GB2364773A

GB2364773A - Improvements In Or Relating To A Light Curtain

Info

Publication number: GB2364773A
Application number: GB0002911A
Authority: GB
Inventors: Richard Walter Lea
Original assignee: RWL CONSULTANTS Ltd
Current assignee: RWL CONSULTANTS Ltd
Priority date: 2000-02-10
Filing date: 2000-02-10
Publication date: 2002-02-06
Also published as: GB0002911D0

Abstract

A light curtain apparatus comprises a transmitter unit and a receiver unit Each of these comprises a casing (2), a lens unit (5) and a board (3) including a plurality of either light emitter diodes (10) or light receiver diodes (10A). The lens unit (5) comprises a plurality of lenses (15) each one aligned with a respective light emitter diode (10) or light receiver diode (10A). The components of each of the transmitter unit and the receiver unit are held in place by a window (7), located in an open face of each casing (2) by a lip (30). The operation of a light curtain may be checked by coding a light beam processor with a predetermined number, incrementing that number differently depending on the fault status of that beam and passing the incremented number to a next beam processor. The fault status of a curtain comprising a series of beam processors is determined from the value of the number output by the final beam processor of the series.

Description

<Desc/Clms Page number 1> IMPROVEMENTS IN OR RELATING TO A LIGHT CURTAIN The present invention relates to a light curtain. More particularly, but not exclusively, it relates to a casing for a light curtain which contains no screws or other related fixing mechanism such as holes through which mating surfaces are fixed together with a device passing through the holes. Furthermore, it relates to an improved design for electronics and optical systems for a light curtain (such as may meet EC standard IEC61496). The main purpose of light curtains such as those of the present invention is in relation to the implementation of a safety light screen for use in safety systems in machinery. Thus it is required that such apparatus is fail safe, impervious to ingress of external contaminants and built to meet cost constraints. The term "light" as in light curtain and otherwise is used herein to refer to any electromagnetic radiation usable with the invention. Infrared radiation is preferred.

It is an object of the present invention to provide a light curtain apparatus which enables such goals to be fulfilled more easily.

According to a first aspect of the present invention, there is provided a light curtain apparatus, comprising a transmitter unit and a receiver unit, each of which comprises a casing, a lens unit and a board including a plurality of means either to transmit or receive light, wherein said lens unit comprises a plurality of lenses each aligned with a respective one of said light transmitting or receiving means.

Preferably, the lens unit locates in place within the casing and the board is aligned substantially exactly by engagement with said casing and said lens unit.

The casing may be closed at one face by a window of material transparent to the light. In this case, the unit may be assembled through said one face.

Seal means may be included between the lens unit and the window.

The casing may be provided with end caps, one of which serves as entry for a power connection cable.

In this case, the end cap may be provided with an electromagnetic interference barrier. According to a second aspect of the present invention, there is provided a method of checking a safety light curtain comprising the steps of coding a beam processing means with a

predetermined number, preferably greater than the number of beams in the light curtain, and if there is no fault, adding one to the predetermined number and passing the new number to a second beam processor and so on.

In this case, if the transmitter is working, the final number returned will be the predetermined number plus the number of beams plus one.

If any beam processor senses a fault condition, it does not increase the received number by one but by the predetermined number plus one.

An embodiment of the present invention will now be more particularly described by way of example and with reference to the accompanying drawings, in which:- Figure 1 shows a transmitter or receiver unit of alight curtain embodying the invention, in separated condition; Figure 2 is a cross-sectional view of a transmitter or receiver unit when assembled; Figure 3 is a schematic view of alight curtain system embodying the invention; Figure 4 is illustrative of some of the electrical features of the invention; and Figure 5 illustrates variations in the lens arrays possible with embodiments of the invention.

Referring now to the drawings, Figure 3 shows the physical arrangement of an operational light curtain embodying the invention in which a plurality of beams of electromagnetic radiation, preferably in the infrared range, are generated and transmitted from a transmitter

unit to corresponding receptors in a receiver unit. If any one beam is broken, a protection system is activated to stop the machine or apparatus to which the light curtain is fitted for safe guarding the operator.

Figures 1 and 2 show the general design of one of the units of the device. Each unit comprises seven different parts, representing a very low component count.

An aluminium extrusion 2 acts as a casing and is shaped to contain numerous features which locate components, form seals and provision for fixing the device to machinery. The shape is shown most clearly in Figure 2.

End caps 1 are adapted to close the ends of the aluminium extrusion. Each cap 1 is a press fit and one doubles as a cable entry point in line with one slot which allows the cable to be directed up, down and back from the device. The cap contains a tapered hole which, when the cable is passed through with an `O' ring over it, becomes trapped and sealed within the cap, acting as cable strain relief and an environmental seal.

A board 3 processes the optical signals. It is fixed in place, at least in part, to a slot in a lens unit 5 described in more detail below. Each optical device is an emitter diode 10 in the transmitter and a receiver diode 10A in the receiver. The principal of operation is that each beam is processed in turn from the first, successively to the last, and a message passed to the next beam in similar succession (at the site of reception or transmission). When a11 the beams have been processed the emerging message contains the information concerning the status of the light curtain. This message may be used to drive an output signal.

End cap 1 where the connection cable 16 is introduced is provided with an electromagnetic compliance (EMC) barrier 4 to block disruptive signals which may be present on the connection cable 16, thus enabling continued operation of the device during disruptions. It also acts to contain disruptive signals generated by the device. The EMC barrier 4 forms a small compartment where suppression of the disruptions is accomplished. Any disruptive emanations inside this compartment are contained by the barrier 4 which has only small holes in it for passing the disruption free wires to the sensitive electronic board 3.

A multifunction lens unit 5 comprises a row of lenses moulded integrally as a plastics material moulding, each lens moulding being adapted to be so aligned with a respective diode 10 or 10A that the light curtain consists of substantially parallel beams. Accurate positioning of the lenses 15 relative to the diodes 10 or IOA is by a series of slots 8, which enable accurate positioning of the lens array 5 relative to the aluminium extrusion 2 with the aid of extrusion feature 14.

Each lens unit 5 includes a pressure feature 13, which clamps the lens unit and the electronic board 3 in position by locating the electronic board firmly within abutments 14 of the casing 2.

A pair of sealing strips 6 prevent the ingress of contaminants when compressed by a window 7 held against the lip 30 of the casing 2. This method provides a double seal point 11 where high compression of the seal strips 6 is made. However, since the seal strip is not compressed as a whole, the forces used for assembly, and the stresses placed on the components are low, reducing the possibility of mechanical distortion which is highly

undesirable in an optical device. A third pressure point on the strip is applied by pressure feature 13 on the lens unit.

The window 7 comprises a plastics material that is transparent to the particular electromagnetic radiation being used and is held in place by lip 30 to accurately control the compression on the sealing strips 6. The lip 30 also acts as a `snap in' device for holding all the components of the light curtain unit together. Thus all components are assembled from the front of the unit, with the insertion of the window 7 being the final operation.

The apparatus includes a transmitter of electromagnetic radiation elements and a receiver of electromagnetic radiation elements, where the elements are arranged to pass from the transmitter to the receiver in a plane to form a detection zone. When an obstruction that is opaque to the particular electromagnetic radiation enters the detection zone, one or more of the receivers will no longer receive its signal and an output is signalled to be turned off In operation the transmitter unit is supplied with electrical power through connection cable 16. The power passes through electromagnetic interference (EMI) filter and barrier 4. The power then passes to a series of LED controls 20 on board 3, the number of which extends across the desired width of the detection zone. The power is used directly to pulse the transmitter diodes 10 and is conditioned to supply the other electronic components (mainly micro- processors) 18. The transmitter micro controllers contain a monitor processor 19 which determines that the transmitting system has no fault. When this is determined it actuates flashing of an indicator 25.

A fault is determined at the transmitter by a series of tests carried out by a beam processor after the beam has been sent. The first beam processor is coded with a predetermined number of the monitor processor - suppose it is 80. If the beam processor has no fault it adds 1 to the number and passes the new number - 81 - to the next processor. Assuming there are less than 81 beams in the device, if there are no faults, the number returned to the monitor will be equal to the number of beams, plus the used number 80. In the case where the number of beams is 60, then if the transmitter is working, the number 141 will be returned. However, if the first beam is bad the processor adds 81 to the number and passes 161 to the next processor.

A further test is performed by the receiver unit. It is conceivable that two beams might transmit simultaneously in error, thus modifying the nature of the detection zone and posing a potential danger. This eventuality is provided for by transmitting coded signals to the receiver. The codes are: Single pulse 24 Double pulses 23 Triple pulses 22 The meaning of these signals is respectively Beam Test, Normal beam and Beam 1. Beam 1 usually sends a triple pulse so that it may be identified by the processor for beam 1 in the receiver. This allows the receiver to lock on to a transmitter without any special connection. Normally the other beams send a double pulse signal. However, when a transmission begins where first beam 1 sends a signal, a test signal is sent by beam 1, which is a single pulse. When this happens the processor for Beam 1 subtracts 80, the predetermined number, from the number it started with. This means that the good number returned to the monitor is equal

to the number of beams (say 60). After beam one has transmitted, beam two transmits normally and so on to the last beam, when the `OK' number 60 is returned to the monitor. A bad beam makes the signal too high and an extra `test' makes the number too low. This cycle is called a scan. After beam one has tested in Scan one, beam two tests in scan two and so on. So if all is well after a number of scans equal to the number of beams every beam has been individually tested.

If a scan takes 10 milliseconds to complete, and there are 60 beams, a blocked beam is detected within 10 ms and a faulty beam is detected within 10 x 60milliseconds = 600 milliseconds.

The receiver unit works in a similar way to the transmitter but does not `self-start'. Beam 1 waits to be triggered by its unique signal. The receivers also use the received code to calculate the numbers passed to the next beam.

The receiver also has the added task of turning off its output if the correct number is not received. The monitor processor determines the correct number by performing a test scan when power is applied and comparing the returned number with a list of legitimate numbers representing the model list of the devices, such as 8 beams, 16 beams, 32 beams (generally being the number of circuit boards multiplied by the number of beams resident on each circuit board). This method of determining the status of the curtain using numeric coding has applications in the field of measurement and other areas where the system can be used to identify which beams are blocked or how many beams are blocked. This data may be sent to other equipment in any format such as RS232 or current loop. Its primary advantage is that

no central control devices are required because all calculations are performed by the beam processors and the accumulated result is the final result needed by the device.

As shown in Figure 4, the lenses of the light curtain are made of optimum shape and separation for achieving long range. This gives a detection capability suitable for the detection of a person's arm, at a beam width of c40mm. By applying a mask to the lens, and thereby limiting the beam width to, say 30mm, the detection capability is improved for detection of hands (at the expense of range). Generally if alight curtain is used over a long range (called perimeter guarding) hand protection is not required.

As may be seen, the present invention provides a method of synchronising emitter and receiver units of alight curtain without using wire connectors or an optical reference beam.

It also allows a method of determining a PCB and its electronic components is OK with an onboard test cycle and an `OK' light. It also includes a method of pulsing light emitting diodes without using a regulated power supply, a method of parting EMI from the operational electronics, and a method of determining that the light curtain is clear without using a centralised control electronics module, either incorporated or separate, from the light curtain. The inventions also permits the obtaining of two detection capabilities for a light curtain by using removable lens masks, and a greater degree of accuracy by allowing both emitter and receiver PCB boards to be placed in line with the optical axis of the device by using cut-outs in the PCB, which co-operate with the unit.

Each transmitter and receiver unit may be finally sealed by sealing the front window of the unit using low force ribs, after assembling multiple lens and electronic assemblies from the front of the unit.

During such assembly, the PCB and lens blocks may be located positively without the use of screws. The window thickness may be defined from variable thickness stock window material.

The invention permits a method of meeting the requirements of EN954 category 3 and IEC61496-2 type using distributed micro controllers.

Claims

CLAIMS 1. A light curtain apparatus comprising a transmitter unit and a receiver unit, each of which comprises a casing, a board including a plurality of means either to transmit or to receive light, and a lens unit which comprises a plurality of lenses each aligned with a respective one of said light transmitting or receiving means. Z. A light curtain apparatus as claimed in claim 1, wherein each casing is a shaped aluminium extrusion. 3. A light curtain apparatus as claimed in any one of the preceding claims, wherein each casing is provided with means to locate in place a respective lens unit, and said lens unit and said casing co-operate to hold a respective board in alignment with the lenses of the lens unit. 4. A light curtain apparatus as claimed in any one of the preceding claims, wherein one face of each casing is closed by a window of material transparent to the light. 5. A light curtain apparatus as claimed in claim 4, wherein seal means are provided between each lens unit and the respective window. 6. A light curtain apparatus as claimed in either claim 4 or claim 5, wherein the window is locatable after the transmitter or receiver unit and the lens unit have been assembled within the respective casing.

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7. A light curtain apparatus as claimed in any one of the preceding claims, wherein each casing is provided with end caps. 8. A light curtain apparatus as claimed in claim 7, wherein one end cap of each casing serves as entry for a respective power cable. 9. A light curtain apparatus as claimed in claim 8, wherein each said one end cap is provided with an electromagnetic interference barrier. 10. A method of checking a safety light curtain comprising the steps of coding a beam processing means with a predetermined number and, if there is no fault, adding one to the predetermined number and passing the new number to a second beam processing means, thence with addition. of one to a third, and so on. 11. A method as claimed in claim 10, wherein the predetermined number is greater than the number of beams in the light curtain. 12. A method as claimed in either claim 10 or claim 11, wherein, if the transmitter is working, the final number returned will be the predetermined number plus the number of beams. 13. A method as claimed in any one of claims 10 to 12, wherein if any beam processing means senses a fault condition, it does not increase the received number by one but by the predetermined number plus one.