5 V IBROTACTILE F EEDBACK IN T OUCHSCREEN I NTERACTION
5.3 Generating Vibrotactile Feedback Using Directional Forces
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environments (against the minimum static load exerted by a fingertip [100gF]). Unlike cheaper and more efficient vibrotactile actuators (ERM, LRA), linear displacement actuators need to function in a dynamically loaded environment, and yet still maintaining minimum micro-displacement levels.
An alternative approach, generally utilized for desktop and tabletop devices with touchscreen input, is to deform parts or the entire touchscreen or display area to create kinetic forces. This is done either by utilizing a flexible design structure or simply be fitting the touchscreen surface with an elastic or deformable transparent overlay. Using actuation mechanisms based on electrically controllable mechanical materials (SMAs or Ferromagnetic) attached to the overlays, it is possible to deform the structure or even the contour of the overlay to relay tactile or kinesthetic information to the user. This technique has the added advantage of physical deformation along with user’s perceptual response to the multimodal experience of kinetic forces, as well as visual transform of the object of the screen. Although this technique can induce haptic imagination, its implementation is limited to large stationary touchscreen based devices. Furthermore, the technique requires substantial energy to generate the necessary forces to transform the touchscreen or attached screen overlay within the necessary timeframe of interaction (usually 150-300ms).
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ORCES The use of overlays moving across a touchscreen in both mobile as well as table top displays is been research considerably. Actuating various textural overlays longitudinally or tangentially on the screen can generate a wide range of feedback sensation, including friction variations. This section explores some of the existing research in creating directional forces on touchscreens and its viability for generating feedback on mobile device (adapted from Farooq et al., 2016b).In recent years, transmitting directional lateral forces accompanying user’s onscreen activity has been a key techniques of creating haptic feedback (McHugh, 2015; Roudaut, 2013; Saga and Raskar, 2013, Saga and Deguchi, 2012; Dandekar et al., 2003). Many studies have been done to characterize the user sensitivity of skin micro-displacements at the fingertip. Most researchers agree that applying shear force to produce in-plane tangential micro-displacements of the skin can be very practical and efficient in providing tactile informative signals in a wide area of applications.
For lateral displacement, Drewing et al., (2005) observed angular resolution thresholds of up to 14-34 degrees, whereas, Vitello et al., (2006) measured thresholds of 30-40 degrees. Placencia et al., (2009) also
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observed direction-dependent sensitivity and found angular thresholds to be around 14 degrees; however, this was done for tangential haptic stimulation. Perhaps, what is interesting is that the above studies revealed different thresholds in different directions, which would suggest that the fingertip is more sensitive to stimuli in the distal direction than in the proximal direction. Either way, the sensitivity of skin micro-displacements at the fingertip, irrespective of the direction, can be utilized to deliver localized and comprehensive tactile information on the touchscreen, which is not possible by shaking the entire device (Dai et al., 2012).
Perceiving Tangential Micro-displacements
Shear force and tangential micro-displacements of the skin are perceived by several types of mechanoreceptors. Birznieks et al., (2001) applied directional forces to the finger pad and recorded that Merkel cells (SA-I), Ruffini corpuscles (SA-II), and Pacini corpuscles (FA-I) afferents, all responded to and encoded directional information (Vallbo and Johansson, 1978). This finding is supported by Vallbo and Johansson’s earlier work, who identified Meissner corpuscles (SA-II) as the primary receptor for skin stretch though other receptor types might also be involved in processing cutaneous sensory input (Olausson et al., 2000; Norrsell et al., 1992).
Similarly, Srinivasan et al., (1990) also identified Meissner corpuscles (SA-II) as the main source of encoding tangential skin micro-displacements.
Additionally, Olausson et al., (1998) also concluded that lateral skin stretch is primarily encoded by Meissner corpuscles (SA-II) afferents while Merkel cells (SA-I) afferents were more sensitive to spatiotemporal stimuli.
Furthermore, research in defining the role of directional stimuli while detecting skin stretch has also been analyzed. Wang and Hayward, (2007), Maeno et al., (1998) and Gleeson et al., (2010) have all tried to categorize and understand tangential skin deformation through measuring and modeling the properties of the human finger pad. Although, researchers may still argue over the resolution of directional spatial stimuli with reference to SA-I and SA-II afferents, they unanimously agree regarding the sensitivity of skin to micro-displacements as well its ability to deliver tactile information.
Directional Forces and Friction Modulation
Until now, the only way of creating a strong sense of directional forces on an interactive surface—in the absence of the mechanical linkages with respect to a reference ground plane—was by modulating the friction coefficient (Muller et al., 2010). When the friction is artificially raised, moving an input device, such as a stylus or a fingertip, over the interactive surface requires a greater force from the user to move the manipulandum.
On the other hand, if the friction coefficient is artificially lowered, moving
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the manipulandum over the interactive surface becomes much easier. By switching between both states, a perception of virtual surface structures (surface edges, virtual fixtures, and textures) can be controllable with respect to the interaction scenario (Daud, 2011; Abbott et al., 2007).
Friction modulation can be achieved in multiple ways. One approach was described by Müller et al. (2010). The researchers developed a stylus containing a steel ball tip and an electromagnetic coil. When moving the stylus over the display surface, the steel ball tip rotates with a minimum amount of friction. By using an electromagnetic brake, the friction force can be modified dramatically. Thus, the force required to move the stylus over the display surface can be modulated.
Another approach was presented by Levesque et al., (2011). Using 26 kHz vibrations produced by a piezoelectric actuator, the authors were able to create a “squeezed air film” that reduces the friction on an interactive display surface. By using this technique, just noticeable difference in friction of approximately 30–40% has been recorded. The presented approach allowed for the simulation of multiple friction levels that could be used for the tactile presentation of different object properties and well-distinguishable levels of the virtual surface.
An alternative approach could allow creating lateral forces in order to simulate virtual edges and surface structures. This concept is based on research done by Robles-De-La-Torre (2002), who described the importance of lateral forces in perceiving, recognizing, and identifying planar shapes. Based on his research, the T-Pad (Tactile Pattern Display through Variable Friction Reduction) was developed by Winfield et al., (2007) and presented at EuroHaptics. The authors utilized piezoelectric actuators to alter the friction coefficient and simulate lateral forces under the fingertip. On the other hand, Kaye (2012) also utilized the slightly different approach of designing an active computer response (output) to the user’s touchscreen inputs. He introduced the possibilities of using saw-tooth-shaped vibrational patterns to create a well-perceived force that could be applied to the user’s fingertip. This allowed the generation of a richer set of sensations when interacting through the mobile touchscreen.
Although the concept discussed by Kaye (2012) is useful in providing tactile signals to the skin, none of the approaches discussed above produced forces strong enough to change the direction of stylus or fingertip movements. The differences in friction force could only be experienced by the user and perceived when s/he initiated the actual movement of the input device (manipulandum). Essentially, most of the solutions discussed can be described as “passive” because they only react to the user’s onscreen activity, instead of actively affecting the user’s input behavior.
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