The present invention relates to detection systems.

Detection systems have been used for detecting the presence of an obstruction in a lift doorway. In the most fundamental form an infrared transmitter is mounted on one side of the doorway and infrared detector is mounted on the other side of the doorway on the leading edge of the door. The optical axis of the transmitter is aligned with the optical axis of the receiver and so a light beam extends across the doorway from the transmitter to the receiver. The receiver thus generates an output signal indicative that the doorway is clear. When an obstruction interrupts the beam, the output from the detector changes to indicate the presence of an obstruction and control means are activated to stop the movement of the door and/or reverse the direction of movemen .

The problem with this arrangement is that as the distance between the detector and the emitter decreases, the intensity of the light received by the detector increases very significantly. If an obstruction entering the doorway during the last stages of closure is slightly translucent eg a human hand, then light passing through the hand will be detected by the detector and so the door will continue to close on the hand. The problem has, to some extent, been overcome by providing additional circuitry which acts to reduce either the beam intensity of the emitter or the sensitivity of the detector as the door approaches closure. This is still unsatisfactory firstly, because only a step wise reduction is achieved and, secondly, because costly additional circuitry is required.

It is an object of the invention to provide an improved detection system.

According to the present invention there is provided a detection system comprising an emitter for emitting a beam of radiation and a detector for detecting the radiation of the emitter, means for varying the radiation path length between the emitter and detector in response to a moving object and simultaneously varying the angle that radiation path makes with at least one of the optical axes of the emitter and the detector and means for varying the radiation intensity detected by the detector as a function of said angle in a sense such, that as the length of the radiation path reduces so also does the radiation intensity.

According to the present invention there is further provided a detection system comprising an emitter for emitting a divergent beam of radiation, a detector having a divergent field of view, the emitter and detector being constrained for relative movement so that the optical axis of the divergent beam and the optical axis of the divergent field of view are maintained substantially parallel to one another with a substantially predetermined spacing between the two axes, the detector being arranged to generate an output signal which is a function of both distance between the emitter and detector along said axes, and the angle between a said axis and the line joining the emitter to the detector, so that as a reduction in the distance between the emitter and detector acts to increase the output signal of the detector, an increase in angle acts to reduce the output signal. Detection systems embodying the present invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings, in which: Figure l is a side elevation of one system; Figure 2 is a graph of distance versus light intensity;

Figure 3 is a polar and coordinate graph of light intensity versus angular displacement for an infrared emitter;

Figure 4 is a side elevation of another system; Figure 5 is a side elevation of the system of

Figure 4 when the emitters and detectors are close together;

Figure.6 is. a side elevation of yet another system; and Figure 7 is a side elevation of a further system.

As shown in Figure 1 an infrared light emitter 2 is mounted on the leading edge of a lift door 4. The optical axis 6 of the emitter extends at right angles to the door and across the door opening. An infrared detector 8 is mounted on a slam post 10 on the opposite side of opening to the leading edge of the door. The optical axis 12 of the detection "zone of the detector 8 extends at right angles to the slam post 10 and across the opening. The optical axes 6 and 12 extends parallel to one another but lie spaced apart by a predetermined distance. The optical axis of the emitter is the axis along which the output is a maximum. The optical axis of the detector is the axis along which the sensitivity is a maximum. The emitter 12 is designed to emit a wide beam of light, the light being most intense along the optical axis 6 and diminishing in intensity with distance from the optical axis.

Figure 3 shows one example of both a polar and a coordinate graph of relative luminous intensity versus angular displacement. As can be seen from the graph, the maximum intensity of 1.00 is along the optical or vertical axis of the graph. The intensity then diminishes with angular displacement from the optical axis reducing to 0.5 or half the maximum intensity at an -angle of 45°. That is to say that on any given point on

the optical axis of the graph the optical intensity is reduced to half by moving to a point on a line which makes 45° with the optical axis and which lies the same distance from the emitter. Preferably the polar characteristic of either the emitter or detector or both, is such that the light intensity reduces to below 60% of the light at the optical axis when the angle to the optical axis is 60°. Emitters and detectors having other polar characteristics can be used.

In operation starting with the door 4 wide open, the line 14 joining the emitter 8 to the detector 2 makes an angle of 0-_ with the optical axis 12. As the door moves towards closure to reach the position 4A, the distance between the emitter and detector is reduced but the angle of δ ₂ between the line 14A and the optical axis 12 will have increased. These two factors act in opposite senses. The diminishment of the distance between the emitter and the detector acts to increase the light intensity received by the detector. However, the increase in angle from 0-^ to δ ₂ acts to reduce the light intensity of the light received by the detector.

Graph 18 of Figure 2 shows the light intensity detected by the detector versus distance from the emitter. As can be seen as the distance reduces from Do (where the door is wide open) towards closure, the light intensity increases progressively to reach a peak or maximum level at a distance apart Dp whereafter the intensity reduces to a minimum at distance apart Dc (where the door is closed) . Under optimum conditions the light intensity at Dc and Do should be the same and the intensity at Dp should only be minimally greater (no more than 60dB of the intensity at Dc and Do) .

If the spacing between the two optical axes is reduced by say half a beneficial effect is still produced

(see graph 20) but the difference between maximum and minimum output signals is increased to about 50%.

Graph 16 by contrast shows the light intensity detected when the optical axis 12 is moved to be level or colinear with the optical axis 6 as is the situation in the prior art. As can be seen the intensity increases progressively as the distance shortens from Do to Dc with no peaking.

The required spacing between the axes 6 and 12 is determined by the optical profile of the detector and/or emitter. Basically, if the beam emitted by the emitter is a narrow angle cone or the detection zone of the detector is a narrow angle cone then the spacing between the axes 6 and 12 can be reduced and vice versa. The object is to arrange the characteristics of the emitter and/or detector and the spacing between their axes such that the output signal from the detector is substantially constant as the door opens or closes.

Because the emitter and detector are vertically offset from one another, the protection secured as the gap between the door and slam post reaches a minimum is improved. This is because the detection zone is no longer just across the gap but is directed more along the gap because of the vertical offset of the emitter and detector.

It will be appreciated that an array of detectors and emitters can be installed along the slam post and leading edge"of the door. As can be seen in Figure 4 three emitters 30, 32 and 34 are provided at spaced intervals along the door 36 and three detectors 40, 42 and 44 are provided along the slam post 38. The vertical off-set between the emitter and detector axes is set to meet the criteria described in connection with Figure 1 . The beam of the emitter 30 is wide enough to impinge on two adjacent de„tectors 40 and 42, and the beam of emitter

32 is wide enough to impinge upon two adjacent detectors 42 and 44. Using suitable switch circuits to switch alternate and intervening emitters ON and OFF sequentially, each detector will receive a signal from only one emitter at any one time. It will be appreciated that with this arrangement the same protection can be achieved as with a conventional detection array but using virtually half the numbers of emitters and detectors. A further advantage arises when the door 36 is approaching the slam post 38. As can be seen from Figure 4 the effective detection beams run more along the gap than across the gap and so can detect narrow fingers in the gap which might otherwise go undetected since the diameter of a finger is generally narrower than the spacing between adjacent detectors.

In a modification shown in Figure 6 like parts in Figure 4 are similarly referenced. In addition to the three emitters 30,32,34 and the three detectors 40,42,44, three further emitters 30A,32A,34A and three further detectors 40A,42A,44A are provided.

Each emitter 30A,32A,34A is mounted on the slam post 38 to face a corresponding emitter 30,32,34 and each detector 40A,42A,44A is mounted on the leading edge of the door 36 to face a corresponding detector 40,42,44. The detection beams are accordingly interlaced. If the emitter/detector set 30A,32A,34A and 40A,42A,44A is independently operated this provides security in the even of one set failing. The provision of two emitter/ etector sets halves the pitch between detection beams.

In yet another modification all the emitters can be mounted one side and all the detectors mounted on the other side.

While the emitter and detectors have been described as infrared emitters and detectors it will be

appreciated that emitters and detectors of other radiation can be used for example ultra sonic radiation.

In the arrangement shown in Figure 7, both the emitter 50 and detector 52 are mounted in side by side spaced relationship on the slam post 48 of a lift doorway. A reflector 54 is mounted on the leading edge of the lift door. The optical axes of the emitter and detector extend parallel to one another. With the door wide open as shown in Figure 7 the light beam from the emitter makes an angle' 0-, with respect to the optical axis of the emitter. As the door closes and the reflector reaches the position 54A shown in Figure 7, the angle_changes to 0 ₂ . The effect achieved is precisely the same-as described in conjunction with the system of Figure 1. In each case the length of the light path between the emitter and detector is changed only in the case of Figure 7 ^' both emitter and detector are constrained against relative movement.

In the case of all the detector systems described, the optical characteristics of at least one of the emitter, detector and reflector must be such that the light intensity diminishes with distance from the optical axis. This can be achieved by using an appropriately profiled lens in conjunction with the relevant component or components. Instead the same effect can be achieved by using an appropriately graded diffuser in conjunction with the relevant component or components.

By using a custom designed polar profile for the lens or diffuser an optimum result can be achieved, ie the signal detected by the detector remains constant irrespective of the length of the light path between the emitter and detector. For example d profiled diffusion could be added on the lens of the emitter or detector away from its axis. While the detector system has been described in

conjunction with a lift door it will be appreciated that it can be adapted for use with other safety systems, for example in connection with robots and moving vehicles.