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author | Andrew Tridgell <tridge@samba.org> | 2014-02-13 14:25:52 +1100 |
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committer | Lorenz Meier <lm@inf.ethz.ch> | 2014-04-05 13:17:39 +0200 |
commit | cdec5e1d052b587a2e2a5d9d3dfef0bc507cc5b9 (patch) | |
tree | d697e601d8457bcef7de917bc7ffad55c6b44ec5 /src/drivers | |
parent | 9719af623d92ceab0c5eebe21906b4d5cf515682 (diff) | |
download | px4-firmware-cdec5e1d052b587a2e2a5d9d3dfef0bc507cc5b9.tar.gz px4-firmware-cdec5e1d052b587a2e2a5d9d3dfef0bc507cc5b9.tar.bz2 px4-firmware-cdec5e1d052b587a2e2a5d9d3dfef0bc507cc5b9.zip |
Included raw differential pressure field
Diffstat (limited to 'src/drivers')
-rw-r--r-- | src/drivers/meas_airspeed/meas_airspeed.cpp | 49 |
1 files changed, 38 insertions, 11 deletions
diff --git a/src/drivers/meas_airspeed/meas_airspeed.cpp b/src/drivers/meas_airspeed/meas_airspeed.cpp index 0b7986383..58b128948 100644 --- a/src/drivers/meas_airspeed/meas_airspeed.cpp +++ b/src/drivers/meas_airspeed/meas_airspeed.cpp @@ -206,20 +206,46 @@ MEASAirspeed::collect() dT_raw = (0xFFE0 & dT_raw) >> 5; float temperature = ((200.0f * dT_raw) / 2047) - 50; - /* calculate differential pressure. As its centered around 8000 - * and can go positive or negative, enforce absolute value - */ + // Calculate differential pressure. As its centered around 8000 + // and can go positive or negative const float P_min = -1.0f; const float P_max = 1.0f; - float diff_press_pa = fabsf((((float)dp_raw - 0.1f * 16383.0f) * (P_max - P_min) / (0.8f * 16383.0f) + P_min) * 6894.8f) - _diff_pres_offset; + const float PSI_to_Pa = 6894.757f; + /* + this equation is an inversion of the equation in the + pressure transfer function figure on page 4 of the datasheet - if (diff_press_pa < 0.0f) { - diff_press_pa = 0.0f; - } + We negate the result so that positive differential pressures + are generated when the bottom port is used as the static + port on the pitot and top port is used as the dynamic port + */ + float diff_press_PSI = -((dp_raw - 0.1f*16383) * (P_max-P_min)/(0.8f*16383) + P_min); + float diff_press_pa_raw = diff_press_PSI * PSI_to_Pa; // correct for 5V rail voltage if possible - voltage_correction(diff_press_pa, temperature); + voltage_correction(diff_press_pa_raw, temperature); + float diff_press_pa = fabsf(diff_press_pa_raw - _diff_pres_offset); + + /* + note that we return both the absolute value with offset + applied and a raw value without the offset applied. This + makes it possible for higher level code to detect if the + user has the tubes connected backwards, and also makes it + possible to correctly use offsets calculated by a higher + level airspeed driver. + + With the above calculation the MS4525 sensor will produce a + positive number when the top port is used as a dynamic port + and bottom port is used as the static port + + Also note that the _diff_pres_offset is applied before the + fabsf() not afterwards. It needs to be done this way to + prevent a bias at low speeds, but this also means that when + setting a offset you must set it based on the raw value, not + the offset value + */ + struct differential_pressure_s report; /* track maximum differential pressure measured (so we can work out top speed). */ @@ -232,6 +258,7 @@ MEASAirspeed::collect() report.temperature = temperature; report.differential_pressure_pa = diff_press_pa; report.differential_pressure_filtered_pa = _filter.apply(diff_press_pa); + report.differential_pressure_raw_pa = diff_press_pa_raw; report.voltage = 0; report.max_differential_pressure_pa = _max_differential_pressure_pa; @@ -328,7 +355,7 @@ MEASAirspeed::voltage_correction(float &diff_press_pa, float &temperature) return; } - const float slope = 70.0f; + const float slope = 65.0f; /* apply a piecewise linear correction, flattening at 0.5V from 5V */ @@ -478,7 +505,7 @@ test() } warnx("single read"); - warnx("diff pressure: %8.4f pa", (double)report.differential_pressure_pa); + warnx("diff pressure: %d pa", (int)report.differential_pressure_pa); /* start the sensor polling at 2Hz */ if (OK != ioctl(fd, SENSORIOCSPOLLRATE, 2)) { @@ -506,7 +533,7 @@ test() } warnx("periodic read %u", i); - warnx("diff pressure: %8.4f pa", (double)report.differential_pressure_pa); + warnx("diff pressure: %d pa", (int)report.differential_pressure_pa); warnx("temperature: %d C (0x%02x)", (int)report.temperature, (unsigned) report.temperature); } |