WO2000025283A1

WO2000025283A1 - Monitoring trolleys at a check-out facility

Info

Publication number: WO2000025283A1
Application number: PCT/NL1998/000624
Authority: WO
Inventors: Andries Cornelis Pasma
Original assignee: Andries Cornelis Pasma
Priority date: 1998-10-28
Filing date: 1998-10-28
Publication date: 2000-05-04
Also published as: AU1177399A

Abstract

For monitoring trolleys (3) at a check-out facility for contents, a trolley (3) in a monitoring position is detected and it is detected whether an article (15, 16, 19) is carried by that trolley (3) by comparing detection signals with at least one rule; and by generating an article present signal if the detection signals meet the rule or rules. The action of detecting the trolley (3) includes detection of a range to the trolley (3) and detecting whether an article (15, 16, 19) is carried by that trolley (3) includes detection of a range to the article (15, 16, 19). The detection signals which are compared include ranges, an article present signal being generated in response to the ranges being in accordance with the rule or rules. A system for carrying out that method is described as well.

Description

TITLE: Monitoring trolleys at a check-out facility

TECHNICAL FIELD

The invention relates to monitoring of trolleys at a check-out facility such as a check-out gate, counter or corridor in a self service store. Monitoring of trolleys at check-out is typically carried out to avoid that objects on the trolleys which are not readily visible for personnel at the check-out, such as a beer crate on a support surface below the basket of the trolley, are taken past the check- out without being paid for.

BACKGROUND ART

A major problem of detecting articles on trolleys at check-out is that articles on trolleys have to be discriminated from other moving bodies passing the checkout. Solving this problem is complicated by the requirement that such systems be relatively low-cost and that the movement of trolleys in the check-out area is not controlled or otherwise predictable. Trolleys may for instance move at different velocities, stop and even move to and fro in an unpredictable manner. Furthermore, it is preferred not to install sensors outside the check-out counter in order to keep installation simple. One known solution to this problem is to provide trolleys with special markers which can be recognized by sensors at the check-out facility. However, this requires the trolleys to be equipped with markers especially for the monitoring purpose. This requires special attention, in particular if the stock of trolleys is supplemented. In practice, the markers can easily be damaged, in particular because little space is available for the markers on the nestable trolleys. It is of course also known to provide articles to be detected with markers, but this is not economically feasible in the field of consumer goods with a low price per unit.

One example of a system for monitoring trolleys at a check-out facility is described in U.S. Patent 4 338 594. According to this document, discrimination between on the one hand articles on a trolley and, on the other hand, the legs of a customer, the frame of a trolley and other items not to be detected is made without using markers by providing an additional transmitter/receiver under and, seen in direction of passage of a trolley, between the transmitter/receivers for detecting articles on the trolley. The transmitter/receivers for detecting articles on the trolley are deactivated in response to the additional transmitter/receiver detecting for instance a foor of a customer, to avoid that objects not to be detected cause an article present signal.

However, such systems causes many false alarms as well, because the additional transmitter/receiver has to be positioned close by the floor to prevent it from detecting the frame of a trolley. At this level it frequently misses the leg and the foot of a customer who, in the course of normal motion, steps over the area covered by the additional transmitter/receiver .

A method according to the introductory portion of claim 1 and a system according to the introductory portion of claim 14 are described in German patent application 29 36 834. According to this document, discrimination between articles to be detected on a trolley and objects to be disregarded is made without using markers by providing an additional sensor for detecting the wires of a basket. The basket is presumed to be detected if at least two detection signals are received within a time interval having a predetermined duration, for instance within 500 milliseconds. A problem of this method of discriminating between presumed trolleys and presumed other objects is, that many false alarms are caused by other multiple signals within the predetermined time interval. However, if the duration of the time interval is shortened and/or the required number of signals is increased, the number of false alarms decreases at the expense of an increased number of not detected - and therefore not monitored - trolleys.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a simple and reliable solution to the problem of discriminating between goods on a trolley and objects to be disregarded in the area of a check-out facility without reverting to the use of markers.

According to the present invention, this object is achieved by carrying out a method for monitoring trolleys at a check-out facility in accordance with the characterizing portion of claim 1. According to another aspect of the invention, this object can be achieved by providing a system for monitoring trolleys at a check-out facility with the characterizing features of claim 14. By detecting range signals representing ranges in areas through which particular portions of the trolley pass and in which articles on the trolley are to be expected, further information regarding the detected objects is obtained, which allows to distinguish more reliably between articles on a trolley and items to be disregarded, on the basis of characteristics of the distance to parts of the trolley and parts on the trolley distinguishing a trolley and an article thereon from items to be disregarded which typically pass the check-out facility. Particularly advantageous modes of carrying out and features of the present invention are set forth in the dependent claims. Further objects, features, advantages and details of the present invention are described and elucidated in the following detailed description with reference to the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a top plan view of portions of check-out counters facing each other and check-out corridors extending along said check-out counters;

Fig. 2 is a partly schematical rear view of a checkout facility;

Fig. 3 is a detailed view of a portion of a shopping trolley in front of sensors in a check-out counter; Fig. 4 is a detailed view of a portion of a human leg in front of sensors in a check-out counter;

Fig. 5 is a combined time/distance - time/presence diagram of an example of detection signals obtained during the passage of a shopping trolley as shown in Fig. 1; Fig. 6 is a combined time/distance - time/presence diagram of an example of a detection signals obtained during the passage of a leg as shown in Fig. 3;

Fig. 7 is a time/distance diagram of an example of detection signals obtained during the passage of a shopping trolley as shown in Fig. 2;

Fig. 8 is a flow chart of an algorithm for determining whether a significant difference between detected ranges has been detected;

Fig. 9 is a detailed representation of detected signals; and

Fig. 10 is a flow chart of an algorithm for determining whether the duration of detected ranges is such that an article present signal should be enabled.

MODES OF THE INVENTION

Figs. 1-4 show various views of a check-out facility of the type typically used in self service stores, where customers leaving the store pass through a passage 1 along a check-out counter 2 with the shopping trolley 3. In an upstream portion of the passage 1 or upstream of the passage the articles the customer wants to buy are unloaded from the trolley 3 onto a conveyor belt 4 to be conveyed to a cashier sitting behind the check-out counter 2 to operate a cash register and to handle payments. Each of the shown check-out counters further includes a scanning window 5, a second conveyor β for transporting registered articles to a pick-up area 7 of the check-out counter 2, and an elevated countertop 8.

Most articles are handed to the cashier via the first conveyor 4, but larger articles, such as crates carrying bottles of beer or soda or boxes of napkins, are usually or at least frequently left on a bottom rack 9 of the trolley 3 under the basket 10 of the trolley 3 and declared to the cashier by the customer telling the cashier that the article in question is present on the bottom rack 9 of the trolley 10. Since the articles on the bottom rack 9 are normally not readily visible by the cashier, it often happens that articles which are not declared are passed by the cashier without being registered by the cashier and, accordingly,, without being paid by the customer. The same applies to smaller articles in the basket 10, in particular if these articles are of a color similar to the color of the baskets

To avoid that articles on trolleys 3 pass the checkout without being noticed by the cashier, the proposed check-out counters 2 are each provided with a system for monitoring trolleys at the check-out facility for contents. Next, only one of these systems is described, the other system being essentially identical to the described one.

The system is provided with trolley sensors A, D for detecting portions of a trolley 3 in a monitoring area 11 and with article sensors for B, C for detecting an article in the monitoring area 11. The sensors are connected to a control system 12 operatively communicating with the sensors A-D for comparing detection signals with rules stored in the control system. The rules can also be stored in an external system with which the control system communicates. The control system 12 communicates with an article present transducer, in this example formed by a flash light 13 in the field of sight of the cashier but not readily visible to the customer in the passage 1. The control system 12 is responsive for activating the flash light 13 in reaction to detection signals meeting the rules mentioned above. It is observed that the rule can of course also include a negation, so that for instance the article present signal is generated if the range signals do not meet the criteria allowing to presume that the obtained signals are not from a trolley carrying an article. The sensors A-D are provided in the form of ultrasound range finders so that not only the presence of an object in front of the sensor can be detected but also the range (distance) to that object. The control system 12 is adapted for comparing the detected ranges with the rules. If detected ranges comply with the rules, the flash light 13 is activated for warning the cashier who should then stand up to look over the counter 2 to see whether indeed an article not declared is present in the trolley 3. Since the flash light 13 is not readily visible to the customer, the monitoring is carried out in an unobtrusive manner not offending customers inadvertently forgetting to mention articles left on the trolley.

In operation, the distance to a bottom frame 14 is measured by the sensor A and the distance to an article 15 on the bottom rack 9 is detected by the sensor B. As can be seen particularly clearly in Fig. 3, the distance detected by sensor B is larger than the distance detected by sensor A. The presence of a significant difference between distances detected by the sensors A and B forms a strong indication of the presence of an article 15 on the bottom rack 9. In practice it occurs very rarely that an article is positioned on the bottom rack in a position flush with the face of the frame portion 14 facing the sensors. This is particularly true for bottom frame portions 14 having a trapezoid shape and a rack 9 of which the support surfaces are at a lower level than the top faces of the frame portion 14. The majority of other objects to be disregarded passing the sensor are formed by legs 17 of customers. If a leg passes the sensors A and B, the range signals generated by the sensors A and B will not differ significantly. Thus, the lack of a difference between detected ranges forms a strong indication that the detected object is to be disregarded.

These considerations are set forth in the rules with which the detection signals including range signals generated by the sensors A and B are compared. An article present signal is generated if the range signals comply with these rules.

In order to reliably detect the horizontally extending frame portion 14 of a trolley 3 closely above wheels 17 of the trolley 3, the trolley sensor A is mounted at a level closely adjacent the floor 18, so that the range signals are obtained by measurements at a level substantially identical to the horizontally extending frame portion 14 of the trolley 3. In order to reliably detect the basket 10 of a trolley 3 without being disturbed by any articles 19 in the basket_ 10, the trolley sensor D is mounted closely below the top edge 20 of the check-out counter 2, so that range signals are obtained by measurements at a level closely below the top edge of the basket 10 of the trolley 3.

Some preferred rules with which received range signals can be compared are discussed with reference to Figs. 5-7 which show some examples of signals which can be received. The dots in Figs. 5-7 represent discrete samples which are obtained from the sensors A-D from a moment t=0. It is observed that the time axes in Figs. 5 and 6 do not coincide, i.e. the signals depicted in Fig. 5 may for instance have been received before or after the signals depicted in Fig. 6. The signals shown in Fig. 7 could but need not have been received simultaneously with the signals shown in Fig. 5. As was discussed above, the presence of a significant distance between a first range signal and a second range signal forms a strong indication of the presence of a trolley 3 carrying an article 15, 16 or 19. Therefore the rules preferably include a minimum value of the difference between the first range signal and the second range signal. Fig. 5 for example shows range signals received upon the passage of a trolley 3 as shown in the right hand passage of Fig. 1. The dot and dash line represents the presence of any accepted signal from sensor A, the wavy line represents the presence of any accepted signal from sensor B, the continuous line A connects range signals received from sensor A and the dashed line B connects range signals received from sensor B. The trolley 3 is oriented parallel to the passage 1 and the frame portion 4 is trapezium-shaped, the narrow end forming the leading end. Therefore, the distances represented by the range signals detected by sensor A gradually decrease from an initial level until the trailing end of the trolley 3 has passed the sensor A. The sensor B first detects a signal when some time has passed after the_ passage of the leading end of the frame portion 14. Since a first portion of the beer crate 16 projects beyond the frame portion 14, the ranges represented by the range signals received by sensor B are initially smaller than the ranges represented by the range signals received by sensor B at the same time or at about the same time. The difference is such that at least for a period of time the absolute value of the difference between the distances A and B is larger than a reference value R. This period of time is designated in Fig. 5 by the equation |ΔX|>R.

Subsequently, the difference between the distance represented by range signals received by sensors A and B decreases to zero, which marks the passage of the position where the crate 16 is vertically flush with the frame portion 14. Then the difference inverses and the absolute value increases again until the equation | ΔX | >R again applies for a period of time ending with the passage of the trailing end of the crate 16. After the crate 16 has passed the sensor B, for a short time, no meaningful range signal is received at sensor B. Finally, the range signal received by sensor B is briefly essentially identical to the range signal received by sensor A. This marks the passage of the posts 21 of the frame of the trolley 3.

Fig. 6 shows an example of a signal which is obtained when a leg 22 of a customer is temporarily in front of the sensors A and B. The signals received by the sensors A and B have a duration essentially identical to each other, because the size of the leg 22 in the direction of the passage is about the same at the levels of the sensors A and B. The range signals have flanks which are less steep than in case of the passage of a trolley, because of the roundness of the leg 22. In this example, the customers stands on the leg in front of the sensors A and B for a short while, which causes the plateau in the range signals A and B, the signals being about equal, but not exactly equal because the face of a leg or the pant covering the leg 22 extends substantially, but not exactly, vertically between the levels of the sensors .A and B. It is to be noted that in many cases the leg is only swayed past the sensors A and B, so that the duration of significant range signals received by the sensors A and B will be very short.

These examples illustrates four rules regarding the range signals received by sensors A and B forming strong indications of the passage of a trolley 3 carrying an article 16 on the bottom rack 9. Firstly, the presence of a significant difference between the ranges detected by the sensors A and B for a significant period of time indicates the passage of a trolley carrying an article. To ensure that distances obtained by measurements taken in essentially the same vertical plane are compared, this rule is applied to range signals obtained simultaneously or directly after each other and from measurements in a common vertical corridor 11 traversing the path of travel 1 of the trolleys 3.

Secondly, that the range signal from sensor A is between an upper and a lower limit value for a first duration longer than a second duration during which the range signal from sensor B is between an upper and a lower limit value also indicates the passage of a trolley carrying an article. Articles are typically shorter in the direction of movement than trolleys. Thirdly, that the range signal from sensor A is between an upper and a lower limit value for a first time interval having first duration and that the range signal from sensor B is essentially identical to the range signal from sensor A during a second time interval within the first time interval and having a second duration which is less than a predetermined fraction of the first duration or which is at least a predetermined amount smaller than the first duration, forms a strong indication of the passage of a trolley, the range signal from sensor B in this case being caused by the post 21 of the frame of the trolley 3 which extends vertically from the horizontal frame portion 14.

Fourthly, that the first range signal has at least a predetermined duration forms a further indication of the passage of a trolley, this duration being coupled to the maximum velocity at which trolleys are normally passed by the sensors .

Preferably, these rules are combined to predict very reliably whether a trolley carrying an article or an object to be disregarded has passed by the sensors. Fig. 7 shows an example of range signals obtained by the sensors C and D during the passage of the trolley 3 as shown in Fig. 2.

The range signal received by sensor D varies quickly and repeatedly between insignificant (or "not accepted") and a range determined by wires of the basket. It is also possible to tune the sensor D such that it picks up reflections from the wires all the time during the passage of a basket. The signals received from sensor C varies quickly and repeatedly between a range determined by the wires of the basket 10 and a range determined by the distance to the article 19 in the basket 10. This example illustrates that the occurrence of a significant difference in ranges detected by the sensors C and D for a significant number of times during a time window is a strong indication of the passage of a trolley 3 carrying an article 19 in its basket 10 as well. Furthermore, this example illustrates that the duration of meaningful range signals from sensor D in the time window is a strong indication of the passage of a trolley 3. Also the difference in total duration of meaningful signals received from the sensors C and D, the duration of meaningful signals from sensor C being shorter than the duration of meaningful signals from sensor D forms a strong indication of the passage of a trolley 3 carrying an article 19 in its basket 10.

Figs. 8 and 10 show examples of flow charts in which some of these rules are applied. Fig. 8 relates to the operation of the sensors to determine the detected ranges and to features of the processing of range signals obtained thereby. The transmission and receipt of pulses by the ultrasound range finders A-D is illustrated by Fig. 9. The algorithm starts with step 22. Then an ultrasound pulse is transmitted (step 23) . Then, it is checked whether a reflection is received at sensor A within a maximum duration of time t_max (step 24) . If no reflection is received at sensor A or if the reflection has been received after the time limit has expired, the next step start a new sampling cycle . The maximum duration of time t_maχ defines the maximum range to be detected by the sensor A. It is observed that if no reflection has been detected by sensor A before expiration of the prescribed time limit t_max, it is not checked whether a reflection is detected by sensor B.

If a reflection is registered by sensor A within the prescribed time limit t_max, the time is read (step 26) . Then, it is checked whether a reflection is received at sensor B within a maximum duration of time t_max (step 27) . If no reflection is received at sensor B or if the reflection has been received after the time limit has expired, the next step is to start a new sampling cycle.

If a reflection is registered by sensor B within the prescribed time limit t_max, the time is read as the time value for sensor B (step 28) . It is then checked whether the absolute value of the difference between the time read for sensor A and for sensor B is smaller than a predetermined threshold value R (step 29) . If the difference has been found to be smaller than R, the next- step is to register that no significant ΔX has been found (step 30) and to start a new sampling cycle. If the difference has been found to be equal to or larger than R, the next step is to register that a significant ΔX has been found (step 31) . Then, the article present signal is enabled (step 32) . Preferably at least one other test (which is described below) is performed to check whether the result is likely to have been caused by the passage of a trolley carrying an article, the article present signal being activated only if both tests result in enabling the article present signal.

A flow chart of the additional test is shown in Fig. 10. The test is started at step 33, for instance as a next step subsequent to detection of a trolley. First, the window of counts within which the test is to be performed is determined at step 34. Initially, this means adding sample results until a predetermined number is reached and then, after a predetermined number of samples has been read, each time discarding the oldest sample result in favor of a newly received sample result. The next step, which is designated as step 24, is the same step as step 24 in Fig. 9, formed by the checking whether a reflection has been received at sensor A within the prescribed time limit. If the result is positive, this is registered at step 35. The next step 27 is again identical to the corresponding step in Fig. 9 and can for instance be carried out by checking the status of a common register position. If a reflection has been received within the prescribed time limit, this is registered as well (step 36) .

Then, the first check of this test is whether the count of registered reflections obtained by sensor A within the moving window is at least equal to a minimum count (step 37) . This check serves to exclude random stray reflections and reflections of a duration which is too short to allow the passage of a trolley at a more or less normal, non- suspect velocity. If the result is negative, a next test cycle is started. If the result is positive, the test continues with the next check.

The next check is whether the count of registered reflections obtained by sensor A within the moving window is smaller than or equal to a threshold limit (step 38). This check serves to identify situations in which a trolley has not been moved continuously past the sensor A, but has been stopped or almost stopped in front of the sensor A. In that case the percentage of readings from sensor A at which also a reading from sensor B is available is not a good measure for the likelihood of an article on the trolley having caused the significant ΔX readings, because the number of readings is likely to have been disturbed by variations in the velocity of the trolley. Therefore, if the count of registered readings from sensor A is above a predetermined threshold level, the article present signal is enabled. However, if the count of registered readings from sensor A is below a predetermined threshold level, it is checked whether the number of readings at which sensor B gave a value is between predetermined percentage levels of the number of readings obtained from the sensor A (steps 40, 41) . If the results of either one of these checks is negative, a next test cycle is started. If the results of both these checks is positive, the article present signal is enabled (step 39) . These checks are based on the consideration, that if a trolley carrying an article to be detected passes the sensors, the count of readings at sensor B should at least be a predetermined proportion of the readings at sensor A. However, there must also have been a difference between the number of readings from sensor A and the number of readings from sensor B, since it virtually never happens, that articles on a shopping trolley have the same size in longitudinal direction as the rail delimiting the bottom rack of the trolley (see for instance Fig. 5).

In connection with this test, it is noticed that the sensors A-D and the monitoring area 11 where the sensors A-D are active are located in a position downstream of areas 42, 43 where customers and trolleys are stand while waiting for another person standing opposite the cashier for making payments and the like. Furthermore, the sensors A-D and the monitoring areas are located upstream of an area 44 opposite the cashier position where customers stand while arranging payments and also upstream of the area 45 where shopping carts stand while the person who has used the cart is making payments. By having the sensors A-D in this intermediate position, in normal day-to-day situations it happens quite rarely that a person or a trolley stops in front of the sensors and thereby makes distinguishing between a trolley with articles and other objects more difficult.

As can be seen in Fig. 1, the sets of transmitter/receivers A-D are each arranged at a side of the monitoring area 11 and in an orientation facing transmitter/receivers A-D of a neighboring monitoring area 11. The control systems 12 for controlling the transmitter/receivers A-D of the neighboring monitoring areas 11 are adapted for activating the transmitter/receivers of the neighboring monitoring areas 11 during alternating time intervals. Thus, it is avoided that pulses sent by other transmitter/receivers disturb measurements made by each of the sensors A-D. 12. To achieve that transmission and reception of signals in the neighboring monitoring areas are carried out during alternating time intervals, the control systems 12 are made to respond to common synchronization signals with different delays. It is also possible to provide that the control systems 12 communicate in a peer-to-peer or master-slave relationship. To avoid the need of installing cables, the communication between control systems is preferable of a wireless form.

That the sensors A-D are provided in the form of ultrasound range finders is advantageous, because such sensors are of sufficient accuracy, capable of operating in a very brightly lit environment, sufficiently compact to be installed in a position in which there is little chance of being noticed by the shopping public and low-cost. Furthermore, such sensors are capable of detecting a range to virtually any material found on or around articles which are sold in a self service store. To further reduce the risk of false alarms, the system according to this example is provided with an infrared sensor E allowing to distinguish the leg of a human or an animal from colder objects, such as an article and a trolley, on the basis of the temperature of that object. As is shown in Fig. 2, the system also includes an input interface 46 for inputting codes representing a presumable cause of the article present signal, for instance by indicating which of the charged items was on the trolley or that the alarm was false. The control system 12 and the input present transducer 13 are arranged for deactivating the input present transducer 13 only in response to input signals received from this input interface 46. Thus, the cashier is forced to indicate the cause of an article present signal and is thereby encouraged not to disregard article present signals. For this purpose, the input interface 46 is provided with a display and a thumb wheel with which lines displayed on the display and representing possible causes of an article present signal can be brought in line with a mark aside the display. To further facilitate the checking of trolleys, a video camera 47 is mounted for recording images from the monitoring area 11. Furthermore, a video display control system 48 and a video display screen 49 are provided. The camera 47, the control system 48 and the video display screen 49 are operatively connected and responsive for freezing an image displayed on the video display 49 in reaction to an article present signal. This makes it easier for the cashier to check a trolley for contents in the event of an article present signal, in particularly if the trolley has been moved on before the cashier was able to react to the article present signal From the above description it will be readily apparent to the skilled person that many other modes of carrying out the invention and embodiments are readily conceivable. For instance, the range finders can be provided in the form of optical or mechanical range finders instead of in the form of ultrasound range finders.

The input interface can for example be provided in the form of a keyboard, a touch screen or a display provided with control buttons along the edges, the function of the buttons in each situation being displayed on the display. Rules can be formed by a fuzzy set of rules, an article present signal being generated if the presumability of the presence of an article is above a predetermined level.

Claims

1. A method for monitoring trolleys (3) at a check-out facility for contents, including the actions of: detecting at least a portion (10, 14, 21) of a trolley (3) in a monitoring area (11) ; detecting whether an article (15, 16, 19) is carried by said trolley (3) ; comparing detection signals with at least one rule; and generating an article present signal if said detection signals meet said at least one rule; characterized in that said action of detecting at least a portion (10, 14, 21) of a trolley (3) in a monitoring position includes detection of a first range signal representing a range to said portion (10, 14, 21); said action of detecting whether an article (15, 16, 19) is carried by said trolley (3) includes detection of a second range signal representing a range to said article (15, 16, 19); said detection signals being compared include range signals; and an article present signal is generated in response to said range signals being in accordance with said at least one rule.

2. A method according to claim 1, wherein said at least one rule includes a minimum value of the difference between said first range signal and said second range signal .

3. A method according to claim 2, wherein said at least one rule applies to range signals obtained simultaneously or directly after each other and wherein said range signals are obtained from measurements in a common vertical corridor traversing a path of travel (1) of said trolleys (3) .

4. A method according to any one of the preceding claims, wherein said at least one rule defines that said first range signal is between an upper and a lower limit value for a first duration longer than a second duration during which the second range signal is between an upper and a lower limit value.

5. A method according to any one of the preceding claims wherein said at least one rule defines that said first range signal is between an upper and a lower limit value for a first time interval having a first duration and that said second range signal is identical to said first range signal during a second time interval within said first time interval and having a second duration which is less than a predetermined fraction of said first duration or which is at least a predetermined amount smaller than said first duration.

6. A method according to any one of the preceding claims, wherein said at least one rule defines that said first range signal has at least a predetermined duration.

7. A method according to any one of the preceding claims, wherein said first range signal is obtained by measurement at a level substantially identical to a horizontally extending frame portion (14) of the trolley (3) .

8. A method according to any one of the preceding claims, wherein said first range signal is obtained by measurement at a level substantially closely below a top edge of a basket (10) of a trolley (3) .

9. A method according to any one of the preceding claims, wherein said article present signal is stopped in response to input signals representing a presumable cause of the article present signal only.

10. A method according to any one of the preceding claims, wherein a video image of the monitoring area is taken and displayed, said displayed video image being frozen in response to an article present signal.

11. A method according to any one of the preceding claims, wherein signals representing ranges over a predetermined value are ignored.

12. A method according to any one of the preceding claims, wherein said range signals are obtained by transmitting a signal from a side of said monitoring area (11) and receiving reflections of said signal at a the same side of said monitoring area (11) and wherein signals are transmitted into two neighboring monitoring areas (11) towards the side of the respective other monitoring area (11) , transmission and reception of signals in said neighboring monitoring areas (11) being carried out during alternating time intervals.

13. A method according to any one of the preceding claims, wherein said range signals are obtained by transmitting and receiving ultrasound pulses.

14. A system for monitoring trolleys (3) at a check- out facility for contents, including: at least one trolley sensor (A, D) for detecting at least a portion (10, 14, 21) of a trolley (3) in a monitoring area (11); at least one article sensor (B, C) for detecting an article (15, 16, 19) in said monitoring area (11) ; and a control system (12) operatively communicating with said sensors (A-D) for comparing detection signals with at least one rule and communicating with an article present transducer (13), said control system (12) being responsive for activating said article present transducer (13) in reaction to said detection signals meeting said at least one rule; characterized in that said at least one trolley sensor (A, D) and said at least one article sensor (B, C) are each formed by a range finder; and said control system (12) is adapted for comparing detected range signals with said at least one rule and responsive for activating said article present transducer (13) in reaction to said range signals being in accordance with said rule.

15. A system according to claim 14, wherein said trolley sensor or at least one of said trolley sensors (A) is mounted at a level closely adjacent a floor (18) , for detecting a range to a horizontally extending frame portion (14) of a trolley (3) closely above wheels (17) of the trolley (3) .

16. A system according to claim 14 or 15, wherein said trolley sensor or at least one of said trolley sensors (D) is mounted closely below a top of a check-out counter (2) for detecting a range to a basket portion (10) of a trolley (3) .

17. A system according to any one of the claims 14-16, wherein said sensors are transmitter/receivers (A-D) arranged at a side of said monitoring area (11) and in an orientation facing transmitter/receivers (A-D) of a neighboring monitoring area (11), the control system or control systems (12) for controlling said transmitter/receivers (A-D) of said neighboring monitoring areas (11) being adapted for activating said transmitter/receivers (A-D) of said neighboring monitoring areas (11) during alternating time intervals.

18. A system according to any one of the claims 14-17, wherein said sensors (A-D) are ultrasound range finders.

19. A system according to any one of the preceding claims, further comprising an infrared sensor (E) .

20. A system according to any one of the claims 14-19, further comprising an input interface (46) for inputting codes representing a presumable cause of the article present signal, said input interface (46) , said control system (12) and said input present transducer (13) being arranged for deactivating said input present transducer (13) only in response to input signals received from said input interface (46) .

21. A system according to any one of the claims 14-20, further including a video camera (47) mounted for recording images from said monitoring area (11) , a video display control system (48) and a video display screen (49), said camera (47), said control system (48) and said video display screen (49) being operatively connected and responsive for freezing an image displayed on the video display (49) in reaction to an article present signal.