A wearable prototype of reflective sensor for non invasive measurement of heart rate

Govardhan Reddy Patancheru is from India.He finished his Bachelors of Technology from JNTU, Hyderabad, India.He finished his Masters Degree from MidSweden Univeristy,Sundsvall, Sweden.He did his Master thesis at Acreo Swedish ICT,Gothenburg,Sweden.

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Chapter 3: Methodology:
This chapter explains the various methods employed for developing the heart rate measurement unit.
3.1 Measuring PPG signals:
From chapter 2, it can be concluded that the PPG signal will be affected by different factors, so the system should be designed such that it minimizes these factors to get a valid measurement.
3.1.1 Proposed methods for building reflective PPG system:
Apart from the things explained in section 2.2.1, distance between the LED and photo sensor also plays a vital role in the output of the PPG sensor system. To overcome the issue of experimenting methods to find out the best distance, the choice made here was to use a component that has built-in phototransistor and LED with a fixed distance between the two, rather than using a separate LED and a phototransistor. The follow-ing are the two such components as explained in section 2.4 and will be used in the investigation to find a better reflective PPG system for the application requirements.
1) TCRT1000-Phototransistor and infrared LED
2) NJL5303R-Phototransistor and green LED
3.2 Improving the raw PPG signal for desired Heart Rate:
The output of photo transistor will be the raw pulse signal that is a combination of the desired pulse signal with the undesired DC and AC components as explained in chapter 2.These undesired components should be ruled out or minimized to make the desired pulse signal.
3.2.1 Minimizing the DC and AC component:
The DC component can be removed by using the high pass filter, while the unwanted AC components as explained in the section 2.3.1.1 can be minimized by using a suitable low pass filter such that it allows the frequencies needed for the desired heart rate.