CHI 2018 Honourable Mention

CHI 2018, April 21–26, 2018, Montréal, QC, Canada

Haptic Revolver: Touch, Shear, Texture, and Shape
Rendering on a Reconfgurable Virtual Reality Controller
Eric Whitmire1 , Hrvoje Benko2 , Christian Holz2 , Eyal Ofek2 , Mike Sinclair2
1 Paul G. Allen School, DUB Group, University of Washington, Seattle, WA
2 Microsoft Research, Redmond, WA
emwhit@cs.washington.edu, {benko, cholz, eyalofek, sinclair}@microsoft.com

Figure 1. (left) Our Haptic Revolver device uses a wheel that raises and lowers and spins underneath the fngertip to render various haptic sensations.
(center) The haptic wheels are interchangeable and can be customized to render arbitrary textures, shapes, or interactive elements. (right) Wheel
features are spatially registered with the virtual environment, so the user can reach out and feel virtual surfaces.

ABSTRACTINTRODUCTION

We present Haptic Revolver, a handheld virtual reality controller that renders fngertip haptics when interacting with
virtual surfaces. Haptic Revolver’s core haptic element is anactuated wheel that raises and lowers underneath the fnger
to render contact with a virtual surface. As the user’s fnger
moves along the surface of an object, the controller spins thewheel to render shear forces and motion under the fngertip.
The wheel is interchangeable and can contain physical textures,
shapes, edges, or active elements to provide different sensations to the user. Because the controller is spatially tracked,
these physical features can be spatially registered with thegeometry of the virtual environment and rendered on-demand.
We evaluated Haptic Revolver in two studies to understand
how wheel speed and direction impact perceived realism. We
also report qualitative feedback from users who explored three
application scenarios with our controller.

Recent advances in display technologies, computer graphics,
and tracking have led to a resurgence in head mounted displaysfor virtual reality. Today’s consumer VR devices are capableof rendering realistic visual and audio content and positionally tracked handheld controllers further improve the sense ofpresence by bringing the user’s hands into the virtual world.
Despite these advances, the ability of such devices to render
the sense of touch is lacking. Haptics on handheld controllers
are limited to vibrotactile stimulation, which is typically used
for notifcation or binary touch events. This lack of cutaneouscues limits the user’s ability to feel contact with a surface andto explore its texture and shape.

ACM Classifcation Keywords

H.5.1. Information Interfaces and Presentation: Multimedia
Information Systems-Artifcial, Augmented, and Virtual Realities; H.5.2 User Interfaces: Haptic I/O
Author Keywords

Virtual Reality; Haptics
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CHI 2018, April 21–26, 2018, Montreal, QC, Canada© 2018 Copyright held by the owner/author(s). Publication rights licensed to ACM.
ISBN 978-1-4503-5620-6/18/04. . . $15.00

For more nuanced haptic rendering, researchers have developed fnger-mounted haptic devices [16, 21, 17, 6, 15, 29, 26,20], glove-based exoskeletons [19, 9, 23, 4], and robotic arm
solutions [1, 14, 13, 10, 12, 24] to render various haptic sensations. These devices either require users to mount or wear
additional devices or require expensive robotic arms with alimited range. Researchers have also explored the use of handheld devices for haptic rendering [3, 31, 18]. Such devices areconvenient to use and they are likely more compatible withexisting VR systems because they can replace the functionalityof existing controllers. However, previous controller-based devices have only focused on rendering a single haptic stimulus(e.g. normal forces or weight distribution).
In this paper, we present Haptic Revolver, a reconfgurable
handheld haptic controller for virtual reality. The device uses
an actuated wheel underneath the fngertip that moves up anddown to render touch contact with a virtual surface and spinsto render shear forces and motion as the user slides along a virtual surface. The device’s wheel is interchangeable and it can

DOI: https://doi.org/10.1145/3173574.3173660

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contain a variety of physical haptic elements, such as ridges,
textures, or custom shapes (Figure 1). These haptic featureson the wheel’s outer surface provide different sensations tothe user as they explore the virtual environment. Because thedevice is spatially tracked, these haptic elements are spatiallyregistered with the virtual environment. As the user explores
a virtual environment, our rendering engine delivers the appropriate haptic element underneath the fnger. For example,
in a virtual card game environment, when a user touches acard, a poker chip, and a table, the device rotates the wheel
to render the appropriate texture underneath the fngertip. As
the user slides along one of these surfaces, the wheel moves
underneath the fnger to render shear forces and motion.
Unlike other haptic devices [3, 6, 23], which always maintaincontact with the fnger, our Haptic Revolver device can selectively contact the fnger. When a user touches a virtual surface,
the wheel rises to contact the fngertip. Because the haptic
wheels on our device are interchangeable, Haptic Revolver can
generalize to many applications. Applications can use custom
wheels with the necessary haptic features. For example, avirtual petting zoo game might use a wheel containing various
textures while a virtual cockpit environment might use a wheelwith input elements such as buttons and switches.
In the following sections, we describe the design and implementation of our device, the techniques we use to render anarbitrary scene, and results from two perceptual studies that
informed these decisions. Our results show that we can change
the wheel speed and direction to render arbitrary scenes without compromising realism and support our technique of rendering 2D motion with a single wheel. We also show several
example applications that highlight functionality of our device
and qualitative feedback from users.
Specifcally, our contributions include:
1. The design of Haptic Revolver, a handheld VR controllerthat renders touch contact, pressure, shear forces, textures,and shapes using a rotating wheel beneath the index fnger;
2. Interchangeable haptic wheels that can be used to rendersurface features and techniques to haptically render any
scene using an arbitrary wheel;
3. The results of two perceptual user studies that inform thedesign of our haptic rendering strategies.
By combining the fundamentals of touch contact, pressure,and shear rendering with the fexibility of haptic wheels that
support arbitrary shapes and textures, Haptic Revolver enables
more accurate haptic rendering for virtual environments.
RELATED WORK

While there is extensive literature on the feld of haptics, we
restrict our review of related work to haptic VR controllers,
wearable haptic devices, and desktop haptic devices.
Haptic VR Controllers

The small form factor, low cost, and low power of vibrotactile actuators have led to their dominance in commercial VR
controllers. The positionally tracked controllers offered byconsumer VR systems (e.g. Oculus Touch or HTC VIVE controllers) include customizable vibrotactile feedback. However,Paper 86

CHI 2018, April 21–26, 2018, Montréal, QC, Canada

the amount of information that can be conveyed by vibrotactile stimulation is limited and its usage is typically limited tosimple touch events or notifcations. Some research efforts
have investigated how to use vibrotactile stimulation to rendersurface textures with a particular focus on how stimulation
parameters impact users’ perception of a surface [8, 22].
Recent academic and commercial efforts have attempted tomove beyond vibrotactile feedback. For example, Benko et al.
demonstrated NormalTouch and TextureTouch, controllers that
render normal forces on the fngertip [3]. Like these devices,
Haptic Revolver also targets haptic sensations at the fngertip,
but we focus on shear forces and texture rendering instead ofnormal forces. Unlike NormalTouch and TextureTouch, which
always maintain contact with the fngertip and modulate force
to simulate touch, the movable wheel in Haptic Revolver isable to fully retract whenever there is no touch contact.
Other efforts have focused on using controllers to render thesensation of holding an object. Zenner and Krüger presented
Shifty, a handheld device that shifts its weight distribution tosimulate holding objects of different weights [31]. Tactical
Haptics designed a controller that simulates friction forcesin the palm due to holding an object using sliding tactors in
the device handle [18]. By moving these tactors on opposite
directions, the controller can simulate torsional forces as well.
Wearable Haptic Devices

In addition to handheld haptic controllers, there are several
options for wearable haptic devices. Glove-based exoskeletonssuch as the Exos [9], Dexmo [19], Cyber Grasp [23], andRutgers Master II [4] all use actuators at the fngers to resist
grasping forces. Though these devices have the advantage ofgrounding at the wrist, they tend to be bulky and require a
nontrivial amount of setup compared to a handheld controller.
Such devices are also unable to render shear forces and motion
underneath the fngertip. In contrast, our device can render thesensation of sliding across a virtual surface.
Finger-mounted haptic devices are another common form factor. These are often strapped or clipped onto one or more
fngers to provide fngertip sensations as the hand moves. In
recent years, researchers have explored rendering contact [21],pressure [17], tilt [6], and shear forces [15, 20] with thesedevices. Yem and Kajimoto developed the FinGAR device,
which uses a DC motor and electrodes to provide skin deformation and vibration [29]. Wolverine is a four-fnger device thatuses braking to simulate grasping rigid objects [7]. Go Touch
VR designed a device that clips onto three fngers and renders
contact and pressure as a user grasps a virtual object [26].
Like the exoskeleton devices, these devices require careful
mounting to the fngers before use. Our Haptic Revolver device renders many of the same sensations as these devices aswell as motion under the fngertip. Moreover, with our device,
one can simply pick it up and begin using it without having
to strap anything to the fngers. However, unlike most controllers, including our device, these fnger-mounted devices
do not restrict hand posture during use. We note that there aremany more examples of fnger-mounted haptic devices. Fora more comprehensive review of such devices, we direct thereader to Pacchierotti et al. [16].

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Finger rest

(not pictured)
Vive Tracker

Servo motor

Raises and lowers wheel

DC motor
Spins wheel

Hapticwheel

Thumb button

Useful for selection

Figure 2. (left) 3D model of Haptic Revolver with a textured wheel attached. (right) Exploded view showing internal components. A servo motor raises
and lowers the wheel, while a DC motor spins the wheel to render motion and shear forces. A button on the side allows the user to select objects andnavigate. The VIVE tracker (not shown) enables 6-DOF spatial tracking of our device

Desktop Haptic Devices

Mechanical Design

In addition to handheld and wearable devices, there are anumber of efforts exploring haptics using environmentally
grounded systems. Robotic arm actuators such as the PHANToM [13], Haptic Master [24], Novint Falcon [14], andHaption Virtuose [10] excel at rendering larger, externallygrounded forces against the fnger or hand. These are often
used in applications such as tele-operated surgery, 3D sculpting, gaming, and interactive training. Recently Araujo et al.’s
Snake Charmer [1] used a robotic arm with custom attachments to render various surface features on demand. Like
Snake Charmer, our Haptic Revolver controller uses interchangeable attachments to render textures, shapes, and active
elements. However, because Haptic Revolver is integrated intoa handheld controller, we remove the need for precise fnger
tracking and have no range limitations. We also use the wheel
to render shear forces and motion under the fngertip. The
Touch Thimble is another example of combining attachmentswith a robotic arm for haptic feedback [12]. In this work,a spring loaded thimble keeps the touch surface suspended
from the fngertip until a virtual surface is contacted. Thoughour Haptic Revolver device does not provide the kinesthetic
feedback that these robotic arms can offer, we do provide thesensation of making and breaking contact with real objects.

We arrived at the design of Haptic Revolver through an iterative process. Each design, shown in Figure 3, improved thefunctionality and ergonomics of the device. Our fnal design,
shown in Figure 2 has two degrees of freedom, each of which
are actuated by a motor. A servo motor (Hitec HS-5070MH)
raises and lowers the wheel assembly along an axis positioned
along the grip of the controller. The wheel assembly is positioned along the axis of the index fnger and consists of a
12 V DC motor (Faulhaber 1524_SR) housed in a 3D printed
mount. The motor includes a 19:1 gearhead and a 4096 count2-channel magnetic encoder. A wheel mount on the end of thewheel assembly allows custom wheels to be easily attached.
With this gear ratio, the motor can spin at 180 rpm, which corresponds to a linear motion underneath the fnger of 565 mm/s,
assuming a 60 mm wheel diameter.

Azmandian et al. showed how retargeting can be used with
passive proxies [2] to reuse the same proxy for multiple virtual
objects. While Haptic Revolver also uses physical proxies on
the wheel, our actuation removes the need for retargeting.
Other desktop devices render fngertip sensations during stationary use [28, 25, 27, 11] or in a limited range [5]. For
example, the Plank is a desktop haptic device that uses a spinning wheel to render friction and various terrain shapes [25].
Other haptic devices use the tilt of a platform under the fnger
to convey surface information [27, 11]. Unlike these devices,
Haptic Revolver is a handheld device that renders multiple
haptic sensations at the fngertip.

The controller is designed so that the index fnger rests in agroove along the wheel axis. This lets the fnger naturally reston the surface of the wheel while preventing horizontal motion
as the wheel spins. For improved ergonomics, the axis of thefnger wheel assembly is offset from the grip handle by 110°.
A tactile button on the side of the device is used for selection and navigation and is in easy reach of the thumb during
normal operation. If desired, there is room on the side of
the controller for additional input elements, such as buttons,a joystick, or a touchpad. For positional tracking we attach
the HTC VIVE Tracker to the end of the device, as shown in
Figure 1 (left). This device integrates easily with the HTC
VIVE head mounted display and Lighthouse tracking system
and reports a 6-DOF position and orientation.

HAPTIC REVOLVER IMPLEMENTATIONTo render contact and motion on the fngertip in a compact
form factor, we chose to use a wheel that raises and lowers androtates in response to its position in the virtual environment. In
the following sections, we describe the hardware and softwareof our system as well as the design of interchangeable haptic
wheels to deliver custom haptic sensations.

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Figure 3. Examples of previous iterations of our device. This iterative
design process led to important design features of our current device.

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SteamVR

PID loop

Middleware
Visualization

Servo control

Figure 5. Interchangeable haptic wheels allow applications to customize
the haptic experience with various shapes and textures. (right) Wheels
with active components use a slip ring in the wheel mount to wire theelectronics in the wheel back to the device.

Interchangeable Haptic Wheels
UDP

Unity
Haptic rendering
strategies

Figure 4. Architecture of the Haptic Revolver device and software stack.
The device is powered by a PSoC, which sends commands to the two motor drivers and communicates with the PC via a USB serial connection.

Although we envision this device eventually being wireless,for simplicity, we use the device with a power and data tether.
In our current implementation, both the power supply andelectronics are external to the device. We note that there isenough room within the device grip for eventual placementof electronics and a battery. The device, including the VIVE
tracker, weighs 237 g, which is comparable to a VIVE Controller (205 g). Table 1 shows additional device specifcations.

WeightMax wheel speed
Wheel diameter
Max motion under fnger (α = 1)
Max force against fnger
Typical power consumption
Peak power consumption

237 g
180 rpm
60 mm
565 mm/s
3.35 N
1.25 W
2.5 W

Table 1. Mechanical and electrical specifcations of our device

Software Architecture

The software architecture is summarized in Figure 4. The
device is controlled by frmware running on a Cypress Programmable System on Chip (PSoC) 5LP. A PID loop on the
PSoC turns the wheel to a specifed rotation from a known
starting angle using an external motor driver (DRV8871). The
PSoC also drives the servo through pulse-width modulation(PWM). The two motors are each powered by separate step-up
power regulators (Pololu U3V50F*). In order to measure theforce exerted by the fnger against the wheel, the PSoC also
monitors the voltage across a 1 Ω shunt resistor in series withthe servo motor. The PSoC interfaces with a PC using a USB
serial connection running at 115 200 baud.
A Python middleware layer on the PC handles communication
with the device, visualization and logging, and communication
with the VR application. The application layer is built with
the Unity 3D game engine. Our Haptic Revolver renderingengine in Unity determines the ideal wheel confguration andstreams the desired settings to the Python middleware using asocket connection.

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Slipring
Plugs into deviceUSB

FirmwareEncoderInternal
wiringVIVE Tracker

Current
sensing

Motor driver

Servo motor
PWM

DeviceDC motor

CHI 2018, April 21–26, 2018, Montréal, QC, Canada

While a simple plastic wheel can simulate touch contact andmotion of the fnger, there are many applications that would
beneft from custom textures or shapes placed on the wheel
to match elements in the virtual environment. Haptic wheels,such as those shown in Figure 5, are designed to slide onto
the wheel mount and can be 3D printed or manufactured fromother materials. The ability to customize wheels allows us torender certain objects with much higher fdelity. For example,
a simple plastic wheel can be easily augmented by affxing
materials with unique textures, such as cloth, rubber, or paper,
which correspond to particular objects in the virtual scene.
Textures such as bumps and grooves can also be printed directly into the wheel itself to render various surface textures.
Coarser shapes printed on the wheel can simulate larger features in the scene. For example, a wheel with a raised region,such as the one shown in Figure 6 can be used to render edges.
With such a simple wheel, a user can feel the boundaries ofa physical button or feel when the fnger slides off the edgeof a surface. Other custom shapes can be designed to match
specifc objects in the scene. In a sculpting application, forinstance, appropriate shapes on the wheel can allow the user
to feel a tool beneath their fnger during use.
Although many sensations can be rendered using passive
wheel elements alone, additional functionality can be achieved
with active wheels containing electronic components. For example, an active wheel can include input elements, such as
buttons and switches that directly map to virtual widgets andadd interactivity. Components such as Peltier elements can be
added to the wheel to create additional haptic sensations thatcan be controlled dynamically.
To deliver electrical contacts onto a spinning wheel, we designed a wheel containing a slip ring, as shown in Figure 5(right). Up to 12 wires attach to elements on the wheel, passthrough the slip ring and out the front of the device, and plugin to a port on the bottom of the device. We created one
such wheel with input elements that include buttons, a switch,
and a joystick to enable interaction with different virtual widgets. We envision future designs of our device to include athrough-bore slip ring in the wheel mount itself. In this design, electrical contacts to the wheel would be made through aphysical connection along the outer face of the mount. This
would move all the electrical components to the interior of thedevice and enable custom active wheels without the overheadof additional wiring.
HAPTIC REVOLVER RENDERING ENGINE

Because each wheel has a limited surface area, haptic elementson the wheel will likely not precisely match the size and po-

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sition of elements in the virtual environment. We developed
a rendering engine to analyze the scene, hand trajectory, andwheel confguration and determine how to control the device.
As there may be competing goals within the scene, the engine
operates by constructing and resolving a set of constraints totake into account dragging motion and the desired orientationof the wheel. At each frame (roughly 90 Hz), we raycast beneath the fnger to determine the nearest collision with a haptic
surface. To minimize jerky movements of the controller, wesmoothly raise the wheel as the fnger approaches a surface.
We use similar penetration compensation techniques as described by Benko et al [3]. As the hand penetrates the virtualsurface, we raise the wheel even further to provide pressure
feedback to the user. Visually, the hand remains at the sameheight.
From the predicted penetration point, we scan left and right todetermine which other haptic elements are nearby. If the user
is making contact with a surface, we add shear constraints tomove the wheel along with the user’s motion. If no contact
is made, we allow the wheel to spin quickly to ensure theconstraints are met. If other haptic elements are nearby, we
add positional constraints to ensure that the features on thewheel align with the virtual elements. In the constraint resolution step, we resolve any shear constraints with positional
constraints to arrive at a desired wheel orientation.
To illustrate this process, consider the simple virtual scene
shown in Figure 6 used with a wheel containing a small raised
region, shown in black. As the user hovers over the blue
surface (left), we ensure the correct texture (shown in blue on
the wheel) is placed beneath the fnger. We also make noteof the nearby raised surface (shown in black) and add that asa constraint. As the user moves closer to the raised surface(center), the edge constraint is given higher priority and thefeature on the wheel is brought close to the fnger. If the user
were making contact with the surface, they would feel theedge in the correct location. Finally, as the user moves onto
the raised region (right), we impose two constraints, one forthe edge in each direction. This effectively scales the gain of
the wheel rotation so that the edges are placed in the correct
spot, regardless of the size of the raised region.
When rendering shear forces during contact with a surface
with no other constraints, a natural option is to spin the wheelsuch that the linear movement under the fnger matches thelinear movement in virtual space. In practice, this leads toquickly running out of room on the wheel before another feature arrives. To balance the realism of the dragging motion
with practical constraints, we choose to reduce the wheel gain,
α, to 0.6. For every 1 cm of fnger motion, we spin the wheelsuch that it travels 0.6 cm beneath the fnger. This value waschosen based on the results of a perceptual study describedlater. It represents the smallest gain before signifcant reductions in realism are observed.
Because our device uses a wheel, it inherently renders motion
in only one dimension (horizontally). While this is appropriate
in many scenarios, it would be ideal to support motion in two
dimensions (horizontally and vertically). Because we noticed
that users were insensitive to the direction of motion under

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Figure 6. (left) When a user hovers over the blue surface, the rendering engine places the appropriate wheel surface under the fnger andbegins to track the nearby edge of the black surface. (center) As the user
approaches the edge, the rendering engine positions the wheel so thatthe edge approaches the fnger. (right) While hovering over the smaller
black surface, the rendering engine adjusts the gain of the wheel so thatthe two edges are rendered correctly.

the fnger when the surface is smooth, we simulate verticalmotion by simply spinning the wheel horizontally. Although
this is orthogonal to the actual direction of motion, prior work
supports the feasibility of this illusion [30]. In this mode, wechoose the direction based on the horizontal component ofvelocity and allow the wheel to switch directions only when
there is a sudden change in hand direction or when the handcomes to a stop. We evaluate the effcacy of this rendering
technique for different types of tracing behavior in Study 2.
Rendering with Custom Wheels

To allow new wheel designs to be easily added without modifying the scene, we created a simple wheel description specifcation, implemented as a JSON fle. This fle contains alist of features on the wheel and where they are located. The
rendering engine uses this wheel description fle to determine
how to control the device. As the fnger approaches a haptic element in the scene, the virtual object reports its desired haptic
feature, such as a soft texture. The rendering engine fnds theappropriate element on the wheel and turns it according to theconstraint resolution steps. If a desired haptic element, suchas a soft texture, is not present on the wheel, the engine will
fall back on a suitable replacement feature, such as a smooth{}"name": "CasinoWheel","features":[{ "start": 0, "stop": 47.7, "height": 1,"texture": "hard", "name": "poker"},{ "start": 47.7, "stop": 90, "height": 0,"texture": "soft", "name": "felt_small"},{ "start": 90, "stop": 135, "height": 0,"texture": "paper", "name": "card"},{ "start": 135, "stop": 360, "height": 0,"texture": "soft", "name": "felt_large"}]

Figure 7. An example wheel description fle used by the rendering engine.
This fle describes the wheel shown in Figure 12.

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texture. Figure 7 shows an example wheel description fle forthe wheel shown in Figure 12.
As additional haptic features are added to the wheel, theamount of wheel space available for any one feature is reduced. This can make it more diffcult to render the sensationof dragging along a surface, as the fnger will quickly collidewith an additional haptic element. To address this challenge,
we developed several strategies for hiding undesired featureson the wheels. These strategies are illustrated in Figure 8.

Wheel Dip

Wheel Reversal

Figure 8. (top) As the fnger approaches an obstacle (indicated here by
the black region on the wheel), the dip strategy causes the wheel to lose
contact with the fnger while an undesired feature remains under thefnger. (bottom) In the reversal strategy, the wheel begins to rotate in the
opposite direction when an undesired feature is encountered.

Wheel Dip: Our frst strategy simply lowers the wheel justbefore a feature would approach the fnger. We ease the wheel
position in and out to create a smooth transition and prevent
jerky behavior. This strategy has the effect of causing the fnger to lose touch with the wheel for a short period of time. We
can actually shorten the time without contact by accelerating
the wheel over the undesired feature once it has lost contact
with the fnger. This can reduce the amount of disturbancecaused to the user.
Wheel Reversal: As an alternative strategy, we simply reverse
the wheel direction before a collision occurs. While dragging
along a surface using a wheel with other elements, the wheel
will rotate back and forth to render the appropriate shear motion while keeping the fnger on the correct region of the wheel,
effectively hiding the other haptic features. This behavior is
supported by our fndings from Study 1, which revealed that
rendering motion in the opposite direction has little impact onperceived realism. Although the overall direction of motion
may not be noticeable, the act of switching directions does
cause a noticeable shear against the fngertip. While this is notentirely unavoidable, we can reduce the frequency of such reversals by reducing the wheel gain, α. This method is also lessnoticeable if the user grips the controller more tightly, which
reduces horizontal fnger wobble during a direction change.
Because these two methods each have their advantages, we
use them both in our rendering engine, depending on the circumstances. When a user is sliding over a surface that has alarge physical size on the wheel, we use the reversal technique.
We observe that this technique causes less disturbance overall
because it never loses contact with the fnger. If the physicalPaper 86

CHI 2018, April 21–26, 2018, Montréal, QC, Canada

region is small or the wheel needs to change orientations to accommodate other constraints, we use the wheel dip techniqueto skip ahead to the desired wheel region.
EVALUATION

To evaluate the fundamental haptic capabilities of Haptic Revolver, we conducted two studies to understand how wheel
parameters impact the realism of the haptic rendering. In the
frst study, we measure the impact of the wheel speed gain
and direction. In the second study, we explore simulating
motion in two dimensions using a single wheel. These studies
helped inform the design of our haptic rendering techniques.
We recruited 12 right-handed participants (10 male, 2 female),age 18 to 48, to participate in both studies. Participants were
instructed about the nature of the study and given a shortoverview of the Haptic Revolver device. Participants then put
on an HTC VIVE head-mounted display and held our Haptic
Revolver device in their right hand and a standard HTC VIVE
controller in their left hand. All studies were conducted while
the participants were standing. Participants briefy explored ademonstration scene where they were able to touch and swipe
on a virtual object before the studies began. Each study tookapproximately 20 minutes and participants were compensatedwith an $8 meal coupon at a nearby cafeteria for their time.
Study 1: Rotation gain

In this study, we sought to understand how the wheel speed
gain and direction impact the perceived realism of the haptic
rendering. When the user’s hand moves a distance of x to theright, the wheel spins such that a distance of αx has passed
underneath the fnger to the left. To most closely match reality,
we would set the gain, α, to 1. However, in some cases, it
is useful to modify the gain to move the wheel to a desiredorientation more quickly or more slowly. We also wanted to
explore how important it is to spin the wheel in the correct
direction. We hypothesized (H1) that a one-to-one mappingfrom virtual motion to wheel motion (a gain of 1.0) would be
most realistic. We further hypothesized (H2) that users would
prefer that the wheel spins in the natural direction (α > 0),
but that they would prefer a reverse spin (α < 0) to no spin(α = 0).
To test this, we asked users to swipe their fnger along thelength of a 50 cm wide virtual surface under 17 different gain
settings from α = −1.6 to 1.6 in increments of 0.2. Participants began the trial by positioning the tip of their fnger withina small sphere on the surface. After exploring the surface forseveral seconds, participants ended the trial by moving theirfnger within the bounds of a second sphere, positioned justabove the surface. We then asked the user to rate the haptic

Figure 9. (left) In the frst study, users slid their fnger horizontally
across a surface. (right) In the second study, users traced a path on asurface. (right, inset) The six paths used in the second study

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To explicitly test this, we displayed a path on a fat surface
and asked users to trace the path in the forward and reverse
directions. Each experimental block consisted of six paths
and fve wheel behaviors for a total of 30 trials, which were
presented in a random order. Participants completed eachblock twice. To explore the effect of path shape, we chosepaths (Figure 9, right) that include a combination of horizontal
and vertical motion as well as a mixture of sharp edges andcurves. Paths were scaled to ft within a 25 cm by 25 cm
square.
In addition to modes that render wheel motion in the horizontal
direction (Motion 1D) and in both directions (Motion 2D), we
introduce three other baseline conditions. Our fve wheel
conditions are:Figure 10. Results of the frst user study showing mean realism ratings
across participants as a function of the wheel speed gain. The error bars
show a 95% confdence interval. A negative gain indicates the wheel was
spun in the opposite direction.

realism by responding to an on-screen prompt. The prompt
asked users "How closely did the haptic rendering match yourvisual impression of the scene?". All responses were collected
on a 5-point Likert scale (1-not at all realistic, 5-highly realistic) by pointing at the desired response on screen and clicking
with the VIVE controller in the left hand. Each block consisted of a single repetition of each gain setting presented in arandom order. Participants completed three blocks each.
Results

Figure 10 shows the average rating across participants for eachcondition. In this plot, we normalize responses such that thehighest and lowest responses for each participant become ’5’
and ’1’, respectively. Participants reported the lowest realism
score for α = 0, or when the wheel never moved. Realism
scores increased as the wheel gain increased, but leveled off
around α = 0.6. Wheel direction had little impact on realism
as shown by the symmetric nature of the graph. When asked
after the study, only two users even noticed that the wheel was
spinning in the reverse direction some of the time. This isconsistent with prior work, which found that the direction ofskin deformation had little impact on realism [30]

• Motion 1D: As the fnger moves horizontally, the wheel
spins with a gain of α = 1. As in the previous study, moving
vertically causes no change to the wheel.
• Motion 2D: Similar to Motion 1D, except the wheel also
spins when the fnger moves vertically. After a suddenchange in direction or when the fnger comes to a near stop,
we reevaluate the spin direction according to the horizontal
component of velocity.
• Off: A control condition in which the wheel does not spinat all. This is equivalent to α = 0 in the previous study.
• Shear 1D: As the fnger moves horizontally, the wheel
turns slightly, causing skin deformation proportional to thehorizontal velocity. Moving vertically causes no change tothe wheel.
• Shear 2D: Similar to Shear 1D, except it also applies skindeformation when the fnger moves vertically. After a sudden change in direction or when the fnger comes to a nearstop, the deformation direction is reset according to thehorizontal component of velocity.
As in the frst study, we then asked users to rate "How closely
did the haptic rendering match your visual impression of thescene?" on a 5-point Likert scale (1-not at all realistic, 5-highly
realistic). We hypothesized that users would fnd the Motion
conditions more realistic than the Shear or Off conditions (H3)and the Motion 2D condition would be most realistic (H4).

These results support several aspects of our rendering techniques. Most importantly, it suggests that as long as the wheel
speed gain is at least 0.6, the gain does not matter much. This
allows for some fexibility to spin the wheel faster or slower
and accommodate other constraints. Second, these results
validate our approach to avoiding features by reversing thewheel direction. Finally, these results do not confict with
our decision to render vertical motion with horizontal motion
under the fnger.
Study 2: Vertical movement

Though Haptic Revolver only spins in one dimension, it isimportant to explore whether we could effectively render motion in two dimensions. Since the results of Study 1 suggestthat spinning in the opposite direction had little impact on perceived realism, we hypothesized that spinning in an orthogonal
direction would also have little impact.

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Figure 11. The results from the second study showing mean realism
ratings across participants as a function of the wheel rendering modeand path drawn. The error bars indicate a 95% confdence interval.

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CHI 2018, April 21–26, 2018, Montréal, QC, Canada

Results

The mean ratings from all participants are summarized in Figure 11. Mann-Whitney U tests show that participants perceived
the Motion conditions (n = 288, median = 0.67) as more
realistic than the Shear conditions (n = 288, median = 0.0,
U = 67382, p < 0.001) and the Off condition (n = 144,
median = 0.0, U = 35494, p < 0.001). Participants also perceived the Shear conditions as more realistic than the Off
conditions (U = 25107, p < 0.001). This confrms H3 andhighlights the importance of rendering more than just skindeformation under the fnger.
Ultimately, we did not fnd strong support for H4 as no signifcant difference was observed between the Motion 1D (n = 144,
median = 0.67) and Motion 2D (n = 144, median = 0.67) conditions in aggregate. However, by breaking down the analysis
by path, we fnd some trends that suggest our 2D rendering
technique is still effective. Participants perceived Motion
2D as more realistic than Motion 1D when tracing a verticalline (medians = 0.33, 0.25, n = 24, U = 178.5, p = 0.021)
and a square path (medians = 0.66, 0.5, n = 24, U = 208,
p = 0.096), paths which both have vertical components. No
signifcant differences were observed with the other paths.
While we expect no differences in the horizontal line, the otherpaths contain signifcant diagonal or curved components. In
these cases, the difference between our Motion 1D and 2D
rendering largely comes down to a difference in speed. For
example, in the diagonal portion of the circular path, the 1D
mode would render the motion at half the speed of the 2D
mode, since only half of the motion lies in the horizontal direction. Since we are not highly sensitive to the magnitudeof motion (as confrmed by Study 1), it is unsurprising thatdifferences were not observed on these paths.
EXAMPLE APPLICATIONS

To explore applications for Haptic Revolver, we built several
scenes and corresponding haptic wheels that highlight different
capabilities of the device. We invited 11 users to try out three
Paper

Soft felt
Hard plastic
Soft felt

Knife toolof these demos in order to elicit qualitative feedback on our
device and rendering techniques. We also refer the reader tothe Video Figure accompanying this paper for a demonstrationof each of these applications.
Card Table (Texture Rendering): The frst application highlights the ability of Haptic Revolver to render different textures
under the fngertip. In this scene, several playing cards andpoker chips lie on a card table within a virtual casino. A user
can touch and drag along the felt table surface, the playing
cards, and the plastic poker chips and feel an appropriate texture beneath the fnger. For this application, we designed awheel containing felt, plastic, and paper. As shown in Figure 12 (left), two felt regions are used in order to render thetransition from paper or plastic to felt in either direction. If
a user presses lightly on one of the virtual objects, the fnger
will slide over it and feel the surface moving beneath it. If auser presses harder, the object will be dragged along with thefnger and the device will render a shear force due to friction.
Painting and Sculpting (Force Sensing): Haptic Revolver
can also turn passive props on the wheel into interactive objects
by sensing the force on the wheel. In this scene, a user canpaint and sculpt a 3D model by choosing between a spraypaint tool, a fnger painting tool, and a sculpting tool. As
shown in Figure 12 (center), the wheel consists of a raised
plastic cylinder to simulate the top of a can of spray-paintand a narrow ridge to simulate the back of a knife. The tooland color can be selected by pointing at the desired element
and clicking with the thumb button. When a tool is selected,
the appropriate haptic element is positioned under the fnger
and left there until a new tool is selected. To use the tool, auser simply presses down on the haptic element beneath thefngertip. The device detects this added force on the wheel andactivates the tool. When fnger-painting, a smooth surface of
the wheel is positioned under the fnger and it spins back andforth during use to render shear forces and motion.

Spraypaint tool

Hard plastic
Indentation

Hard plastic

Figure 12. (left) A card table demo that highlights our ability to render different textures. The wheel used in this demo consists of two regions of soft
felt, a hard plastic ridge, and a small section of paper. When the user touches an object in the scene, the appropriate texture is placed underneath thefngertip. (center) A painting and sculpting demo that highlights the ability to render shapes and sense the force applied to the wheel. The wheel usedin this demo consists of a raised nub and a ridge to simulate holding tools. The user presses on the wheel to activate the tool. The model can be exploredby touch. (right) A keyboard demo that highlights our ability to render edges and shapes. The wheel used for this demo consists of nine raised plastic
regions with grooves in between. When a user approaches the edge of a key, the edge of a groove is placed under the fnger.

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TactilebuttonPush button
Rocker switch
Joystick

Figure 13. A demo with a DJ mixer board that highlights our ability to
put interactive elements on the wheel. The wheel in this demo consists ofseveral physical UI elements wired up to the device. When a user touches
a virtual UI element, not only do they feel the shape of a similar physicalelement, but they can physical interact with the widget.

Keyboard (Shape Rendering): In this application, we highlight the ability of Haptic Revolver to render custom shapes
under the fngertip and improve the experience of using virtual
buttons with a purely passive wheel. In this scene, an onscreen keyboard allows a user to enter text by pressing on thevirtual key with their fnger. As shown in Figure 12 (right) thewheel for this scene consists of nine raised plastic ridges, eachapproximately the size of a standard key on a keyboard. Eachraised "key" is separated by a small indentation to simulate
the gap between keys. When the user presses on a virtual key,
the rendering engine ensures the edges for the physical ridgealign with the edges of the virtual key (see Figure 12, right).
The gap can be felt by touching between two virtual keys (seeFigure 12, bottom right). In fact, a user can lightly brush along
an entire row of the keyboard and feel a rapid succession of
bumps, much like one would feel on a real keyboard.
DJ Mixer (Active Wheels): While the previous applications
have highlighted the capabilities of Haptic Revolver using
only passive wheels, additional functionality can be added
through the use of active electronic elements. In this DJ mixer
application, we haptically render a virtual DJ mixer using awheel with active buttons, a rocker switch, and a low profle
two-axis joystick. Each widget on the mixer board is linked
to a physical widget on the device. Buttons and switches
have a direct mapping on the device. A joystick on the wheel
metaphorically maps to virtual knobs and dials. Pushing thejoystick in a particular direction rotates the knob to the same
direction. Sliders are rendered using a switch on the wheel,
though a simple passive object would suffce as well. Whena user touches the thumb of the slider, the tactile switch is
positioned under the fnger. Much like the dragging behavior
in the card table demo, moving the slider causes the haptic
element under the fnger to tug against the skin, rendering
frictional shear forces. When the slider reaches its extreme
point, the haptic element begins to slide off the fnger.

CHI 2018, April 21–26, 2018, Montréal, QC, Canada

sculpting scene. For simplicity and timing, we omitted thefourth DJ mixer scene. Upon arriving, users were given an
introduction to the device and head-mounted display, and allowed to become accustomed to our device in a simple tutorial
scene. Over the next thirty minutes, participants tried the threescenes using both our device, with the appropriate wheel foreach scene, and a standard HTC VIVE controller. The VIVE
controller vibrated upon contact with a virtual surface.
To elicit reactions to our device, participants explored a scenethrough a guided walkthrough and then provided feedback
through a semi-structured interview about their experience.
Questions focused on the haptic realism of various aspects ofthe scene, preferences related to the rendering of both devices,and usability aspects of our device. To enable a more quantitative comparison between devices, we also asked users torate the haptic rendering ("How well did the haptic rendering
match your visual impression of the scene") of each device on
a 5-point Likert scale after each scene. Participants spent 3-5
minutes exploring each combination of scene and controller.
Participants experienced both controllers within a scene before
moving on to the next scene. We randomized the presentation
order of both the scenes and the devices. Participants werecompensated with an $8 meal coupon for their time.
Results

Participants were generally excited about our device and appreciated that it could render more than just vibrations. Many
participants remarked that while using the Haptic Revolver,
they felt like they were actually touching the surface. In thecard table scene, P10 remarked, "It actually felt like I was moving my fnger along a felt table". P3 noted that when touchinga surface, our device responds based on how they move theirhand, which felt much better than the vibrotactile controller.
While using the vibrotactile notifcation, many users remarked
that it was not the sensation they were expecting. Some users
noted that while it did not feel realistic, they still appreciatedthe vibration feedback as an indication of touch. Users also
rated our device more realistic than vibrotactile notifcation
with consistent median ratings of 4 for our device and 3 forvibrotactile (n = 11) in each scene. Wilcoxon signed-rank
tests between the realism responses showed that these differences were signifcant for the card table (T = 0, p = 0.003),

User Feedback

To better understand the performance of our device, we invitedan additional 11 users (10 male, 1 female) from our institution
who had not tried the device before to provide feedback on
its use. We sought to understand how our Haptic Revolver
device compares to standard vibrotactile notifcation. During
the study, participants tried three of our example applications:the card table scene, the keyboard scene, and the painting andPaper 86

Figure 14. Quantitative results of our feedback elicitation study showing
mean realism ratings across participants for our Haptic Revolver device
and vibrotactile notifcation. The error bars indicate a 95% confdence
interval. All differences are signifcant at the 0.05 signifcance level

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keyboard (T = 6, p = 0.016), and sculpting scenes (T = 0,
p = 0.003).
This study also revealed opportunities to improve future iterations of the device. Several participants commented on thenoise made by the motors. This can be improved in future
iterations by using higher quality brushless motors. Some
users found our reversal technique for avoiding sections of thewheel distracting, particularly when moving slowly. For example, P4 did not realize the reversal technique was intentionaland commented "the motion didn’t always match up with the
motion of the fnger". In our testing, a user’s sensitivity to thereversal can be signifcantly reduced by ensuring the fnger ispressed down against the groove on the controller, minimizing
the horizontal sway during the reversal. Adding an elastic
fnger strap to the device may help ensure the fnger restsin the ideal place. In the keyboard demo, many participants
attempted to explore the sides of the keyboard, which werevisually rendered as a smooth surface. Participants were surprised when they could feel edges while touching this surface.
For generality, wheel designs should include a larger smoothregion that can be utilized as a general purpose touch surface.
A few unexpected observations arose from this study. First, weobserved differences between users with VR experience andnovice or frst-time VR users (seven participants had used a
VR system for less than an hour). The attention of novice users
was largely consumed by the novelty of the VR experienceand visual aspects of the scene. With these users, it was more
diffcult to elicit feedback related to the haptic rendering.
Interestingly, we also observed that users started to commenton more nuanced haptic features while using our device. For
instance, P4 noted that for the poker chip in the card tablescene, they could feel the texture, but it was not exactly likea poker chip. As another example, P7 said "It doesn’t feelexactly like spray-paint, but this is cool. I have a nice sense that
I’m holding it." We suspect that because our device rendered
textures and shapes in an attempt to match reality, users had
higher expectations and noticed subtle imperfections, similar
to the uncanny valley effect observed in 3D animation. Whilevibrotactile notifcation provided touch feedback, it was clearly
not realistic and users did not expect it to be. This raises
interesting questions about what level of detail is appropriate
in haptic rendering for VR.
Finally, we observed that not all users preferred greater realism in the experience. For example, P8 initially felt shocked
when they could feel our device rendering surface textures andremarked, "It was hard to get used to it touching my fnger. It
felt more real, but that was a bad thing. If I had more time I
could’ve gotten used to that". This suggests that while it isimportant to explore methods of accurately rendering haptic
feedback, realism may not be the only design goal.
DISCUSSION, LIMITATIONS, AND FUTURE WORK

Haptic Revolver goes beyond vibrotactile notifcation to explore what rendering multiple haptic sensations can add to the
VR experience. While a simple haptic wheel can render touchcontact and motion under the fnger, the use of interchangeable haptic wheels allows applications to design custom haptic

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CHI 2018, April 21–26, 2018, Montréal, QC, Canada

experiences. We envision some haptic wheels to be general
purpose, while others may be tailored to provide highly realistic experiences for certain applications. When an end userpurchases a new application that would beneft from a customized haptic experience, it could come with its own haptic
wheel. Future work could explore actuating the wheel axis forward and backward to automatically switch between multiple
wheels installed on the device.
Our ability to place electronic components on the wheel significantly broadens the design space of haptic wheels. We chose
to focus on interactive elements such as buttons and switches
because these are commonly found in virtual environments.
Our choice of physical widgets supports a wide range of virtual elements such as those found in a airplane cockpit or a
car dashboard. However, electronic elements placed on thewheel are not limited to these physical controls. For example, a Peltier element on the wheel could enable temperature
feedback. Adjustable mechanical components could enableadditional fexibility by dynamically changing the wheel in
response to the virtual environment.
We add additional interactivity to haptic wheels by sensing
the force the user applies. In one of our example applications,
we use this as a binary indicator of pressure to activate thespray-paint. Further interactivity could be added by using thecontinuous pressure signal. For example, a user might touchwith varying pressure to control the spread of the paint stream.
In our keyboard demo, we improve the realism of the scene byadding ridges so that the user can feel the edges and shape of
the keys. Users enjoyed this and remarked on its realism, but
it is currently unclear whether this can improve performanceon the keyboard in any way. Exploring how physical haptic
feedback impacts task performance is an interesting avenue
for future work.
Lastly, though we made an effort to design our device for users
with varying hand sizes, individual differences cause subtle
changes in how the fngertip rests on the wheel. This results ina different resting height for the wheel which, for some users,
was manifest as physical contact before virtual contact was
made. Future versions could include a proximity sensor in thewheel to automatically calibrate the correct height.
CONCLUSION

Haptic Revolver is a general-purpose handheld VR controllerthat goes beyond vibrotactile stimulation to render touch contact with virtual surfaces, motion along a surface, textures, andshapes using interchangeable haptic wheels. By customizing
wheels for the virtual environment, designers can use Haptic
Revolver to render realistic haptic feedback on the fngertip.
We demonstrated techniques to render motion along a surface
in two dimensions and adapt a particular wheel for use in arbitrary scenes. We conducted two user studies to inform andvalidate the design of our haptic rendering techniques and athird study to elicit qualitative feedback from participants. We
believe that Haptic Revolver offers high-fdelity haptic rendering with clear advantages over vibrotactile solutions and wehope others will build upon our design to continue enabling
better haptic experiences for VR.

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