DE10104418A1 - Distance sensor has shared reference beat frequency paths reduces equipment errors - Google Patents

Abstract

A distance sensor (1) transmits a frequency modulated light beam (3) to an object (2) for comparison of the reflected (5) beam with a reference beam (10) modulated at different frequency in receivers (6, 12) with beat frequency comparison by counting (23).

Description

The invention relates to a distance sensor according to the preamble of the An saying 1.

Such a distance sensor has a sensor that emits transmitted light rays and a receiver receiving receiving light beams. The sen Light rays strike a target located at a certain object distance Object and are guided from there to the receiver as reception light beams. To determine the distance, the transit time of the transmitted light beams becomes the object and evaluated back to the distance sensor.

The distance measurement is based on the phase measurement principle. For this purpose, the transmitted light beams are amplitude modulated with a frequency f ₁ . The amplitude modulation of the transmitted light beams takes place by means of an oscillator arranged in front of the transmitter.

Another oscillator generates a reference signal with the frequency f ₂ , this frequency being almost the same size as the frequency f ₁ . The oscillators are synchronized via a synchronization circuit. The phase difference between the received signals at the output of the receiver, modulated with the frequency f ₁ , and the reference signal thus provides a measure of the object distance of the object measured with the transmitted light beams.

To determine the phase difference, the received signals and the reference signals are fed to a mixer and superimposed there. The output signal of the mixer is fed to a demodulator. The frequency of the output signal at the demodulator corresponds to the difference Δf from the frequencies f ₁ and f ₂ .

A start-stop counter is designed to determine the phase difference used counter. The synchronization circuit sends a synchronizer sationsimpuls to the counter, as soon as that at the outputs of the oscillators generated signals are in phase. Through this synchronization pulse the counter is started. The counter is stopped again as soon as the first following the synchronization pulse the zero crossing of the output of the Demodulator pending output signal is detected. That determined Time interval provides a measure of the phase difference between the received signal and reference signal and thus a measure of the transit time of the transmitted light beams to the object.

A disadvantage of this measurement method is that the size of the counter determined time interval not only by the transit time of the transmitted light beams is determined.

The size of the time interval also depends on the electrical signal run times in different electrical components of the distance sensor. This includes in particular one necessary for the operation of the transmitter Sender regulation as well as the receiver.

These electrical signal runs are used to eliminate such measurement errors times recorded in correction tables and when converting the determined Time interval recorded in the corresponding distance value as a correction value.

However, the signal propagation times are due to environmental influences, such as Ambient temperature example, not constant. Furthermore arise due to component tolerances of the corresponding electrical components also sample-dependent fluctuations in signal transit times. such  Fluctuations in the signal transit times can not with the correction values or only recorded incompletely so that when performing phase measurement an undesirably large, systematic measurement error ver remains.

The invention has for its object a distance sensor of the beginning of the type mentioned so that the most accurate possible distance determination is guaranteed.

The features of claim 1 are provided to achieve this object. Advantageous embodiments and expedient further developments of the Erfin tion are described in the subclaims.

The distance sensor according to the invention has a transmitter light emitting transmitter and a receiver light receiver. The amplitude-modulated transmission light beams with a frequency f _{1 are} guided over a measurement path to an object arranged in an object distance and reflected as reception light beams on the latter and guided to the receiver. The distance sensor also has a reference transmitter and a reference receiver, reference transmitter light beams emitted by the reference transmitter and having an amplitude f ₂ amplitude-modulated are guided as reference reception light beams to the receiver and to the reference receiver over a reference path in each case. Some of the transmitted light beams are directed to the reference receiver via a further reference path. At the output of the receiver beat measurement signals and at the output of the reference receiver beat reference signals are each with a beat frequency Δf = | f ₁ - f ₂ | generated. To determine the object distance, means are provided for determining the phase difference between the beat measurement signals and the beat reference signals.

Since the transmitter light beams and the reference light beams in sym metrically both on the receiver and on the reference receiver are carried out in the evaluation of the output of the recipient and the reference receiver pending beat measurement signals and Beat reference signals component-related time differences in the electri components of the transmitter and the reference transmitter and their respective circuits completely eliminated and therefore do not lead to systematic Measurement errors.

This leads to a high measuring accuracy of the distance sensor according to the invention sors.

It is also advantageous that synchronization for the modulation of the transmitter and the reference transmitter is not necessary. To modulate the Transmitter and the reference transmitter is preferably a separate oscillator used. Since synchronization of the oscillators is not necessary, a corresponding connection for this is not required.

Due to the optical superimposition of the received light beams and the reference received light beams in the receiver and in the reference receiver, signals are generated at their outputs, which vary according to the modulations of the received light beams and the reference received light beams with a carrier frequency f = ½ (f ₁ + f ₂ ). In addition, the signal amplitudes vary with the beat frequency Δf = | f ₁ - f ₂ |.

In this case, in the involutes varying with the beat frequency Δf Signal amplitudes the transit time-related phase difference of the transmitted light beams contain. Only the one with the beat frequency is used to determine the distance frequency varying low frequency portion of the signals used.  

A major advantage here is that component-related runtime fluctuations in the receiver and in the reference receiver by the factor Δf / f less on the low-frequency signal component than on the high-frequency, with of the carrier frequency varying signal component are transmitted. Here lies the carrier frequency f typically in the MHz range, while the Schwe exercise frequency is in the kHz range.

By evaluating the never varying with the beat frequency the frequency beat measurement signals and beat reference signals a transformation of the phase measurement into the low frequency range. Consequently the influence of component fluctuations of the receiving electrical Components reduced by the factor Δf / f and the measuring accuracy of the Distance sensor increased accordingly.

This advantage is reinforced by the fact that, due to the low excess transfer of the runtime fluctuations of the signals to the measurement result narrowband gain in the receiver and in the reference receiver generated signals is possible, whereby a high signal-noise Ratio is obtained.

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

Fig. 1: Block diagram of an embodiment of the distance sensor according to the invention.

FIG. 2a: timing diagram of the radiation to the reference receiver of the distance sensor impinging received light beams and reference received light.

FIG. 2b: timing diagram of the receiver of the distance sensor auftref fenden received light beams and reference received light beams.

Fig. 2c: Time diagram of the signals at the output of the reference receiver.

Fig. 2d: timing diagram of the signals at the output of the receiver.

Fig. 1 shows the basic structure of the distance sensor 1 of the invention. The distance sensor 1 is integrated in a housing, not shown. To determine the object distance of an object 2 arranged at a distance from the distance sensor 1 , a transmitting light beam 3 emitting transmitter 4 and a receiving light beam 5 receiving receiver 6 are provided.

The transmission light beams 3 emitted by the transmitter 4 are guided through a transmission optics 7 and strike the object 2 . From the object 2 , the transmitted light rays 3 are reflected back as received light rays 5 and guided to the receiver 6 via an optical receiver 8 . The transmitting optics 7 and the receiving optics 8 are integrated in the wall of the housing and each consist of a lens.

In addition to the transmitter 4 and the receiver 6 , the distance sensor 1 has a reference transmitter 10 emitting reference transmission light beams 9 and a reference receiver 12 receiving reference reception light beams 11 .

The transmitter 4 and the reference transmitter 10 each consist of a laser diode, which are preferably of identical design.

The receiver 6 and the reference receiver 12 each consist of a photodiode, the photodiodes also being of identical design.

As can be seen from FIG. 1, the two transmitters 4 are arranged downstream beam deflecting means. The beam deflection means comprise two beam splitter mirrors 13 , 14 and two deflection mirrors 15 , 16 .

A first beam splitter mirror 13 is arranged downstream of the transmitter 4 . The part of the transmitted light beams 3 passing through the beam splitter mirror 13 is guided to the object 2 , the path covered by the transmitted light beams 3 forming a measurement path. The reflected reflected from the object 2 Emp light rays 5 are guided back to the receiver 6 via the receiving optics 8 and a first order steering mirror 15 . The part of the transmitted light beams 3 reflected at the beam splitter mirror 13 is guided to the reference receiver 12 along a first reference path.

A second beam splitter mirror 14 is arranged downstream of the reference transmitter 10 . The portion of the transmitted light beams 3 reflected on this beam splitter mirror 14 is guided along a second reference path to the reference receiver 12 . The part of the transmitted light beams 3 passing through the beam splitter mirror 14 is guided to the receiver 6 via a second deflecting mirror 16 along a third reference path.

The distance of object 2 is determined according to the phase measurement principle. To carry out the phase measurement, the light beams 3 emitted by the transmitter 4 are amplitude-modulated with a frequency f ₁ . The transmission light beams 3 are modulated by means of a first oscillator 17 connected to the transmitter 4 .

The reference transmission light beams 9 emitted by the reference transmitter 10 are amplitude-modulated with a frequency f ₂ . The modulation takes place by means of a second oscillator 18 arranged upstream of the reference transmitter 10 . The oscillator 18 is operated asynchronously to the first oscillator 17 .

The frequencies f ₁ and f ₂ are in the megahertz range and are almost of the same size, the frequencies being chosen such that the frequency difference Δf = | f ₁ - f ₂ | is preferably in the kHz range.

To carry out the distance measurement, the transit time of the transmitted light beams 3 is recorded over the measurement path. Essentially, the phase of the transmitted light rays 3 arriving as receiver light rays 5 on the receiver 6 is compared with a defined reference.

To form a reference, the transmission light beams 3 or reference transmission light beams 9 guided over the three reference paths are used, which strike the receiver 6 or reference receiver 12 as reception light beams 5 or reference reception light beams 11 . The decisive factor here is that the lengths of the reference sections have fixed, time-invariable conditions. The reference sections run completely inside the housing.

On the receiver 6 and the reference receiver 12 there is an optical superimposition of the incident received light beams 5 and reference received light beams 11 .

The signals present at the output of the receiver 6 and the reference receiver 12 are each fed to an amplifier 19 , 19 '. At the output of the amplifier 19 , 19 ', a demodulator 20 , 20 ' is connected, which is arranged downstream of a filter 21 , 21 '. A Schmitt trigger 22 , 22 'is connected to the output of the filter 21 , 21 '.

These components downstream of the receiver 6 and the reference receiver 12 form two evaluation channels, which are preferably formed identically.

The outputs of the Schmitt trigger 22 , 22 'are connected to a counter 23 formed as a start-stop counter, the output of which is connected to an evaluation unit 24 . The evaluation unit 24 consists of a micro controller or the like.

The counter 23 is also connected via a feed line to the oscillator 18 assigned to the reference transmitter 10 . This oscillator 18 is used to clock the counter 23 .

The operation of the distance sensor 1 according to the invention is explained in the following using the time diagrams according to FIG. 2.

Fig. 2a shows the time course of the reference receiver 12 amplitude modulated impinging received light beams 5 and Referenzemp fang light beams 11. The frequency f ₁ modulated with the amplitude of the ge via one of the reference paths for reference receiver 12 led received light beams 5 is indicated by U _ref1. The amplitude modulated with the frequency f _{2 of} the reference _{reception light beams} 11 guided to the reference _receiver 12 via a further reference _path is designated U _ref2 . In the example shown in FIG. 2a, the maximum value of the amplitude U _ref1 is greater than the maximum value U _ref2 . For the sake of clarity, it is assumed in the present example that the phase difference Δϕ between U _ref2 and U _ref1 Δϕ _ref = 0.

FIG. 2b shows the time course of the light incident on the receiver 6 the amplitude modulated received light beams 5 and reference received light beams 11. In this case, U ₁ denotes the amplitude of the received light beams 5, which is modulated with the frequency f ₁ , of the received light beams 5 guided over the measuring section to the object 2 .

The modulated with the frequency f ₂ amplitude of the reference reception light beams 11 guided over a reference path to the receiver 6 is marked with U ₂ be. The maximum amplitude of U ₁ is again greater than the maximum amplitude of U _{2 in} accordance with the amplitude conditions according to FIG. 2a. Due to the transit time of the transmitted light beams 3 to the object 2 , the amplitude U ₁ with respect to U ₂ is phase-delayed by Δϕ, the phase shift Δϕ = 180 ° in the present example.

The signal U _{Gref shown} in FIG. 2 c is obtained by superimposing the received light beams 5 and the reference light 11 on the reference receiver 12 . Correspondingly, the signal U _G shown in FIG. 2d is obtained by superimposing the received light rays 5 and the reference received light rays 11 on the receiver 6 .

The signals U _G and U _Gref each oscillate with a high-frequency carrier frequency f, which is given by the relationship f = ½ (f ₁ + f ₂ ). In addition, a beat is created by the respective received light beams 5 and reference receive light beams 11 superimposed on the receiver 6 and reference receiver 12 , so that the amplitude maxima of U _G and U _Gref with a beat frequency Δf = | f ₁ - f ₂ | vary.

The transit time of the transmitted light beams 3 to the object 2 and thus the object distance are determined from the low-frequency beat component of the signals U _G and U _Gref .

For this purpose, the signals U _G , U _Gref are first amplified in the associated amplifier 19 , 19 'and then demodulated in the demodulator 20 , 20 '. The high-frequency component of the signals U _G , U _{Gref is} eliminated, so that low-frequency beat signals are obtained at the output of the demodulator 20 , 20 '.

At the output of the demodulator 20 'assigned to the reference receiver 12 , a beat reference signal is obtained, the course of which corresponds to the course of the envelope E _{ref of} the signal U _Gref shown in FIG. 2c.

Correspondingly, a beat measurement signal is obtained on the demodulator 20 assigned to the receiver 6 , the time course of which corresponds to the course of the envelope E of the signal U _G shown in FIG. 2d.

Since the beat measurement signal and the beat reference signal have arisen in a symmetrical manner by a beat of the receiving light beams 5 and the reference receiver 12 impinging on the receiver 6 and the reference receiver beam 11 , the phase shift between the beat measurement signal and the beat reference signal corresponds to the phase shift of the signals U ₁ and U ₂ and thus the transit time delay, which is caused by the transit time of the transmitted light beams 3 to object 2 and back to distance sensor 1 .

The Schmitt triggers 22 , 22 'and the counter 23 serve as means for determining the phase difference. By means of the reference receiver 12 downstream of the Schmitt trigger 22 ', the zero crossings of the Schwebungsreferenzsig Nals be detected. Correspondingly, the zero crossings of the beat measurement signal are detected by means of the Schmitt trigger 22 arranged after the receiver 6 .

These measured variables form the input variables for the counter 23 . The counter 23 is then used to determine the time difference between the time of a zero crossing of the beat reference signal, which is denoted by t ₀ in FIG. 2c, and the time of the subsequent zero crossing of the beat measurement signal, which is denoted by t ₁ in FIG. 2d , This time difference provides a direct measure of the phase difference between the beat measurement signal and the beat reference signal. In the evaluation unit 24 , the object distance of the object 2 is calculated from the phase difference.

LIST OF REFERENCE NUMBERS

1

Distance sensor

2

object

3

Transmitted light beams

4

Channel

5

Receiving light rays

6

receiver

7

transmission optics

8th

receiving optics

9

Reference end beams

10

reference stations

11

Reference received light beams

12

reference receiver

13

Beamsplitter mirror

14

Beamsplitter mirror

15

deflecting

16

deflecting

17

oscillator

18

oscillator

19

amplifier

19

'Amplifier

20

demodulator

20

'Demodulator

21

filter

21

'Filter

22

Schmitt trigger

22

'Schmitt trigger

23

counter

24

evaluation

Claims (16)

1.Distance sensor with a transmitter light-emitting transmitter and a receiving light-receiving receiver, the amplitude-modulated transmitter light beams having a frequency f _{1 being} guided over a measuring distance to an object arranged in an object distance and being reflected as receiving light beams thereon and guided to the receiver, and wherein from the propagation time of the transmitted light beams and received light beams, the object distance is determined, characterized in that a reference transmitter (10) and a reference receiver (12) are provided, wherein light emitted from the reference transmitter (10), with ei ner frequency f ₂ amplitude-modulated reference light beams (9 ) ü via in each case one reference path are guided as reference received light beams (11) to the receiver (6) and to the reference receiver (12), and part of the transmitted light beams is guided (3) via a further reference distance to the reference receiver (12) that, at the Output of the receiver ( 6 ) beat measurement signals and at the output of the reference receiver ( 12 ) beat reference signals each with a beat frequency Δf = | f ₁ - f ₂ | are generated, and that means for determining the object distance are provided for determining the phase difference between the beat measurement signals and the beat reference signals. 2. Distance sensor according to claim 1, characterized in that for the generation of the beat measurement signals and the beat reference signals the receiver ( 6 ) and the reference receiver ( 12 ) each have a demodulator ( 20 , 20 ') downstream. 3. Distance sensor according to claim 2, characterized in that the means for determining the phase difference have two Schmitt triggers ( 22 , 22 ') and a counter ( 23 ), each with a Schmitt trigger ( 22 , 22 ') the beat measurement signal and the beat reference signal is fed, and the outputs of the Schmitt triggers ( 22 , 22 ') are fed to the counter ( 23 ). 4. Distance sensor according to claim 3, characterized in that by means of the Schmitt trigger ( 22 , 22 ') the zero crossings of the Schwebungsmesssig signals and the beat reference signal are determined. 5. Distance sensor according to claim 4, characterized in that the time difference between a zero crossing of the beat reference signal and the subsequent zero crossing of the beat measurement signal is determined by means of the counter ( 23 ). 6. Distance sensor according to one of claims 1-5, characterized in that this is integrated in a housing in which the reference stre run completely. 7. Distance sensor according to claim 6, characterized in that stationary beam deflecting means are provided along the reference paths for deflecting the transmitted light beams ( 3 ) and the reference transmitted light beams ( 9 ). 8. Distance sensor according to claim 7, characterized in that the beam deflection means of deflection mirrors ( 15 , 16 ), prisms and / or beam splitter mirrors ( 13 , 14 ) are formed. 9. Distance sensor according to one of claims 1-8, characterized in that the frequencies f ₁ and f _{2 are} in the MHz range, and that the beat frequency Δf is in the kHz range. 10. Distance sensor according to claim 9, characterized in that an oscillator ( 17 , 18 ) is provided for generating the modulation of the transmitted light beams ( 3 ) and the reference light beams ( 9 ). 11. Distance sensor according to claim 10, characterized in that the Os zillatoren ( 17 , 18 ) are operated asynchronously. 12. Distance sensor according to one of claims 1-11, characterized in that the transmitter ( 4 ) and the reference transmitter ( 10 ) consist of identically formed laser diodes. 13. Distance sensor according to one of claims 1-12, characterized in that the receiver ( 6 ) and the reference receiver ( 12 ) consist of identically formed photodiodes. 14. Distance sensor according to claim 13, characterized in that the receiver ( 6 ) and the reference receiver ( 12 ) are each followed by an amplifier ( 19 , 19 '), the amplifiers ( 19 , 19 ') being formed identically. 15. Distance sensor according to one of claims 2-14, characterized in that the demodulators ( 20 , 20 ') and Schmitt triggers ( 22 , 22 '), which are the receiver ( 6 ) and the reference receiver ( 12 ) nachgeord net , are identical. 16. Distance sensor according to one of claims 3-15, characterized in that the counter ( 23 ) is connected to an evaluation unit ( 24 ) in which the object distance is determined from the output signals of the counter ( 23 ).