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- PWM: the motor input signal.
- const_: a constant term representing friction.
- W(s)_: transfer function between PWM signal and speed.
- v: 2D speed of the car.
- y': monodimensional speed obtained by projecting the 2D speed of the car on the path.
- y: monodimensional position of the car obtained by projecting its 2D position on the path.
- ym: monodimensional position of the car measured by cameras.
- Da: actuator disturbances on the control variable.
- Dslope: the test-bed is not perfectly flat but it is slightly inclined. This has been found to considerably affect the speed caused by slopes of the test-bed.
- Dslip: disturbances on the speed caused by the coupling with the lateral dynamics of the car (i.e. with the steer).
- Dproj: projecting the 2D speed on the path is modeled here as a disturbance.
- Djump: the path followed by cars is composed by a polygonal chain. This means that when the car goes from a segment to the next one, there is an interval of time during which the car position is projected on the same exact point of the path. For example, in Figure Djump.png the car goes through the trajectory A-B-C-D. While in the arc B-C, the vehicle position is projected on the path always in the same point P. Similar considerations can be done when a car cuts a corner. This disturbance is modeled with a negative (or positive in case the vehicle cut the corner) step that corrects the position each time the vehicle changes the segment of a path. Essentially, this is a measure of the difference between the length of the nominal path and the lenght of the actualy trajectory that the car follows.
- Dm: measurement error of the cameras.
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Djump
The path fig8 is composed by 649 control points for a total length of 9465.53 millimeters. The angle difference between two consecutive segments is delta = 0.9 degrees. We set the maximum distance of the car from the path to be xmax = 30cm (which is realistic with the current steer control performances). Assuming the car x coordinate (distance from the path) to be constant at xmax, we can compute Djump = 649 * delta * xmax = 3058.34mm which is the 32.31% of the path length.
DISTRUBANCES ANALYSIS CONCLUSION
Work in Progress. The complete analysis still have to be performed. Still the partial conclusion I found are driving the research.
It is clear thus that the steer control performance affects considerably the behavior of the carvehicle. Consequently lots of efforts should be done to improve it. Besides the designing of the controller, the ability of the car to stay close to the path is affected by the measurement error of cameras. Thus, limiting these disturbances should follow two directions:
- steer controller design;
- camera measurement error correction.
CPS
CAMERA MEASUREMENT ERROR CORRECTION
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