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Piezo film sensor for capture of arterial wave pulse - Implementation

Module by: Akhila Tadinada

III) Implementation

The setup used to record the arterial wave pulses had three main functional units

  1. Sensor
  2. Amplifier /filter circuit
  3. Data Acquisition board and real time software.

1. Sensor

Commercially available blood pressure monitors use pressure sensors for measuring arterial pressure. Pressure sensors require an inflatable cuff and pressure transducers. We plan to use piezoelectric sensors in the proposed implementation. The sensor that was chosen is FLDT – 028K by Measurement Specialties Inc.The sensor was chosen for the following reasons [2] –

(a) it is very sensitive to low level mechanical movements

(b) it has an electrostatic shield located on both sides of the element (to minimize 50/60 Hz AC line interference)

(c) it is responsive to low frequency movements in the 0.7 - 12 Hz range of interest

(d) the foil size is about right (1 inch / 2.54 cm long)

(e) the electrodes are integrated with the sensor and laminated too

Figure 1
Figure 1 (graphics1.png)
Fig 2: FLDT -028K sensor provided by Measurement specialties Inc

2. Filter/Amplifier circuit

The filter/amplifier circuit had a power supply of 3.6 – 4.5 V to the board is and it consumes 100uA. The output is monitored with via test points located on the board.

Figure 2
Figure 2 (graphics2.jpg)

Fig 3. Filter/amplifier circuit [8]

The circuit has a three-pole low pass filter with a lower (-3 dB) cutoff frequency at about 12-13 Hz. The main purpose of the low-pass filter is to prevent unwanted 50/60 Hz AC line interference from entering the sensor. The internal capacitance of the piezo film element also forms a high pass filter that has a wide band frequency response so the filter attenuates any extraneous sound waves or vibrations that get into the piezo element. The DC gain is about +30 dB.

The circuit has very high input impedance of around 10 megaohm. This lowers the cutoff frequency of the piezo filter to 0.7 Hz .The front end of the filter/amplifier circuit uses an op-amp follower in parallel with a 10 Megohm parallel resistor [8] .

3. Data Acquisition

National Instruments DAQ USB 6009 and LabView software version 8.0 were used to record the analog signal output from the amplifier filter circuit. The data acquisition system converts the analog input to digital output which is then fed to the Labview software that records it real time data in a file. The sampling rate was set to 2000 Hz and the value for samples to read is 50.

Figure 3
Figure 3 (graphics3.png)

Fig4: National Instruments DAQ system and Labview software.

IV) Experimental Setup

Lab experiments were conducted using the above mentioned setup .We conducted the following experiments to characterize the sensors.

  1. The sensor was put firmly in place by wrapping it in place using a 3M breathable tape.
  2. To keep the sensor in place and exert external pressure on it a foam backing was placed on top of it.
  3. To exert more pressure a Velcro wrapping was tied around the wrist.
  4. In another set of experiments a child blood pressure cuff was put around the wrist and the mid arm. The pressure was increased from 20-120 mm Hg. This experiment was conducted to determine the optimal pressure around the sensor at which prominent peaks can be determined.

5. In the above mentioned experiments the sensor was placed approximately close to where the wrist and the midarm pulses were located. Experiments were also conducted by displacing the sensor from the pulse location.

graphics4.jpg graphics5.jpg

(a) (b)

Figure 4
Figure 4 (graphics6.jpg)

(c)

Fig 5: (a) The child cuff used for exerting external pressure (b) Different kind of wraps used – velcro, 3M tapes and foam backing (c) experimental setup comprising of the 2 amplifier filter circuits for mid arm and wrist and the NI DAQ board

Figure 5
Figure 5 (graphics7.jpg)

Fig 6: Subject has two sensors taped each at the wrist and the mid arm respectively. A child cuff is placed at the mid arm to provide external pressure when needed. The sensor output goes through the electrodes to the amplifier/ filter circuit.

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