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Passive Haptics: Variable Asperity
by Using Damping Brake of DC Motor
Takumu Okada, Shogo Okamoto, and Yoji Yamada
Department of Mechanical Systems Engineering
Nagoya University, Japan(b)(a)

Abstract—A passive haptic interface that is inherently safe andstable and can present variable virtual asperity was developed.
This interface delivers passive resistance force by switchingon and off the damping brake of a DC motor without being
driven. We conducted identification experiments for six levelsof asperity stimuli to investigate human perceptual properties
against passive asperity. Variable asperity was found to be
successfully discriminated into several levels at approximately70%. Furthermore, information transfer was calculated to be
1.60 bits.

RCrankII. PASSIVE HAPTIC INTERFACE USING DAMPING BRAKE
OF DC MOTOR
Figure 1(a) shows a photograph of the haptic interface basedon the damping brake of a DC motor. A DC motor wasfixed on an aluminum frame and a crank with a handle wasconnected to its output shaft. Although this device uses a DCmotor, it is not driven. The device uses only the brake forceof the damping brake of the DC motor. The device presented
force sensation passively by controlling the ON/OFF process

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Computer-controlledswitch (TOSHIBA,
TB6643KQ, H bridge)

Handle

MDC motor(c)BrakeONI. I NTRODUCTION
Haptic interfaces, wherein the hardware itself comprises
passive elements, have the advantage of inherent safety, energy
efficiency, and low-cost. Such haptic interfaces have beendeveloped thus far [1], and are expected to be applied in
virtual reality systems, which may be used by a number ofunspecified users. We developed a passive haptic interface
based on the damping brake of a DC motor [2], [3]. Thedevice can present surface asperity using the resistive forceof the damping brake. The asperity is varied by changing theintervals of presentation of the brakes. Applications of suchasperity stimuli include force feedback for steering wheels
to convey information about road conditions to drivers andforce feedback for dial controllers to adjust the volume.
Thus far, the characteristics of human perception of asperity
stimuli presented by a periodic resistance force have been
investigated [4]. In the research, the main purpose was tospecify the parameters that influence the perception of asperity. On the other hand, discrimination characteristics ofstimuli are important for accurate information transmission.
However, identification experiments or the investigation of
the discrimination thresholds of asperity stimuli presented bypassive haptic interfaces have not been conducted. In thisstudy, we performed an identification experiment for six levelsof asperity stimuli to investigate the utility of passive asperitystimuli as tools for information transmission.

LGeared DC Motor (RE-40, Maxon
motor, Reduction ratio: 12)

OFFAngle

Fig. 1. Haptic interface based on the damping brake of a DC motor. (a)Overview of the device. (b) Short circuit of a DC motor controlled by switch.
(c) Switch operation for presenting asperity stimuli.

of the damping brake while an operator rotated the crank
attached to the motor. The ON/OFF process of the damping
brake was controlled using the motor driver (See Fig. 1(b)),
and the rotation angle of the motor was measured by anencoder mounted on the motor. As shown in Fig. 1(c), byswitching the ON/OFF process of the brake at regular angularintervals, the braking and release of the operator’s hand wasrepeated, and as a result, the operator perceived virtual asperity
from the periodic force.
III. E XPERIMENT
We conducted an identification experiment for six levelsof asperity stimuli to investigate the discriminative ability ofhumans against the passive asperity stimuli.
A. Stimuli
Six levels of asperity stimuli were from S1 , which was thefinest stimulus, to S6 , which was the coarsest stimulus. As
the change in spatial frequency is dominant in the perceptionof asperity [4], we expressed different asperities by changing
the spatial frequency of the ON/OFF process of the brake.
Although our haptic interface can switch the ON/OFF processof the brake finely, such stimulus is too fine to be recognized
as an asperity. Further, very long intervals of the brakes
were not perceived as an asperity stimulus. Then, we decided
the minimum and maximum intervals of brakes that can beperceived as asperity. The ON/OFF process of the brakes
was repeated at intervals of 0.15 deg in S1 , and 7.3 deg in318

2017 IEEE 6th Global Conference on Consumer Electronics (GCCE 2017)

IT indicates the quantity of information transmitted by thereceived stimuli. The average IT is given by)(∑P (Si | Rj ),(1)
IT =P (Si , Rj ) log2
P (Si )
i,j

TABLE IC ORRECT ANSWER RATE AND INFORMATION TRANSFER .

Participants
Correct answer rate (%)
Information transfer (bit)

A
68.3
1.58

B
65.8
1.32

C
73.3
1.68

D
69.2
1.55

E
81.7
1.88

Ave.
71.7
1.60

where P (Si ) is the occurrence probability of Si , P (Si , Rj )is the joint probability of Si and Rj and P (Si | Rj ) is theconditional probability of Si given Rj . The average IT value
for all participants was 1.60 bits.

TABLE IIAVERAGE CONFUSION MATRIX .

S1S2S3S4S5S6R1
0.67
0.04
0.02
000R20.33
0.5
0.18
000R300.45
0.61
0.11
0.01
0R400.01
0.18
0.81
0.25
0.02

R5000.01
0.08
0.71
0.14

R600000.03
0.84

E. Discussion

S6 . We designed the remaining stimuli, S2 –S5 , as the spatial
frequency of the ON/OFF process of the brakes increased
linearly on the logarithmic axis from S1 to S6 ; that is, as thenumber of ON/OFF processes per rotation angle decreased inan equal ratio from S1 to S6 .
B. Participants
The participants were five volunteers. Auditory cues weremasked by headphones playing a pink noise during the experiment. The participants experienced asperity by rotating thecrank at any speed.

As the average IT value for asperity stimuli was 1.60 bits,
our device can present at least three levels of identifiable
asperity: 21.60 ∼ 3.0. It is known that IT values depend on theexperimental design [5]. Especially, the number of stimuli usedin the experiment influences the IT values. The less numberof stimuli may result in greater IT values. There may be room
for increasing the IT value by rearranging the experimental
conditions in terms of the number of stimuli.
IV. C ONCLUSION
We investigated the characteristics of the human perceptionof asperity stimuli passively presented by the haptic interface.
The average correct answer rate of identification experiments
for six levels of asperity stimuli was 70%. Furthermore, theaverage IT value was 1.60 bits; that is, the device can present
at least three levels of identifiable asperity. From the above,
we can use the asperity stimuli presented by a passive haptic
interface as tools for information transmission.

C. Task

ACKNOWLEDGMENTS

The participants performed the identification task for six
levels of asperity stimuli. They identified the asperity of thestimulus by stating numbers 1–6. For each participant, the total
number of trials was 120, including 20 trials for each of thesix stimuli. We divided the experiment into 4 sessions; eachhad 30 trials, including 5 trials for each stimulus. The orderof presented stimuli was randomized in each session. Before
the first session, we conducted a 3 min training session for theparticipants to memorize each stimulus. In this session, theycould experience the six types of stimuli as many times as
they desired. Similarly, we also had a 1 min training session
before the other sessions. Breaks of several minutes were given
between each session.
D. Results
Table I indicates the correct answer rates for each participant. The correct answer rate was the highest for participant
E with 81.7%, lowest for participant B with 65.8%, andthe average rate was 71.7%. The experimental results weresummarized in a 6 × 6 confusion matrix about the stimuli(S1 –S6 ) and the responses (R1 –R6 ). The average answer ratesof the five participants are presented in Table II. Diagonalelements (Si , Ri ) indicate the probabilities that responses
were coincident with the presented stimuli. We calculated
the information transfer (IT) from the confusion matrix. The

978-1-5090-4045-2/17/$31.00 ©2017 IEEE

This study was in part supported by ImPACT (ToughRobotics Challenge) and JSPS Kakenhi (15H05923).
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[3] Takumu Okada, Shogo Okamoto, and Yoji Yamada. Passive haptics:
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