CHI 2018 Paper

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

CLAW: A Multifunctional Handheld Haptic Controller for
Grasping, Touching, and Triggering in Virtual Reality
Inrak Choi1,2 , Eyal Ofek1 , Hrvoje Benko1 , Mike Sinclair1 , Christian Holz1
1 Microsoft Research, Redmond, WA, USA
2 Stanford University, Stanford, CA, USA
irchoi@stanford.edu, {eyalofek, benko, sinclair, cholz}@microsoft.com

Figure 1. CLAW VR controller provides articulated movement and force feedback actuation to the user’s index finger which allows for convincing haptic
rendering of: (a) grasping, (b) touching, (c) rendering virtual textures, and (d) triggering.

ABSTRACTINTRODUCTION

CLAW is a handheld virtual reality controller that augments
the typical controller functionality with force feedback andactuated movement to the index finger. Our controller enables
three distinct interactions (grasping virtual object, touchingvirtual surfaces, and triggering) and changes its corresponding
haptic rendering by sensing the differences in the user’s grasp.
A servo motor coupled with a force sensor renders controllable
forces to the index finger during grasping and touching. Using
position tracking, a voice coil actuator at the index fingertip
generates vibrations for various textures synchronized with
finger movement. CLAW also supports a haptic force feedbackin the trigger mode when the user holds a gun. We describe thedesign considerations for CLAW and evaluate its performancethrough two user studies. The first study obtained qualitative
user feedback on the naturalness, effectiveness, and comfort
when using the device. The second study investigated the easeof the transition between grasping and touching when using
our device.

The capabilities of current interaction devices to render meaningful haptics lag far behind their abilities to render highly
realistic visual or audio content. In Virtual Reality (VR) andAugmented Reality (AR) scenarios, there is a strong need for
haptic devices that let the users feel, touch, push, grab andmanipulate virtual objects around them with the similar tactile
and force stimulation that the users are familiar with in thereal world. To make them effective in a variety of VR/AR
scenarios, there is also a need for such devices to allow for freemovement in space, i.e., to not have the devices be grounded,
but instead be held or worn on the user’s body.

ACM Classification Keywords

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

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

To address these challenges, researchers have proposed various types of hand-held or hand-worn haptic devices such as
exoskeleton hand gloves for grasping [1], fingertip devices forshear force and weight [27], vibrotactile devices for textures[19], controller type devices for touching [4], and exoskeleton haptic suits for kinesthetic feedback to the entire arms[5]. While potentially effective in rendering specific typesof haptic feedback, the main shortcoming of these solutions
is that they are mostly single purpose devices, unable to render a range of feedback required for a variety of envisioned
VR/AR scenarios. For example, the 3RRS device [9] can render shear and pressure at each fingertip, but is unable to render
forces between fingers and is thus unable to effectively simulate grasping soft or hard objects. In contrast, we argue thatcreating an effective VR/AR haptic controller device requires
the design to be multi-purpose in nature.
In this paper, we present the design of a novel haptic controller,
named CLAW, that augments a typical handheld controller
functionality with force feedback and actuated movement to
the index finger (Figure 1). The primary design principle ofour controller is to provide a multi-purpose controller that contains both the expected functionality of a VR controller (i.e.,
buttons, 6DOF movement control, thumb joysticks, trigger)

DOI: https://doi.org/10.1145/3173574.3174228

Paper 654

Page 1CHI 2018 Paperand enables a variety of haptic renderings for the most commonly expected hand interactions: grasping, touching, andtriggering. Our CLAW controller changes its corresponding
haptic rendering by sensing differences in the user’s grasp
and situational context of the virtual scene. Besides shooting
feedback, CLAW can act as an active trigger button. By selecting particular finger force and positions, CLAW can simulate
various trigger release points, non-linear responses and haptic
behaviors such as a click. The force sensor can also act as ananalog input function, e.g., interacting with a slider, changing
attributes of a paint brush or pen in a drawing program.
Our paper makes the following three contributions:
1. The design and implementation of a handheld haptic controller capable of rendering three types of haptic feedback
in a single multi-purpose device. Three types of haptic
feedback include: (a) finger forces when grasping virtual
objects, (b) rendering of a virtual object’s shapes, stiffness,
extent, and textures and (c) realistic trigger feedback whileshooting.
2. The idea and the implementation of switching haptic rendering modes based on changes in the user’s grip.
3. Two evaluations of the CLAW controller with respect to theoverall effectiveness in use and ease of transition between
grasping and touching modes.
In the rest of this paper, we first review the state-of-the-art inhandheld haptic technologies and commercial VR controllers.
We discuss design considerations for a VR haptic controllerthat integrates both existing controller functions and multiple
haptic sensations. We then introduce our CLAW prototype,built in an iterative fashion based on those design requirements.
Finally, we discuss the results of two user studies with our
CLAW controller. The first study collected qualitative feedback from participants using our controller to manipulate avariety of virtual objects. Users performed and switched between various interactions, such as picking, squeezing, touching, rubbing, and triggering. The second study explored theeffectiveness of our grasp-based method for switching between the pick and place task and pushing a virtual button task.
The results from our experiments show that participants werehighly effective and confident when manipulating virtual objects with CLAW and that switching between different haptic
modes was quick and mostly error-free.
RELATED WORK

We differentiate between two types of haptic devices:grounded and ungrounded devices [16], where the former
are devices fixed to environment (e.g., desk mounted) while
the later are attached, held or worn on the human body (e.g.,hand). Although grounded devices such as PHANToM [21],HIRO [14], or SPIDAR [23] are effective in rendering externalkinesthetic forces to the user’s hands in VR, they restrict theuser’s movement to a small working area which is contrary to
our goal of free movement in space. Therefore, we focus ourreview of related work to ungrounded haptic interfaces only,

Paper 654

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

and, in particular, to wearable and mobile devices that provide
haptic feedback to the VR user.
Exoskeleton Haptic Devices

Exoskeleton gloves render kinesthetic feedback when the userholds or touches a virtual object. There have been manyefforts to make exoskeleton gloves lightweight, small, andbetter-performed using various mechanisms. Researchers developed haptic gloves using tendon-driven mechanisms [1],passive actuation mechanisms with magnetorheological fluidfor various force feedback [6], simple mechanical brakes forlow cost [11, 15], a pneumatic actuator for higher force toweight ratio [7], jamming technologies for variable stiffness
control with passive actuation [28, 40]. A gripper style device (force feedback to the thumb and index finger) was also
developed for teleoperation [18].
Exoskeleton haptic devices render compelling grasping andtouching feedback for every finger of the user’s hand. However, they have some limitations in a practical way. They
require long time to put on or take off. They are cumbersome
and constrain the user’s motion. They are expensive becauseof the number of actuators required. These factors limit theircommercial use for VR.
Fingertip Haptic Devices

More recently, researchers examined devices that can bemounted on the user’s fingers, stimulating cutaneous haptic
feedback to the mechanoreceptors on fingertip skin [17]. Pacchierotti et al. developed a belt mechanism with two motors.
By actuating the two motors in same directions or opposite
directions, they can render shear force and normal pressure on
the fingertip [25]. Minamizawa et al. also used this mechanism to stimulate the sensation of gravity [22]. Tsagarakis etal. developed a device directly contacting between fingertip
skin and two motor shafts for the sensation of slipping andshear forces [33]. Solazzi et al. used a voice coil actuator tomove a plate for the sensation of contact [29]. Multi-linkage
wearable fingertip devices render the slopes of virtual surfaces[9, 27]. Yem et al. combined a mechanical linkage and arrayof electrodes to generate both shear force and texture [38].
Culbertson et al. attached a voice coil actuator at fingertip
to simulate virtual kinesthetic sensation using asymmetricvibration [12].
Fingertip haptic devices are lighter than exoskeletons andrender various types of cutaneous feedback. These devices
lack of ability to create kinesthetic feedback because they
are not connected through mechanical linkages. Also, theyneed to be placed on the fingertip to receive desired haptic
feedback which may take time to put on and off. Applying
kinesthetic forces to these actuators may also compromise
their effectiveness. These features may not be desirable forgeneral use.
Handheld Haptic Feedback

Handheld devices have gained popularity, in particular, incommercial VR and AR systems. The main reason is that they
are easy and convenient to use; A user may hold a controller tointeract with a virtual object and quickly lay it aside to interact

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with real objects, for example a keyboard with bare hands.
Also, handheld devices are stable during interaction because
users hold them while most wearable devices rely on gloves
or straps.
Although most available haptic feedback is limited to simplistic vibrotactile actuation, recent works tries to enrich theirhaptic rendering capabilities. Researchers developed various
handheld haptic devices using a palm stretch mechanism tosimulate transient kinesthetic feedback [26], weight-shiftingmechanisms to simulate the sensation of holding objects ofdifferent mass distributions [31, 39], actuated gimbals for
simulating external force [24, 35], and a linearly vibrating
mechanism to give perceived heaviness [3]. Most of thesedevices render haptic feedback to the user’s palm or entire
hand but they do not apply haptic feedback to fingers so thatthey are not capable of high fidelity rendering for grasping andtouching.
There are handheld haptic controllers interacting with theindex finger. MacLean et al. attached a rotating wheel for
creating shear force [20]. Benko et al. used tilt-platform andvertically actuated pin array to convey normal surface andtextures underneath the user’s index fingertip while the user
moves the device in space [4].

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

Design Rules for an ideal VR Haptic Controller

Based on the considerations above, we made a set of design
requirements for an ideal VR haptic controller (Table 1). We
consider these design rules for our prototype.
The features expected for an Ideal VR Haptic Controller

1 Handheld for ease of use
Input buttons on the controller manipulated by thumb and a
2 trigger manipulated by the index finger for shooting games.
3 6 DOF tracked in space
4 Untethered

5 Ergonomically comfortable

The features we would like to add

1 Ability to render shapes of virtual objects
Ability to render forces from touching or grasping virtual objects
such as elastic and inelastic stiffness, damping, inertia, shear
2 force, and weight
3 Ability to render textures of virtual surfaces
4 Realistic haptic effect for triggering

5 Ability to understand some natural manipulation gestures
Table 1. Our design requirements for a VR haptic controller.

While previous devices in this category rendered only one kindof haptic feedback such as shear force [20], normal force [4],and textures [4], we integrate multiple haptic feedback into asingle multi-purpose device, guided by the user’s grip. Song etal. looked at thumb gestures as a guide for prehensile manipulation tasks [30], while CLAW compares thumb gestures forprehensile and non-prehensile manipulation tasks, enabling
untrained users to naturally navigate the controller affordances
depending on the task.
In this paper, we leverage the mobility and the flexibility offered by today’s available handheld controllers. Our proposed
device incorporates most of the capabilities of existing commercial controllers yet enabling new functionalities that will
increase the haptic realism and allows the user to do holdobjects, touch surfaces and manipulate them using naturalinteraction.
DESIGN AND IMPLEMENTATION OF CLAW

The design goal of our haptic controller is for its seamless use
in a virtual environment. It should provide human-scale forces
in rendering 3D shapes as well as the sensation of holding
a rigid or soft object between the user’s fingers. It should
generate various textures when users rub virtual surfaces with
their index finger. It should also enable a gun trigger sensation.
It should provide all the above in a compact and lightweight
form factor to facilitate unencumbered mid-air operation.
The main challenge in designing our controller was integrating the default operation provided by current commercial VR
systems (e.g., pointing, button selections, user locomotion,
trigger) with the haptic output to render virtual shapes andforces experienced from grasping. By maintaining the functionalities of existing controllers in our design, we leverageusers’ familiarity to facilitate a smooth transition, as well as
the ability to plug into existing use-cases and applications.

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Figure 2. (Top Left) Overall system configuration and components. (TopRight) Actual shape of the controller and its motion range. (Bottom)
Mechatronis components block diagram with detailed pinout information.

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

4) to render kinesthetic feedback, i.e., the sensation of force,
actuation and displacement of the index finger. CLAW integrates an VCA on the finger mount for cutaneous rendering on
the fingertip as shown in Figures 1 and 2. As the user makes
contact with virtual objects in the scene, CLAW opens its arm
accordingly to move the index finger outward, keeping it on
the boundary of the virtual surface in the same 3D position.
Refer to Figure 1 (b).

Figure 3. We repeated the prototyping process multiple times for a better
ergonomic design and performance. Four CAD models are shown as
examples.

Variable
Value
Max. Force
Up to 30N
S�ﬀness range Up to 10N/deg (5.73N/mm)
Mo�on range 45°, 0.1° resolu�on for posi�on control
0 ~ 50N range, 88 Hz sampling rate, 0.023mN
Force sensing resolu�on, 0.048mN noise (std. dev.)
Admi�ance display. 333Hz PD control loop by
Force control extrapola�ng force values.
Hand tracking Vive tracker
Weight420g (335g w/o Vive tracker)
Dimensions
90*170*190 mm
Power draw
30mA (idle), 1A (max. force) @ 5V

The rotating arm of the controller is mechanically powered bya Hitec HSB-9370TH servo motor. We designed all connecting
components using CAD and printed them in PolyJet on an
Objet Connex 3 printer. A Teensy 3.2 USB microcontroller
PCB is mounted in the controller handle and controls the servo.
To make an ergonomic haptic controller for various hand sizes,
we designed an arm with adjustable length. We interated
through the designs shown in Figure 3 to optimize ergonomics.
CLAW achieves most of our initial design guides, but three
items remain. CLAW is currently not untethered, does notrender shear forces to the user’s finger, and it does not generatea sense of weight. At this time, we prioritize the other features
and these are remaining as future work.
For power consumption, we measured idle current of 30 mA at
7.4v and maximum current of 1A for maximum force exerted.
We believe these are within the capability of an untethered, battery operated controller in the future, even with the additional
need of powering a 6DOF tracker and radio link.

Table 2. Technical specifications of the controller.

The CLAW VR Controller

Based on our design rules, we designed and built CLAW to
extend and integrate the functions of existing VR controllers
and multiple haptic feedback modes. Figure 2 (Top) shows
the general appearance and the arrangement of mechatronic
components. Figure 2 (Bottom) shows the block diagram andgeneral connection information of mechatronic components.
CLAW is handheld where users grab the handle grip with theirmiddle, ring, and little fingers, placing their thumb either at
the side on the thumb rest or on top of the controller. The
controller features a proximity sensor for detecting thumbpositions and several buttons and a thumbstick for explicit
user input. Users place their index finger into the opening at
the end of the arm, where a voice coil actuator (VCA) under
the fingertip renders textures. A force sensor just above theindex finger mount senses user input force, including touch,grasping, and triggering. An additional revolute hinge withspring return on the force sensor implements CLAW’s trigger
mechanism. A servo motor drives the arm, which connects tothe main body through another revolute joint.
The handle grip encloses a Teensy 3.2 microcontroller, an
HX711 ADC board for sensing force, and a DRV8833 motor
driver to power the VCA at the index fingertip and the linear
resonant actuator (LRA) in the handle grip. We mounted an
HTC Vive tracker at the bottom for 6DOF tracking. Two cables
provide power and USB communication with the controller.
The core element of CLAW is the rotating arm as shown in
Figure 2 (also see the green part of the controller in Figure

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In essence, CLAW is a combination of a handheld controller
base, an index finger exoskeleton, and a fingertip vibro-tactile
actuator. Table 3 shows a comparison between CLAW andstate-of-art haptic devices for VR. To our knowledge, this is a
first handheld controller that integrates this functionality intoa single compact design.
Mode Switching Via Thumb Grip Sensing

Our high-level goal is to allow for multi-purpose haptic rendering without requiring a complex method to change between
different modes. For touching and rubbing interactions, theuser’s thumb is usually not on the thumb rest. While pinchingor grabbing objects, the thumb is on the thumb rest, aligning with the index finger, to produce a counter-force to theindex finger as shown in Figure 4. The counter-motion ofboth fingers supports simple grasping or pinching tasks (prehensile) while the index finger alone supports touching tasks(non-prehensile) [8, 13]. While the user touches virtual objects
using only their index finger, we provide a convenient rest areaon the front of the device (where the joysticks and buttons arelocated), to ensure the thumb is out of the way.
The thumb position and the VR application intent determine
the operation mode of the device. To sense the thumb, we
integrated an optical proximity sensor (QRE1113) into thecontroller’s thumb rest. The sensor detects when the thumb ison the thumb rest which covers the sensor as shown in Figure
1 (a) and Figure 4 (Top-left). In this mode, the thumb aligns
with the index finger as in a pinching or grasping attitude. This
switches the controller into ‘Grab’ mode, during which thecontroller physically adjusts to the size of the object between

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

Easiness of Mounting

Functions of existing controllers(Trigger, directional joystick etc.)

CLAW

Easy (handheld)

Yes

Vive Controller

Easy (handheld)

Yes

Grabity [10]Schorr et al. [28]Reactive Grip [27]CyberGrasp [13]Hard (Mounted to two
fingertip)
Hard (Mounted to two
fingertip)
Easy (handheld)

Hard (Mounted towhole hand)

No
No
Yes
No

Haptic Feedback
Kinesthetic feedback for grasping, touching, and triggering.
Vibration normal to fingertip for textures.
Simple vibrotactile feedback only.

Kinesthetic feedback for grasping (binary) and simulating weight.
Vibrotactile feedback for touching.

Tactile feedback (sensory substitution) for grasping, touching,simulating edges and weight.
Simulating external forces in grip motion only.

Kinesthetic feedback for grasping and touching.

Table 3. Comparison of CLAW and existing devices. CLAW integrates all functionality found across previous devices into a single, handheld controller
by offering multiple interaction modalities and rendering the corresponding haptic feedback.

(closed) position, which cannot be altered by the user throughforce input. Once the controller comes into contact with avirtual object, the arm moves the user’s finger according to
the amount of object penetration and simultaneously renders
normal forces based on the stiffness of the object.

Thumb Positon

Gun in Hand

Figure 4. CLAW operates in three haptic modes: (Top-left) When theuser tucks away the thumb, off the thumb rest, the controller is in its default ‘Touch’ mode. (Top-right) The proximity sensor (red) detects whenthumb and index finger align, and switches the controller into ‘Grab’
mode. (Middle) When the user has ‘grabbed’ a gun, the rotating arm(green) locks in place and mimics a gun, and the finger module (pink)acts as a trigger. (Bottom) The haptic mode is decided based on theuser’s thumb position and VR scenario.

the index finger and the thumb in the virtual world along with
its stiffness in response to squeezing force input. When the
thumb is not on the thumb rest (does not align with the index
finger), the controller switches back to ‘Touch’ mode andrenders surfaces and textures that lie under the index fingertip
as shown in Figure 4 (Top-right).
The block diagram in Figure 4 details how CLAW changes its
operation mode. In the initial ‘Touch’ mode (user’s thumb is
off the proximity sensor), CLAW resets the arm to a minimum

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When users are in ‘Grasp’ mode with their thumb covering
the proximity sensor, they can open and close their index
finger freely. Once they grasp a virtual object, the servo motor
impedes motion at the object’s grasp width, rendering grasping
forces based on the stiffness of the object. If the object a user
has picked up is a gun, the controller enters ‘Trigger’ modeand locks the arm in a fixed open position. At this point, users
may only bend the finger hinge to trigger, which, when above
a force of 4N, generates a quick back and forward motion
from the servo, simulating an automatic weapon, proximity
for haptic trigger feedback in addition to vibration feedback
under the index finger. To drop the gun and return to ‘Touch’mode, a user moves their thumb off the sensor (i.e., drops a
gun).
Force Control

One of the important features of CLAW is that it generates
forces in a closed-loop fashion by sensing the forces applied by
the user’s index finger. Therefore, not only does the controllerchange the index finger position for rendering the shapes ofvirtual objects, it also renders various stiffness of grasped
objects.
Most existing haptic devices [1, 21] are impedance displaysthat sense positions for input and render force as output. The
structure of an impedance display is simple: a closed-loop
position control system that does not require a force sensor.
However, it should be highly backdrivable with low inertia.
The gear ratio is limited so it requires a large and heavy motor
to create human-scale force. CLAW is an admittance display[2, 34], sensing force as input and rendering position as output(as opposed to mere force). While CLAW needs a force sensor
and a force closed loop on top of a position loop, it does not
require backdrivability. CLAW can have high inertia with acompact and lightweight geared motor. We thus believe that
this admittance display is a better type for a controller form
factor haptic device.

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INTEGRATING CLAW INTO A VR ENVIRONMENT

CLAW connects to a PC through USB (serial link) and powered through a separate 5V and 2A power adapter. The baudrate is 2,000,000 bits per second. A HTC Vive headset is used
for VR display. Unity 2017 software is used for rendering VR
scenes, and our display update rate is 90 Hz.
There are several ways in which our touching and graspinginteractions could be implemented in a VR environment (e.g.,using collision dynamics, or kinematic control). While CLAW
is capable of supporting all such methods, we discuss our
implementation below, in addition to discussing the details of
the haptic rendering that CLAW enables.

Figure 5. The avatar hand in Unity’s VR scene when (a) grabbing, (b)touching, and (c) triggering. Note: the green rays emanating from thefingertips are hidden from the VR user and only visualize directions fordetermining collisions in VR.

Grasping: Force Between Thumb and Index Finger

For grasping interactions (i.e., when CLAW is in the graspingmode), our VR simulation tests for possible contacts below the
thumb and index finger by checking for collisions against a ray
originating at each fingertip and pointed towards the opposing
finger. If each one of those rays intersects the same virtual
object within a small tolerance buffer (1cm) from the fingertip,
we consider this virtual object grabbed (Figure 5 (a)), andfrom that point on, render its position and orientation underkinematic control. While not completely physically correct,
this kinematic approach to grasping makes this interaction
easier and more controllable, while still giving the user theability to grip, squeeze and move virtual objects in a naturalmanner.
When rendering varying elastic stiffness of grasped objects, weuse Hooke’s Law for spring behavior to compute the amountof visual deformation needed based on the force exerted on the

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0Distance [mm]5 10 15 20 25 30 35 40 45 50 55 60 65 70 75
10 N/deg(5.73 N/mm)

25

Force [Newton]

The force sensor (strain gauge on a cantilever beam, 0-5kg)
measures the force values sampled by the HX711 ADC board.
The 24bit ADC board has enough resolution (0.000023N with
0.000048N one standard deviation noise), but it is limited at
88Hz sample rate. To minimize the problem of a low sampling
rate compared to the much higher servo position update rate,
we derive new force values by using a simple slope extrapolator: calculate the amount of the change from the previous force
values but at the higher sampling rate. For the position servo
input, we use a simple PD controller operating at 333Hz andtune the gains empirically for quick response with no instability. We use a commercial servo motor (Hitec HSB-9370TH) in
which the position loop is run internally closed loop but open
loop externally, receiving new position commands at 333Hz.
Table 2 shows the detailed specifications of CLAW.

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

2.75 N/deg(1.58 N/mm)

201.49 N/deg(0.85 N/mm)

150.9 N/deg(0.52 N/mm)

100.53 N/deg(0.30 N/mm)

50.25 N/deg(0.14 N/mm)

00 N/deg(0 N/mm)

0

5

10

15

20

25

30

Angle [Degree]

35

40

45

Figure 6. Various stiffness rendering through CLAW.

CLAW by the index finger. We also haptically render similar
spring behavior on the controller itself.
Under closed-loop force control system, CLAW generates up
to 30N of grasping force and up to 10 N/degree (5.73 N/mm)of stiffness. Note that stiffness can be inelastic (like clay) or
elastic (like a spring) or a combination. Using this feature, we
can simulate various stiffness of different materials like rigid
objects (wood, metals) and soft objects (clay, rubber balls,
sponges) and even non-linear properties. To evaluate how correctly the controller renders stiffness, we measured forces withdifferent elastic stiffness. Figure 6 shows seven force curves
with different levels between the minimum stiffness (0 N/mm)
and the maximum stiffness (5.73 N/mm). We generated thesecurves from actual data sensed by the calibrated strain gauge
force sensor and the position input to the servo. We see that theforce curves keep consistent stiffness (slopes of force curves)
over the entire motion range, although with a small dead bandfor each curve ( 0.5N), likely due to static friction, gear slop,
hysteresis of the servo encoder, etc. inside the servo motor.
CLAW also senses the force imparted by the index fingertip with a 0.000023 N resolution and 0.000048N noise (onestandard deviation) and controls positions with 0.1°(0.17mm)
resolution. This actuation capability is generally beyond human perception as shown in previous work: The just noticeable
difference (JND) of length discrimination between the thumband index finger is 1-3mm, the JND of grasping force is 710%, and the JND of compliance during squeezing motion is22% [32, 37]. More rigorous measurements of the perceived
stiffness require a future human subject experiment.
From the measurements above, we show that our controller
with closed-loop force control renders variable stiffness forrigid and soft materials. However, not every object in thereal world has a perfect elastic stiffness like the force curves
shown in Figure 6. This hysteresis effect changes the force
curves. The pushing forces and recovering forces are notmatched. In Figure 7, we simulated three force curves withdifferent hysteresis levels. With elastic stiffness rendered withhysteresis, CLAW generates more realistic grasping forces.

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4

5

Distance [mm]678

9 10 11 12 13 14 15“Large”
Hysteresis

20

Force [Newton]“Medium”
Hysteresis

15“Small”
Hysteresis

10

DesiredForce5

Direction
0

0

1

2

3

4

5

Angle [Degree]

6

7

8

9

1

2

3

4

5

Distance [mm]678

9 10 11 12 13 14 15

14“Stiff”

Switch Button

Force [Newton]

12
10“Soft”

Switch Button

8
6Desired
Force

4
2
0

Direction
0

1

2

3

4

5

Angle [Degree]

6

7

8

9

Figure 8. Virtual switch forces rendering.

Surface Rendering: Elevation of the Index Finger

In ‘Touch’ mode, users feel the shapes of virtual objects
through the position change and the forces applied to theirindex fingertip. In the VR simulation, we build on the height
rendering process of NormalTouch [4]: When the user makes
contact with virtual objects in the scene, CLAW adjusts theangle of the index finger arm to lift the index finger corresponding to contact with the surface. This causes the user’s
finger to follow the surface of the virtual object rather thanpenetrating it upon contact as shown in Figure 5 (b). The
actuation mechanism by which CLAW renders normal forcesin ‘Touch’ mode is the same as in ‘Grasp’ mode.
A variant of this rendering is to simulate a clickable button
with sizable hysteresis, i.e., one that produces a ‘click’ when
actuated. We derived two force curves for a “stiff” click andone for a “soft” click as shown in Figure 8. Dotted lines
are the desired force curves. Through application of thesecurves, users feel a stiff elastic force as they push the button.
When they reach the hysteresis threshold, they encounter alower spring force, causing a click sensation as well as a lower
return force until they reach the 2nd hysteresis point, at whicha higher spring force is encountered.

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“Rough” Texture0(P400 grit sandpaper)

−1
0

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4

6

8

10

1

“Rougher” Texture0(P220 grit sandpaper)

−1
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10

1

“Roughest” Texture0(P80 grit sandpaper)

−1
0

2

4

6

Distance [mm]810Figure 9. Various roughness rendering using the voice coil actuator under the fingertip mount.

Texture Rendering: Spatial Vibrations of the Index Finger

Figure 7. Various hysteresis curve rendering through CLAW.

016

Voltage [V]1Voltage [V]025

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

Voltage [V]

CHI 2018 Paper

In ‘Touch’ mode, CLAW simulates various textures by actuating a voice coil actuator on the fingertip. The microcontroller
in CLAW plays back pre-generated haptic patterns. The texture actuator loop runs at 14kHz, and data is played back withspeeds corresponding to the user’s calculated finger speed andrequired amplitude information (i.e., grit size of the surface).
The Vive Tracker reports hand (and hence fingertip) positions
to the PC at 120 Hz. From Unity, we calculate and report
the scalar velocity of the fingertip with respect to the touchedsurface to CLAW’s microcontroller at 90 Hz. As mentioned
above, CLAW plays back the corresponding stored haptic
pattern to the actuator. CLAW modulates the playback speed
proportional to the velocity of the fingertip motion and adjusts
the playback amplitude according to other parameters such as
overall texture roughness, applied force, etc. This allows users
to feel fingertip-normal vibrations simulating various textures
when they stroke a virtual surface.
The three graphs shown in Figure 9 are examples of textures
CLAW can generate. We simulated different grit size sandpapers (P400, P220, P80). The microcontroller plays back thehaptic pattern with 500 random (colored noise) values for themagnitude.
Trigger Mode for Grabbed Objects

Most existing VR controllers rely on a vibration actuator for
haptics, when the user pulls the trigger. To generate a realistic
trigger feeling with objects that afford this type of interaction,
we use the servo motor on the controller arm and the LRA
inside the handle grip.
When the user grabs an object that can be triggered (e.g., awater pistol), our VR environment switches CLAW into a dedicated ‘Trigger’ mode as described above (Figure 5 (c)). The
controller from that point on handles the trigger input and reports it back to the VR environment to produce corresponding
animations. Similarly, the controller returns to ‘Touch’ mode
when the user switches their thumb accordingly, informing the
VR system to reflect this state.
In ‘Trigger’ mode, the controller arm is locked to a fixed position and users can either pull the trigger or drop the item in thismode. The force sensor at the fingertip measures the trigger
force and once the trigger force is higher than a threshold (4N),

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

Figure 11. Study 2 Grab Task: A participant needs to grab the white
cylinder and align it with the red target.

Figure 10. Haptic playground in Study 1 contains different objects withdifferent haptic qualities.

CLAW “discharges” the weapon as follows: the servo motor
decreases the arm angle quickly with its maximum speed while
the LRA simultaneously turns on and vibrates the handle. Because the arm angle decreases fast, the user’s trigger force
drops below the threshold, resetting the arm to the original
position quickly and turning off the LRA. This combinationof the servo motor and LRA actuation produces stronger andmore realistic trigger haptics than the vibro-tactile feedback
sensations produced by traditional controllers.
EVALUATIONS

We performed two experiments to investigate the following
core questions regarding our CLAW controller:
1. How effective is the CLAW controller in rendering a variety
of different haptic sensations?
2. How easy and reliable is it for users to switch between
different haptic modes of the controller?
Participants

To answer these questions, we recruited 12 participants (6female) with ages ranging from 26 to 61 (M=32, SD=9.4). All
our participants were right handed, as our prototype controller
was designed for a right-handed user.
Each participant was asked to perform two experiments, after
which they filled a questionnaire, and when finished, they were
given an $8 gift coupon.
Study 1: Qualitative Feedback for CLAW Interactions

Participants were introduced to the CLAW controller, andwere shown a physically simulated virtual playground (Figure10) containing a range of objects that can be manipulated by
grabbing and touching:• A set of cubes, cylinders and balls that can be grabbed by
the user when using the controller in ‘Grab’ mode, moved
and stacked, and pressed on, or pushed by the index finger
in ‘Touch’ mode.

Figure 12. Study 2 Touch Task: A participant needs to tap the red target.

• A seesaw model that can be used for object stacking, or forpressing in ‘Touch’ mode.
• A gun that can be grabbed by the user, and then be used toshoot objects in the scene.
• A model of a car that can be touched to explore its shape or
be grabbed and stacked.
• Two unmovable gray boxes that exhibit textured haptics ofregular spaced grooves when the user’s index finger slides
along their surface.
To familiarize participants with CLAW’s capabilities, the experimenter demonstrated one operation with each of the above
objects to each participant. Then, each participant put on a
HTC Vive HMD, and used the CLAW controller in the scene
for 10 minutes while freely manipulating objects and verbally
expressing opinions.
Study 2: Task Switching Evaluation of Grab vs. Touch

Our second experiment was designed to evaluate how reliable
and easy is our mode switching technique based on sensing
the thumb location. To do so we designed an experiment that
required the participant to frequently switch between grabbing
and touching modes. Participants performed a series of 50
tasks of two types (described below in detail) with the CLAW
controller, while wearing a HTC Vive HMD.
The first task required the participants to use the grab modeof the controller by moving their thumb to lie in front of theirindex finger, grab a white cylinder, drag it until it aligns witha red target red of same size (Figure 11) and then release it.

• A set of cubes with different level of softness that the user
may grab, stack, and squeeze.

The second task requires the participants to move their thumbaway from the tip of their index finger to bring the controller toa ‘touch’ mode, and then push on a red circular button labeled“Push” (Figure 12).

• Several buttons that the user may press, in ‘Touch’ mode,and turn small lights on and off.

The experiment contained 25 grab tasks and 25 touch tasks in arandom order. Completing each task would have immediately

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CHI 2018 Paper

presented the participant with the next task, until the experiment is finished. The entire experiment took the participant
less than 3 minutes to complete.
After finishing both Study 1 and 2, participants were given
a questionnaire consisting of 30 questions related to theirexperience, each to be ranked on a 1-7 Likert scale. The entire
study lasted approximately 30 minutes.
Results and Discussion
Study 1 Results

Table 4 shows the result of the first part of our questionnaire,
which directed at participant’s experience in Study 1 and used
the standard Presence Questionnaire (PQ) by Witmer andSinger [36]. According to the typical scoring of PQ, we report the total and average values, for 5 aggregated categories:“realism”, “ability to act”, “quality of interface”, “ability to
examine” and “self-evaluation of performance”, as well as thescore overall. All average scores were all in the positive range(4-7).
Examining the participants comments from Study 1 (playground) and specifically the qualitative results aggregated inTable 5, it is clear that the ability to pick up objects was themost favorite task. Participants gave positive comments, suchas “Best experience at grabbing objects in VR I’ve tried better than gloves.” “Soft vs. Rigid objects was great!”.
Touching operation also received positive scores, but lower
than grabbing. Several reasons may explain this. First, while
holding an object between the user’s fingers was a realistic andsatisfying experience, the realism of touching a surface is limited by the ungrounding of the controller. CLAW moves thetip of the index finger to avoid solid object penetration, but wecannot stop the user from pushing the controller into the surface, which is when the rendering stops. We thus implemented‘penetration compensation’ [4] to render surface shapes during
such user behavior, even if the user has pushed the controller
inside an object. However, we left this compensation disabledduring our user study as it impacts the participant’s ability
to pick up an object that is close to the surface. Re-enabling
this may increase the practical dynamic range of our surface
rendering, and thus enhance the user’s satisfaction.
Secondly, we detect touch by the index finger by examining
a ray originating at the tip of the finger in the direction of thefinger (similar to [4]). If this ray hits an object close to thefinger, we render a touch event by pushing the index finger
back. While in reality the user finger may hit objects along it
entire finger, e.g., pushing an object by the side of the finger,
this limitation of our current implementation may limit therealism of touching. Although most touch events are indeed
performed using the fingertip, we will widen detection of touch
events in future versions.
The gun operation was a favorite of the users. The movement of the controller arm along with a vibrotactile rumble
generated an impression of recoil not available by current
commercial controllers.
CLAW renders texture with limited fidelity in its current implementation due to the latency of our VR setup. CLAW itself isPaper 654

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

Aggregate categories
Total (All ques�ons)
Realism (Ques�ons
7+8+9+10+11+14+17)
Ability to act (Ques�ons
1+2+8+9)
Quality of interface (Ques�ons
14+17+18)
Ability to examine (Ques�ons
11+12+19)
Self-evalua�on of performance(Ques�ons 15+16)

Total Avg. Score Std.Dev.
107.3
5.6
0.6

37.8

5.4

0.7

24.2

6

0.6

12

4

0.7

18.2

6.1

1

12.5

6.3

1

Table 4. Study 1 (haptic playground) assessment using Presence Questionnaire [36].

Question

1
2
3
4
5
6
7

How well were you able to pick up an
object in the VE?
How well could you explore the VE
using touch?
How well were you able to switch
between touching and picking?
How well were you able tomanipulate and ﬁre the gun?
How realis�c was the gun ac�on?
How well were you able todisambiguate surface texture?
How realis�c was it to feel diﬀerent
textures?

Avg. Score Std. Dev.
6.3

0.9

4.5

0.8

5.7

1.1

6.2
5.6

1.3
1.4

4.3

2

4.3

1.7

Table 5. Task specific questions from Study 1 and average scores (on ascale 1-7, 7 being best).

capable of rendering high-fidelity textures at 14kHz, but Unity
and the Vive tracker report hand position and motion updates
at slow rates only (120Hz and 90Hz, respectively). This is
too low for spatial rendering, but could be remedied by using
faster haptic software (e.g., CHAI3D) and a better tracking
system (e.g., OptiTrack Prime17W). Integrating the 6 DOF
tracking system into CLAW would also save the roundtrip
through the PC and improve CLAW’s current operation.
Study 2 Results

The second experiment looked at the ability of new users to
use a multi-modality controller, by requesting the users to
execute fast tasks of changing modality. The participants wereable to execute 50 tasks with an average of 27 flips between
grabbing and touching tasks.
On average, participants executed grabbing task in 5.9 seconds (SD=2) while touching a button task, which was a lesscomplicated task, was completed in 2.9 seconds (SD=1.6).
We measured the time that it takes for a user to flip the mode of
the controller, by looking at the last frame before the executionof the task, where the user thumb was in the wrong position.
This time includes the reaction time of the user, understanding that the mode of the task has changed, the movement of
the thumb from pinching position to touching or vice versa,
and any mistake done after it. The average time to move

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CHI 2018 Paper

from pinching position to a touching position was 1.4 seconds(SD=0.9), while the average for the move from touching position to a pinching mode was 3.6 seconds (SD=1.5). One
possible reason for the longer time it takes to move into pinching mode, is that the thumb must move to a specific locationin front of the index finger tip, and above the thumb sensor
for this transition to register. The other direction, on the other
hand, only requires the thumb to move away a bit from thesensor to detect its absence.
When asked to assess their ability to switch between modes,
on a 1-7 Likert scale, the participants ranked it very positiveat 5.6 (SD=1.3). Overall, no participant reported any issues or
confusion about switching between modes and none of themhad concerns on the reliability of this method.
In summary, we received overwhelmingly positive feedbackon the use of CLAW in a variety of VR manipulation tasks.
The feedback from our studies (while mostly qualitative innature) confirms that CLAW is an effective device for multiple
interactions in VR and that our thumb grip mode switching
method is a reliable and easy to learn method that follows theuser’s natural tendencies when interacting with real objects.
LIMITATIONS AND FUTURE WORK

While we have designed CLAW with VR scenarios in mind,
it could easily be used in AR. For example, having a fully
tracked, handheld, wireless CLAW would permit the user tointeract with virtual objects situated in the real world.
However, this future version would require several improvements to our current implementation. For example, our device
is not yet wireless. Although all mechatronic components are
already embedded inside the controller, the device is tetheredbecause of the communication and power supply. Incorporating Bluetooth wireless communication and integrating abattery would enable fully wireless operation. With a 7.4V
1000mAh li-po battery, the controller would run continuously
at least 1 hour or more.
Our current CLAW controller is prototyped only for the righthand use. While our design could easily be mirrored forthe left hand, we have not yet done so due to our limited
prototyping time and budget. Once we have two controllers
for both hands, we plan to explore two-hand manipulation
tasks, such as holding a larger object or shooting an arrow.
Although the current controller is not capable of renderingkinesthetic feedback to the user’s arms, it would be interesting
to explore interactions when performing two handed tasks
using two CLAWs.
The size of the user’s hand also has an impact on their ability
to effectively use CLAW. The current device has a lengthadjustable part on the index finger arm designed to accommodate different index finger’s lengths. However, our current
implementation requires an additional tool (screwdriver) to
make an adjustment and the amount of length adjustment isstill not enough for everyone, especially children. Future work
should improve this mechanism for the length adjustment,
making it possible to adjust without extra tools. Furthermore,
we should consider designing a smaller version for smaller
hands.

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

Another improvement in the overall quality of the device
would be to use different manufacturing and plastics in the device body. While our current prototype consists of 3D printed
parts due to its rapid prototyping nature, those parts are toobrittle and can break if the device falls or is mishandled (acommon problem in VR where the user does not see the device
while wearing a headset). We are eager to try ABS material
with a FDM printer or injection molding in future iterations.
The CLAW is designed to be a multi-purpose haptic device,
but there are still aspects of the haptic interaction with thehand that it does not render. For example, future work should
consider adding an extension to create the sensation of slip
and friction (similar to the fingertip haptic devices proposedby [9, 33]. We can adapt such a skin stretch mechanism in
combination with the voice coil actuator for a more convincing
shear force rendering. For rendering kinesthetic sensation toarms, we can also adapt an asymmetric vibration mechanism or
actuated gimbals inside the controller as some handheld haptic
devices have demonstrated [3, 35]. Another consideration
would be to incorporate Peltier elements to render heat/coldsensations at the fingertips or palm.
Finally, CLAW currently depends on a relatively high-costbrushless servo motor ( $150), to enable very-quiet, fast, hightorque interactions. We are inspired by the recent prototypes[10, 11] that explored the use of active braking in facilitating effective grasping manipulations. Incorporating a brake,
instead of a servo, might offer a path towards cheaper, more
power efficient implementation which would also make iteasier to commercialize the device.
CONCLUSION

In this paper, we present the design of a novel haptic controller,
named CLAW, that augments a typical VR handheld controller
functionality with force feedback and actuated movement to
the index finger. The primary design principle of our controller
is to provide a multi-purpose controller that contains both the
expected functionality of a VR controller (i.e., buttons, 6DOF
movement control, thumb joysticks, trigger) and enables avariety of force and tactile renderings for the most commonly
expected hand interactions: grasping, touching, and triggering.
We also contribute a way to switch between haptic modes,
by sensing the differences in the user’s grasp as well as thesituational context of the virtual scene. Our user evaluations
show that CLAW is highly effective in a variety of interactions
with participants reporting high scores on realism, control, andmanipulation ability, as well as low error rates when switching
haptic modes. We hope that our CLAW controller inspires a
new generation of VR handheld controllers that provide higher
fidelity haptics (beyond vibro-tactile) on a single compact
multi-purpose device.
ACKNOWLEDGMENTS

This work was part of the haptic intern project at Microsoft
Research. We would like to thank Mar Gonzalez-Franco,
Ed Cutrell, Andy Wilson, Merrie Morris, Christopher Berger,
Evan Strasnick, Eric Whitmire, and Yuhang Zhao for insightful
conversations.

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Paper 654

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

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