In this post I will talk about the blood pressure measurement subsystem

The main components of this subsystem are

  1. the heart beat detector
  2. a continuous rotation servo
  3. 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

 

                                                            10 - Blood pressure measurement.png

In order to make test easier, I built a frame where all the components will be placed

 

IMG_20150429_122138.jpg

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

 

IMG_20150429_122323.jpg

 

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

 

IMG_20150429_124919.jpg

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...

 

IMG_20150429_125027.jpg

 

Finally, the servo that pulls the elastic rubber has been added

IMG_20150429_130219_1.jpg

 

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...