X-Git-Url: https://git.gag.com/?a=blobdiff_plain;ds=sidebyside;f=src%2Futil%2Fmake-altitude;fp=src%2Futil%2Fmake-altitude;h=716aa8a8786569f66a087b5276303f6e4a004652;hb=9513be7f9d3d0b0ec29f6487fa9dc8f1ac24d0de;hp=0000000000000000000000000000000000000000;hpb=3bfe8df44b575ca430ffaa051e20faa955a06c03;p=fw%2Faltos diff --git a/src/util/make-altitude b/src/util/make-altitude new file mode 100644 index 00000000..716aa8a8 --- /dev/null +++ b/src/util/make-altitude @@ -0,0 +1,283 @@ +#!/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 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); +}