Revision 1:d01abb5e6f52, committed 2011-10-17

Comitter:

gbeardall

Date:

Mon Oct 17 10:41:37 2011 +0000

Parent:

0:2fdc6095d7b4

Commit message:

Changed in this revision

--- a/main.cpp	Wed Sep 07 07:27:01 2011 +0000
+++ b/main.cpp	Mon Oct 17 10:41:37 2011 +0000
@@ -1,3 +1,10 @@
+/*
+ * Honeywell Pressure Sensor
+ *
+ */
+ 
+#include <math.h>
+
 #include "mbed.h"
 
 DigitalOut myled(LED1);
@@ -34,14 +41,19 @@
         unsigned char data[4];
         i2c.read((id<<1)|1, (char*)data, sizeof(data));
         //printf("%02X %02X  %02X %02X\n\r", data[0], data[1], data[2], data[3]);
+        
         int st = data[0]>>6; // status
         int bd = ((data[0] & 0x3f) << 8) | data[1]; // bridge data
         int td = ((data[2]<<8) | data[3]) >> 5; // temp data
         
-        float p = ((float)bd-1638)*(15-0)/(14745-1638)-0; // convert to psi
-        p = p * 68.9475729; // psi to hPa
+        // P: 14 bits
+        // Cmin=2^14-1*.1 Cmax=2^14-1*.9
+        // Pmin=0 Pmax=15
+        float p = (((float)bd-1638)*(15-0)/(14745-1638))+0; // convert raw data to psi
+        p = p * 68.9475729; // convert psi to hPa
         
-        float t = (((float)td*200)/2047)-50; // convert to deg C
+        // T: 11 bits (Cmax=2047=2^11-1)
+        float t = (((float)td*200)/2047)-50; // convert raw data to deg C
         printf("%d: %.2f hPa  %.2f C\n\r", st, p, t);
         
         wait(1);
@@ -55,3 +67,76 @@
     } // while
     
 } // main
+
+
+// http://en.wikipedia.org/wiki/Barometric_formula
+const float layers[][4] = {
+//  Height[m], Pressure[Pa], Temp[K], Lapse[K/m], Subscript
+        0,     101325.0,     288.15,  -0.0065,    // 0
+    11000,      22632.1,     216.65,   0.0,       // 1
+    20000,       5474.89,    216.65,   0.001,     // 2
+    32000,        868.019,   228.65,   0.0028,    // 3
+    47000,        110.906,   270.65,   0.0,       // 4
+    51000,         66.9389,  270.65,  -0.0028,    // 5
+    71000,          3.95642, 214.65,  -0.002      // 6
+};
+
+enum { L_HEIGHT=0, L_PRESS, L_TEMP, L_LAPSE };
+
+const float g = 9.80665;
+const float M = 0.0289644;
+const float R = 8.31432;
+const float Ta = -273.15; // Absolute zero [deg C]
+
+// Search for subscript which matches pressure range
+// Choose appropriate formula and plug in values
+// Lookup derived h in table and check subscript is still same, otherwise recalculate
+// Add height correction
+
+// Pass in pressure and temperature from sensor
+// Return height[m]
+float pressureToHeight(float p, float t) {
+    float h = -999; // error
+    int b;  
+    int tries;
+
+    // Search backwards thru layers for our pressure
+    for(b = 6; b >= 0; --b) {
+        if(p <= layers[b][1]) break;
+    }
+    if(b < 0) b = 0; // adjust so can have -ve heights
+    int b1 = b;
+    
+    // Only retry limited times
+    for(tries = 10; tries; --tries) {
+          
+        float Hb = layers[b][0];
+        float Pb = layers[b][1];
+        float Tb = layers[b][2]; //??? Tb = t - Ta;
+        float Lb = layers[b][3];
+        
+        if(Lb != 0.0) 
+            //h = (log(g*M/R/Lb)/log(p/Pb)*Tb - Tb)/Lb + Hb; // Equation 1 (non-zero lapse rate)
+            h = (Tb/exp(R*Lb/g/M*log(p/Pb))-Tb)/Lb + Hb;
+        else 
+            h = -log(p/Pb)*R*Tb/g/M + Hb; // Equation 2 (zero lapse rate)
+        
+        // Search again for derived height
+        for(b = 6; b >= 0; --b) {
+            if(h <= layers[b][0]) break;
+        }
+    
+        if(b == b1) break; // gave same subscript, so we can stop
+        b1 = b;
+        
+    } // for - tries    
+    
+    if(tries <= 0) h = -999; // error
+
+        
+    return h;    
+
+} // pressureToHeight
+
+/*end*/
+