In this post I will talk about the blood pressure measurement subsystem
The main components of this subsystem are
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...