MotorBeat Acoustic Communication for Home Appliances via Variable Pulse Width Modulation

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Proc. ACM Interact. Mob. Wearable Ubiquitous Technol., Vol. 6, No. 1, Article 31. Publication date: March 2022.
MotorBeat: Acoustic Communication for Home Appliances via
Variable Pulse Width Modulation
WEIGUO WANG,
Tsinghua University, China
JINMING LI,
Tsinghua University, China
Y
U
AN
HE
∗
,
T
singhua
Univ
ersity,
China
XIUZHEN GUO,
Tsinghua University, China
YUNHAO LIU,
Tsinghua University, China
More and more home appliances are now connected to the Internet, thus enabling various smart home applications. However,
a critical problem that may impede the further development of smart home is overlooked: Small appliances account for the
majority of home appliances, but they receive little attention and most of them are cut off from the Internet. To fill this gap,
we propose MotorBeat, an acoustic communication approach that connects small appliances to a smart speaker. Our key idea
is to exploit direct current (DC) motors, which are common components of small appliances, to transmit acoustic messages.
We design a novel scheme named Variable Pulse Width Modulation (V-PWM) to drive DC motors. MotorBeat achieves the
following 3C goals: (1)
Comfortable
to hear, (2)
Compatible
with multiple motor modes, and (3)
Concurrent
transmission. We
implement MotorBeat with commercial devices and evaluate its performance on three small appliances and ten DC motors.
The results show that the communication range can be up to 10 m.
CCS Concepts:
•
Human-centered computing
→
Ubiquitous and mobile computing systems and tools
.
Additional Key Words and Phrases: Acoustic Communication, Electric Motor, Smart Speaker, Home Appliance
ACM Reference Format:
Weiguo Wang, Jinming Li, Yuan He, Xiuzhen Guo, and Yunhao Liu. 2022. MotorBeat: Acoustic Communication for Home
Appliances via Variable Pulse Width Modulation. Proc. ACM Interact. Mob. Wearable Ubiquitous Technol. 6, 1, Article 31
(March 2022), 24 pages. https://doi.org/10.1145/3517255
1
INTRODUCTION
The recent years have witnessed rapid advances in smart home [18, 60]. Many home appliances are now connected
to the Internet and thus are endowed with interesting capabilities to improve the user experience. The users can
interact with them remotely, and these appliances themselves can cooperate in providing a seamless service [30].
Despite the trend of smart home, there are still a considerable number of home appliances cut off from the
Internet, especially small appliances. Home appliances can be mainly classified into three categories [72]: small
appliances, major appliances (i.e., white goods), and consumer electronics (i.e., brown goods). In contrast to major
∗
Corr
esp
onding
author
Authors’ addresses: Weiguo Wang, wwg18@mails.tsinghua.edu.cn, Tsinghua University, Beijing, China; Jinming Li, li-jm19@mails.tsinghua.
edu.cn, Tsinghua University, Beijing, China; Yuan He, heyuan@tsinghua.edu.cn, Tsinghua University, Beijing, China; Xiuzhen Guo,
guoxiuzhen94@gmail.com, Tsinghua University, Beijing, China; Yunhao Liu, yunhao@mail.tsinghua.edu.cn, Tsinghua University, Beijing,
China.
Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that
copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first
page. Copyrights for components of this work owned by others than ACM must be honored. Abstracting with credit is permitted. To copy
otherwise, or republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee. Request permissions from
permissions@acm.org.
©
2022 Association for Computing Machinery.
2474-9567/2022/3-ART31 $15.00
https://doi.org/10.1145/3517255
31
Proc. ACM Interact. Mob. Wearable Ubiquitous Technol., Vol. 6, No. 1, Article 31. Publication date: March 2022.
31:2
•
Wang et al.
Table 1.
The sales, prices, ratios, and status indicators of small appliances.
Group Keyword
Sales
1
(k)
Price ($)
Ratio2
Status Indicators
3
Electric Toothbrush
384.4
66.9
3%
B th
Hair Trimmer
280.4
16.4
0%
Electric Razor
194.4
60.9
0%
Hair Dryer
182.6
46.5
0%
mode
Rice Cooker
632.0
120.9
0%
Kit h
Electric Mixer
151.5
36.1
0%
Coffee Maker
135.6
94.4
0%
Blender
116.5
210.6
0%
replace-blade [49]
Li i Humidifiers
366.9
45.2
3%
R Massager
340.2
90.8
1%
Bd
Vacuum Cleaner
278.1
196.5
5%
Fan
198.0
68.9
2%
charge-battery, working-mode
Average
271.8
87.8
1.2%
-
1
The sales are calculated as the total number of customer reviews.
2
The ratio denotes the percentage of small appliances that can transmit data.
3
The status indicators are collected from user manuals, technical specifications, or LED lights of appliances’ appearances.
appliances such as refrigerators and washing machines, small appliances mainly refer to portable or semi-portable
household devices. Examples include electric toothbrushes, blood pressure monitors, and fans.
One basic fact is that small appliances now account for a major proportion of home appliances, and this
proportion is expected to continue to grow. According to statistics in 2020 [62], the sales volume of small
appliances is 8.3x larger than that of major appliances in the USA. Besides, small appliances are dedicated to
meeting consumers’ various and fine-grained demands, while major appliances mainly focus on accomplishing
necessary housework tasks. This means that the demand for small appliances increases with the quality of life.
However, the vast majority of small appliances are not connected to the Internet. We investigate three groups
of small appliances, shown in Table 1. We separately select the four most popular appliances in each group based
on their sales. The names of these appliances are used as keywords to search for the top 100 best-selling items on
Amazon [3]. Table 1 lists the ratio of small appliances that are connected to the Internet. As we can see, most
small appliances are isolated from the Internet. The average connection ratio is only 1.2%.
In summary, an overlooked but critical problem is that small appliances account for the majority of home
appliances, but they receive little attention, and few of them are connected to the Internet.
In this paper, we present a new approach MotorBeat that connects small appliances to the smart speaker. As
illustrated in Figure 1, the idea is to exploit electric motors, which are widely installed on small appliances, to
talk to the smart speaker in the acoustic channel. The idea of MotorBeat stems from two key observations:
The first observation is that nearly 80% of small appliances contain an electric motor
1
. This observation ensures
that MotorBeat can be widely applied to most small appliances without significant hardware modification. The
second observation is that smart speakers are now very popular, which have access to the Internet and contain
always-on microphones. Smart speakers can then behave as free gateways for appliances by receiving and
relaying their acoustic messages to the Internet.
The ability of small appliances to talk to the smart speaker will give birth to many applications. (1) Small
appliances can report their statuses, such as ON-OFF state, remaining battery, and working mode. Table 1 lists
1
We check all 56 default small appliances presented on Amazon [4], and 44 of them contain an electric motor (i.e., 44/56
≈
78.6%).
MotorBeat: Acoustic Communication for Home Appliances via Variable Pulse Width Modulation
•
31:3
Proc. ACM Interact. Mob. Wearable Ubiquitous Technol., Vol. 6, No. 1, Article 31. Publication date: March 2022.
Fig. 1. An illustration of MotorBeat.
the detailed status indicators that small appliances may report. Based on the ON-OFF state of, for example, your
electric toothbrush, smart home systems can know when and how long you brush your teeth, and remind you
when you forget to do so. Similarly, smart home systems can also warn you to turn off some appliances, like
hair dryers, with abnormal running time to avoid fire [26]. (2) Such ability may also be attractive to appliance
manufacturers. If the status of their products can be continuously acquired, they can track the entire life cycle of
each appliance [12, 51], based on which they can provide better after-sale service and improve the future product
designs. (3) From a general perspective, smart home systems can explicitly or implicitly perceive more contextual
information of home environments [2, 40], such as human activity, appliance usage, and further our daily life.
We notice that a recent pioneering work, Bleep [8], also exploits motors to enable UAVs to communicate
with each other. The solution of Bleep is acceptable in industrial environments, but is unreasonable in home
environments: First, Bleep encodes information in linear chirps and generates sounds that are unfriendly to
human ears. Second, Bleep equips each UAV with an additional microphone to sense channels and choose an idle
channel. In our case, small appliances hardly contain a microphone to avoid collisions, and it’s unrealistic to
additionally equip each appliance with a microphone. In our home environments, MotorBeat should achieve the
following 3C (Comfortable, Compatible, and Concurrent) goals:
Comfortable to hear. Small appliances are designed to serve customers. The quality of experience is
critical for them. Therefore, MotorBeat should not disturb the users while transmitting acoustic messages.
Compatible
with multiple motor modes. The motors of small appliances typically have multiple working
modes, and these modes could be changed unpredictably by the users at any time. This requires that the
transmission of MotorBeat should not affect the function of these motors, and the transmission itself should
not be disturbed by users.
Concurrent
transmission. Small appliances typically have no microphone to monitor the transmission of
other appliances. This means that collision detection or collision avoidance is infeasible for small appliances,
and collisions are inevitable in our scenario. To tolerate the collisions, MotorBeat should allow concurrent
transmission and support decoding from collided signals.
To achieve the above
3C
goals, MotorBeat introduces a novel modulation technique, Variable Pulse Width
Modulation (V-PWM). At a high level, small appliances drive the voltage of their motors with a pseudo-random
(i.e., variable) switching frequency. By doing so, the harmonics of the acoustic signals, which sound uncomfortable,
are significantly reduced (
Comfortable
). Furthermore, even though the switching frequency is variable, the
duty cycle of the voltage can be configured online without impeding the transmission, thus supporting multiple
working modes of motors (
Compatible
). Last, we assign each appliance with a unique V-PWM symbol. These
symbols are orthogonal to each other. Similar to Code-Division Multiple Access (CDMA), multiple appliances are
allowed to transmit concurrently. The MotorBeat receiver is able to separately detect and decode messages of
these appliances (Concurrent). Our contributions are as follows:
Hair Dryer Fan
Blood Pressure
Monitor
Electric
Toothbrush
Coffee
Machine
Smart Speaker
Blender
Humidifier
•
•
•
31:4
•
Wang et al.
Proc. ACM Interact. Mob. Wearable Ubiquitous Technol., Vol. 6, No. 1, Article 31. Publication date: March 2022.
We propose MotorBeat, a novel motor-based communication paradigm that enables small appliances to
talk to a smart speaker. By doing so, we can connect small appliances to the Internet.
We disclose the acoustic characteristics of DC motors, and show the opportunity to communicate based
on them. We introduce a novel modulation technique, V-PWM, to drive the motors and achieve
3C
goals
required in the real world.
We implement MotorBeat and evaluate its performance on three small appliances (electric toothbrush,
blood pressure monitor, and fan) and ten different DC motors. The results show that MotorBeat can be
widely applied to small appliances. The communication range can be up to 10 m, ensuring that MotorBeat
can cover a standard apartment.
Roadmap.
Sections 3 and 4 introduce how a DC motor works and its acoustic characteristics. Section 5 gives
MotorBeat’s overview. In Sections 6 and 7, we elaborate on the design of transmitter and receiver, respectively.
Section 8 discusses some practical issues. Section 9 presents the implementation and evaluation results. Section 2
discusses the related work. Sections 10 and 11 respectively discuss and conclude this work.
2
RELATED WORK
Motor-Based Communication. Bleep [8] is the work closest to ours. Bleep modulates the sounds of UAV
motors to enable UAVs to communicate in the acoustic channel. Bleep increases or decreases the switching
frequency of PWM voltage every 50ms to transmit acoustic up-chirp or down-chirp signals. To capture and
decode the acoustic signals, Bleep equips each UAV with an additional microphone.
Ripple [54, 55] exploits Linear Resonant Actuators (LRAs) in smartphones to achieve motor-accelerometer
communication. Similarly, VibroComm [73] utilizes LRAs to transmit vibration messages to gyroscopic sensors,
thus achieving targeted and explicit communication. Differing from the existing works that use AC motors,
MotorBeat’s design is based on DC motors. The existing works allow to modulate both the magnitude and (or)
frequency of vibrations, and have much more space for modulation. In comparison, MotorBeat satisfies more
constraints in modulation, so as to preserve the original function of small appliances.
Side-Channel of Vibration.
Acoustic signals in the side-channel of vibration have received research attention
in recent years. For example, ViBand [34] and SecureVibe [32] explore bio-acoustics sensed by wearable devices
like smartwatch for interaction and secure communication, respectively. Deaf-Aid [19] utilizes ultrasonic signals
to make the gyroscope resonate and then to convey information. Those side-channels can also be used for device
authentication [14, 37, 42, 71, 75], object identification [24], drone identification [52], near-field communication
[43], earshot communication [39] and inter-vehicular communication [53]. Some existing works demonstrate
that the side-channels can be used to recover information from input/output devices such as keyboards [6, 25, 38],
printers [7], and screens [21, 29]. In this paper, we disclose the acoustic characteristics of DC motors, and
demonstrate the feasibility of using motors to transmit acoustic messages by modulating the voltage.
Timing-Based Encoding.
As we will see soon, MotorBeat can also upload bits by embedding bits into intervals
between V-PWM symbols. This method is inspired by the following works: ONPC [41] uses the timing of 802.11
frames to convey information. WiChronos [56] encodes information in the time interval between two narrow-
band symbols to achieve low-power communication. FreeBee [31] achieves cross-technology communication via
embedding bits into beacons by shifting transmission timing. Encoding information within the beacon timing is
also studied in optical and UWB communications[16, 20, 22].
Random Pulse Width Modulation. Our V-PWM is inspired by random pulse width modulation which is
originally designed for voltage-controlled power electronic converters [9, 35, 64]. Because this modulation can
disperse the concentrated energy of harmonics over a wide frequency range, many works utilize it to mitigate
acoustic noise [10, 33], electromagnetic interference [63, 65], or mechanical resonance frequency [11].
•
•
•
MotorBeat: Acoustic Communication for Home Appliances via Variable Pulse Width Modulation
•
31:5
Proc. ACM Interact. Mob. Wearable Ubiquitous Technol., Vol. 6, No. 1, Article 31. Publication date: March 2022.
0.63
(a)
(b)
Time (ms)

/
/
≈
− /
τ
const
T
OFF
T
ON
Fig. 2.
DC motor structure.
PWM Voltage
Rotation Speed
(c)
Fig. 3. PWM voltage vs. speed.
3
PRIMER
3.1
DC Motor
A direct current (DC) motor
2
shown in Figure 2 consists of two main components: stationary part (stator) and
rotating part (rotor). The stator contains permanent magnets on the side, and the rotor comprises coils. The
working process of a DC motor is as follows. According to the Lorentz law, when the DC motor is powered, an
additional magnetic field will be created inside the rotor. The rotor is then attracted or repelled by the magnets
on the stator. Two electronic brushes connect the rotor to the voltage. These brushes can switch the direction of
the coil current during rotation and thus switch the polarity of the electromagnet. By doing so, the rotor can
keep rotating in the same direction.
3.2
PWM Voltage
Pulse Width Modulation (PWM) voltage is widely adopted to drive small DC motors for its simplicity. PWM
voltage is a periodic signal with two states, ON and OFF, and is described by two parameters: (1)
duty cycle
󰜔
and
(2)
switching period
󰑱

. Here, duty cycle
󰜔
=

%, where
󰑱

and
󰑱

denote the durations of states ON
and
OFF
within
a
perio
d,
r
espe
ctively
(
se
e
F
i
g
u
r
+
e
3
(
a
)).
Another
parameter
switching
perio
d
󰑱
is
define
d
as
󰑱
+
󰑱
.
The
DC
motor
can
r
otate
steadily
when
the
switching
fr
e
quency
󰑽
satisfies
the
follo
wing
condition:
1
󰑽
=
󰑱
1
≫
󰜧
(1)
 
where 󰜧 denotes the time constant3 of a DC motor and it can be used to quantify how fast a DC motor
responds to the input voltage (see Figure 3(a)).
One way to interpret Equation 1 is to model the DC motor as a circuit with a resistor and an inductor in series,
which is equivalent to a first-order low pass filter with cut-off frequency 1
󰜧

. Therefore, as long as the input
v
oltage
fr
e
quency
󰑽
is
larger
than
the
cut-off
fr
e
quency
1
󰜧
,
the
DC
motor
will
r
otate
steadily
.
The advantage of PWM is that by merely changing the duty cycle 󰜔, the appliance can easily control its
motor’s rotation speed. In Figure 3(b), the duty cycle is 50%, and the switching frequency is 50x larger than that
in Figure 3(a). After a short transient period (0-40ms), the motor exhibits a steady speed, 50% of its maximum
speed. In Figure 3(c), the duty cycle increases to 75%, and the steady speed increases to 75% correspondingly
4
.
2
Here, we take brushed motors as an example to introduce DC motors.
3
Time constant is formally defined as the time the motor takes to reach its 63% (
1
1 ) maximum speed.
4
In practice, the rotation speed is not a linear function of the duty cycle.
Coil
Stator
Magnet
Rotor
Brush
0.5
0.75
摘要：

Proc.ACMInteract.Mob.WearableUbiquitousTechnol.,Vol.6,No.1,Article31.Publicationdate:March2022.MotorBeat:AcousticCommunicationforHomeAppliancesviaVariablePulseWidthModulationWEIGUOWANG,TsinghuaUniversity,ChinaJINMINGLI,TsinghuaUniversity,ChinaYUANHE∗,TsinghuaUniversity,ChinaXIUZHENGUO,TsinghuaUniver...

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