The present invention relates to a production line and to a trolley for use in the production line.

Since the introduction of the assembly line, it has been a paradigm that for cost-effective mass production, workpieces shall be subjected to identical processing steps by being conveyed from one workstation, where one pro cessing step is carried out, to a next workstation, where a next processing step is carried out, thus applying identical processing steps to all workpiec es and outputting identical finished articles at the last one of the work stations. Ideally, such workstations will be adjacent to each other, so that a workpiece, after having been processed in one workstation, can be for warded to the next with minimum loss of time. Still, time can be lost in such a production line when cycle times of the workstations differ, and a fast workstation is compelled to wait for a subsequent, slower workstation to finish a task before it can forward there a workpiece that has been pro cessed by it, and accept a further one.

Computer control has allowed more flexible production schemes, in which workpieces may be conveyed along different paths between workstations using trolleys which can move more or less unconstrained on a workshop floor wherever there is space enough for them to pass. This allows a work station that has finished its job on a given workpiece to discharge that workpiece to a trolley, and immediately accept a new one, whereas the trolley can convey the workpiece to the any one of several workstations dedicated to the next processing step, that happens to be idle or will soon finish its current workpiece. In such a production line, efficiency can suffer because trolleys need to recharge and are therefore not available, or if re- charging trolleys block the path of busy ones. Of course, a shortage of op erative trolleys can be prevented by providing a large number thereof, but the larger their number is, the more likely they are to block one another’s way. Charging trolleys can be prevented from blocking the busy ones by providing charging stations remote from the work stations, but charging trolley will still occupy considerable space that might be put to better use, and moving to the charging stations and back also keeps the trolleys occu pied unproductively.

The object of the present invention is therefore to provide a highly efficient production lines and trolleys for use in such a production line.

According to a first aspect of the invention, this object is achieved by a pro duction line for processing a workpiece in successive steps, the production line comprising a plurality of stations and a plurality of trolleys, each of which comprises a first connector, said plurality of stations comprising at least one workstation adapted for carrying out therein an associated one of said steps and at least one charging station comprising a second connector for providing a commodity to one of said trolleys when connected to the first connector of said trolley, wherein at least one of said trolleys comprises a second connector which communicates with the first connector.

When said trolley has its first connector connected to the second connector of a charging station, not only can the trolley take in the commodity provid ed by the charging station, but it can also forward it to a second trolley which docks to its second connector. Thus, the number of charging stations can be reduced, and the space that these would otherwise require can be put to more productive use. In a preferred embodiment, at least one of said workstations is also a charging station. This allows a trolley to take in the commodity while it is occupied at the workstation, thus reducing the time spent unproductively on charging. The larger the fraction of workstations is that are also charging stations, the more frequently the trolley can recharge without having to abandon productive work, and the smaller is the fraction of downtime spent at a station which is only a charging station. Ideally, the fraction of work stations that are also charging stations is so large stations which are only charging stations can be done without.

The commodity to be recharged can be any of electric power, data, fuel, pressurized gas or vacuum. Preferably it comprises at least one of electric power and pressurized gas. The possibility to recharge frequently at sever- al workstations visited in the course of an operating cycle of the trolley re duces storage capacity requirements, which reduces cost and weight of the trolleys.

By making the second connector of said trolley identical to the second con- nector of one of said charging stations, it is ensured that the trolley can recharge by docking directly to a charging stations or by docking to another trolley which is already charging there. Thus no time is lost waiting for an opportunity to recharge. Further, when the charging station also is a work station, the trolley can thus make good use of the time it spends waiting while the workstation is busy with a workpiece conveyed by the other trolley docked between it and the workstation.

In a particularly preferred embodiment, the storage space of the trolley is a dust-proof compartment having at least one port adapted to be coupled in a dust-proof manner to mating ports of said at least one workstation. Such a dust-proof compartment is particularly for transporting freshly painted work- pieces, i.e. in a production line wherein the processing step associated to at least one of the workstations is a painting step.

The ports of the trolley and/or of the workstations preferably comprise a slidable door. Slidable doors can be opened after the ports of the trolley and of the workstation have been placed in mating relationship, so that en try of dust through gaps between the ports can be minimized, or can be prevented altogether by a slightly positive pressure in the compartment and/or the workstation.

The door should comprise a flexible sheet. By displacing the sheet continu ously around an edge of the compartment from a face side facing the work station to e.g. a sidewall or a roof of the compartment, the slidable door can fill almost the entire face side and can nevertheless be opened without in- creasing the dimensions of the trolley.

In order to facilitate loading and unloading workpieces, the trolley may comprise a rail portion for displaceably supporting the workpiece. When this rail portion is coupled to a rail of a workstation, the workpiece can be trans- ferred easily and smoothly along the rail and the rail portion.

In the production line, more than two workstations should have ports by which a workpiece can be transferred to or received from the trolley. Thus, the trolley can take up a workpiece at the port of one workstation and choose a workstation to which the workpiece is to be delivered, or collect workpieces from several workstations and deliver these to a same target workstation.

In order to minimize idle time of a workstation, the workstation should have at least two ports to which a trolley can be coupled, so that loading a pro cessed workpiece on one trolley and unloading the next workpiece to be processed from another trolley can take place simultaneously at different ports.

According to a second aspect of the invention, the object is achieved by a trolley comprising a motorized steerable undercarriage, a storage space for a workpiece, a first connector for receiving a commodity from an external source and a second connector for outputting said commodity to an exter nal sink. The external source can be a charging station or another trolley that is docked to a charging station; the sink, if one is connected, will gen- erally be another trolley of the same type.

Preferably, the trolley is an autonomous guided vehicle, AGV.

Autonomy of the trolley can imply that the trolley, in an unloaded state, is adapted to identify a workstation that has finished or is about to finish a processing step associated to it, to navigate to said workstation and to be loaded there with a workpiece after said processing step has been finished on it. Analogously, the trolley may be adapted to, when loaded, identify a next processing step to be carried out on the workpiece, to identify a next work station associated to said next method step and to drive the workpiece to said next workstation. The identification of said next processing step may be based on data the trolley charges from the workstation while being docked to it.

Preferably the first and second connectors are located on opposite sides of said undercarriage so that when on one side of the undercarriage the first connector faces a station to which the trolley is docked, the second con- nector on the opposite side is free to accommodate a second trolley. In order to enable efficient docking, the side where the first connector is locat ed should be a front side of the trolley in a normal displacement direction thereof, so that a connection can be established simply by the trolley ap proaching the charging station in its normal displacement mode, and the second trolley can connect to the second connector at the rear side by moving in the same displacement mode.

Preferably, the first and second connectors are hermaphroditic, so that the trolley can make contact to a charging station with whichever one of its connectors happens to be in front when the trolley approaches the station. The trolley should comprise storage means for the commodity in communi cation with said first connector.

As mentioned above, the storage space of the trolley can be a dust-proof compartment. In that case, the commodity should comprise pressurized gas, and the trolley should comprise means, in particular, valves, for meter ing said pressurized gas into the compartment. Thus entry of dust into the compartment can be prevented even if the compartment isn’t hermetically sealed, since air escaping through gaps of the compartment will prevent dust from entering through these or through the at least one port.

Where the trolley comprises a rail portion for displaceably supporting the workpiece, the rail portion may be adapted to be connected to the rail por tion of a second, identical trolley. In that way, a workpiece can be conveyed from said second trolley to a workstation or vice versa via the first trolley, without any of the trolleys having to move and to interrupt charging.

For the trolley, it is also advantageous to have a plurality ports in order to minimize the amount of maneuvering needed for coupling to a workstation port. Preferably, there are two ports provided at opposite sides of the dust- proof compartment, in association to the first and second connectors of the undercarriage. To enable the trolley to identify a next method step to be carried out on the workpiece loaded into it and to identify a target workstation for said work- piece, each workpiece should have associated to it a schedule of method steps to be carried out on it, which is accessible to the trolley.

For accessing the schedule, the trolley may comprise a reader, for example a RFID reader, for reading data from a data carrier of a workpiece. The data may comprise the schedule itself, or a reference by which the sched ule can be retrieved from a database.

Each workstation may have a recording device associated to it for writing into the schedule of a workpiece processed by it a record identifying the workstation and/or its associated method step, so that when the workpiece is loaded onto a trolley, the trolley can find out the next method step due to be carried out on the workpiece, and find an appropriate workstation to de liver the workpiece to.

For good maneuverability the undercarriage may comprise a plurality of casters that are drivable independently and are rotatable around a vertical axis.

Further features and advantages of the invention will become apparent from the subsequent description of embodiments, referring to the appended drawings.

Fig.1 is a schematic diagram of a conventional production line

Fig. 2 is a schematic diagram of a first production line according to the invention;

Fig. 2 is a schematic diagram of a second production line according to the invention; Fig. 4 is a schematic diagram of a third production line according to the invention; Fig. 5 is a diagram of a trolley used in the production lines of Fig. 2,

3 and 4; and

Fig. 6 is a schematic section of the trolley’s undercarriage. Fig. 1 is a schematic diagram of a conventional production line for painting car bodies. Workstations of the production line comprise paint stations 1-6. Paint station 1 is for applying a primer to the white car body, stations 2 and 3 for applying a base coat to an inside and an outside of the car body, re spectively, and stations 5, 6 for applying a clear coat inside and outside.

While for painting a car body in a single colour, paint stations 1-3, 5 and 6 will be sufficient, a further paint station is necessary if a customer’s wish for a multicolour body is to be satisfied, e.g. if the roof is to have a colour dif ferent from the rest of the car body. Integrating such a further paint station into a linear path of the car bodies would lead to an increase in processing time for all car bodies, and would leave the further paint station idle most of the time. Therefore, the production line of Fig. 1 provides a track switch 7 where a car body can be sent to paint station 4, if required, whereas all others go straight to paint stations 5, 6, and a track switch 8 where paths of the car bodies having undergone painting in station 4 and of those that ha ven’t reunite.

When one layer of paint has been applied, and before applying a next one, the car bodies are flashed, i.e. volatile components of the layer are given time to evaporate, and baked. The production line of Fig. 1 therefore has baking stations 9 between paint stations 1 and 2, 10 between paint stations 3 and 4, 11 between track switch 8 and paint station 5 and 12 behind paint station 6. Flashing stations 13 are provided between stations 1 and 9, sta tions 9 and 2, station 3 and track switch 7, and various other places as shown in Fig. 1. Due to the track switches 7, 8, not all stations of the production line can be directly adjacent and sealedly coupled to each other. Wherever tracks 14 for conveying the car bodies extend outside a workstation, they have to be tightly shrouded in order to ensure that no dust will settle on the car bodies during conveyance.

Fig. 2 illustrates a production line according to the invention. The work stations of this production line are the same as in Fig. 1 , and the steps of the painting process carried out by them are also identical. Therefore, the same reference numerals as in Fig. 1 will be used for the workstations, and their description will not be repeated here.

As in Fig. 1 there are three groups of adjacent workstations, the first com prising paint stations 1, 2, 3 and baking station 9, the second comprising baking station 10 and paint station 4, and the third comprising paint stations 5, 6 and baking stations 11, 12. Between the groups, no tracks are provid ed. Instead, there is a flat floor surface between the groups where vehicles can move freely in two dimensions, unconstrained by tracks.

Paint station 3 has an outlet port 15 for ejecting a car body after having applied the base coat to its outside. A trolley 16 is shown coupled to the outlet port 15 for receiving the car body. When the car body has been load ed on the trolley 16, the trolley 16 drives it to the next station where the car body is to be processed. In the case shown, this is either baking station 11 or baking station 10. There is no flashing station at the outlet side of paint station 3, since the time the car body spends being ferried on board the trolley serves the purpose of a flashing stage. The trolley 16 has an inlet port 17 for coupling to the outlet port 15 of paint station 3, and an outlet port 18 opposite the inlet port 17, which can be coupled to inlet ports 19 of baking stations 10 or 11, in order to unload the car body.

Ports 15, 19 of workstations 3, 10 face each other, so that the trolley 16 can move from one to the other along a straight line 20, whereas moving from the paint station 3 to baking station 11 involves a U-turn 21. In Fig. 2, the U- turn 21 is wider than necessary to reach workstation 11, at the end of the U-turn, a second image 16’ of trolley 16 is shown in a parking position lat erally offset with respect to inlet port 19 so that another trolley 22 can finish unloading a car body into workstation 11 and then move away. Such a situ ation may arise since when a car body has been processed in paint station 4, it and the next car body finished by paint station 3 will compete for a place in baking station 11. Use of the trolley allows to keep one (or more) car bodies waiting for admission to baking station 11 , whenever the need arises.

Fig. 3 illustrates a second embodiment of a production line which exploits the possibilities offered by the trolleys more systematically than that of Fig 2. There is only one flashing station 13 left in this production line; all others have been replaced by trolleys. While only two trolleys 16, 22 are shown in Fig. 3, their number may be larger, as necessary e.g. to serve as temporary storage for car bodies that have to wait for a next processing step. The number of trolleys is smaller than the number of flashing stations 13 in Fig.

2, since times needed for flashing are shorter than those for painting or baking. Thus one trolley, by transporting car bodies between more than one pair of stations, can replace more than one flashing station, saving space and cost.

Painting stations 1-6 are identical to those of Fig. 1 and 2 and serve the same purpose. Obligatory paint stations 1-3, 5 and 6 are located on one side of a central corridor 23, facultative paint station 4 and baking stations 9-12 on the other. When primer has been applied to a car body in paint station 1 , one of the trolleys 16, 22 will come and shuttle it across the corri dor 23 to baking station 9. Trolley 16 is shown carrying a car body from baking station 9 to paint station 2. Since paint stations 2, 3 are arranged in line, the trolleys have to fetch a car body that got its base coat applied in stations 2, 3 at the outlet port 15 of station 3 facing away from corridor 23 and convey it from there to one of baking station 11 or, if the body is to be painted in more than one color, baking station 10.

Baking station 10, flashing station 13 and paint station 4 are arranged adja cent to each other so that a car body that was delivered to baking station can be conveyed to paint station 4 without the use of a trolley. Trolley 22 is shown fetching a car body from paint station 4 and delivering it to baking station 11. Alternatively, it might deliver it once more to baking station 10. By thus sending the car body once more through paint station 4, as many different colours as desired can be applied to parts of the car body, with no additional investment in machinery required. T rolley 22 is delivering its car body to a port 15 of baking station 11 facing away from corridor 23, since, coming from paint station 4, it is the fastest and easiest port to reach. I.e. every port 15, 19 of station 11 can serve both as an inlet or an outlet port, as needed. If at the time when trolley 22 has to deliver its car body, baking station 11 is occupied while baking station 12 is not, trolley 22 is adapted to detect the fact and to deliver to baking station 12 instead.

When a car body that previously passed through paint stations 2, 3 and, eventually, 4, is taken out of baking station 11, it is conveyed to paint sta tion 5 by a trolley not shown. From the outlet port 15 of paint station 6, it is conveyed back to any of baking stations 11, 12 that happens to be free. After treatment in this last baking station, the painting process is finished, and the car body is conveyed to a subsequent assembly line.

Fig. 4 illustrates a further optimized embodiment of the production line. Here, all transport of the car bodies is done by the trolleys, from loading the white body into paint station 1 to unloading the fully painted body from one of the baking stations. Pairs of stations 2, 3 and 5, 6 for painting inside and outside of the car bodies have been replaced by combined stations 2/3 and 5/6, respectively, each of which is designed to paint inside and outside. Since the time required for processing a car body in each of the combined stations 2/3 and 5/6 is longer than in the single-function stations 2, 3 and 5, 6 of the previous embodiments, more than one of each type is provided. Thus, a processed car body can be unloaded from one of e.g. stations 2/3 and replaced by a new one while the other is occupied processing one car body, and one trolley 16 or 22 is sufficient to serve them both.

Paint stations 1, 2/3 and 5/6 and baking stations 9-12 are disposed at op posite sides of corridor 23. All are loaded and unloaded through ports fac ing the corridor 23. Since it is no longer necessary to convey the car bodies through the workstations, conveying means of the workstations can be dis pensed with; it is sufficient if the trolleys have conveying means with an operating range sufficient to place a car body appropriately within the work stations 1-6, 9-12 and to withdraw it from there. At least some of the baking stations 9-12, e.g. stations. 9 and 10, have heaters 24 that are displaceable or have independently controllable seg ments 25 in order to apply heat locally to a part of a car body that is to re ceive a further paint layer on top of the base coat in paint station 4. Thus, when a car body is taken out from one of stations 2/3 and is destined for paint station 4, the trolley will have to take it to one of stations 9, 10; else it can take it to any one of baking stations 9-12 that is or will soon be availa- ble. Also, car bodies that are removed from stations 4 or 5/6 can be baked in any of stations 9-12.

Fig. 5 is a schematic view of one of the trolleys, e.g. trolley 16. The trolley has an undercarriage 26 which supports a box shaped compartment 27 for receiving a car body 28 on a substructure 29.

The undercarriage 26 comprises a rectangular chassis 30 with four casters

31 at its corners. Each caster 31 comprises a wheel 32, a first electric mo- tor 33 driving the wheel 32, and a second motor 34 for rotating the wheel

32 and its associated motor 33 around a vertical axis. Motors 33, 34 of the four casters 31 are controlled independently by an on-board computer 35, enabling the trolley to drive in curves, to rotate on the spot or to change its direction of displacement arbitrarily while maintaining its orientation.

The undercarriage further carries a pressurized air container 36 for pressur izing compartment 27 by dust-free air and a battery pack 38 for powering the motors 33, 34, the computer 35 and a magnetic valve 47 which controls airflow from container 36 to compartment 27.

The computer 35 has a wireless interface 37 for communication with other computers of the production line and with a transponder 39 attached to the car body 28. The compartment 27 is delimited by pillars 40 at its four corners, by a roof

41 carried by the pillars 40 and by sidewalls extending between the pillars 40 along the two longer sides of the chassis 30. At the two shorter sides, slidable doors serve as inlet and outlet ports 17, 18. Leaves of the doors are flexible or articulate, so that the doors can be opened without the door leaves protruding beyond the outline of the compartment 27. For example, the door leaves may be formed as a roller blind 42 that is guided in grooves of the pillars 40 and can be raised and lowered by rotation of a sprocket 43 controlled by computer 35. The left port 17 is shown in an open state in which most of the roller blind 42 extends along the roof 41 ; the right-hand port 18 is closed with an end of the roller blind 42 touching the chassis 30. When the trolley 16 is in an idle state, computer 35 scans for wireless mes sages indicating a workstation where a car body is ready for transport or will soon be. The requester sending such a message may be the work station itself or a central computer controlling operation of the workstations. When a message is received, trolley 16 calculates the distance or the time needed to reach the workstation and places a bid indicating the result.

Among several such bids, requester chooses the one from the trolley where distance or time is smallest, and sends a return message to said trolley 16 indicating that its bid is accepted. The trolley 16 then navigates to the work station indicated, and docks one of its port 17, 18 to a port of the work- station.

When facing ports of the workstation and of the trolley open, a positive pressure in compartment 27, upheld by air flowing from container 36 through valve 47 ensures that air will blow out through gaps between the ports, so that no dust can reach the compartment 27 while car body 28 and substructure 29 are transferred into it. After having received the car body 28, port 17 closes again.

Displacement of the car body 28 within compartment 27 may be facilitated by a plurality of rollers 44 rotatably mounted in an upper surface of chassis 30 for supporting the substructure 29. For actively moving the car body, some of the rollers may be motorized.

Instructions concerning the destination of the car body 28 can be provided directly by the central computer. Alternatively, the trolley’s computer 35 retrieves from transponder 39 either an identifier which it can use to query the processing status of the car body 28 from the central computer, or it retrieves the processing status of the car body directly from its transponder 39.

The processing status identifies steps of the painting process to be applied to the car body 28, their order of execution, the workstations in which they are carried out and, where appropriate, processing data such as an identifi er of a colour to be applied, a region of the car body where the colour is to be applied, and the like. Whenever a particular step of the painting process has been carried out, it is labeled as done in the processing status. Based on this processing status, the computer 35 identifies the next step to be carried out, and the workstations capable of carrying it out. It then sends a request for allocation of a processing time slot to all these. The work stations respond by communicating their next available time slot, and the computer 35 chooses the one requiring least waiting time, sends a reply message booking this time slot and sets out to bring the car body 28 to the work station it has booked. In the meantime, air is flowing continuously from container 36 into compartment 27.

Docking to that workstation can be done by either port 17 or 18. For dock- ing, each port of the trolley 16 has a mechanical coupling 45 associated to it, e.g. a pivotable hook, which is adapted to lock to a mating hook 49 of the workstation by a positive fit when the trolley 16 approaches the workstation, so that the trolley is prevented from moving relative to the workstation while a car body is being transferred between the two. Fig. 6 shows couplings 45, 49 of the trolley 16 and of a workstation, e.g. 1, slightly offset in a lateral direction merely to facilitate distinction between the two. Further, an electri cal connector 46 is provided by which, while the trolley 16 is docked to the workstation 1 , mains electric power is transferred to it via the workstation 1. Energy-intensive tasks of the trolley 16 such as transferring the car body between the workstation 1 and the trolley 16, heating or cooling the com partment 27 may be enabled only while the trolley 16 is thus connected to power mains 50. Thereby, load on the battery 38 is reduced, and the productivity of the trolley 16 is improved since the fraction of its operating time spent charging the battery 38 is reduced. Times the trolley 16 spends connected to a workstation can thus be used for recharging the battery 38 via a powerline 58 extending through the undercarriage 26. Since the amount of electric power transferred to the battery 38 must just be sufficient to enable operation of the trolley 16 until it reaches the next workstation having a mains connector 50, the battery 38 can be small and light-weight. Not all of the different types of workstations described above have to be equipped with such a mains connector, but in practice, a large fraction of them will be.

If desired, the electrical connector 46 may also be used for transferring data from the workstation to the computer 35 of the trolley 16, e.g. the pro cessing status of the car body 28 carried by the trolley, after the workstation has carried out its associated processing step on the car body.

Similar to electrical connector 46, a pneumatic connector 48 is provided adjacent to each port 17, 18 of the trolley 16, so that whenever the trolley 16 is docked to a workstation 1 equipped with a mating pneumatic con- nector 51 , the container 36 is refilled. As shown in Fig. 6, the pneumatic connector 48 associated to port 17 comprises an inlet port 52 and an outlet port 53. The inlet port 52 mates with connector 51 of workstation 1. An air duct 54 extending inside undercarriage 26 from inlet port 52 to container 36 comprises a check valve 55 which prevents air from leaking from inlet port 52 when unconnected. An air duct 56 extending between outlet port 53 and container 36 comprises a valve 56 which opens only when outlet port 53 mates with some external inlet port. Since workstation 1 has no such inlet port, the valve 56 associated to port 17 is closed. On the side of undercarriage 26 located below port 18, there is an identical assembly of electrical and pneumatic connectors 46, 48 facing connectors of an approaching trolley 22. The assemblies are inversion symmetric so that the assembly of port 18 might connect to workstation 1 just in the same way the assembly of port 17 does. When trolley 22 advances further to wards trolley 16, their mechanical couplings 45 engage and lock both to each other in a position where their electrical connectors 46 make contact, and wherein inlet and outlet ports 52, 53 of the two trolleys 16, 22 engage. Although each inlet port only couples to an outlet port of the other trolley and might therefore be regarded as a male/female couple, the pneumatic connector 48 as a whole is hermaphroditic. In the docked state, the battery of trolley 22 can recharge from mains pow er fed through to it by power line 58 of trolley 16. The valve 57 on port 18 side of trolley 16 is open due to outlet port 53 engaging inlet port 52 of trol ley 22, so that container 36 of trolley 22 can refill, too, while trolley 22 is waiting for trolley 16 to finish its task at workstation 1.

When trolley 16 has finished its task, trolley 22 can back off in order to al low trolley 16 to move away and then dock to workstation 1 in its place. In a preferred embodiment, and as outlined in Fig. 5, mechanical couplings 45 and connectors 46, 48 of the trolleys are mounted on supports 59 which are displaceable with respect to their respective undercarriage 26 along the engagement direction of connectors 46, 48. When docking to a workstation or to another trolley, the support 59 is in an advanced position shown on the port 18 side of Fig. 5. When trolley 16 has finished its task at work station 1 , it withdraws both supports 59 into a retracted position as shown on the port 17 side of Fig. 5. Thanks to casters 31, the trolley 15 can then leave its position in front of workstation 1 in a direction orthogonal to said engagement direction, i.e. it can move out of the gap between workstation

I and trolley 22 without requiring cooperation by trolley 22. This is particu larly advantageous in case that several trolleys are queuing up before a same workstation, as considered above with respect to the baking station

I I in the discussion of Fig. 2. The task of the trolley might be just to deliver a car body 28 to a work station, or to fetch it from there after processing in the workstation. In each case a time-consuming docking process has to be carried out in order to ensure that the car body can be unloaded from or loaded into the trolley properly. Such a two-fold docking is inevitable if a trolley is to be re-used in a new task as soon as it has discharged its payload at a workstation, and another trolley may come to fetch it after processing by the workstation is finished. In that case, in order to minimize idle time of a workstation, it may be necessary to provide the workstation with two ports, so that when pro- cessing of a car body in the workstation is about to finish, and one trolley that will carry away the finished car body is docking to one of the ports, an other, carrying a new car body to be processed, may be docking to the oth er. With the trolley and the production line of the present invention, it is more efficient to have a trolley wait at a workstation while a car body it has brought is being processed, and to have the same trolley take it away again when processing is finished. Thus the time spent in docking processes is reduced by half, and since the time the trolley spends docked to the work station is made efficient use of, the productivity of the trolley can be just as high as or higher than that of a trolley which from time to time has to aban- don the production line in order to be recharged. Moreover, when after pro cessing the car bodying is loaded onto the trolley that brought it, a high efficiency of the workstation can be achieved even if the workstation has just a single port, which significantly reduces the cost of the workstation.

Reference numerals

1-6 workstation (paint station) 7-8 track switch

9-12 workstation (baking station) 13 workstation (flashing station) 14 track

15 outlet port

16 trolley

17 inlet port

18 outlet port 19 inlet port

20 straight line

21 U-turn

22 trolley

23 corridor 24 heater

25 segment

26 undercarriage

27 compartment

28 car body 29 substructure

30 chassis

31 caster

32 wheel 33, 34 motor 35 computer

36 pressurizing means

37 wireless interface 38 battery pack

39 transponder

40 pillar

41 roof 42 roller blind

43 sprocket

44 roller

45 mechanical coupling

46 electrical connector 47 magnetic valve

48 pneumatic connector

49 hook

50 power mains

51 pneumatic connector 52 inlet port

53 outlet port

54 air duct

55 check valve

56 air duct 57 valve

58 power line

59 support