Patent Publication Number: US-11036295-B2

Title: Electrostatic slide clutch

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application No. 62/426,163 filed Nov. 23, 2016, the entirety of which is hereby incorporated herein by reference. 
    
    
     BACKGROUND 
     A state-of-the-art virtual reality (VR) or mixed reality (MR) holographic system may immerse a user in a convincing alternative reality, where visual and auditory aspects of virtual objects are represented in a true-to-life manner. 
     SUMMARY 
     Examples are disclosed that relate to electrostatic slide clutches that may be used for tactile feedback in mixed reality and virtual reality settings. One example provides an electrostatic slide clutch comprising a first electrode, a second electrode oriented parallel to the first electrode, an electrically insulating structure, and a controller. The electrically insulating structure comprises a heterogeneous surface structure and is disposed on the first electrode on a face of the first electrode opposing the second electrode. The controller is electrically coupled to the first electrode and to the second electrode and configured to apply a variable voltage between the first and second electrodes to influence a normal force between the first and second electrodes. 
     This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. Furthermore, the claimed subject matter is not limited to implementations that solve any or all disadvantages noted in any part of this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows aspects of an example display system including a body-movement restriction device. 
         FIGS. 2 and 3  show aspects of an example electrostatic slide clutch of a body-movement restriction device. 
         FIG. 4  shows aspects of an example electrically insulating structure of an electrostatic slide clutch. 
         FIGS. 5 and 6  show aspects of other electrically insulating structures of an electrostatic slide clutch. 
     
    
    
     DETAILED DESCRIPTION 
     While mixed reality and virtual reality systems may provide an immersive visual and auditory experience, such systems may not provide a satisfying tactile experience for the user, in which virtual objects feel like the real objects they represent. Thus, examples are disclosed that relate to electrostatic slide clutches that may be used for tactile feedback in mixed reality and virtual reality settings. 
       FIG. 1  shows aspects of an example display system  10  configured to present to a user  12  a lifelike virtual reality environment. The display system  10  as illustrated is used to support virtual reality game play, but may be used in numerous other scenarios as well. Display system  10  includes a headset  14  and a haptic device  16 . The headset  14  includes a stereoscopic display  18  configured to display virtual imagery in the field of view of the user. In some examples, the stereoscopic display is see-through, enabling real-world and virtual imagery to be admixed in the user&#39;s field of view. This approach may be referred to as augmented or mixed reality. In headset  14 , signal encoding the virtual display imagery is sent to the stereoscopic display via on-board computer  20 . The on-board computer includes at least one processor  22  and associated electronic memory  24 . 
     Leveraging communications componentry arranged in headset  14 , on-board computer  20  may be coupled communicatively to one or more off-board computers on a network. Thus, the virtual display imagery that user  12  sees may, in some examples, be composed and/or rendered by an off-board computer. In other examples, the virtual display imagery may be composed and rendered on-board. 
     Headset  14  includes stereophonic loudspeakers  26  that enable user  12  to experience immersive VR audio. Haptic device  16  of display system  10  is configured to further augment the immersive experience by providing lifelike contact sensation responsive to user interaction with the virtual environment. In particular, the haptic device may be configured to provide a mechanical resistive contact sensation in response to detection of contact between the body of the user and a virtual display object projected into a field of view of the user. For instance, the mechanical resistive contact sensation may be provided whenever the hand of user  12  intersects virtual display object  28 . 
     For example, haptic device  16  may optionally include a skin-pressure simulation portion  30 A. The skin-pressure simulation portion is a component of the haptic device configured to apply touch-simulating pressure to the user&#39;s skin in the vicinity of contact with a virtual display object. The skin-pressure simulation portion may include an actuator configured to apply pressure in a direction normal to the surface of the skin. The skin-pressure simulation portion may include a piezoelectric or voice-coil type actuator, for example. In order to determine when to apply the pressure, the skin-pressure simulation portion is communicatively coupled to one or more computers of display system  10 , such as on-board computer  20  of headset  14 . More specifically, the computer hosts a model of the virtual environment and also tracks the position of the user&#39;s hand with the real-world coordinates of that model. When contact between the hand and a solid virtual object is indicated, the actuator is energized and pressure is applied to the skin. The actuator is de-energized when contact is no longer indicated. Accordingly, the skin-pressure simulation portion may be configured to give a static, rather than a dynamic actuation. 
     Although skin-pressure simulation portion  30 A may simulate the sensation of the touch of a virtual object on the user&#39;s skin, this aspect alone may not provide a satisfactory contact sensation, as it would not offer a resistive effect of object contact on the user&#39;s skeletal joints. Contact with an actual solid object, by comparison, would result in the sensation of kinematic resistance to attempted movement through the object, which would be felt at the joints. For example, the joints of the fingers, in attempting to move the fingers through a solid object, would experience at least some reactive force from the object, with reference to Newton&#39;s third law. 
     To simulate this sensation, haptic device  16  includes at least one body-movement restriction portion  32  in the form of an electrostatic slide clutch. In the example shown in  FIG. 1 , a first body-movement restriction portion  32 A is arranged in a glove worn on the hand of user  12 , and a second body-movement restriction portion  32 B is arranged in a sleeve worn around the arm. A sleeve supporting a body-movement restriction portion may be worn alternatively or additionally on the leg or other body part. Body-movement restriction portion  32  may be used either with or without skin-pressure simulation portion  30  in various implementations. 
       FIG. 2  shows aspects of electrostatic slide clutch  34  of body-movement restriction portion  32 A. In general, the electrostatic slide clutch is configured to vary the sliding and/or static frictional force between two substrates  36  movable translationally with respect to each other. In the example of  FIG. 2 , first substrate  36 A is coupled to the skin on a first side of a skeletomuscular joint  38 , and second substrate  36 B is coupled on a second, opposite side of the skeletomuscular joint. 
     Configurations differing from that of  FIG. 2  are also envisaged. In one example, the body-movement restriction portion may take the form of a tube around the finger that becomes stiffer to restrict movement. In yet another example, substrates may be arranged as overlapping scales on a medieval armor glove. 
       FIG. 3  shows additional aspects of electrostatic slide clutch  34 . The electrostatic slide clutch includes a first electrode  40 A and a second electrode  40 B oriented parallel to the first electrode. In  FIG. 3 , the first electrode is formed on or bonded to a first substrate  36 A and the second electrode is formed on or bonded to a second substrate  36 B. Each of the first and second substrates is secured to the skin of the user, or otherwise closely coupled to an articulable region of the body. More generally, one or both of the first and second electrodes may be formed on or bonded to a substrate, and one or both of the substrates may be secured to the skin. In other examples, an electrode may be applied directly to the skin, such that no distinct substrate is required for that electrode. 
     In some examples, one or both of first electrode  40 A and second electrode  40 B includes an electrically conductive continuous or discontinuous (e.g., patterned) film applied to an underlying substrate using solution-process methods. Examples of electrically conductive films include ink-jet, screen-printed, gravure printed, or flexographic-printed films. Other examples include slot-die coatings and spin coatings of conductive films. Graphite, silver, or gold films, for example, may be applied. In other examples, a metal-film electrode may be formed by vacuum deposition methods such as physical vapor deposition (PVD) one or more metals, such as aluminum or copper, etc. onto a substrate. Chemical vapor deposition, electroless plating, electroplating, atomic-layer deposition, and/or other forms of material deposition may also be used. Suitable electrode substrates may be rigid or flexible, depending on the detailed configuration of electrostatic slide clutch  34 . 
     In some examples, one or both of the first and second electrodes  40  may be flexible. A flexible electrode may be formed from a woven or non-woven conductive fabric, for example. In some examples, one or both of the first and second electrodes may have relatively high sheet resistance, as very little current is drawn through the electrodes during operation of electrostatic slide clutch  34  (vide infra). 
     Continuing in  FIG. 3 , an electrically insulating structure  42  is disposed on first electrode  40 A on a face of the first electrode opposing the second electrode  40 B. In the illustrated example, the electrically insulating structure is arranged in slidable contact with the second electrode. In other examples, electrostatic slide clutch  34  further comprises a second insulating sheet disposed on the second electrode on a face of the second electrode opposing the first electrode. There, the first and second insulating sheets may be arranged in slidable contact with each other. 
     Electrostatic slide clutch  34  includes a slide guide  44  configured to guide relative sliding motion of the first and second electrodes. The slide guide may include a slot for one or both of the first and second electrodes. In the example of  FIG. 3 , the slide guide is fixedly coupled to first electrode  40 A and includes a slot for second electrode  40 B and the associated substrate. More generally, the slide guide may be fixedly coupled to either of the first and second electrodes or to a substrate thereof. 
     Electrostatic slide clutch  34  includes a controller  46  electrically coupled to first electrode  40 A and to second electrode  40 B and configured to apply a variable voltage between the first and second electrodes, to influence a normal force between the first and second electrodes. Positive voltage may be applied to the first electrode relative to the second electrode, or vice versa. In some examples, an alternating voltage of suitable frequency may be used, to facilitate dynamically changing the force applied during sliding operation. 
     Applying voltage of either polarity causes an amount of unbalanced charge to form on the opposing surfaces of electrodes  40 , which draws the electrodes together via the Coulomb force. In the illustrated example, the Coulomb force is balanced by a reactive (i.e., third law) normal force between electrically insulating structure  42  and second electrode  40 B. Increasing normal force brings about a corresponding increase in both static and sliding friction forces between the electrically insulating structure and the second electrode. 
     In some examples, the material properties of electrically insulating structure  42  may be selected for suitable resistance to sliding wear and to provide a suitably high coefficient of static and/or sliding friction, so that an increase in the normal force between the electrically insulating structure and second electrode  40 B restricts the relative sliding motion of the first and second electrodes. This action may cause, in some scenarios, adhesion between the two surfaces. 
     To ensure that electrostatic slide clutch  34  imparts resistive force at the appropriate time, controller  46  is communicatively coupled to one or more computers of display system  10 , such as on-board computer  20  of headset  14 . Contact between the user&#39;s body and virtual objects in the virtual or mixed reality environment is determined with reference to the virtual environmental model hosted by the computer and to tracked body positions. 
     Counterforce to separate the electrodes and relieve the applied frictional force may be provided, for example, by a small deviation from planarity of the electrodes and associated substrates, which acts as a spring. In the example of  FIG. 3 , slide guide  44  accommodates only one pair of opposing first and second electrodes. In other examples, the slide guide may accommodate a plurality of pairs of opposing first and second electrodes, to achieve increased frictional force for a given applied voltage. 
     Electrically insulating structure  42  may be of a thickness selected to achieve a suitably strong normal force at a given voltage, and thereby reduce the operating voltage for operation of electrostatic clutch  34 . Both the electric field strength and the charge are inversely proportional to the distance between the electrodes, so the normal force is inversely proportional to the square of the distance and directly proportional to the surface area of overlap between the first and second electrodes. In some implementations, roughening, texturing, and/or shaping the electrodes over an appropriate length scale may amplify the effective overlap area. 
     In the examples envisaged herein, electrically insulating structure  42  includes a dielectric material. The dielectric material may be selected to exhibit a high dielectric strength in order to support a large electric field without suffering breakdown. Moreover, the dielectric material may exhibit a high dielectric constant, to maximize the normal force at a given voltage. Increasing the dielectric constant of the dielectric material increases proportionately the surface charge on first and second electrodes  40 , resulting in a proportional increase in the normal force at a given voltage. In one example, a 10 μm thick electrically insulating poly(vinylidenefluoride-trifluoroethylene-chlorofluoroethylene)) (P(VDF-TrFE-CFE)) sheet provides adequate normal force with 150 volts applied between the first and second electrodes. In some examples, the dielectric material may exhibit a dielectric constant from 3 or above. In some examples, the dielectric material may exhibit a dielectric constant between 20 and 60. 
     In some examples, the dielectric material of electrically insulating structure  42  may be a homogeneous, sheet-forming material, such as a polymer of suitable dielectric constant and dielectric strength. In other examples, the electrically insulating structure may be a solid material composite (i.e., a mixture) comprising the dielectric material dispersed and immobilized within a polymer matrix. For example, the dielectric may include finely divided aluminum oxide dispersed in poly(vinylidenefluoride) (PVDF). Table 1 lists physical properties of various example dielectric materials. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Dielectric Materials 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                 Dielec- 
                 Dielec- 
               
               
                   
                   
                   
                   
                 tric 
                 tric 
               
               
                 Material a   
                 VD c   
                 Static b   
                 Sliding b   
                 Constant 
                 Strength d   
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 PTFE 
                   
                 0.04 
                   
                 2.1 
                 500 
               
               
                 Al 2 O 3 / 
                 yes 
                 1.05-1.13 
                 1.4 
                 9.34 
                 16 
               
               
                 Sapphire 
               
               
                 SiO 2   
                 yes 
                   
                   
                 3.9 
               
               
                 Si 3 N 4   
                 yes 
                   
                   
                 7.5 
               
               
                 Dragon Skin 
                   
                   
                   
                 2.7-3   
                 13 
               
               
                 Medium 
               
               
                 P7670 
                   
                   
                   
                 2.7-3   
                 30-80 
               
               
                 Polyimide 
                   
                 0.63 
                 0.48 
                 3.4 
                 303 
               
               
                 Polyurethane 
                   
                   
                   
                 7.1-30  
               
               
                 PVC 
                   
                   
                   
                 3.5-8   
               
               
                 PE 
                   
                   
                   
                 2.5 
               
               
                 HTT C1 
                 yes 
                   
                   
                 20-25 
               
               
                 HTT T1 
                 yes 
                   
                   
                 60 
                 80 
               
               
                 SU8-2000 
                 no 
                   
                   
                 4.1 
               
               
                 SU8-3000 
                 no 
                   
                   
                 3.2 
               
               
                 Plexiglas 
                   
                   
                   
                 1.9 
               
               
                 Pyrex 
                 no 
                 0.9-1     
                 0.4 
                 4.7 
               
               
                 Natural rubber 
                 no 
                 1-4 
                 0.5-1 
                   2-2.5 
               
               
                 BaTiO 3   
                 yes 
                   
                   
                  1200-10000 
               
               
                 PET 
                   
                   
                   
                 2.9 
                  50-100 
               
               
                 PFA 
                   
                   
                   
                 2.1 
               
               
                 Parylene 
                 yes 
                   
                   
                  2.5-2.67 
               
               
                 ZrO 2   
                 yes 
                   
                   
                 10-23 
               
               
                 (Pb,La)(Zr,Ti)O 3   
                 yes 
                   
                   
                 440 
               
               
                 (PLZT) 
               
               
                 Ta 2 O 5   
                 yes 
                   
                   
                 25 
               
               
                 TiO 2   
                 yes 
                   
                   
                  80-170 
               
               
                   
               
               
                   a Dragon Skin Medium is a product of Smooth-On, Inc. of Macungie, PA. P7670 is a product of Wacker Chemie AG of München, Germany; Kapton Mylar, Teonex, Tetoron, and HFF are products of Dupont of Wilmington, DE; HTT C1 and HTT T1 are products of Arkema of Colombes, France; SU8-2000 and SU8-3000 are products of Microchem Corp. of Westborough, MA. 
               
               
                   b Coefficients of static and sliding friction. 
               
               
                   c Yes indicates that the material is amenable to vapor deposition. 
               
               
                   d Dielectric strength in units of volts per micrometer. 
               
            
           
         
       
     
     Electrically insulating structure  42  may be formed on or bonded to first electrode  40 A in any suitable manner. In some examples, as noted above, first electrode  40 A may include a conductive fabric. Here, the dielectric material of electrically insulating structure  42  may permeate the conductive fabric of the first electrode. In other examples, the electrically insulating structure may be blade-cast or spin-cast, for example, to a thickness of 1 μm, or deposited on the first electrode by vapor-phase deposition. Both physical vapor deposition and chemical vapor deposition are envisaged. Table 1 provides non-limiting examples of dielectric materials amenable to vapor-phase deposition. 
     In some examples, electrically insulating structure  42  may be subject to surface treatment. For example, a chemical surface treatment may be used to modify coefficients of static and/or sliding friction, or to increase the wear resistance or dielectric strength of the electrically insulating structure. Physical surface treatment such as mechanical abrasion may be used to roughen the electrically insulating structure to increase the frictional coefficients, or to prepare the sheet for chemical surface treatment, for example. Second electrode  40 B may also be subject to chemical and physical surface treatment, for similar advantages. 
     In some examples, the electrically insulating structure may have a heterogeneous surface structure (which also may be referred to as a composite surface structure) having zones differing in one or more of dielectric composition, matrix composition, surface treatment and/or surface relief. The length scale of the heterogeneity (i.e., the size of the various zones) is not particularly limited; the length scale may differ from one embodiment to the next and may range from microns to tens of millimeters. In the example shown in  FIG. 4 , the heterogeneous surface structure of electrically insulating structure  42 ′ includes a millimeter- to micrometer-scale pattern of such zones  48 . As noted above, zones of other dimensions are also envisaged. The illustrated example shows a checkerboard pattern of alternating zones  48 A and  48 B, as a non-limiting example of a patterning of zones. This approach may facilitate fine-tuning of the dielectric constant and surface properties, such as the frictional coefficients. Strictly or substantially two-dimensional zone patterns may be used in some examples. In other examples, the zone pattern also may extend normal to the electrically insulating structure. One such example is shown in  FIG. 5 . Here, the pattern of zones of electrically insulating structure  42 ″ includes surface features interlocking three-dimensionally with complementary surface features of second electrode  40 B″. This configuration increases the effective frictional forces between the first and second electrodes. In some examples, zone patterns may be formed on the electrically insulating structure  42 ″ and/or second electrode  40 B″ via a micromolding or embossing procedure. In other examples, the zone patterns may be formed by lithography and/or etching. In the examples provided above, the heterogeneous surface structure of the electrically insulating structures is on the micrometer-to-millimeter scale. 
     In some examples, a motion restricting apparatus may comprise a heterogeneous surface structure in the form of macroscopically separate frictional and dielectric surfaces.  FIG. 6  shows an example of such a structure. Separating frictional surface  48 B from dielectric surface  48 A lessens wear on the dielectric surface with repeated actuation of electrostatic slide clutch  34 . This approach also may facilitate subjecting the frictional and dielectric surfaces to different surface treatments. For example, the dielectric surface may be treated with a lubricant to lessen wear, while the frictional surface may remain unlubricated, in order to preserve the high frictional coefficients. In the illustrated example, second electrode  40 B′″ includes a low-friction zone  50 A opposing the dielectric surface of the electrically insulating structure and a high-friction zone  50 B opposing the frictional surface of the electrically insulating structure. In these and other examples, the first and second electrodes may be shaped so as to provide a region of overlap of a predetermined or changing geometry as the electrodes move against each other. For instance, non-rectangular electrodes may provide a changing area of overlap (and normal force, therefore) as one electrode slides over the other. 
     No aspect of the foregoing description should be understood in a limiting sense, for numerous variations, extensions, and omissions are contemplated as well. In some configurations, for example, haptic device  16  may be non-wearable, but instead integrated into a control, such as a dial. The electrostatic slide clutch of the haptic device may provide variable resistance to the turning of the dial, or give the sensation of a series of detents, for example, pursuant to resistive applied via a controller  46 . 
     In other examples, a body-movement restriction portion in the form of an electrostatic slide clutch may be used in medical, rather than virtual or mixed reality applications. Controllable body-movement restriction may be useful for the patient rehabilitating from a skeletomuscular injury such as back/knee injury, or brain injury such as stroke. In other examples, a body-movement restriction portion may be used as an active restraint for management of seizures or potentially dangerous body movements associated with autism, psychiatric disorders, or acute substance abuse. 
     A body-movement restriction portion in the form of an electrostatic slide clutch may also be amenable to various industrial-safety applications. In one example, machine vision or other environment-sensing componentry may be used to assess whether persons are in close proximity to potentially dangerous machinery. Persons wearing an appropriate body-movement restriction portion operatively coupled to the environment-sensing componentry may be protected from drawing too close to the machinery, extending a hand or finger into a cutting device, etc. In other examples, a body-movement restriction portion worn by a worker may be configured for skeletal strain mitigation. When the worker is carrying a heavy object, the body-movement restriction portion may lock into place, providing additional resistance to motion and relieving stress on the worker&#39;s fingers, wrists, arms, legs, and other skeletal joints. 
     In still other examples, an electrostatic slide clutch as described above may be used in a variety of electromechanical applications. The clutch may be used to provide positive braking for a servomechanical (e.g., a robotic, soft robotic, and/or ball-joint) actuator. Additionally, each of the electrostatic slide clutch examples described herein also may be used as a sensor, as the electric current drawn through the device at constant voltage bias is related to the rate of change of the area of overlap between the opposing electrodes, and of the portion of the overlap area occupied by the dielectric. As such, the structures described herein may be used both to provide a controllable resistance to motion and to provide sensor output regarding an extent of motion. In one example, measurement of the capacitance across the opposing electrodes may provide an indication of the extent of motion. More generally, the controller of the electrostatic slide clutch may be configured to provide an output responsive to an amount of charge provided to the first or second electrodes in response to relative motion of the first and second electrodes. 
     Another example provides an electrostatic slide clutch comprising: a first electrode; a second electrode oriented parallel to first electrode; an electrically insulating structure disposed on the first electrode on a face of the first electrode opposing the second electrode, the electrically insulating structure having a heterogeneous surface structure including a pattern of zones differing in one or more of dielectric composition, matrix composition, surface treatment, and surface relief; and a controller electrically coupled to the first electrode and to the second electrode and configured to apply a variable voltage between the first and second electrodes, to influence a normal force between the first and second electrodes. 
     In some implementations, the electrically insulating structure is a first electrically insulating structure, the electrostatic slide clutch further comprising a second insulating sheet disposed on the second electrode on a face of the second electrode opposing the first electrode, and wherein the first electrically insulating structure and second electrically insulating structures are arranged in slidable contact with each other. In some implementations, the electrically insulating structure includes a dielectric material. In some implementations, a dielectric constant of the dielectric material is between 20 and 60. In some implementations, the dielectric material is dispersed in a polymer matrix. In some implementations, the dielectric material includes aluminum oxide. In some implementations, the electrically insulating structure comprises a vapor-phase deposited film. In some implementations, the electrically insulating structure comprises a treated surface. In some implementations, the pattern of zones includes surface features interlocking three-dimensionally with complementary surface features of the first electrode, to increase an effective frictional force between the first and second electrodes. In some implementations, the heterogeneous surface structure includes macroscopically separate frictional and dielectric surfaces. 
     Another example provides a haptic device of a virtual reality system, the haptic device comprising: a body-movement restriction portion in the form of an electrostatic slide clutch, comprising: a first electrode disposed on a first substrate; a second electrode disposed on a second substrate, where each of the first and second substrates is closely coupled to an articulable region of the body; an electrically insulating structure disposed on the first electrode on a face of the first electrode opposing the second electrode; and a controller electrically coupled to the first electrode and to the second electrode and configured to apply a variable voltage between the first and second electrodes, to influence a normal force between the first and second electrodes. 
     In some implementations, the body-movement restriction portion is configured such that an increase in the normal force restricts the relative sliding motion. In some implementations, the electrically insulating structure is arranged in slidable contact with the second electrode. In some implementations, the electrically insulating structure includes a dielectric material, wherein the first electrode includes a conductive fabric, and wherein the dielectric material permeates the conductive fabric of the first electrode. In some implementations, one or both of the first and second electrodes is flexible. In some implementations, the heterogeneous surface structure of the electrically insulating structure includes a millimeter- to micrometer-scale pattern of surface features interlocking three-dimensionally with complementary surface features of the first electrode, to increase an effective frictional force between the first and second electrodes. Some implementations comprise a plurality of pairs of opposing first and second electrodes, to achieve increased frictional force for a given applied voltage. In some implementations, the electrostatic clutch is arranged in a glove or sleeve of the virtual reality system. In some implementations, the first substrate is coupled to the skin on a first side of a skeletomuscular joint of the user, and the second substrate is coupled to the skin on a second, opposite side of the skeletomuscular joint. 
     Another example provides a virtual reality system comprising: a body-movement restriction portion in the form of an electrostatic slide clutch, comprising: a first electrode disposed on a first substrate; a second electrode oriented parallel to first electrode, the second electrode disposed on a second substrate, where each of the first and second substrates is closely coupled to an articulable region of the body; an electrically insulating structure disposed on the first electrode on a face of the first electrode opposing the second electrode; and a controller electrically coupled to the first electrode and to the second electrode and configured to apply a variable voltage between the first and second electrodes, to influence a normal force between the first and second electrodes in response to detection of contact between the body of the user and a virtual display object projected into a field of view of the user. 
     It will be understood that the configurations and/or approaches described herein are exemplary in nature, and that these specific embodiments or examples are not to be considered in a limiting sense, because numerous variations are possible. The specific routines or methods described herein may represent one or more of any number of processing strategies. As such, various acts illustrated and/or described may be performed in the sequence illustrated and/or described, in other sequences, in parallel, or omitted. Likewise, the order of the above-described processes may be changed. 
     The subject matter of the present disclosure includes all novel and non-obvious combinations and sub-combinations of the various processes, systems and configurations, and other features, functions, acts, and/or properties disclosed herein, as well as any and all equivalents thereof.