This article reviews the technology behind creating artificial touch sensations and the relevant aspects of human touch. We focus on the design and control of haptic devices and discuss the best practices for generating distinct and effective touch sensations. Artificial haptic sensations can present information to users, help them complete a task, augment or replace the other senses, and add immersiveness and realism to virtual interactions. We examine these applications in the context of different haptic feedback modalities and the forms that haptic devices can take. We discuss the prior work, limitations, and design considerations of each feedback modality and individual haptic technology. We also address the need to consider the neuroscience and perception behind the human sense of touch in the design and control of haptic devices.

Virtual reality systems would benefit from a compelling force sensory substitute when workspace or stability limitations prevent the use of kinesthetic force feedback systems. We present a wearable fingertip haptic device with the ability to make and break contact in addition to rendering both shear and normal skin deformation to the fingerpad. A delta mechanism with novel bias spring and tether actuator relocation method enables the use of high-end motors and encoders, allowing precise device control: 10 Hz bandwidth and 0.255 mm RMS tracking error were achieved during testing. In the first of two experiments, participants determined the orientation of a stiff region in a surrounding compliant virtual surface with an average angular error of 7.6 degree, similar to that found in previous studies using traditional force feedback. In the second experiment, we evaluated participants' ability to interpret differences in friction. The Just Noticeable Difference (JND) of surface friction coefficient discrimination using our skin deformation device was 0.20, corresponding with a reference friction coefficient of 0.5. While higher than that found using kinesthetic feedback, this demonstrates that users can perceive differences in surface friction without world-grounded kinesthetic forces. These experiments show that three DoF skin deformation enables both stiffness and friction discrimination capability in the absence of kinesthetic force feedback.

One of the main barriers to immersivity during object manipulation in virtual reality is the lack of realistic haptic feedback. Our goal is to convey compelling interactions with virtual objects, such as grasping, squeezing, pressing, lifting, and stroking, without requiring a bulky, world-grounded kinesthetic feedback device (traditional haptics) or the use of predetermined passive objects (haptic retargeting). To achieve this, we use a pair of finger-mounted haptic feedback devices that deform the skin on the fingertips to convey cutaneous force information from object manipulation. We show that users can perceive differences in virtual object weight and that they apply increasing grasp forces when lifting virtual objects as rendered mass is increased. Moreover, we show how naive users perceive changes of a virtual object's physical properties when we use skin deformation to render objects with varying mass, friction, and stiffness. These studies demonstrate that fingertip skin deformation devices can provide a compelling haptic experience appropriate for virtual reality scenarios involving object manipulation.

When a person interacts with an environment using a tool, he/she receives tactile information in the form of fingerpad skin deformation. Different interaction forces and torques on the tool cause different skin deformation patterns on the fingerpads. We designed a 6-degree-of-freedom tactile device that creates similar skin deformation patterns on the fingerpads. The device communicates force and torque information by translating and rotating skin deformation tactors relative to the fingerpads. An experiment was conducted to determine participants' ability to use skin deformation tactile cues to perform a peg-in-hole insertion task. Results show that participants can use the tactile cues to reduce interaction force and torque, and they use the tactile force cues to reduce interaction force more than they use the tactile torque cues to reduce interaction torque. Rendering force and torque cues simultaneously causes device saturation and degrades user performance. These results suggest that additional training may help participants use the skin deformation torque cues, and motivate a tactile device design that decouples force and torque skin deformation rendering to minimize device saturation. Fingerpad skin deformation is a promising form of tactile feedback to convey force and torque information in teleoperation systems such as robot-assisted surgery, where force feedback may be undesirable due to stability and safety concerns.

During multi-lateral collaborative teleoperation, where multiple human or autonomous agents share control of a teleoperation system, it is important to be able to convey individual user intent. One option for conveying the actions and intent of users or autonomous agents is to provide force guidance from one user to another. Under this paradigm, forces would be transmitted from one user to another in order to guide motions and actions. However, the use of force guidance to convey intent can mask environmental force feedback. In this paper we explore the possibility of using tactile feedback, in particular skin deformation feedback, skin deformation feedback to convey collaborative intent while preserving environmental force perception. An experiment was performed to test the ability of participants to use force guidance and skin deformation guidance to follow a path while interacting with a virtual environment. In addition, we tested the ability of participants to discriminate virtual environment stiffness when receiving either force guidance or skin deformation guidance. We found that skin deformation guidance resulted in a reduction of path-following accuracy, but increased the ability to discriminate environment stiffness when compared with force feedback guidance.

During tool-mediated interaction with everyday objects, we experience kinesthetic forces and tactile sensations in the form of vibration and skin deformation at the fingerpad. Fingerpad skin deformation is caused by forces applied tangentially and normally to the fingerpad skin, resulting in tangential and normal skin displacement. We designed a device to convey 3-degree-of-freedom (DoF) force information to the user via skin deformation, and conducted two experiments to determine the devices effectiveness for force-feedback substitution and augmentation. For sensory substitution, participants used 1-DoF and 3-DoF skin deformation feedback to locate a feature in a 3-DoF virtual environment. Participants showed improved precision and shorter completion time when using 3-DoF compared to 1-DoF skin deformation feedback. For sensory augmentation, participants traced a path in space from an initial to a target location, while under guidance from force and/or skin deformation feedback. When force feedback was augmented with skin deformation, participants reduced their path-following error over the cases when force or skin deformation feedback are used separately. We conclude that 3-DoF skin deformation feedback is effective in substituting or augmenting force feedback. Such substitution or augmentation could be used when force feedback is unattainable or attenuated due to device limitations or system instability.

During robot-assisted minimally invasive surgery, surgeons perform challenging dexterous tasks, including the manipulation of soft tissue and suture tying. In the absence of environment force sensing of tool-tissue interaction forces to provide force feedback, surgeons must rely on visual feedback to modulate the grip force they apply on the environment. Clinical systems, like the da Vinci Surgical System (Intuitive Surgical, Inc.), use physical springs to provide closing resistance on the gripper degree-of-freedom (DOF) of the master manipulator. This feedback provides increasing force resistance as the gripper is closed. To determine the effect of master manipulator gripper stiffness on performance in a teleoperated manipulation task, we designed a new and open source gripper, the OmniGrip. The OmniGrip attaches to a SensAble Phantom Omni (now available as Geomagic Touch), replacing the stySensAble Phantom Omnilus end effector, and providing the ability for user programmable force characteristics. We conducted a study in which participants used an OmniGrip to teleoperate a Raven II surgical robRaven II surgical robotic systemotic system in a pick-and-place task. Increasing the stiffness of the OmniGrip resulted in reduced interaction forces at the slave-side environment. Additionally, these interaction forces were significantly lower when the OmniGrip as compared to when using the Phantom Omni stylus.

When we use a tool to explore or manipulate an object, friction between the surface of the tool and the fingerpads generates skin stretch cues that are related to the interaction forces between the tool and the object. In this study, we emulate these naturally occurring skin stretch cues in order to convey force direction and magnitude information to users during teleoperation. We hypothesize that skin stretch feedback is a useful substitute for kinesthetic force feedback in force-sensitive teleoperated tasks. In this study, ten participants performed teleoperated palpation to determine the orientation of a stiff region in a surrounding artificial tissue using five feedback conditions: skin stretch, force, reduced gain force, graphic, and vibration. When participants received skin stretch feedback, they localized the stiff region as well as with force feedback, with no increase in task completion time. Additionally, participants receiving skin-stretch feedback localized the stiff region statistically significantly more accurately than those using vibration feedback. Although participants using skin stretch exhibited higher interaction forces than when using force, vibration, and graphical feedback, skin stretch statistically significantly decreased interaction forces compared with reduced gain force feedback. Thus, skin-stretch feedback is a compelling substitute for force feedback and may be useful in scenarios where force feedback is reduced or infeasible.

Teleoperated robots are used in a variety of applications. The immersive nature of the teleoperated experience is often limited by a lack of haptic information. However, in many applications there are difficulties conveying force information due to limitations in hardware fidelity and the inherent tradeoffs between stability and transparency. In situations where force feedback is limited, it is possible to use sensory substitution methods to convey this force information via other sensory modalities. We hypothesize that skin stretch feedback is a useful substitute for kinesthetic force feedback in force-sensitive teleoperated tasks. We created and tested a tactile device that emulates the natural skin deformation present during tool mediated manual interaction. With this device, experiment participants performed teleoperated palpation to determine the orientation of a stiff region in a surrounding artificial tissue using skin stretch, force, reduced gain force, graphic, or vibration feedback. Participants using skin stretch feedback were able to determine the orientation of the region as accurately as when using force feedback and significantly better than when using vibration feedback, but also exhibited higher interaction forces. Thus, skin stretch feedback may be useful in scenarios where force feedback is reduced or infeasible.

During manual interactions, we experience both kinesthetic forces and tactile sensations. Friction and normal force between the fingerpads and the tool/interaction surfaces cause shear and normal deformation of the skin. Capitalizing on this observation, we designed a 3-degree-of-freedom (DoF) tactile device that is grasped by a user and can render both tangential skin stretch and normal deformation on the skin of the user's fingerpads. Tactile feedback from the device is delivered in a manner consistent with natural tactile cues from manual interaction. An experiment assessed the accuracy with which users can locate the center of a contoured hole on a virtual surface. The task was completed under four conditions: the cases of skin deformation and force feedback, with both 3- and 1-DoF feedback in each case. With 3-DoF feedback, users located the hole faster and more accurately than with 1-DoF feedback, for both force and skin deformation feedback. These results indicated that users were able to interpret the additional DoF cues provided by our 3-DoF tactile device to improve task performance.

During tool-mediated interactions with objects, we experience force and fingerpad skin stretch resulting from shear forces caused by friction between the fingerpad skin and the stylus. When probing an object, for the same penetration distance, a stiffer object causes a larger load force and, thus, greater fingerpad skin stretch. We hypothesized that rendering additional artificial skin stretch together with force will increase perceived stiffness. We created a Skin Stretch Stylus that renders skin stretch through tactor displacement, attached it to a force-feedback device, and performed a study to characterize the effect of tactor displacement-induced skin stretch on stiffness perception. Results showed that adding artificial skin stretch causes additive augmentation of perceived stiffness across a range of surface stiffness, and the addition is a linear function of tactor displacement gain. However, intersubject variability in the estimated slope coefficient was large. We propose a model that explains the additive effect and suggests potential sources for the intersubject variability. We conclude that augmenting force feedback with skin stretch can increase users' perception of stiffness, but the effect is user-specific. Such augmentation may be useful in virtual environment and teleoperation scenarios when force feedback gains must be kept low to prevent feedback-induced instabilities, or when force feedback is limited due to actuator force limits.

Skin stretch is a novel haptic feedback method that can provide a human operator with information about the magnitude and direction of an applied force. To evaluate the potential for skin stretch feedback to be used as a sensory substitute for kinesthetic (force) feedback in robotic teleoperation systems, a study was conducted to measure the ability of users to discriminate environment stiffness using varying levels of fingerpad skin stretch instead of force feedback. A new, high-fidelity skin stretch feedback device was developed that imposes tangential fingerpad skin stretch in proportion to the intended level of force feedback. In psychophysical experiments, users received skin stretch feedback with magnitude proportional to the users' penetration depth into a virtual wall. Users' stiffness discrimination capability using skin stretch was comparable to that of using force feedback. Furthermore, larger skin stretch cues were perceived by users as portraying greater stiffness without any advance training, which indicates that skin stretch feedback would be an intuitive sensory substitute for force feedback. Thus, skin stretch feedback is a promising method for conveying kinesthetic force information in applications such as robot-assisted surgery, where high levels of force feedback may not be desirable due to stability or safety concerns.

When interacting with everyday objects, we experience kinesthetic force feedback as well as various forms of cutaneous tactile feed-back. Skin stretch is part of the cutaneous tactile experience that is caused by friction between the skin and the grasped object. Interacting with stiffer objects causes larger force, and results in a larger amount of skin stretch. Therefore, we hypothesize that adding artificial fingerpad skin stretch to kinesthetic force feedback will increase users' perception of stiffness. A tactile display called the Skin Stretch Stylus was designed to augment kinesthetic force feedback with skin stretch feedback. The change in users' stiffness perception due to the addition of skin stretch feedback is quantified through a two-alternative forced-choice paradigm, method of constant stimuli experiment. In this experiment, subjects compared the stiffness of virtual springs with kinesthetic force feedback augmented with skin stretch feedback versus virtual springs with only kinesthetic force feedback. Results show that the addition of skin stretch causes a significant increase in the perception of stiffness, and this effect increases with higher amount of applied skin stretch. These results indicate that skin stretch feedback could be used to augment perceived stiffness in situations where it is not possible to increase force feedback gains. Such scenarios include teleoperation systems where force feedback gains must remain low to ensure stability, and haptic devices with limited actuator force.