GB2232283A - Object movement control - Google Patents

Abstract

An object transfer system comprises a path defined by a conveyor, along which one or more pallets 2 or other objects are movable. Each pallet 2 includes code retaining means comprising a plurality of holes 34A capable of supporting discrete code elements 34 to form a code readable by sensing means PC1, PC2 and PC3. When the code carried by the pallet is read, an interface generates a control signal to stop the pallet and also generates a report signal to update a central control system. This ensures precise control of the pallet's movement. <IMAGE>

Description

OBJECT MOVEMENT CONTROL The present invention relates to methods of and apparatus for controlling the movement of an object along a path.

During the movement of an object along a path, it may be necessary to stop the object at a number of work stations along the path and then, after work has been performed at a particular work station, to allow movement of the object to recommence so that the object can move to the next work station. Normally, nowadays, the movement of an object along a path is under computerized control e.g. by a supervisory system such as a Program Logic Control (PLC) which is programmed to send a stop signal such that the movement of the object will be stopped at the particular work station. However, due to a number of factors, e.g. high speed of movement, the object can override the computerized control system such that a stop signal is not sent or the stop mechanism fails whereby the object misses one or more work stations.

For example in conventional systems, e.g. those using disc-activated proximity switches, it can take as long as 0.5 seconds for the computerized control system to respond and to initiate a stop signal for feeding to the stop mechanism.

In the absence of a stop signal, the computerized control system has no way of knowing that the object has not stopped at a given work station, and therefore that the work assigned to that particular work station has not been carried out. This can have extremely disadvantageous consequences in some cases. In one such case, the object is passive, for example a pallet carrying a work piece for forming into an automobile component. The pallet is moved by a conveyor along the assembly line path where robots are disposed at the work stations, so that each robot can perform a specific operation on the work piece when the work piece reaches each respective work station.

However, should the pallet fail to stop at any particular work station, due to failure of the computerized control system to send a stop signal or due to failure of the stop mechanism itself, then the specified operation will not be performed and any remaining operations performed at the succeeding work stations will be out of sequence. As a result, expensive machine tools may be damaged with consequential costly replacement, repair and down-time and, also, one or more work pieces may be rendered useless and therefore may have to be scrapped, resulting in further expense.

In another case, the object is self-propelled and may have an inboard computer control system. An example is a cleaning robot which is guided along a path such as a metallic strip in the floor, and is stopped at a succession of work stations to perform cleaning operations such as emptying of waste bins. In this case, failure of the computerized control system or inboard computer to send a stop signal will result in a cleaning operation not being carried out at the correct position, the disadvantages of which are obvious.

In a further case, the self-propelled object may be a robot in a nuclear plant, in a radioactive zone, or in a contamination-free environment.

In a still further case the object may move along a path of, for example, sinuous or grid-like form to retrieve objects from, or to place objects into, store areas or compartments in a warehouse.

In yet another case the movable object may be a passenger-carrying car in an amusement park or theme park and the computerized control system may be programmed to initiate work in the form of certain amusements or effects at the work stations along the path by sending an appropriate signal instead of or in addition to sending a stop signal.

Applicants have experimented with bar codes to identify passive objects such as pallets. However, bar code readers can only read and cannot write. Thus, the bar code reader cannot initiate any further action.

Moreover, the computerized control system cannot cause the bar code reader to produce any initiating signal or stop signal. In other words, the only function that a bar code reader can perform is to inform the control system of the identity of a moving pallet by reading the associated bar code as the pallet approaches a particular work station. Thus, although the bar code may still be read and a particular pallet identified even if the pallet overrides the computerized control system, say by high speed of movement, the bar code reader cannot directly produce a stop signal because it can only read. The pallet may therefore move on to the next station and damage may occur before an operator can intervene.Even when a signal does go back to the computerized control system from the bar code reader, the computer may not be able to generate a stop signal quickly enough to prevent the object overriding the stop mechanism. Other disadvantages of bar code reader systems are that they are expensive and unreliable since the bar codes can easily be damaged, obscured or otherwise interfered with by the dirt, oil, grease, metal swarf and so on which may abound in a factory or other industrial environment.

Applicants have also tried colour codes, which are cheaper than bar codes, but have found that these do not work reliably, particularly in the factory environment. For example, colour codes are also susceptible to being obscured by dirt.

Thus, it has long been Applicants' object to provide a cost effective, fast acting (of the order of nano-seconds), reliable control method and apparatus e.g. for passive pallet coding, which is not only suitable for the harsh environment of a factory assembly line, and can easily interface with existing computerized control systems such as PLC's, but is sufficiently flexible to be used for other cases, such as outlined hereinabove, and has repeatability and variability.

Having applied considerable thought, time and effort, and having carried out numerous experiments, Applicants believe that they have discovered a principle which is that a computerized control system can be bypassed with a device arranged to generate a stop signal or other signal with great speed and precision, to report to the computer that the stop or other signal has'been generated, and also, optionally, to provide the computer with object code data.

In order to carry this principle into effect and in accordance with one aspect of the invention, the movement of an object is controlled by an interface for a computerized control system, which interface is read/write responsive to an object code.

By means of the invention not only can the interface directly produce a stop or other initiating signal without involving the computerized control system, but also the or each object can be uniquely coded which means that each object can be identified from other objects such as pallets if there are other objects whose movement is being controlled along the path, because in such a case other objects would have different codes.

The code can be read by an appropriate sensing means which forms part of the interface and which is disposed along the path. The sensing means enables the interface directly to produce a signal for stopping the object or for initiating particular operations or effects at a work station.

In accordance with another aspect of the invention, an object for movement along a path is provided with a plurality of discrete metallic elements for co-operation with sensing means to initiate interfacing read/write operations for bypassing a computerized control system.

This aspect of the invention provides an object code which can work in all normal industrial, manufacturing and service environments, no matter how dirty or highly contaminated, without being affected by the environment, and'which is shock resistant, hose proof and weather proof and can withstand ultrasonic cleaning.

Moreover, by the use of at least two discrete metallic elements, which are suitably insulated from each other, commercially available sensing means may be used. The sensing means is conveniently of the inductive type, such as inductive proximity switches, and provides signals to a read/write interface for processing in a manner to be described.

The number of metallic elements will be dependent upon the number of object code combinations required, and will be restricted by the amount of available, useful space on the object. For example, provision for up to two metallic code elements will provide four unique code combinations, whereas provision for twelve metallic code elements will provide up to 4,096 unique code combinations. In normal practice 200 unique code combinations will be adequate. However, it should be appreciated that the number of unique code combinations is infinite and depends only upon the size of the object and the size and spacing of the code elements.

Advantageously, the code elements are disposed in an array, preferably a linear array, with there being at least one line or sub-array of elements for data sensing means for coding and one line or sub-array of elements for clock sensing means for control. Each clock and data line is conveniently provided with its own sensor.

The metallic elements may be of any suitable form, e.g. of plate-like form such as discs or of elongate form such as pins or studs which are fixed to the object in any suitable manner. Preferably, where the object is wholly or partially of plastics or other suitable magnetic field isolating material, the metallic elements are fitted in recesses or holes formed in the plastics or other isolating material.

The holes are preferably drilled slightly undersize so that the elements are a resilient push fit therein.

The metallic elements are preferably flush fitted with the surface of the object, which surface is preferably flat, whereby the movement of the object over the sensing means can be arranged such that the exposed face of each element is located at the same or substantially the same distance from the sensing means it passes over and with which it co-operates.

In the case of a passive object such as a pallet used for the assembly of a work piece such as an automobile component, the metallic elements may be carried by a flat under surface which permits co-operation with sensing means carried by a support on a conveyor on which the pallets are moved.

To ensure that the metallic elements are correctly located with respect to the sensing means when the object is moving, in order to guard against the object overriding a work station by not activating the sensing means, compensating means is advantageously provided. The compensating means may be operative to prevent tilting of the object in one direction or another, which could otherwise interfere with the operation of the sensing means.

Since inductive sensors are preferred, which act as inductive proximity switches, the metallic elements are preferably of a ferrous material, such as stainless or mild steel. The latter is preferred, because it is more effective. The ferrous elements are suitably carried by an isolating material which is non-ferrous, e.g. of plastics material such as polyethylene. Alternatively, the metallic elements may be of brass, copper or aluminium.

The provision of a data line and a clock line has the advantage of being fail-safe since, if a data line sensor is not working, a stop signal will still be produced by the interface and, moreover, an error signal will be sent to the control system computer which will have the effect of preventing release of the stop mechanism. In one embodiment suitable for passive objects such as pallets used for the assembly of a work piece such as an automobile component, the metallic elements may be arranged in several lines, e.g. one or two data lines and one clock line with the clock line conveniently being located alongside one or between two data lines.

Thus, with two types of sensors, i.e. a data line sensor and clock line sensor forming part of the interface or read/write circuit, when all of the metallic elements have passed the sensors, and data from the data line elements has been stored in data latches of the interface, a stop signal is directly sent to the stop mechanism and/or any other command signal may be sent. A signal is fed to the control system computer to tell it that the data is ready, whereupon the control system computer requests information from (sends a writing request to) the interface circuit and, on receiving an appropriate data code from the interface, e.g. a code confirming the object's identity, the control system computer releases the object stop, and the interface circuit is ready to receive data from the next object.

The invention therefore provides a high speed acting (of the order of 500 nano-seconds) computer bypass which can read the metallic elements via the sensors, store information in data latches, initiate a direct stop or other signal, report data ready to the computer, and write an object code after which the computer can act on the data received to release, or otherwise deal with, the object as the case may be.

In order that the invention may be more readily understood, an embodiment thereof will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic plan view of an assembly line for automobile components, and incorporating a conveyor, under the control of a computerized control system; Figure 2 is a plan view of a passive object in the form of a pallet for carrying workpieces along the assembly line of Figure 1; Figure 3 is an end view of the pallet of Figure 2, viewed in the direction of the arrow C; Figure 4 is a plan view of a part of the conveyor and showing sensing means; Figure 5 is a section taken along the line V-V of Figure 4; and Figure 6 is a circuit diagram of an interface for the computerized control system.

Referring to Figure 1, there is shown an assembly line path 1 for manufacturing automobile components, and along which a succession of passive objects in the form of pallets such as 2 are moved by a conveyor 3 driven continuously at high speed by electric motors 11, 12, 13 and 14. The pallet 2 is moved by the conveyor 3 along the path 1, in the directions indicated by the illustrated arrows, and under the control of a computerised control system, constituted by a PLC which is diagrammatically illustrated at 4.

Work is carried out on the workpiece at a number of workstations 5,6 and 7,8. The workstation 9 is a rework station for reworking a workpiece in the event that a particular operation has not been carried out at a previous work station.

At the work stations 5,6 and 7,8 as well as the rework station 9 if needs be, the pallet 2 is stopped by stop mechanisms A,B and G,H and E respectively.activated by stop signals in a manner to be described. Work is then carried out by respective robots whose moving arms act within respective robot areas 10 and 15 or the rework robot area 16, as the case may be. Once a particular operation has been completed at a particular work station, the associated stop mechanism receives a release signal in a manner to be described and the pallet 2 can then be moved on to the next work station to be stopped for another operation.

Additional stop mechanisms C and D are associated with the rework robot area 16, F is associated with the work robot area 15 and I is associated with the work robot area 10. The stop mechanisms D, F and I work in co-operation with diverter mechanisms D1, F1 and I1 respectively to enable a pallet to be diverted into the respective robot areas and stop mechanism C acts as a check and works in co-operation with stop mechanism D and diverter mechanism D1 in the event that a rework operation is necessary.

In order to ensure that pallet 2 is stopped by each stop mechanism in turn, e.g. that the speed of the pallet does not avoid the PLC 4, and thus that the consequential aforementioned disadvantages are avoided, a plurality of interfaces 20 to 28 are provided. These interfaces are operable to bypass the PLC 4 and send a stop signal to the associated stop mechanism in a far shorter time than it would take for the PLC 4 to send a stop signal as is the case in conventional systems.

The interfaces 20 to 28 are so-called because they interface with the PLC 4, i.e. they not only initiate a stop signal but also report to the PLC 4 that the stop signal has been sent and that pallet code data is ready for the computer to receive. Thus, each interface 20 to 28 is read/write responsive to an object code carried by the pallet 2, with each object code being unique to the particular pallet.

In order to understand the nature of the object code of the preferred embodiment of this invention, reference will now be made to Figures 2 and 3.

The pallet 2 comprises a generally octagonal body 30 made of moulded plastics material such as polypropylene. The undersurface of pallet 2 carries guide wheels 31 of a suitable metal such as steel which engage with the conveyor 3 of Figure 1 and project beyond the body 30 to form the leading and trailing ends 32 and 33 respectively of the pallet 2.

The object code is provided by a plurality of discrete metallic elements in the form of metal studs 34, preferably of mild steel, which are fitted in undersize holes 34A drilled in the undersurface 35 of the body 30 of the pallet 2. As will be apparent from Figure 3, the exposed surfaces 36 of the studs 34 lie flush with the undersurface 35.

The studs 34 are arranged in a linear array of three sub-array lines 36, 37 and 38, of which the lines 36 and 38 are for data sensing means for coding and the line 37 is for clock sensing means for control. The pallet 2 illustrated has sixteen holes 34A, five in each of the lines 36 and 38 and six in line 37. This particular pallet 2 has eleven studs 34 arranged as illustrated to provide its unique code, whereas other pallets will have different numbers of data studs, in different arrangements, to provide them also respectively with unique codes.

Sensing means in the form of inductive proximity switches PC1, PC2 and PC3 form part of the interfaces 20 to 28 with the switches PC1 and PC3 co-operating with the data lines 36 and 38 and the switch PC2 co-operating with the clock line 37.

Referring now to Figures 4 and 5, let us assume that these show a part of the conveyor 3 which is in the region of the interface 20 and stop mechanism A of the work station 5 in the robot area 10. It should be appreciated, however, that these Figures could show a part of the conveyor in the region of any of the other interfaces 21 and 28.

Mounted in each of the support members 37 of the conveyor 3 are carriers 38 supporting two blocks 39A and 39B One of the blocks, 39B, is situated on one side of the conveyor 3 and supports the proximity switches PC1, PC2 and PC3. The other block, 39A, is situated on the other sie of the conveyor 3 and directly opposite block 39B, thereby to resist any tendency for an object moving along the conveyor 3 to tilt away from the block 39B as it passes thereover.

Furthermore, in order to ensure that the studs 34 are sensed correctly by the switches PC1, PC2 and PC3, the blocks 39A and 39B are resiliently loaded by means of springs 40 engaging in recesses 41 in the blocks and co-operating with supports 42. Advantageously, the blocks 39A and 39B are faced with a friction-reducing layer such as PTFE to minimize friction as the object passes thereover.

As will be appreciated from Figures 4 and 5, the inductive proximity switches PC1, PC2 and PC3 are disposed beneath the path of movement of the pallets so that the studs 34 co-operate with them. As the pallet 2 passes over the proximity switches of the interface 28, for example, the interface is responsive to the code represented by the studs 34 to activate the stop mechanism I because it generates a stop signal. The interface 28 then reports to the PLC 4 that data is ready and the PLC 4 requests data from the interface 28. When data is received, the PLC 4 stores the code indicative of the pallet 2, sets the diversion route I1 to the robot area 10 and first work station 5, and releases stop I to allow the pallet 2 to move to the work station 5.When the pallet 2 gets to the work station 5, the proximity switches of another interface 20 co-operate with the studs 34 and the procedure is repeated. The PLC 4 double-checks the code for a particular stage of the work cycle of the workpiece 2a to see that it is the correct pallet, and then starts the robot. The robot reports to the PLC 4 that the work at the particular work station 5 has been completed and puts the workpiece back onto the pallet. The PLC 4 then sets up the next route and releases the stop mechanism B. Thus, the pallet moves on to the workstation 6 where it is stopped by a stop signal from the interface 21 and the process is repeated.

When the pallet reaches the interface 22, a check is made to see whether or not the work to date has been completed and, if not, the interface 23 initiates a stop signal to stop mechanism D and co-operates with the diverter D1 to divert the workpiece to the rework station 9. Otherwise, the pallet is moved on to the next work station prior to being stopped and checked by interface 25 to set up diverter F1 to divert the pallet to the workstations of robot area 15.

Each interface 20 to 28 including the inductive proximity switches PC1, PC2 and PC3 is connected by lines such as 42 (only two such line connections shown) to the PLC 4 and comprises an interface circuit such as is shown in Figure 6 to which reference will now be made.

The inductive proximity switches are shown at PC1, PC2 and PC3. The data inputs 'A' and 'B' received from data lines 36 and 38 are fed through optoisolators OP1 and OP3, and Schmitt inverters 50 and 51 to data latches IC6 and IC7 where the data is stored. From there, data passes via inverter buffers 52 and 53 to Darlington drives 54 and 55 to Data 'A' and Data 'B' outputs connected to the PLC 4.

The proximity switch PC2 receives the clock input from the clock line 37 which passes via the optoisolator OP2, two Schmitt inverters 56,57 and a clock pulse counter IC1 which pulse counts pulses to a count of six. Then, the pulse counter IC1 triggers stretcher circuit 58 which extends the data ready pulse to approximately two seconds. One output from the stretcher circuit 58 passes through an inverter buffer 59 and thence to a Darlington drive 61 to provide a data ready signal to the PLC 4. Another output from the stretcher circuit 58 passes directly to a Darlington drive 60 to provide a stop signal to the 'stop out' output.

An important feature of the interface circuit of Figure 6 is its ability to receive a strobe input from the PLC 4 through the input PC4. This input is connected through an optoisolator OP4, a Schmitt inverter 62, a gate 63 and a Schmitt inverter 64 to the data latches IC6 and IC7. Thus, the pulsed strobe output signal from the PLC 4 tells the interface to clock the stored data from the data latches IC6 and IC7 to the data inputs of the PLC 4.

Optionally, the interface circuit can be provided with additional facilities. One such facility is a component detection input PC5, which is fed through an optoisolator OP5 to logic circuitry 65 and thence to a Darlington drive 66 to provide a 'component detect' output. Another facility provides remote stop inputs 'A' and 'B' (PC6 and PC7) which are connected, respectively, to Schmitt inverters 67 and 68 and to Darlington drives 69 and 70 to provide 'remote A' and 'remote B' outputs. This facility caters for cases where the stop mechanisms are remote from the interface.

In order to monitor the inputs to the interface circuit and, in particular, to ensure that the sensors (the inductive proximity switches) are operative, the interface circuit is provided with the illustrated test lamps constituted by an array 71 of LED's.

Should there be a situation where there is a power failure for any reason, the interface is provided with a circuit 72 which ensures that the components of the interface circuit are reset to zero.

It should be appreciated that the invention is not limited to the embodiment herein described but includes all modifications or variations falling with its scope.

Claims (40)

1. An object transfer system comprising a path along which one or more objects are movable, wherein the or each object includes code retaining means situated on a code-carrying portion and capable of supporting a plurality of discrete code elements to form a code that is readable by code sensing means associated with the path.

2. An object transfer system according to claim 1, wherein the code retaining means comprises a plurality of discrete retainers disposed on the code-carrying portion.

3. An object transfer system according to claim 2, wherein each retainer is a recess or hole set into the code-carrying portion and capable of receiving a code element.

4. An object transfer system according to claim 3, wherein each code element is a resilient push fit in its associated recess or hole.

5. An object transfer system according to any preceding claim, wherein the or each code element lies substantially flush with the surface of the code-carrying portion, when supported by the retainers.

6. An object transfer system according to any preceding claim, wherein the surface of the code-carrying portion is substantially planar.

7. An object transfer system according to any preceding claim, wherein the code-carrying portion is positioned on the object such that its surface faces towards a track defining the path.

8. An object transfer system according to claim 7, wherein the code sensing means is situated on the track such that the code-carrying portion passes over the code sensing means as the object moves along the track.

9. An object transfer system according to claim 8, wherein the code sensing means is biased towards the code-carrying portion.

10. An object transfer system according to claim 8 or claim 9, wherein the code sensing means is situated on one side of the track and the track is provided with compensating means situated on the other side of the track, opposite the code sensing means.

11. An object transfer system according to any preceding claim, wherein the code retaining means is arranged such that the plurality of code elements, when supported thereby, are disposed in an array.

12. An object transfer system according to claim 11, wherein the array includes at least one linear sub-array.

13. An object transfer system according to claim 12, wherein the array includes a plurality of linear sub-arrays and the code sensing means includes a plurality of sensors each dedicated to a respective sub-array, the arrangement being such that each linear sub-array passes over the sensor dedicated thereto as the code-carrying portion passes over the code sensing means.

14. An object transfer system according to claim 12 or claim 13, wherein one linear sub-array represents a clock code and another linear sub-array represents a data code.

15. An object transfer system according to any preceding claim, wherein the code elements are pins or studs.

16. An object transfer system according to any preceding claim, wherein the code elements are of ferrous metal.

17. An object transfer system according to claim 16, wherein the code elements are of mild steel.

18. An object transfer system according to any preceding claim, wherein at- least the code-carrying portion of the object is of plastics material.

19. An object transfer system according to any preceding claim, wherein the code sensing means includes one or more inductive sensors.

20. An object transfer system according to any preceding claim, and including at least one interface which interacts with a central control system and which is arranged such that, when the code is read by the code sensing means, a control signal is produced to control the movement of the object along the path, and a report signal is produced to update the central control system.

21. An object transfer system, substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.

22. An object for use in an object transfer system constructed in accordance with any preceding claim.

23. An interface for interaction with a control system to control the movement of an object along a path, including means, responsive to a code associated with the object, to generate a control signal to control the movement of the object, and to generate a report signal to update the control system.

24. An interface according to claim 23, wherein the control signal is generated substantially immediately when the code is read.

25. An interface according to claim 23 or claim 24, wherein the control signal is a stop signal to be passed to a stopping device.

26. An interface according to any of claims 23 to 25, including a data store in which the report signal is stored when the code is read and means for generating a data ready signal to be input to the control system when the report signal is so stored.

27. An interface according to claim 26, including means responsive to a request signal from the control system to write the report signal from the data store to the control system.

28. An interface according to any of claims 23 to 27, wherein the report signal is representative of the code associated with the object.

29. An interface according to any of claims 23 to 28, including a strobe input allowing clock synchronisation with the control system.

30. An interface according to any of claims 23 to 29, and providing a path for a component detection signal.

31. An interface according to any of claims 23 to 30, and providing a path for a signal from a stop mechanism.

32. An interface according to any of claims 23 to 31, including input test means.

33. An interface according to claim 32, wherein the input test means includes an input test lamp associated with each input, which test lamp is illuminated when a signal is received at an associated input

34. An interface according to any of claims 23 to 33, including reset means for resetting the interface data components to zero in the event of power failure.

35. A method for controlling the movement of an object along a path, comprising reading a code associated with the object, generating a control signal independently of a central control system to control the movement of the object, and generating a report signal to update the central control system.

36. A method according to claim 35, wherein the control signal is generated substantially immediately when the code is read.

37. A method according to claim 35 or claim 36, wherein the control signal is a stop signal to be passed to a stopping device to stop the object.

38. A method according to any of claims 35 to 37, including storing the report signal when the code is read, and generating a data ready signal for input to the control system when the report signal is stored.

39. A method according to claim 38, including writing the report signal from store to the control system.

40. A method according to any of claims 35 to 39, wherein the report signal is representative of the code associated with the object.