Device and method for taking pictures of objects moving on a conveyor system

Abstract

Device, in particular code reader, for recording images of objects (14) moving on a conveyor device (12), wherein the device has a line sensor (10), in particular a line camera, which can scan the objects line by line to generate image data of the surface, and a speed determination device (26;30) which can determine the movement speed of the objects (14) relative to the line sensor (10) or their conveyed distance, wherein an evaluation unit (26) is implemented on a programmable logic module of the line sensor (10), which is configured to adapt the recording frequency of the line sensor (10) to the movement speed, and wherein the speed determination device (30) is a conveyor controller of the conveying device (12) which specifies the speed profile of the conveying device (12) and which notifies the line sensor (10) of a changed speed profile from a defined time.

Description

[0001] The invention relates to a device for recording images of objects moved on a conveyor device according to the preamble of claim 1 and a corresponding method according to the preamble of claim 12.

[0002] For the automation of logistics applications, such as the automatic sorting of objects, objects are provided with a code that is read by a code reader. A common practical application is barcodes with corresponding barcode scanners. However, other coding methods are also used for identification, such as two-dimensional codes like a Maxicode or an Aztec code, or labels decoded using optical character recognition (OCR). To read such codes, images of the objects with the codes are taken.

[0003] In an automated reading device, such as baggage handling at airports or the automated sorting of parcels in logistics centers, the objects are conveyed past the code reader and image data of the objects with the codes arranged on them is obtained by scanning line by line.

[0004] The conveyor on which the objects are moved does not always move at a constant speed. This is particularly the case with tray sorters, where each tray of the conveyor holds only one object at a time. When loading such a tray sorter, objects are transported to the trays using so-called drop-in conveyors. If no tray is currently free, the drop-in conveyor must be slowed down or stopped. A code reader mounted above the drop-in conveyor, which scans objects line by line, then only produces distorted images because the relative movement of the object to the code reader is irregular. This problem occurs not only on drop-in conveyors, but with any change in the speed of the conveyor on which the objects are located.However, distortions in the recorded image data make decoding more difficult, especially when conventional text recognition programs are required to evaluate the images to decode a label such as an address.

[0005] US Pat. No. 6,749,110 B2 discloses a scanning system with an oscillating line sensor. The line is aligned in the conveying direction above an object stream. The image created by combining the lines is distorted due to the object movement in between.

[0006] JP 2003 / 042972A discloses an image recording device for inspecting objects on a conveyor that captures images line by line. The image recording frequency is adjusted to the conveyor speed.

[0007] US 2003 / 0206305 A1 describes a measuring device for conveyed objects which has a line sensor which operates at a fixed scan rate.

[0008] It is therefore an object of the invention to further develop a barcode scanner with line-by-line acquisition of image data in such a way that it delivers distortion-free or less distorted images.

[0009] This object is achieved by a device and a method for capturing images according to claims 1 and 12, respectively. By adapting the recording frequency of the line sensor to the movement speed, the resolution of the image data can remain independent of the movement profile. Downstream image evaluations, including decoding or text recognition, can work with image data free of interference, distortions, and other artifacts caused by varying conveyor speeds. This simplifies evaluation and, above all, avoids misreadings, thus increasing the reading rate, so that complex and costly manual post-processing of the automated sorting is less frequently required.

[0010] The invention is based on the principle of detecting and reacting to disturbances in the smooth movement of the conveyor system. The density of the recorded image lines then no longer depends on the speed of the conveyor, but ultimately on the distance the objects are conveyed, which represents the correct measure for a distortion-free and uniformly resolved image. Therefore, the speed determination device can also be understood as indicating the distance traveled without a direct time reference. Since a time reference is nevertheless established via the recording frequency, a speed measure is ultimately used as a basis in this case as well.

[0011] The evaluation unit is preferably configured to adaptively adjust the recording frequency such that the line sensor records image data from equidistant lines of the object. Here, the reference is the object via its conveying speed; the image lines are thus equidistant and evenly distributed across the object. This results in a uniform image resolution in the direction perpendicular to the lines, corresponding to an image recorded in a fictitious situation with a constant conveying speed and a constant recording frequency. The evaluation is thus independent of interference due to fluctuations in the conveying speed.

[0012] In a preferred embodiment, the line sensor is designed to be set to a basic recording frequency with associated recording parameters, which corresponds in particular to a maximum conveying speed of the conveyor, and the evaluation unit is designed to differentially specify the adjustment of the recording frequency and / or the recording parameters to a movement speed that is different from the maximum conveying speed. Such recording parameters can be brightness, amplification factors, exposure times, or the like. The system is thus set up with a basic setting that corresponds to the normal state of the smoothly running conveyor. According to the invention, this basic state does not require intervention.Only when a conveyor speed deviating from the baseline is detected, the frequency is adjusted and, if necessary, one or each frequency-dependent recording parameter is modified, for example, the exposure time is shortened to accommodate a higher recording frequency and the gain is increased accordingly.

[0013] The speed-determining device is a conveyor control of the conveyor system, which specifies the speed profile of the conveyor system. The conveyor control can transmit data about the speed profile to the evaluation unit, which includes, in particular, a current speed, a duration of a subsequent acceleration phase, a start of the acceleration phase, and / or a final speed after completion of the acceleration phase. Here and in the following, acceleration is to be understood in the physical sense, thus including deceleration or braking in the case of negative acceleration. Furthermore, an acceleration phase at constant acceleration is overdetermined with the initial speed, final speed, start, and duration of the acceleration phase; therefore, the transfer of only some of these parameters is sufficient.If data about a phase of non-uniform movement of the conveyor is transferred directly from the conveyor control, on the one hand, an actual measurement of the speed can be omitted, and on the other hand, data about the change in speed is already available well in advance.

[0014] Especially with tray sorters, it is common practice to decelerate and accelerate not in an arbitrary manner, but with a precisely defined acceleration phase. This approach is conceivable for any conveyor. A practical example of such a predefined acceleration phase is when the conveyor control system advantageously allows for the setting of a maximum conveyor speed of 2 m / s, a decelerated conveyor speed of 0.5 m / s, and an acceleration phase of 500 ms at an acceleration of 3 m / s. 2from maximum conveyor speed to decelerated speed and vice versa. This parameter set is used especially for common tray sorters and their infeed conveyors.

[0015] The conveyor device is preferably a tray conveyor or its infeed conveyor. With a tray conveyor, the problem of a tray already filled, requiring a speed adjustment, can easily arise. Therefore, the invention is particularly suitable for tray conveyors because it can compensate for distortions in the image data caused by the acceleration phases that regularly occur.

[0016] In a particularly advantageous development, the temporally changing recording frequency for a given speed profile of an object is stored in a table in the evaluation unit as time intervals between each two recordings, with the entries in the table describing, in particular, a hyperbola. In this way, the device can be prepared in advance for specific speed changes of the conveyor. For this purpose, the table can be calculated, entered, or read in via an interface or network connection in a preliminary phase, but also online, during commissioning, or during a maintenance break.The hyperbola as a table of values ​​stored in the table is particularly advantageous because it is the appropriate function for treating a phase with uniform acceleration due to the inversely proportional relationship between recording frequency and period, i.e. the time between two recordings, and the proportional relationship between recording frequency and conveyor speed for recording a uniformly resolved image.

[0017] Even more preferably, the evaluation unit is designed so that entries in the table are predefined, in particular by input, reading, or calculated based on a predefined speed profile, in particular as hyperbolas from a maximum conveyor speed, a decelerated conveyor speed, and a uniform acceleration phase in between, specified by a start time, end time, and / or acceleration. Thus, the invention can independently generate the table from data describing the acceleration phase, thus facilitating setup.

[0018] The evaluation unit is preferably designed to use a table value multiple times to map longer acceleration phases and / or to cause the line sensor to stop recording when the movement speed is zero, i.e. the conveyor is stopped. According to the first aspect, the table can be more coarsely resolved to save memory requirements and computational effort by using each value multiple times. According to the second aspect, the device is able to react appropriately to a belt stop where further recordings of image data cannot provide any useful information. When the belt starts up again, a smooth transition can be ensured because this is nothing more than an acceleration phase with an initial speed of zero, which can be absorbed according to the invention like any other acceleration phase.

[0019] In a further preferred embodiment, an additional table is provided in the evaluation unit, which contains time intervals for the recording frequency in the event that the sign changes during an acceleration phase, i.e., an acceleration reversal occurs, to ensure a smooth transition of the time intervals. Without such an additional table, the recording frequency plotted against time could exhibit a kink, which would be reflected in the image data as a disturbing distortion.

[0020] The evaluation unit is at least partially implemented on a programmable logic module, in particular an FPGA, of the line sensor. The line sensor advantageously has at least one input with which the logic module can be instructed to start and execute the adjustment of the recording frequency via a predefined speed profile. The invention is thus essentially implemented in a separate module, which can be triggered by a simple start command to compensate for disruptive influences caused by changes in the conveyor speed. This enables simple, cost-effective, and modular integration of the solution according to the invention into existing reading systems. Of course, an existing logic module can also be equipped with the functionality according to the invention instead of an additional one.

[0021] The method according to the invention can be further developed in a similar manner and thereby exhibits similar advantages. Such advantageous features are described by way of example, but not exhaustively, in the subclaims following the independent claims.

[0022] The invention will be explained in more detail below with regard to further features and advantages, using exemplary embodiments and with reference to the accompanying drawings. The figures of the drawing show: Fig. 1 a schematic three-dimensional representation of an embodiment of the device according to the invention above a conveyor belt with several objects moving thereon; Fig. 2 a representation of the temporal dependence of the recording period, i.e. the inverse recording frequency, and the conveying speed in a braking and acceleration phase; Fig. 3 a representation according to Fig. 2 in the special situation that the conveyor is accelerated again during the braking phase; and Fig. 4 a flow diagram of the inventive adaptation of the recording frequency to the conveying speed using the example of a braking phase with constant acceleration.

[0023] Fig. Figure 1 shows, in a schematic three-dimensional representation, the arrangement of an embodiment of a line sensor 10 according to the invention above a conveyor belt 12, on which objects 14 are moved past the sensor 10 in a direction indicated by arrows 16. The objects 14 carry information 20, which is detected and read by the line sensor 10.

[0024] The line sensor 10 has a lens 22 (shown only in outline) with imaging optics and a line sensor 24, for example a CMOS or CCD line. The line sensor 24 provides image data of the objects 14 in a reading plane 18. These image data, captured line by line as the objects 14 continue to move through the reading plane 18, are combined by a camera controller 26 into an overall image to enable image evaluation. Possible image evaluations include the decoding of the information 20, such as barcodes, two-dimensional codes, or text recognition, by the controller 26 or in a downstream, Fig. 1. As an alternative to a line-scan camera with a line-shaped image sensor 24, another technical implementation can also be provided as the line sensor 10, such as scanning the line or any other known optoelectronic method.

[0025] A communication node 28 is connected to the controller 26, for example as part of a data network or as a bus system, which receives data about the conveyor belt 12 from a conveyor controller 30 and forwards it to the controller 26. The conveyor controller 30 is responsible, among other things, for controlling the movement of the conveyor belt 12, i.e., accelerating, decelerating, starting, stopping, or maintaining it at a specific conveyor speed. As an alternative to connecting the controller 26 to the conveyor controller 30, it can also be provided to measure the speed or position of the conveyor belt 12 with a dedicated sensor, which is arranged upstream of the line scan camera 10 or integrated into it, such as an incremental encoder, a displacement encoder, or even an upstream optoelectronic sensor.

[0026] In a specific embodiment of the invention, the conveyor belt 12 is operated at a constant maximum speed of 2 m / s during normal operation. The conveyor control 30 is capable of reducing the speed to 0.5 m / s and then returning it to the maximum speed. The speed change occurs in a braking or acceleration phase of 500 ms at a constant acceleration of ±3 m / s. 2This is particularly necessary if the conveyor belt 12 is a feed belt for a tray sorter, which must be decelerated if no tray is currently free. However, the invention is not limited to this specific speed profile of the conveyor 12, but is capable of generating undistorted image data at any constant and non-constant speed by adapting the recording frequency of the line sensor 10 to the current speed and, associated with it, to the distance traveled by the object 14 between two recordings by the line sensor 10.

[0027] The adjustment of the recording frequency will now be explained using a speed profile as shown in Fig. 2. Instead of the recording frequency, the actually relevant value is its inverse, i.e. the recording period or the time that elapses between two recordings of the line sensor 10. This can be handled directly by the controller 26 by waiting for the required recording period after a recording. Therefore, in Fig. 2 In addition to the dashed speed profiles, the inverse value of the recording frequency, namely the recording period T, is shown with a solid line instead of the recording frequency.

[0028] If you follow the speed profile (dashed line) and the corresponding recording period (solid line) in Fig. 2 over time, the conveyor belt 12 initially moves at a maximum speed of 2 m / s, while the line sensor 10 scans the conveyed objects 14 line by line at a basic recording frequency represented in arbitrary units. At a point in time represented by the first vertical dotted line, the conveyor belt is accelerated at a constant rate of 3 m / s 2decelerated within 500 ms to a decelerated speed of 0.5 m / s. To ensure that the image data of the objects 14 are uniform during this deceleration phase as well, i.e., the image lines are distributed equidistantly across the objects 14, the recording period increases hyperbolically during the deceleration phase. While the conveyor belt subsequently moves at the decelerated speed of 0.5 m / s, the line sensor 10 records image data with an increased, constant recording frequency per line. At a later point in time, the conveyor belt 12 is uniformly accelerated back to its original maximum speed of 2 m / s (third and fourth dashed vertical lines), with the recording period being shortened hyperbolically during the acceleration phase, entirely analogous to the deceleration phase.

[0029] In this way, an adverse effect of the non-constant speed of the conveyor 12 is compensated by a corresponding adjustment of the recording period or the recording frequency, and the image data of the image composed by the controller 26 are distortion-free, thus revealing no difference from a fictitious situation in which the conveyor belt 12 is moving at a constant speed without acceleration phases.

[0030] A special situation arises when, as indicated by the dot-dash line, the conveyor comes to a complete standstill beyond the reduced speed. In this situation, the recording period must be adjusted using a correspondingly extended hyperbola and set to infinity during the belt stop, i.e., the system waits until the conveyor 12 starts moving again. The restart of the conveyor 12 represents a further acceleration phase during which the recording frequency can be adjusted for a distortion-free image, as shown.

[0031] In order to implement the adjustment of the recording frequency in a particularly simple technical manner, the line sensor 10, with its recording parameters such as brightness or exposure time, as well as the recording frequency, is set to a basic state which corresponds to the maximum speed of the conveyor 12. If the speed changes, the recording frequency is adjusted by differentially adding a component to the recording period of the basic state which corresponds to the adjustment. The recording parameters are also adjusted accordingly to the changed recording period. The underlying maximum speed can be specified or determined adaptively during operation. For this determination, the speed data can be determined by the controller 26 itself or transmitted from the conveyor controller 30 via the message node 28.

[0032] Fig. 3 is one of the Fig. 2 Analog representation to illustrate a special situation in which acceleration occurs again during deceleration. The dashed section of the acquisition period plotted over time shows a kink at the point where the sign of the acceleration reverses. This kink would lead to a distortion of the image data. Since the line sensor 10 is informed of the kink in advance via the conveyor control 30, the acquisition period can be smoothed, as shown in the enlarged section 34, thus avoiding the distortion.

[0033] Based on the Fig. 4, a method for adjusting the recording frequency is presented below. This method is based on a table in which values ​​for suitable recording periods for a given speed profile are stored. For constant acceleration, the table is a table of values ​​for a hyperbola; for other speed profiles, the table must be calculated from the inverse relationship between recording frequency and recording period and the relationship between the speed of the conveyor 12 and the recording frequency, which is proportional to a desired uniform resolution of the image data. Such a calculation can be performed in advance and the table stored in a non-volatile memory; however, the controller 26 can also be capable of independently calculating the table from a speed profile.Further configuration procedures consist of entering or reading the value table via a network connection or an operator interface.

[0034] First, in a first step S1, during a phase in which the conveyor 12 moves at a maximum conveyor speed, the line sensor 10 operates at a base recording frequency. In a second step S2, the line sensor 10 receives a message from the conveyor controller 30 announcing a changed speed profile starting at a specified time t0. In the example shown, this is a braking phase, the parameters of which the conveyor controller 30 transfers to the controller 26 of the line sensor 10. In a third step S3, which consists of a loop divided into substeps S3a-S3d, the table with the recording periods associated with the speed profile is processed starting at time t0 until the braking phase is over.

[0035] For this purpose, one row is recorded per loop iteration (S3a), the next recording period or waiting time is read from the table (S3b), the corresponding time is waited (S3c), and then checked whether the end of the table has been reached (S3d). If the end of the table has not yet been reached, the loop starts again with a recording of the next row (S3a).

[0036] When the end of the table is reached, the loop is exited and a further step S4 checks whether the braking phase has also ended according to the parameters transferred in step S2, or whether the speed is constant again after the current measurement. If this is the case, further image data is generated line by line with a recording period that corresponds to the last table value and thus a recording period appropriate to the final speed reached. If, on the other hand, the braking phase was not yet ended in step S4, a check is carried out (S6) to see whether the conveyor 12 has been stopped. If this is the case, further recordings are stopped until the conveyor 12 starts up again in a new acceleration phase. Otherwise, the braking phase is not yet ended, even though no more values ​​are provided in the table.Then, as in step S5, the process continues with the last table value for the recording period because, although this may not be the ideal value, it is certainly the best available value.

[0037] The described run through an acceleration phase can be a deceleration phase, a true acceleration phase with positive acceleration, or a combination of both with single or multiple reversals of motion. The procedure is the same in each case because the required recording periods are provided in the table. This also applies to the special case of acceleration from a standstill of the conveyor 12, which offers no special features from the perspective of the method according to the invention.

[0038] Implementation via a value table is only a particularly simple implementation, and the invention is not limited to this. For example, it is also conceivable that the speed or the distances traveled by the conveyor 12 are determined dynamically by the line sensor 26 or an upstream sensor, and the next recording period is calculated from this online or with a time offset.

[0039] In a particular embodiment of the invention, the control 26 as a whole or only that part which carries out the method according to the invention, in particular according to Fig.4, including the value table, is implemented on a dedicated logic component, for example, an FPGA. It is then sufficient to inform this logic component of the present speed profile, and it returns the appropriate recording periods in chronological order. For this purpose, a whole series of tables corresponding to various speed profiles, braking phases, or acceleration phases can be stored specifically in the FPGA. The invention can then be easily integrated into existing systems by using a simple switching command to cause the FPGA to output adjusted recording periods, which are used as the waiting time, instead of a constant waiting time between two recordings.

Claims

[1] Device, in particular a code reader, for recording images of objects (14) moved on a conveyor device (12), the device comprising a line sensor (10), in particular a line camera, which can scan the objects line by line to generate image data of the surface, and a speed determination device (26;30) which can determine the movement speed of the objects (14) relative to the line sensor (10) or their conveyed distance, wherein an evaluation unit (26) is implemented on a programmable logic module of the line sensor (10), which evaluation unit is designed to adapt the recording frequency of the line sensor (10) to the movement speed and wherein the speed determination device (30) is a conveyor control of the conveyor device (12), which predetermines the speed profile of the conveyor device (12) and which announces a changed speed profile to the line sensor (10) from a specified point in time.; [2] Device according to claim 1, wherein the evaluation unit (26) is designed to adapt the recording frequency such that the line sensor (10) records image data of equidistant lines of the object (14). [3] Device according to claim 1 or 2, wherein the line sensor (10) is designed to be set to a basic recording frequency with associated recording parameters, which in particular corresponds to a maximum conveying speed of the conveying device (12), and wherein the evaluation unit (26) is designed to differentially specify the adaptation of the recording frequency and / or the recording parameters to a movement speed which is changed compared to the maximum conveying speed to the line sensor (10). [4] Device according to one of the preceding claims, wherein the conveyor control (30) can transmit data about the speed profile to the evaluation unit (26), which data in particular contain a current speed, a duration of a following acceleration phase, a start of the acceleration phase and / or a final speed after completion of the acceleration phase. [5] Device according to claim 4, wherein the conveyor control (30) is configured to set a maximum conveyor speed of 2 m / s, a delayed conveyor speed of 0.5 m / s and an acceleration phase of 500 ms at an acceleration of 3 m / s 2 from maximum conveying speed to decelerated speed and vice versa. [6] Device according to one of the preceding claims, wherein the conveying device (12) is a tray conveyor or its weft conveyor. [7] Device according to one of the preceding claims, wherein in the evaluation unit (26) the time-varying recording frequency for a predetermined speed profile of an object (14) is stored in a table as time intervals between two recordings, the entries in the table describing in particular a hyperbola. [8] Device according to claim 7, wherein the evaluation unit (26) is designed such that entries of the table are predetermined, in particular by inputting or reading in, or are calculated on the basis of a predetermined speed profile, in particular as hyperbolas from a maximum conveying speed, a delayed conveying speed and a uniform acceleration phase therebetween, which is specified by start time, end time and / or acceleration. [9] Device according to claim 7 or 8, wherein the evaluation unit (26) is designed to use a table value multiple times in order to map longer acceleration phases and / or to cause a recording stop of the line sensor (10) when the movement speed is zero, i.e. the conveyor device (12) is stopped. [10] Device according to one of claims 7 to 9, wherein a further table is provided in the evaluation unit (26), which contains time intervals for the recording frequency in the event that the sign changes in an acceleration phase, i.e. an acceleration reversal takes place, in order to ensure a smooth transition of the time intervals. [11] Device according to one of the preceding claims, wherein the line sensor (10) has at least one input with which the logic module can be instructed to start and carry out the adjustment of the recording frequency via a predetermined speed profile. [12] Method for recording images of objects (14) moving on a conveyor device (12) and for decoding codes (20) arranged on the objects (14), wherein the objects (14) are scanned line by line to generate image data and wherein the speed of movement of the objects (14) relative to the line sensor (10) or their conveyed distance is determined, wherein the recording frequency of the line-by-line scanning is adapted to the speed of movement by a conveyor control of the conveyor device (12), which predetermines a speed profile for the conveyor device (12), announcing a changed speed profile from a specified time to an evaluation unit (26) implemented on a programmable logic module of the line sensor (10), which is used to adapt the recording frequency. [13] Method according to claim 12, wherein the speed profile contains a current speed, a duration of a subsequent acceleration phase, a start of the acceleration phase and / or a final speed after completion of the acceleration phase, and wherein in particular a maximum conveying speed of 2 m / s, a delayed conveying speed of 0.5 m / s and an acceleration phase of 500 ms at an acceleration of 3 m / s 2 from maximum conveying speed to decelerated speed and vice versa. [14] Method according to claim 12 or 13, wherein the time-varying recording frequency for a predetermined speed profile of an object (14) is stored in a table as time intervals between two recordings, wherein the entries in the table describe in particular a hyperbola and are calculated from a maximum conveying speed, a delayed conveying speed and a uniform acceleration phase in between, which is specified by start time, end time and / or acceleration. [15] Method according to one of claims 12 to 14, wherein a table value is used multiple times to map longer acceleration phases, and / or wherein a recording stop is initiated when the movement speed is zero, i.e. the conveyor device (12) is stopped, and / or wherein a further table is provided which contains time intervals for the recording frequency in the event that the sign changes in an acceleration phase, i.e. an acceleration reversal takes place, in order to ensure a smooth transition of the time intervals.

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