Position information, produced by the image recognition, is
subjected to delays and signal outages before it reaches the control
input of the quadrocopter. One major drawback of direct visual
servoing is the need for the target to stay inside the field of
view of the camera. In the case of the quadrocopter under direct
visual servo control, one must be aware of the unstable nature
of this aircraft. This nature requires a functioning visual servoing
loop, which is directly dependent on the visibility of the target
by the camera. Therefore, an indirect visual servo-control was
developed that uses a combination of local position tracking with
an integrated IMU unit and an image-based position estimation
and filtering with a Kalman filter. The additional dynamic delay
estimation and compensation system was included in the position
filtering process.
As the quadrocopter, as a system, includes nonlinearities, it is
common practice to employ the Extended Kalman filter (illustrated
in Fig. 6), where a linear approximation is only used for solving
the Riccati equation, a result of which is the Kalman gain. The full,
nonlinear model is used in the propagation of the estimate and in
computing the predicted sensor outputs [34]. This would introduce
a heavy load on the on-board, high-level processor and thus was
not selected for our application. Therefore, a nonlinear part of
the quadrocopter system (mainly the nonlinear coordinate system
transformation from the quadrocopter’s coordinate system K to
the target coordinate system T ) was decoupled from the linear
part of the system and replicated on the path of the acceleration
measurement vector aK entering the Kalman filter (Fig. 7) in order
to enable aT to enter the Kalman filter directly. This enabled us to
employ a basic (linear) form of the Kalman filter that presents a
much lighter load to the processor.
Position information, produced by the image recognition, is
subjected to delays and signal outages before it reaches the control
input of the quadrocopter. One major drawback of direct visual
servoing is the need for the target to stay inside the field of
view of the camera. In the case of the quadrocopter under direct
visual servo control, one must be aware of the unstable nature
of this aircraft. This nature requires a functioning visual servoing
loop, which is directly dependent on the visibility of the target
by the camera. Therefore, an indirect visual servo-control was
developed that uses a combination of local position tracking with
an integrated IMU unit and an image-based position estimation
and filtering with a Kalman filter. The additional dynamic delay
estimation and compensation system was included in the position
filtering process.
As the quadrocopter, as a system, includes nonlinearities, it is
common practice to employ the Extended Kalman filter (illustrated
in Fig. 6), where a linear approximation is only used for solving
the Riccati equation, a result of which is the Kalman gain. The full,
nonlinear model is used in the propagation of the estimate and in
computing the predicted sensor outputs [34]. This would introduce
a heavy load on the on-board, high-level processor and thus was
not selected for our application. Therefore, a nonlinear part of
the quadrocopter system (mainly the nonlinear coordinate system
transformation from the quadrocopter’s coordinate system K to
the target coordinate system T ) was decoupled from the linear
part of the system and replicated on the path of the acceleration
measurement vector aK entering the Kalman filter (Fig. 7) in order
to enable aT to enter the Kalman filter directly. This enabled us to
employ a basic (linear) form of the Kalman filter that presents a
much lighter load to the processor.
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