Arduino NANO (6) Heart rate sensor

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Arduinoセンサーキットに、Heart rate sensorある。

血中ヘモグロビンの近赤外線吸収の性質を利用して脈拍パルス検出するらしい。

Arduinoとの接続は3線(GND、VCC、アナログ入力)のみ。

「参考1」をみて、まず簡単の試す。ただA0の値を表示。

// Pulse Monitor Test Script
int sensorPin = 0;
void setup() {
   Serial.begin(9600);
}
void loop ()
{
   while(1)
   {
     Serial.print(analogRead(sensorPin));
     Serial.print('\n');
   }
}

指先センサー装着

シリアルプロッターに見える上昇カーブ

安定した状態これはなにもわからない

指はセンサーから離れる時の下降カーブ。

「参考1」を参考して、Smoothingしても、同じ判別不能。困った。

#define samp_siz 4
#define rise_threshold 5
// Pulse Monitor Test Script
int sensorPin = 0;
void setup() {
   Serial.begin(9600);
}
void loop ()
{
   float reads[samp_siz], sum;
   long int now, ptr;
   float last, reader, start;
   float first, second, third, before, print_value;
   bool rising;
   int rise_count;
   int n;
   long int last_beat;
   for (int i = 0; i < samp_siz; i++)
     reads[i] = 0;
   sum = 0;
   ptr = 0;
   while(1)
   {
     // calculate an average of the sensor
     // during a 20 ms period (this will eliminate
     // the 50 Hz noise caused by electric light
     n = 0;
     start = millis();
     reader = 0.;
     do
     {
       reader += analogRead (sensorPin);
       n++;
       now = millis();
     }
     while (now < start + 20);  
     reader /= n;  // we got an average
     // Add the newest measurement to an array
     // and subtract the oldest measurement from the array
     // to maintain a sum of last measurements
     sum -= reads[ptr];
     sum += reader;
     reads[ptr] = reader;
     last = sum / samp_siz;
     // now last holds the average of the values in the array
     // check for a rising curve (= a heart beat)
     if (last > before)
     {
       rise_count++;
       if (!rising && rise_count > rise_threshold)
       {
         // Ok, we have detected a rising curve, which implies a heartbeat.
         // Record the time since last beat, keep track of the two previous
         // times (first, second, third) to get a weighed average.
         // The rising flag prevents us from detecting the same rise 
         // more than once.
         rising = true;
         first = millis() - last_beat;
         last_beat = millis();
         // Calculate the weighed average of heartbeat rate
         // according to the three last beats
         print_value = 60000. / (0.4 * first + 0.3 * second + 0.3 * third);
         Serial.print(print_value);
         Serial.print('\n');
         third = second;
         second = first;
       }
     }
     else
     {
       // Ok, the curve is falling
       rising = false;
       rise_count = 0;
     }
     before = last;
     ptr++;
     ptr %= samp_siz;
   }
}

こんな図形になって、周囲の雑音とか酷いかな?

参考:

  1. https://www.hackster.io/Johan_Ha/from-ky-039-to-heart-rate-0abfca

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