The invention relates to a method for the separation or spacing distance measurement with an active optical sensor arrangement according to the preamble of the patent claim 1. The inventive method is especially suitable as an evaluation method for optical precrash sensors in vehicles.
In order to improve the safety in road traffic, optical sensor arrangements are increasingly installed as obstacle warning systems in vehicles, which, for supporting or assisting the driver and the occupant protection system, predominantly detect the immediate surrounding environment in front of the moving vehicle and warn of danger sources such as, for example, stationary or moving obstacles on the roadway. For this purpose, for example so-called CV sensors (Closing Velocity, approaching speed) are used, which work based on a spacing distance measurement by means of pulse transit time methods and sampling of the received signals. Thus, the following explanations relate to this application, but are also utilizable in connection with other sensors of comparable type.
In the determination of the distance of an object from the sampled measured values of a CV sensor, a drift of the result is caused by several effects:
For the calibration of the distance measurement, a reference measurement with an object in a known spacing distance is needed for each operating condition.
The difference between the calculated distances of the known object and the calculated distances to a different object then always or constantly corresponds to the actual distance difference between both objects. By addition of the distance of the known object, the absolute distance of the unknown object is determined. Thereby, the spacing distance measurement is calibrated.
Previously the approach is pursued, for example, to utilize a temperature dependent correction function or table for the calculated distance values. For this purpose, a calibration of each sensor is necessary within the scope of the production due to component dispersions or scatterings. Other effects other than temperature drift are not taken into account in that context, or signify a considerable additional effort or expenditure (for example in connection with dependence on the emitter power).
Furthermore, an apparatus and a method for the distance measurement in a vehicle are described in the DE 195 41 448 A1. From that it is known to determine the distance by evaluation of the signal flank. It is taught, to mask or screen out all close range reflections through a time-variable threshold value level. This time-variable threshold, however, causes errors in the spacing distance determination, which must be corrected with great effort or expense.
The object underlies the invention, to set forth a method for the distance measurement with an optical sensor arrangement according to the preamble of the claim 1, with which an automatic distance calibration is made possible.
This object is achieved by a method with the characterizing features set forth in the claim 1.
The method according to claim 1 comprises the advantages, that an automatic calibration of the distance can be carried out in a simple manner and by simple means, without hereby giving rise, as known from the state of the art, to new errors for the distance measurement, which on their part must again be corrected with effort or expense.
Advantageous embodiments of the method according to claim 1 are set forth in the dependent claims.
The invention will now be explained in connection with an example embodiment with the aid of the drawing.
It is shown by:
a: a diagram with the time sequence or progression of the brightness or intensity of interfering or stray light correction values from a stray light correction and a reference distance ascertained from these stray light correction values;
b: a diagram with the time sequence of the brightness of corrected sampled values, the reference distance from
The
Besides a previously carried-out stray light correction, the inventive solution is based additionally on the following assumptions:
In connection therewith, the mentioned stray light components can be interpreted as reflected light of a very close (fictitious) object and thus used as a reference measurement or reference distance 2. The actual spacing distance of this fictitious reference object is conditioned or dependent on the construction and can be stored as a fixed parameter in the sensor.
In
Because the stray light component is dependent also on external factors, the following measures should be provided for avoiding false or erroneous measurement results:
If one of these plausibility tests turns out negative, either the last reliable distance correction value or a standard setting can be reverted to, or the reference distance is determined as a function of previous reference distances that were ascertained with positive plausibility test. As a further calibration or compensation, a fixed offset that is permanently stored or ascertained during the production can be provided as the reference distance 2.
In stationary measurements, that is to say for a stationary vehicle, an invalid (that is to say too large) reference distance is ascertained; for this operating range, however, no measurements of the CV-sensor are specified, because a resting object represents no collision danger. If nonetheless a further object approaches with a high speed, a speed can nevertheless be ascertained, because the signal component due to the moving object, is not suppressed by the stray light correction. Merely the measured distance is given out too low.
If, in contrast, the previously resting object disappears out of the field of view of the sensor, the values of the stray light correction are adapted very quickly to the relevant values. Thereby, a correct reference distance can again be determined.
The proposed algorithm is also suitable under certain circumstances for a coarse indirect measurement of the temperature in connection with a known temperature dependence of the distance drift. This would also obviate a possibly needed temperature sensor.
Since the described algorithm builds on a stray light correction, both algorithms are to be tuned or coordinated with respect to one another. Especially, a sufficiently slow behavior of the adaptation of the stray light correction is to be provided.
If no reliably arising, or only too-small stray light components of the previously mentioned type are present, an additional component can be fed-back through suitable optical layout or design.
Number | Date | Country | Kind |
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101 63 925.2 | Dec 2001 | DE | national |
Filing Document | Filing Date | Country | Kind |
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PCT/DE02/04400 | 11/30/2002 | WO |