Virtual Reality
https://doi.org/10.1007/s10055-020-00452-8

ORIGINAL ARTICLE

Passive haptics: greater impact presented by pulsive damping brakeof DC motor and physical indices for perceived impact
Takumu Okada1 · Shogo Okamoto1· Yoji Yamada1

Received: 25 April 2019 / Accepted: 4 June 2020
© Springer-Verlag London Ltd., part of Springer Nature 2020

Abstract
This study investigated the perceptual characteristics of pulsive brakes presented by passive haptic interfaces. A passivetype haptic interface based on the damping brake of a DC motor was used to generate impact; this has merits of inherent
safety and energy efficiency. This haptic interface expresses impacts by resisting the operator’s hand via the resistive force
generated by a damping brake. In terms of impulse or momentum, maximum impact was achieved by continuously operating the damping brake after colliding with a virtual object. We found that instantaneous release of the brake immediately
after collision increases the perceived impact. We computed several physical indices associated with the force against thehand as well as the hand velocity and investigated their relationships with the perceived magnitudes of the impacts. A highcorrelation was found between the absolute change ratio of the hand velocity and the perceived impact, which suggests that
instantaneously releasing the brake is effective in terms of impact perception. Our findings indicate that the performance of
passive haptic interfaces can extend physical limits, and the range of applications can be expanded by incorporating humanperceptual characteristics.
Keywords Passive haptic interface · Impact perception · Damping brake

1 Introduction
Haptic interfaces, in which the system is passively controlled
in accordance to an external force, have been investigated bymany researchers with a focus on high inherent safety andenergy efficiency (Goswami et al. 1990; Davis and Book
1997; Scilingo et al. 2003). As most objects surrounding
us are static, haptic interactions with them can be achievedby using reactive resistances. For instance, surface texture
can be expressed using a passive haptic interface that features resistive force (Minsky et al. 1990; Okada et al. 2017,2018). Passive force interfaces are promoted as safe andstable force-feedback devices for wearable robots or applications developed for untrained users (Hirata et al. 2007;
Koyama et al. 2002; Winter and Bouzit 2007; Kikuchi et al.
2009; Asbeck et al. 2015; Ohashi et al. 2017). In the present
manuscript, haptic systems that reduce the kinetic energy of
the user are termed as passive haptic systems, regardless of* Shogo Okamoto
shogo.okamoto@mae.nagoya‑u.ac.jp
1

Department of Mechanical systems engineering, Nagoya
University, Furo‑cho, Chikusa‑ku, Nagoya, Japanthe type of mechanical elements or actuators used to produce
forces.
However, because passive haptic interfaces do not generate any energy in order to apply it against the operator, it isdifficult to model impact-based feedback, which requires alarge output force. This may become problematic when passive haptic interfaces are used to generate feedback forces
for teleoperation robotics or virtual reality systems, in which
dealing with information about contacts or collisions with anobject (Lim et al. 2007; Seth et al. 2011; Hachisu and Kajimoto 2017; Wu et al. 2017; Culbertson and Kuchenbecker2017) is important. Using active haptic interfaces is effective
to express impact, provide a large impulsive force to an operator (Vander Poorten and Yokokohji 2007; Constantinescu
et al. 2005), or rapidly increase the reactive force (Lawrence
et al. 2000; Han and Choi 2010; Ikeda and Hasegawa 2009).
It is believed that the physically maximum impact in passive
haptic interfaces can be achieved by initiating the maximum
brake or resistive force at the moment of collision. However,this method is not necessarily the best for presenting a perceptually maximum impact.
In a previous study, we devised a passive haptic interface
based on the damping brake of a DC motor (Okada et al.

13Vol.:(0123456789)

Virtual Reality

2016a, b). For this haptic interface, physically maximum
impact is achieved by the continuous function of the damping
brake following collision with a virtual object. We previously
empirically demonstrated that a brake with preceding pulsive
brakes increases the operator’s perception of impact (Okada
et al. 2016a, b). However, only two types of pulsive brakes
were tested therein, and the mechanism of the perception was
not discussed. In this study, to determine the kind of pulsivebrake operation that can evoke enhanced perceived impact, we
compared the physically maximum brake and seven types ofpulsive brakes using a passive haptic interface based on a DC
motor. Furthermore, to investigate the link between physicalstimuli and impact perception, we calculated some physicalquantities from contact force and hand velocity at the momentof collision and analyzed the correlation between the physicalquantities and the perceived impact.
Pulsive brakes generate high-frequency components inpresenting the impact. Such high-frequency force or vibrotactile feedback superimposed upon quasi-static reactive forces,
following Hooke’s law, delivers more accurate information
regarding the colliding virtual objects in terms of the type ofmaterial and stiffness (Wellman and Howe 1995; Okamura
et al. 2001; Hwang et al. 2004; Kuchenbecker et al. 2006).
For instance, Hwang et al. (2004) demonstrated that transient
reactive forces that nullified the momentum of a contactor as
quickly as possible increased physical and perceived stiffness
compared with a wall, simply represented by Hooke’s law.
Vibrotactile actuators that produce high-frequency components more easily than force displays are also effective forthe same purpose (Wellman and Howe 1995; McMahan andKuchenbecker 2009). The present study realizes similar principles by using a passive method, whereas the earlier studies
used active force or vibrotactile displays.
A promising application of the present study is, for example, a user interface for assistive robots such as a walker, i.e.,a hand-guided four-wheeled cart for those with locomotive
disabilities. Some of the pre-existing robots (e.g., RT. Works,
https​://www.rtwor​ks.co.jp/, 1, Dec., 2019) are equipped withelectromagnetic motors to not only counteract the self-weightof a walker robot, but also prevent abrupt acceleration, among
other purposes. In addition to these functions, passive haptic
forces can be utilized to guide and navigate the walker’s user
via a combination of impact and other stimuli, including virtual bumps or friction on the road. Another application of such
systems is in gaming and entertainment systems, for which theinherent safety shall be realized.

13

R

L
M

Computercontrolled switch

Fig. 1  Computer-controlled short circuit of the DC motor. Adaptedfrom Okada et al. (2016a, b)

2 Apparatus: passive haptic interface basedon damping brake of a DC motor
2.1 Principle: damping brake of a DC motor
We utilized the damping brake of a DC motor as the passive
element of the haptic interface (Okada et al. 2016a, b). The
damping brake’s function is activated in a DC motor when
its terminals are shorted. Figure 1 shows a shorted circuitof a DC motor with a computer-controlled switch. R and L
are the resistance and inductance of the circuit, respectively.
When the circuit is shorted by the switch, while the motor isrotating, a brake torque is generated by the back electromotive force of the motor. The relationship between the brake
torque 𝜏(t) and the rotation speed of the motor 𝜔(t) is basedon Kirchhoff’s law and is given as follows:

L d𝜏(t)K2+ 𝜏(t) = − 𝜔(t),
R dt
R

(1)

where K is the torque constant of the motor. Provided
that the inductance L is negligible, the brake torque 𝜏(t)is proportional to the rotation speed 𝜔(t), in other words,
the damping brake of the DC motor functions as a viscous
resistance. This effect can be used, for example, to improve
the system passivity to avoid undesired oscillation (Mehling
et al. 2005).

2.2 Passive haptic interface
Figure 2 shows the passive haptic interface that uses thedamping brake of a DC motor. The DC motor (RE-40,
Maxon motor, Switzerland, reduction ratio: 12, inductance: 0.025 mH, resistance: 0.115 Ω , torque constant:
16.4 mNm/A) was vertically fixed on the frame, and its
output shaft was connected to a crank with a handle. Shortcircuiting of the motor was controlled by a mechanical relay(G6L, Omron, Japan, resistance: less than 0.1 Ω). The rotation angle of the motor was measured using a 1024-pulse
optical encoder. The electrical time constant of the motor

Virtual Reality(a)

Virtual object expressedby a damping element

Crank

Geared DC Motor

3-axial force sensor
on the crankFig. 2  Passive haptic interface based on the damping brake of a DC
motor. Adapted from Okada et al. (2016a, b)

DC motor(Brake off)

Brake

(b)

(c)

On

Off

0
Time
Stepwise brake

Force at hand: f/Hand velocity: v

HandleVelocity

Resistive
force

0

0
Time
Change in force/velocity

Fig. 4  Stepwise brake that achieves physically maximum impact.
a Process of collision with a virtual object expressed by a damping
element. b Operation of the stepwise brake. c Change in force andvelocity caused by the stepwise brake. t = 0 signifies the time at the
moment of impact

DC motor(Brake on)

Top view of the device
Fig. 3  Presentation of impact using the damping brake of a DC
motor. Adapted from Okada et al. (2016a, b)

was 0.21 ms, and the responsivity of the relay was 1.1 ms.
The total time of these is the delay from the initiation of
stimuli to the abrupt changes in force and velocity. We did
not perceive any effects from contact chattering. An operator
rotating the crank from front-to-back experiences an impact
at an angle of 90 deg, as shown in Fig. 3. At that point, thecircuit is shorted, and the motor applies a brake abruptly.
The force applied against the hand of the operator wasmeasured using a force sensor (USL06-H5, Tec Gihan,
Japan) between the handle and crank. We used the force
component along the direction of hand motion for the analysis. The linear drift of the sensor outputs was removed byusing the sensor values immediately before and after eachtrial.

2.3 Physically maximum impact
When presenting impacts by using passive haptic interfaces, resistive force is generated against the movement of
the operator (see Fig. 4a). Therefore, a physically maximum impact is achieved by continuously resisting the

movement of the operator; more specifically, by increasing the impulse and thus decreasing the momentum of theoperator’s hand as much as possible. Thus, for presenting
impacts based on the damping brake of the DC motor, weused a stepwise brake that continuously activates at the
moment of collision, which is the best method to present
physically maximum impact, as shown in Fig. 4b. Owing
to this brake, the force applied on the operator and thehand velocity continue to increase and decrease after thecollision (see Fig. 4c), resulting in maximization of theimpulse or derived change of momentum.

3 Methods
We previously discovered that a pulsive brake that momentarily releases the brake immediately after a collision candeliver greater impact perception than a stepwise brake(Okada et al. 2016a, b). As shown in Fig. 5b, releasing
the brake leads to a transient drop of resistive force and atransient rise of hand velocity. Thus, in the pulsive brake,
the impulse applied toward the operator is smaller thanthat in the stepwise brake. This suggests that the impact
perception is disparate from physical aspects. In our previous experiment, only two types of pulsive brakes wereused and the mechanism of impact perception was not discussed. In this study, we varied the number and durationof the pulse brakes as well as the release time of the brake,
and quantitatively compared the perceived impact between
these pulsive brakes.

13Virtual Reality(a)Pulse brake

Brake

On

Release of the brake
Off

0
Time
Operation of pulsive brake(b)
Force at hand: f/Hand velocity: v

Force at hand: f/Hand velocity: v

Velocity
Increase
Force

0

Decrease

0Time

Velocity
Increase in thedecrease rate
Decrease in theincrease rate

Force

0

0

Time

Change in force/velocity
Fig. 5  Pulsive brake that instantaneously releases the brake immediately after impact. a Braking operation for the pulsive brake. b
Change in force and velocity caused by the pulsive brake. Releasingof the brake causes a decrease in force or increase in velocity (leftside) and reduction in the rate of increase of force or increase in therate of decrease in velocity (right side)

On

Off

On

Off

0

Time (ms)080(b) (e) 1-pulse type stimuli
On

OffTime (ms)80(f), (g) 2-pulse type stimuli
Brake

Brake

(h) 3-pulse type stimulus
Brake

Brake

(a) Stepwise stimulus

On

Time (ms)

80

(b)

0

Time (ms)80

Stimuli

Stimuli
3

3

(f)

3

(g)

4

(c)

5

5

(d)

6

6

(e)

9

3

Fig. 6  Brake operations for eight types of impact stimuli (a)–(h)3.1 Impact stimuli
Eight types of impact stimuli were compared in this experiment. Figure 6 shows the brake operations for each impact
stimulus. Stimulus (a) was a stepwise brake that continuously activated the brake for 80 ms, which led to the13

3.2 Participants
Thirteen naive participants (11 males and 2 females; allright-handed) took part in the experiment. We obtained
informed consent from all participants before theexperiment.

3.3 Experimental procedureOff0physically maximum impact. Stimuli (b)–(h) included one ormore pulse brakes generated by releasing of the brake before
the main stepwise brake. Stimuli (b)–(e) included one pulsive brake. For instance, in stimulus (b), the first brake duration lasted 3 ms, followed by releasing of the brake for 3 ms,
and then presentation of the main brake. For the stimuli that
had more than two pulse brakes, presentation and release of
the brake were repeated at a certain period. Stimuli (f) and(g) had two pulse brakes each presented with a period of
6 ms and 8 ms. Stimulus (h) had three pulse brakes presented
with a period of 5 ms. We unified the total duration of every
stimulus to 80 ms.
The duration and intervals of the pulse brakes weredevised from trial and error. When the release time of thebrake is overly long, either impact perception occurs multiple times or the system vibrates. Such stimuli are qualitatively different from a step-wise brake. Hence, we limited
the release time of the brake to less than 6 ms, with whichsuch sensations were not evoked. In addition, when the timefrom the start of the stimulus to the main brake is overly
long, such stimuli provide an apparently different sensationfrom the impact,1 which is an instantaneous event caused
by a collision with an object. Thus, we regulated the timeby the main brake to less than 16 ms. The seven types ofpulsive brakes were designed to show variation under theseregulations. Note that these specifications depend on theexperimental apparatus, as discussed in Sect. 7.4.

In the experiment, we investigated the impact stimuli thatdelivered large impact perception based on Scheffe’s paired
comparison method (Scheffe 1952). In each trial, the participants compared a pair of stimuli (stimulus X, stimulusY) in terms of the magnitude of perceived impact on fivescales, from −2 (larger in X) to 2 (larger in Y). The participants could experience each stimulus as many times as they
desired during the experiment. Typically, they experienced
each stimulus three or four times before making a judgment.
The contact force and hand velocity during the last impact
experienced for each stimulus were automatically recorded
and used for subsequent analysis. Each participant evaluated1

For example, some feel a moderate collision with a deformableobject.

Virtual Reality
8 P7 = 56 pairs, which is the number of all possible combina-

tions of the stimuli in the case where the order of the stimuli(which stimulus is labeled as X (or Y)) is considered. We
divided the experiment into two sessions, with both sessions
having 28 paired stimuli randomly presented. A break of afew minutes was given between sessions.
Through preliminary experiments, it was found that when
the collision speed of the crank at the 90 deg point was slow,
the difference between the stimuli was vague. Therefore,
in the present experiment, we regulated the collision speedof the crank to faster than 10 rad/s and did not start thestimuli when the collision speed fell below the limit. Notethat the minimum collision speed for discerning the stimuli
is unknown. Participants had a few minutes to be familiar
with this speed condition beforehand. The average collision
speed was in the range 10.8–18.3 rad/s for all participants inthe main tasks. The stimuli were presented virtually every
time despite this regulation.

4 Physical indices for impact perception
In this study, we also aimed to investigate a physical index
consistent with the impact perception. For this purpose, wemeasured contact force and hand velocity during a collision and extracted some feature physical quantities from thedata. Then, we investigated correlations between the physical quantities and the impact perception obtained in the psychophysical experiment.

4.1 Proposition: absolute change of resistive force
or velocity affects impact perception
We propose, as an index of impact perception, physicalquantities relating to the change in contact force and thechange in hand velocity just after the impact on the basis of
the studies about presentation of hardness by using haptic
interfaces.
In order to improve the sensation of hardness perceived
from virtual objects by using haptic interfaces wherein
the output force is limited, increasing the rate of the force
output just after contact is effective (Lawrence et al. 2000;
Han and Choi 2010; Hauser and Gerling 2018). In thecase of impact perceived from collision with an object,
it is also expected that the greater the force applied in ashort period, the greater the perceived impact. For presenting impact using a brake, continuous operation of thebrake after the collision maximizes the increasing rate of
the force. Thus, the stepwise brake appears to be the best
method to deliver large impacts. In the previous experiment (Okada et al. 2016a, b), however, a pulsive brake,
in which resistive force momentarily decreased after thecollision, delivered a larger perceived impact than the
stepwise brake. If the enhancement of impact perception is
not caused by increasing the rate of force itself but ratherchanging rate of force, the force does not have to necessarily continue to increase. Hence, we investigated whether
the absolute change of force, which includes decreasing
the force too, affects perceived impact. This cumulative
absolute change of force is expressed as follows:J|ḟ | (Δt) =∫0

Δt|ḟ (t)|dt.

(2)

Here, f(t) represents the contact force at hand after t ms froma collision. The unit of Δt is ms.
Applying a large momentum to the operator at the
moment of collision is effective for presenting a large
impact, by using active-type haptic interfaces (Vander
Poorten and Yokokohji 2007). On the other hand, passive
haptic interfaces present impact by using resistive force
and deriving momentum from the operator because theycannot generate momentum independently. Assumingthat deriving the operator’s momentum in a short period
induces great impact perception, continuously functioning
the stepwise brake from the moment of collision shouldpresent the maximum impact. This is because the decreasing rate of hand velocity, i.e., that of momentum of thehand, becomes maximum for a stepwise brake as shown in
Fig. 4. However, a pulsive brake, wherein the hand velocity momentarily increases just after a collision, as shownin Fig. 5b (left), induced a larger impact than the stepwise
brake in our previous experiment (Okada et al. 2016a, b).
This result indicates the possibility that an abrupt increaseof hand velocity caused by instantaneous release of thebrake contributes to the impact perception. Once the brakeis instantaneously released just after a collision, the operator’s hand is accelerated by their own output force being
generated against the resistive force of the brake. Hence,passive haptic interfaces can also realize the process, inwhich momentum abruptly increases, by using the pulsive
brake. Then, we hypothesize that the perceived impact isenhanced by the following phenomena: an abrupt decreasein momentum caused by the resisting hand movement, andan abrupt increase of momentum owing to the momentary
release of the resistive force. Based on this hypothesis, we
propose, as an index of perceived impact, the integral ofabsolute change of the hand velocity immediately after thecollision, which is defined as follows:

J|v|̇ (Δt) =∫0

Δt|v(t)|dt.
̇(3)Here, v(t) represents the hand velocity after t ms following
the collision.

13

Virtual Reality

4.2 Physical indices for magnitude of impact

Stepwise

We investigated the following physical quantities representing the physical magnitude of impact as candidates for aphysical index consistent with the human impact perceptionin addition to J|ḟ | (Δt) and J|v|̇ (Δt).
–
–
–
–
–

Small
impact01-pulse type2-pulse type3-pulse type(h) (f) (g) (b) (d) (c)(a)(e)
1

2

Large
impact3

Fig. 7  Psychophysical scale of the impact stimuli

Maximum force value, fmax
Time taken for the force to reach its maximum value, tmax
Minimum velocity value, vmin
Time taken for velocity to reach its minimum value, tmin
Resistive force after Δt ms from the collision, f (Δt)(increase of force)– Decrease in velocity during Δt ms from the collision,
vd (Δt)– Impulse during Δt ms from the collision, I(Δt)– Energy consumption during Δt ms from the collision,E(Δt)

that the collision speeds for stimuli (f) (11.5 ± 3.0 rad/s,
mean and standard deviation) and (h) (11.4 ± 2.8 rad/s)
tended to be greater than the others. As described later, these
two types of stimuli were perceived as weaker than the others. Hence, for clearly feeling the impacts, the participants
might have collided with these stimuli at speeds faster thanthose for the other types of stimuli. The stimuli (a) (10.8 ±1.7 rad/s) and (e) (10.8 ± 1.8 rad/s) recorded the smallest
collision speeds.

vd (Δt), I(Δt), and E(Δt) are defined as follows, respectively:

5.2 Perceived impact

vd (Δt) = v(0) − v(Δt),I(Δt) =

E(Δt) =

∫0

(4)

Δt

∫0

f (t)dt,(5)Δtf (t)v(t)dt.

(6)

In the event that the contact force monotonically increases
during Δt ms from the collision, I(Δt) is an identical index toJ|ḟ | (Δt). The decrease in momentum during Δt ms from themoment of collision can be expressed by using hand mass
m and hand velocity v as follows:m(v(0) − v(Δt)) = mvd (Δt).

(7)

Therefore, given that the hand mass is constant, the decreasein velocity vd (Δt) is an qualitatively identical index to thedecrease in momentum.

5 Results
5.1 Collision speed for each stimulus
The mean collision speed for the eight types of stimuli was
11.2 rad/s; it did not statistically differ among the stimuli
types (ANOVA, F(7, 96) = 1.89, p = 0.080). For this computation, we computed the mean collision speeds for individual participants, i.e., 104 (13 participants × 8 stimuli) meanvalues were used for the statistic test. Nonetheless, we found

13

Following Scheffe’s method (Scheffe 1952), the scores ofstimuli were calculated within individual participants. For
example, in a trial, when stimulus X was judged substantiallygreater than stimulus Y, 2 points were assigned to stimulus
X. The scores were then accumulated among trials; basedon these scores, the stimuli were then ranked. In the case ofa tie rank, the average ranks were assigned. Table 1 shows
the ranking of the scores of stimuli for each participant.
Stimulus (e) had the highest total rank, followed by stimulus (c) as the second highest, and stepwise stimulus (a) as
the third highest. The ranks of the eight types of stimuli differed significantly (Friedman test, 𝜒 2 (7) = 58.76, p < 0.01).
The question is whether stimuli (c) and (e) were felt greaterthan stimulus (a). Hence, as post hoc tests without p value
correction, we compared stimuli (a) and (e) and stimuli (a)and (c), with the result showing that there was a significant difference between stimuli (a) and (e) (Friedman test,𝜒 2 (1) = 8.33, p = 0.0038), whereas there was not a difference between stimuli (a) and (c) ( 𝜒 2 (1) = 1.0 , p = 0.31).
This indicates that stimulus (e) delivered a larger impact
perception than the stepwise brake (a), which achieved thephysically maximum impact.
In the later analysis, to compare some physical indices ofbrake stimuli, which are ratio scales and impact perception,
we converted the above results into psychophysical scales(Scheffe 1952), which can be treated as interval scales. Figure 7 shows the psychophysical scales of the stimuli obtained
from the results for all participants. The larger the psychophysical scale of a stimulus, the greater the impact perception the stimulus presented. Stimulus (e) had a larger psychophysical scale than the stepwise stimulus (a). There wasa tendency for the lower number of pulse brakes deliveredVirtual Reality
Participants

A
B
C
D
E
F
G
H
I
J
K
L
M
Total

(a) stepwise

(b)

(c)

(d)

(e)

(f)

(g)

(h)

5
4
5
5
1.5
3.5
2
3.5
2
1
3
6
3
3

1
6
3
3
4
7.5
4
3.5
4
5
6.5
4
6
5

3
1.5
4
4
1.5
3.5
1
5
1
3
4
2.5
4.5
2

6
3
1.5
1.5
6.5
5
5
2
5
4
2
2.5
4.5
4

4
1.5
1.5
1.5
3
1
3
1
3
2
1
1
1
1

7
8
7
7
6.5
7.5
8
8
8
7
8
5
8
7

2
5
6
6
5
2
6
6
6
6
5
7
2
6

8
7
8
8
8
6
7
7
7
8
6.5
8
7
8

(c) 1-pulse (5 ms)

0

20
10

80
40
Time (ms)0(e) 1-pulse (9 ms)

0

0

0
0

40
80
Time (ms)0(g) 2-pulse (4 ms)
Force at hand (N)

10

40
80
Time (ms)8040Time (ms)(h) 3-pulse
30

20
10
0

0

10

80
40
Time (ms)

30

20

20

0

(f) 2-pulse (3 ms)
Force at hand (N)

10

10

80
40
Time (ms)

30

30
20

20

0

0
0

30
Force at hand (N)10Force at hand (N)20(d) 1-pulse (6 ms)

30

30
Force at hand (N)Force at hand (N)(a)(b) 1-pulse (3 ms)

30

Force at hand (N)Ranking

Force at hand (N)Table 1  Rank of the eight typesof impact stimuli by individual
participants. The total rank atthe bottom was obtained byranking the rank sum among all
participants for each stimulus

20
10
0

0

40
80
Time (ms)04080Time (ms)Fig. 8  Examples of force at the hand for eight types of impact stimuli (a)–(h). Each stimulus started at t = 0. The collision speed in each case
was in the range of 10–11 rad/s. Ten samples are displayed for each stimulus

to result in the greater impact perception. We utilize thesecontinuous scales in after-mentioned correlation analysis.

5.3 Correlation between physical indicesand impact perception
Figures 8 and 9 show, for each stimulus, the examples ofchanges in the contact force at the operator’s hand duringcollision measured by the force sensor and examples ofchanges in the hand velocity during collision calculated
from the measurements recorded by the encoder. t = 0 is thetime at the moment of collision. For stepwise stimulus (a),the contact force abruptly increased immediately after thecollision, and moderately decreased after the force reached
its peak. The hand velocity abruptly decreased immediately after the collision, and then moderately decreased.

13

40
80
Time (ms)(e) 1-pulse (9 ms)

10

5

0

0

40
80
Time (ms)

15

5

0

80
40
Time (ms)0(f) 2-pulse (3 ms)

10

5

0

0

40
80
Time (ms)15(c) 1-pulse (5 ms)

10

5

0

0

40
80
Time (ms)(g) 2-pulse (4 ms)

10

5

0

0

40
80
Time (ms)Angular velocity of hand (rad/s)

0

10

15

15

Angular velocity of hand (rad/s)

0(b) 1-pulse (3 ms)

Angular velocity of hand (rad/s)

515Angular velocity of hand (rad/s)

10Angular velocity of hand (rad/s)

15(a) stepwise

Angular velocity of hand (rad/s)

Angular velocity of hand (rad/s)

15

Angular velocity of hand (rad/s)

Virtual Reality

15(d) 1-pulse (6 ms)

10

5

0

0

80
40
Time (ms)(h) 3-pulse

10

5

0

0

40
80
Time (ms)Fig. 9  Examples of angular velocity of the hand for eight types of impact stimuli (a)–(h). Each stimulus started at t = 0. The collision speed ineach case was in the range of 10–11 rad/s. Ten samples are displayed for each stimulus
Table 2  Correlation coefficients
R between physical indices andpsychophysical scale of the
impact stimuli. Computed by
the samples with the collision
speeds of 10–11 rad/s

fmax

tmax

vmin

tmin

Δt

I(Δt)

E(Δt)

f (Δt)

vd (Δt)J|ḟ | (Δt)

J|v|̇ (Δt)

0.49

0.39

0.09

0.03

5 ms
10 ms
15 ms
20 ms
25 ms
30 ms

0.61
0.62
0.63
0.62
0.61
0.56

0.12
0.37
0.32
0.31
0.26
0.25

0.60
0.70
0.47
0.48
0.28
0.00

0.66
0.84
0.34
0.46
0.00
0.24

0.61
0.76
0.80
0.72
0.50
0.46

0.60
0.91
0.92
0.78
0.60
0.54

For pulsive stimuli, the contact force and the hand velocity
fluctuated because of releases of the brake. The stimuli withthe longer release time of the brake and with the later timingof the first release of the brake tended to show the greaterchanges in force and velocity when the brake was released.
Specifically, for stimuli (c), (d), (e), and (g), decreases in theforce and increases in the velocity were observed when thebrake was released, as shown in Fig. 5b (left). On the other
hand, for stimuli (b), (f), and (h), decreases in the increase
rate of force and increases in the decrease rate of velocity
were observed, as shown in Fig. 5b (right).
The physical quantities introduced in Sects. 4.1 and 4.2
were calculated for the 192 samples (at least 12 repetitions
for each stimulus) that had collision speeds of 10–11 rad/s.
The recorded examples are shown in Figs. 8 and 9 . Notethat the physical indices should not be compared among substantially different collision speeds because of the nature

13

of a damping brake. The physical quantities expressed in
the form of integration were calculated for different integral
values of Δt from 5 ms to 80 ms with 5 ms increments.
Table 2 shows the correlation coefficients between thephysical quantities that were computed using the samplesof 10–11 rad/s and the psychophysical scale of impact perception. For the physical quantities expressed as functionsof Δt , only the results for Δt ≤ 30 ms are shown because
the correlations with the impact perception were small forΔt > 30 ms. The correlation with the impact perception was
the largest for J|v|̇ (15) with R = 0.92 followed by J|v|̇ (10)with 0.91, vd (10) with 0.84 and J|ḟ | (15) with 0.80. Figure 10
shows the relationships between the psychophysical scales
and J|v|̇ (15) and vd (10). The correlation coefficients for fmax,
tmax , vmin , and tmin , relating to the peak values of force orvelocity, were below 0.5. The correlation coefficients forenergy consumption expressed as temporal integration of theVirtual Reality

Decrease of hand velocity (m/s):(a)(a) (e)0.6(g) (d)
(f)

0.4

(c)

0.2
0

(h)

0

(b)

11—13 rad/s
10—11 rad/s
2

1

3

Psychophysical score of impact

Integral value of absolute changeof hand velocity (-):(b)(d) (a)
(b)

400

(e)

(f)

(c)

200
(g)

0

0

6 Follow‑up study(h)

11—13 rad/s
10—11 rad/s

1

2

3

Psychophysical score of impact
Fig. 10  Relationship between the observed psychophysical scale ofthe stimuli and physical indices for perceived impact. a Decrease inhand velocity for Δt = 10 ms: vd (10). b Integral value of absolutechange of hand velocity for 15 ms: J|v|̇ (15). Each dot shows the meanvalue of the index among samples for each stimulus (a–h) with standard deviation. Filled and open circles are for the collision velocitiesof 10–11 rad/s and 11–13 rad/s, respectively. For the computation ofopen circles, 150 samples whose collision speeds were in the range of
11–13 rad/s were used
Table 3  Rank of the seven typesof impact stimuli used in thefollow-up experiment. The total
rank at the bottom was obtainedby ranking the rank sum among
all participants for each stimulus

Participants

A
C
E
F
O
P
Q
R
S
T
Total

product of contact force and hand velocity were all below
0.4.
Among the physical quantities that we investigated, thephysical quantity that showed the largest correlation with
the impact perception was neither the maximum value ofthe force, the impulse, nor the change of momentum; it was
the integral of the absolute change of velocity. Thus, in thecase of presenting impact by using a passive haptic interface,large impact perception might be delivered by momentarily breaking off the resistance just after a collision, whichcauses absolute change of momentum due to the accelerationand deceleration of the hand. The correlation coefficientsfor energy consumption were smaller than those for otherphysical quantities, including force only or velocity only,
indicating that when perceiving impact, we utilize force orvelocity alone, or their linear combination, rather than theinteraction between them.

Stimulus (e) was judged as the greatest impact in the aforementioned experiment. One question is whether there exist
stimuli that present the impact greater than stimulus (e).
Assuming that such stimuli may exist in the vicinity of stimulus (e), we newly prepared for five one-pulse stimuli thatwere close to stimulus (e) of which (t1 , t2 ) = (9, 3). These
stimuli (i)–(m) were (t1 , t2 ) = (9, 5), (11,3), (7,3), (10,4), and(8,2). By using these five types of stimuli, step-wise stimulus(a), and stimulus (e), we performed a paired-comparison test.
The experimental procedures were same as those in Sect. 3.
For this experiment, 10 males (right-handed) were invited.
Four of them participated in the experiments in Sect. 3.
Table 3 shows the ranks assigned by individual partici-

Ranking
(a)

(e)

(i)

(j)

(k)

(l)

(m)

3
6
3
5
6
3
4
2.5
3
3
4

2
1.5
2
1
2
1
3
2.5
4
1.5
2

1
1.5
1
4
3
2
1
4
1
1.5
1

4.5
7
4.5
6
7
5.5
5
6
5.5
7
7

7
4
6
3
1
7
6
7
5.5
4.5
5

4.5
3
4.5
2
4
4
2
1
2
4.5
3

6
5
7
7
5
5.5
4
5
7
6
6

pants. Stimulus (i) exhibited the highest rank, followed by13

Virtual Reality

Decrease of hand velocity (m/s):

(a)
(m)

(a)

0.6
(j)

0.4

(i)

(k)

(l) (e)

0.2
0

0

1

2

Psychophysical score of impact

Integral value of absolute changeof hand velocity (-):

(b)
(i)

(a)

400

(m)

(k)
(e)

(j)

200

0

0

(l)

2
1
Psychophysical score of impact

Fig. 11  Psychophysical scores and physical indices for perceived
impact (mean and standard deviations) in the follow-up study. a
Decrease in hand velocity at t = 10 ms: vd (10). b Integral value ofabsolute change of hand velocity for 15 ms: J|v|(15)̇(e), (l), (a), (k), (j), and (m). Significant differences were
found among the seven types of stimuli ( 𝜒 2 (6) = 36.64 ,p < 0.001, Friedman test). As post hoc tests without p values
correction, we compared step-wise stimulus (a) and stimuli(i), (e), and (l), which were ranked higher than stimulus (a).
The ranks of stimuli (i) and (e) were higher than those forstep-wise stimulus (a) ((i) versus (a): 𝜒 2 (1) = 6.4, p = 0.011,(e) versus (a): 𝜒 2 (1) = 5.4, p = 0.020). Stimuli (l) and (a) did
not exhibit significant differences (𝜒 2 (1) = 0.4 , p = 0.53).
There were no significant differences among stimuli (i), (l),and (e) ((i) versus (l): 𝜒 2 (1) = 3.6, p = 0.058, (i) versus (e):
𝜒 2 (1) = 0, p = 1.00, (l) versus (e): 𝜒 2 (1) = 1.6, p = 0.21).
On the basis of these results and analyses, stimulus (i)was found to present an equally large impact as stimulus(e) and both of them yielded greater impact perception thanstimulus (a). Nonetheless, the stimuli that yield a statisticallygreater impact perception than stimulus (e) were not found.
̇forSimilar to Sect. 5, we computed vd (10) and J( |v|)(15)
these stimuli. For the computation, we used 145 trials, ofwhich collision speeds were 10–11 rad/s including at least
12 stimuli for each type of stimulus. Figure 11 shows therelationships between these two types of indices and the psychophysical scores of seven types of stimuli. Note that these
psychophysical scores are not directly compatible with those

13

computed in Sect. 5 because the stimulus set used in theexperiments were different between two experiments. Unlike
previous experiment, vd (10) did not exhibit a strong correlation with the psychophysical scores ( R = 0.34). vd (10) is
the decrease in the hand velocity at t = 10 ms, and the seventypes of stimuli did not substantially vary except for stimulus(k). This is reasonable because the relies of the brake started
at 7–11 ms for the six types of one-pulse stimuli and they did
not produce substantial differences before 10 ms. In contrast,
J(|v|)
̇ (15) and the psychophysical scores exhibited a higher
correlation ( R = 0.86 ) because it covers the hand velocity
until 15 ms.

7 Discussion
7.1 Similarity between impact and hardness
perception
For presentation of hardness by using active-type haptic
interfaces, a rapid increase of contact force just after a collision is more important than the maximum value of the force(Lawrence et al. 2000; Han and Choi 2010; Hauser and Gerling 2018), which is consistent with the results of the present
study. In this experiment, f(5) and f(10), representing theincrease of the force just after the collision, showed higher
correlations with the impact perception than fmax and tmax ,which pertain to the peak values of the force. Moreover, it isalso true that a decrease in hand velocity, vd (5) and vd (10),immediately after the collision showed higher correlations
with the impact perception than vmin and tmin , which pertain
to the peak values. These results indicate that for presenting
impact with passive haptic interfaces, the change rates offorce and velocity just after a collision are more important
than their peak values.
These results may be related to the time that elapses
before each physical quantity is observed after the collision. Not only in f (Δt) and vd (Δt) , but also in the otherphysical quantities expressed as functions of Δt , there weretrends in which the correlation coefficient was largest for Δt
below 20 ms and was smaller for the greater Δt . On the other
hand, the physical quantities pertaining to the peak valuesof force and velocity that had small correlation coefficientswere recorded after the point in time when some indiceswith high correlation coefficients were recorded. As shownin Figs. 8 and 9 , the peaks of the force were observed typically at around 30 ms from the time of collision and thoseof the velocity were typically observed at around 80 ms inmost trials. Thus, the magnitude of the impact presented
by using a passive haptic interface can be judged using theinformation obtained before the force and the velocity reach
their peaks.

Virtual Reality

7.2 Change of force/velocity versus integralof absolute change of force/velocity
Under the condition that Δt is larger than 5 ms, the correlation with the impact perception was higher for J|ḟ | (Δt)
than for f (Δt), and higher for J|v|̇ (Δt) than for vd (Δt). These
results support the propositions in Sect. 4.1.
f (Δt) and J|ḟ | (Δt) showed differing correlation coefficients especially after 10 ms, because the contact force
turned downward as a result of the pulse brakes for many
pulsive stimuli. Hence, in cases where the force decreases
because of the pulse brake, the absolute change of force
may explain the impact perception more than the increaseof force. This suggests the possibility that not only theincrease in force but also the decrease in force contribute to
the enhancement of impact perception.
Comparing vd (Δt) and J|v|̇ (Δt) in the same manner, these
indices also showed large differences in correlation coefficients after 10 ms when the effects of the pulse brakes
appeared. The high correlations between J|v|̇ (Δt) and theimpact perception also support the same postulate.
Figure 10a, b shows the values of both indices for eachstimulus. The results for vd (10) and J|v|̇ (15) are shown as
they exhibited the largest correlation coefficients with theimpact perception among different Δt values. The decreasein hand velocity vd (10) was the largest for stimulus (a)because continuous functioning of the brake maximizes theimpulse to the hand. For the same reason, the earlier releaseof the brake tended to lead to a smaller vd (10) value. On
the other hand, the integral value of the absolute change of
velocity was the largest for stimulus (e), which induced the
maximum impact perception in the experiment. In Fig. 9,
for step stimulus (a), the hand velocity moderately changed
after the velocity reached its local minimum value around10 ms after the collision; on the other hand, for stimulus (e),abrupt changes in velocity continued owing to pulse brakes,
resulting in the large values of J|v|̇ (15). Moreover, the pulsive
stimuli having more than two pulse brakes (stimuli (f), (g),and (h)) showed little changes in velocity because the timings of the on and off switching of the brake were so quickthat the hand velocity lagged behind.

7.3 What affects perceived impact?
We theorize that J|v|̇ (10) or J|v|̇ (15) , which show high correlations with the impact perception ( R > 0.9 or 0.86),
can be effective indices in stimulus deign for presenting
impact via passive haptic interfaces. However, there is noevidence to conclude that humans actually perceive impact
from these physical quantities. In fact, although stimulus(d) was perceived as having the fourth largest impact in theexperiment, it took the second largest values for J|v|̇ (15) ,next to stimulus (e) in Fig. 10, showing an inconsistency
with the experimental results. Following our intuition, it isalso unlikely that humans judge the magnitude of impact
by using only information for a short period of 10–15 ms
after the collision. In reality, humans may use broader
information in terms of time: in order to judge the magnitude of an impact, humans probably analyze information
until the contact force reaches its peak.
One speculation of the effects of pulsive brakes is thatthe intermittent braking effectively stimulates sensory
organs. Presenting pulse brakes just after the collision,
the muscles of the arm and finger and the tendons extend
or contract with acceleration and deceleration of the hand.
This may result in repeated activations of mechanoreceptors in the flexor and extensor muscles and tendons in ashort period, thus contributing to greater impact perception. Muscle spindles could have responded to braking
pulses used in the experiments considering their response
characteristics (Burke et al. 1976). A possible reason why
the correlation with the impact perception is larger for
J|v|̇ (Δt) than for J|ḟ | (Δt) is that the changes in hand velocity
more directly relate to the extension and contraction of thearm and finger than that in the contact force. Nevertheless,there is no physiological evidence that a rapid change inforce caused such activation of muscles in our study.
Another speculation is based on the fact that actual collision produces high-frequency components. The presentation of high-frequency component at the moment of collision with virtual objects increases the experienced realismor hardness (Wellman and Howe 1995; Okamura et al.
2001; Hwang et al. 2004; Kuchenbecker et al. 2006; Fiene
and Kuchenbecker 2007; Higashi et al. 2016, 2018, 2019;
Culbertson and Kuchenbecker 2017). The neural basis of
this effect is unclear; however, humans know that the natural frequencies of stiffer objects are generally higher andour daily experiences may underlie this effect. Nonetheless, it is difficult to pursue this effect by using our experimental setup. For passive haptic interfaces, the higher thefrequency of the pulse brakes, the smaller the changes inforce and velocity. Consequently, comparing the effects
of different frequency components while maintaining thechanges in force and velocity is difficult. Moreover, somestimuli having a single pulse are not suitable for frequency
analysis. From the above, in order to discuss the effects offrequency of stimuli on the impact perception, we need touse active haptic interfaces and fairly compare different
frequency contents. As the objective of this research is
to investigate the impact perception presented by passive
haptic interfaces, we refrain from discussing the effects ofstimuli frequency in detail here.

13Virtual Reality

7.4 Generality of the study
The generality of the present experiment remains to bestudied. Pulsive brakes can also be generated by actuators
other than DC motors; however, it is not clear whether thesame principle holds for those other actuators and therefore should be investigated in future studies. The temporalcharacteristics of stimuli are mainly dependent on the time
constant and responsivity of the motor used in the experimental apparatus. As aforementioned, the pulsive brakes
included a delay of 1–2 ms. Hence, integral intervals of
the indices must be changed as needed when using products that show quite different response performances fromthat of the products we used. In fact, we obtained different
experimental results in previous experiments with a different
experimental setup (Okada et al. 2016a, b). In the experiment, a 2-pulse type stimulus was perceived as having alarger impact than stepwise and 1-pulse type stimuli. In theexperiment, a FET (TB6643KQ, TOSHIBA), showing very
high responsivity (below 1 µs), was used as the switch of
the short circuit in place of a relay2. Furthermore, as aforementioned, the impact is not effectively experienced when
the collision speed is small, considering the principle of thedamping brake. Hence, the hardware specification including
the length of links and gear friction should have some effecton the perception of pulsive brakes. Large gear frictions may
mask the damping brake and diminish the effect of the passive brakes.
In summary, when using different experimental setups
to present impacts passively, the parameters of the impact
stimuli should be designed on the basis of the physical indices of which integral intervals are modified depending on
the responsivity performance of the setup.

8 Conclusion
In this study, we investigated the perceptual characteristics
of pulsive brakes presented by passive haptic interfaces. The
physical index that showed a high correlation with the perceived impact was the integral value of the absolute changeof hand velocity, rather than the maximum value of the contact force or its impulse. The rapid change in the hand velocity after the collision was substantially related to the impact
perception. This result shows that the temporal increaseof hand velocity as well as its decrease caused by a pulse
brake just after the collision can also improve the impact
perception. The abilities of passive haptic interfaces canbe improved more than its physical limits by incorporating
human perceptual characteristics with passively presented
2

We stopped using a FET because of its leak current.

13

stimuli. Our findings contribute to the expansion of the range
of applications for passive haptic interfaces, which are inherently safe and energy-conservative.
Acknowledgements This study was in part supported by ImPACT(Tough Robotics Challenge) and MEXT Kakenhi (15H05923).

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