Prolonging VR Haptic Experiences by Harvesting Kinetic Energy
from the User
Shan-Yuan Teng

University of Chicago

Jacqueline Chen

University of Chicago

K. D. Wu

University of Chicago

Pedro Lopes

University of Chicago

Figure 1: We propose a new technical approach to wearable haptics that requires no battery, yet it provides active haptic
feedback. The key is (a) our motion harvester engineered from a geared-DC motor paired with a custom clutch, worn on theuser’s elbow. This enables haptics in long experiences, such as this “VR survival” that lasts for hours, which is not possible
with existing haptic devices that render strong and power-hungry haptics. Using our approach, a haptic device harvests all thenecessary energy from the user’s own movement. However, the resistance felt while harvesting would become a distraction
to the VR user as it does not match their experience. Our concept solves this by adjusting the VR world to render a situationin which the user expects to feel resistance, such as rowing this boat. (b) When its supercapacitors are charged, the device
informs the VR and releases its clutch, so that the user no longer feels the resistance of the harvester; simultaneously, the VR
renders a situation without resistance (e.g., walking on an island). The benefit is that the VR can “spend” the recently harvested
energy with on-demand haptics (e.g., vibration, electrical stimulation). (c) If the battery is close to being depleted, this cycle
repeats, ad infinitum; without the user even needing to be aware of the harvesting. This prolongs VR haptics beyond theirusual short-lived durations and frees the user from large batteries & power tethers.

ABSTRACT
We propose a new technical approach to implement untethered VR
haptic devices that contain no battery, yet can render on-demand
haptic feedback. The key is that via our approach, a haptic device charges itself by harvesting the user’s kinetic energy (i.e.,
movement)—even without the user needing to realize this. Thisis achieved by integrating the energy-harvesting with the virtual
experience, in a responsive manner. Whenever our batteryless haptic device is about to lose power, it switches to harvesting mode(by engaging its clutch to a generator) and, simultaneously, thePermission to make digital or hard copies of all or part of this work for personal orclassroom use is granted without fee provided that copies are not made or distributedfor profit or commercial advantage and that copies bear this notice and the full citationon the first page. Copyrights for components of this work owned by others than theauthor(s) must be honored. Abstracting with credit is permitted. To copy otherwise, or
republish, to post on servers or to redistribute to lists, requires prior specific permission
and/or a fee. Request permissions from permissions@acm.org.
UIST ’22, October 29–November 02, 2022, Bend, OR, USA© 2022 Copyright held by the owner/author(s). Publication rights licensed to ACM.
ACM ISBN 978-1-4503-9320-1/22/10. . . $15.00
https://doi.org/10.1145/3526113.3545635

VR headset renders an alternative version of the current experience that depicts resistive forces (e.g., rowing a boat in VR). As aresult, the user feels realistic haptics that corresponds to what theyshould be feeling in VR, while unknowingly charging the device
via their movements. Once the haptic device’s supercapacitors arecharged, they wake up its microcontroller to communicate with the
VR headset. The VR experience can now use the recently harvested
power for on-demand haptics, including vibration, electrical or mechanical force-feedback; this process can be repeated, ad infinitum.
We instantiated a version of our concept by implementing an exoskeleton (with vibration, electrical & mechanical force-feedback)that harvests the user’s arm movements. We validated it via a user
study, in which participants, even without knowing the device washarvesting, rated its’ VR experience as more realistic & engaging
than with a baseline VR setup. Finally, we believe our approachenables haptics for prolonged uses, especially useful in untethered
VR setups, since devices capable of haptic feedback are traditionally
only reserved for situations with ample power. Instead, with our approach, a user who engages in hours-long VR and grew accustomed

UIST ’22, October 29–November 02, 2022, Bend, OR, USA

to finding a battery-dead haptic device that no longer works, will
simply resurrect the haptic device with their movement.

KEYWORDS
haptics, virtual reality, energy harvesting
ACM Reference Format:
Shan-Yuan Teng, K. D. Wu, Jacqueline Chen, and Pedro Lopes. 2022. Prolonging VR Haptic Experiences by Harvesting Kinetic Energy from theUser. In The 35th Annual ACM Symposium on User Interface Software andTechnology (UIST ’22), October 29–November 02, 2022, Bend, OR, USA. ACM,
New York, NY, USA, 18 pages. https://doi.org/10.1145/3526113.3545635

1

INTRODUCTION

In the past decades, interactive devices became mobile, moving intouser’s pockets and body, thanks to power-efficient computation
and advances in battery technology. While sensing in devices now
can be realized in a power-efficient manner [1, 18, 29, 48], output(e.g., displays or haptics) still remains a power-hungry factor in
mobile devices [4, 19, 42]. Haptic feedback, specifically strong forcefeedback, needs to push against the user to generate its effect, thus
demanding even more power [10, 12, 31] (typically orders of magnitude above the power required for a sensor [48]). Therefore, most
haptic devices are tethered to the powerline or require large andcumbersome batteries. While researchers have explored alternative
actuators (e.g., muscle stimulation instead of mechanical motors; orbrake-based actuators) for the sake of power efficiency, even thesedevices’ batteries will likely last far less than a whole day of usage[17, 31].
We propose a new technical approach to implement untethered
Virtual Reality (VR) haptic devices that contain no battery, yet canrender on-demand & strong haptic feedback. The key is that via
our approach, a haptic device charges itself by harvesting the user’s
kinetic energy (i.e., movement)—even without the user needing torealize this.
Figure 1 depicts how we achieve this: we integrate kinetic energyharvesting directly into the virtual experience, in a responsive manner. For example, in our approach, whenever a batteryless haptic
device is about to lose power, it switches to harvesting mode (byengaging its custom electropermanent magnetic clutch to a generator) and, simultaneously, the VR headset renders an alternative
version of the current VR experience that depicts resistive forces(e.g., rowing a boat in VR). As a result, the user feels realistic haptics that corresponds to what they should be feeling in VR (i.e., itshould “feel hard” to row a boat), while unknowingly charging thedevice via their movements. Then, once the haptic device’s supercapacitors are charged (which charge/discharge much faster than
traditional batteries), the device’s microcontroller communicateswith the VR headset. The VR experience can now use the recently
harvested power to request more on-demand haptics, including
vibration, electrical or mechanical force-feedback; this process canbe repeated, ad infinitum.
We instantiated, explored, and validated a version of our concept
by implementing an exoskeleton that harvests elbow movements
and uses this energy to render on-demand haptic feedback intermittently, e.g., vibration, electrical & mechanical force-feedback.

Shan-Yuan Teng et al.

We validated this via technical evaluation and a user study, inwhich participants (even without knowing the device was harvesting) rated a VR experience as more realistic and engaging using
our device than with a baseline VR setup.
Finally, we believe our technical approach is fundamentally different from devices powered by batteries, as it affords new usesof haptics for prolonged use-cases, which are especially useful in
untethered VR setups, since devices capable of haptic feedback are
traditionally only reserved for situations with ample power. With
our approach, a user who engages in hours-long VR and grew accustomed to finding a battery-dead haptic device that no longer
works, will simply resurrect the haptic device with their movement.
Moreover, our approach enables new ways to use haptic devices
unthinkable for battery-powered devices today: namely, walk-up
use. Even if the user forgot to charge the haptic devices or change
their batteries, our technique enables these to wake up rapidly after
the first interactions.

2

OUR APPROACH: HARVESTING THE
USER’S KINETIC ENERGY TO PROLONG VR
HAPTICS

To demonstrate our novel concept of harvesting the user’s kinetic
energy to prolong the lifetime of VR haptics devices, we engineered
an exoskeleton worn on the user’s arm that demonstrates one
possible instantiation of our technical approach. This exoskeleton,
depicted in Figure 1, can be charged by the user during VR evenwhen it loses all its power. Then, it uses this harvested energy
to render subsequent haptic effects. The fact that our approachis cyclical (the device automatically returns to harvesting always
before losing power and always informs the VR accordingly) allows
any device and VR experience built around our approach to run
for extremely long periods of time, including being picked up after
any arbitrary period of inactivity (the device will always switch toharvesting when power is low).
We first summarize the key technical insights that make ourapproach feasible: (1) Harvest kinetic energy from the user.
While most intermittent-computing interactive devices typicallyharvest energy from the surrounding environment, such as solar,
thermal, or ambient vibrations [38], these harvesting approaches
are only suited to systems that operate on very low-power—in fact,
most of these are sensing systems since sensors tend to requireless power than their actuator counterparts (e.g., sensing a touch
via capacitive sensing requires less power than delivering a haptic
touch via electro-tactile stimulation or motor-based haptics). In
contrast, we focus on a batteryless device designed for strong haptic
sensations (e.g., vibration, electrical & mechanical force-feedback).
Unfortunately, approaches that harvest small amounts of power donot scale to the magnitudes required for strong haptic sensations;thus, we turn to harvesting kinetic energy from the user as it provides a suitable power efficiency ratio [11]. (2) Supercapacitors
instead of batteries. This is the technical insight that enables
our approach to quickly charge with sufficient energy to generate strong haptic effects because, supercapacitors charge fasterthan batteries, and more importantly, balance energy storage withcharge and discharge times. While supercapacitors do not holdas much energy as a comparably sized lithium-ion battery, theyProlonging VR Haptic Experiences by Harvesting Kinetic Energy from the User

trade capacity with power density [26], which allows for fast charging/discharging speed (orders of magnitude faster than a battery ofsimilar capacity). (3) Conceal the harvesting in the VR experience. Moreover, kinetic harvesting alone would pose a serious
problem for VR. Since any time that such a device would try toharvest energy, users would notice the increased force required tomove (resistance from moving against the harvester) and feel thisas a distraction—this increase in resistance does not match their VR
experience. Our concept solves this by adjusting the VR world torender a situation in which the user expects to feel resistance—thisallows the resistance felt while harvesting to go unnoticed by theuser.
Now that we, succinctly, laid out the principles (see Technical
Evaluation as well as Implementation for more details) that allowed
our device to render haptic feedback without a battery, we demonstrate its application in a VR example.

3

WALKTHROUGH: A VR SURVIVAL
EXPERIENCE WITH HOURS OF HAPTICSWITHOUT BATTERIESTo help readers understand the applicability of our technical approach to achieving batteryless haptics, we demonstrate it in a VR
experience that makes use of a range of haptic sensations, including
tactile- and force-feedback. This VR user is wearing a device thatuses our technical approach to enable non-stop haptics in theirarm. The device takes the form of a forearm exoskeleton that pivots
around the elbow joint. While our device has no batteries, it canharvest the user’s kinetic energy into supercapacitors via a motion harvester engineered from a geared-DC motor paired with acustom-made switching clutch, which powers the VR experience’
haptic effects.
At the start of this VR experience, users find themselves washedashore on an unknown island. Their objective is to find food, water,and batteries to radio call a rescue team, as depicted in Figure 2.
Note that many of these interactions could be made more immersive
using two of our devices on both arms.

UIST ’22, October 29–November 02, 2022, Bend, OR, USA

starting the VR experience, the VR software attempts to contact
the haptic device using Bluetooth. However, the haptic device has
been idle for an unknown amount of time, potentially days, andtherefore has no power. This would be a frustrating situation forexisting wearable haptic devices that quickly deplete their batteries[20]. However, our batteryless device can provide non-stop haptics
experiences by harvesting the user’s kinetic energy on-demand.
When the VR software does not receive a response from the haptic
device in 300ms, it assumes the device is not charged and it isin harvesting mode, i.e., its clutch is engaged and the user’s arm
movements are connected to the device’s high-transmission gear,
which converts kinetic energy into electricity using a DC-motor
and our harvesting circuitry. As such, the VR software immediately
initiates the pre-programmed “harvesting” sequence depicted in
Figure 3 (a) the tide rises and the user has to swim; (b) as the user
moves their arm, they feel the resistance of swimming in water. This
resistance is caused by our harvester’s gears. The user is feeling
passive haptics, which is being converted into usable energy, by
charging supercapacitors on board of our device. While this might
appear simplistic in hindsight, it is a key contribution in our work—
dynamically changing the VR experience to coherently justify why theuser is feeling the strong resistive forces from the harvester’s gears.
In fact, using this insight, participants in our User Study did not
notice that the haptic device was harvesting their energy.

Figure 2: (a) A VR user with our batteryless haptic device
worn on their arm. (b) At the start of our VR experience, theuser finds themselves on a desert island. Their goal is to findwater (coconuts), food (carrots), and batteries to radio call a
rescue.

Figure 3: (a) When VR fails to connect to our batterylesshaptic device, the VR assumes the device needs charging(and is in energy-harvesting mode) and renders a tide rising.
(b) The user swims and feels resistance, without even knowing
they are charging the device.

Bootstrapping a haptic device without any battery. As the
VR experience starts, the user is stranded on a desert island. Upon

Charging on-demand = charging if haptics is needed. The
VR experience can request haptic power on demand depending on

UIST ’22, October 29–November 02, 2022, Bend, OR, USA

events the user might encounter next, or what the designer intended
the game narrative to be. For instance, if the user quickly charges
the haptic device by swimming fast, the haptic device will harvest
sufficient energy to boot its internal micro-controller and circuitry.
At this point, the haptic device communicates via Bluetooth to the
VR software and transmits its amount of internal power, sendinga message with the voltage reading of its supercapacitors. If the
VR software deems this to be sufficient, it can stop the harvesting
and continue the narrative. Figure 4 depicts how this is achieved inour VR experience by the VR software spawning the next island(from the VR experience’s pre-defined narrative) in the vicinity of
the user. The user now swims to the island and the VR headset
requests the haptic device to stop the resistive force. The haptic
device responds by releasing its electro-magnetic clutch, which
disconnects the user’s arm from the harvesting gear. The user now
moves their arm without any resistance and the device is fully
charged and ready for delivering on-demand haptic effects.

Shan-Yuan Teng et al.

is achieved by sending electrical muscle stimulation to the user’s
forearm muscles, which causes them to involuntarily contract.

Figure 5: (a) As the user hits this coconut tree, they feel vibrations on their hand (via a linear resonant actuator in the arm
device); the energy required for this haptic sensation was,unknowingly, just harvested by the user as they swam to theisland. (b) Now, the user picks up the coconut and smashes itagainst a rock. As they hit the rock, they feel force feedback
via electrical muscle stimulation—again, the energy required
for this haptics was, unknowingly, harvested by the user as
they swam to the island.

Figure 4: (a) The haptic device is now charged and responds
to the VR of its charge capacity. The VR experience thenrenders an island. (b) The user walks on the island, so VR
requests the device to stop harvesting mode: now the user’s
arm feels less resistance.
Spend harvested energy on active haptics. The user now
explores the island to find food, water, or batteries to survive. Now,
with the charged supercapacitors, different kinds of haptic feedback
can be rendered. The user finds coconut trees and bangs their fist
against the tree, to make the coconuts fall. Each time they hit thetree they feel a vibration. This is achieved by using the energy previously harvested from the user’s kinetic movements. Subsequently,
the user picks up the coconut and breaks it by smashing it against
a spiky rock. When they crack the coconut on the rock, our device provides a force-feedback sensation to render the impact. This

Handling the uncertainty of user’s behavior = return tocharging mode. All the interactions we depicted so far are dynamic. In other words, if the user gets lost on the island or remains
idle for a long time (or even puts down the game and returns later),the supercapacitors will slowly discharge (see Technical Evaluation
for measurements). However, this is not a problem for our approach:the key is that VR environments are computer-generated in real-time,
and thus are easier to alter than physical systems (e.g., less disruptive for the user’s sense of immersion than stopping and swapping
batteries in a haptic device). As such, if the supercapacitors discharge, our haptic device will always engage the clutch (returning
itself to charging mode) and always notifies the VR software. Upon
receiving this message, the VR software immediately instantiates
the next available “charging” sequence, such as our “tide raises”
example from Figure 3 or “shaking the crab off the user’s arm” from
Figure 6.

Prolonging VR Haptic Experiences by Harvesting Kinetic Energy from the User

Figure 6: (a) The device senses low power, enters energyharvesting mode, and informs the VR, which responds byrendering a crab jumping to the user’s hand. The user shakes
off the crab by moving their arm, which charges the device,unknowingly. (b) When the device is charged, it leaves theenergy-harvesting mode and informs the VR, which respondsby rendering the crab let go.

Creating variation in harvesting experiences. Previously,
when the device was about to run out of power, the VR rendered atide that washed the user onto the sea (harvesting = swimming).
However, the VR designer can incorporate more cyclical VR scenesthat justify the felt harvesting resistance with more variety. In
Figure 7, after exploring the island for a long time (which mostly
involves “spending” energy enjoying haptic effects such as thecoconuts breaking), if the device does not urgently need power, VR
can instruct the user to walk to the boat where they start rowing—as the user moves their arm to row, they are now harvesting. Morepower can be used when they explore the next island.
Surplus of harvested energy? More haptics. Our VR experience can leverage a haptic device that is fully charged, despite
no immediate need for haptics. While the VR software could simply ignore this, a VR designer could take advantage of this energy
surplus and create multiple versions of the same experience thatprovide more realistic haptic experiences. (Obviously, these scenes
could also be required as per the experience’s narrative, and thus
the player would be required to experience a charging sequence
prior to those; but, in this section, we explore the situation where
there’s a surplus of energy). Figure 8 shows an example, while theuser is paddling the boat, the VR experience spends the surplus ofenergy by rendering dense reeds, in which the user experienceseven higher resistance than the harvesting resistance (which isachieved by braking the DC harvester via shortcutting it).

UIST ’22, October 29–November 02, 2022, Bend, OR, USA

Figure 7: (a) VR demands resistive force (device enters energyharvesting mode) for this rowing interaction; (b) VR demands
leaving energy-harvesting mode after the user lands on another island to walk again.

Figure 8: If there is a surplus of energy, the VR software
launches one of the pre-made experience that can make useof this extra energy for a more immersive version of thecurrent experiences. In this case, the user experiences paddling through dense reeds, which they experience a higher
resistance as they paddle (which is achieved by braking theDC harvester itself).
Just-in-time charging for quick haptic sensations. At last,
the user explores the final island to find batteries to power theiremergency radio. Again, this scene could take advantage of previously harvested power, but we choose to depict a worst-casescenario: the user has been idling and the haptic device has sufficient power for Bluetooth communication, but not enough forany haptic sensations. However, the next VR experience is to feel

UIST ’22, October 29–November 02, 2022, Bend, OR, USA

Figure 9: The VR experience includes a haptic effect in which
when the user inserts the last battery in their radio and feels
an electrical tingling. However, if the haptic device is low
on power (a) the VR experience dynamically renders a justin-time harvesting sequence, with a crab that latches onto
the user’s arm. As the user shakes off the crab, the clutch
is engaged and harvests for a brief moment. This way, thehaptic device gathered enough energy just-in-time for the
final (b) electrical tingling sensations to be felt.

Shan-Yuan Teng et al.

an electrical tingling as the user’s radio comes back alive uponinserting the battery. As such, the VR designer created a short,just-in-time charging experience, that ensures that there is always
available power for the user to feel the electrical tingling sensation.
When the user approaches the last battery, the VR software checks
the available power reported by the haptic device. If it is below
the requirement for the electrical tingling (which is rendered via
electrical muscle stimulation), a crab appears next to the battery.
As the user reaches for the battery, the crab jumps at them andlatches to their arm. The VR software instructs the clutch to activateand harvest the user’s arm movements in this very short “charging” sequence. The user shakes their arm twice, which generates
enough power for the final electrical tingling effect. The VR scene
immediately stops the harvesting and causes the crab to be flung
off the user’s arm. Finally, the user places the battery in their device
and experiences the electrical tingling as the radio wakes up andcalls for help.
While we depicted only a few key moments in this VR survival
experience, this VR experience runs for a long time; Figure 10
depicts the entire VR experience’s events, including a cold start(the device has absolutely no power), all harvesting VR sequences(scenes where the user is harvesting), spending sequences (scenes
where the user receives on-demand haptics) and their inner loops(that connect harvesting to spending sequences).

4

CONTRIBUTION, BENEFITS & LIMITATIONS

Our main contribution is a novel technical approach for haptic
devices that contains no battery and yet can deliver haptic sensations, even strong active haptics (e.g., vibration, electrical- or

Figure 10: Flow of our exemplary VR experience, including when it renders the required harvesting and spending sequences.

Prolonging VR Haptic Experiences by Harvesting Kinetic Energy from the User

UIST ’22, October 29–November 02, 2022, Bend, OR, USA

Figure 11: (a) Our wearable haptic device, which is untethered and batteryless (here, we show a version for the right arm). (b)Our device with a kinetic energy harvester, which features our custom electropermanent magnetic clutch (EPM). Close-up sideview of the clutch: (b) In free mode, the clutch gear (with permanent magnets) is held by EPM and not engaged with the gear on
the forearm, thus the elbow movement is not transmitted to the harvester. (d) To enter energy-harvesting mode, a single electricpulse (∼20V) triggers EPM to switch its polarity, and as a result, repels the clutch gear to slide on the shaft and engage with gearon the forearm. To disengage the gear, simply apply another electric pulse through EPM that runs in the opposite direction.
force-feedback). Our key technical insight to enable this contribution is our implementation of a kinetic harvester with sufficient
power efficiency to render haptic effects longer than the time theuser spent harvesting. Secondly, our key conceptual contribution
is that, while harvesting alone is not sufficient (as the increased
resistance from the harvester would create a distraction to the VRexperience), we solve this by adjusting the VR world to render asituation in which the user expects to feel resistance.
The benefits of our approach include: (1) Technical approachto realize immersive haptics for long VR experiences—contrast the fact our device can virtually last for days, with thetypical lifetime of a wearable haptic device; moreover, in our study,
we found that our device was more immersive than today’s baseline
VR setup; (2) Technical approach that frees the user from large
batteries & power tethers—contrast this with the abundance of
haptic devices connected to large batteries or powerlines; (3) Users
do not need to notice the harvesting—because we use the VR
dynamically to conceal the resistance felt by the harvester, users
can use our device unaware of its inner workings; in fact, in ouruser study participants did not realize our approach was harvesting
their movements. (4) Haptic “walk-up use”—our device can siton the shelf for virtually any amount of time and will “wake up”
once the user starts the VR experience and starts moving; and, (5)Re-using power for heterogeneous haptic experiences—ourdevice harvests kinetic energy, but can deliver power to a wide

range of actuators; we demonstrate how our implementation converts this energy into vibrations, electro-tactile, electrical muscle
stimulation, and braking-based resistance.
Our approach is not without its limitations: (1) Content creation for intermittent haptics—our approach only supports haptic feedback that happens intermittently (up to a few minutes, rather
than continuously for hours). This is a consequence of the switching between on-demand haptics and harvesting energy from theuser’s movements (which in our concept also provides haptics, but
typically not on-demand as these are triggered by the haptic device, rather than by the VR content). This necessary switch fromon-demand haptics to harvesting haptics, requires VR designers
to create harvesting sequences, i.e., alternative moments in the
VR experience that can be activated ad-hoc when the haptic device loses all power and returns to harvesting mode. (2) Longer
experiences—another limitation of our approach is that it leads tolonger experiences, since when using our device, the user performs
additional actions designed to charge the device if the power isrunning low. Finally, (3) Physical exertion; our approach relieson physical movements, as such it also induces higher physicalexertion and might not be suitable for all users or situations.

5

IMPLEMENTATION

We detail our prototype batteryless haptic device (Figure 11) with
its custom energy-harvesting and the VR interactive system. To

UIST ’22, October 29–November 02, 2022, Bend, OR, USA

Shan-Yuan Teng et al.

accelerate researchers interested in reproducing this device, alldesigns are made available and open-source1 .

5.1

Arm-worn exoskeleton haptic device

We implemented an exoskeleton that is untethered and batteryless,shown in Figure 11(a). It measures 42 cm in length (adjustable)
and weighs 680 g (resistive gears & clutch 325g, exoskeleton 270g,
harvester 35g, PCB 30g, electrodes & straps 20g). It harvests kinetic
power from elbow movements, since at every triceps/biceps curl,
ample mechanical energy is available to be turned into electricity.
Also, the accompanying resistive haptic feedback on the elbow iseasy to integrate into VR interactions since the hands are often
involved in VR experiences (e.g., rowing a boat, lifting heavy objects, pushing against walls, etc.). We detail each component of our
exoskeleton in the following sections.
Harvester mechanics. The core of our kinetic energy harvester
is a 3-phase brushless DC motor (ZSFD-F1HD) with gear ratio
1:20.25, which we re-purposed as a power generator with rated
output current 700mA. To incorporate a clutch (switching on/off
energy-harvesting mode, which we describe later) into the gear
set, we added two 12-teeth gear (20 degree-teeth; gear 𝜙 28mm)
for transmission, and a 15-teeth clutching gear (20 degree-teeth;
gear 𝜙 34mm), fabricated with PLA using FDM 3D printing, 12mm
thick, fit into ABS rods (𝜙 8mm). The power generator, transmission
gears, and clutching gears are mounted on the upper arm. The lower
arm’s half-gear (20 degree Metric, 60-teeth, 120 mm Pitch, 𝜙 124mm,
fabricated with 3mm-aluminum by water jet cutting) is attacheddirectly to the exoskeleton brace, which increases the torque ratio
by a factor of 5 when engaged with the generator. The enclosure of
the upper arm gears is fabricated with 2mm aluminum plates (water
jet cutting) and secured with M3 screws; finally, these are clamped
onto an MOSCARE elbow brace with adjustable upper/lower arm
length along with adjustable straps.
Switching harvester on/off using a custom clutch. To enableon-demand kinetic energy harvesting, ideally, the actuation of the
clutch should take require harvesting a few arm movements. Thisis difficult to achieve since most types of clutches (e.g., motorized
friction clutch) require actuating large motors to engage/disengage.
In the process of designing our clutch, we implemented six different
clutch designs (some depicted in Figure 12): a DC-motor horizontal linear-clutch, DC-motor vertical linear-clutch, one-directionalratchet clutch, one-directional spring-clutch, DC motor frictionclutch, but found these would require too much power to actuatecompared to a magnetic clutch.
To clutch in an energy-efficient way, our custom magnetic clutch
is based on electro-permanent magnet (EPM). In EPMs, the polarity
can be switched by an electric pulse but remains stable for a longtime [14, 25, 39] (contrast this with a solenoid, which only pulls
when power is supplied). Our Figure 11(b) depicts our EPM clutch,
consisting of a clutch gear that has a ring of permanent magnets(neodymium, 𝜙 5mm, 3mm thick, 242 gauss), sliding on a plastic
shaft to avoid magnetic materials. To move the clutch, the polarity
of EPM is switched to attract (disengage) or repel (engage) the
clutch gear on the shaft. Importantly, even if the gear ends up not
perfectly aligned (did not slid all the way) the clutch will still slide
1 http://lab.plopes.org/#harvesting-haptics

Figure 12: Four (out of six) examples of clutch designs that
we implemented but proved inferior to our final EPM clutch
design.
in to engage when the user moves the arm by just a small amount,since the EPM continues to push the clutch gear. Our EPM (Alnico
Grade 5, 𝜙 5mm, 15mm length) was wrapped in a coil with 80
windings (28 AWG). Finally, the magnetic field was measured at 59
gauss in both polarities, after switching.
Microcontroller. The complete circuit is layout in our custom
PCB. Refer to the schematics in Figure 13. The control center is
NRF52840 Dongle (Nordic Semiconductor), which is a Bluetooth
Low Energy (BLE) enabled low-power microcontroller (1.6-3.6V,
measuring 1.3mW under 1.8V, while BLE connected). A supercapacitor (0.33F) is used to store the harvested energy. The available
power is sensed by the microcontroller through an analog reading
pin. To lower the power consumption, we lowered the Bluetooth advertising rate to 100ms. We utilized UART for BLE communication.
Harvesting circuitry. We connect the output of the DCgenerator to a 3-phase rectifier composed of six Schottky diodes(PD3S0230-7), high-voltage shunt diodes (20V MMSZ5250CT1G
and 25V SZMMSZ5253BT1G), and a regulator (3.3V for low-powersuper-capacitors, NCP1117ST33T3G). This generated power is thenfed into five parallel capacitors, each with its capacity and voltage
rating, including three electrolytic capacitors (330𝜇F, 25V) and two
EDLC (electrostatic double-layer capacitors) supercapacitors (0.1F
and 0.33F, 5.5V). Again, we chose supercapacitors rather than traditional batteries (e.g., LiPo, Li-Ion, NiMH) since supercapacitors
charge faster than batteries, and more importantly, balance energy
storage with charge and discharge times. While supercapacitors do
not hold much energy as a comparably sized battery, they trade off
capacity with charging/discharging speed [26] —this is the technicalinsight that enables our approach to quickly charge with sufficientenergy to generate strong haptic effects. We will now explain therole of each of our five parallel capacitors, depicted in Figure 14.
Discharging power into haptic effects. Two 330𝜇F 25V electrolytic capacitors are charged/discharged to engage and disengage
our EPM-clutch (∼20V, with peak current 6 A discharged in 2ms).
With just a couple of biceps curls, we can harvest the needed 20Veven without requiring a step-up circuit to engage/disengage theProlonging VR Haptic Experiences by Harvesting Kinetic Energy from the User

UIST ’22, October 29–November 02, 2022, Bend, OR, USA

Figure 13: (a) Our custom PCB containing energy harvesting and haptic control circuits. (b) Schematics of our PCB, V+ depicts
the DC power which can range from 0-35V when charged by the arm, regulated into sub-voltages for the microcontroller andactuators.
(EMS) actuator, which can render either electro-tactile or forcefeedback, depending on the stimulation time. For EMS output, weadded a current regulator (LT3092) that we configured to limit theoutput current to 33mA (as most EMS systems that actuate thebiceps do not require more than this [31, 32]). The stimulation
signal is created by the microcontroller via a P-channel MOSFET(Si2371EDS) and an N-channel MOSFET (SQ2310ES-T1_BE3), at
50Hz with 200 microseconds pulse-width. Moreover, we added twoparallel EDLC supercapacitors (0.1F and a 0.33F, 5.5V, PowerStor)—
these are the highest storage capacitors in our circuit. The largest(0.33F) constantly discharges onto the microcontroller and the remainder (0.1F) discharges, on-demand, onto the vibration device.
We actuate this linear resonant actuator (LRA, C10-100, 65mW at
2V, Precision Microdrives) using an N-channel MOSFET (SQ2310EST1_BE3) driven it at its resonant frequency (170Hz) using the microcontroller.
Finally, any capacitors’ voltage drops gradually over long-timeframes; thus. the voltage of supercapacitors and the EMS capacitor
is sensed, by using a voltage divider circuit and the microcontroller.
We estimate the voltage of the EPM capacitor by reading the EMS
capacitor as a workaround for limited analog reading pins available
on this microcontroller dongle.

5.2
Figure 14: (a) The various available haptic output provided by our device. (b) Diagrammatic view of each subcapacitor/supercapacitor depending on its haptic output.

clutch. To control the EPM we modified a traditional H-bridge
circuit to allow charging up two capacitors at the same time but discharging at different times, ensuring there is power when it needs
to disengage/engage the clutch. The H-bridges are manually implemented using 20V N-channel MOSFETs (SQ2310ES-T1_BE3) and
20V P-channel MOSFETs (SQ2351ES-T1_GE3), which can withstanda peak current of 12A. Next, the remainder 25V 330𝜇F electrolytic
capacitor stores energy for our simple electrical muscle stimulation

Available haptic feedback modes

We wanted our prototype device to be representative of the wide
variety of haptics that our approach can enable. In our prototype,four actuators can be driven (motor braking, magnetic clutch, electrical stimulation & vibration motor) enabling six different haptic
sensations, which we present in ascending energy-consumption
order:
1. Resistive force feedback (energy-harvesting). This iswhen the clutch is engaged with the kinetic harvester and makes theuser feel resistance in their movements. We measured the torque
to be 1.29 N·m. Since this is the default state of our device (i.e.,left alone it will eventually lose power and switch to this mode) it
consumes the least power.
2. Stronger resistive force-feedback (braking). This is when
the clutch is engaged and we brake the motor by shortcutting it,
which we achieve by discharging the EPM engage-capacitor into

UIST ’22, October 29–November 02, 2022, Bend, OR, USA

the motor terminals. We measured torque of 1.61 N·m, higher thanin harvesting—this allows users to feel force-feedback sensationsof higher resistance [2].
3. Free (disengaged). Upon disengaging the clutch from thekinetic harvester, the user experiences virtually no resistance fromthe device (0.05 N·m).
4. Electro-tactile (using EMS). By actuating our simple electrical muscle stimulation circuit for very short periods, such as 10ms,
we render an electrical tingling sensation similar to electro-tactile
sensations [22]. In our main VR experience, we delivered these
impulses via two electrodes on dorsal side of user’s forearm.
5. Force feedback (using EMS). By actuating our simple electrical muscle stimulation circuit for longer periods, such as 100 ms
or longer, we render involuntary muscle contractions sensation
similar to force-feedback [30]. In our main VR experience we deliver
this stimulation via the same electrodes as for electro-tactile (samecircuit) close to the wrist extensor muscles. These can be attached
to other parts of the body for different applications. Note that our
EMS implementation is simplistic. First, the difference between ourelectro-tactile and EMS is how long the actuation is (long induces
muscle twitches; very short feels like electro-tactile). Second, our
EMS approach induces only small movements since it uses only 25V.
However, this still depicts the range of haptics that can be easily
added to our harvesting-based approach. Obviously, our simple
EMS sub-circuit can be swapped for more powerful EMS circuits,
such as bioSync [36].
6. Vibration. We create vibrations using a linear resonant actuator (LRA) attached to a strap on the user’s palm. It can be attached
to other parts of the body for different applications.

5.3

VR-side implementation

We developed our VR experiences using Unity 3D and render these
via an Oculus Quest 1. The VR experiences are rendered in a laptop
but displayed in the VR headset via Oculus Link connection over
Wi-Fi. The haptic device sends all its messages over Bluetooth (BLE),
which arrive at the laptop. We capture these Bleak (handles the
low-level BLE connection) and we convert them messages to OpenSound Control (OSC), which are delivered via TCP to Unity3D.
While typical OSC implementations utilize UDP, we switched to
the TCP protocol to ensure the delivery of messages.
VR searching for haptics device. At the start of our VR, ourUnity3D class (that can be added to any Unity3D project that wishes
to extend or use our approach), automatically starts searching forthe haptic device via BLE. If the device does not respond within
300ms, it assumes the device is out of power and in harvesting mode.
Thus, the VR experience triggers the next available “charging scene”,
which in our Walkthrough, was the swimming scene.
Anatomy of a “charging” VR scene. The idea behind these
VR scenes that justify the presence of the resistive force from theharvester is that they are cyclical, i.e., they can be looped or instantiated in sequence until the VR receives confirmation that theamount of power is sufficient. We implemented such scenes inUnity3D using two design strategies: (1) transport-to-charging-area;and (2) in-place-charging. For transport-to-charging-areas, these arescenes that take place in a specific place (literally a software scene
as determined by Unity3D). We transport the user to this scene

Shan-Yuan Teng et al.

by, for instance, fading the previous scene (e.g., we use a thick fog
to transport them from “swimming” from the island to swimmingin the open ocean). Now that the user is in this scene, the scene
can loop itself (or be infinite/procedurally generated) while it constantly is charging the device. Examples of this scene included also
our “rowing the boat”, which even includes procedurally generated
clouds and rocks on the water, to ensure the user feels there is a lotof variety—while, strictly speaking, they are caught in the charging
loop (again, users in our User Study did not realize this at all). As
for in-place-charging scenes, these are events that happen in place
where the user is currently at. We recommend these, especially,
when needing to harvest smaller amounts of power for the next haptic effect. Examples of this scene include our “shake-the-crab” scene,
which we simply trigger on-demand when needed by toggling its
visibility.
Power thresholds for each scene. Each scene or haptic effectin our VR experiences extends from our “spending” or “charging”
Unity3D classes. Any VR event that extends from a “spending” classwill behave dynamically and will require the designer to indicate
what type of feedback (from our six possible types, at fixed durations). Using this information, our Unity3D class automaticallyestimates the required power from our measurements (see Technical
Evaluation). If the power threshold is not satisfied (under voltage)
this will cause the next “charging scene” to appear. For example, in
our Walkthrough application, the threshold is set to 2.4V, which wemeasured to last for about 5 minutes (on idle). We also determineda low power threshold in our program to trigger energy-harvesting
mode in the device and render the respective scene (e.g., “tide rises”
scene). We set the threshold of the supercapacitor for the microcontroller to be 1.85V as the microcontroller will shut off with 1.7V
remaining in supercapacitor.
Communicating available power. Unity requests voltage readings from our device every 100ms. Since supercapacitors consist of
two-layers, the voltage sensed by the microcontroller can be higher
than what is charged, especially in a relatively quick charging in
kinetic harvesting; thus, to obtain a reliable reading of the current
voltage, our Unity3D class employs a 20-sample moving average.

6

TECHNICAL EVALUATION

When engineering energy-harvesting devices it is paramount tocharacterize their behavior with respect to how long they take tocharge and for how long they operate (discharge) under different
conditions (idle vs. delivering haptics).
Apparatus. Measurements were performed using our prototype(described in Implementation) connected to a factory-calibrated
precision-multimeter (0.01 mV resolution, accuracy of 0.02%, 4 1⁄2
digits of precision and USB logging). For the technical evaluation,
unless where noted, all the movements were performed using biceps/triceps curls at a period of 2 seconds by a participant with noarm injuries but a relatively low muscle mass (biceps diameter 22
cm, much below the average of 34.3 cm [13]).
Kinetic harvester’s electrical output. Figure 15 depicts thevoltage of our harvester in two exemplary situations: (a) a ∼30V
peak from a slow biceps curl (∼1.4 seconds per complete flexion/extension) and the ∼35V peaks from a faster biceps curl (∼1second per complete flexion/extension). Moreover, we measured

Prolonging VR Haptic Experiences by Harvesting Kinetic Energy from the Usera current of 70mA averaged across one complete elbow movement (one elbow flexion followed by one elbow extension) at theslower pace, and 65mA at the faster pace—this minimal difference
illustrates how we tuned the gear ratio to be effective at slower
movements.

Figure 15: Voltage (rectifier output) of our harvester for two
consecutive elbow movements (flex/extend for each movement).
Measuring harvesting for the microcontroller. Microcontrollers require a stable power source to remain operational. In
Figure 16(a), we depict a cold-start of our haptic device. In this exemplary run, it took ∼30 seconds to charge the 0.33F supercapacitor
to 1.8V, which wakes up the NRF52840 microcontroller (includingBluetooth). However, this 1.8V region is close to the shutdown voltage (around 1.7V), as such, our device remains in harvesting mode
(clutch engaged) until the voltage is at least 2.4V at this supercapacitor. As depicted in Figure 16(a), it took ∼1 minute of harvesting
to reach this point, at which microcontroller clutches off to leave
harvesting mode. Left alone, this stable mode would last for ∼5 minutes; thus, the idle efficiency is 1:5 (1 minute of harvesting provides
5 minutes of idling).
Measuring harvesting for clutch operation & braking
mode. While the previous measurements evaluated how fast our
implementation can wake up the microcontroller (∼30 seconds),
we now turn to measuring how long it takes to harvest sufficientenergy to engage/disengage the clutch. Since the microcontroller
is already charged, the two clutch capacitors are also charged. As
depicted in Figure 16(b)(1), at the 5s mark, we discharge the first
EPM capacitor, which clutches off (leaving harvesting mode). The
device is now in free mode and the user feels no resistance. Notethat our second EPM capacitor is also charged, which we trigger
to re-enter the energy-harvesting mode at the 12s mark, in Figure

UIST ’22, October 29–November 02, 2022, Bend, OR, USA

16(b)(2). Now, both capacitors are discharged, and the system does
not have sufficient energy to move the clutch; however, the system
is in harvesting. As shown at the 13s mark, the user starts charging
by moving their arm: a single elbow movement charges both capacitors immediately; it is already ready to leave the energy harvestingor deliver haptics on-demand. This depicts a fast turn-taking efficiency (i.e., 1 movement charges the clutch switching). In Figure16(b)(3) depicts a later case in which the device activates the braking mode (which feels harder to move than in harvesting mode).
Because the clutch capacitors are already charged after a single
movement, our device can render the braking mode by discharging
the second EPM capacitor to shortcut the DC motor. The capacitor is charged again when the shortcut is off, as shown in Figure16(b)(4). Finally, we found that, while in idle mode, the capacitorsconserve adequate charge for clutch operations for ∼5 minutes.
Measuring harvesting for electrical muscle stimulation.
In Figure 16(c) depicts the efficiency of harvesting power to drive
our simple electrical muscle stimulation subcircuit. We found thata single arm movement can charge its capacitors sufficiently for 11
electrical stimuli (300 ms each) to be delivered to the user. Since
our electro-tactile circuit is the same, its performance is similar.
The VR experience can request another arm movement when more
EMS-based effects are needed. This depicts also a quick turn-taking
from our system (1 arm movement, 11 EMS impulses). Moreover,
we found that while in idle mode, the EMS capacitors conserveadequate charge for EMS stimulation for ∼5 minutes.
Measuring harvesting for vibration. in Figure 16(d) depicts
the efficiency of harvesting power to drive our vibration subcircuit.
We found that from the microcontroller’s stable mode, we can deliver 56 vibrations on our LRA (300ms each) before these become
too weak, which happens when the LRA voltage drops below 1V.
We also found that harvesting for vibrations takes longer whencompared to clutch, braking, or EMS. We found that ∼30s of harvesting are required to charge up to ∼2.1V, which allows delivering∼37 vibrations. Note this was to be expected, since we used a large
supercapacitor for vibration (0.1F) but not for EMS (330𝜇F). Yet, thisstill results in an efficiency of 30s of harvesting to ∼1 minute ofvibration. Finally, we found it to conserve adequate charge for theirvibration operations for ∼24 hours.
Limitations. The operation of any force-based device (including kinetic-based harvesters) depends on the operators’ force. As
such, we advise to take our results as illustrative of a typical performance of our device rather than a lower (e.g., users with lessmuscle force) or higher (e.g., users with more muscle force) boundfor its performance.
Summary. We characterized the efficiency of our device. We
found that for all haptic modes, users will tend to spend less timein harvesting mode (where they provide energy by means of their
movement) than in enjoying on-demand haptics. In fact, we foundthat our microcontroller requires ∼30s-1min of charging for 5 minutes of operation; our vibration circuit can be charged in ∼30s andlasts for one minute of vibration; and, finally, our clutch’s operation,
as well as the EMS stimulator, can be charged in a few seconds (i.e.,these are charged in a single arm movement). Moreover, if left alone,
our device tends to discharge in ∼5 minutes; however, it will always
return to its harvesting mode (sending a message to the VR andclutching), so it is always ready to be picked up by a user and will
always wake-up on charge.

UIST ’22, October 29–November 02, 2022, Bend, OR, USA

Shan-Yuan Teng et al.

Figure 16: (a) Charging and discharging profile of our 0.33F supercapacitor used to power the haptic device’s microcontroller.
(b) Charge/discharge profile of the two capacitors that switch our haptic device’s EPM clutch and the braking mode. (c)Charge/discharge profile of the capacitor used for electrical muscle stimulation, of 300ms in duration. (d) Charge/discharge
profile of the super-capacitor used for vibrations, of 300ms in duration.

Prolonging VR Haptic Experiences by Harvesting Kinetic Energy from the User

7

USER STUDY

In our study, we evaluated whether wearing our haptic harvesting
device impacted actual VR experiences. To do this, we created along VR experience (the island survival experience shown in our
Walkthrough), in which players could be immersed for longer periods. This is the type of experience that would deplete existing
haptic devices, and, at some point, the user would find themselveswith just input but not enough power for haptics. As such, in thisstudy, we compared our harvesting approach to a baseline condition where the participants use only input controllers. Moreover,this study was conducted with incomplete-disclosure, i.e., our participants consented that “something about this study” was not told
until the complete study was done. In fact, we did not inform ourparticipants that our device had no battery nor that it was
harvesting their movements. This study was approved by our
institutional ethics committee (IRB22-0467).
Hypotheses. Our hypotheses were as follows: (H1) our approach would lead to more realism than the baseline condition, since we hypothesized that the addition of haptics would befelt as more realistic because—even though our participants would
be required to exert more force to harvest the haptics’ power—our
harvesting technique charges while simultaneously rendering passive haptics that are, in themselves, also realistic. Moreover, we
hypothesized that, with our approach, participants would (H2) feel
their senses are more engaged, which is a key feature in immersion [41]. Furthermore, we expected that (H3) our approach
would lead to more fatigue than the baseline condition since
the harvesting of our haptics comes at the expense of the user’s
physical exertion. Because of the latter, we also hypothesized thatthe (H4) experience duration would increase with our approach. Still, despite this expected physical exertion and longer
gameplay, we further hypothesized that (H5) participants wouldfind this exertion more enjoyable with haptics than in thebaseline condition.
Preview of study results. We found that our device increased
the realism and sensory engagement, which suggests it is usefulfor increasing the immersion of VR experiences, especially forvery long VR experiences that could not easily befit from wearable
haptics. Moreover, we also confirmed two expected limitations ofour device, i.e., it makes the experiences longer and more physicallydemanding (since the charging is via physical movement).

7.1

UIST ’22, October 29–November 02, 2022, Bend, OR, USA

harvesting condition, the experimenter discharged all the capacitors
so that the device started with zero power.

7.2

Participants

We recruited ten right-handed participants (M=22 years old, SD=2;
five self-identified as female and five as male). Four of them had
prior experience with VR, but none had experience with our deviceor haptics beyond vibration in controllers. It is important to note
that, all participants did not have prior knowledge about the devicein terms of its harvesting capability—as aforementioned, this information was withheld from participants until the end of the study(Incomplete Disclosure).

7.3

Tasks

We utilized our island survival VR experience (see Walkthrough).
The experience was configured to be shorter than our walkthroughby requesting participants to find three coconuts, three carrots, andthree batteries. Pilot tests were used to inform how long the task
should last. We designed it to last ∼10 minutes in baseline condition.
To prevent a sequence effect in which participants memorize thelocation of the objects and puzzles, we changed the order of thefirst two islands and locations of the objects; moreover, we alsocounterbalanced the condition order. With participants’ consent,
we recorded their VR screen for later labeling of events and time
duration. After each condition, participants were asked to rate theperceived realism, enjoyment, sensory engagement, and physicalfatigue on a 7-point Likert scale. The experimenter conducted semistructured interviews with the participants to collect subjective
feedback. Breaks were given in between the condition.
Finally, only at the end of the interview, we disclosed thatour device charged by harvesting kinetic energy.

7.4

Results

Figure 17 depicts our main findings, which were analyzed using
paired t-tests.

Interface conditions and apparatus

Conditions. Participants were asked to play our island survival VR
in two conditions: (1) harvesting-device, in which they wore ourharvesting device on the arm and held input controllers to navigate
and interact with objects; and (2) controllers-only, in which where
they did not wear our device and used controllers to navigate andinteract with objects. Note that we only use controllers for sensing
purposes in both conditions and we did not use the built-in vibration
motor in controllers, since we were interested in rendering all
haptics using the power harvested by the participant without theneed for batteries. Interface condition order was counterbalanced
across participants.
Apparatus. An inside-out 9DOF tracking VR headset (Oculus
Quest 1) with controllers in both hands, and our device. In the

Figure 17: Study results, error bars show standard errors.
H1 (realism). First, we found that participants rated significantly higher (F(9)=5.6, p<.0005) realism in harvesting device(M=5.3, SD=0.7) than controllers only (M=3.8, SD=0.9). This finding
suggests that our H1 was confirmed (our approach led to more
realism).

UIST ’22, October 29–November 02, 2022, Bend, OR, USA

H2 (sensory engagement). We found the participants rated
significantly higher (F(9)=7.7, p<.0005) in sensory engagementwith harvesting device (M=5.7, SD=0.8) than with controllers only
(M=3.7, SD=0.9). This finding suggests that our H2 was confirmed (our approach led to more sensory engagement),
which is a key factor in immersion [41]. Taken together, our H1and H2 suggest that our approach to generating haptic feedback
from power harvested from the user, improved the realism of
VR experiences.
H3 (fatigue). We found the participants rated significantly
higher (F(9)=5.7, p<.0005) in physical fatigue with harvesting device (M=5.4, SD=1.5) than with controllers only (M=2.6, SD=1.6).
This confirmed our H3 (a harvesting approach leads to more
fatigue), and confirms this as a limitation of our approach.
H4 (duration). We found that participants took significantly
longer (F(9)=4.9, p<.0005) to complete the experience with theharvesting-device (M=18.3 min, SD=4.41) than with the controlleronly (M=11.3 min, SD=2.75). This was to be expected since whenusing our device, the user performs several additional actions (e.g.,more rowing, swimming, etc.) while they charge the device in casethe power is running low. This supports our H4 (harvesting increases the duration of the VR experiences) and confirms thisas a limitation of our approach.
H5 (enjoyment). We did not find a significant difference(F(9)=1.6, p=0.14) in enjoyment even though harvesting device(M=5.5, SD=1.2) was rated higher than controllers only (M=4.7,
SD=1.2). As such, our H5 was not supported.

Shan-Yuan Teng et al.

felt like "I was pinched" (P4, P10), "a shock” (P2, P10), and “kinda
sharp” (P3). In fact, P2 and P4 subsequently tried to avoid the next
crabs to prevent them from getting pinched. Inserting batteries felt
“some electricity” (P2) and "tingly" (P3) and, similarly, participants
recalled the coconut’s EMS impact as "crashing” (P9) and "tingly”(P3). Participants also positively recalled their vibration experience.
All participants felt it when hitting the palm trees; one participant stated that “vibration pulled me into the world, especially thethunking on the tree” (P8).
Baseline. Compared to the baseline, without haptic feedback,all participants mentioned it felt less real and less engaging to theirsenses. For instance, they said, “I did find myself missing the stimuli
even though it hurt” (P8), “without the thing on my arm rowing
was easier” (P10), or “I was just looking around [instead of feeling]
for this condition” (P6).
Study conclusion. Our qualitative and quantitative findings
suggest that our approach can prolong VR haptic experiences that
lend themselves well to intermittent haptics. Furthermore, we foundthat our harvesting approach might go unnoticed by unknowing
participants, which are likely to feel the passive haptics from theharvesting as a source of realism and sensory engagement, leading to immersion even during harvesting periods. Naturally, we
also found that our approach to prolonging haptics leads to more
physical exertion and to longer experiences.

8
7.5

Subjective feedback

Disclosure of harvesting. No participant realized our device washarvesting their movements. After disclosure of the working principle of the harvesting device, all participants found it unexpected
and felt the integration was well executed. For example, P2 realized
why “[I] had to row more to reach the islands!”, P4 commented“makes sense now that I think about it”; P6 commented “wow, thatis unexpected”; and P8 expressed “definitely it [the integration]
works”.
Harvesting-device. All participants mentioned that they felt theresistive force from our device when rowing the boat, and that thisalso contributed to the realism of VR. Regarding this, participants
stated, for instance, that "it felt heavy" (P2) and "heavy when I
moved my arm" (P3) and “the fact that you have to work your arm
for it to go increased the realism” (P7). Some limitations were alsobrought up: P5 added that our device did not replicate the exact“physical experience of paddling since resistance in other joints is
missing”, and P8 mentioned it was “less fatigued than rowing in reallife”. However, most participants felt it to be enjoyable, e.g., “therowing made it more enjoyable" (P9). The rising tide on the island
happened at least once to seven (out of 10) participants—in fact,
one participant experienced it twice because they wandered for alonger time exploring the island. The remaining two participants
did not experience flooding because they found items quickly and
immediately left the current island in search of the next one.
On-demand haptics. As for the remainder, on-demand, haptic
sensations, most participants had a vivid impression of their experience with the electro-tactile feedback. Participants stated the crab

EXPANDING OUR CONCEPT TO MORE
FORM-FACTORS AND APPLICATIONS

To further expand our approach, we explored additional applications, form factors, and other actuators in this section.
Fire-training simulator. To use our approach in a VR experience, the key is to design harvesting sequences that are adequate forthe user’s expected experiences and can be triggered seamlessly.
Figure 18 depicts another illustration of using our approach in VR,with the example of a fire-training simulator, in which users mustput out a fire in a building with an unknown room layout: (a) shows
the start of this experience. (b) The user yields a heavy ax to break
down a door of the building on fire. As the user moves the heavyax, they feel resistance; while, at the same time, they are charging
our device from a cold-start (no power). Once the microcontroller
wakes up, the haptic device communicates to the VR experience.
(c) The VR experience responds and breaks the door into pieces,
instructing the clutch’s release—the user now moves freely into
the first room, they are no longer harvesting. Then, the user puts
out fires in this room, and the VR experience requests on-demand
haptics: such as (d) the feeling of flames (electro-tactile) or (e) thevibration from a fire extinguisher. Anytime that the haptic device
is about to lose power, it extinguishes the fires in the current roomand shows another locked door, which the user will have to ax
down (harvesting) to reach the next room on fire.
Endless VR running. Different usage of our haptic device by
attaching it to the knee joint is depicted in Figure 19. In this VR
experience, users play a running VR game, akin to endless VR
runners where users walk in place. However, our VR endless runneris complete with haptic feedback, even for hour-long game runs.

Prolonging VR Haptic Experiences by Harvesting Kinetic Energy from the User

UIST ’22, October 29–November 02, 2022, Bend, OR, USAreleased—the user now moves freely and runs on solid ground;they are no longer harvesting. Then, as users run through different
sections of the terrain, the VR experience requests on-demand
haptics, such as (d) grass brushing the user’s legs (electro-tactile onlegs), (e) a sandy terrain (vibration), or (f) running on mud (brakingmode, high resistance). Anytime that the haptic device is about tolose power, it informs the VR experience, which renders anothersection of the shallow water, so that the device can harvest energy
from the user’s leg movements.
Other haptic actuators. We believe our approach is not limitedby the actuators used in our current prototype. One can envision aplethora of haptic actuators to provide richer haptic experiences,such as pressure, temperature, wind/airflow/fluid. In our preliminary explorations, we found that our device could successfully
drive: (1) a small DC motor (26:1 Gearmotor Polulu) for ∼5 seconds, which can be used for pressure or skin stretch feedback; (2)a solenoid (ZMF-1632d) for 100ms, which can be used to create
impact; and finally, (3) a small Peltier element (CP081030-M) for∼5 seconds, which can be used for thermal feedback; the latter
required an additional capacitance of 0.2F due to its high power
consumption.
Figure 18: This VR experience immerses the user in a simple
fire-fighting exercise, complete with haptic sensations.

99.1

Figure 19: Our batteryless haptic device is worn on the footin a VR running experience with haptic feedback.
Figure 19 depicts: (a) The user wearing our device runs in place.
(b) As they run inside water through a shallow stream, they feelan added resistance from our harvester. (c) As always, once themicrocontroller wakes up and reads a stable power, the haptic
device communicates to the VR experience, which responds andstops rendering the shallow water and instructs the crutch to be

RELATED WORKThe work presented in this paper builds primarily on the field of
haptics, with particular emphasis on sensations that require highenergy devices, such as force-feedback (motor-based exoskeletons,
friction-based haptics, etc.). Moreover, we take inspiration fromrecent approaches to batteryless computing, which have predominantly focused on harvesting energy from the user or the environment for sensing applications. Finally, as our technical approachrequires not only a hardware component (our clutch & kinetic
harvester) but also a dynamic VR environment, we also review
prior work on adaptive VR systems that render scenes to users inreal-time depending on different constraints (e.g., space).

Batteryless computing devices

The growth of ubiquitous computing devices, internet of things(IoT) and wearable devices, has brought attention to the problem ofpowering devices that cannot reach a powerline or include a large
battery. As such, many researchers across HCI and electrical engineering have been exploring new ways to power devices without
the need for batteries.
Wireless transfer & RF backscattering. While power can betransferred wirelessly using Tesla coils, the power required can bedramatically high (i.e., this approach’s efficiency is low) and thecoils tend to be very large even at close distances (also removing theheat from the coils is nontrivial). As such, to power up small devices,
radio frequency (RF) backscattering is a popular technique, in which
the batteryless device’s antennas pick up RF and convert it into
power [1, 35]. In this way, multiple devices can be powered using
one RF transmitter, making the receiving devices easier to set up
around users [49]. One widespread usage of this is RFID technology,
which leverages the same concept for powering ID badges andtransit cards. For interactive devices, researchers utilized RFID for
sensing gestures on surfaces [27] and tangible props [21]. In fact, we
take inspiration from a recent endeavor to push batteryless devices

UIST ’22, October 29–November 02, 2022, Bend, OR, USA

to the tactile domain, such as an NFC-powered tactile device [47].
Overall, these devices based on wireless-transfer/RF are promisingand popular for sensors using small amounts of current, but theserequire installation of a transmitter with a large battery or tethered
to wall power—these are not ideal for mobile haptics applications
such as free-walking VR.
Energy harvesting. Without instrumenting the device witha power supply or without instrumenting the environment withpower transmitters, the energy must be harvested directly from theenvironment or its user. Next, we overview the most common techniques to realize this and their application area (for a comprehensive
review of energy harvesting, see [46]). For powering small devices,
solar ambient light or heat can be harvested. However, using small
solar panels (the kind that would fit in a user or haptic device)requires constant and ample sunlight, but lighting conditions varythroughout the day. Therefore, harvesting using small solar panels
is mostly suitable for sensors that operate at low power and in anintermittent manner [16]. As for harvesting energy from the human
body, body heat and kinetic energy from movements can be used[50]. Body heat can be harvested using Peltier elements, which are
exciting in that they harvest passively (the user does not need toengage in a harvesting behavior). However, their efficiency is low
in the small sizes that are wearable, e.g., even very large skin areas
covered can only generate 𝜇W of power [24]— not sufficient for
driving large haptic devices. A source of larger energy efficiency isbody movement. On a smaller scale, even micro-movements can be
harvested via MEMS energy harvesting devices, e.g., piezoelectric[15, 28] and triboelectric [44] harvesters—again, these are promising but outside the range needed for driving strong haptics. Finally,
to harvest more power from the body, large body movement fromthe joints can be harvested using electromagnetic generator (e.g., a
DC motor)—the most powerful way to harvest kinetic energy fromthe body [11].
As our goal is to harvest energy for haptic feedback, where thepower ranges from mW to W [10, 31, 42], and considering themobility of the device (i.e., untethered), we see kinetic energy as themost promising power source.

9.2

Energy harvesting for interactive
applications

HCI researchers have been exploring concepts to blend energy
harvesting in our everyday appliances and devices [3, 37]. Here, we
specifically overview energy harvesting applications for interactive
devices.
For example, Peppermill [43] is a handheld input knob that is
powered by the user’s motions. Paper Generator [23] harvests powerfrom the user rubbing an electret harvester on paper for lighting
LEDs or driving an e-ink display. Battery-free Gameboy [45] integrates energy harvesting into an interactive system, utilizing solar
panels and button clicks to power an interactive game, even bootstrapping between power losses—we take conceptual inspiration inthis as well, since we engineered our device to be able to bootstrap
itself from absolutely no battery (by ensuring the default state isharvesting mode). Interactive Generator [2] is a handheld knob thatharvests energy when the user turns the knob, which it uses for
RF communication to the interactive application. Moreover, while
it does not provide a wide range of haptic feedback (e.g., vibration or others), it does provide on-demand resistance changes, by

Shan-Yuan Teng et al.

shortcutting the terminals of its DC motor—we take inspiration inthis to realize our braking force. Furthermore, SPIN [5] integrates
triboelectric nanogenerators into wearables that power LEDs anda buzzer. In fact, more recently, a similar approach has even beenused to generate electro-tactile feedback [40]. However, the output
power is still low at the 𝜇A level, while stronger stimulations tend
to require mA level intensities. As such, with these previous harvesting approaches, electrical muscle stimulation or vibration has
not been demonstrated and might not be readily achievable.
Generally, these previous devices incorporated energy harvesting by harvesting for a short duration of time (e.g., a button push[45], or a twist of a knob [2, 43]), which limits the amount of harvested energy (i.e., short harvesting = small energy). As such, thissmall amount of energy limits applications of these techniques todevices using relatively low power when compared to more powerhungry haptic actuators (e.g., vibrations, EMS, etc.). While we takeinspiration from all these approaches, we take two conceptual turns:(1) harvesting large movements for much longer—thus harvestinglarger output currents that are sufficient to power stronger haptic
devices; and (2) concealing the harvester’s side-effects (i.e., resistancethat users would feel as distracting) as part of the VR experience &interactions.

9.3

Adaptive VR systems

Our system adapts the user’s VR experiences in real-time to properly integrate the energy harvesting in the experience. This is inspired by works that utilize VR to adjust to different constraints(e.g., space), while preserving immersion. For instance, Scenograph[33] adjusts the VR experience dynamically according to the empty
space available in the room, while preserving the same VR narrative. VirtualSpace [34] takes this further by enabling multiple users
to overload the same limited physical space; to achieve this, thesystem generates alternative VR scenes in run-time (such as rendering an effect in a particular location, or making a new “power up”
to appear), which causes users to move, even without needing torealize that there are sharing the space with other users. Moreover,motivated by the high power demand for haptic feedback, a seriesof research works have been exploring the idea of using humans
as actuators in VR systems [6–9]. Specifically, in Mutual Turk, VR
users, without even knowing other users are present, trade haptic
forces with one another. Again, in this system, the VR experience
also dynamically renders the content according to the motion of
the other person to render realistic haptic feedback. Like these approaches, we take inspiration from VR experience’s ability to be
dynamically generated in real-time and use it to justify the resistive
force felt while wearing our harvesting device.

10

DISCUSSION ON BATTERYLESS HAPTIC
DEVICESEven though there exists plenty of research on haptic devices, they
are rarely mobile. One prominent issue is their high power consumption [12], leading them to be often tethered; or to have shortbattery lives if not having cumbersome batteries (e.g., CLAW [10]estimates the use of 1000mAh LiPo to yield only ∼1 hour). This
happens because devices generating haptic feedback require more
power, typically orders of magnitude above, than a sensor.

Prolonging VR Haptic Experiences by Harvesting Kinetic Energy from the User

While researchers have explored alternative actuators for thesake of power efficiency (e.g., using muscle stimulation insteadof mechanical motors; or using brake-based actuators instead of
motor-based actuators), even these alternative devices still featurebatteries that will likely last far less than a whole day of usage. For
example, Dexmo [17] uses a 800mAh LiPo battery, which allows it
to provide haptic feedback for 4h, but only provides resistive force.
Similarly, Impacto [31] uses a 1050mAh LiPo battery, which allows
it to provide haptic sensations for ∼200 seconds. In fact, as devices
attempt to simulate richer sensations, they require adding more
haptic modalities, which implies even more power consumption
and shorter uptime. We see an urge of seeking alternative solutions
to batteries in haptic devices.
We demonstrate that integrating intermittent energy harvestingis a viable approach for VR. While the aforementioned batterypowered Impacto [31] provides ∼2000 haptic sensations (each100ms), we estimate, with our approach, these sensations can be
replicated with 3 seconds of charging prior to each stimulation(during runtime, i.e., after the process of bootstrapping the microcontroller and communications). Our device can provide ∼2000 ofthese sensations within 100 minutes, and it can continue doing thisfor as long as the user intends, 10 hours, etc. For Impacto to match
this, it would need more than its current battery (1050mA LiPo). For
example, to prolong its usage to 2.5h, it would require a 3150mA
LiPo, weighing ∼0.5kg (three times heavier than the original battery). Naturally, adding more weight to wearables is undesirable as
it creates fatigue, hinders movement, and creates unwanted haptic
sensations. Moreover, to make matters worse, even this now heavier battery (lasting 2.5h) would still need to be charged at somepoint—the bigger the battery, the longer the charging time.
We believe that without batteries, haptic devices can be moremobile and more readily available, without users needing to worry
about having enough charge. Finally, while we focused on the extreme case of having no batteries at all, one can also integrate batteries in our approach—in this way, the device can provide continuous
haptic feedback when its batteries have charged, or switch to ourintermittent harvesting approach when its batteries are depleted.

11

CONCLUSIONS & FUTURE WORK

We propose a new technical approach to implement untethered VR
haptic devices that contain no battery, yet can render on-demand
haptic feedback. The key is that via our approach, a haptic device charges itself by harvesting the user’s kinetic energy (i.e.,
movement)—even without the user needing to realize this. Thisis achieved by integrating the energy-harvesting with the virtual
experience, in a responsive manner. We instantiated a version of
our concept by implementing an exoskeleton (with vibration, electrical & mechanical force-feedback) that harvests the user’s arm
movements to power haptics intermittently. We validated this device by means of a user study, in which participants (even without
knowing the device was harvesting their movement) rated a VR
experience as more realistic & engaging using our device than witha baseline VR setup. We believe our technical approach affords
new uses of haptics for prolonged use-cases, especially useful in
untethered VR setups, since devices capable of haptic feedback are
traditionally only reserved for situations with ample power. Instead,

UIST ’22, October 29–November 02, 2022, Bend, OR, USAwith our approach, a user who engages in hours-long VR and grewaccustomed to finding a battery-dead haptic device that no longer
works, will simply resurrect the haptic device with their movement.
Moreover, our approach enables new ways to use haptic devices
unthinkable for battery-powered devices today: namely, walk-up
use. Even if the user forgot to charge the haptic devices or change
the batteries, our technique enables these to work rapidly during
the interaction.
As for future work, we envision how researchers might expand
our approach to integrate different haptic actuators (e.g., Peltierelements) or create new renditions of our approach for other bodyparts (e.g., neck, shoulder, and so forth).

ACKNOWLEDGMENTS
We would like to thank Josiah Hester for his valuable feedback
and Ken Nakagaki’s generosity for us to use the water jet cutter at
AxLab.

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