WO1987005115A1

WO1987005115A1 - Passive optical detector

Info

Publication number: WO1987005115A1
Application number: PCT/SE1987/000094
Authority: WO
Inventors: Robert Andersson; Bo Strandman; Morgan Fagerman
Original assignee: SILVERGRANEN INNOVATION AB
Current assignee: SILVERGRANEN INNOVATION AB
Priority date: 1986-02-25
Filing date: 1987-02-25
Publication date: 1987-08-27
Anticipated expiration: 1988-08-25
Also published as: CN87101582A; AU7035387A

Abstract

A passive optical detector having photocells that allow optical detection, solely using existing ambient light, e.g. daylight or the normal illumination of a space. This is achieved in that the optical detector includes two passive photocells (11, 12) coupled in series with a central output (1) therebetween. The photocells are arranged such that their respective incident ray paths are either mutually parallel or converging. A Schmitt trigger (71, 72) is arranged after an amplifier (61, 62) which is connected to each photocell (11, 12), the outputs of the triggers (71, 72) are connected to their respective inputs on an OR gate (3), the output of which is connected to the relay means (4).

Description

Passive Optical Detector Technical Field

The present invention relates to an optical detector of the kind disclosed in the preamble to the following claim 1. Background Art

Known optical detectors are sensitive to ' disturbance, since they operate with an artificial light source directed towards a photocell. Vibrations can upset the setting light source-photo- cell, and thus endanger the function of the detector.^' In addition the photocell and light source must always be on their respective sides of the passage along which objects for detection shall pass. For long detection distances they require a relatively strong light source, and in such a case ambient light can affect the photocell and cause malfunction of the detector. It should be noted here that in connection with such photocells and artificial light sources it may be necessary to use mirrors, where according to practical experience, the reflected light is only about 50 % of the incident light. It has been attempted to use pulsed light in order to make the photocell insensitive to ambient light, but this makes the indicators slower and has a deleterious effect on indication of movement direction and speed.

The mentioned optical detectors may be used for providing optical detection of a movement, e.g. in connection with goods on a conveyor belt, escalator, shop doorway, detection of movement at a large distance, indication of movement velocity etc. Disclosure of Invention

The present invention has, inter alia, the task of eliminating or mitigating the disadvantages with known optical detectors. It further has the task of providing a detector of the kind mentioned in the introduction that is not sensitive to vibration, that has a long life, that allows easy installation and that covers long distances.

This is achieved by the invention being given the distinguishing features disclosed in the characterising portion of claim 1.

The inventive optical detector is intended to replace apparatus of the type operating with a specially illuminated photocell, where the objects to be detected interrupt the emitted light beam between source and photocell.

The inventive^' principle is based on the realisation that much better reliability and great simplification are achieved if the special light source can be dispensed with. Accordingly, the invention utilises prevailing light, daylight, indoor illumination or other existing, ambient light. This simplifies the structural implementation of the thus passive optical detector, and eliminates the need of service and adjustment.

Not one but two elements, working in pairs and sensitive to visible or invisible light are used, e.g. CdS cells, solar cells, phototransistors or photodiodes. The term "photocell" is used hereinafter for such light-sensitive elements, both in the description and claims.Optically the two photocells are mutually directed towards a far distance target. Both photocells are electrically connected in series, and the photo detector signals are taken off at the connection between the photocells, either directly or via capacitors or resistors, to an input on a signal processing circuit having a zero-pass amplifier which, through a signal formation step is activating a relay circuit coupled to relay means or the like. The zero-pass amplifier may include a branch circuit for the respective photocell being associated to an amplifier. Then the signal formation step in the respective branch circuit comprises a Schmitt trigger or the like, and after inverting the signal from one of the triggers their output signals are applied to an OR gate included in the relay control circuit. The output of the OR gate is connected to ^•a relay means or the like, which in turn provides the necessary operational indication or control, although this is not a subject of the invention. With the passive optical detector in accordance with the invention, movements over large surfaces can be sensed in a simple and reliable way by two or more pairs of photocells being connected for coaction with electronic circuits. It is thus possible effectively to monitor a large continuous area or alternatively separate points within this area. Preferred Embodiments

The invention will now be explained in detail with reference to the accompanying drawings illustrating embodiments, without restricting the invention to them and where:

Figure 1 is a wiring diagram for a passive optical detector in accordance with the invention and with resistance-changing light-sensitive elements. Figure 2 is a wiring diagram of a modified embodiment.

Figure 3 is a diagram of a Schmitt trigger. Figure 4 illustrates another input to the circuit with current generating light-sensitive cells and

Figure 5 illustrates an embodiment of the invention where a plurality of photocell pairs is used for monitoring an extensive area.

In a basic embodiment of the invention, illustrated by the wiring diagram of Figure 1 , there are two series-connected photocells 11., 12 with a terminal 1 therebetween. The photocells 11, 12 are supplied with plus and minus operating voltages via respective resistors 21 and 22 to connections facing away from the central terminal 1. The detection signal itself is taken from the central terminal 1 between the photocells 11, 12 and is applied to an in¬ put 10 of a signal processing circuit 100. This circuit includes a zero-pass amplifier 106 and a signal formation step 107. In a preferred embodiment of the invention, the circuit 100 also includes a branch circuit 41 - 71 and 42 - 72, for the respective photocell 11, 12. The amplifier 106 then includes two branches, each with its respective capacitor 41, 42 or resistor 51, 52 connected to an amplifier 61, 62. With the aid of the capacitors or resistors the amplifiers are balanced such that the amplifier of one branch amplifies a voltage variation tending towards plus, while the other amplifies a voltage variation tending towards minus. The signal formation step 107 is" connected after the amplifiers 61, 62 and is provided with a Schmitt trigger 71, 72 for the respective branch, the signal from one of the triggers being inverted, so that the output signals have the same polarity. This may be accomplished either by one of the amplifiers having an inverting output or by an inverting means 2 being connected after one of them. The outputs of both triggers 71 , 72 are connected to each the input on an OR gate 3. The OR gate will thus give a signal only if there is a difference in the amount of light incident on one of the photocells as compared with that incident on the other photocell. The time during which the difference must be present for a detection signal to be sent can be easily modified by suitable dimensioning of the capacitors 41, 42 or resistors 51, 52. A resistor 31 , 32 may be arranged in parallel with the photocells in Figure 2 for the purpose of stabilisation. These resistors are suitably high- oh ic, in the megaohm order of magnitude. The output of the OR gate 3 is connected e.g. to a reed relay 4 or an optocoupler, to provide galvanic separation of the optical detector from the apparatus that are to be connected to, and controlled by it. One skilled in the art is capable of further modifications. A further Schmitt trigger 5 can be arranged between the OR gate 3 and the relay means 4, as in Figure 2.

The inventive optical detector is intended to be driven by batteries or the like, and if CMOS electronics are used the current consumption will be very low, i.e. in the order of magnitude of some microamperes, thus affording very long battery life.

Figure 3 is a Schmitt trigger diagram, the triggering point being denoted by T. An associated "•amplifier is turned such that both its positions FI and FII are respectively distinctly to the right and left of the trigger point T, thus providing reliable triggering on going from FI to FII. For turning the amplifiers 61, 62 to suitable positions they are provided with their respective tuning resistor 81 , 82 as illustrated in Figure 2. The moving outputs of the resistors are connected to the input of the respective amplifier 61, 62. Alternatively as will be seen from Figure 5, a single tuning resistor 8 may be connected the positive input of one amplifier and the negative input of the other, the mentioned in¬ puts of both amplifiers then being connected to the supply plus or minus potential via respective fixed resistors.

A modification of the circuit input will now be described in connection with Figure 4, where the photocells which change their resistances are re¬ placed by solar cells 91, 92, generating current or voltage, a working example being accounted for here. By only schematically illustrated optics known per se, two solar cells 91, 92 are directed at an acute angle to an object P having a given light intensity. Alternatively, the solar cells may also be directed parallel to or diverging towards their respective points P and P', which may be equally as bright or somewhat differently bright. Minor differences can be tuned out with the aid of a "potentiometer means 95, 96. A low-gain amplifier 93, 94 has been connected after the solar cell 91, 92, one of the amplifiers, here 94, having an inverting output.

When an object F passes in the direction R past both solar cells 91 , 92 the beam I is interrupted first, the cell then changing its output current i1 relative that i2 of the other cell 92. Current i1 is supplied to the amplifier 93, and current i2 to amplifier 94. If it is assumed that the object F is darker than the point P, the current i1 will be less than before. This means that the previously adjusted balance between the currents voltages existing from the amplifiers 93, 94 will be upset, resulting in a corresponding change at the output 96 of the potentiometer 95, i.e. in the case here the voltage taken off changes towards minus. If the object was lighter than the point P, or the beam II was interrupted first, this voltage would have been changed towards plus. The change is propagated via the capacitors 41, 42, amplifiers 61, 62 and trigger circuits 71, 72 to both inputs of the OR gate 3, one of these being given a signal, whereby the OR condition is met, and the output of the OR gate sends a signal to the relay 4 to trigger it. The wiring diagrams illustrated on the draw¬ ing as examples can be modified further by one skilled in the art with measures known per se. Accordingly, indication can be achieved showing whether beam I or beam II is interrupted first, and whether the interrupting object is lighter or darker than the point P, thus enabling, indication of direction. It is also possible to select or adjust the brightness of points P, P' as desired in one or other respect. Furthermore, it is possible to improve and/or adjust reaction sensitivity by suitable selection of components.

With the solution proposed by the invention, there is obtained a passive movement detector which ismore or less indpenedent of the light source, apart from needing some form of light for it to function. It has also been found by practical trials that requirements for light are very low. The optical detector in accordance with the invention uses the light available on site and adjusts itself to the light conditions prevailing at the indicating instant. Since the light may vary, seen as a whole, the inventive optical detector functions just as well in weak as in strong light. It is practically completely insensitive to light variations at the points P.

There is no trouble if the light comes from an old, deficiently functioning fluorescent tube that is constantly going on and off. This kind of light source could be quite inadequate for conventional photocells operating with a directly incident light beam. Should this beam vary too much, the photocell of the conventional apparatus takes this to mean that the beam has been interrupted, and sends a signal corresponding to this effect. Since the present invention uses photocells 11, 12 coupled in pairs, which have their incident light paths, mutually parallel or arranged at a mutual acute angle, thus signifying that one photocell can continuously receive a stronger light while the other photocell can continuously receive a weak light, there is achieved by the invention the essential difference that the optical detector solely detects the difference in signals from both photo¬ cells, i.e. whether one of them suddenly receives a definitely less amount of light .compared with the other one. Practical trials have also shown that signal differences within such short times as a few milliseconds are quite sufficient to enable the optical detector to send an operating signal.

Since the inventive optical detector adjusts itself to prevailing light conditions, it is excellen for detecting objects or movements at large distances when associated with suitable optical means. Such optical means are well known in the prior art and are not part of the invention, since they can be provided by one skilled in the art within the scope of his competence. He can also easily modify the inventive optical detector to enable it to indicate the speed of an obje^'ct detected by it, thus turning it into a movement detector and the indication of the movement direction to or from can also be enabled in a conventional manner. The speed of the reaction is solely dependent on photo¬ cell sensitivity.

A further development of the passive optical detector in accordance with the invention is illustrated in Figure 5, and is intended for sensing moving objects over a large area. More specifically, this detector may be used for indicating that only one person at a time is within a certain area.

This further developed indicator thus includes a plurality of pairs of photocells 11 - 12 (64 such pairs 11 - 12 being shown in the Figure) uniformly distributed over the area to be monitored. The photo¬ cells can be arranged in a roof over the area, the roof taking the form of one with a flat surface provided with a duct for each photocell 11, 12, of which there are 128 in this case. By giving a suitable length to the respective duct and appropriate directions to the ducts for respective pairs of photocells, the first criterion for the invention is fulfilled, i.e. that each pair of photocells shall be optically directed such that each receivers light . rays that are parallel to those for the other photo¬ cell, or such rays mutually forming an acute angle. The signals from the photocell pairs 11 - 12 are sampled periodically, and the sampling is regulated by a switching circuit 102 controlled by a clock circuit 103. The signal from a sampled photocell pair 11 - 12 is taken to a data collection circuit 101, which feeds an input 10 on a signal processing circuit 100. A memory circuit 104 is connected to the output of the signal processing circuit 100 for evaluating each signal message from it. To determine the number of objects/persons for which the detector shall send a signal, there is a quantity circuit 105 connected to the output line from the signal processing circuit 100, it being possible to feed the output signal to a relay or an optocoupler 4, as with the embodiments described in connection with Figures 1 and 2. There are two outputs on the quantity circuit 105, the upper output in Figure 5 is intended to signal the state "more than one person" and the lower output the state "more than two persons" are within the monitored area. The quantity circuit 105 may be setable for the desired quantity of objects/persons it is desired to monitor. The circuit wiring for data collection and evaluation is made up from electronic integrated circuits. Thus IC1 - IC11 represent 8-bit analog MUX/DEMUX's, IC12 and IC18 8-bit NAND gates, IC13 and IC16 so-called ripple counters (seven bits, CD 4024) , IC14 amplifiers of type 324, IC19 and IC20 OR gates, IC15 a NAND gate of quadruple type with double inputs, both having a Schmitt trigger, and IC17 a memory with 2 x 4 bits.

Moving objects/persons may thus be sensed in a given area by the optical detector accordring to Figure 5. The area may be a room or a corridor provided with a door which, on its being opened can cause a resetting signal RES to the detector, so that opening the door causes detection to start from an initial state. It should be clear to one skilled in the art that other electronic evaluation circuits can be coupled into the system, as well as there being different possibilities of locating the photocell pairs 11 - 12. The latter may have different mutual positional relationships, both in ceiling and walls, for sensing different area configurations and in three dimensions. It must also be considered obvious that the embodiments of Figures 4 and 5 can be combined for remote detection, it being possible to utilise daylight or artificial light (IR or laser) for illuminating the target points P, P'.

The inventive optical detector is entirely insensitive to the light variations which may exist at the site monitored. Accordingly, the effects of sun or shadow in cloudy weather or the illumination from lighting do not deleteriously affect the signals from the detector. The radiation sensed by the photocells may be given a desired configuration with the aid of suitable auxiliary means.

It has been found that the inventive detector can be made without being sensitive to vibration and it can be optically robust. In addition, very cheap electronics can be used to realise the invention, enabling the detector to be marketed at a low price. For example, the signal processing circuit 100 can have its zero-pass amplifier 106 and signal formation step 107 each implemented as an EXCLUSIVE-OR gate which, via a relay control circuit including a further EXCLUSIVE-OR gate, activates a transistor, which thus f nctions as the mentioned relay 4.

Claims

1. A passive optical detector with photocell (11, 12) and amplifier (61, 62) connected thereto as well as a relay means (4) or the like, characterized by photocells (11, 12) coupled in pairs and in series, with a central output (1) therebetween, said photocells (11, 12) having their optical ray paths either mutually directed parallel or converging, said photocells being adapted to sense ambient, existing light, e.g. daylight, said central output (1) being connected to an input (10) on a signal processing circuit (100) having a zero-pass amplifier (106) which, via a signal formation step (107) is adapted to activate a relay circuit (3, 5) having its output coupled to the relay means (4) or the like.

2. Optical detector as claimed in claim 1, characterized in that the zero-pass amplifier (106) includes a branch circuit (41 - 71 and 42 - 72) for the respective photocell (11, 12) with which is also associated an amplifier (61, 62), the signal formation step (107) in the respective branch circuit comprising a Schmitt trigger (71, 72) or the like, connected thereto, and in that the relay control circuit (3, 5) includes an OR gate (3) having its inputs connected to the outputs of the branch circuits (41 - 71, 42 - 72) and its output connected to the relay means (4) or the like.

3. Optical detector as claimed in claim 2, characterized in that one of the amplifiers (61, 62) has an inverting output, or has its output coupled to inverting means (2) .

4. Optical detector as claimed in claim 2 or 3, characterized in that a capacitor (41, 42) and/or a resistor (51, 52) is/are connected between each photo- cell (11, 12) and associated amplifier (61, 62). 1 3

5. Optical detector as claimed in one or more of claims 2 - 4, characterized in that tuning resistors (81, 82;8) are arranged on the input side of the amplifiers (61, 62), such as to keep the signals on the outputs of the respective amplifier (61, 62) on either side of the triggering point of the connected Schmitt trigger (71 , 72)

6. Optical detector as claimed in claim 5, characterized in that the moving output of the tuning resistor (81, 82) is connected to the input of the associated amplifier (61, 623..

7. Optical detector as claimed in one or more of the preceding claims, characterized in that there is a high-ohmic resistor (31, 32) connected in parallel with each photocell(11 , 12).

8. Optical detector as claimed in one or more of the preceding claims, characterized in that the photocells comprise current-generating cells (91, 92), e.g. solar cells, the current (i1 , i2) from the cells being applied to the terminals of a resistor (95) connected between the outputs of a low-gain amplifier (93, 94) and having a moving output (96) for the output signal from the detector, said low- gain amplifier (93, 94) being optional and connected after the photocells (91, 92), one of said amplifiers having an inverting output. (Figure 4).

9. Optical detector as claimed in any one of the preceding claims, characterized in that two or more photocell pairs (11 - 12) are connected to the signal processing circuit (100) for sensing changes in existing ambient light illuminating a large monitoring area and/or at different points in a monitoring area. (Figure 5) .

10. Optical detector as claimed in claim 9, characterized in that each photocell pair (11 - 12) is separately connectable to the signal processing circuit (100), preferably by a connection circuit (102) adapted for periodically connecting the separate photocell pairs to the signal processing circuit.

11. Optical detector as claimed in claim 10, characterized in that the photocell pairs (11 - 12) are connected to a data collecting circuit (101), the output of which is connected to the input (10) of the signal processing circuit (100) , the output of the latter being connected to a memory circuit (104) for evaluating the signal message given by each photocell pair (11 - 12).

12. Optical detector as claimed in claim 9 - 11, characterized in that a quantity-determining means (105), which may be programmable, is connected to the memory circuit (104) for predetermining the number of photocell pairs (11 - 12) that must give an actuated signal message of obtaining an output signal to the relay means (4) or the like.