This invention relates to obstruction detection apparatus and in particular it relates to apparatus for the detection of obstructions in the path of moving bodies such as moving doors on lifts (elevators) or trains.

Such doors are often controlled automatically and should be able to detect the presence of an obstruction such as a body in their path and cease their motion. It is important with a lift door to ensure that the door closes before the lift is raised or lowered, but also to ensure that the door does not hit or even frighten a passenger when closing. It is also useful that at least two independent protection devices are included in the door mechanism, both of which fail to safety, i.e. with the doors open. Typically one mechanism may use light beams whilst the other monitors the torque on the door. It is preferable to monitαr two different zones of the doorway area, as shown in Figure A . Zone A is a thin zone across the opening and following the door closing path. An obstruction to this path causes the doors to be 'held' so that a person can enter or leave the lift. Zone 3 is immediately in front of the door and is broader than zone A. It extends for a distance greater than the stopping distance of the door and prevents the door from hitting a person going through the entrance. According to the present invention there is provided an obstruction detection apparatus comprising a transmitter and a receiver mounted on one side of a zone to be monitored and having substantially a common field of view and a reflector mounted on the other side of the zone and arranged to

lie within at least a part of the said field of view to reflect a signal from the transmitter back to the receiver, said reflector being endowed with a reflectivity substantially higher than any other body at which the transmitter is normally directed, and means for detecting a reduction in the signal received by the receiver to indicate the presence of an object interposing the path from the transmitter to the receiver via the reflector. Preferably, the reflector lies within only a part of the field of view and the apparatus includes means for detecting an increase in the signal received by the receiver, which increase is indicative of the entry of an object relatively close to the receiver and in the portion of the field of view other than that in which the reflector lies.

According to the present invention there is further provided movable door apparatus comprising an array of transmitter/receiver pairs, each pair comprising a transmitter and receiver in proximity and sharing substantially a common field of view, respective pairs being mounted in spaced apart relationship along the leading edge of a door: a reflection means mounted, adjacent the surface upon which the door closes to lie, within at least part of the field of view of each transmitter/receiver pair, the reflection means having a substantially higher reflection coefficient than any other surface which normally lies within said field of view and means for detecting a reduction in the signal received by one or more receivers to indicate the presence of an object interposing the door and the reflection means.

Preferably, the reflection means lies within only a part of the field of view and means are

provided for detecting an increase in the signal received by one or more receivers to indicate the entry of an object in the part of the field of view other than that in which the reflection means lies. The respective transmitter/receiver pairs may be actuated and monitored in turn by multiplexing.

Preferably, the reflector or reflection means is a retroreflector but any other reflector, such as a plane mirror, may be used provided care is taken to avoid unwanted reflections reaching the receiver. Advantageously the retroreflector is in the form of a screen of spherical beads or appropriately faceted bodies. Advantageously, the field of view of the or each pair is arranged so that a greater portion of it lies on a chosen side of the doorway.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, in which:

Figure 1 shows a plan view of a schematic lift door apparatus:

Figure 2 shows a front view of the door apparatus: Figure 3 shows a plan view of a further embodiment of the present invention: and

Figure A shows the zones in a doorway which it is desirable to monitor.

Referring to Figures 1 and 2 a lift door 1 is mounted adjacent a lift frame 2. Further doors 3 are provided at each landing and, in use, both a respective landing door 3 and the lift door 1 must be open before a passenger can enter or leave the lift. Mounted in vertically spaced apart relationship on the door are a plurality of transmitter/receiver

pairs 4- These may be of any suitable type such as infra-red, ultra-sonic, etc., but are arranged such that each transmitter and receiver are proximate or integrated in one unit and share substantially a common field of view F. The transmitter and receiver are therefore not focussed. On the other side of the door in Figures 1 and 2 a fixed slam post 5 is shown. Alternatively, this could be another door for a system which uses two doors in a bi-parting system. Vertically mounted on the slam post 5 is a long thin retroreflective strip 6 approximately parallel to the edge of door 1. This may comprise for example a plurality of corner cubes or could have a coating of retroreflective paint, of the type commonly used for road signs. Any other type of retroreflective medium may be applied, for example a screen with translucent spherical beads embedded to form an outer edge. Alternatively, a reflector such as a plane mirror may be used. It is desirable that the reflector or retroreflector be a substantially better reflector than any other surface in the surrounding area. The retroreflector has the effect of reflecting any radiation back in the direction from which it came. Thus, substantially all radiation emitted in the plan (horizontal) angular region R of Figure 1 is reflected back onto itself and hence onto the receiver part of transmitter/ receiver 4. Similarly, any radiation in the vertical angular region S for transmitter/receiver 4a of Figure 2 is reflected back onto itself. When an obstruction such as a hand 7 is placed within the zone between transmitter/receiver 4 and retroreflector 6, less radiation is reflected back. Accordingly, the signal received at transmitter/ receiver 4 is reduced. This reduction in signal can

be detected by standard and known techniques such as thresholding or by using comparators or delays for example, and used to provide a signal to instruct the door either not to close or, if it is already closing, to stop closing and open. Hence any obstruction in the direct closing path of the door will immediately inhibit closure of the door.

A thresholding unit 20 is shown in Fig.1, which unit receives a signal from the transmitter/ receiver pair 4 and, if the signal is less than a predetermined threshold and/or greater than a second predetermined threshold, outputs a control signal to inhibit closure, or begin opening the door, as appropriate. Such units are well known. Other signal detecting apparatuses may, of course, be used.

The transmitter/receiver pairs 4 are spaced by a distance d down the side of door 1. Advantageously, this distance is around 50 mm which is approximately the diameter of a child's wrist so that any part of a body will normally always be in the field of vision if that body strays within the closing path of the door. The transmitter/receiver pairs are unfocussed and, as shown in Figure 2. overlap each other. Hence if an object is placed in front of the retroreflector 6 it will usually cause a reduction in the signals received by at least two receivers 4. A hand placed at position 7b in Figure 2 will cause a reduction in part of the signal from transmitter/ receiver 4a and also in part of the signal from transmitter/receiver 4b directly above

4a. By measuring the output of each receiver in each respective transmitter/receiver pair 4 and noting the relative changes between them, the position of an obstruction can be detected to a high degree of accuracy. For instance, if the signal at receiver 4a

is reduced by half, and that at receiver 4b by one quarter, then the object must be between 4a and 4b, but nearer 4a.

The transmitter/receiver pairs 4 may preferably be activated in turn, i.e. multiplexed, and the received signals compared with each other. This is especially desirable if reflectors other than retroreflectors are used, to prevent unwanted radiation being received. Although the retroreflective strip is shown mounted on a slam post it may alternatively be provided on the edge of a door in a bi-parting system.

Any radiation from transmitters 4 which falls outside the zone defined by the retroreflective strip 6 will be either reflected away by the surface upon which it eventually falls or, more typically, absorbed or greatly attenuated. Thus, if an object is placed within this region outside the zone defined by strip 6, the object will, if it is close enough to the transmitter/receiver pair, reflect radiation back and will have the effect of increasing the signal received. Accordingly, in a zone close to the transmitter/receiver pair and determined by the power of the system and the reflectivity of any obstruction, the monitored field of view is wider than that further from the pair and an obstruction will make its presence felt by an increase in the signal. Hence, the system according to the invention automatically monitors two different zones simultaneously but independently, avoiding the need for two or more different types of sensor. The zones can correspond to desired zones A and B in Figure 4. This bi-zone monitoring is desirable because the closing of a door may be alarming to a passenger,

particularly if it is a very young or very old passenger who is close to the door, but not directly in its closing path.

If a lift is to be used during busy periods it will often be filled to capacity. In such cases it will usually be necessary for one or more persons to stand very close to the edge in a corner of the lift. It is undesirable in such circumstances for the obstruction sensing apparatus to sense such a person. Figure 3 shows an embodiment of the invention in which such detection is avoided by rotating the field of view of the transmitter/ receiver pairs about a vertical axis such that the central plane of the field of view is at an angle 0 to the closing direction of the door. The angle 0 is such that the area of detection in which an increase in signal occurs when an object is placed in the field of view is just in front of the car door 1 and landing door 3- The transmitter/ receiver pairs are still able to "see" the slam post or second door 5, and hence the retroreflector, and to detect a reduction in signal indicative of an obstruction in the direct closing path. If an object to be detected enters the detection zone outside that defined by retroflective strip f> then an increase in signal is seen.

A reference transmitter/receiver pair may preferably be provided outside the detection zone to act as a comparison reference for the remaining pairs-. Alternatively, a reference signal can be generated by averaging all the received signals.

The present invention avoids the need for careful focusing of transmitter/receiver pairs since all the pairs are unfocused and have a conical volume of vision directed at a particular angle ^' . In some

applications it is possible to restrict the angle of vision. This may be achieved by using partially focused transmitters and receivers or by the use of shutters for restricting the angle of vision. Since only a passive retroreflector is required on the right side of the door in the Figures considerable cost savings are achieved since active circuits need only be provided on one side of the door opening. Thus wires linking the two sides of the opening are obviated and a reduction in power supply consumption is achieved.

It is further preferable that the retroreflector 6 is housed in an identical enclosure to the transmitter/receiver circuits. These circuits are often housed in an elongate channel shaped member which extends down the length of the door. By also mounting the retroreflective means in a similar member, cost is greatly reduced and the appearance of the system improved. A guarding channel-shaped member also serves to protect both the transmitter/ receiver pairs and the retroreflective means.

The invention is also applicable in control or monitoring systems, or with robots, machine guards and other applications related to conveying or material handling. The invention can also be used in intruder detection systems.