Tasbi: Multisensory Squeeze and Vibrotactile
Wrist Haptics for Augmented and Virtual Reality
Evan Pezent†‡ , Ali Israr† , Majed Samad† , Shea Robinson† ,Priyanshu Agarwal† , Hrvoje Benko† , and Nick Colonnese†
Abstract—Virtual and augmented reality are poised to deliver
the next generation of computing interfaces. To fully immerseusers, it will become increasingly important to couple visual
information with tactile feedback for interactions with the virtualworld. Small wearable devices which approximate or substitutefor sensations in the hands offer an attractive path forward. In
this work, we present Tasbi, a multisensory haptic wristband capable of delivering squeeze and vibrotactile feedback. The device
features a novel mechanism for generating evenly distributed andpurely normal squeeze forces around the wrist. Our approachensures that Tasbi’s six radially spaced vibrotactors maintainposition and exhibit consistent skin coupling. In addition to experimental device characterization, we present early explorations
into Tasbi’s utility as a sensory substitution device, employing
squeeze, vibration, and pseudo-haptic effects to render highly
believable virtual button interactions.

I. I NTRODUCTION AND BACKGROUND
Advances in motion tracking, optics, displays, and computer
graphics have revolutionized head mounted displays (HMD)
for virtual reality (VR) and augmented reality (AR). With
increasing research and development into AR/VR, these devices are positioned to reinvent computer interfaces in ways
similar to the cellphones and personal computers that preceded
them. The addition of haptic feedback to AR/VR is appealing,
since touch feedback can close the action-confirmation loop,
reduce cognitive load, and increase performance [1]. However,most widely used AR/VR interfaces today leave much to be
desired in terms of haptic feedback as they offer little morethan simple vibration at best. While more advanced grasping,fingertip, and glove type devices [2], [3] show promise, they
currently face a number of challenges regarding encumbrance,
locating actuators, grounding forces, and power requirements.
Because of these unresolved complexities, more practical andimminent devices are desired to render feedback in AR/VR.
Wristband or bracelet type devices are an attractive location
for AR/VR interfaces and offer several advantages. First,
wristbands allow for a reasonable design space in terms ofacceptable weight, size, and power needs. Secondly, wrist
bands leave the hands free, which is particularly importantfor AR as they allow the hands to manipulate the physicalworld unhindered. Finally, wrist bands are socially acceptableand even fashionable. A variety of feedback modalities havebeen explored in wrist and arm haptic devices, with two of
the most common being vibrotactile feedback and squeeze (orcompressive) feedback.
† Facebook Reality Labs, Redmond, WA 98052, USA. ‡ Mechatronics
and Haptic Interfaces Laboratory, Dept. of Mechanical Engineering, Rice
University, Houston, TX 77005, USA.

Figure 1. Tasbi is a novel multisensory haptic wristband capable of delivering
squeeze and vibrotactile feedback for a variety of uses in AR/VR.

A. Vibrotactile Feedback
Several research groups have studied placing vibrotactors
on the wrist and forearm. Chen et al. [4] compared placing
planar arrays of vibrotactors on the dorsal versus the volarside of the wrist. Matscheko et al. [5] further studied arranging
vibrotactors dorsally versus radially, concluding that placing
tactors around the circumference of the wrist was best. Following their advice, Carcedo et al. [6] developed a wristband
with radially spaced tactors. They tested variations of a bandwith 3, 4, 5, and 6 equally spaced vibrotactors, and found thatusers can reliably detect up to five vibration motors around thewrist. However, their design did not attempt to isolate vibrationtransfer between adjacent motors. Hong et al. [7] addressedissues of vibration transfer by separating tactors with thinelastic thread. They concluded that in this configuration, up to
eight tactors can increase accuracy in a directional guidancetask. While results of tactor placement and discretization arevaried, it is generally agreed that ensuring proper contact withthe skin is critical to achieving optimal vibration transfer.
B. Squeeze Feedback
Squeeze feedback is thought to be less attention demanding
than vibration [8], provides more intimate feedback similar
to how one human might get the attention of another [9],[10], and may elicit affective or emotional responses [11].
Most squeezing devices employ one or more motors to tension
bands around the arm [10], [12]–[16], and are characterized bygenerating both inward and tangential forces (i.e. skin-stretch)on the skin. Other devices have used cams and linkages tocreate purely compressive forces [17]–[19]. Gupta et al. [20]addressed the size concerns of the aforementioned devices

OBJECT
MANIPULATIONSOCIAL
INTERACTIONSGAMING &VIRTUALTRAINING

USER
INTERFACES

Figure 2. Tasbi can enable a variety of interactions with the virtual world. Vibrotactile feedback may substitute for fingertip contact with virtual buttons
or other types of user interfaces, while squeeze may convey the weight or stiffness associated with manipulating virtual objects. Squeeze and vibration canfurther add to telepresence and remote social interactions such as hand shaking or holding, and can also provide immersive feedback for gaming and training.

by employing shape-memory alloys. However, this approachrequired high power and insulation to shield users from heat.
Pohl et al. [21] used pneumatically actuated pockets to create
uniform compression akin to a blood pressure cuff, but thislow bandwidth approach could limit AR/VR interactions.
C. Multisensory Feedback
Most of the devices discussed so far were developed for
notification type interactions, and thus can only offer limited
experiences for AR/VR. One way in which wristband devices
can be made more applicable to AR/VR is by enabling
multisensory feedback. Providing both squeeze and vibration
could deliver a richer cue set, higher information throughput tothe user, and a more diverse variety of interaction primitives.
Despite the high volume of research into vibration andsqueeze feedback alone, a relatively limited number of devices
have implemented both squeeze and vibration into a multisensory device. Dunkelberger et al. developed the MISSIVE[22], an arm device with separate dedicated bands for vibrationand squeeze to render language through skin. Aggravi et al.
presented a forearm device [23] that incorporated squeeze andfour vibrotactors into the same band, but did not address thefact their design causes tactors to translate on the skin, which
would likely confuse users.
D. Contribution
In this paper, we present Tasbi (Tactile And Squeeze
Bracelet Interface), a multisensory haptic wristband for
AR/VR (Fig. 1). We hypothesize that combining robust formsof squeeze and vibration feedback into a single device will
enable a richer haptic experience (Fig. 2). Our design (SectionII) introduces a novel squeezing mechanism which addresses
issues with previous squeezing devices, namely size and theability to generate entirely normal forces, while maintaining
high force output and bandwidth. The device incorporates a six
vibrotactor band which has been specially designed to eliminate undesired translation and vibration transfer. In Section III
we characterize squeezing capabilities and provide estimatesof force and bandwidth useful to future designers. Section
IV presents initial explorations into using Tasbi for sensory
substitution, and details a unique virtual button interaction
which leverages squeeze, vibration, and control-display ratio
based pseudo-haptics. Finally, in Section V we present ourconclusions and future plans.

II. D EVICE D ESIGN
The design of Tasbi was constraint driven. We aimed
to achieve a wrist-watch form factor of approximately
50×50×15 mm in size with a total weight less than 200 grams,
without sacrificing force output and bandwidth. Further, we
decided the device should emit little audible noise to avoid
annoying or confusing users. Finally we deemed it necessary
to keep total power consumption around 2 W so as to notdissipate an uncomfortable amount of heat on users’ skin.
A. Squeeze Mechanism
While a few novel approaches to squeeze actuation exist,
most devices use a similar scheme where one or more rotational actuators are used to directly wind a band element into
an actuator housing [10], [12]–[16]. While this approach isstraightforward, it presents two main issues (Fig. 3-a). First,
directly tensioning the band itself gives rise to an unequal
distribution of forces where there are concentrated tangential
shear forces on the sides of the arm, and smaller normal
forces on the underside. Furthermore, this results in non-trivial
squeeze force losses due to friction between the band and skin.
Second, because this method causes the band to translate along
the skin, it does not allow for embedding vibrotactile elementsin the band since they would consequently translate too.
Maintaining the radial positions of the vibrotactors is key since
their movement would decrease user identification rates andpossibly cause discomfort. Some devices have circumvented
this issue by using two separate bands: one for generating
squeeze, and one for housing vibrotactors [22]. However, thisapproach is less than ideal for a wrist-watch form factor andcomplicates donning and doffing the device.

(a)tactortensioning
mechanism(b)

tensioning
mechanismband

tactor

cord

wrist

wrist

smoothpin

Figure 3. (a) Typical constricting-band approaches to squeeze produce nonuniform and tangential forces which would cause embedded tactors to shift.
(b) Our decoupled approach aims to produce pure, uniform normal forces.

Tasbi solves these problems by decoupling the squeeze
actuation from the wrist band and vibrotactors. This is accomplished by means of small diameter, flexible UHMWP
cord, (trade name Dyneema/Sprectra) which wraps circumferentially around the exterior of the band (Fig 3-b). Tensioning
this cord, not the band, creates squeeze forces. Friction isminimized by separating contact between the cord and bandwith smooth polished stainless steel pins placed directly above
each vibrotactor. This mechanism results in cord tension beingtransmitted as an inward force approximately normal to thevibrotactor. Because friction between the pin and cord is small,little tangential force should be transmitted to the band, andas a result each vibrotactor should maintain its radial position
around the circumference of the wrist. Eliminating friction andtangential forces also allows for a smaller actuator, since mostpower is ideally converted to pure normal squeeze force.
Early testing suggested that approximately 10 N of cord
tension would be required to achieve an appropriate range ofsqueeze stimuli. Several tensioning mechanisms and actuators
were initially considered. For ease of implementation andcontrol, a electromechanical approach, as opposed to pneumatic or other exotic approaches, was decided. We use a 12mm brushed DC motor coupled to a 100:1, 13 mm strainwave gear unit from Harmonic Drive. These drives offereda set of characteristics we deemed necessary to fully realize
Tasbi: (1) a high torque reduction, (2) compactness superior
to conventional gear units, and most importantly (3) lessaudible noise due to having zero backlash. The DC motor andHarmonic gear unit are contained within a 50×30×15 mm
housing unit which rests on the dorsal side of the user’s wrist.
Attached to the output of the gear unit is a 10 mm diameter,
two-sided spool. Both ends of the cord terminate to the eitherside of the spool so that the take-up rate is doubled. The cord
is redirected internally over additional smooth pins to exit at
the center of the main housing.

Figure 4. Exploded view of Tasbi (color added for clarity). The squeeze
mechanism consists of a 12 mm DC motor (a) and a 13 mm Harmonic Drive
gearbox (b) which drives a two-sided spool (c) to create tension in a UHMWP
cord (d). Control feedback is provided through an optical encoder consisting ofa reflective code wheel (e) and optoelectronic sensor (f). The sensor is epoxied
into the rear housing panel (g) which also contains two 10-pin connectors forall electronics. The drive assembly (a-e) drops into the main housing (h) andis secured in place with a housing lid (i). Each vibrotactor unit (j) contains
a 10 mm LRA vibrotactor and a smooth stainless steel pin to convert cord
tension into normal force. Vibrotactor units are clipped into elastic sidings(k) and secured with lids (l).

B. Vibrotactile Band

C. Feedback and Control

The band contains six individual vibrotactor units. Each unit
consists of a plastic housing in which the vibrotactor is press
fit. The vibrotactors themselves are 2.5 VAC, 10 mm linear
resonant actuators (LRA) capable of 150 to 200 Hz. Directly
above the vibrotactor is a groove for one of the aforementionedsmooth pins. The distance from skin to the pin was optimized
so that the UHMWP cord would clear and not rub against theuser’s skin. A lid secures each vibrotactor and pin in place.
Each tactor unit is clipped in between elastic polyurethane
sidings via T-shaped joints. Tactor power cables are embedded
within the rubber siding and enter the main housing throughopenings on both sides. The elasticity and geometry of thesidings allows the band to be stretched over the user’s handduring the donning and doffing process and reduces vibrationtransfer between adjacent tactor units. Tasbi has an nominal
inner circumference of approximately 150 mm, equal to the50th percentile female wrist circumference. Thus for mostusers, the band provides a light amount of passive squeeze
to ensure a comfortable initial fit.

Closed-loop squeeze control is achieved via incremental
encoder feedback placed on the motor side of the mechanism
and an externally located servo controller operating in current control mode. The encoder and controller are interfaced
through a host PC using a Quanser Q8-USB digital acquisition(DAQ) board and the C++ based Mechatronics Engine andLibrary. The motor position loop is closed in software with aproportional-derivative (PD) control law running at 1000 Hz.
All six vibrotactors are controlled through a MOTU 24Aosound card connected to the host PC over USB. We developed custom synthesizer software to generate ASDR (AttackSustain-Decay-Release) amplitude envelopes with adjustable
pitch, modulation, and waveform to deliver a wide rangeof vibration stimuli. Commands for squeeze and vibration
can be sent to a server application over UDP or sharedmemory, enabling plug-and-play with VR content creation
software such as Unity or Unreal Engine. The bracelet itself isconnected to the external motor driver, DAQ, and sound cardthrough two 10-pin connectors on its back panel.

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Figure 5. (a) Representative responses for each torque step. (b) Steady statenormal vs. tangential forces for each vibrotactor. All curves represent a mean,
with standard deviation represented by shaded areas.

Figure 6. (a) Motor position response to a 1-to-15 Hz chirp command forboth free and user worn conditions. (b) Magnitude of the experimental transfer
function. The closed-loop bandwidth is 9.7 Hz, defined by the -3 dB cutoff.

III. S QUEEZE C HARACTERIZATION

To estimate bandwidth of the squeezing mechanism andcontroller, we commanded a chirp from 1 Hz to 15Hz to the
PD motor position loop. Since a total motor displacement
between 1,000 and 3,000 degrees was found to be typicalacross most users, the peak-to-peak amplitude of the chirp
was set to a representative 2,000 degrees. We ran the test in
both a user worn case and a free (i.e. suspended in air) case to
see how actually wearing the device would impact bandwidth.
Both responses versus the commanded input are shown in Fig.
6-a. Aside from higher noise content, the worn condition is
generally the same as the free. The -3 dB magnitude cutoff
reveals the bandwidth to be approximately 9.7 Hz (Fig 6-b),
which, based on our usage thus far, is more than sufficient.

We next characterized Tasbi’s squeezing capabilities. We
created a fixture containing two ATI Nano17 F/T sensors, one
which could be relocated under any tactor housing, while theother remained fixed under the main housing for referencing.
The device was stretched over the fixture with the band
center axis oriented upward so that gravity would not affectnormal and tangential force measurements. The tensioning
mechanism, initially with the cord loose, was commanded tostep to a certain percentage of the max motor torque, hold forthree seconds, and then return to the loose position. Forcemeasurements were taken underneath the main and tactor
housings in both the normal and tangential directions. This
procedure was repeated ten times for each of ten torque levels
from 10% to 100% of the maximum motor torque (3.21 mNm).
The full test was repeated for each of the six tactor housings.
Fig. 5-a shows the mean force response for each torquelevel under a representative tactor. For torque levels above
50% there is a noticeable relaxing effect, most likely due tothe material properties of the plastic housings and UHMWP
tensioning cord. Torque levels below 20% produce little tono force output, revealing some dead band in the tensioning
mechanism due to friction in the drive components. Fig. 5-bshows the “steady state” (i.e. average of the last half-secondof the force response) normal and tangential forces undereach tactor. Importantly, we can see that there is negligible
tangential force, with the force distribution being almost
entirely normal and thus satisfying our design goals. The more
proximal tactor housings have a higher normal force than the
distal tactor housings, which is likely due to cord tension dropoff between adjacent tactors as a result of pin friction. Thereis also some bias, with left-side normal forces being higher,
but generally we see a linear region from 20% to 70% torque.

IV. M ULTISENSORY VR I NTERACTIONS
Tasbi’s most compelling use is its ability to substitute forinteraction forces arising from hand and finger interactions
with virtual objects. To demonstrate this, we prototyped avirtual push button (Fig. 10). The onset of interaction istriggered through vibrotactile cues rendered when the user’s
avatar finger collides with the surface of the button. As the userpresses the button inward, proportional squeeze is delivered
through the band to convey stiffness. By changing the rateof squeeze, the perceived stiffness can be altered. A second
vibrotactile stimulus is rendered when the button is fully
depressed. Similar to a “god-object” [24], the avatar hand is
projected onto the button surface so that it never appears topenetrate. The illusion of different stiffness levels is also affected by changing the control-display (C/D) ratio between theuser’s real and avatar hands, such that a stiffer button requires
more real-world displacement. Within the haptics community,
this visual effect falls under “pseudo-haptics” [25]. Users
commented that the combination of vibrotactile, squeeze, andC/D stimuli created a highly intuitive and believable effect.

(a)

timer

lightsThe Standard button always displayed Standard Kh and Kv ,while the other button could be in one of three conditions:(b)

1. vibration

(c)

(d)

display
control

2. squeeze

A random Comparison Kh and Standard Kv
Standard Kh and a random Comparison Kv
Congruent Comparison Kh and Kv values

Haptics
Visuals
Congruent

3. vibration

Figure 10. (a) Virtual environment as seen through the HMD. A trial
timer was shown along with lights indicating press counts. (b) A vibrotactile
stimulus is rendered when the finger and button collide. (c) Tasbi begins
to squeeze proportionally as the button is displaced, with stiffer buttons
displaying higher squeeze rates. (d) The hidden control represents the user’s
actual position, while the display is projected back on to the button surface.
Stiffer buttons exhibit a greater discrepancy between the control and display.
A second vibrotactile stimulus is rendered when the button bottoms out.

Thus each condition provides both squeeze and C/D stimuli,
but in the Haptics condition only squeeze is used to vary thestiffness across trials, while in the Visuals condition only C/Dratio stimuli is used to vary the stiffness. In the Congruent
condition both squeeze and C/D ratio stimuli vary across trials,
matching in difficulty level. Variations of all three conditions
were randomly interleaved across 108 trials such that eachpossible combination was represented six times. The placement of the Standard button was also randomized to either
the left or right side. All experimental stimuli included thesame vibrotactile stimulus for button contact and bottomingout. Subjects were given seven seconds and two presses per
button to make their decision, which they were encouraged tomake as soon as they were confident. A total of 12 subjects
participated (6 males, ages 25 to 44, mean 30) in accordancewith WIRB #20182617.

A. Experiment Design

B. Results and Discussion

To understand how squeeze haptics and C/D ratios contributed to the button-press, we conducted a method of constants style experiment where users were required to select the“stiffest” of two buttons each trial. Stiffness was parameterized
by the combination of Kh , the stiffness depicted throughhaptic squeeze in units of squeeze per virtual displacement,
and Kv , the stiffness depicted through the visual C/D ratio
in units of physical displacement per virtual displacement.
Thus, higher Kh increased the rate of squeeze as the button
was pressed, while higher Kv increased the distance users
had to reach their physical hand to achieve a particularbutton displacement. For both Kh and Kv , seven values werechosen: a Standard value, three Comparison values below
the Standard (-Easy, -Medium, -Hard), and three Comparison
values above the Standard (+Easy, +Medium, +Hard). Values
were determined during a pilot such that users differentiatedbuttons displaying Easy Kh and Kv values from the Standard
with 100% accuracy, Medium values with 75% accuracy, andHard values slightly above 50% accuracy.

To analyze the results, we assessed the percentage of correct
responses overall, the Point of Subjective Equality (PSE),
and the Just Noticeable Difference (JND). The accuracy ofparticipant responses was assessed by counting the numberof times that they correctly chose the stiffer button for the
Easy, Medium, and Hard comparison levels (Fig. 11-a). A twoway ANOVA with repeated measures revealed main effects
for both difficulty (F (2, 22) = 11.7, p < .001) and condition(F (1.3, 14.4) = 25.11, p < .001). Contrasts with a Bonferroni correction showed the Congruent condition produced
significantly more accurate responses than the other conditions(t(11) = 8.7, p < .001 for Easy, t(11) = 4.8, p = .001 forMedium, and t(11) = 4.7, p = .001 for Hard). Furthermore,
the Congruent condition showed considerably less variance,suggesting more consistent responses overall.
To obtain the JND and PSE values for each condition,
we plotted the proportion of correct responses between theStandard and each comparison at the group-level. We then fit
this data to three psychometric curves (Fig. 11-bc) using a(b)

P (Report Comparison Stiffer)

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Figure 11. (a) The percent of correctly identified button stiffnesses across all conditions and difficulties. The Congruent condition produced the significantly
more accurate responses overall. (b-c) Psychometric curves fitted to the group-level data. The Congruent condition displays a higher slope than either of theother conditions. All error bars represent the standard error of the group-level mean.

JND, K v

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R EFERENCES
Congruent Visuals

Congruent Visuals

Figure 12. Just Noticeable Difference (JND) and Point of Subjective Equality(PSE) values obtained from psychometric curves in Fig. 11-bc. The Congruent condition displays smaller JND values, and all conditions’ PSE value
corresponds well with the Standard values.

Generalized Linear Model (GLM) regression. While we cansee that the visual stiffness Kv was not perfectly titrated tothe same difficulty as Kh (despite our best efforts in the pilot),
there are several interesting observations. First, we can see thatthe curve is steeper in the Congruent condition, resulting in asmaller JND compared to either of the other two conditions.
Fig. 12 shows that the JND decreased by 2-fold for Kh when
the C/D ratio also varied, and by 10-fold for Kv when squeeze
also varied. The PSE was not affected by the manipulation, andcorresponds well with our standard values. These results showthat varying both the visual and haptic modalities together
produces more accurate responses than varying either alone.
V. C ONCLUSIONS AND F UTURE W ORK
To conclude, we have presented Tasbi, a novel haptic
wristband featuring multisensory squeeze and vibration for
AR/VR interactions. The device has been shown to produce
evenly distributed forces up to 15 N and 10 Hz radially aroundthe wrist. Importantly we have demonstrated that our design
eliminates tangential shear forces which would have otherwise
presented problems for band embedded vibrotactors. Finally,
we presented a proof of concept application in which squeeze,vibration, and a psuedo-haptic effect were utilized to create ahighly believable finger-button interaction.
There are several avenues we are interested in exploring
for future work. On the hardware side, the addition of force
sensing would enable an even broader range of interactions
and the ability to intelligently respond to disturbance forcesfrom users (e.g. automatically loosening/tightening when auser pulls/pushes the device). Incorporating inputs such as
buttons or a capacitive surface would allow for use cases
which require bidirectional communication between the userand device. Further miniaturization and integration of batteries
and wireless electronics is also of long-term importance, which
may necessitate exploration of more exotic actuation methods.
From a user standpoint, we are most interested in continuing
work into sensory substitution. The experiment presented here
involving button rendering should be expanded to include
varying levels of vibrotactile stimuli as an effect, and othertypes of force interactions should be explored. Finally, in
addition to determining vibrotactor identification rates andsqueeze threshold levels, we are interested in researching
how active squeeze may enhance vibrotactor-skin coupling toensure consistent stimuli across users.

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