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47 lines
2.8 KiB
Markdown
47 lines
2.8 KiB
Markdown
# Static Pressure Buildup
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Air flowing over an enclosed vehicle can cause the _static pressure_ to change within the canopy/hull.
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Depending on the location of holes/leaks in the hull, you can end up with under or overpressure (similar to a wing).
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The change in pressure can affect barometer measurements, leading to an inaccurate altitude estimate.
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This might manifest as the vehicle losing altitude when it stops moving in [Altitude](../flight_modes_mc/altitude.md), [Position](../flight_modes_mc/position.md) or [Mission](../flight_modes_mc/mission.md) modes (when the vehicle stops moving the static pressure drops, the sensor reports a higher altitude, and the vehicle compensates by descending).
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The problem is particularly visible on multicopters because fixed wing vehicles move with a more constant airspeed (and it is the airspeed deltas that are noticeable).
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One solution is to use foam-filled venting holes to reduce the buildup (as much as possible) and then attempt dynamic calibration to remove any remaining effects.
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::: tip
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Before "fixing" the problem you should first check that the Z setpoint tracks the estimated altitude (to verify that there are no controller issues).
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:::
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::: info
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While it is possible to remove the barometer from the altitude estimate (i.e. only use altitude from the GPS), this is not recommended.
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GPS is inaccurate in many environments, and particularly in urban environments where you have signal reflections off buildings.
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:::
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## Airflow Analysis
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You can modify the hull by drilling holes or filling them with foam.
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One way to analyse the effects of these changes is to mount the drone on a car and drive around (on a relatively level surface) with the hull exposed to air/wind.
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By looking at the ground station you can review the effects of movement-induced static pressure changes on the measured altitude (using the road as "ground truth).
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This process allows rapid iteration without draining batteries: modify drone, drive/review, repeat!
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::: tip
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Aim for a barometer altitude drop of less than 2 metres at maximum horizontal speed before attempting software-based calibration below.
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:::
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## Dynamic Calibration
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After modifying the hardware, you can then use the [EKF2_PCOEF\_\*](../advanced_config/parameter_reference.md#EKF2_PCOEF_XN) parameters to tune for expected barometer variation based on relative air velocity.
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For more information see [Using PX4's Navigation Filter (EKF2) > Correction for Static Pressure Position Error](../advanced_config/tuning_the_ecl_ekf.md#correction-for-static-pressure-position-error).
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::: info
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The approach works well if the relationship between the error due to static pressure and the velocity varies linearly.
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If the vehicle has a more complex aerodynamic model it will be less effective.
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:::
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## See Also
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- [OEM/Factory Configuration](../advanced_config/oem.md)
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