, the stiffness JND is a fixed proportion of the combined frequency response of stiffness and mass; 2) the frequency response magnitude of the damping JND is a fixed proportion of the frequency response magnitude of the combined system (the mass-spring-damper system).@en

This study revisits the active manipulator developed for manual aircraft control. The active manipulator sends the force applied by the pilot to the aircraft while feeding back the aircraft rotational velocity bymeans of its deflection angle. We find that the activemanipulator, in comparison with the conventional passive manipulator, greatly facilitates target following and disturbance rejection in compensatory tasks. We also find that greater improvements in task performance are associated with higher forcing-function bandwidths. The findings are accounted for by the fact that the active manipulator changes the effective controlled element dynamics into an integrator, and integrates the disturbance rejection into the neuromuscular system. Furthermore, we demonstrate that the haptic feel of the activemanipulator depends on the dynamics of the aircraft. With further exploration, we reveal that human haptic perception of the active manipulator could be characterized by mass-spring-damper properties. @en

Most haptic interfaces developed for aircraft control provide haptic support as an additional force on the control manipulator. This study revisits the active manipulator, which is a design concept that is different from but complementary to existing haptic interfaces. This control device sends the force that the pilot exerts on it to the aircraft while feeding back the aircraft rotational rate by means of its deflection angle. It is found that, in comparison with the conventional passive manipulator, the active manipulator greatly facilitates target following and disturbance rejection in compensatory tracking tasks. Furthermore, larger improvements in task performance are associated with higher forcing-function bandwidths. These findings are accounted for by the fact that the active manipulator changes the effective controlled-element dynamics into integratorlike dynamics while at the same time integrating disturbance rejection into the neuromuscular system. However, the high-frequency disturbances acting on the aircraft present in feedback about the aircraft state adversely affect the operational effectiveness of the active manipulator. Based on the experimental findings and results from the passivity theory, a lead-lag filter is designed and evaluated, which mitigates this effect without affecting task performance.@en