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Heart Rate Signal Analysis

Various Heart Rate Signal

Various Heart Rate Signal

In this article outlined analysis of heart rate signals. A heart rate signal is obtained from the pulsesensor sensor.

This Sensor has been discussed in some of the previous articles:

The addition of this article is a filtering with LPF to help smooth the signal of the sensor results.

 

Heart rate Sensor from pulsesensor.com

The signal processing stage is as follows:

The software for the data measurements on the Arduino is as follows: https://github.com/waskita/embedded/tree/master/atmega-detak-jantung-profiling

The software for the analysis of signals is as follows: https://github.com/waskita/embedded/blob/master/analisis-detak-jantung/heart-beat-analysis-1kHz.ipynb

Here’s a signal flow block diagram on this system:

Here is details of the signal flow description of the system

Signals of measurement results from ADC
The original signal is enlarged

The purpose of the sensor is to measure one’s heart rate.

There are a few things that complicate heart rate calculations:

The ADC signal measurement results are still less flat, so to make the smoother filtering carried out at a frequency cut off of 4 Hz. The cut-off frequency of 4 Hz is selected considering the maximum heart rate of a normal person is 220 per minute, or equivalent to 3.667 Hz. So the signal above the frequency 3.667 Hz is not required. 

Filter using a digital filter of IIR 4 pole. The 6 pole filter produces an unstable filter, and finally 4 pole is used. IIR filter types so that calculations are simpler, since this system will be implemented on microcontrollers with limited computing capabilities.

Design filter using Iowa Hills IIR Filter Designer Version 6.5 as follows:

Implementation of filters with Python language, using the filter structure “Nth Order Coeffiecients”.

The structure of the IIR (Infinite Impulse Response) filter used is as follows:

The filter result is as follows (red signal).

Filter result signal with low pass with FC = 4 Hz

The measurement Hasi signal contains a DC signal offset, so the average is not 0. To eliminate the DC offset, is done filtering with LPF at 200 mHz. Filter 2 pole IIR.

The filter results are as follows (red signal):

LPF 0.2 Hz Signal Filter result

Signal without offset is obtained with the filter result signal 4 Hz reduced by the filter result signal at 0.2 Hz.  The result is a signal (blue color) in the image below.

The blue signal is then seared (rectified), so there is a red signal.

Offset signal results and directives

 

The redirect signal is filtered again with the 0.2 Hz filter, to help peak detection. Peak detection is somewhat difficult, because of the 4 Hz filter results There is still a small peak. With the signal of a referral result is expected to detect high peaks only.

Signal Combination

The final process is peak detection. Only high peaks are counted.

Pulse top detection of heart rate

Cover

From experiments made, analog and digital filtering can help shape the signal so that it is easier to process.

Reference

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