piezoelectric sensor

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INTRODUCTION:
This paper summarizes the experiments conducted using piezoelectric sensors to obtain heart
pulses by non-invasive methods. The project had two main phases. In the first phase a twochannel
low power amplifier and filter circuit was designed. This amplifier is then used to amplify
and filter the signals acquired from the piezoelectric sensors. It was specifically designed for
battery powered operation from a single 3V coin cell and consumes just 200 uA of current. The
device has two channel analog inputs; one for the wrist location and one for the mid-arm section
of the arm. The signals were obtained simultaneously and outputted as analog signals. The
amplifiers used in the circuit were instrumentation amplifiers, which have superior performance
over discrete op-amp designs. Initially, Measurement Specialties SDT1-028K type piezoelectric
sensor was used. However, this type of sensor did not yield very good results. It was difficult to
make a good coupling to the skin because of the mechanical design of this sensor. The sensors
were placed over the skin and tape was used to fix them in place as well as provide some
coupling to the skin. Although some waveforms were obtained with this method, it was very
difficult to get good waveforms and it was also found that the repeatability of the tests needed
major improvement. The reasons of poor results with the previous method can be summarized
as follows:
a. The sensor coupling to the skin was poor.
b. The sensor was very position sensitive.
c. The method to obtain the pulses needed improvement.


PCB Design:
The PCB was custom designed. The board dimensions are 2.1 inch (5.33 cm) x 1inch (2.54
cm). The material used for the PCB is polyimide based which makes it flexible to take on the
contour of the arm. Battery is placed on the board along with the other components


Explanations of some of the terminology used in this paper:
Backing up/down: This is a shortcut term used to clarify how the sensor was coupled to the
skin. Backing up means, the sensor itself touches the skin and the backing faces away from the
skin. Velcro is then placed on top of the sensor. The configuration is skin-sensor-backing-velcro.
Plastic up/down: The sensor has two different sides. If it is viewed in bright light, it can be seen
that one side is shiny and the other is dull. The shiny surface is the plastic protective cover that
is placed by the manufacturer. The dull side has the sensor exposed. Plastic down means that
this surface is touching the skin and vice versa.
Backing/No backing: A firm foam material was attached to the sensor so that some support
was given to the sensor. The sensor itself does not have this material and this made the sensor
more rigid. When the Velcro was tightened over the sensor, the sensor pushed down on the
skin yielding better coupling to the skin and stronger signals. This term simply refers to if such a
support was used to obtain signals or not.