Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
1999-02-11
2002-11-19
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S231180
Reexamination Certificate
active
06483104
ABSTRACT:
TECHNICAL FIELD
The invention relates to a position sensor, specifically an angle of rotation sensor with a CCD line.
BACKGROUND OF THE INVENTION
CCD sensors are known, such as is described in the application brief by the Texas Instruments under the title “linear products TLS 214 integrated opto-sensor” in connection with the
FIGS. 1 through 5
shown there. Details of the integrated TLS 215 opto-sensor were published in section 4.19 through 4.28 of the Texas Instruments product description and under the heading “TSZ 215, 128x1 integrated opto-sensor” under number SOES 005A—May 1993.
The known CCD line has a row of composite elements called pixels arranged side by side. Starting from an initial state in which the pixels are drained of all electrical charge, these composite elements are then charged in proportion with the light flux striking them. The varying charge on the pixels resulting from the difference in light distribution striking them corresponds to different voltage values on the pixels. At a predetermined, periodically repeated point in time, the voltages of all pixels are transmitted in parallel to the respective memory elements of a shift register, so that the voltage distribution on the individual memory elements in the shift register corresponds to the light flux supplied to the respective individual pixels. By means of suitable electronics, the composite elements (pixels) are then reset at their initial charge free state and again charged by the incident light, while at the same time the voltage values are read incrementally in series out of the memory elements of the shift register, and the analog values are converted to digital values. Thus, for example, 64 to 128 digital values are ultimately available, describing the light flux striking the respective pixels during the past time interval. With the help of a carrier with coded apertures where the light distribution is allowed to pass through corresponds to the position of the carrier with respect to the CCD line, it is thus possible to draw conclusions about the instantaneous position of the carrier during the measurement process on the basis of the measured light and voltage distribution. In the example described here, this applies to the angle of rotation of a coded disk, but also to the description of a longitudinal position of a carrier, when the linear position of the carrier is described with the help of an optically light-transmitting code. The use of such optically light-transmitting code has already been described in German Patent 195 32 903.1. The essential difference in comparison with the code disk described there is that the light flux penetrating the disk is measured not with a CCD line, but with one or more diodes. The invention is therefore based on a position sensor, in particular an angle of rotation sensor of the generic type in the definition of the species in claim
1
. One disadvantage of such a sensor device is that during a sampling interval of the light flux distribution of the light flux passing through the code disk, the code disk undergoes a considerable shift with a rapid turn of the steering wheel. In other words, there is a change in the allocation of individual light-transmitting fields on the code disk with respect to the pixel, so that the light flux distribution of the incident light changes during a collecting period. This results in blurred grey scale value transitions, so that the image of the light distribution due to the code disk on the CCD line is blurred. To reduce the blurred grey scale value transitions, one might attempt to keep the number of code words and hence the code as large and as rough as possible. A rough code also serves to reduce sensitivity to dirt and to simplify code disk production. The object of the present invention is to greatly improve the known system by improving the grey scale value transitions through other measures and increasing the sensitivity of the sensor.
The invention consists in principle of greatly shortening the lighting duration of the code element, i.e., of the rotation sensor of the code disk. The code element is lighted only briefly, approximately in the manner of a stroboscope. One disadvantage of the invention could be that the light flux output and thus the charge picked up by the collecting elements is comparatively low. This situation can be remedied where the light source is operated so that the rated output is obtained for the entire period. Thus, in other words, a large amount of light is delivered by the light source for short periods of time. To be able to determine unambiguously the position of the code element after the charging process on the basis of the reduced light flux, the on-time of the light source is preferably synchronized with the sampling time and the readout timing. Tests have shown that the cyclic on-time corresponding to the features according to claim
3
should amount to less than 5% of the cycle time and, better yet, less than 1% of the cycle time.
To further improve the measurement accuracy of the sensor of the generic type the code element (i.e., the code disk for an angle of rotation sensor) is provided with code fields, which correspond to only two states. In other words, the code element is provided with code fields that either are opaque or have a defined light transmission. The light transmission corresponds to states 0 and 1 of a multistep code, where the geometric arrangement is such that increasing numbers of memory elements (pixels) are either covered by a code field or receive the light flux. It would also be conceivable to assign a separate code element to each pixel. The proposed invention serves the goal of greater security, in order to be able to decide in borderline cases whether or not a sufficient light flux has been sent to a group of pixels. The decision regarding whether or not it was a light-transmitting code field or an opaque code field is made much more reliably than when only a single pixel is present per code field. Furthermore, multiple code fields (for example, 12 code fields) are scanned by the CCD line and weighted by the sensor, with several pixels again being assigned to each code field, e.g., between 10 and 11 pixels (this is obtained, for example, by using a 12-step maximum code and a code line with 128 pixels). Using a larger number of pixels per code element or coding step of a multistep code has the advantageous effect that a light suppressing code field or a light flux transmitting code field can be recognized even when the pixel voltage, which determines the flank of the image, assumes an intermediate value. This is the case, for example, when the edge of a code field only partially covers a pixel or when stray radiation at the edges of the code field distorts the flanks readable on the pixels.
When the position sensor for determining the angle of rotation according to the invention is used in particular to determine the angle of rotation of a steering wheel, it is preferable to form the code fields of the code disk by adjacent sectors of a circle with the same sector angle In principle, the code element may be formed from a code disk on which light-transmitting and opaque sectors of a circle are arranged adjacent to one another like pieces of a pie in a sequence determined by the code. The code fields may follow one another on an ring, as if only sections inside the ring were cut from the aforementioned pieces of pie. In this case it is advisable to have the CCD line run at a tangent to the ring and above or below the code disk over the ring. Since the line is essentially a straight line, while the circular sector is curved, the light flux allowed to pass through by the light transmitting code fields also depends on the position of the respective pixels on the line. Suitable corrections can be made here accordingly to compensate for the regular reduction in light flux. In a further development of the invention, a maximum code is selected. This maximum code is characterized in that the code elements arranged in an arc in an ordered sequence of code elemen
Benz Jurgen
Ruff Achim
Le Que T.
Valeo Schalter und Sensoren GmbH
Vincent Paul
LandOfFree
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