Diff: main.cpp
- Revision:
- 1:d01abb5e6f52
- Parent:
- 0:2fdc6095d7b4
diff -r 2fdc6095d7b4 -r d01abb5e6f52 main.cpp
--- 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*/
+