Method and apparatus for determining the fill level of containers
BACKGROUND OF THE INVENTION 
     This invention relates to a method and apparatus for determining if a container, such as a glass bottle, is filled with material to above or below a prescribed level. 
     In a known method for determining the fill level of containers, the containers are transported consecutively through a beam of penetrating radiation emanating from a radioactive isotope source. A radiation detector unit is located on the opposite side of the container from the radiation source. The detector unit converts the radiation photons to electric pulses. The pulses are summed within a definite measuring interval and the sum is compared with a reference or threshold value. The sum of the counts will be less than threshold value if the radiation beam has been attenuated by the presence of material in the container above the prescribed level and the pulse count sum will exceed the threshold value if the material level in the container was below the level of the radiation beam so as to not contribute to attenuation. In the latter case, the comparator provides a signal which causes an ejection device to operate and remove the underfilled container from the production line. 
     To define the measuring interval, it is necessary to determine the point in time at which the container begins to pass through the radiation beam. The ultimate determination as to whether the ejection threshold has been reached is critically dependent upon accurate definition of the measuring interval. In one previously known method, the measurement of the pulse rates is triggered or initiated when the container begins to pass through a light beam which is directed toward a photoelectric device. In this arrangement, the duration of the measuring interval is assumed to be constant. 
     Using a photoelectric device to determine the beginning of a measuring interval has several disadvantages. One disadvantage is that the light beam must be realigned with the photoelectric detector for containers such as glass bottles, plastic bottles and cans having different sizes and shapes. With any type of container it is necessary to initiate the measuring interval at exactly that point in time at which the leading external surface of the container moves into the radiation beam. Additional problems result when a change is made from inspecting a material level in containers which have clear walls to containers which do not such as cans made of metal. 
     Another problem with prior art fill-level determining apparatus is that measuring accuracy is affected by the speed with which the containers pass through the penetrating radiation beam. When the containers move slowly, the pulse count measuring interval may have already expired and yet the container may still be in the path of the beam. Hence, a loss of information needed for determination of the fill-level occurs. At the other extreme, when the containers pass through the penetrating radiation beam at very high speeds, the containers may be out of the beam already before the end of the defined pulse counting or measuring interval which again affects accurate determination or calculation of the fill-level adversely. 
     To eliminate the effect of these variables, it has been necessary in known methods and apparatus to coordinate the measuring area with the measuring time interval by adjusting container transit speed and by adjusting the level of the beam or the height of the container to account for variations in diameter of different containers in order to permit reliable differentiation between underfilled and overfilled containers. 
     A further disadvantage of using a photoelectric device is that it increases the cost and complexity of the system and requires periodic attention and adjustment. 
     Use of an inductive circuit breaker as a proximity switch instead of a photoelectric device has been proposed but this creates adjustment problems which are more serious than those resulting from use of a photoelectric device. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a method and apparatus that is simple and accurate and avoids the regulation and adjustment problems that heretofore resulted from switching from inspection of one type of container to another and avoids the effects of variables in containers of the same type and geometric and speed variables as well. 
     In accordance with the invention, accuracy is greatly improved by an improved way of determining the beginning and end of the pulse counting or pulse rate measuring interval. 
     Stated in another way, changes in the counting rate at different times when the container is passing through the beam is used to determine the beginning and end of the useful pulse counting or measuring interval. Since the beginning and end of the measuring interval may be acquired from the counting rate itself, no information is lost as the entire course of the pulse rate over the cross section of the container becomes available for evaluation. Another benefit is that the effect of statistical fluctuations of the pulse counting rate are minimized. 
     Accordingly, it may be said that because of the new method, the measuring interval is adjusted automatically to the optimum attainable for accurate fill-level determination independently of the container passage speed and the geometry of the containers. It is only necessary to assume a suitable ejection threshold for determination of the fill-level for a particular surface mass of the container and the particular filler material. 
     An advantageous feature of the new method provides for defining a shortened measuring interval which selects out the particular points in time during which only the wall of the container is in the penetrating ray beam. This is in recognition that the reduction in the pulse rate is caused solely by means of the absorption or scattering of the gamma rays in or on the container wall and consequently is independent of the level of the material in the container. The shortened measuring interval, that is, an interval that corresponds to less than the width of the container, ensures that only such numbers of pulses are used for fill-height determination as may result from irradiation of the central area of the container where pulse rates differ significantly between full and underfilled containers. This means that statistical fluctuations of the counting rates within the measuring interval are also accounted for and it means greater accuracy in determining when the counters are to be triggered or enabled and disabled. 
     How the foregoing and other more specific objects of the invention are achieved will be evident in the description of illustrative embodiments of the method and apparatus which will now be set forth in reference to drawings. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graphical representation of pulse counting rates as ordinates and time as the abscissa for an associated bottle which is presently in the penetrating radiation beam; 
     FIG. 2 is a block diagram of one type of apparatus for carrying out a first version of the method according to the invention; and 
     FIG. 3 is a block diagram of another apparatus for carrying out a second version of the method according to the invention. 
    
    
     DESCRIPTION OF A PREFERRED EMBODIMENT 
     The first embodiment of the invention will be explained in reference to FIGS. 1 and 2. In FIG. 2, a source that provides a penetrating radiation beam such as a gamma ray beam is represented by the block marked 10. One of the containers in a series of containers whose fill level is to be determined is marked 20. The containers are typically bottles although they may be cans as well. The containers pass through the penetrating radiation beam which is represented by an undulating arrowheaded line. The emergent beam which has been attenuated by an intervening container is received in a detector unit symbolized by the block marked 11. The detector puts out electric pulses at a rate corresponding to the rate of incoming gamma ray photons. The detector unit may be of a well-known type comprised of a scintillation crystal, a photomultiplier tube and a pulse height selector which are not illustrated specifically. 
     Referring now to FIG. 1, the counting rates N of the pulses that are output from the detector 11 are plotted in in a graph as ordinates vs. time t on the abscissa. A cross-section of a container 20 having a wall thickness D is disposed next to the plot. When the container is moved through the radiation beam at constant speed the counting rates, N, will vary with time and, accordingly, with bottle container position as shown in the diagram. 
     One may see that after the front edge of container 20 enters the path of the rays, the pulse rate from the detector unit 11 drops to a minimum since at this time the maximum length A of absorption by the container wall is present. Thereafter, the pulse counting rates for a filled container 20 varies or follows the level indicated by the line marked b and with an empty or underfilled container the counting rates vary in respect to time in accordance with the line marked a. The absorption or scattering of the gamma rays in the irradiated material, when such material intervenes, produces the differences in the counting rates. 
     After the container 20 passes through the radiation beam, the pulse rate rises again to its orginal level. This level N 0  is accordingly the rate at which the pulses are put out by detector unit 11 when no container 20 is in the penetrating radiation beam. 
     The manner in which the pulse counting or measuring interval is determined in accordance with the invention will now be discussed. 
     Referring to FIG. 1, N 0  is the pulse count rate with no bottle or container in the radiation beam. The drop in the pulse rate to a value N 1  results from the wall of the container beginning to pass through the beam. N 1  is designated a first trigger or threshold level. This threshold pulse rate is sensed by a threshold discriminator represented by the block marked 12 in FIG. 2. Sensing of the threshold initiates pulse counting as will be explained. The trigger threshold N 1  must necessarily be selected so that it lies outside of the statistical fluctuations of the pulse rate N 0 . The point in time t 1  at which the pulse rate falls below trigger threshold N 1 , therefore, marks the beginning of the measuring interval M as it is designated in the embodiment of the invention which is being discussed presently. Sensing of the first threshold at t 1  by discriminator 12 in FIG. 2 results in an output signal from the discriminator. The output of the discriminator is in a circuit with a first AND gate 14 and a second AND gate 161 which respond to input of the threshold signal by becoming enabled to gate clock pulses and pulses corresponding to photons, respectively. A clock pulse counter 15 is used and it is provided with clock or timing pulses from a clock pulse generator 13. The output of AND gate 161 is coupled to an adder circuit 162. When threshold is sensed and the beginning of the measuring interval M occurs, the adder circuit 162 counts all of the pulses delivered from the detector unit 11. Simultaneously with occurrence of the first threshold or count triggering event, counter 15 begins to count clock pulses or unit time pulses from generator 13. 
     Referring further to FIG. 1, after the container 20 passes through the radiation beam, the pulse rate N finally reaches a second trigger threshold N 2  which again is sensed by the threshold discriminator 12. Then the threshold discriminator output changes level to disable AND gates 161 and 14 so that they no longer output detected pulses and clock pulses, respectively. The number of detector pulses counted then remains unchanged in the adder 162 and the number of clock pulses counted remains unchanged in counter 15. 
     The contents of the adder circuit 162 and the time unit counter 15 are delivered to a divider circuit represented by the block marked 17. The divider circuit forms the quotient of these two counts and, accordingly, the average pulse rate in the measuring interval M. This measured average pulse rate is then fed to one input of a comparator 19 whose other input receives a signal value representative of a container fill reject threshold S from a threshold reference value storage device 18. The container reject threshold S is selected in such manner that if it is exceeded in the comparator by the quotient count, it is indicative of the beam not being attenuated by material in the container and of the material level being below the desired level. If the quotient is below the reference threshold value it is an indication that the material in the container is filled to at least the minimum required level. Stated in another way, if the measured pulse rate corresponds to the curve b in FIG. 1, the average pulse rates in the measuring interval M will lie below the rejection threshold S and, consequently, comparator 19 will not deliver an output signal. If the beam is not attenuated by intervening fill material, curve &#34;a&#34; will be in effect and the comparator 19 will output a signal. This signal is used to control a well-known container rejection or ejection device, not shown, which removes the improperly filled container from the line of containers progressing through the inspection beam. 
     The embodiment of the invention just described is advantageous in that a very simply constructed signal processing unit 16 can be used since the pulse rates during the measuring interval M are just added. This embodiment provides reliability and accuracy of level evaluation which is satisfactory for most practical applications. 
     Another embodiment of the invention which provides even greater accuracy will now be described in reference to FIGS. 1 and 3. 
     On inspection of the counting rate curves in FIG. 1 one may see that the two curves a and b extend almost congruently as long as the container wall structure with the thickness D moves through the path of the penetrating radiation beam. This means that at the beginning and end portions of the measuring interval M, substantially no usable information can be obtained as to whether the material in the container is above or below the desired level. Moreover, the thicknesses of the wall of different containers may vary such that anomalous information is yielded. According to the invention, provision is made for defining a shortened measuring interval M&#39; for evaluating fill level which permits exclusion of anomalous information. As shown in the graph, first the pulse rates are advanced from a point in time t 1  which lies one time span Δt 1  after the point of time t 1  where the first trigger threshold N 1  falls below the curve. Correspondingly, the measuring interval will be ended at a point of time t 2  &#39; which lies in a time span Δt 2  before the point of time t 2  where the second trigger threshold N 2  is exceeded. Thus the measuring interval lies between t&#39; 1  and t&#39; 2 . This shortened measuring interval M&#39; covers the time in which the middle area of the container 20 passes the radiation beam from source 10. The central area of interest is shown cross-hatched in FIG. 1. 
     Hence the indefinite information on the pulse rates N at the beginning and end areas of the container 20 is eliminated. Thus only those parts of the pulse rate curves for overfill and underfill wich clearly differ from one another are used as in this area the conditions of container underfill and fill produce the maximum differential between pulse rates. The greater differences results in more accurate evaluation of the level of the material in the container. 
     To obtain and use a shortened measuring interval M&#39; a processing circuit 16 is shown in FIG. 3 is employed. For determination of the points of time t 1  &#39; and t 2  &#39; practically the entire pulse rate must be stored, and indeed, not cummulatively as in the addition circuit 162, but individually. 
     The FIG. 3 embodiment, therefore, provides a signal processing unit which has multiplication circuits 164 and 166 and an addition circuit 167 similar to the addition circuit 162 with a read-write memory 163 now being included. 
     The pulse rates provided by the detector 11 are entered consecutively cyclically (the devices required for the cyclic storage are not shown for the sake of simplicity) in the electronic read-write memory unit 163 and are pushed further with each cycle so that after the container 20 passes through the beam the course of the pulse rates in FIG. 1 is stored in the electronic read-write memory 163 as is shown symbolically in FIG. 3. 
     As in the first embodiment, the clock pulse counter 15 counts the clock or unit time pulses during the measuring interval M. The point of time t 1  and t 2  of the beginning or end, respectively, of the measuring interval M are marked in the electronic read-write memory 163 at the pertinent pulse rate. The addresses of the count rates in the memory 163 are accordingly numbered in a continuous fashion from 1 to y from the point of time t 1 . When counter 15 becomes enabled by discriminator 12, the counter output is input to three multiplication circuits 164, 165 and 166 which cooperate to shorten the measuring interval M to the measuring interval M&#39;. 
     The first multiplication circuit 164 multiplies the total number of the time units by a factor x, where x lies between 0 and 0.5. Thus the starting point of time t 1  as in FIG. 1 is established. 
     Correspondingly (with symmetrical reduction), the second multiplication circuit 165 multiplies the time units by a factor (1-x), so that the end point of time t 2  is determined. The addresses of the memory unit contents (pulse rates) whose addresses lie between both addresses given out by the two multiplication circuits 164 and 165 are added in the addition circuit 167. At the output of the addition circuit 167, therefore, the total pulse rate is available during the measuring interval M&#39;. Because of the shortened measuring interval and the corresponding reduction of clock pulses or time units during the interval, the total number of time units given off by the clock pulse counter 15 in the measuring interval M are not used. Therefore, a third multiplication circuit 166 is used. It multiplies the clock pulses counted by the time unit counter 15 by the factor (1-2x), and thereupon calculates the number of clock pulses or time units in the measuring interval M&#39;. 
     In FIG. 3 as in FIG. 2, the total pulse rate and a time unit number may be supplied to divider circuit 17 so that at the output of the divider circuit the average pulse rate in the shortened measuring interval M&#39; is plottable. Further processing of the signals as with the threshold reference value indicator 18 and the comparator 19 is similar to what has been described in connection with the FIG. 2 embodiment and need not be repeated. However it may be noted that for the FIG. 3 mode, another rejection threshold S&#39; will be selected. 
     The FIGS. 2 and 3 block diagrams are sufficient for explaining the fundamental construction of the apparatus according to the invention. In this connnection, addition devices which do not touch on the essence of the invention are not shown for the sake of clarity. 
     In particular, in the second embodiment illustrated in FIG. 3, on account of the large memory capacity needed it is advantageous to carry out the entire operation with a programmable microcomputer, not shown. A Zilog Z80 microprocessor is suitable. 
     The second embodiment of the method permits taking into account flucuations or drift in the pulse rate N 0  and accordingly permits after-regulation of the trigger thresholds N 1  and N 2  and the rejection threshold S&#39;. In other words, the indicated thresholds may be raised or lowered, respectively, in the same relation as the maximum counting rate N 0  increases or decreases. For instance, in a definite time interval between two containers the pulse rate from the detector may be measured and compared with the previous N 0 . If significant differences occur, a special control circuit, not shown, may be used to energize or trigger the threshold discriminator 12 and the reference threshold value indicator 18 to adapt the trigger thresholds N 1  and N 2  and the rejection threshold S. 
     In summary, the invention provides a simple method for determining the fill level of containers with great accuracy. Avoidance of external trigger devices such as photoelectric devices or proximity switches permits elimination of periodic and time-consuming adjustments of the system components.