Diff: MQ2.cpp

Revision:

2:05d00b8b071d

Parent:

0:a5033edf6975

Child:

3:ed294e830904

--- a/MQ2.cpp	Thu Oct 05 12:03:38 2017 +0000
+++ b/MQ2.cpp	Fri Oct 06 07:38:25 2017 +0000
@@ -36,36 +36,62 @@
     return MQGetGasPercentage(MQRead()/10,GAS_SMOKE);
 }
 
+/****************** MQResistanceCalculation ****************************************
+Input:   raw_adc - raw value read from adc, which represents the voltage
+Output:  the calculated sensor resistance
+Remarks: The sensor and the load resistor forms a voltage divider. Given the voltage
+         across the load resistor and its resistance, the resistance of the sensor
+         could be derived.
+************************************************************************************/ 
 float MQ2::MQResistanceCalculation(int raw_adc) {
    return (((float)RL_VALUE*(1023-raw_adc)/raw_adc));
 }
 
-float MQ2::MQCalibration() {
+/***************************** MQCalibration ****************************************
+Input:   mq_pin - analog channel
+Output:  Ro of the sensor
+Remarks: This function assumes that the sensor is in clean air. It use  
+         MQResistanceCalculation to calculates the sensor resistance in clean air 
+         and then divides it with RO_CLEAN_AIR_FACTOR. RO_CLEAN_AIR_FACTOR is about 
+         10, which differs slightly between different sensors.
+************************************************************************************/ 
+float MQ2::MQCalibration() {                                                    // This should be done in 'clean air'
   float val=0;
   for (int i=0;i<CALIBARAION_SAMPLE_TIMES;i++) {                                //take multiple samples
     val += MQResistanceCalculation(_pin.read_u16()>>6);
     wait_ms(CALIBRATION_SAMPLE_INTERVAL);
   }
   val = val/CALIBARAION_SAMPLE_TIMES;                                           //calculate the average value
-  val = val/RO_CLEAN_AIR_FACTOR;                                                //divided by RO_CLEAN_AIR_FACTOR yields the Ro                                                                           //according to the chart in the datasheet 
+  val = val/RO_CLEAN_AIR_FACTOR;                                                //divided by RO_CLEAN_AIR_FACTOR yields the Ro according to the chart in the datasheet 
   return val; 
 }
 
-/*
- * Sample the MQ2
- */
+/*****************************  MQRead *********************************************
+Input:   mq_pin - analog channel
+Output:  Rs of the sensor
+Remarks: This function use MQResistanceCalculation to caculate the sensor resistenc (Rs).
+         The Rs changes as the sensor is in the different consentration of the target
+         gas. The sample times and the time interval between samples could be configured
+         by changing the definition of the macros.
+************************************************************************************/ 
 float MQ2::MQRead() {
   int i;
   float rs=0;
-  int val = _pin.read_u16()>>6;
   for (i=0;i<READ_SAMPLE_TIMES;i++) {
-    rs += MQResistanceCalculation(val);
+    rs += MQResistanceCalculation(_pin.read_u16()>>6);
     wait_ms(READ_SAMPLE_INTERVAL);
   }
   rs = rs/READ_SAMPLE_TIMES;
   return rs;  
 }
 
+/*****************************  MQGetGasPercentage **********************************
+Input:   rs_ro_ratio - Rs divided by Ro
+         gas_id      - target gas type
+Output:  ppm of the target gas
+Remarks: This function passes different curves to the MQGetPercentage function which 
+         calculates the ppm (parts per million) of the target gas.
+************************************************************************************/ 
 float MQ2::MQGetGasPercentage(float rs_ro_ratio, GasType gas_id) {
     switch(gas_id) {
         case GAS_LPG: return MQGetPercentage(rs_ro_ratio,LPGCurve);
@@ -75,6 +101,15 @@
     } 
 }
 
+/*****************************  MQGetPercentage **********************************
+Input:   rs_ro_ratio - Rs divided by Ro
+         pcurve      - pointer to the curve of the target gas
+Output:  ppm of the target gas
+Remarks: By using the slope and a point of the line. The x(logarithmic value of ppm) 
+         of the line could be derived if y(rs_ro_ratio) is provided. As it is a 
+         logarithmic coordinate, power of 10 is used to convert the result to non-logarithmic 
+         value.
+************************************************************************************/ 
 int MQ2::MQGetPercentage(float rs_ro_ratio, float *pcurve) {
   return (pow(10,(((log(rs_ro_ratio)-pcurve[1])/pcurve[2]) + pcurve[0])));
 }
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