Patent Publication Number: US-7214954-B2

Title: Method for operating optical sensors

Description:
BACKGROUND OF THE INVENTION 
   The invention concerns a method for operating an optical sensor for measuring a physical quantity, as well as the corresponding sensor. 
   In general, one must define a so-called scale value for measurement devices with which a physical parameter is being measured and which operate pursuant to a calibration certificate. The scale value corresponds to the smallest indicated dimensional unit and is used to define the measurement accuracy. The measurement results may only be indicated in multiples of the scale value. By definition, scale values should only be selected in steps of 1, 2 and 5 times 10 n  (n is an integer). The definition is mandated by national authorities, for example, in Germany, by the Physikalisch Technische Bundesanstalt, which follow the recommendations of the OIML (International Organization of Legal Metrology). 
   Each calibrated measurement device has one of these scale values and is suitable for applications with a measurement accuracy corresponding to that scale value. 
   A problem occurs when the measurement accuracy cannot be constant because it is influenced by external conditions. For example, such a situation is encountered when making automatic length or volume measurements with an optoelectronic sensor, such as a laser scanner, of goods being transported by a conveyor. The length measured in the transport direction by a stationary sensor depends on the transport speed. Upon change in that speed, it might become necessary to employ a different sensor calibrated with a different scale value. Thus, the measurement layout at the conveyor either has a limited range of application or it must have several sensors with different scale values, which is correspondingly expensive and cumbersome. 
   SUMMARY OF THE INVENTION 
   Based on this state of the art, it is an object of the invention to provide an improved method and sensor which are more diversified and can cover larger measurement ranges. 
   In the method of the present invention for measuring a physical quantity by means of a defined scale value governed by a calibration certificate, wherein the measurement accuracy of the quantity being measured depends at least on one external condition and a particular scale value is used for the measurement depending on the measurement accuracy, the scale value of the optoelectronic sensor can be switched in response to a change in the measurement accuracy caused by an external condition. 
   In this way, a large measurement range can be covered by a single sensor. The use of different sensors for different conveyance speeds, for example, is no longer necessary. 
   Since the sensors used, for example on conveyors for the measurement of volume, require a certification, it is additionally no longer necessary to certify several systems for different transport speeds when using the method and/or the sensor of the present invention. It is sufficient to certify a system with a single optoelectronic sensor made according to the invention, whose scale value can be switched. 
   It should be pointed out that the conveyance speed is only a convenient example of many parameters for the external condition so that the advantages can be more clearly explained. The present invention can also be used with other systems with calibratable quantities. 
   Thus, in one embodiment of the invention, the external condition may be derived from a geometrical property of the object. For example, if it is necessary to measure the volume of not only square-shaped luggage and packages, but also bags or pouches of essentially any given shape on a conveyor belt at an airport or the like, one can use the invention, i.e. use one and the same sensor, to measure both the square-shaped goods with high measurement precision as well as bags with a lesser measurement accuracy. In the past, such an arrangement required different measurement systems. With the invention, this task can be accomplished with a single sensor or sensor system. 
   The same benefits occur when measuring goods that are separated from each other, i.e. individual goods arranged at intervals from each other, such as pieces of luggage or packages, with a high measurement accuracy, as well as when measuring goods with lesser accuracy when they touch or overlap each other. The external condition then is a function of the position of the object. The position of the objects, whether they are touching, lying next to each other, are stacked, or lie one on top of the other, can also be automatically detected by the sensor, as is described, for example, in published German patent application DE 102 26 663. 
   For certain arrangements it may alternatively be advantageous to enter the external condition into the sensor manually, rather than automatically. 
   Especially when the external condition is detected automatically, it is advantageous that the scale value is also switched automatically as a function of the external condition, for then no manual intervention is required and the entire measurement process occurs automatically with the desired measurement accuracy. 
   Advantageously, and as already mentioned with reference to one embodiment of the invention, at least one parameter that represents the external condition is detected and the scale value is automatically switched when a defined change in the parameter occurs. The parameter in one preferred usage of the invention is the transport speed of a conveyor and the sensor measures at least one dimension, such as the length of the goods being transported. 
   An optoelectronic sensor of the invention for measuring the physical quantity has means of detecting the physical quantity in dimensional units of a calibratable scale value and includes an arrangement for switching the scale value. 
   Advantageously, the scale value can be automatically switched, so that no manual intervention is necessary, e.g. upon change in the transport speed or when the goods being measured have different shapes or positions. Then, regardless of the external condition, the correct quantity will be measured with the desired measurement accuracy. 
   In order that the sensor can itself and preferably automatically derive the external condition from a geometrical property of the object, or from the position of the object in the embodiments of the invention already described above, a corresponding arrangement is included in the sensor. Such an arrangement is described in the already mentioned DE 102 26 663 publication, to which reference is hereby made, and can consist of electronic components and/or corresponding software. 
   When the external condition can be represented by a parameter, an arrangement is advantageously provided for detecting this parameter. 
   In a simple example of usage, this parameter is the speed of a conveyor and the sensor measures at least one dimension, e.g. the length of goods on the conveyor. 
   The invention is explained below in more detail. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a layout for measurement of geometrical dimensions of goods on a conveyor with an optoelectronic sensor made according to the invention; 
       FIG. 2  is a diagram that illustrates the dependency of a scale value on the transport speed of the conveyor; 
       FIGS. 3–6  are top views which show alternative arrangements of the goods on the conveyor; and 
       FIGS. 7–9  are side views which show arrangements of the goods on the conveyor. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   An optoelectronic sensor  10  according to the invention is arranged above a conveyor  12 , on which goods such as packages  14  are being transported in any desired direction  16 . The packages  14  are automatically measured by the sensor  10  during their transport to determine their volume. The measurement results can be indicated on a display unit  18  and/or they can be fed to a suitable signal output for further processing. 
   The optoelectronic sensor  10  can be a laser scanner, such as is marketed by the assignee of this invention, SICK AG of Germany, under the designation LMS  200 , which scans the package  14  in familiar manner with a laser beam  24 . The volume can ultimately be determined from the angle and distance information of the reflected laser beam. 
   When determining the volume of a square-shaped package  14 , it is necessary to determine its height h, width and length L. The accuracy of the length measurement in the transport direction  16  is, as mentioned above, dependent upon the transport speed fg. Since measurement devices for the determination of volume require calibration certificates, such as specified in the German Calibration Statute or the Canadian Weight and Measurement, a scale value as discussed above is defined. 
   Furthermore, a device or arrangement  20  is provided for detecting the speed of the conveyor, so that a signal corresponding to the speed fg can be fed over a line  26  to the optoelectronic sensor  10 , or to an evaluation unit assigned to the sensor. The device  20  can be a separate part of the measurement arrangement, or it can be part of the optoelectronic sensor  10 . Furthermore, a switch  22  is provided for switching the scale value as a function of the transport speed fg. 
   The dependency of the scale value on the transport speed fg is shown in  FIG. 2 . Below transport speed fg 1 , a scale value of 1 is set. At a transport speed larger than fg 1  and smaller than fg 2 , the scale value is 2, and at a transport speed larger than fg 2  a scale value of 5 is set. The switching of the scale values takes place automatically so that, regardless of the transport speed fg, the length L and ultimately the volume will always be determined with the correct scale value and the required measurement accuracy. 
   In other embodiments of the invention, the external condition can be derived in other ways as will be explained with reference to  FIGS. 3–6  and  7 – 9 . 
   For example, the volume of packages  14 . 1  and  14 . 2  being transported by conveyor  12  can be measured without problem and with a constant measurement accuracy when the packages are being delivered separately and at an interval from each other, i.e. individually. In reality, however, packages often touch each other, as is shown for example in  FIGS. 3–6 . Packages  14 . 1  and  14 . 2  of  FIGS. 3 and 4  touch and those of  FIGS. 5 and 6  are side by side. The packages can also lie on top of each other, as is shown in  FIGS. 7 and 8 . The arrangements of  FIGS. 7 and 8  show stacked and those of  FIG. 9  show leaning package arrangements. 
   If one of these situations occurs, it is difficult for the sensor to recognize the individual packages  14 . 1  and  14 . 2  and the measurement is correspondingly poor. In the already mentioned German patent publication DE 02 26 663, a method is described for recognizing individual packages under such conditions and measuring them separately. However, the measurement will not have the same accuracy as when the packages are separate from each other. 
   Therefore, when one of these conditions occurs, i.e. one of these external conditions of touching, stacked, etc., the scale value is switched in accordance with the invention as a function of the external condition. 
   It is conceivable that, for example, the touching condition is assigned a different scale value than the stacked condition, and the latter, again, is given a different scale value than the leaning condition. 
   Furthermore, the external condition can be derived from the form of the delivered goods. Thus, for example, square-shaped luggage can be measured with higher measurement accuracy than bags with an irregular external form, so that the scale value is adjusted in accordance with the invention as a function of the external shape of the goods. 
   Since the sensor of the present invention has means  28  of recognizing the external condition, the sensor  10  can be automatically switched to a particular desired measurement accuracy, i.e. to the corresponding scale value in the certified system. The means  28  of recognizing the external condition are described in detail in the earlier mentioned DE 102 26 663 publication and comprise, for example, the recognition of the position and/or shape of the delivered goods. The essential part of these means  28  of recognition is intelligent software which can evaluate the reflections of the scanning light beam.