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Re: [Paparazzi-devel] hff filter in ins subsystem

From: Felix Ruess
Subject: Re: [Paparazzi-devel] hff filter in ins subsystem
Date: Tue, 11 Jun 2013 13:56:31 +0200


well, the problem is that we don't know the true linear body accelerations... So one assumption is that the current attitude estimate is good enough to rotate the acceleration measured by the IMU in body frame to the LTP (LocalTangentPlane) frame. Assuming this is correct, the acceleration due to gravity is only in the z-component and b2_hff_[xy]dd_meas only contains the linear trajectory accelerations.

As Gautier already said, this was a quick'n dirty solution to some specific problems (gps lag and low accel noise) some years ago.

If you have a more powerful microcontroller (like the STM32F4 with FPU) on your autopilot, running the ahrs_mlkf (multiplicative linearized Kalman Filter in quaternion formulation) will give you better attitude estimates and hence should also improve the results of hff. But this combination hasn't really been extensively tested so far.
Keep in mind that even then hff is still not a very nice solution and if you have enough computing power a "real" INS (which estimates position, velocity, attitude and biases in one filter) will definitely give you better results. Mainly due to hardware limitations this was not implemented in Paparazzi so far.

Cheers, Felix

On Tue, Jun 11, 2013 at 9:50 AM, Gautier Hattenberger <address@hidden> wrote:

Sorry, there is no hidden branch with a magical ins filter. This filter is a quick hack made some years ago, and it makes a lot of assumptions that are easily broken. It is probably giving you some benefit if you have a bad gps, low noise on gyro/accel and if you fly in near hover most of the time.
If your noise on accelerometers is too high, maybe you should just try to fly without the filter (gps only): <subsystem name="ins"/>
Any better filters are welcome !


Le 11/06/2013 08:12, address@hidden a écrit :
hi gautier,

i have been flying with the master branch with a quadrotor. Have problems with the autonomous navigation, i have oscillations while reaching the waypoint and tried to tune all parameters related with horizontal side but no success. I think it may be related with the speed and position estimates of the hff filter because i see there is a huge noise on the speed and position signals. While reading the Kalman algorhytms on the hff filter, i see the variable of "b2_hff_xdd_meas" is used as the measured acceleration of the body to drive the Kalman filter. But this variable includes the whole accelaration including the ones with related to the orientation angles. I guess we have to use an accelearation of the true body lineer accelarations on the x and y directions. But i do not know how to derive these accelerations because the acc sensor will always measure the gravitational accels and the centrifugel accels together. Trying to extract the effect of the gravitational accels from the acc sensor readings does not work(i have lots of tests) because there is a little phase delay between the real orientation angles of body(also the orientation angles of the acc sensor) and the angles we derived with a fusion of gyro and accel. Do you have an idea about this topic, or is there any upgrade/branch dealing with the "ins", i have missed? Thanks..

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