01-03-2013, 04:59 PM
Development of the ring sensor for healthcare automation
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1. Introduction
As the population of aged people increases, close
and continuous monitoring becomes more important.
Real-time, continuous monitoring would allow not
only for emergency detection of an abrupt change of
the patient health condition but also for long-term
assessment to establish the right dose and timing of
medication. Especially, an ambulatory system that
would allow long-term monitoring of otherwise difficult
and non-compliant patients such as demented
elderly people is highly in demand. A couple of compact,
continuous monitoring devices have been developed
[1,2] for elderly care. However, these devices
have not been widely accepted due to the lack of
functionality and comfort for wearers.
To answer these needs, we have developed a compact,
non-obtrusive telemetered wearable patient
monitoring device in a ring configuration. Fig. 1
shows a conceptual diagram of the ring sensor. The
ring sensor is equipped with optoelectric components
that allow for long-term monitoring of the patient’s
arterial blood volume waveforms and blood oxygen
saturation non-invasively and continuously [3,4].
These signals are transmitted to a home computer
for diagnosis of the patient’s cardiovascular conditions.
This continuous monitoring system can provide
unique and useful information for preventive diagnosis
in which long-term trends and signal patterns are
more important. The ring sensor is completely wireless
and miniaturized so that the patient can wear the
device comfortably 24 hours a day.
The ring sensor, however, is inevitably susceptive
to a variety of motion and ambient light artifacts. The
pulse wave signals from the optical sensors are often
distorted by the noise so significantly that even
the pulse rate cannot be calculated from the signals.
To capture the pulse rate even from the noisy signals
consistently, we have developed an efficient algorithm
using a signal correlation method, in which periodic
signals such as pulse waves can be reconstructed from
the original signals contaminated by a random noise.
In this paper, we provide detailed descriptions of the
hardware and software of the ring sensor. Also, the algorithm
to capture the pulse rate from distorted pulse
waves is presented with an experimental verification.
Unique features of the 24 hour patient monitoring system
using the ring sensor will be discussed at the end.
2. Concept of the ring sensor
A finger ring is a unique form of wearable sensors,
and probably, the only thing that the majority of people
will accept to wear at all times. To monitor a patient
24 hours a day continually, a miniaturized sensor
in a ring is a rational design choice. Other personal
ornaments and portable instruments, such as ear rings
and wrist watches, are not continually worn in daily
living. When taking a shower, for example, people remove
wrist watches. Bathrooms, however, are one of
the most dangerous places in the home. More than
10,000 people, mostly hypertensives and the elderly,
die in bathrooms every year. Miniature ring sensors
provide a promising approach to guarantee the monitoring
of a patient at all times. Also, a ring configuration
provides the anatomical advantage of having
Package
The ring sensor consists of a ring with LEDs and a
photodiode, a four-layer printed circuit board (PCB)
for signal processing, another four-layer PCB for wireless
transmission, and two batteries. The signal processing
and the wireless transmission were separated
to reduce severe interference between the two functions.
Two batteries are sandwiched between the two
PCBs to supply the power to the two circuits. It has
been found that the two circuits have to be powered
separately to eliminate signal interference. I/O connections
are distributed on the edge of the boards, providing
the connections for power supplies, LEDs and
programming. Four screws are used in the four ears
on the boards to provide mechanical fixtures for the
boards. All the circuitry on the boards are protected
by optical epoxy after fabrication and debugging.
LEDs and photodiode
One red LED and two infrared LEDs are used as the
light sources. The peak wavelength of the red LED is
660 nm, and that of the infrared LEDs is 940 nm. The
photodiode has the peak wavelength of 940 nm and the
spectral sensitivity ranges from 500 to 1000 nm, which
meets our needs. The voltage drop of the red LED is
1.6V and that of the infrared LEDs is 1.2V, and two
infrared LEDs are connected in serial. We used LEDs
in a die form and the diameter is less than 0.1 mm.
First-stage amplifier
The first stage amplifier must be fast enough to keep
in pace with the flickering speed of the LEDs, which
means that it must have a high slew rate. On the other
hand, it is not desirable if this amplifier consumes a
lot of power. We chose an OPA336 surface mount
style amplifier from Burr-Brown. This amplifier has
0.03 V/ms of slew rate which is quite high for a 20 mA
low power amplifier. Furthermore, this amplifier is designed
to be used as a pre-amp for photodiode, which
also satisfies our need.