DE19852173A1 - Operating method for optical sensor for detecting objects forms sum and difference values from near and far sensors and evaluates to set binary output - Google Patents

Abstract

The sensor includes a light transmitter (4) and a receiver (6). The receiver has a near element (8) and a far element (9), whose outputs (UnahA, UfernA) are fed to an evaluation unit which provides a binary output that is on if an object is detected, and off if no object is detected. The sum (UsumA = UnahA + UfernA) and difference (UdiffA = UnahA - UfernA) of two outputs are formed. The binary output assumes an on state if UdiffA is greater than zero. The binary output assumes an off state if UdiffA is less than S2, where S2 represents a threshold value less than zero, or if UsumA exceeds another threshold S1 that is greater than zero. An Independent claim is also included for an optical sensor.

Description

The invention relates to a light sensor and a method for its operation according to the preamble of claim 1 and according to the preamble of An Proverbs 6

In known light pushbuttons, the one emitted by a transmitter and by egg nem object reflected back transmission light on the near and the far element Recipient. From the at the exit of the near and far element in the form of The received signal becomes the differential signal educated. This difference signal is evaluated with a first threshold value. If the difference signal exceeds this threshold, the switch changes gear to the ON switching state. So that the switching process is not through the on threshold voltage is triggered unintentionally, the threshold is above of the noise peak. In addition there is a second one, below the first one Threshold value is provided. The value of this threshold value is slightly above 0 volts. If the difference signal falls below this second threshold value, the switching state changes again at the switching output to the value OFF.

A disadvantage of this device is that the response distance at which a The switching state of the device changes from the surface area The quality of the object, especially on its reflectivity, strongly depends.

The darker the object surface, the wider the response distance at which the switching output changes to the ON switching state Distances shifted. This systematic measurement error forms the black White error of the device.

Furthermore, an edge error can also occur in such a device to step. This error occurs when the transmitter's broadcast spot is partially on an object and partly on the background behind it  meets. If the background is considerably lighter than the object, the proportion of the disproportionate in the difference signal from the background signal takes into account what can lead to the object not being recorded. Vice a bright object against a dark background may reverse be recognized as being in the surveillance area even though it did is still out of the surveillance area.

The black and white error can be eliminated in such light sensors be that instead of a difference signal, the quotient of the received signals of the near and far element is formed. However, the edge still remains error. Another disadvantage is that the quotient formation by a micropro processor is time-consuming, or as an analog circuit a large circuit effort required.

The invention has for its object in a device of the beginning mentioned type, the safest possible object detection with the lowest possible Achieve circuit effort.

To solve this problem, the features of claims 1 and 6 are vorese hen. Advantageous embodiments and expedient developments of the Invention are described in the subclaims.

In its simplest embodiment, the light scanner according to the invention has a transmitter and a receiver with a near element and a Fernele element. The transmitted light beams emitted by the transmitter, which reflect back as received light beams from an object to the receiver, generate the output signals U _nahA and U _fernA at the outputs of the near and _{far elements} . Both the difference U _diffA = U _nahA - U _fernA and the sum U _sumA = U _nahA + U _{fernA of} the output signals are formed in the evaluation unit. The switching output changes to the "on" switching state as soon as U _diffA <0. A change to the switching state "off" takes place as soon as U _diffA <S2, where S2 <0, or as soon as U _sumA <S1, where S1 <0.

The main advantage of the light switch according to the invention is that this has almost no black and white error.

This is due, on the one hand, to the fact that the transition to the "on" switching state takes place when the condition U _diffA = 0 is fulfilled, that is to _say that U _nahA and U _fernA are of the same size. This condition is fulfilled for objects at a predetermined distance regardless of the degree of reflection of their surface. For the transition to the switching state "off", it is checked whether the difference U _{diffA falls below} the threshold value S2, where S2 <0 and is selected such that noise voltages do not lead to the unintentional switching off. In the event that the object is laterally removed from the monitoring area, the threshold value S2 is not fallen below, as a result of which a second switch-off criterion is required. If there is no object or the background is dark, the sum U _{sumA drops} below the threshold value S1, which has the effect of the “off” switching state.

It is also advantageous that only the sum and the difference of the output _signals U _nahA and U _{fernA have to be} formed in the signal evaluation _{according to the invention,} which can be achieved by simple analog circuits.

In a particularly advantageous embodiment, U _sumA and U _{diffA are checked} periodically in succession using a sequence of evaluation _rules .

The evaluation unit first checks whether a first condition (rule 1 )

U _sumA <S1

is met, with S1 being a threshold value with S1 <0. If this condition is met, the switching output of the light button takes the switching state "off". This Switching status corresponds to a free beam path of the light button.

However, if the first condition is not met, a check is made to determine whether another condition (rule 4 )

U _diffA <0

is satisfied. If this condition is met, the switching output takes the switching state "on" on. This switching state corresponds to the detection of an im Monitoring area of the object located on the light button.

If this condition is not met, it is checked whether the further condition (rule 5 )

U _diffA <S2

is satisfied.

If this condition is met, the switching output takes on the switching state "from a. In the other case, the switching status remains unchanged.

Two evaluation rules, namely rules 1 and 5, are used to switch to the "off" switching state. This multiple _check of U _sumA and U _diffB ensures safe switch-off behavior under any object and background conditions. The switching hysteresis can be specified by the threshold value S2. This threshold value is expediently chosen such that no switching process is triggered by self-noise of the electronic components of the light sensor and offset voltages.

In a second embodiment of the invention, the light scanner points to the detection on objects in the near range on a second transmitter, which between the first and the second transmitter is arranged. It is more appropriate as the beam guide chosen so that the transmitted light beams of the transmitter do not overlap in the monitoring area covered by the light sensor.

In the signal evaluation according to Claim 3 generated by the light emitted by the transmitters transmit light pulses output signals U _nahA, U _Ferna and U _NaHB, U _REMOTE are detected separately, and white error black of the light sensor is almost completely eliminated in this case, the. In addition, objects arranged at a very short distance from the distance sensor can be detected with the second transmitter.

The evaluation according to claim 4 also achieves a reliable one Detection of object edges. This can also cause incorrect switching edge defects are largely eliminated.

The invention is explained below with reference to the drawings. It demonstrate:

Fig. 1 block diagram of a first embodiment of the light switch according to the Invention.

FIG. 2 flow chart of the evaluation rules for the light sensor according to FIG. 1.

Fig. 3 shows signal waveforms of the output signals at the receiver of the light sensor of FIG. 1.

Fig. 4 block diagram of a second embodiment of the light switch according to the Invention.

Fig. 5 shows pulse diagrams of the radiation emitted by the transmitters of the light sensor shown in FIG. 4 transmit light pulses,

• a) first transmitter,
• b) second transmitter.

Fig. 6 flowchart of the evaluation rules for the light sensor of FIG. 4.

Fig. 7 shows signal waveforms of the output signals at the receiver of the light sensor of FIG. 4.

Fig. 8 waveforms of different combinations of the output signals at the receiver of the light sensor according to FIG. 4 in the case of an object laterally penetrating into the monitoring area.

Fig. 1 shows a first embodiment of the Lichtta sters 1st invention Objects 2 are detected with the light sensor 1 within a predetermined monitoring area. For this purpose, the light sensor 1 has a transmitter 4 emitting light beams 3 and a receiver 6 receiving light beams 5 . The transmitter 4 and the receiver 6 are arranged from one another side by side in a housing 7 . A transmitter optics, not shown, is arranged downstream of the transmitter 4 . A receiving optics, also not shown, is arranged upstream of the receiver 6 . The transmitter 4 is preferably formed by a light emitting diode. The receiver 6 has a near element 8 and a remote element 9 , which can each be formed by a photodiode. The near element 8 forms the end of the receiver 6 facing the transmitter 4 and, as shown in FIG. 1, preferably has a larger photosensitive surface than the remote element 9 . If the object 2 is at a great distance from the light scanner 1 , the received light rays 5 that are reflected back by the object 2 predominantly strike the remote element 9 . As the object distance becomes smaller, the received light rays 5 increasingly hit the near element 8 until finally all received light rays 5 hit the near element 8 . The minimum object distance at which objects 2 can still be detected is essentially determined by the distance between transmitter 4 and receiver 6 .

With the light sensor 1 , objects 2 can be detected within a monitoring area which is limited by the scanning distance to large distances. The scanning range is defined by a predetermined signal ratio of the output signals on the local and remote element 8. 9 In order to adjust the scanning distance, the receiver 6 or the receiving optics is arranged in the housing 7 such that it can be moved in the longitudinal direction of the receiver 6 .

The transmitter 4 and the receiver 6 are connected to an evaluation unit 10 . The transmitter 4 is preferably operated in pulse mode via the evaluation unit 10 , so that it periodically has transmitted light pulses with a predetermined pulse-pause ratio.

The output signals U _nahA pending at the output of the near element 8 and the output _signals U _fernA pending at the output of the remote _element 9 are each amplified in a preamplifier 11 , 12 and then supplied to the evaluation unit 10 .

The evaluation unit 10 has a subtractor 13 and an adder 14 , each of which is followed by an amplifier 15 , 16 . The outputs of the amplifiers 15 , 16 are routed to a computing unit 17 , which is part of the evaluation unit 10 . The computing unit 17 preferably consists of a microcontroller.

The output signals at the amplifiers 15 , 16 are read into the arithmetic unit 17 via analog-digital converters, not shown.

The analog-digital converters are components of the computing unit 17 . These sample and hold elements, which are also not shown, are connected upstream, so that the output signals on the amplifiers 15 , 16 are detected simultaneously in the computing unit 17 .

In the subtractor 13 the difference U _diffA = U _nahA - U _{fernA of} the output signals at the near 8 and far element 9 is formed, while in the adder 14 the sum signal U _sumA = U _fernA + U _{nahA is} formed. In this case, only a small preamplification of the output _signals U _{near A} , U _fern A takes place in the pre-amplifiers 11 , 12 , so that the output _{signals are} still below the output _{limits of} the preamplifiers 11 , 12 . The actual signal amplification takes place in the amplifiers 15 , 16 downstream of the adder 14 and the subtractor 13 .

The difference and sum _signals U _diffA and U _{sumA are} each simultaneously read into the computing unit 17 by the sample and hold elements and evaluated there.

A switching output 18 , a warning output 19 and a parameter input 20 are connected to the outputs of the computing unit 17 . Via the binary switching output 18 , the switching state of the light button 1 is output ben. The switching output 18 assumes the switching state "on" when an object 2 is in the monitoring area. Otherwise, the switching output 18 assumes the switching state "off".

Warning or fault messages can be output via the warning output 19 . Parameter values can be read into the computing unit 17 via the parameterization input 20 . These parameter values can be stored in a parameter memory 21 connected to the computing unit 17 .

To determine the switching state of the light button 1 , the sum and difference _signals U _sumA , U _{diffA are} periodically checked in succession using a sequence of evaluation _rules . The flow chart of these evaluation rules is shown in FIG. 2.

According to rule 1 , it is first checked whether the condition

U _sumA <S1

is satisfied. S1 is a threshold value with S1 <0, the amount of S1 preferably being chosen so large that it is not exceeded by the intrinsic noise or by offset voltages of the electronic components of the Lichtta sters 1 .

If the condition according to rule 1 is fulfilled, the switching output 18 assumes the switching state "off". Otherwise it is checked whether the condition according to rule 4 is fulfilled, namely whether

U _diffA <0

is. If this condition is met, the switching output 18 assumes the switching state "on". Otherwise it is checked whether the condition

U _diffA <S2

in accordance with rule 5 .

Here, S2 forms a threshold value with S2 <0, the amount of S2 being selected so that the threshold value S2 is not undershot by intrinsic noise or by offset voltages of the electronic components of light sensor 1 .

When this condition is met, the switching output 18 assumes the switching state "off". In the other case, the switching state remains unchanged at the value that was assumed before the check according to rule 5 .

In Fig. 3 shows typical waveforms of U and U _{Suma diffa} are shown as a function of object distance. The condition for the switch-on process according to rule 4 is chosen so that a transition to the switching state "on" then follows when U _nahA and U _{fernA are the} same size. It is particularly advantageous that this switching point is essentially independent of the _{degree of} reflection of the surface of an object 2 , since the signal _curves of U _diffA for different reflectances _intersect the same point at U _diffA = 0.

The switching point according to rule 5 for the transition to the switching state "off" is not exactly at U _diffA = 0 but at S2, so that here a dependency of the switching point on the reflectance of the object 2 is given. However, since the threshold is very close to zero, this dependency is low.

In addition, as a further switch-off rule 1 is provided in which not the difference signal U _Diffa but the sum signal U _Suma is evaluated. The level of this threshold is chosen so that a safe transition to the switching state was "off" both with small signal amplitudes for objects 2 with low reflectance and with a weakly reflecting background.

In Fig. 4, a second embodiment of the Lichtta sters 1 of the invention is shown. The structure of this light button 1 is essentially identical to the structure of the light button 1 according to FIG. 1. In contrast to this, the light button 1 according to FIG. 4 has, in addition to the first transmitter 4, a second transmitter light beam 3 a emitting transmitter 4 a which objects 2 are detected in the close range. For this purpose, the second transmitter 4 a is arranged between the first transmitter 4 and the receiver 6 .

The transmitters 4 , 4 a are preferably arranged in such a way that the beams of the transmitted light beams 3 , 3 a emitted by the transmitters 4 , 4 a do not overlap or only overlap within the monitoring range. Preferably, the beams of the transmitted light beams 3 , 3 a run parallel and at a short distance from each other.

The transmitters 4 , 4 a each periodically emit a sequence of transmitted light pulses. The transmitters 4 , 4 a are alternately activated by the evaluation unit 10 so that the objects 2 are alternately scanned by the transmit light pulses of the two transmitters 4 , 4 a. This is illustrated in Figures 5a and 5b. The emission of transmission light pulses from a transmitter 4 , 4 a takes place in the transmission pauses of the other transmitter 4 , 4 a.

As shown in FIGS. 5a and 5b, the second transmitter 4 a each emits a light pulse in the transmission pauses of the first transmitter 4 . In contrast, the first transmitter 4 emits two transmission light pulses P1, P2 in the transmission pauses of the second transmitter 4 a, in quick succession, the first transmission light pulse P1 having a considerably lower amplitude than the second transmission light pulse P2.

The evaluation unit 10 checks whether at least one of the output signals U _nahA and U _fernA , which are obtained when the first transmission light pulse P1 is emitted, are above at least one threshold value. In an advantageous embodiment, it is checked whether the sum of the output signals is above a threshold value. If this is the case, these output signals are used for further evaluation and the emission of the subsequent transmission light pulse P2 is suppressed. If, on the other hand, the output signals are below the threshold value, these output signals are rejected and the second transmitted light pulse P2 is emitted. Then, the output signals generated by the second transmitted light pulse are used for the further evaluation.

In principle, this method can also be applied to a number of different sen Light pulses P1, P2. . . PN with different amplitudes be stretched.

This transmission mode allows the transmission power of the transmitter 4 , 4 a to be adapted very quickly to changes in the amplitudes of the output signals as a result of a variation in the object distance and the reflectance of the object surfaces. It is particularly advantageous here that a re setting of the transmission power can be dispensed with. Such a transmission mode can also be selected for the light sensor 1 according to FIG. 1.

In order to determine the switching state of the light sensor 1 of FIG. 4 are in the evaluation unit 10, the output signals at the near-8 and point at infinity 9 U _nahA, U _REMOTE which are obtained with activated first transmitter 4, and the output signals U _NaHB, U _REMOTE at the near - 8 and remote element 9 separately registered and evaluated with activated second transmitter 4 a. For this purpose, the sum and difference signals of the output signals are again formed, whereby

U _sumA = U _nahA + U _fernA
U _diffA = U _nahA - U _fernA
U _sumB = U _near + U _farB
U _diffB = U _nearB - U _farB

is.

These sum and difference signals are in turn evaluated using a sequence of evaluation rules. The flow chart of these evaluation rules is shown in FIG. 6.

Rules 1 , 4 and 5 correspond to the evaluation rules according to FIG. 2. In addition to these rules, further rules 2 and 3 are inserted in the flow chart between rules 1 and 4 .

Accordingly, if the condition according to rule 1 is not met, the next condition is checked according to rule 2 whether

U _diffB <S3

is. S3 is a threshold value with S3 <0, the amount of S3 in the we considerably corresponds to the amount of S2 or S1.

If this condition is met, the switching output 18 assumes the switching state "on". Otherwise it is checked whether the following conditions according to rule 3 are fulfilled:

Wa <0 and U _sumB <S1 or Wb <0
in which
Wa = 0.5 U _sumA - U _sumB
Wb = 0.5 U _sumB - U _sumA

If the conditions according to rule 3 are met, the switching state of the switching output 18 remains unchanged. Otherwise, a further check according to rules 4 and 5 takes place analogously to the flow chart according to FIG. 2.

In Fig. 7 typical waveforms of U _{SUMA, SUMB} U _diffa and U, U _diffB are shown as a function of object distance. Since only objects 2 in the close range can be detected with the second transmitter 4 a, the signal curves U _sumB , U _{diffB obtained} with this transmitter 4 a _extend over a smaller distance range compared to the signal _curves U _sumA , U _diffA .

The evaluation of the signals U _sumA , U _diffA with rules 1 , 4 and 5 corresponds to the illustration in FIG. 2. In addition, the signals U _{diffB are evaluated} using _rule 2 . In addition to rule 4, rule 2 defines a second condition for the transition to the switching state "on". Rule 2 defines the condition for a safe switch-on for objects 2 in the close range. In contrast, rule 4 defines the transition to the "on" switching state for larger distances.

With the evaluation according to rule 3 , faulty switching of the light button 1 can be avoided if the transmission light emitted by the transmitters 4 , 4 a 3 strikes an object edge.

This case occurs in particular when an object 2 enters the beam path of the light sensor 1 transversely to the beam axis of the transmitted light beams 3 .

If, for example, an object 2 enters the beam path laterally at a distance just above the scanning distance, this must not lead to a transition to the switching state "on", since the object 2 is beyond the scanning distance. However, if the object 2 enters the beam path from the side on which the transmitted light rays 3 falling on the object edge increasingly strike the near element 8 , then a transition to the switching state "on" occurs when the object edge is detected. As soon as the object 2 is completely in the beam path of the transmitted light beams 3 , the transmitted light beams 3 hit the object 2 completely , so that the proportion of the received light beams 5 falling on the remote element 9 predominates again and the switching output 18 returns to the correct switching state "off" changes.

Rule 3 suppresses the short-term switching state changes which are erroneously caused by the detection of object edges. An object edge is present when the condition according to rule 3 is met. In this case, the original switching state is retained to suppress incorrect switching.

The mode of operation of rule 3 is explained in more detail in FIG. 8. The upper _{diagram shows} the course of the sum _signals U _sumA , U _sumB as a function of the path q that an object 2 _traverses across the beam path of the light sensor 1 .

Since the transmitters 4 , 4 a are laterally offset in the light sensor 1 , an penetrating object 2 is detected by the two transmitters 4 , 4 a at different times or in different object positions. As a result, the waveforms U _sumA and U _{sumB are} offset from one another. This fact is used for the detection of the object edges.

At the object position q ₁ , the signal U _{sumA has} already reached its maximum, while U _{sumB is} still below the threshold value S1. This means that the object 2 is detected by the first transmitter 4 , but not by the second transmitter 4 a.

Accordingly, a zero crossing is obtained by forming the difference Wa = 0.5 U _sumA - U _sumB in the vicinity of q ₁ at q _{1 '} . The area between q ₁ and q _{1 '} defines with great accuracy the area within which the object edge of the object 2 immersed in the beam path is detected.

Analogously, the object edge of the object 2 emerging from the beam path between q ₂ and q _{2 'is} detected by Wb.

The light sensor 1 shown in Fig. 4 is particularly suitable for safety applications. This is mainly due to the fact that this light sensor 1 has two transmitters 4 , 4 a.

A suitable combination of the evaluation rules according to FIG. 6 can be used to check whether one of the transmitters 4 , 4 a has failed.

The conditions according to rule 3 are particularly suitable for checking the failure of a transmitter 4 , 4 a. If, for example, there is an object 2 in the beam path within the scanning range and the second transmitter 4 a has failed, the conditions according to rule 3 are met.

Then it is additionally checked during a plurality of runs of the evaluation rules according to FIG. 6 whether a certain number of switching state changes is registered. If this is the case, there is almost certainly no detection of a single object edge, so that the conditions according to rule 3 have only been met due to a failed transmitter 4 a.

If a failure of a transmitter 4 , 4 a was found, a fault message is output via the warning output 19 .

Claims (13)

1. Method for operating a light scanner for the detection of objects in a surveillance area with at least one transmitter light-emitting transmitter and one receiver-receiving light beam with a local element and a remote element, the output signals U _nahA and U _{fernA of} which are fed to an evaluation unit which _controls a binary switching output , which assumes the switching state "on" for an object detection and otherwise assumes the switching state "off", characterized in that in the evaluation unit ( 10 ) the difference U _diffA = U _nahA - U _fernA and the sum U _sumA = U _nahA + U _{fernA of} the output _{signals is} formed that the switching output ( 18 ) assumes the switching state "on" if the condition U _diffA <0 it is met, and that the switching output ( 18 ) assumes the switching state "off" if the Condition U _diffA <S2 is fulfilled, with S2 (S2 <0) forming a threshold value, or the sum U _{sumA being} a further one a threshold value S1 (S1 <0) exceeds. 2. The method according to claim 1, characterized in that in the evaluation unit ( 10 ) U _sumA and U _{diffA are} periodically checked in succession with a sequence of evaluation _rules , that the switching output ( 18 ) assumes the switching state "off" if a first Condition (rule 1 )
U _sumA <S1
is fulfilled, whereby S1 (S1 <0) forms a first threshold value, otherwise it is checked if another condition (rule 4 )
U _diffA <0
is fulfilled, the switching output ( 18 ) assuming the switching state "on" when this condition is met, and otherwise a check is carried out to determine whether another condition (rule 5 )
U _diffA <S2
is satisfied that when this condition is met, the switching output ( 18 ) assumes the switching state "off" and otherwise the switching state remains unchanged. 3. The method according to claim 2, characterized in that between the transmitter ( 4 ) and the receiver ( 6 ) a second transmitter ( 4 a) is arranged, the transmitter ( 4 , 4 a) alternately activated by the evaluation unit ( 10 ) and emit at least one transmission light pulse P1, P2 during their activation, that in the evaluation unit ( 10 ) the difference U _diffB = U _nahB - U _fernB and the sum U _sumB = U _nahB + U _{fernB of} the by the second transmitter ( 4 a ) generated output _signals U _nahB , U _fernB on the near ( 8 ) and _{far element} ( 9 ), that if rule 1 is not met , the evaluation unit ( 10 ) checks whether another condition (rule 2 )
U _diffB <S3
is fulfilled, with S3 (S3 <0) forming a third threshold value that when this condition is fulfilled the switching output ( 18 ) assumes the switching state "on" and that otherwise rule 4 is checked in the evaluation unit ( 10 ). 4. The method according to claim 3, characterized in that the linkage of the following conditions (rule 3 ) is checked for the detection of object edges if rule 2 is not met in the evaluation unit ( 10 ):
0.5 U _sumA - U _sumB <0 and U _sumB <S1 or 0.5 U _sumB - U _sumA <0,
that when these conditions are met, the switching state of the switching output ( 18 ) remains unchanged, and that if these conditions are not met, rule 4 is checked. 5. The method according to claim 4, characterized in that to check the failure of a transmitter ( 4 , 4 a) is checked whether the condition according to rule 3 is met, and whether a predetermined when activating the transmitter ( 4 , 4 a) Number of switching status changes is registered. 6. light scanner with at least one transmitting light beam emitting Sen and a receiving light receiving receiver with egg nem near element and a remote element, the transmitter and the receiver being arranged next to each other at a distance, for carrying out the method according to any one of claims 1-5, thereby ge indicates that the output _signals U _nahA , U _fernA present at the output of the near ( 8 ) and _{far element} ( 9 ) are _{fed to} an evaluation unit ( 10 ) with an adder ( 14 ) and a subtractor ( 13 ), the adder ( 14 ) the sum U _sumA and in the subtractor ( 13 ) the difference U _{diffA of} the output _signals U _nahA and U _{fernA is} formed, and the values of U _sumA and U _{diffA are compared} with threshold values stored in the evaluation unit ( 10 ), and that in dependency this comparison, a binary switching output ( 18 ) connected to the evaluation unit ( 10 ) assumes a predetermined switching state. 7. Light sensor according to claim 6, characterized in that it has a second transmitter ( 4 a), which is arranged between the first transmitter ( 4 ) and the receiver ( 6 ) so that the beams of the transmitters ( 4 , 4th a) emitted transmission light beams ( 3 , 3 a) run at a predetermined distance from each other in the monitoring area. 8. Light scanner according to claim 7, characterized in that the optical axes of the transmitted light beams ( 3 ) of the transmitter ( 4 , 4 a) run parallel to one another. 9. Light sensor according to one of claims 7 or 8, characterized in that the transmitters ( 4 , 4 a) each periodically emit a sequence of transmitted light pulses, the transmitters ( 4 , 4 a) being activated alternately by the evaluation unit ( 10 ) . 10. Light sensor according to one of claims 6-9, characterized in that the transmitter ( 4 ) emits two transmitted light pulses within a period, the amplitude of the first transmitted light pulse P1 signifi cant smaller than the amplitude of the second transmitted light pulse P2 subsequently emitted. 11. A light sensor according to claim 10, characterized in that the emission of the second transmission light pulse P2 emitted by the transmitter ( 4 , 4 a) is suppressed by the evaluation unit ( 10 ) if the amplitudes of U _{near B} and obtained during the emission of the first transmission light pulse P1 / or U _{fernB exceed} a predetermined threshold. 12. Light sensor according to one of claims 6-11, characterized in that the position of the receiver ( 6 ) is adjustable transversely to its optical axis for adjusting the scanning distance. 13. Light sensor according to one of claims 6-12, characterized in that it has a warning output ( 19 ).