Toward a Wearable Tactile Sensory Amplification Device:Transfer Characteristics and Optimization
Mercedes Chartier1 , Neha Thomas2 , Yitian Shao3 and Yon Visell4
Abstract— This project aims to create a wearable device
capable of electronically enhancing touch, in the same waythat a hearing aid enhances the auditory sense. Our prototype
device utilizes electronic sensors, in the form of wide bandwidth
MEMS accelerometers, that are worn on the finger to capture
touch-induced skin vibrations, which are instrumental to haptic
perception. Active, analog electronics are used to filter andamplify the resulting signals without digitization. A compact
recoil actuator is used to accurately reproduce these signals,
effectively enhancing the transient touch information that isproduced when the finger contacts or slides against an object. In
order to optimize the performance of this device, we measured
and analyzed the frequency-domain transfer characteristicsof this system. The results were used to determine sensor
and actuator locations and orientations that would minimize
feedback, increasing the range of feasible amplification gains.
This study may help to guide the design of sensory prostheses
and haptic displays worn on the upper limb. Such devices could
be beneficial for individuals with impaired touch sensation due
to peripheral neuropathy, or for those with prosthetic limbs.

Advances in haptic technologies for stimulating the senseof touch hold the potential to aid individuals with peripheral
sensory impairments, and to assist those who may have losta limb by helping to restore touch sensation in a prosthesis.
Conscious percepts of touch are enabled by receptors thattranslate contact-dependent mechanical and thermal stimuli
into neural impulses that are transmitted through the nervous
system to the brain [2]. A major outstanding challenge inhaptic engineering is to reproduce touch-like stimuli via anelectronic interface, and many methods for doing so aredescribed in the literature, and are exemplified in commercialproducts.
Despite the potential applications in sensory aids, far lessattention has been devoted to the possibility of electronicallyprocessing and enhancing tactile information that is felt
by the skin during natural haptic interaction. The approachdescribed in this Work-in-Progress contribution is inspiredby previous research by Yao et al., in which the authors
created an instrument for surgical procedures that could
amplify the vibrations felt between the instrument tip anda surface [4]. The authors demonstrated that using thishandheld instrument, users were able to perceive lesions intissue that would otherwise go unnoticed. We adopt a similar

approach, in which we treat the hand itself as an instrument,
and ask whether wearable sensors and actuators may beemployed to amplify touch-elicited mechanical signals in theupper limb.
Prior research has demonstrated that touch-elicited mechanical signals can propagate widely in the hand, can bereadily recorded in beyond the wrist, that these signals
encode haptic properties of objects that are touched by thefinger, and that these signals are salient to tactile perception[1], [3]. Here, we propose to supplement this channel with anelectronic device that can further amplify the transmission oftactile information at the finger to proximal areas of the hand,
thereby amplifying touch sensation. We designed a compact,
wearable accelerometer to be worn on the finger, a widebandwidth vibrotactile actuator to be worn on the proximal
part of the hand, and analog electronics to amplify signals
from the former device so that they might be reproduced by
the latter (Fig. 1). In preliminary work described here, wehave measured the transfer characteristics of this system fordifferent locations of the actuator, as described below, in order to guide the design of this electronic tactile amplification
system.
The prototype tactile amplification device consists of anaccelerometer (Model ADXL335, Analog Devices Inc.), analog electronics, and a wide bandwidth vibrotactile actuator(Haptuator Mark II, Tactile Labs). The accelerometers xand y- axes have a bandwidth extending to 1600 Hz, andwith z-axis 550 Hz. A high pass filter (cutoff frequency 80Hz) was designed and implemented in the analog domain
to pre-condition the signal, removing motor behavior fromthe acceleration signal and constraining the signal to theoperating frequency range of the actuator. This was followedin cascade by an analog low pass filter (cutoff frequency 400Hz) which served to minimize channel noise while preserving the frequency range of greatest sensitivity. The signal isthen processed through a two-stage amplifier, providing an

1 Electrical and Computer Engineering Department, Drexel University, PA19104, USA mcc359@drexel.edu
2 School of Biomedical Engineering, Drexel University, PA 19104, USA

nehatk17@gmail.com

3 Electrical and Computer Engineering Department, University of California, Santa Barbara, CA 93106, USA syt@ece.ucsb.edu
4 Electrical and Computer Engineering Department and Media Arts &Technology Program, University of California, Santa Barbara, CA 93106,USA yon@mat.ucsb.edu

Fig. 1.

The diagram of determining the transfer function H(f ).

amplification gain of approximately 78. The gain was set byresistors, and the entire system was powered by coin cellbatteries. Acceleration (input) signals and actuator (output)
signals were collected for further analysis using a USB
Data Acquisition device (Analog Discovery, Digilent, Inc.),
and were analyzed using software (Matlab, The MathWorks,Inc.).
We measured the transfer characteristics of this system asindividuals wore it on their hand. One female and one maleDrexel University student, 18 and 24 years old, volunteered
to participate in our measurements. Both wore the device
on their left hand. Three index finger positions, tip, middle
and side, and nine actuator positions on the hand and wrist
were selected for testing the three axes of the accelerometer(Fig. 2). For each configuration, three trials with different
hand postures were explored, in order to account for posturedependent variations in mechanical impedance: Relaxed,
Open (Flat), and Closed (Flexed) hand position. We collected
data in two stages. In a first step, we measured the signal
amplitude received at the sensor when the actuator wasdriven with variable frequency. The actuator signal consistedof a frequency-modulated chirp, with bandwidth from zero to
500 Hz (Fig. 1). The input to the actuator and the output fromthe accelerometer were recorded for each gesture. In a secondstep, a similar configuration was used to compute a transfer
function for a system in which the sensor was directly
coupled to the actuator. In it, the actuator was tied to a string,so that the former could vibrate freely. An accelerometer wasfirmly attached to the actuator. The actuator input and theaccelerometer output were recorded and the transfer function
measured using the chirp method, as in step one.
We used this data in order to compute a transfer function
H(f ) between the sensor (accelerometer) locations and theactuator locations for each of the 81 configurations of thesensor-actuator system. This could be inferred from thefollowing relation:
H(f ) =Y (f )A(f ), G(f ) =X1 (f )G(f )X2 (f )

(1)

where Y (f ) is the accelerometer output and X1 (f ) is actuator input, recorded to calculate the feedback. G(f ) is theactuator transfer function acquired from dividing actuator
output A(f ) by actuator input X2 (f ), which were bothmeasured in the second set-up. The conversion from time
to frequency domain was achieved via the fast Fouriertransform algorithm. The transfer functions were comparedusing a frequency vs. (voltage/voltage) line graph (Fig. 3),
with V/V was plotted in dB(V/V) (20 log10(V/V)). Rootmean square sum of values of the gain of the transfer functionwere computed a bandwidth from 80 to 500 Hz.
A fundamental limitation on the level of amplification thatcan be achieved with such a device arises due to feedback
from the actuator to sensor, which occurs when the gain
is sufficiently high, as the output from the actuator travels
through the hand and back to the accelerometer, or input. We
evaluated the different sensor and actuator configurations inorder to assess which would be most immune to feedback, anFig. 2. Summary of transfer function H(f ) for 81 Possible Configurations.

Fig. 3.

Selected Configurations and Transfer Functions.

important criterion when designing a sensory amplification
system.
As shown in Fig. 3, the optimal configuration proved tobe that in which the y-axis of acceleration at the tip of
the index finger was used to drive an actuator that was at
the dorsal surface of the wrist oriented transversally to thelimb. The RMS feedback amplitude in this configuration was29.9 dB(V/V), which was the second-lowest RMS overall
and the lowest RMS using the accelerometer on the tip ofthe finger. The latter was the best accelerometer position
overall (for both wearability and feedback minimization).
This configuration allows for stable amplification, yielding anadditional 8.2 dB relative to the mean across configurations,and 17.6 dB relative to the worst case scenario.
In future work, we hope to further refine the design andergonomics in order to realize a practical wearable prototype,to extend the concept to multiple fingers, and to evaluate thisdevice in practical applications of touch amplification.
This work was supported by NSF awards No. 1446752
and 1527709.
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