4 * Pressure Sensor Model, version 1.1
6 * written by Holly Grimes
8 * Uses the International Standard Atmosphere as described in
9 * "A Quick Derivation relating altitude to air pressure" (version 1.03)
10 * from the Portland State Aerospace Society, except that the atmosphere
11 * is divided into layers with each layer having a different lapse rate.
13 * Lapse rate data for each layer was obtained from Wikipedia on Sept. 1, 2007
14 * at site <http://en.wikipedia.org/wiki/International_Standard_Atmosphere
16 * Height measurements use the local tangent plane. The postive z-direction is up.
18 * All measurements are given in SI units (Kelvin, Pascal, meter, meters/second^2).
19 * The lapse rate is given in Kelvin/meter, the gas constant for air is given
20 * in Joules/(kilogram-Kelvin).
23 const real GRAVITATIONAL_ACCELERATION = -9.80665;
24 const real AIR_GAS_CONSTANT = 287.053;
25 const int NUMBER_OF_LAYERS = 7;
26 const real MAXIMUM_ALTITUDE = 84852;
27 const real MINIMUM_PRESSURE = 0.3734;
28 const real LAYER0_BASE_TEMPERATURE = 288.15;
29 const real LAYER0_BASE_PRESSURE = 101325;
31 /* lapse rate and base altitude for each layer in the atmosphere */
32 const real[NUMBER_OF_LAYERS] lapse_rate = {
33 -0.0065, 0.0, 0.001, 0.0028, 0.0, -0.0028, -0.002
35 const int[NUMBER_OF_LAYERS] base_altitude = {
36 0, 11000, 20000, 32000, 47000, 51000, 71000
40 /* outputs atmospheric pressure associated with the given altitude. altitudes
41 are measured with respect to the mean sea level */
42 real altitude_to_pressure(real altitude) {
44 real base_temperature = LAYER0_BASE_TEMPERATURE;
45 real base_pressure = LAYER0_BASE_PRESSURE;
48 real base; /* base for function to determine pressure */
49 real exponent; /* exponent for function to determine pressure */
50 int layer_number; /* identifies layer in the atmosphere */
51 int delta_z; /* difference between two altitudes */
53 if (altitude > MAXIMUM_ALTITUDE) /* FIX ME: use sensor data to improve model */
56 /* calculate the base temperature and pressure for the atmospheric layer
57 associated with the inputted altitude */
58 for(layer_number = 0; layer_number < NUMBER_OF_LAYERS - 1 && altitude > base_altitude[layer_number + 1]; layer_number++) {
59 delta_z = base_altitude[layer_number + 1] - base_altitude[layer_number];
60 if (lapse_rate[layer_number] == 0.0) {
61 exponent = GRAVITATIONAL_ACCELERATION * delta_z
62 / AIR_GAS_CONSTANT / base_temperature;
63 base_pressure *= exp(exponent);
66 base = (lapse_rate[layer_number] * delta_z / base_temperature) + 1.0;
67 exponent = GRAVITATIONAL_ACCELERATION /
68 (AIR_GAS_CONSTANT * lapse_rate[layer_number]);
69 base_pressure *= pow(base, exponent);
71 base_temperature += delta_z * lapse_rate[layer_number];
74 /* calculate the pressure at the inputted altitude */
75 delta_z = altitude - base_altitude[layer_number];
76 if (lapse_rate[layer_number] == 0.0) {
77 exponent = GRAVITATIONAL_ACCELERATION * delta_z
78 / AIR_GAS_CONSTANT / base_temperature;
79 pressure = base_pressure * exp(exponent);
82 base = (lapse_rate[layer_number] * delta_z / base_temperature) + 1.0;
83 exponent = GRAVITATIONAL_ACCELERATION /
84 (AIR_GAS_CONSTANT * lapse_rate[layer_number]);
85 pressure = base_pressure * pow(base, exponent);
92 /* outputs the altitude associated with the given pressure. the altitude
93 returned is measured with respect to the mean sea level */
94 real pressure_to_altitude(real pressure) {
96 real next_base_temperature = LAYER0_BASE_TEMPERATURE;
97 real next_base_pressure = LAYER0_BASE_PRESSURE;
101 real base_temperature;
102 real base; /* base for function to determine base pressure of next layer */
103 real exponent; /* exponent for function to determine base pressure
106 int layer_number; /* identifies layer in the atmosphere */
107 int delta_z; /* difference between two altitudes */
109 if (pressure < 0) /* illegal pressure */
111 if (pressure < MINIMUM_PRESSURE) /* FIX ME: use sensor data to improve model */
112 return MAXIMUM_ALTITUDE;
114 /* calculate the base temperature and pressure for the atmospheric layer
115 associated with the inputted pressure. */
119 base_pressure = next_base_pressure;
120 base_temperature = next_base_temperature;
121 delta_z = base_altitude[layer_number + 1] - base_altitude[layer_number];
122 if (lapse_rate[layer_number] == 0.0) {
123 exponent = GRAVITATIONAL_ACCELERATION * delta_z
124 / AIR_GAS_CONSTANT / base_temperature;
125 next_base_pressure *= exp(exponent);
128 base = (lapse_rate[layer_number] * delta_z / base_temperature) + 1.0;
129 exponent = GRAVITATIONAL_ACCELERATION /
130 (AIR_GAS_CONSTANT * lapse_rate[layer_number]);
131 next_base_pressure *= pow(base, exponent);
133 next_base_temperature += delta_z * lapse_rate[layer_number];
135 while(layer_number < NUMBER_OF_LAYERS - 1 && pressure < next_base_pressure);
137 /* calculate the altitude associated with the inputted pressure */
138 if (lapse_rate[layer_number] == 0.0) {
139 coefficient = (AIR_GAS_CONSTANT / GRAVITATIONAL_ACCELERATION)
141 altitude = base_altitude[layer_number]
142 + coefficient * log(pressure / base_pressure);
145 base = pressure / base_pressure;
146 exponent = AIR_GAS_CONSTANT * lapse_rate[layer_number]
147 / GRAVITATIONAL_ACCELERATION;
148 coefficient = base_temperature / lapse_rate[layer_number];
149 altitude = base_altitude[layer_number]
150 + coefficient * (pow(base, exponent) - 1);
156 real feet_to_meters(real feet)
158 return feet * (12 * 2.54 / 100);
161 real meters_to_feet(real meters)
163 return meters / (12 * 2.54 / 100);
167 * Values for our MP3H6115A pressure sensor
169 * From the data sheet:
171 * Pressure range: 15-115 kPa
172 * Voltage at 115kPa: 2.82
173 * Output scale: 27mV/kPa
176 * 27 mV/kPa * 2047 / 3300 counts/mV = 16.75 counts/kPa
177 * 2.82V * 2047 / 3.3 counts/V = 1749 counts/115 kPa
180 real counts_per_kPa = 27 * 2047 / 3300;
181 real counts_at_101_3kPa = 1674;
183 real count_to_kPa(real count)
185 return (count / 2047 + 0.095) / 0.009;
195 read_record(file in) {
197 File::fscanf(in, "%c %x %x %x\n",
198 &r.type, &r.time, &r.a, &r.b);
202 real g_count = 264.8;
204 count_to_g(real count)
206 return (15792 + g_count - count) / g_count;
218 real[...] accelerometer;
220 real sinc(real x) = x != 0 ? sin(x)/x : 1;
222 real gaussian(real x) = exp(-(x**2)/2) / sqrt(2 * pi);
224 load "/usr/share/nickle/examples/kaiser.5c"
226 real[...] convolve(real[...] d, real[...] e) {
227 real sample(n) = n < 0 ? d[0] : n >= dim(d) ? d[dim(d)-1] : d[n];
228 real w = (dim(e) - 1) / 2;
231 for (int o = -w; o <= w; o++)
232 v += sample(center + o) * e[o + w];
235 return (real[dim(d)]) { [n] = c(n) };
238 real sum(real[...] x) { real s = 0; for(int i = 0; i < dim(x); i++) s += x[i]; return s; }
240 real[...] filter(real[...] d, int half_width) {
241 # real[half_width * 2 + 1] fir = { [n] = sinc(2 * pi * n / (2 * half_width)) };
242 real M = half_width * 2 + 1;
243 real[M] fir = { [n] = kaiser(n, M, 8) };
244 real fir_sum = sum(fir);
245 for (int i = 0; i < dim(fir); i++) fir[i] /= fir_sum;
246 return convolve(d, fir);
249 real gravity = 9.80665;
251 real[...] pressure_value, accelerometer_value;
254 void readsamples(file in) {
255 setdim(pressure_value, 0);
256 setdim(accelerometer_value, 0);
257 while (!File::end(in)) {
258 flight_record r = read_record(in);
260 clock[dim(clock)] = r.time / 100;
261 pressure_value[dim(pressure_value)] = r.b;
262 accelerometer_value[dim(accelerometer_value)] = r.a;
269 int size = dim(accelerometer_value);
270 real[size] accelerometer = { [n] = gravity * (count_to_g(accelerometer_value[n]) - 1.0) };
271 real[size] barometer = { [n] = pressure_to_altitude(count_to_kPa(pressure_value[n] / 16) * 1000) };
272 real[size] filtered_accelerometer = filter(accelerometer, 8);
273 real[size] filtered_barometer = filter(barometer, 128);
275 real[...] integrate(real[...] d) {
277 for (int i = 0; i < dim(ret); i++)
278 ret[i] = i == 0 ? 0 : ret[i-1] + (d[i-1] + d[i]) / 2 * (clock[i] - clock[i-1]);
282 real[...] differentiate(real[...] d) {
284 for (int i = 1; i < dim(ret); i++)
285 ret[i] = (d[i] - d[i-1]) / (clock[i] - clock[i-1]);
290 real[size] accel_speed = integrate(filtered_accelerometer);
291 real[size] accel_pos = integrate(accel_speed);
292 real[size] baro_speed = differentiate(filtered_barometer);
293 real[size] baro_accel = differentiate(baro_speed);
295 for (int i = 0; i < size; i++)
296 printf("%g %g %g %g %g %g %g %g %g\n",
298 filtered_barometer[i] - filtered_barometer[0], accel_pos[i],
299 baro_speed[i], accel_speed[i],
300 baro_accel[i], filtered_accelerometer[i],
301 barometer[i] - barometer[0], accelerometer[i]);