CHI 2018 Paper

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

Haptic Links: Bimanual Haptics for Virtual Reality Using
Variable Stiffness Actuation
Evan Strasnick1

Christian Holz2

Eyal Ofek2

Mike Sinclair2

Hrvoje Benko2

1 Stanford University

2 Microsoft Research

Stanford, USA
estrasni@stanford.edu

Redmond, USA{cholz, eyalofek, sinclair, benko}@microsoft.com

Figure 1. (a) Three prototype Haptic Links attached to commercial HTC VIVE controllers. From left to right: Layer-Hinge, Chain,
Ratchet-Hinge. (b-c) A survival game in which a Haptic Link rigidly locks handheld controllers in the shape of a two-handed gun.
Overlay added in post-production for visualization.
ABSTRACT

We present Haptic Links, electro-mechanically actuatedphysical connections capable of rendering variable stiffness
between two commodity handheld virtual reality (VR) controllers. When attached, Haptic Links can dynamically alter
the forces perceived between the user’s hands to support thehaptic rendering of a variety of two-handed objects andinteractions. They can rigidly lock controllers in an arbitrary configuration, constrain specific degrees of freedom ordirections of motion, and dynamically set stiffness along acontinuous range. We demonstrate and compare three prototype Haptic Links: Chain, Layer-Hinge, and RatchetHinge. We then describe interaction techniques and scenarios leveraging the capabilities of each. Our user evaluation
results confirm that users can perceive many two-handed
objects or interactions as more realistic with Haptic Links
than with typical unlinked VR controllers.
Author Keywords

Haptic Links; virtual reality; variable stiffness; haptics;
two-handed; controller;
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CHI 2018, April 21–26, 2018, Montreal, QC, Canada© 2018 Copyright is held by the owner/author(s). Publication rights licensed to ACM.
ACM 978-1-4503-5620-6/18/04…$15.00
https://doi.org/10.1145/3173574.3174218

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ACM Classification Keywords

H.5.1 [Information Interfaces and Presentation]: Multimedia Information Systems-Artificial, Augmented, and Virtual
Realities; H.5.2 [User Interfaces]: Haptic I/O.
INTRODUCTION

Researchers have made significant advancements in thedesign of haptic controllers for virtual reality (VR), resulting in a variety of methods for rendering tactile and kinesthetic sensations in the hand [3,6,10,38]. However, the majority of this work prioritizes single-handed interactions,despite the prevalence of bimanual interactions in our lives[14]. That is, though many haptic controllers provide local
feedback for a single hand interacting with an object, theycannot render forces between the hands. For example, whena user drives with a virtual steering wheel or swings a virtual golf club with handheld controllers, these interactions
lack the physical constraints imposed by the real object.
In this paper, we focus not on the design of a VR haptic
controller, but on the design of a haptic connection between
controllers. We present Haptic Links, electro-mechanicallyactuated physical connections capable of rendering variable
stiffness between two commodity handheld VR controllers.
When attached, Haptic Links can dynamically alter theforces perceived between the user’s hands to support thehaptic rendering of a variety of two-handed objects andinteractions. They can rigidly lock controllers in an arbitrary configuration, to, for example, make the controllers
feel and behave like a two-handed tool or weapon (see Figure 1(c)). They can constrain specific degrees of freedom ordirections of motion between the controllers, such as whenturning a crank or pulling a lever. They can even set stiffness along a continuous range, to render friction, viscosity,

Page 1CHI 2018 Paperor tension. In these ways, Haptic Links augment existing
handheld controllers with realistic mechanical constraints,
making interaction and game play in VR scenarios more
immersive and tangible.
This paper presents the following contributions:
1. The design and implementation of three Haptic Link
devices (“Chain,” “Layer-Hinge,” and “Ratchet-Hinge”)
enabling variable stiffness feedback between commodity handheld VR controllers;
2. Technical and user evaluations outlining the tradeoffs ofeach Haptic Link design and showcasing the potential ofeach in the haptic rendering of different object types;
3. Several two-handed interaction techniques and scenarios
leveraging Haptic Links to improve bimanual haptic
rendering in VR.
RELATED WORK

Haptic interfaces for VR interaction encompass a diverse
set of devices and form factors. Wearable devices such as
gloves and full-body suits present simulated tactile sensations to the skin, often utilizing vibrotactile or electrical
feedback [12,20,50]. Exoskeleton-type devices are similarly
worn, but mechanically impart forces or constrain motion
around the fingers, hands, or arms to render haptic sensations on a variety of scales and resolutions [5,11,42,44].
Because they do not require the user to hold a device, both
wearables and exoskeletons free the user’s hands and fingers for direct interactions with the virtual environment.
Another widely studied class of haptic system is the actuated force-feedback arm, such as the PHANToM family of
devices [16,23,26,29]. These arms typically make use of agrounded physical reference in the user’s environment. As aresult, they can impart net forces on the user to render large
haptic forces and collisions, but often at the cost of reduced
mobility and operating space. In contrast, encountered-typedevices such as robotic arms and drones [2,47] follow theuser as they move through the environment, making physical contact as needed to render haptic cues. Though they
offer mobility and support large interaction spaces, challenges in predicting and reacting to the user’s movements
often result in issues of latency.
More recently, another form factor—the handheld controller—has emerged as the dominant mode of interaction in
consumer VR systems, such as in the Oculus Rift [30] andthe HTC VIVE [17]. Though handheld controllers greatly
simplify the tracking and input requirements of VR systems, their haptic capabilities are frequently limited to integrated vibrotactile motors. They may also include physicalinputs which provide passive feedback, such as triggers,
buttons, and joysticks. Recent work has targeted the controller form factor in enabling rich haptic interactions suchas touch, grasping, weight, and texture [3,9,10].
Bimanual Haptics in VR

Despite the range of technologies capable of rendering haptic sensations on the hands in VR, systems that provide

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

feedback between the hands are less widely studied. Typicalbimanual tasks vary greatly in the positioning, orientation,and relative motion of the hands [14]. The increased distances, stronger forces, and numerous degrees of freedom inbimanual scenarios make many conventional haptic solutions infeasible. Grounded force-feedback systems, described above, can support bimanual feedback when used inpairs by rendering forces on each manipulandum in a coordinated manner [32,41]. Such dual articulated arm setups
are most frequently used in cases of robotic telemanipulation [18,21] or in surgical simulation [7,24], where precise
bimanual actions are crucial. However, the size, cost, andcomplexity of these setups prohibit their widespread usage
with commodity VR systems.
An alternative solution uses passive controller augmentations to improve the haptic presentation of a specific virtual
object. For example, a number of commercially sold attachments anchor VR controllers in the form factor of a
two-handed gun, enabling users to feel realistic handling inapplications that utilize rifles or other such weapons[34,44]. Similar attachments exist for golf clubs [45], musical instruments [31], and more. Though simple and compelling, these passive attachments are static, making them lesseffective for rendering multiple types of objects, or objects
that are deformable or customizable.
Variable Stiffness Feedback

We consider variable stiffness actuation as a possible
mechanism by which to enable general-purpose bimanual
feedback in VR. Variable stiffness as a feedback techniqueis well studied. Approaches include electromechanical actuators, jamming, rheological fluids, shape memory alloys(SMAs), and low-melting point alloys. Detailed reviews ofthese techniques can be found in [25] and [43].
Already, researchers have applied several of these variable
stiffness techniques in the design of haptic interfaces. Mechanical exoskeletons utilize belts and cables to stiffen individual joint motion [42]. Wearable interfaces have employed jamming to restrain finger movements in a glovetype form factor [39], as well as in the design of full-limb
restraints [27]. Similarly, MR fluid brakes have been usedin the design of a haptic glove for VR [4]. Other researchers
have leveraged shape-memory alloys and electromechanical approaches to enable variable stiffness interactions with mobile devices [15,28]. The Haptic Links described in this paper apply these variable stiffness actuation
techniques on a larger scale to dynamically brake the relative motion between two handheld controllers.
DESIGN AND IMPLEMENTATION

The implementation of our Haptic Link vision began withnumerous design considerations. These included: stiffness
when actuated, flexibility when relaxed, resolution of stiffness, weight, bulk, moment of inertia, actuation speed,
power consumption, noise, and range of motion. The ideal
Haptic Link would be virtually undetectable to a user prior

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to actuation, but could stiffen quickly, strongly, and precisely as needed.
Of the studied approaches for rendering variable stiffness,
we first ruled out both SMAs and low-melting point alloys,
which have prohibitively long actuation periods, and
ER/MR fluids, whose solutions tend to settle out over timeleading to decreasing effectiveness. Particle jamming interfaces showed promise with relatively high stiffness gains,particularly when reinforced with internal frames or skeletons [8,27,46]. However, such reinforcements reduce theflexibility of the joint when unjammed, and the volume ofmatter required adds significant weight to the mechanism.
Layer jamming was perhaps more promising, as researchers
have successfully used it to produce light, flexible manipulators with significant stiffening capabilities [19,37]. However, for both jamming techniques, actuation remained unsuitably slow, and the large pumps required ultimately led
us to focus on electro-mechanical stiffening approaches.
Existing electromechanical techniques to produce variable
stiffness could easily achieve sufficiently high braking torques as well as quick and precise stiffness control. However, due to the size, weight, and power constraints of ourtask, using motors and brakes to directly oppose the user’s
torque quickly became infeasible. Thus, our designs shiftedto investigate alternative joints and mechanisms which
could be indirectly actuated using smaller motors.
Our exploration yielded three prototype Haptic Links(Figure 1(a)), all capable of allowing and halting the 6-DOF
motion of handheld VR controllers. Each design has
tradeoffs and advantages over the others, making them best
suited for different applications. We envision that designersof VR experiences could choose the Haptic Link that best
meets their needs out of many options, allowing users toquickly attach the recommended Haptic Link to their controllers prior to entering the virtual world.
We chose to design our Haptic Links to fit the commercially available HTC VIVE [17] controllers, due to the ease of
constructing mounting geometries either for the annular
tracking region or the protruding base. All Haptic Links
affix to these controllers without additional modifications.
Each Haptic Link connects via ribbon cable to a regulated
power supply and to a Teensy 3.2 microcontroller [33]which controls the actuators on the device. VR applications
can communicate with this controller via serial to dynamically update the stiffness of each joint in the Haptic Link.
Both our VR experiences and our user evaluation were implemented in the Unity 2017 game engine.
Chain Device

The Chain prototype (Figure 2), utilizes a highly articulated
chain composed of ball-and-socket elements. A strong cableis threaded through the length of the chain and tethered to alinear actuator on each end. With the linear actuators extended, the chain is kept loose such that the user can arbitrarily move the controllers in 3D space. When the linear

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Linear actuator
Cable
Ball-and-socket element

Figure 2. The Chain prototype stiffens an articulated chain bypulling tight a cable threaded through ball-and-socket elements.

actuators retract the cable, the ball-and-socket elements arecompressed into one another, increasing the friction at eachjoint in the chain. As a result, the entire chain stiffens, fixing the current spatial relationship between the two controllers.
The Chain prototype uses two Actuonix L12-R Micro Linear Servos [1] each capable of 80 N maximum force at 6.5mm/s. The ball-and-socket elements were 3D printed usinga PolyJet material, and the cable is an ultra-high-molecularweight polyethylene string (1 mm diameter, 160 kg breaking force, 4.8% breaking elongation). The linear servos aremounted onto laser cut Polyoxymethylene pieces and oriented coaxially with the ends of the chain. Each of thesepieces then mounts onto two 3D printed conical frusta
which clamp down on either side of the annular portion of
the HTC VIVE controller.
The Chain prototype was designed for unrestricted 6-DOF
motion which can globally stiffen in any configuration.
Though it can render non-binary stiffness, precise control
over the intermediate range is difficult, as stick-slip motion
between the individual elements results in a nonuniform
perception of stiffness. Models of the frictional forces in theinterdependent ball-joint elements are beyond the scope of
this paper, but details can be found in [37].
Layer-Hinge Device

The second prototype (Layer-Hinge, Figure 3) consists of
two main components: ball joints to allow rotation of thecontrollers, and a hinge controlling the distance between
them. Each controller is connected at its base via a 3D
printed mount to a ball joint capable of 360° pan and 60°
tilt rotation. A FEETECH FS5115M servo [13] (180° rotation in .48 s, 15.5 kg-cm torque) drives the set screw oneach ball joint, locking and unlocking rotation of the controller. The hinge component consists of a series of interleaved layers of laser cut cast acrylic. An additional
FS5115M servo drives a bolt threaded through the layers
and a captive nut on the other end. As the nut cannot rotate,

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

Ball joint
Servo driving set screw

Servo driving
set screw

Ball joint

Carbon fiber rod

Pawl

Interwoven layers

Servo driving bolt
Figure 3. The Layer-Hinge prototype consists of locking balljoints to control rotation and a hinge leveraging the friction
between layers to control distance between controllers.

the rotational motion of the bolt compresses the layers, increasing the overall friction of the hinge based on the number of layers in contact. The hinge and ball joint pieces areconnected by carbon fiber rods (21.5 cm length, 1.27 cm
diameter).
With three distinct points of actuation, the Layer-Hinge
prototype has the advantage of selectively locking individual degrees of freedom in the motion of the controllers. For
example, if the hinge is locked but the ball joints remain
free, the controllers can rotate at a fixed distance apart,much like joysticks. Further, the friction of each joint canbe controlled with relative precision, allowing the device torender a continuous range of stiffness values in both thehinge and ball joints. The resistive force in the hinge can bemodeled with the relationship𝐹 = 𝜇𝑛𝑝𝐴
where 𝜇 is the static coefficient of friction of the cast acrylic, 𝑛 is the number of contact layers (12), 𝑝 is the appliedpressure, and 𝐴 is the contact area of each layer.
Ratchet-Hinge Device

The third prototype (Ratchet-Hinge, Figure 4) uses thesame ball joint components beneath the controllers as in theLayer-Hinge prototype, but replaces the hinge with a dual-

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Servo actuating pawl

Gear

Figure 4. The Ratchet-Hinge prototype uses locking ball joints
and a dual-ratchet hinge. Each of the opposing pawls can be
individually disengaged to control both directions of motion.

ratchet mechanism capable of independently braking inward or outward motion. The ratchet mechanism is cut from
Polyoxymethylene and features two pawls set against acentral gear at opposite angles. A Hitec HS-35HD Nano
Servo [36] with an attached cam disengages each pawl,
freeing the corresponding direction of motion. With both
pawls engaged, the gear is fixed, and with both disengaged,
the gear can rotate freely. When one pawl is disengaged, thegear can ratchet against the engaged pawl in one direction,
but rotation in the opposite direction further engages theremaining pawl and halts the motion.
The directionally-selective capabilities of the RatchetHinge prototype enable unique force-feedback interactions,such as the rendering of a midair impassable surface. However, each ratchet can only engage or disengage in a binary
fashion, and thus the prototype trades away the intermediate
stiffness capabilities of the Layer-Hinge prototype.
TECHNICAL EVALUATION

Table 1 compares the technical specifications of each Haptic Link, and Figure 5 shows experimentally obtained
torque-angle curves of the prototypes when fully stiffened.
Torque-angle measurements were taken using a lathe byanchoring one end of the joint to the spindle, attaching theother end in series with a force gauge to the slide, and recording measurements while stepping back the slide (Figure6). Measurements were taken with the arms of the proto-

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Chain

Layer-Hinge

Ratchet-Hinge

Weight (g) (without controllers)

673

793

651

Weight distribution

Evenly distributed

Concentrated at the base of thecontrollers and at the hinge

Concentrated at the base of thecontrollers

Actuation speed (s) (0-100%)

1.0

Hinge: .65, Ball Joint: .50

Hinge: .13, Ball Joint: .50

Maximum distance between controllers (cm)

54

63

68

Stiffness control

Continuous

Continuous

BinaryTable 1. Technical specifications of each Haptic Link prototype.

1.6

Torque (Nm)

1.4
1.2

Slip

1
0.8

Slip

Slip

0.6

LAYER-HINGE (Hinge)
RATCHET-HINGE (Hinge)
CHAIN
Ball Joint

0.4
0.2
0

0

2

4

6

8

10

12

Angle (degrees)
Figure 5. Torque-angle curves for each actuated component of the prototypes. The Ball Joint curve applies to both the Layer-Hinge
and Ratchet-Hinge prototypes. The hinge of the Ratchet-Hinge was not driven to failure (due to its having a critical failure mode),
whereas all other components were driven until torque remained constant.

types parallel to each other and perpendicular to the applied
force. Both the Chain prototype and the hinge of the LayerHinge prototype reached maximum torques of 1.1 Nm before slippage, whereas the ball joints used in the LayerHinge and Ratchet-Hinge prototypes yielded only .7 Nm.
The hinge of the Ratchet-Hinge prototype holds until mechanical failure of the ratchets. Due to our having only asingle Haptic Link of each design, we did not test destructive failure modes.

on the stiffness of the joint, but ranges from 1-5 W on each
prototype. In addition, we can instead configure the Haptic
Links to push the maximum stiffness further by holding atstall torque. In this case, the total power consumption depends upon the time spent at maximum stiffness, for which
the stall power consumption is around 4.8-7.2 W on all prototypes. The motors may grow warm if left at stall for significant periods, though no heat-related issues were everdetected during the course of this work.

The prototypes can be configured to operate in a powerefficient mode or to optimize for the maximum possible
stiffness. For most purposes, the actuators can be set suchthat the static back drive force is used to hold the joint when
fully stiffened. In this case, power exceeds the standby
power (.05-.1W) only when moving the servos (i.e. changing the stiffness). This transient consumption varies based

USER EVALUATION

SpindleHaptic Link

Force Gauge

Slide

We designed an evaluation to investigate the following
questions:
1.

2.

Can inter-controller variable stiffness feedback contribute to a more realistic haptic rendering for twohanded objects and interactions than that of conventional (unlinked) controllers?
Which of the Haptic Link prototypes are most effective
in providing realistic haptic renderings for different
types of objects?

Methodology

We recruited 12 participants from our organization (ages25-49, 1 female) to help in answering these questions. Participants were asked to rate their perceptions of object realism using each Haptic Link and a control pair of unlinked
HTC VIVE controllers. For each trial, participants firstaligned their controllers into the proper positions for theobject using visual indicators. Once correctly positioned,
Figure 6. Setup for measuring Torque-angle curves on a lathe.
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Figure 7. Objects explored in the user evaluation. Left to right, top to bottom: RIFLE, BOW, TROMBONE, PISTOLS. Overlay
added in post-production for visualization.

the virtual controller models disappeared and were replacedby the target object. Haptic rendering then began, and theparticipant was allowed up to 30 seconds to freely explore
the object. Once finished exploring, the participant responded to the following question on a 1-5 Likert scale:“How much did it feel like you were holding and handling areal object?” The participant selected a response using alaser pointer tool, and then continued to the next trial.
We presented four objects (described below) across four
device conditions (Chain, Layer-Hinge, Ratchet-Hinge, andunlinked VIVE controllers). Each participant explored eachobject twice on each condition, for a total of 32 trials. Trials
were grouped into device blocks, such that participants explored all objects in one device condition before switching
to another device. The order of object presentation was randomized within a device block, and device blocks werecounterbalanced across participants.
After each device block, participants rated aloud qualitative
aspects of the current device by agreeing with statements on
a 1–5 scale (1 = “Strongly disagree”, 5 = “Strongly agree”).
The aspects rated were comfort (“I found the controllers
comfortable to use”) and movability (“I found that I could
move the controllers as desired or expected”).
To prevent the effects of visual bias on the ratings of any
device condition, participants did not see any of the controllers until the end of the study. The devices were hidden
prior to the arrival of the participant, and participants kept
on the HMD throughout the study as devices were taken

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from and placed into their hands. Participants wore headphones playing brown noise to prevent listening for any
noises from motor activations.
Virtual Objects

We presented participants with the following four virtual
objects (Figure 7). These objects were selected to cover arange of possible stiffness requirements and motion types,
to fully explore the capabilities of each Haptic Link. Table
2 describes how stiffness was applied to each Haptic Link
to render these objects. Standard vibrotactile feedback on
the unmodified VTVE controllers was used across all objects and conditions. We refer the reader to the accompanying Video Figure to see these objects in greater detail: RIFLE: A two-handed rifle was rendered such that theparticipant’s right hand held the trigger, while their left
hand rested beneath the forestock.
 BOW: A recurve bow and arrow were rendered such thatthe participant’s left hand held the bow at its grip while
the right hand held an arrow. After setting the arrow inthe bow’s nock, the participant held the trigger while retracting their right hand to pull back the bowstring andarrow. Releasing the trigger fired the arrow. Slight vibrations were felt while pulling back the bowstring.
 TROMBONE: A trombone was rendered such that thebell section was held in the left hand while the righthand held the slide. Moving the right hand towards andaway from the left hand controlled the motion of the virtual slide. The slide would not move past its first position (too close) or beyond the end of the tubing (too far)Page 6

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Chain

Layer-Hinge

Ratchet-Hinge

RIFLE

Rigidly locked

Rigidly locked

Rigidly locked

BOW

Increasing stiffness as the userdraws bow

Left (bow hand) ball joint
locked; increasing stiffness
as the user draws bow

Left (bow hand) ball joint
locked; outward motion
braked when fully drawn

TROMBONE

Small baseline stiffness

Small baseline stiffness inhinge; ball joints locked

Ball joints locked; directional brake as user exceeds
max/min slide

PISTOLS

Unlocked

Unlocked

Unlocked

Table 2. Specific stiffness applied to each Haptic Link to render the objects in the user evaluation.

to prevent unrealistic renderings or separation of thepieces. Slight vibrations were felt as the slide moved.
 PISTOLS: Two pistols were rendered to track each of
the participant’s controllers individually.
Hypotheses

The RIFLE served to inform whether Haptic Links improve
the realistic presentation of rigid two-handed objects. The
BOW primarily explored dynamically increasing stiffness
values in a continuous range (for applicable Haptic Links).
The TROMBONE investigated constrained degrees of freedom in motion as well as directionally selective braking(for applicable Haptic Links). Finally, the PISTOLS served
as a control condition to determine whether the presence ofthe Haptic Link detracts from the haptic presentation ofotherwise disjoint objects.
Thus, for each object, post-hoc comparisons were madebetween each Haptic Link condition and the unlinked VIVE
controllers. In particular, the Haptic Links were hypothesized to be perceived as more realistic in the RIFLE, BOW,

and TROMBONE conditions, and less realistic in the
PISTOLS condition.
Results and Discussion

Figure 8 and Table 3 summarize the ratings for each deviceobject combination, as well as overall comfort and movability responses from post-device feedback. Using Wilcoxon
Signed-Ranks Tests, we assessed the comparisons between
each Haptic Link and the unlinked VIVE controllers foreach condition. We used Dunnett’s Test as a follow-up
measure to account for multiple comparisons against thecontrol group.
Participants considered all Haptic Links to be significantly
more realistic than unlinked controllers for the RIFLE object, suggesting that variable stiffness haptics can indeed
improve the haptic rendering of rigid two-handed objects.
Mean ratings were also higher for all Haptic Links with the
TROMBONE but failed to reach adjusted statistical significance. Ratings were highest for the Ratchet-Hinge protoVIVE

Chain

LayerHinge

Ratchet- χ-crit
Hinge

RIFLE

2.75

3.54
3.57
3.83
p = .017* p = .005* p = .001*

.67

BOW

3.79

3.54
p = .314

3.65
p = .367

3.71
p = .528.64

TROMBONE

2.75

3.21
p = .098

3.08
p = .2353.38
p = .020.73

PISTOLS

3.63

2.83
3.21
p = .026* p = .100

3.63
p = .973.68

OverallComfort

4.7

3.7
3.2
3.3
p = .003* p = .003* p = .005*.7

Overall
Movability

3.9

3
3.3
p = .042* p = .086

.93.7
p = .594* = Significance persists following multiple comparisonsadjustment using Dunnett’s test (minimum significant
distance between group means listed as χ-crit)
Figure 8. Box plots aggregating object-device realism ratingsand overall device ratings.

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Table 3. Mean ratings and p-values for object-device realism
and overall device ratings. p-values represent comparisons tounlinked VIVE controllers within the same category.

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type, which is expected given that it was the only prototype
capable of rendering the outward stop matching the end of
the range of motion of the visual slide component.
The results from the BOW trials are more surprising, as
ratings were similar across all conditions. Follow-up interviews with participants indicated that they were highly sensitive to the nuances of each Haptic Link’s motion in thecontext of such a complex action. Among the issues noted
were the lack of a true spring force in the bowstring, thetime to fully release stiffness after firing, and the inability
to pull straight back on the hinged devices while holding
the bow upright. In contrast, when using the unlinked controllers, participants rarely critiqued the experience at all,despite the complete lack of feedback between the bow andbowstring. One possible conclusion is that users are more
willing to accept no feedback in the rendering of an object(perhaps due to familiarity) than incorrect feedback.
Finally, only the Chain prototype had significantly lower
ratings than the unlinked controllers for the PISTOLS object. Participants largely attributed these sentiments to alimited range of motion due to the chain’s length.
Participants rated comfort as significantly higher for theunlinked controllers than for the Haptic Links. Weight was
the most frequently mentioned issue, as well as the feelingthat the Haptic Link could be felt protruding if the controllers were held close and at a particular angle. However,some participants noted that despite the impact on comfort,
the added weight improved the realism of the virtual objects. Others specifically mentioned that the linked controllers felt less familiar in their hands, which suggests a follow-up evaluation with longitudinal design to explore theeffects of familiarity. Finally, Movability ratings were significantly decreased only for the Chain prototype. As highlighted during the PISTOLS trials, the shorter length of the
Chain prototype resulted in an unsatisfactory range of motion for scenarios requiring total freedom of each controller.
Given these results, we can begin to consider guidelines fordesigners that suggest the usage of different Haptic Links
for different objects and interactions. For example, both theLayer-Hinge and Ratchet-Hinge models are effective inswitching between objects that require rigid locking andfree motion, such as picking up and wielding various static
objects. While the Chain device suffers from a decreased
effective length and slower actuation speed, it can be freely
positioned in different shapes and is safe to grab, having no
exterior actuated elements along the chain. Thus, we mightsuggest its use in wheels, levers, ropes, and other arbitrarily-shaped tools that users can grasp in different positions.
APPLICATIONS AND EXTENSIONS

Along with user evaluation, we explored the potential of
our Haptic Link attachments for supporting richer bimanualinteractions in VR through several techniques.

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Unlocked

LockedFigure 9. Summoning a vehicle using gestural input (left), thendriving with a rigid steering wheel (right).
Object Summoning

As our Haptic Links can currently only provide resistive
forces (that is, they cannot actively move controllers into agiven configuration), a key question is how to get the user’s
controllers positioned appropriately to render a particular
object. Similar to existing work leveraging gestures thatmimic physical input [40], our first approach—referred to
as “Summoning”—allows the user to assume a desired object at will by mimicking its shape with their controllers.
We explored this technique in the context of a virtual racing
game (Figure 9), where the user can Summon either a car ora motorcycle around them by placing their controllers in the
shape of a steering wheel or handlebars respectively. Oncein position, the Haptic Link rigidly locks the controllers,
thus rendering the steering wheel/handlebars which the user
can rotate as if steering a real vehicle. By pressing a buttonon the controller, the Haptic Link relaxes, and the vehicle
disappears. With this technique, users can easily switch
between objects on the fly by Summoning them through thecorrect pose of the two controllers.
In a similar fashion, we also created a zombie shooter game
utilizing the rifle and dual pistol weapons featured in theuser study (Figure 1(b-c)). By default, players can shootpistols in each hand, but by holding their hands out as if
wielding a rifle, the pistols are replaced by the rifle weapon,
which shoots farther and faster. Upon transition, the Haptic
Link rigidly locks the controllers in the rifle configuration.
A button on the controller returns to the pistols, allowing
the user to switch between weapons at will. Through Summoning, Haptic Links allow users to wield an indefinite
number of objects with different haptic presentations.
Object Retrieval Indicators

With Summoning, the user can take up a given object at any
time—as if the object is always with them. An alternative
style of interaction is manual object acquisition: the object
exists in a particular location and the user must approachand pick it up. To ensure that the controllers are properly
arranged for haptic rendering in this style of interaction, we
use a set of visual indicators that represent the proper position and orientation of each controller to pick up the object.

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

Guided indicators

Unlocked

InwardLockedFigure 11. Using directionally-selective braking to grasp objects with two hands. As the controllers meet the object, inward motion is braked. Overlay created using post-processing.

Figure 10. Using guided indicators (top) to retrieve a wrench
object (bottom).

We created a plumbing scenario (Figure 10) demonstrating
the use of these indicators, in which users tighten down anut on a pipe using a wrench. One controller holds thewrench while the other holds the pipe. To begin the interaction, the user must approach the pipe, grab the pipe below
the nut with one hand, and position their other hand at thehighlighted zone in the air above the nut. When correctly
positioned, the wrench appears, and the Haptic Link provides increasing stiffness on the downswing, while releasing the stiffness as the user returns on the upswing.
Grasping Virtual Objects

By tracking the controllers and locking them in a directionally-selective fashion as they move through virtual space,
the Ratchet-Hinge prototype can render impassable virtualsurfaces. We explored this capability in the context of twohanded grasping interactions by creating a virtual playground where users grasp objects of different sizes between
their hands (Figure 11).
Controller Grounding

Again leveraging the directional capabilities of the RatchetHinge prototype, we can anchor one controller to a fixed
point, and then using the Haptic Link to provide grounded
force feedback to the other: In short, the user grounds either
on-body (the user anchors a controller to their body) or externally (the user anchors a controller to a fixed surface inthe room). Then, we can repurpose the variable stiffness
actuation of the Haptic Link to halt motion of the remaining
controller in the user’s hand. For example, if the user
grounds one controller at their waist, we can render solid

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Figure 12. Grounding one controller at the waist (top) or on an
external surface (bottom), to enable grounded force-feedback
interactions with the other controller via the Haptic Link.

walls and objects by braking outward motion of the controller as it contacts each surface. We fabricated an additional
controller mount allowing one of the linked controllers to
attach either on-body or externally (Figure 12), and thendesigned a virtual environment where users could reach out
and explore walls in front of them.
LIMITATIONS AND FUTURE WORK

Despite the capabilities we have shown, the Haptic Link
prototypes have limitations requiring further iteration. Froma mechanical standpoint, users mentioned weight, bulk, andrange of motion as noticeable detractors, all of which canbe improved through modifications to the design, materials,
and fabrication. As our prototypes were primarily built withlaser cut or 3D printed components, switching to manufac-

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CHI 2018 Paper

tured components should improve not only weight and sturdiness, but precision and robustness of the friction-based
mechanisms as well. Additional investigation is needed to
fully understand the ergonomic impacts of Haptic Links,
such the added moment of inertia in different controller
poses, motion constraints given different sizes and attachment sites of the Haptic Link, as well as potential muscle
fatigue from extended use.
From an actuation standpoint, the maximum stiffness ofmost joints—though enough to present rigidity to the user—can be overcome through the user’s full strength. Similarly, some users noted that a faster speed of actuation
could improve the rendering of quick transitions—for example, the releasing of tension on the bowstring. We can
improve both the maximum braking torque and the actuation speed by using more performant motors and by iterating on the designs of our joint mechanisms. However, ourresults also suggest the need for further evaluation to identify how much—or how little—stiffness is needed to perceive the controllers as a unified object, and how quickly
actuation must occur to render compelling interactions.
Finally, several modifications could make Haptic Links
more convenient for use with a commodity VR setup, suchas making them adjustable in length, removing the need fora wired connection, and creating an attachment mechanismthat allows them to quickly snap onto and off of the controllers. Longevity of the friction-based mechanisms is also a
concern: over the course of our development and evaluation, we occasionally found the need to recalibrate the motors to adjust for wear in the frictional surfaces.
More generally, the resistive nature of Haptic Links limits
the range of possible types of force feedback that they canpresent. Specifically, Haptic Links cannot provide inertial
force feedback, meaning that while they can introduce forces between the hands, they cannot impart net forces onto
them. Further, Haptic Links are incapable of rendering thespring forces found in many objects. If stiffness is applied
as a controller moves in one direction, there is no restoring
force in the opposite direction. Future Haptic Links could
include additional components such as gyroscopic mechanisms or springs to render such forces, and designers could
choose these Haptic Links as needed by their VR scenarios.
Beyond improvements to the devices themselves, we aim to
explore as future work how Haptic Links can influence theperception of other haptic techniques and phenomena inbimanual scenarios. For example, recent work has examined illusory haptics in VR [22,35], finding that the vibrotactile feedback in most handheld controllers can beleveraged to present stimuli that seem to originate fromdifferent locations on a virtual object held between thehands. As our results show that users can perceive a twohanded object more realistically with a Haptic Link, we caninvestigate whether the Haptic Link also improves the robustness of these spatio-haptic illusions. We can also explore the power of visual dominance in expanding the ca-

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

pabilities of Haptic Links. For example, can a sufficiently
visually compelling experience lead users to perceive
spring forces or inertial forces that cannot be rendered by
the Haptic Link? Or, by effectively redirecting users’ focus
and movement, can we design the interaction such that theusers themselves provide these forces from the other controller? We find these open questions to be exciting future
work as we continue to iterate on the design of our Haptic
Links.
CONCLUSION

Haptic Links demonstrate the potential to improve the haptic rendering of two-handed objects and interactions in VR
using inter-controller variable stiffness feedback. The multiple implementations of Haptic Links yield different capabilities and advantages for object rendering. Our evaluationshows that Haptic Links can improve the perceived realismof two-handed objects without significantly detracting fromthe rendering of normal interactions requiring disjoint controllers. Finally, the interaction techniques we introduce
leverage Haptic Links to provide more compelling haptic
experiences in VR.
Virtual reality has become increasingly immersive, leaving
us with a growing need for authentic haptic interactions.
Haptic Links offer designers of VR experiences a wide
range of new haptic tools that work seamlessly with the
handheld controllers of commodity VR systems. While ourprototypes represent just a starting point in the design offuture Haptic Links, we find their early success encouraging for the exploration of a new class of devices which can
rapidly augment existing controllers to provide a customized haptic experience.
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