Prototype Design

The methodology is based in the creation and design of several prototypes and architectures in order to measure and compare the performance of every one.

The first prototype was based in the board Arduino Uno R3 and the e-Health Shield as the hardware components.

The software used was the Arduino IDE version 1.0.6 and the e-Health library version 2.2 because due a compatibility problem the code for the e-Health Sensor Platform cannot be compiled with the last version of Arduino IDE 1.6.1. or if the compilation did  work, some sensors especially the pulsioximeter sensor return wrong values.

Also for the first prototype was used the library ChibiOS/RT, an embedded real time operative system  in order to run concurrently two or more process in the Arduino CPU and try to get different sample frequency for every sensor.

With the first prototype working was analysed the code of the e-Health library for each sensor.

Pulsioximeter Sensor

This pulsioximeter is based in an infra-red sensor adapted to connect to e-Health board. The sensor has a display led (light emitting diode) screen which is connected pin by pin to the Arduino using a wire to the board. The sensor can get two biometric variables: SPO2 that is the level in percentage of the blood oxygen and BPM that is the bit per minute or heart frequency.

The code related to pulsioximeter is in the file eHealth.cpp of the library lines 515-545, in this code is possible to see that the values of the sensor are obtained measuring the level of every led in the display then for the lecture is necessary to read all the leds to get two variables. Also is possible to read in the line 537 that the pulsioximeter has a minimal sample frequency of 300 microseconds because the code apply a 300 microseconds delay between the lectures of every leds. And then apply an addition formula to obtain the values.

Galvanic Skin Response (GSR) Sensor

The GSR sensor get the measuring of the electrical conductance of the skin, which varies with the moisture level. This is of interest because the sweat glands are controlled by the sympathetic nervous system, so moments of strong emotion, change the electrical resistance of the skin. Skin conductance is used as an indication of psychological or physiological arousal, The Galvanic Skin Response Sensor (GSR - Sweating) measures the electrical conductance between 2 points, and is essentially a type of ohmmeter (e-Health Platform documentation).

The conductivity of the skin is measured at fingers of the palm. When a high level of sweating take place the electrical resistance drops down.
The sensor is connected to the Analog A2 pin of Arduino. This pin is the sensor value and convert all the input to an analogue value between 0 and1023.

With this sensor is possible obtain three variables

• SkinConductanceVoltaje: Defined as (sensor value * 5.0)/1,023 with a minimal sample frequency of 2 milliseconds
• SkinConductance: Defined as 2((SkindConductanceVoltaje – 0.5) / 100,000)
• SkinResistance: Defined as 1/conductance with a minimal sample frequency of 4 milliseconds