Diff: MQ4.cpp

Revision:

0:401959f72658

Child:

1:d64cac41351a

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/MQ4.cpp	Tue Jul 17 04:34:21 2018 +0000
@@ -0,0 +1,119 @@
+#include "MQ4.h"
+
+void MQ4::begin(){
+    Ro = MQCalibration();
+}
+
+/*
+ * Reads data from MQ4.
+ *
+ * Param data: the pointer to fill.
+ */
+void MQ4::read(MQ4_data_t *data){
+   data->lpg = MQGetGasPercentage(MQRead()/Ro,GAS_LPG);
+   data->h2 = MQGetGasPercentage(MQRead()/Ro,GAS_H2);
+   data->ch4 = MQGetGasPercentage(MQRead()/Ro,GAS_CH4);
+}
+
+/*
+ * reads data, returns LPG value in ppm
+ */
+float MQ4::readLPG(){
+    return MQGetGasPercentage(MQRead()/Ro,GAS_LPG);
+}
+
+/*
+ * reads data, returns H2 value in ppm
+ */
+float MQ4::readH2(){
+    return MQGetGasPercentage(MQRead()/Ro,GAS_H2);
+}
+
+/*
+ * reads data, returns CH4 value in ppm
+ */
+float MQ4::readCH4(){
+    return MQGetGasPercentage(MQRead()/Ro,GAS_CH4);
+}
+
+/****************** 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 MQ4::MQResistanceCalculation(int raw_adc) {
+   return (((float)RL_VALUE*(1023-raw_adc)/raw_adc));
+}
+
+/***************************** 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 MQ4::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 
+  return val; 
+}
+
+/*****************************  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 MQ4::MQRead() {
+  int i;
+  float rs=0;
+  for (i=0;i<READ_SAMPLE_TIMES;i++) {
+    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 MQ4::MQGetGasPercentage(float rs_ro_ratio, GasType gas_id) {
+    switch(gas_id) {
+        case GAS_LPG: return MQGetPercentage(rs_ro_ratio,LPGCurve);
+        case GAS_H2: return MQGetPercentage(rs_ro_ratio,H2Curve);
+        case GAS_CH4: return MQGetPercentage(rs_ro_ratio,CH4Curve);
+        default: return -1.0;
+    } 
+}
+
+/*****************************  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 MQ4::MQGetPercentage(float rs_ro_ratio, float *pcurve) {
+  return (pow(10,(((log(rs_ro_ratio)-pcurve[1])/pcurve[2]) + pcurve[0])));
+}
+// Return Ro calculated;
+float MQ4::get_Ro() {
+  return Ro;
+}
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