Technical field

The present disclosure relates to a production system and to methods of data collection and data analysis in a production system. The disclosure finds particular application in automated or even autonomous production systems.

Background

In industrial production, there is a need for provision of performance data on various levels in a production system, e.g. in order to identify potential problems, such as cycle time balancing problems, machines needing maintenance, and breakdowns.

Moreover, there is a need for determining work in progress in various stages of the production process and for identifying bottlenecks in the production system.

In centrally controlled production systems, such as PLC based systems, such data can be derived, but only if the system is programmed to retrieve and save the data. Hence, in order to meet a change in the need for data logging, extensive reprogramming of the PLC software may be needed. Such reprogramming is not only time consuming and expensive, but may at worst interfere with the actual operation of the system.

Another obstacle to the provision of production system data is the fact that machine sensor data may not be readily accessible for collection. This may be due to a lack of sensors and/or accessible interfaces with the machines.

Yet another obstacle is the fact that there may be different types of machines at different operation stations, such that there is no uniform interface for data collection.

Hence, there is a need for a data collection system which is flexible, which can handle all types of operation stations and which can be implemented with a minimum of interference with existing control systems. Summary

An object of the present disclosure is to provide a system which overcomes or at least alleviates the problems discussed by way of introduction.

A particular object is to provide a system which allows for data collection with a minimum of interference with existing control systems.

The invention is defined by the appended independent claims, with embodiments being set forth in the dependent claims, in the following description and in the drawings.

According to a first aspect, there is provided a production system, comprising a conveyor system, configured to convey a plurality of workpiece carriers, at least some of which carrying at least one workpiece, a plurality of operation stations, each configured to perform at least one operation on at least one of the workpieces. The conveyor system is configured to convey the workpiece carriers between the operation stations. The conveyor system comprises a highway conveyor, in the form of an endless conveyor adapted for transporting and circulating said workpiece carriers, and at least two operation unit conveyors, each in the form of an endless conveyor adapted for transporting said workpiece carriers. Each operation unit conveyor is associated with at least one of the operation stations, such that the operation unit conveyor is configured to convey the workpiece carriers from the highway conveyor to the associated operation station. Switch devices are arranged at each intersection between the highway conveyor and the operation unit conveyors to control the flow of the workpiece carriers at the respective intersection. At least one auxiliary workpiece carrier sensor is provided at at least some of the conveyor intersections. The auxiliary workpiece carrier sensors are configured to detect arrival of a workpiece carrier at the intersection and to provide an event message to a controller. The event message comprises a time stamp. The controller is configured to receive the event messages, determine lengths of time periods between corresponding event messages from one of the auxiliary workpiece carrier sensors, and classify the time periods based on their lengths into one of at least two classes, said classes comprising “normal operation” and “non-normal operation”.

A workpiece carrier can be any type of carrier which is adapted for being transported on the conveyor and of carrying the workpiece or workpieces at issue. Examples of carriers may include relatively small carriers, such as carriers for integrated circuits or circuit boards; or relatively large carriers, such as carriers for engines, gear boxes, chassis for machines or vehicles, or the like. The carrier may also be arranged to hold a workpiece suspended below the conveyor, as is common, e.g. in painting applications.

A conveyor may be any type of conveyor that allows circulation of workpiece carriers, such as a chain type conveyor, or an arrangement of belts or the like that enable circulation in an endless loop. Such conveyors are known as such and are available in a multitude of widths. Length and paths of each conveyor can be determined arbitrarily based on what is needed and on the space available.

The workpiece carriers may be releasably engageable with the conveyors based on gravity-induced friction.

Hence, one of the workpiece carriers may be stopped without the motion of the conveyor, and thus of the other workpiece carriers currently transported on that conveyor, being affected.

The term “circulating” means that the conveyor is endless and adapted for allowing workpiece carriers to follow the conveyor around its track. It is understood that an endless conveyor can be created by what is truly a single endless conveyor chain, or by a plurality of conveyor chains or belts, which together provide an endless path, along which workpiece carriers may be circulated.

An operation station may be a station arranged to provide any type of manufacturing-related operation, such as machining (e.g. lathing, cutting, milling, drilling, grinding), bonding (welding, soldering, brazing, gluing, melt bonding, etc.), inspection (e.g. measuring, scanning), surface treatment (e.g. painting, blasting, polishing, electro-plating, PVD or other type of material deposition), assembly, testing or packaging. The operation may be wholly or partially automated, or entirely manual.

An “auxiliary sensor” is a sensor that does not form part of a central control system, such as a PLC system, and which is used only to collect data.

A “time stamp” is an indication of point in time at which the even took place. For example, the time stamp may be an indication of an actual point in time, e.g. on a yyyymmddhhmmss format, or a relative indication of a point in time, e.g. since startup of the system, or the like.

The denominations “normal” and “non-normal” are understood as schematic designations only indicating a normal and non-normal operation, respectively. Other designations with the same meaning may be used.

The production system described above provides for data collection to be achieved without any need for interfering with a central control system of the operation stations (e.g. machines, robots) or of the conveyor system as such. That is, data may be collected independently without disturbing other parts of the system.

The controller may further be configured to determine a “normal operation” time length for an operation associated with one of the auxiliary workpiece carrier sensors based on at least one of a lowest time period length, a most common time period length, and an average time period length.

That is, a lowest time period length may be used to determine the “normal operation” time length. The lowest time period may be identified as a time period that is common enough for it not to represent a workpiece carrier that accidentally passes the operation unit conveyor too fast. Alternatively, the lowest time period may be determined in respect of an operation that is known to have functioned properly.

As another option, a most common time period length may be used to identify the “normal operation” time length.

As yet another option, an average time period length may be used to identify the “normal operation” time length. The controller may further be configured to determine time period length for at least one of the classes based on the normal operation time, optionally with a predetermined variance.

The controller may further be configured to determine time length limits for at least one class representing “non-normal operation” based on the “normal operation” time length with an additional multiplier or time added.

The controller may further be configured such that the “non-normal event” class comprises the classes “idle” and “breakdown”, whereby the monitoring device is configured to classify the time periods into one of at least three classes.

The controller may further be configured to accumulate at least some of the time periods classified in one of the classes over a predetermined time period.

For example, all normal operation time periods may be accumulated into a total normal operation time for an operation station during a predetermined time interval, such as a day, a week, a month, etc.

For example, all non-normal operation time indicating idle time may be accumulated into a total idle time for an operation station during a predetermined time interval, such as a day, a week, a month, etc.

For example, all non-normal operation time indicating breakdown time may be accumulated into a total breakdown time for an operation station during a predetermined time interval, such as a day, a week, a month, etc.

The controller may comprise a central controller, which is configured to control the operation of the highway conveyor, the operation unit conveyors and optionally the operation stations.

Flence, the central controller even though it may have a feedback function from sensors forming part of the control system, does not communicate with the auxiliary workpiece carrier sensors.

The controller may comprise a monitoring controller, which is configured to receive the event messages from the auxiliary workpiece carrier sensors.

The event message may comprise a sensor ID. The event message may comprise a workpiece ID and/or a workpiece carrier ID.

Hence, it is possible to track a workpiece between different sensors.

The auxiliary workpiece carrier sensor may be configured to detect whether a workpiece carrier carries a workpiece, and wherein the event message further includes an indication on whether the event message relates to an empty or loaded workpiece carrier.

The event message may further comprise an event type indication.

The auxiliary workpiece carrier sensor may further be configured to determine the event type indication, said event type indication being selected from at least two of

“diverted”, when the workpiece carrier is being diverted from the highway conveyor to one of the operation unity conveyors by the associated switch controller;

“passing, not this operation”, when the workpiece carrier is caused to pass the switch control device since the next operation for the workpiece carrier does not match the associated operation unit conveyor;

“passing no priority”, when the workpiece carrier is caused to pass the switch control device even though the associated operation unit conveyor corresponds to a next operation for the workpiece carrier and is available;

“merging”, when the workpiece carrier is being returned from one of the operation unit conveyors conveyor to the highway conveyor by the associated switch controller;

“passing full”, when the workpiece carrier is caused to pass the switch control device and the operation unit conveyor is full of workpiece carriers; and

“passing empty”, when the workpiece carrier is caused to pass the switch control device and the workpiece carrier is empty.

The labels “diverted”, “passing, not this operation”, “passing no priority”, “merging”, “passing full” and “passing empty” are understood as schematic designations only indicating the respective action at a switch station and the reason to it. Other designations with the same meaning may be used. In particular, codes may be used to represent the various events. The workpiece carriers may be provided with a tag and the auxiliary workpiece carrier sensor may further comprise a tag reader and/or writer.

A tag can be any type of tag that can be read by a machine. Preferably the tag has a writable and/or erasable memory. The tag may communicate through any type of interface, such as, but not limited to wireless interfaces like Wi-Fi, Bluetooth, RFID or NFC, or through electrical or optical interface. Any type of memory technique can be used.

In the system, the tag may be configured to store information on workpiece next destination and optionally on workpiece. The address may be configured such that it is possible to derive which conveyor the workpiece carrier is supposed to be on.

The tag only needs to contain information indicating what type of workpiece is being transported, or if the carrier is empty, and on which station the workpiece is to go next. In an even more simplified system, handling only one type of product, there may be no need for workpiece type to be indicated in the memory.

The auxiliary workpiece carrier sensor may comprise a first sensor, which is arranged on one of the operation unit conveyors, immediately upstream of the associated switch device.

Flence, the auxiliary workpiece carrier sensor may be configured to detect a workpiece carrier leaving one of the operation unit conveyors.

The auxiliary workpiece carrier sensor may comprise a second sensor, which is arranged on one of the operation unit conveyors, immediately downstream of the associated switch device.

Flence, the auxiliary workpiece carrier sensor may be configured to detect a workpiece carrier entering one of the operation unit conveyors.

The auxiliary workpiece carrier sensor may comprise a third sensor, which is arranged on the highway conveyor, immediately downstream of the associated switch device.

According to a second aspect, there is provided a production system, comprising a conveyor system, configured to convey a plurality of workpiece carriers, at least some of which carrying at least one workpiece, and a plurality of operation stations, each configured to perform at least one operation on at least one of the workpieces. The conveyor system is configured to convey the workpiece carriers between the operation stations. The conveyor system comprises at least two conveyors, each in the form of an endless conveyor adapted for transporting and circulating said workpiece carriers, and at least one switch device arranged at an intersection between the conveyors to control the flow of the workpiece carriers at the respective intersection. At least one auxiliary workpiece carrier sensor is provided at at least one of the conveyor intersections. The auxiliary workpiece carrier sensors is configured to detect arrival of a workpiece carrier at the intersection and to provide an event message to a controller, the event message comprising a time stamp, and an event type indication. The controller is configured to receive the event messages, and derive a workpiece carrier status for at least one of the conveyors, based on the event messages.

A workpiece carrier status may be an indication of the amount of workpiece carriers that are present in at least one of the conveyors. This may, in turn, indicate a buffer size or a work in progress at the respective conveyor. It may also indicate a flow of empty carriers in the reverse direction, including how many such empty carriers are present at the respective conveyor.

The event type indication may be selected from at least two of: “diverted”, when the workpiece carrier is being diverted from one of the conveyor to another one of the conveyors by the associated switch controller;

“passing, not this operation”, when the workpiece carrier is caused to pass the switch control device since the next operation for the workpiece carrier does not match the other conveyor;

“passing no priority”, when the workpiece carrier is caused to pass the switch control device even though the other conveyor corresponds to a next operation for the workpiece carrier and is available;

“merging”, when the workpiece carrier is being returned from one of the conveyors another one of the conveyors by the associated switch controller;

“passing full”, when the workpiece carrier is caused to pass the switch control device and the other conveyor is full of workpiece carriers; and “passing empty”, when the workpiece carrier is caused to pass the switch control device and the workpiece carrier is empty.

The conveyors may comprise at least two highway conveyors, and at least one flow balancing conveyor, wherein said flow balancing conveyor provides an exclusive connection between the highway conveyors, and wherein the switch device is arranged at an intersection between one of the highway conveyors and the flow balancing conveyor, to control the flow of the workpiece carriers between said one of the highway conveyors and the flow balancing conveyor.

The highway conveyors and the flow balancing conveyor may be free from operation stations.

The flow balancing conveyor is also free from intersections with operation unit conveyors, such that it only interacts with highway conveyors.

The conveyor may comprise a highway conveyor having a short-cut route, configured such that workpieces may circulate over only a portion of the highway conveyor, and the switch device may be arranged at a connection between said highway conveyor and the short-cut route, to control the flow of the workpiece carriers between said one of the highway conveyors and the short-cut route.

The conveyors may comprise at least one highway conveyor and at least one operation unit conveyor, wherein the operation unit conveyor is associated with at least one operation station, such that the operation unit conveyor is configured to convey the workpiece carriers from the highway conveyor to the associated operation station, and wherein the switch device is arranged at an intersection between one of the highway conveyors and the operation unit conveyor, to control the flow of the workpiece carriers between said one of the highway conveyors and the operation unit conveyor.

The controller may be configured to receive the event messages, derive a workpiece carrier status for at least one of the conveyors, based on the event messages, said workpiece carrier status indicating at least one of: number of completed workpiece carriers in said conveyor, number of non-completed workpiece carriers in said conveyor, and number of empty workpiece carriers in said conveyor. The controller may comprise a central controller, which is configured to control the operation of the conveyors, and optionally the operation stations.

Controlling the conveyors includes controlling any devices, which control the flow on or between conveyors, such as switch devices, stop devices, tag writers, etc.

The controller may comprise a monitoring controller, which is configured to receive the event messages from the auxiliary workpiece carrier sensors.

The event message may comprise a sensor ID.

The event message may comprise a workpiece ID and/or a workpiece carrier ID.

The auxiliary workpiece carrier sensor may be configured to detect a workpiece carrier leaving one of the conveyors.

The auxiliary workpiece carrier sensor may be configured to detect a workpiece carrier entering one of the conveyors.

The auxiliary workpiece carrier sensor may be configured to detect a workpiece carrier passing a switch device while remaining on the conveyor.

The production system may further comprise a second auxiliary workpiece carrier sensor, arranged at the same intersection as the auxiliary workpiece carrier sensor, wherein the auxiliary workpiece carrier sensor and the second auxiliary workpiece carrier sensor are selected such that two of the following functions are provided for the same intersection: detection of a workpiece carrier leaving one of the conveyors, detection of a workpiece carrier entering one of the conveyors, and detection of a workpiece carrier passing a switch device while remaining on the conveyor.

The production system may further comprise a third auxiliary workpiece carrier sensor, arranged at the same intersection as the auxiliary workpiece carrier sensor and the second auxiliary workpiece carrier sensor, said third auxiliary workpiece carrier sensor being selected such that all three of said functions are provided for the intersection. The workpiece carriers may be provided with a respective tag and the auxiliary workpiece carrier sensor may further comprise a tag reader.

Brief description of the drawings

Fig. 1a-1b are schematic diagrams of a portion of a production system.

Fig. 2 is a schematic chart for one operation, that can be derived based on the present invention.

Fig. 3 is a schematic chart for one conveyor, that can be derived based on the present invention.

Fig. 4 is a schematic diagram of an alternative conveyor configuration. Detailed description

Figs 1a and 1b schematically illustrate a portion of a production system, wherein endless conveyor sections 11, 12, 13, 21, 22, 23, 24 are configured to transport workpieces, preferably carried by workpiece carriers WPC, between operation stations.

In Fig. 1a, empty workpiece carriers are illustrated by a square and loaded workpiece carriers are illustrated by a square with an inscribed circle.

The production system portion comprises first and second main conveyor sections 11, 12 and a flow balancing conveyor section 13.

The main conveyor sections are interconnected by the flow balancing conveyor section 13 in the sense that workpiece carriers may pass from one of the main conveyor sections 11, 12 to the other via the balancing conveyor section 13.

Additional main conveyor sections may be added, wherein, optionally, each any, but not necessarily all, pairs of adjacent main conveyor sections may be connected via a flow balancing section.

Each of a plurality of operation unit conveyor sections 21 , 22, 23, 24 is connected to one of the main conveyor sections 11, 12.

The operation unit conveyor sections 21, 22, 23, 24 are configured to receive a workpiece carrier from the main conveyor section 11 , 12, to convey it to its associated operation station 31, 32, 33, 34 and then to return it to the main conveyor section 11 , 12 when it has been processed by the operation station 31 , 32, 33, 34.

Each operation station may be designed to carry out one or more operations on an arriving workpiece, such as, but not limited to, machining (cutting, grinding, milling, lathing, etc.), measuring, assembly, bonding (welding, brazing, gluing), painting, loading, or unloading.

It is noted that one, two or more operation unit conveyors 21 , 22, 23, 24 may be connected to each main conveyor section 11, 12.

At each connection between conveyors, there is provided a switch device 41 , 42, 43, 44, 45, 46. Switch devices are known as such and one example of a switch device is disclosed in EP3121136A1.

Moreover, at each connection between conveyors, there is provided at least one, possibly two or three, auxiliary workpiece carrier sensors 51 , 61 , 81, 52, 62, 82, 53, 63, 83, 54, 64, 84, 55, 65, 85, 56, 66, 86.

The auxiliary workpiece carrier sensors may be workpiece return sensors 51 , 52, 53, 54, 55, 56, which are configured to detect a workpiece or workpiece carrier leaving the conveyor section associated with the workpiece return sensor.

When applied to an operation unit conveyor 21 , 22, 23, 24, workpiece return sensors 51 , 52, 53, 54 may be positioned on the respective operation unit conveyor 21, 22, 23, 24, immediately upstream of the associated switch device 41 , 42, 43, 44.

When applied to a flow balancing conveyor 13, workpiece return sensors 55, 56 may be positioned on the respective flow balancing conveyor 13, immediately upstream of the associated switch device 45, 46 connecting the flow balancing conveyor 13 to a highway conveyor 11, 12.

Alternatively, or as a supplement, the auxiliary workpiece carrier sensors may be workpiece receive sensors 61 , 62, 63, 64, 65, 66, which are configured to detect arrival of a workpiece carrier in the conveyor section associated with the workpiece receive sensor.

When applied to an operation unit conveyor 21 , 22, 23, 24, workpiece receive sensors 61 , 62, 63, 64 may be positioned on the respective operation unit conveyor 21 , 22, 23, 24, immediately downstream of the associated switch device 41 , 42, 43, 44.

When applied to a flow balancing conveyor 13, workpiece receive sensors 65, 66 may be positioned on the respective flow balancing conveyor 13, immediately downstream of the associated switch device 45, 46 connecting the flow balancing conveyor 13 to a highway conveyor 11, 12.

Alternatively, or as a further supplement, the auxiliary workpiece carrier sensors may be workpiece pass sensors 81 , 82, 83, 84, 85, 86, which are configured to detect passage of a workpiece on a highway conveyor 11, 12 associated with the workpiece pass sensor.

When applied to an operation unit conveyor 21 , 22, 23, 24, workpiece pass sensors 81 , 82, 83, 84 may be positioned on the respective highway conveyor 21, 22, 23, 24, immediately downstream of the associated switch device 41 , 42, 43, 44.

When applied to a flow balancing conveyor 13, workpiece pass sensors 85, 86 may be positioned on the respective flow balancing conveyor 13, immediately downstream of the associated switch device 45, 46 connecting the flow balancing conveyor 13 to a highway conveyor 11, 12.

Any auxiliary workpiece carrier sensor 51 , 61 , 81 , 52, 62, 82, 53, 63,

83, 54, 64, 84, 55, 65, 85, 56, 66, 86 may include a tag reader and, which may be configured to read a tag to identify a workpiece carrier and optionally to derive data from the workpiece carrier.

Based on tag data, it is possible to derive e.g. an indication of where the workpiece is to be transported. The tag data may thus have been updated at an operation station 31 , 32, 33, 34.

It is also possible to derive based on tag data whether a workpiece carrier is empty or full.

In the alternative, a separate detector may be provided as part of the auxiliary workpiece carrier sensor 51 , 61 , 81 , 52, 62, 82, 53, 63, 83, 54, 64,

84, 55, 65, 85, 56, 66, 86, in order to derive whether the workpiece carrier is full or empty.

Fig. 1b illustrates the same production system portion as Fig. 1a, but with some reference numerals removed and others added, for clarity. Fig. 1b illustrates a central controller 71 , which is connected to operation stations 31 , 32, 33, 34 and optionally to switch devices 41 , 42, 43, 44, 45, 46, in order to receive sensor signals and/or status signals and in order to provide control signals for controlling the flow of workpieces in the conveyor system and optionally for controlling also the production operations.

In one embodiment, both operation stations 31, 32, 33, 34 and switch devices 41 , 42, 43, 44, 45, 46 are controlled by the central controller 71 , as would be the case in a PLC controlled system or in a semi-autonomous system, e.g. as disclosed in EP3418834A1.

In another embodiment, only the operation stations 31 , 32, 33, 34 are controlled by the central controller 71 , as would be the case wherein an autonomous conveyor system is applied, e.g. as disclosed in EP3214024A1.

In yet another embodiment, the central controller 71 may be abolished altogether, where an autonomous conveyor system is used, as mentioned above, and wherein also the operation stations are autonomous.

The auxiliary workpiece carrier sensors 51, 61, 81, 52, 62, 82, 53, 63, 83, 54, 64, 84, 55, 65, 85, 56, 66, 86 are connected to a monitoring device 72, such that data from the sensors can be received at the monitoring device. Communication between the auxiliary workpiece carrier sensors and the monitoring device 72 may be achieved through a wired or wireless data interface.

The monitoring device 72 may be a computer with appropriate communication hardware and software. A memory device may be provided for storing event messages received, or data corresponding to event messages. A processor may be provided, along with software for analyzing the event data, e.g. as will be described in the following.

According to a first concept, a monitoring system is presented, which has the capability to determine operation station performance, e.g. by determining cycle time, idle time and/or breakdown time, and to identify anomalies at each operation station.

In this concept, at least a workpiece return sensor 51 , 52, 53, 54 is provided at those switch devices 41 , 42, 43, 44 associated with operation units 31 , 32, 33, 34 that are to be monitored. The workpiece return sensor 51 , 52, 53, 54 will detect arrival of a workpiece carrier at the associated switch device 41 , 42, 43, 44, and, provided the workpiece carrier is allowed to merge onto the highway conveyor 11 , 12, an event message is sent to the monitoring device 72, containing a time stamp, and optionally an indication of what type of event occurred, such as a merge event, indicating that the workpiece carrier is returned to the distribution highway conveyor 11, 12. The message may also contain a sensor ID, such that the event message can be associated with the right sensor.

In this case the event message will indicate “merging”, since the workpiece carrier is being returned from one of the operation unit conveyors conveyor to the highway conveyor by the associated switch controller.

Hence, in its most basic form, the workpiece return sensor 51 , 52, 53, 54 will only generate a merge event, indicating that a finished product is leaving the associated operation unit conveyor. This is significant, since the measurement thus provided does not only take into account the cycle time of the operation unit, but of the entire operation cycle, including the transport of the workpiece from and to the highway conveyor.

The monitoring device 72 will receive event messages indicating time stamps for points in time where workpiece carriers have left the operation unit conveyor.

Under normal operation, the time between event messages will correspond to the total cycle time for that operation station.

Hence, a time for a normal operation may be determined as e.g. a most common time (often referred to as “mode”) between events (with some tolerances), or as an average time between events. It may also be possible to determine time for normal operation based on e.g. a calibration run, where it is known that the times between events corresponds to a normal operation. Again tolerances may be applied.

With time for normal operation being known, it is possible to assign time for non-normal operation, i.e. anomalies.

In one embodiment, a distinction is made only between normal operation and non-normal operation. With the time for a normal operation (Tnormai) being known, the time for non-normal (Tnon-normai) operation may be derived as the time between two events (T2-T1 ) less the time for a normal operation (Tnormai).

For a normal operation: Tnormai = T2-T1

For a non-normal operation: Tnon-normai ⁼ T2-Ti-Tnormai

Flence, for each operation station, the total time of normal operation may be derived as an accumulation of those times between events that correspond to normal time period lengths, and total time of non-normal operation may be derived either as the remaining time, or as the sum of times representing normal operation.

Optionally, the time for non-normal operation may be further broken down into e.g. idle time and breakdown time.

Idle time (Tidie) may be defined as the time for a normal operation with a certain addition, in relative or absolute time.

For example, idle time may be defined as time between consecutive events that is 105-130 % of the time for normal operation.

That is, if the time between two consecutive events exceeds 105 % of the time for normal operation but is below 130 % of the time for normal operation, the difference between the time between the events (T2-T1) and a normal operation (Tnormai) may be defined as idle time.

Hence, if 105 % of Tnormai < T2-T1 < 130 % Of Tnormai, then Tidie ⁼ T2-T1-

Tnormal

Breakdown time (Tbreakdown) may be defined as the total time between two consecutive events, provided that total time exceeds the breakdown time threshold.

Hence, if T2-T1 > 130 % of Tnormai, then Tbreakdown — T2-T1 The monitoring device 72 may accumulate normal operation time, idle time and breakdown time, which may be presented as a performance measurement of each operation station 31 , 32, 33, 34, including its associated operation unit conveyor 21, 22, 23, 24.

Fig. 2 schematically illustrates a performance diagram for an operation unit.

Optionally, the workpiece return sensor 51 , 52, 53, 54 may comprise a tag reader and/or a workpiece detector, such that it can read a tag off the workpiece carrier. Based on the tag, the workpiece return sensor 51, 52, 53, 54 can determine whether there is a workpiece on the workpiece carrier, and if there is a workpiece on the workpiece carrier, the workpiece return sensor 51 , 52, 53, 54 may determine whether the workpiece carrier is to be merged onto the highway conveyor 11, 12 or whether it is to remain on the operation unit conveyor 21 , 22, 23, 24, e.g. because it has not yet been processed by the associated operation unit 31 , 32, 33, 34. Hence, a further event may be provided, such as “recirculate”.

Optionally, a workpiece receive sensor 61 , 62, 63, 64 may be provided to detect arrival of a workpiece carrier to the operation unit conveyor 21 , 22, 23, 24 associated with the workpiece receive sensor.

The workpiece receive sensor 61 , 62, 63, 64 will detect arrival of a workpiece carrier at the operation unit conveyor 21 , 22, 23, 24, and an event message is sent to the monitoring device 72, containing a time stamp and optionally an indication of what type of event occurred, such as a divert event, indicating that the workpiece carrier is received at the operation unit conveyor 21 , 22, 23, 24. The message may also contain a sensor ID, such that the event message can be associated with the right sensor.

Based on data from a workpiece return sensor 51 , 52, 53, 54 and a workpiece receive sensor 61 , 62, 63, 64, it is possible to determine the number of workpiece carriers present on an operation unit conveyor 21 , 22, 23, 24, and thus the size of a local buffer associated with an operation unit 31, 32, 33, 34.

Optionally, the workpiece receive sensor 61 , 62, 63, 64 may comprise a tag reader and/or a workpiece detector, such that it can read a tag off the workpiece carrier. Based on the tag, the workpiece receive sensor 61 , 62, 63, 64 can determine whether there is a workpiece on the workpiece carrier or if it is empty.

By comparing workpiece ID between the workpiece return sensor and the workpiece receive sensor, it is possible to identify a workpiece which is being recirculated, e.g. since it has not yet been processed at the operation station.

Further optionally, a workpiece pass sensor 81 , 82, 83, 84 may be provided on the highway conveyor 11, 12, immediately downstream of a switch device 41 , 42, 43, 44, to detect passage of a workpiece carrier past the switch device 41 , 42, 43, 44.

The workpiece pass sensor 81 , 82, 83, 84 will detect arrival of a workpiece carrier on the highway conveyor 11, 12, and an event message is sent to the monitoring device 72, containing a time stamp and optionally an indication of what type of event occurred. The message may also contain a sensor ID, such that the event message can be associated with the right sensor.

In this case, the event may indicate “diverted”, since the workpiece carrier is being diverted from the highway conveyor to an operation unity conveyor by the associated switch controller.

The workpiece pass sensor 81 , 82, 83, 84 may comprise a tag reader and/or a workpiece detector, such that it can read a tag off the workpiece carrier. The event type indication may thus be sent depending on the content of the tag and/or on the result of a workpiece presence detection.

For example, where a plurality of identical operation stations are available, a workpiece carrier travelling on a highway conveyor 11, 12 may be rejected by a switch device 41 , 42, 43, 44, even though he workpiece carried by that workpiece carrier is destined for the operation, such that the workpiece carrier may instead proceed to another one of the identical operation stations. In this case, the event message may indicate “passing no priority”, since the workpiece carrier is caused to pass the switch control device even though the associated operation unit conveyor corresponds to a next operation for the workpiece carrier and is available. Another example is where a workpiece carrier travelling on the highway conveyor 11, 12 passes a switch device 41 , 42, 43, 44 because it is not intended to be processed by the operation unit 31 , 32, 33, 34 associated with that switch device. In this case, the event message may indicate “passing, not this operation”, since the workpiece carrier is caused to pass the switch control device because the next operation for the workpiece carrier does not match the associated operation unit conveyor.

Yet another example is where a workpiece carrier travelling on the highway conveyor is refused to enter the associated operation unit conveyor, even though it is intended to be processed by the associated operation unit, because the operation unit conveyor is full. In this case, the event message may indicate “passing full”, since the workpiece carrier is caused to pass the switch control device and the operation unit conveyor is full of workpiece carriers.

Yet another example is where the workpiece carrier is found to be empty, and for this reason is not allowed to enter an operation unit conveyor 21 , 22, 23, 24. In this case, the event message may indicate “passing empty”, since the workpiece carrier is caused to pass the switch control device and the workpiece carrier is empty.

Based on the event messaged described above, it is possible to provide data on events at each individual sensor.

For example, it is possible to show over time how many events of each kind are generated. When operating at a certain capacity, and fully functioning, it may be expected that a certain number of each event type will occur during a certain time period. Deviations may thus be made easily visible.

It is further possible to compare event messages from various auxiliary workpiece carrier sensors at one intersection, e.g. with respect to workpiece ID.

For example, if a “passing...” event is received from a pass sensor 81, 82, 83, 84, 85, 86, but is not preceded by any “merging” event from the respective return sensor 51 , 52, 53, 54, 55, 56, then it can be deduced that that particular workpiece carrier passed the switch without being diverted or merged. That is, it remains on the same highway or flow balancing conveyor.

If instead the “passing...” event was preceded (e.g. within a predetermined time span or for a certain workpiece ID) by a “merging” event, then it can be deduced that a merge has taken place.

Likewise, if an event message from a return sensor 51 , 52, 53, 54, 55, 56 preceds (within a predetermined time span or for a certain workpiece ID) an event message from a receive sensor 61, 62, 63, 64, 65, 66, then it can be deduced that a workpiece remains on the same conveyor, e.g. because it was not allowed to leave.

According to a second concept, a monitoring system is presented, which has the capability to determine work in progress and/or to determine buffer sizes and/or to monitor bottleneck protection.

In this concept, at least a workpiece return sensor 55, 56 is provided at those switch devices 45, 46 associated with a flow balancing conveyor 13 that is to be monitored.

According to this concept, the workpiece return sensor 55, 56 will detect the arrival of a workpiece carrier immediately upstream of a switch device 45, 46 that controls the flow between a highway conveyor 11, 12 and a flow balancing conveyor.

Again, this workpiece return sensor 55, 56 may issue a “merge” event message whenever a workpiece carrier arrives that is to be merged from the flow balancer 13 onto a highway conveyor 11, 12. The message may also contain a sensor ID, such that the event message can be associated with the right sensor.

That is, workpiece carriers which are present on a flow balancing conveyor 13 and which carry a workpiece that is intended to proceed downstream to the next highway conveyor 12, will generate a merge message.

Optionally, empty workpiece carriers that are intended to proceed upstream to the previous highway conveyor 11 , may generate a “merge empty” event message. It is noted that all other features described with respect to the workpiece return sensors 51, 52, 53, 54 associated with operation unit conveyors are applicable to the workpiece return sensors 55, 56 associated with the flow balancing conveyor 13.

Optionally, a workpiece receive sensor 65, 66 may be provided to detect arrival of a workpiece carrier to the flow balancing conveyor 13.

The workpiece receive sensor 65, 66 will detect arrival of a workpiece carrier at the flow balancing conveyor 13, and an event message is sent to the monitoring device 72, containing a time stamp and optionally an indication of what type of event occurred, such as a divert event, indicating that the workpiece carrier is received at the flow balancing conveyor 13. The message may also contain a sensor ID, such that the event message can be associated with the right sensor.

By comparing event messages indicating arrivals and returns to sensors associated with a flow balancing conveyor 13, it is possible to obtain a status indication of the flow balancing conveyor, such as the number of workpiece carriers present on the flow balancing conveyor 13, and optionally, how many full and empty workpiece carriers, respectively, are present.

Further optionally, a workpiece pass sensor 85, 86 may be provided on the highway conveyor 11, 12, immediately downstream of a switch device 45, 46 associated with a flow balancing conveyor 13, to detect passage of a workpiece carrier past the switch device 45, 46.

The workpiece pass sensor 85, 86 will detect arrival of a workpiece carrier on the highway conveyor 11, 12, and an event message is sent to the monitoring device 72, containing a time stamp and optionally an indication of what type of event occurred. The message may also contain a sensor ID, such that the event message can be associated with the right sensor.

It is noted that the tag data on a workpiece carrier may indicate what the next processing step for that workpiece is, which may indicate whether the workpiece is to remain on the present highway conveyor 11 or whether it is to proceed to a subsequent highway conveyor 12, analogous with what was discussed referring to the operation units 31 , 32, 33, 34. Hence, event messages may be generated by the workpiece pass sensor 85, 86 analogous with what was discussed with regard to those workpiece pass sensors 81 , 82, 83, 84 associated with the operation unit conveyors 21 , 22, 23, 24.

By comparing event messages indicating workpieces entering, leaving and staying on a highway conveyor 11, 12, it is possible to determine the status of that highway conveyor, e.g. in terms of the amount of buffer present on the highway conveyor 11, 12.

In a series of highway conveyors 11, 12, separated by flow balancing conveyors 13, it is possible to compare event messages from sensors associated with different flow balancing conveyors 13.

Moreover, it is possible to associate counts at the various auxiliary workpiece carrier sensors with loop times (i.e. the time for one lap) for the respective conveyor.

Fig. 3 schematically illustrates the number of workpiece carriers (y- axis) over time (x-axis) for one conveyor, with the dashed lines indicating upper and lower thresholds, with the upper threshold indicating too many workpiece carriers present on the conveyor and the lower threshold indicating too few carriers present on the conveyor.

For example, in the case of the highway conveyor 11 to flow balancing conveyor 13 junction, by counting the number of workpiece carriers that come from the highway conveyor 11 and pass the switch device 45 (i.e. “passing full”), it can be determined how many workpiece carriers are completed but still left on the first highway conveyor 11 , even though they should pass over to the flow balancing conveyor 13 and then on to the second highway conveyor 12.

Similarly, the number of workpiece carriers present on the flow balancing conveyor can be determined, as can the number of workpiece carriers (finished or unfinished) preset on any one of the operation unit conveyors.

Thus, the amount of work in progress can be accurately monitored and analyzed. Moreover, it is possible to provide an alarm when the amount of work in progress exceeds or falls below the thresholds. Referring to fig. 4, there is schematically illustrated yet another situation, where auxiliary workpiece carrier sensors may be employed.

In fig. 4, there is illustrated a highway conveyor 14, which may replace or supplement the highway conveyors 11, 12 illustrated in figs 1 a and 1 b. This highway conveyor 14 has a short cut conveyor 141 , which provides a short cut between two portions of the highway conveyor 14, thus enabling a shortened circulation path, which may be used for finished products that are to be shifted out at the intersection 45.

This short cut conveyor 141 may be realized by a switch device 47, connecting the highway conveyor 14 to the short cut 141. The switch device may comprise a tag reader for determining the status of the load of an arriving workpiece carrier. For example, all finished workpiece carriers may be diverted into the short cut 141 while all non-finished workpiece carriers are allowed to proceed on the highway conveyor 14. A first auxiliary workpiece carrier sensor 57 associated with the short cut 141 may thus be configured to detect a workpiece carrier leaving the short cut 141 and thus merge onto the highway conveyor 14.

A second auxiliary workpiece carrier sensor 67 associated with the short cut 141 may be configured to detect a workpiece carrier entering the short cut 141 , after having been diverted from the highway conveyor 14.

A third auxiliary workpiece carrier sensor 87 associated with the short cut 141 may be configured to detect a workpiece carrier passing the intersection 47 without entering the short cut 141.