In this post I will talk about the blood pressure measurement subsystem
The main components of this subsystem are
- the heart beat detector
- a continuous rotation servo
- a force sensor resistor
The idea behind this experimental blood pressure measurement is to replace the arm cuff that is inflated by an air compressor with an elastic rubber pulled by a servo motor. When the rubber exercise a force that is exactly equal to the pressure of the blood, the blood stops flowing through the finger. At that moment, I can read the force the finger is squeezed y means of the FSR. Force can then be converted to a pressure value by applying a simple linear conversion
The following flowchart gives an overview of the whole process
In order to make test easier, I built a frame where all the components will be placed
Note the plastic strip that will pull the elastic rubber back when the pressure measurement process has finished
Then I put the Force Sensor Resistor in place
On top of FSR, I placed the heart beat sensor. I enclosed the sensor in foam to protect the FSR (and the patient finger as well..). Then, I placed the elastic rubber
Then I added a microswitch to detect when the finger is in place. Since Atmel 32U4 is not that monster, this makes me save a lot of computational power when there is nothing to measure...
Finally, the servo that pulls the elastic rubber has been added
The implementation, is easy because I just need to put some software pieces together. I already implemented the heart beat detection algorithm and the FSR reading procedures. Moving the servo is very easy thanks to the servo library included in the Arduino IDE.
I will probably need some experiments to determine the PWM that allows the best trade-off between measurement speed and accuracy.
Also, some measurements on different subjects and a reference instrument (a blood monitor system you can buy in any pharmacy) will be reuqired to determine the force-to-pressure conversion law
After that, I hope I will finally able to measure my blood pressure...
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