+++ /dev/null
-#!/usr/bin/nickle -f
-/*
- * Pressure Sensor Model, version 1.1
- *
- * written by Holly Grimes
- *
- * Uses the International Standard Atmosphere as described in
- * "A Quick Derivation relating altitude to air pressure" (version 1.03)
- * from the Portland State Aerospace Society, except that the atmosphere
- * is divided into layers with each layer having a different lapse rate.
- *
- * Lapse rate data for each layer was obtained from Wikipedia on Sept. 1, 2007
- * at site <http://en.wikipedia.org/wiki/International_Standard_Atmosphere
- *
- * Height measurements use the local tangent plane. The postive z-direction is up.
- *
- * All measurements are given in SI units (Kelvin, Pascal, meter, meters/second^2).
- * The lapse rate is given in Kelvin/meter, the gas constant for air is given
- * in Joules/(kilogram-Kelvin).
- */
-
-const real GRAVITATIONAL_ACCELERATION = -9.80665;
-const real AIR_GAS_CONSTANT = 287.053;
-const int NUMBER_OF_LAYERS = 7;
-const real MAXIMUM_ALTITUDE = 84852;
-const real MINIMUM_PRESSURE = 0.3734;
-const real LAYER0_BASE_TEMPERATURE = 288.15;
-const real LAYER0_BASE_PRESSURE = 101325;
-
-/* lapse rate and base altitude for each layer in the atmosphere */
-const real[NUMBER_OF_LAYERS] lapse_rate = {
- -0.0065, 0.0, 0.001, 0.0028, 0.0, -0.0028, -0.002
-};
-const int[NUMBER_OF_LAYERS] base_altitude = {
- 0, 11000, 20000, 32000, 47000, 51000, 71000
-};
-
-
-/* outputs atmospheric pressure associated with the given altitude. altitudes
- are measured with respect to the mean sea level */
-real altitude_to_pressure(real altitude) {
-
- real base_temperature = LAYER0_BASE_TEMPERATURE;
- real base_pressure = LAYER0_BASE_PRESSURE;
-
- real pressure;
- real base; /* base for function to determine pressure */
- real exponent; /* exponent for function to determine pressure */
- int layer_number; /* identifies layer in the atmosphere */
- int delta_z; /* difference between two altitudes */
-
- if (altitude > MAXIMUM_ALTITUDE) /* FIX ME: use sensor data to improve model */
- return 0;
-
- /* calculate the base temperature and pressure for the atmospheric layer
- associated with the inputted altitude */
- for(layer_number = 0; layer_number < NUMBER_OF_LAYERS - 1 && altitude > base_altitude[layer_number + 1]; layer_number++) {
- delta_z = base_altitude[layer_number + 1] - base_altitude[layer_number];
- if (lapse_rate[layer_number] == 0.0) {
- exponent = GRAVITATIONAL_ACCELERATION * delta_z
- / AIR_GAS_CONSTANT / base_temperature;
- base_pressure *= exp(exponent);
- }
- else {
- base = (lapse_rate[layer_number] * delta_z / base_temperature) + 1.0;
- exponent = GRAVITATIONAL_ACCELERATION /
- (AIR_GAS_CONSTANT * lapse_rate[layer_number]);
- base_pressure *= pow(base, exponent);
- }
- base_temperature += delta_z * lapse_rate[layer_number];
- }
-
- /* calculate the pressure at the inputted altitude */
- delta_z = altitude - base_altitude[layer_number];
- if (lapse_rate[layer_number] == 0.0) {
- exponent = GRAVITATIONAL_ACCELERATION * delta_z
- / AIR_GAS_CONSTANT / base_temperature;
- pressure = base_pressure * exp(exponent);
- }
- else {
- base = (lapse_rate[layer_number] * delta_z / base_temperature) + 1.0;
- exponent = GRAVITATIONAL_ACCELERATION /
- (AIR_GAS_CONSTANT * lapse_rate[layer_number]);
- pressure = base_pressure * pow(base, exponent);
- }
-
- return pressure;
-}
-
-
-/* outputs the altitude associated with the given pressure. the altitude
- returned is measured with respect to the mean sea level */
-real pressure_to_altitude(real pressure) {
-
- real next_base_temperature = LAYER0_BASE_TEMPERATURE;
- real next_base_pressure = LAYER0_BASE_PRESSURE;
-
- real altitude;
- real base_pressure;
- real base_temperature;
- real base; /* base for function to determine base pressure of next layer */
- real exponent; /* exponent for function to determine base pressure
- of next layer */
- real coefficient;
- int layer_number; /* identifies layer in the atmosphere */
- int delta_z; /* difference between two altitudes */
-
- if (pressure < 0) /* illegal pressure */
- return -1;
- if (pressure < MINIMUM_PRESSURE) /* FIX ME: use sensor data to improve model */
- return MAXIMUM_ALTITUDE;
-
- /* calculate the base temperature and pressure for the atmospheric layer
- associated with the inputted pressure. */
- layer_number = -1;
- do {
- layer_number++;
- base_pressure = next_base_pressure;
- base_temperature = next_base_temperature;
- delta_z = base_altitude[layer_number + 1] - base_altitude[layer_number];
- if (lapse_rate[layer_number] == 0.0) {
- exponent = GRAVITATIONAL_ACCELERATION * delta_z
- / AIR_GAS_CONSTANT / base_temperature;
- next_base_pressure *= exp(exponent);
- }
- else {
- base = (lapse_rate[layer_number] * delta_z / base_temperature) + 1.0;
- exponent = GRAVITATIONAL_ACCELERATION /
- (AIR_GAS_CONSTANT * lapse_rate[layer_number]);
- next_base_pressure *= pow(base, exponent);
- }
- next_base_temperature += delta_z * lapse_rate[layer_number];
- }
- while(layer_number < NUMBER_OF_LAYERS - 1 && pressure < next_base_pressure);
-
- /* calculate the altitude associated with the inputted pressure */
- if (lapse_rate[layer_number] == 0.0) {
- coefficient = (AIR_GAS_CONSTANT / GRAVITATIONAL_ACCELERATION)
- * base_temperature;
- altitude = base_altitude[layer_number]
- + coefficient * log(pressure / base_pressure);
- }
- else {
- base = pressure / base_pressure;
- exponent = AIR_GAS_CONSTANT * lapse_rate[layer_number]
- / GRAVITATIONAL_ACCELERATION;
- coefficient = base_temperature / lapse_rate[layer_number];
- altitude = base_altitude[layer_number]
- + coefficient * (pow(base, exponent) - 1);
- }
-
- return altitude;
-}
-
-real feet_to_meters(real feet)
-{
- return feet * (12 * 2.54 / 100);
-}
-
-real meters_to_feet(real meters)
-{
- return meters / (12 * 2.54 / 100);
-}
-
-/*
- * Values for our MP3H6115A pressure sensor
- *
- * From the data sheet:
- *
- * Pressure range: 15-115 kPa
- * Voltage at 115kPa: 2.82
- * Output scale: 27mV/kPa
- *
- *
- * 27 mV/kPa * 2047 / 3300 counts/mV = 16.75 counts/kPa
- * 2.82V * 2047 / 3.3 counts/V = 1749 counts/115 kPa
- */
-
-real counts_per_kPa = 27 * 2047 / 3300;
-real counts_at_101_3kPa = 1674;
-
-real fraction_to_kPa(real fraction)
-{
- return (fraction + 0.095) / 0.009;
-}
-
-
-real count_to_kPa(real count) = fraction_to_kPa(count / 2047);
-
-typedef struct {
- real m, b;
- int m_i, b_i;
-} line_t;
-
-line_t best_fit(real[] values, int first, int last) {
- real sum_x = 0, sum_x2 = 0, sum_y = 0, sum_xy = 0;
- int n = last - first + 1;
- real m, b;
- int m_i, b_i;
-
- for (int i = first; i <= last; i++) {
- sum_x += i;
- sum_x2 += i**2;
- sum_y += values[i];
- sum_xy += values[i] * i;
- }
- m = (n*sum_xy - sum_y*sum_x) / (n*sum_x2 - sum_x**2);
- b = sum_y/n - m*(sum_x/n);
- return (line_t) { m = m, b = b };
-}
-
-real count_to_altitude(real count) {
- return pressure_to_altitude(count_to_kPa(count) * 1000);
-}
-
-real fraction_to_altitude(real frac) = pressure_to_altitude(fraction_to_kPa(frac) * 1000);
-
-int num_samples = 1024;
-
-real[num_samples] alt = { [n] = fraction_to_altitude(n/(num_samples - 1)) };
-
-int num_part = 128;
-int seg_len = num_samples / num_part;
-
-line_t [dim(alt) / seg_len] fit = {
- [n] = best_fit(alt, n * seg_len, n * seg_len + seg_len - 1)
-};
-
-int[num_samples/seg_len + 1] alt_part;
-
-alt_part[0] = floor (fit[0].b + 0.5);
-alt_part[dim(fit)] = floor(fit[dim(fit)-1].m * dim(fit) * seg_len + fit[dim(fit)-1].b + 0.5);
-
-for (int i = 0; i < dim(fit) - 1; i++) {
- real here, there;
- here = fit[i].m * (i+1) * seg_len + fit[i].b;
- there = fit[i+1].m * (i+1) * seg_len + fit[i+1].b;
- alt_part[i+1] = floor ((here + there) / 2 + 0.5);
-}
-
-real count_to_fit_altitude(int count) {
- int sub = count // seg_len;
- int off = count % seg_len;
- line_t l = fit[sub];
- real r_v;
- real i_v;
-
- r_v = count * l.m + l.b;
- i_v = (alt_part[sub] * (seg_len - off) + alt_part[sub+1] * off) / seg_len;
- return i_v;
-}
-
-real max_error = 0;
-int max_error_count = 0;
-real total_error = 0;
-
-for (int count = 0; count < num_samples; count++) {
- real kPa = fraction_to_kPa(count / (num_samples - 1));
- real meters = pressure_to_altitude(kPa * 1000);
-
- real meters_approx = count_to_fit_altitude(count);
- real error = abs(meters - meters_approx);
-
- total_error += error;
- if (error > max_error) {
- max_error = error;
- max_error_count = count;
- }
-# printf (" %7d, /* %6.2g kPa %5d count approx %d */\n",
-# floor (meters + 0.5), kPa, count, floor(count_to_fit_altitude(count) + 0.5));
-}
-
-printf ("/*max error %f at %7.3f%%. Average error %f*/\n", max_error, max_error_count / (num_samples - 1) * 100, total_error / num_samples);
-
-printf ("#define NALT %d\n", dim(alt_part));
-printf ("#define ALT_FRAC_BITS %d\n", floor (log2(32768/(dim(alt_part)-1)) + 0.1));
-
-for (int i = 0; i < dim(alt_part); i++) {
- real fraction = i / (dim(alt_part) - 1);
- real kPa = fraction_to_kPa(fraction);
- printf ("%9d, /* %6.2f kPa %7.3f%% */\n",
- alt_part[i], kPa, fraction * 100);
-}
projects / fw / altos / blobdiff