GB2182904A - A positioning mechanism - Google Patents

Abstract

A positioning device comprises a pair of devices (50, 52) having extendible and retractable elements which have well-defined stroke limits and are collinear with each other. One device (50) is fixed and the other is mounted on a slideway (54) and carries a gate which can be displaced selectively into anyone of three well-defined positions by appropriate operation of devices (50, 52) to obtain different combinations of extension and/or retraction of the elements. The mechanism is used for diverting articles such as pellets from one feed channel (14) into a selected one of a number of feed channels (24, 26, 28) in accordance with the results of an inspection carried out as the articles progress along the channel (14). <IMAGE>

Description

SPECIFICATION Positioning mechanism This invention relates two a positioning mechanism having digital characteristics, the device being capable of effecting positioning of a component in any oneofatleastthreewell-defined positions.

According to the present invention there is provided a positioning mechanism comprising a number of devices having elements which are each extendible and retractible between well-defined stroke limits relative to the respective device and are substantiallycollinearwith each other, one ofthe devices being fixed and another of the devices carrying a component to be positioned atwell-defined discrete positions along a line parallel to the elements, and means for selectively operating the devices to obtain different combinations of extension and/or retraction ofthe elements, each combination corresponding to a different well-defined position of said component.

One application ofthe invention is in apparatus for conveying articles, such as pellets, where there is a requirementforthe articles to be diverted into differentfeed lines.

One form of apparatus embodying a positioning mechanism according to the invention will now be described by way of example onlywith reference to the accompanying drawings, in which: Figure ? is a schematic plan view of a pelletstacking apparatus for use in circumstances where all of the pellets are of the same type; Figure2 is an enlarged view of part ofthe appar atusofFigure 1; Figure 3 is a plan view showing one form of pellet divertermechanismapplicabletotheembodiment of Figure 1 and also to the other embodiment described herein; Figure 4 is a front elevation ofthe diverter mechanism; Figure 5 is an end elevation of the diverter mechanism; Figure 6is a viewsimilarto Figure 1 but showing an alternative embodiment;; Figure 7isa planview,with parts removed, of a selector mechanism; Figure 8 is a sectionalviewtaken in the direction 8-8 in Figure7; Figure9isasectionalviewtaken inthedirection 9-9 in Figure8; and Figure 10 is a sectional view taken in the direction 10-10 in Figure 8.

Referring to Figure 1, pellets are off-loaded from trays onto a moving belt 10 by an off-loader mechanism generally depicted by reference numeral 12.

The belt 10travels inthedirection of arrow A and, initial Iy, the pel lets travel in spaced relation within the confines of a guide lane 14 and with their axes parallel to the direction of belt movement. The pellets move through a measurement station 16 in which the lengths are measured preferably by a noncontacting method and while the pellets are in motion. The measuring device may be a laser scanning arrangement such asthat manufactured underthe brand name "Laser Mike" byTechmat Company of 6060 Executive Boulevard, Dayton, Ohio, 45424, USA.The individual measurements are fed to a microprocessor-based control unit 18which is preprogrammed with data relating to the required overall length of the stack and also the target length of the stack at one or more intermediate points during its assembly.

Depending upon the result ofthe measurement, each pellet is channelled, underthe control of control unit 18, by a diverter mechanism 22 into one of three lanes 24, 26, 28, herein called the oversize, intermediate-size and undersize lanes respectively. The criteriaforsegregation of the pellets intotheselanes will in general be governed bythefactthatthestack will be assembled primarily from intermediate-size pellets and the oversize and undersize pellets are principally intended to be used for corrective action in the event that the partially completed stack drifts away from the target length.Thus, the classification ofthe pellets by the control unit 18 is governed bythe need to maintain an adequate supply of intermediate size pellets and the control unit is therefore programmed to update the size ranges corresponding to undersize, oversize and intermediate-size to meet this need. For example, the control unit 18 may be arranged to ensure that say 50% of pellets are channelledtothe intermediate-size lane 26 andthe remaining 50% are shared between the oversize and undersize lanes 24,28.For this purpose, the control unit 18may be arranged to keep an inventory ofthe pellet distribution between the lanes 24, 26,28, the size criteria being automatically adjusted when necessaryto ensure an adequate supply of pellets in each lane. Alternatively, as illustrated in Figure 2, instead of an inventory being kept by the control unit 18, the lanes 24, 26, 28 may be provided with sensor means, eg. photoelectric detectors 30, for providing signalsto unit 18 indicating the extentto which each lane is occupied by pellets enabling control unit 18 to divert more or less pellets to each lane automatically by adjusting target lengths.

The lanes 24, 26, 28 direct the respective pellets to a selector mechanism 32 which is controlled by the unit 18 and serves to hold up pellets in the lanes 24, 26,28 and release them selectively into a single lane 34 constituting the stacking zone of the apparatus.

The pellets thus released collect end-to-end in a stack which can be constrained against movementwith the belt 10 at various points during formation ofthe stack by a series of stops 36-39 which are extendable into and retractable from the stacking lane 34 under the control of unit 18. Associated with the stacking lane 34, there is a stack-length measuring system 40 for detecting the position of the trailing end ofthe growing stack when the latter makes successive contact with each of the stops 36,37,38 and 39. The system 40 may, like device 16, be a laser-scanning device having a well-defined scanning field upon which the trailing end ofthe growing stack eventually encroaches afterthe stack initially contacts stop 36 and subsequently, in succession, stops 37,38 and 39.

The microprocessor-based control unit 18 is pro grammedtoco-ordinatethe positionsofthestops 36-39 with detection of the trailing end of the stack within the field of view of the scanning system 40 and thereby derive length measurements for the partly-completed stack at each ofthe stop positions 36-39. In this way, length measurements forthe stack are obtained at different points in its assembly, eg.

halfway (stop 36), three-quarters completed (stop 37),fourpelletsfrom completion (stop 38) and one pellet short of being fully completed (stop 39). Each ofthese measurements is compared by the unit 18 with corresponding target values so that any drift away from the target length can be detected atvarious points during assembly of the stack. If any deviation occurs at stops 36-38, the control unit 18com- putes the combination of oversize or undersize and/or intermediate size pellets to be supplied to the stackto correct for such drift as the stack continues to grow towards the size at which the next length measurement is taken.The required combination of pellets is released into the stacking lane 34 by appropriate operation ofthe selector mechanism 32 underthecontrol of unit 18.

Afterthe stack has advanced to the stop 39 and any corrective action has been taken in selecting the appropriate pellet size for completion ofthe stack, pellet supply into lane 34 is terminated, stop 39 is retracted and the completed stack is transferred to a tray in loading mechanism 42. The completed stack length can be subsequently checked and if it lies outsidethe acceptable tolerance range the stack may be rejected and the pellets recycled or it may be transferred to a reject tray for corrective action by manual means, using a glove box if the nature ofthefuel material demands this.Aftertransfer ofthe completed stack from lane 34, stop 36 is extended into lane 34and pellet supply via the selector mechanism is resumed to build-up afresh stack in the same manneras described above.

One embodiment of the divertermechanism 22 is illustrated in Figure 3,4 and 5to which reference is now made. As mentioned previously, the diverter mechanism 22 is operable to divert pellets in lane 14 (defined by rails 26) to one ofthe lanes 24,26 or28 depending on the result ofthe measurement made at the device 16. In this embodiment, the diverter mechanism is constructed as a three-position actuator having three well-defined settings in which the guide section 47 of its diverting gate 48 is in registry with lanes 24,26,28 respectively. The mechanism comprises a pair of elements 50,52 having a well-defined stroke length, one (50) of which is fixed and the other (52) of which is sl idable transversely of direction A along a slide 54 and carries the diverting gate 48.The two elements 50,52 are coupled together in such a way that when both are at their minim um stroke set- tings, the guide section 47 is in registry with the lane 24; when one is at its full stroke setting and the other at its minimum, the section 47 is in registry with the lane 26; and when both are at the full stroke settings, the section 47 is in registrywith the lane 28. The el ements50,52 may for example com prise fl uid pow- ered piston and cylinder devices arranged with their piston rods coupled together or they may comprise solenoids arranged with their armatures coupled together.

The diverter mechanism 22 operates under the control of unit 18, the arrangement being such that the guide section 47 defined by rails 56 is normally in registry with lane 14. When a pellet passes through the measuring device 16 it is allowed to continue into the guide section 47 until the pellet is enti rely within the section 47 (as registered for example by a suit able sensor 58 connected to the control unit 18) whereupon the gate 48 is shifted (if necessary) to align the section 47 with the appropriate lane 24,26, 28 so that the pellet can continue without interrup- tion through the section 47 and into the selected lane.After the pellet has cleared the section 47 (as registered by sensors 60 connected to the control unit 18),the gate 48 is restored (if necessary) into alignment with the lane 14 in preparation for receiving the next pellet.

The foregoing description relates to the fabrication of pellet stack in circumstances where all of the pel lets are ofthe same type. However, in some circum- stances, it may be necessary to assemble a stack from two or more different types of pellet in such a way that certain types of pellet occupy specific positions along the length ofthe stack. Thus, fuel for a known form of gas cooled graphite moderated nuclear reactor comprises plain pellets and pellets formed with a circumferential groove into which the sheath of the fuel pin is subsequently deformed to lock the pellet stack axiallyto the pin.To enable corrective action to be taken to ensure the stack length is within an acceptabletolerance rangeaboutthe target length, the practice has been to manufacture the plain pellets in two sizes, long and short, so that stack adjustment can be effected by substituting long for short pellets or vice versa.

An embodiment of the invention for use in handling pellets in such circumstances is illustrated in Figure 6. This embodiment is similarto that of Figures 1 and 2 in many respects and like reference numerals aretherefore used to indicate likecomponents and structures. The control unit 18 and its connections to various parts ofthe apparatus have been omitted in Figure 6 forth sake of clarity. The short and long plain pellets are supplied to apparatus via off-loading mechanisms 70,72 respectively which feed pellets from trays, such as that indicated by reference 74, onto the belt 10 downstream ofthe diverter mechanism 22 via curved guides 76,78 which lead into guide lanes 80,82 running alongside lanes 24,26,28 to the selector mechanism 32. The long and short plain pellets are not measured by measuring device 16 since they have already been classified into specific size ranges. The grooved pellets are fed by the off-loading mechanism 12 and followthe route previously described in relation to Figures 1 and 2 and consequently lanes 24,26 and 28 serve to accumulate oversize, undersize and intermediate size grooved pellets, the classification into these size ranges being effected in the manner described previously so as to ensure that an adequate supply ofthe different grooved pellet sizes is always available.

The stack of pellets formed in stacking lane is assembled by operation of the selector mechanism 34underthecontrol of unit 18 in accordance with a preprogrammed sequence of long, short and grooved pellets so that the grooved pellets will occupy the desired positions along the length ofthe fuel pin after insertion ofthe stack into the sheath.

The unit 18 may store a range of different pellet sequences since the sequence may be required to differ from one pin to another and, in this event, the required sequence may be preselected by entry of command data via an operator console ofthe unit 18.

As the stack grows in lane 34, its length is periodically checked as before by means of the stops 36-39 and the measuring system 40 to detect any drift away from the target length. In this embodiment, the necessary corrective action can be taken by selecting a suitable combination of pellet sizes from the lanes 24,26,28,80, 82 taking into account also the need to secure the correct number and positioning ofthe grooved pellets a long the stack. In practice,thecor- rective action may only involve the use ofthe long and short plain pelletsfollowingthefirstand second measurements at stops 36 and 37 and the different grooved pellet sizes may be employed either alone or in combination with long and short plain pellets afterthe measurement at stop 38 has been made.

Oneform of selector mechanism for use in the apparatus of Figure 6 (or in Figure 1 with modification involving a three lane input instead of five lanes) is shown in Figures 7,8,9 and 10towhich reference is now made. As shown, the mechanism comprises a base 90 on which a housing structure 92 is pivoted by pivots 94 so thatthe housing 92 can be pivoted by means of knob 86 between the in-use position shown and a raised position for maintenance purposes. Beneath the housing 92, the base 90 is divided by a set of parallel rails 98 into lanes which, atthe right hand side, register with and form continuations of the lanes 24,26,28,80,82 (not shown).To the left ofthe housing 92 the lanes are merged by guides 100,102 into a single lane 104 which registerswitt the stacking lane 34. For each lane defined byrails98,the housing 92 mounts a pellet gate 106 and at least one pellet stop 108, the pellet gates 106 being downstream ofthe pellet stops 108 in the direction A of pellet travel.

As shown in Figure 10, each pellet gate 106 com prises a bifurcated vertically displaceable gate plate 110whose legs project through openings 112 in a support 1 14ofthe housing 92 and slide in grooves 115 in the rails 98 which, in turn, are secured to the underside of the support 114. The plates 110 are each movable underthe control of unit 18 by an actuator 118, such as a pneumatic piston and cylinder, between the lowered position shown in which it impedes pellet motion along the respective lane (see pel let 120 in Figure 10) and a raised position in which it is clearofthe pellet and allows passage ofthe pellets into the converging section 122.The pellet stops 108 are constructed and operate in a similarfashion to the pellet gates 106 except that the bifurcated stop plates 124 have legs shaped to embrace the pellets so that, when lowered, the stop plates 124 do notim- pede pellet motion but, when raised,theyliftthepel- lets upwardly and into contact with the underside of the support 114. In this way, raising of a pellet againstthe support 114 halts its movement and, because the distance it is raised is only a fraction ofthe pellet diameter, all of the pellets following the raised pellet are also blocked.

In use, the pellet gates 106 are used to accumulate pellets in the lanes 24,26,28,80 and 82 and release them underthe control ofthe unit 18. The pellet stops 108 are used underthecontrol of unit 18to determine the number of pellets released each time the associated pellet gate 106 is opened. Thus, for example, where a lane has two pellet stops 108, the downstream stop 108 may be used to stop the first pellet following the leading pellet so that only the latter is released when the pellet gate 106 is opened.

The upstream stop may be used to stop the second pellet following the leading pellet so that ifdesired the leading and next following pellet may be re leased by appropriate operation ofthe pellet gate 106 and the downstream pellet stop 108whilethe remaining pellets in that lane are held up by the upstream stop. Afterthe leading two pellets have been released,the gate 106 may be closed and the upstream stop 108 operated to allow the remaining pellets to move uptothe gate 106 in readinessforthe next pellet-releasing operation which may involve the release of one ortwo pellets.

Although the invention has been described herein with particular reference to the formation of stacks of nuclear pellets where the acceptability of the stack for subsequent loading into a fuel pin sheath is based on a length criterion, it will be understood that a weight criterion may be employed alternatively in which case the references to measuring length should be read as referring to weight. The invention also has application to articles otherthan nuclear fuel pellets.

Claims (10)

1. A positioning mechanism comprising a number of devices having elements which are each extendible and retractible between well-defined stroke limits relative to the respective device and are substantially collinearwith each other, one of the devices being fixed and another ofthe devices carrying a component to be positioned atwell-defined discrete positions along a line parallel to the elements, and means for selectively operating the devices to obtain different combinations of extension and/or retraction ofthe elements, each combination corresponding to a differentwell-defined position of said component.

2. A mechanism as claimed in Claim 1 in which the stroke lengths of said elements are substantially the same.

3. A mechanism as claimed in Claims 1 or 2 in which each device comprises a fluid-powered piston and cylinder.

4. A mechanism as claimed in Claim 1 or 2 in which each device is electromagnetically-operable.

5. A mechanism as claimed in Claim 1,2, or 4 in which there are two of said devices coupled together via said elements.

6. A mechanism as claimed in any one of Claims 1-5 in which the or each other device is mounted on a slide for movement relative to the fixed device.

7. A mechanism as claimed in any one of Claims 1-6 including a number of article feed lines extending transversely of the axis of collinearity and spaced apartfrom one another in the direction of said axis, the arrangement being such thatsaid component is aligned with a respective feed line when in each of said well-defined positions.

8. A mechanism as claimed in Claim 7 in which there is atleastone articlefeed line on one side of said axis and at leasttwo article feed lines on the other side.

9. A mechanism as claimed in Claim 8 including means for inspecting a predetermined characteristic of the articles being fed from said one side, the selec tively operating means being responsive to the in spection means to effecttransfer,via said compo nent, of an article from said one side into alignment with a selected feed line on the opposite side.

10. A positioning device substantially as here- inbefore described with reference to, and as shown in, Figures 3 to 5 of the accompanying drawings.