TECHNICAL FIELD This invention relates to gap scanning. In particular it is concerned with the scanning of a gap typically but not exclusively, one of variable size where the occurrence of an intrusion requires a modification of an operation involving the gap.

An example of a gap scanning requirement arises in connection with a sliding door of a lift. For safe operation as the door, and so the gap closes, the gap is scanned to detect whether something is intruding into the gap. In the event scanning reveals the existence of an intrusion into the gap then operation of the door modified to ensure that the intruding object is not trapped by the closing of the gap. Typically the scanning can be carried out by way of an array of emitters on one side of the gap. Each emitter in turn periodically sends out an electromagnetic beam (such as an infra red beam) and a complementary array of receivers on the other side of the gap detects the arrival of the emitted beams. The arrays can be mounted either a fixed distance apart ('a fixed gap') to look across a gap progressively closed by a door or doors. Alternatively one or other of the arrays can be mounted on the leading edge of a movable door so that arrays are effectively located at or near the lateral boundaries of the gap (a 'variable gap') as it opens or closes.

Another example of gap scanning arises in relation to the safe operation of a machine tool such as a sheet metal press having a working area which is protected by one or more machine guards. Provision of access to the working area requires one or more of the guards to be opened so as to leave a gap. Access may be required for a number of reasons typically to enable material or tooling to be fed to or removed from the working area or for cleaning or for an adjustment to be made. For as long as the gap is being scanned and an intrusion in the gap is detected such as the arm of an operator then operation of the press is inhibited until the intrusion is removed. In what follows the term 'lift door' is used to cover installation having: a single door; or an installation involving a door in two or more sections which is operable to move between an open position when access is made available to and from the interior of the lift and a closed position when such access is barred by the extended door sections; or an installation involving two door members (one or both of which can be sectioned) which can be drawn apart and closed together. Access is by way of a gap defined between an edge of the door (whether of one or more sections) and a part of the lift system structure or between the edges of two oppositely opposed door members.

BACKGROUND ART A known problem with self regulated lift operation is the need for a potential user to be clear as to the consequences of a particular action arising in the course of lift operation. For as long as a lift door is open the ability to pass into and out of the lift car is readily apparent. Once a lift door starts to close an inexperienced, un¬ coordinated or diffident user may becomes unsure whether it is safe to pass through the closing gap or whether, for example, they should operate a control typically, if outside the lift, a call button or, if inside the lift, a 'do or open' button or finally whether to just stand back and wait for another lift. Given that a user does choose to enter when the gap is closing and does not clear the gap then, for safe lift operation, it is known to provide sensing means whereby the closure of a lift door can be stopped or reversed in the event that somebody or something is still within the gap when the door is closing. In an existing system it is known to provide a leading edge of a lift door with a projecting strip or pad which is displaced, on contacting something in the path of the closing door, resulting in the generation of a signal providing for the closing motion of the door to be stopped and then reversed. Any point on the length of the strip or pad will respond to contact.

In another known system a lift door is equipped with a series of electromagnetic wave (typically infra red) beam emitters with a corresponding collection of receivers for the emitted beams on the opposite side of a gap with the door open with each receiver being directed towards an emitter. A variant on this system include one where emitters (and receivers) are provided on both sides of the gap. In either case an invisible network of emitted beams is caused to extend across the entry gap to the lift. As the door closes the entry of an intruding object into the closing gap results in a blanking of beam transmission between at least one emitter/receiver pair. Such blanking results in the generation of a control signal providing for the closing motion of the door to be stopped and reversed. In this case it will be apparent that an intrusion sensitive area represented by an emitted beam is spaced apart from its neighbour in the emitter series. In this way the gap in the path of a closing door has extending across it a sequence of spaced beams sensitive to an intruding object interposed with a sequence of insensitive regions between the beams. If an article intrudes into an insensitive region then a door control signal will not be generated. Consequently a sufficient number of emitter/receiver pairs need to be provided to ensure that the probability of any credible intrusion cutting at least one beam is high.

DISCLOSURE OF INVENTION According to a first aspect of the present invention there is provided a gap scanning system for a variable sized gap bounded on a first side by a first member and on the other side to the first side by a second member movable relative to the first member wherein a plurality of emitters of a beams of electromagnetic radiation, such as of infra red, are directed from one side of the gap of variable size to the other side to the one side where a beam from one emitter in the plurality can be received by a complementary receiver in a plurality thereof; characterised in that the number of emitters in the plurality thereof differs substantially from the number of receivers in the plurality thereof and the plurality containing the greater number of emitters or receivers is located on the first side of the gap. According to a first preferred version of the first aspect of the present invention the first member is movable relative to the second member.

According to a second preferred version of the first aspect of the present invention or of the first preferred version thereof the number of emitters in the plurality thereof is greater than the number of receivers in the plurality thereof.

According to a third preferred version of the first aspect of the present invention or of the first preferred version thereof the number of emitters in the plurality thereof is less than the number of receivers in the plurality thereof.

According to a second aspect of the present invention there is provided a lift door system incorporating a gap scanning system according to the first aspect or any preferred version thereof.

According to a first preferred version of the second aspect of the present invention the lift door system comprises lift (34) having a lift door (32) with a leading edge (31) serving as a first vertical boundary of a gap (33) providing access into the lift (34); a frame edge (35) of the lift installation serving as a second vertical boundary to the gap 33; the leading edge (31) housing a series (36) of M infra red receivers (typically receiver 37); the frame edge (35) housing a complementary series (38) of M infra red transmitters (typically transmitter 39) characterised in that in that M > N or M < N.

According to a second preferred version of the first preferred version of the second aspect of the present invention the lift door system is further characterised by a control unit (40) linked to the series (36) of receivers by a data link (42) and to the series (38) of transmitters by a data link (41) so as to provide for controlled opening and closing of the lift door (32) in response to data received and transmitted along links 41, 42 in accordance with predetermined programs. Typically the system is further characterised by a control unit (40) linked by cable (43) to an operating system (44) for opening and closing the door (32) and by link (45) to a drive system (46) operating a hoist for the lift (34).

BRIEF DESCRIPTION OF DRAWINGS A known type of lift installation together with an exemplary embodiment of the present invention will now be described with reference to the accompanying drawings of part of a lift installation of which each drawing is a diagrammatic view of a lift entry with a lift door.

For purposes of comparison with the present invention Figure 1 shows a known type of lift installation where, seen from the outside, leading edge 11 of a lift door 12 acts a right-hand vertical boundary of a gap 13 providing access into a lift car 14. Frame edge 15 of the lift installation serves as a left hand vertical boundary of the gap 13. In this case leading edge 11 of the door 12 houses a series 16 of twenty four infra red receivers (typically receiver 17). Frame edge 15 houses a complementary series 18 of twenty four infra red transmitters (typically receiver 19).

A control unit 20 is linked to the series 18 by data link 21 and to series 16 by data link 22 so as to provide for controlled opening and closing of the lift door 12 in response to data received and transmitted along links 21, 22 involving data relating to the size of a gap governed by way of the lift door in accordance with predetermined programs. The control unit 20 is also linked by cable 23 to an operating system 24 for opening and closing the door 12 and by link 25 to a drive system 26 operating a hoist for the lift car 14.

MODE FOR CARRYING OUT THE INVENTION Figure 2 shows a lift installation in accordance with the present invention with some similarities to the installation described in connection with Figure 1 Leading edge 31 of a lift door 32 acts a right hand vertical boundary of a gap 33 providing access into lift car 34. The lift door 32 is shown already extending part way into gap 33. Frame edge 35 of the lift installation serves as a left hand vertical boundary of the gap 33 and in this case leading edge 31 of the door frame 103 houses a series 36 of N (in this case N = 13) infra red receivers (typically receiver 37). The gap 33 has as a left hand frame edge 35 which houses a complementary series 38 of M (in this case M = 24) infra red transmitters (typically transmitter 39). The installation makes use of a series 36 of N receivers having only thirteen members in contrast to the series 18 of twenty four transmitters of Figure 1.

A control unit 40 is linked to the series 36 by data link 42 and to series 38 by data link 41 so as to provide for controlled opening and closing of the lift door 32 in response to data received and transmitted along links 41, 42 in accordance with predetermined programs. The control unit 40 is also linked by cable 43 to an operating system 44 for opening and closing the door 32 and by link 45 to a drive system 46 operating a hoist for the lift car 34.

INDUSTRIAL APPLICABILITY A number of gap scanning benefits have been found to arise from the present invention by making use of a series of receivers which are less in number than the emitters making up the corresponding series of transmitters. As the door 32 closes in the gap 33 the density of the infra red beams does not differ to a significant extent from that arising from the transmitter /receiver configuration shown in Figure 1 so that despite the reduced number of receivers there is no significant reduction in the sensitivity to intrusions in the gap especially as the gap is half closed or more. A significant difference between the beam patterns of Figure 1 and Figure 2 lies in the fact that the beam pattern of Figure 1 is essentially symmetrical whereas that of Figure 2 is asymmetric. This applies whether the boundaries of the gap are defined by one moving and one fixed edge or by two moving edges. If one assumes that two beams are projected from each transmitter in series 38 (one to each of adjacent receivers in series 36) then it can be said that there is a pattern of beams in the gap 33 made up, in total, of forty eight beams. This compares favourably with the result of applying a similar consideration to the system shown in Figure 1 with its equal numbers of transmitters and receivers. In the case of Figure 1 assume again two beams are projected from each transmitter in series 18 (one to each of adjacent receivers in series 16) then it can be said that there is a pattern of beams made up, in total, of forty seven beams. Thus there is a slight increase in the number of beams in the pattern of Figure 2 in comparison with that of Figure 1 despite a nearly 50% reduction in the number of sensors in series 39 (Figure 2) by comparison with those in series 19 (Figure 1).