DE10350489A1 - Optical sensor employs phase shifting as a means to determine the distances of objects - Google Patents

Abstract

The invention concerns an optical sensor (1) to determine the distances of objects (2) in a surveillance area. The optical sensor employs a transmitter (4), which emits light beams, an amplitude modulation unit, a receiver (6), which receives the light beams (5) reflected by the objects and a data interpretation assembly, which determines the phase shift of the emitted light beams to the received light beams as a measure for the distance of the object. Furthermore, an electrical reference distance is introduced to generate a reference-phase reference for the registered phase shifts in the data interpretation assembly.

Description

The The invention relates to an optical sensor.

such Optical sensors are used to determine distances of objects in a surveillance area. The distance determination is carried out according to the phase difference method. The optical sensor has for this purpose a transmitting light beam emitting Transmitter and a receiving light beams receiving receiver. To carry out the phase measurement becomes amplitude modulation to the transmitted light beams imprinted with a given frequency. The reflected from an object Receiving light beams have a corresponding amplitude modulation on, but this is according to the light transit time of the optical Sensor to the object and back to the optical sensor out of phase to the amplitude modulation of Transmitted light beams. In an evaluation unit, the phase difference the transmitted light beams and the received light beams. Out this phase difference then becomes the distance of the object to the optical Sensor calculated.

From the DE 198 11 550 A1 For example, such an optical sensor operating according to the phase difference method is known. This optical sensor has an existing digital circuit components circuit arrangement for generating frequency signals that are needed to perform the phase measurement.

The with the transmitter and receiver conducted Distance measurements are each related to a reference measurement, for this purpose, the transmitted light beams over an optical reference path to the recipient guided are. The optical reference path has an optical switch on, by means of which in a predetermined clock the transmitted light beams to carry out of reference measurements to the receiver supplied become.

adversely Here is the high design complexity of the optical sensor to carry out the Distance measurement. In particular, it requires continuously for each distance measurement to be performed optical Reference measurement undesirable high effort.

Of the Invention is the object of an optical sensor of the aforementioned type such that with this at as low as possible constructive effort an accurate distance determination is made possible.

to solution This object, the features of claim 1 are provided. advantageous embodiments and appropriate training The invention are described in the subclaims.

Of the according to the invention optical Sensor is used to determine distances of objects in a surveillance area and has a transmitter emitting light-emitting emitters. Likewise includes the optical sensor is a modulation unit, by means of which the transmitted light beams be amplitude modulated. Furthermore, the inventive optical Sensor a receiver for receiving the reflected as received light beams from an object Transmitted light rays and an evaluation unit, in which the Phase shift of the transmitted light beams and the received light beams as a measure of the object distance is determined. The inventive optical Sensor finally includes an electrical reference path for generating a reference phase reference for the in the evaluation registered phase shifts.

One significant advantage of the optical sensor according to the invention consists in that by the electrical referencing of the phase measurement optical components to form an optical reference path and thus necessary Means for electro-optical conversion of signals for carrying out a Reference measurement omitted.

With the electrical according to the invention Referencing will thus be a significant component reduction and thus a corresponding cost reduction in the production of the achieved optical sensor.

In a particularly advantageous embodiment, to carry out the phase measurement, the transmitted light beams and thus also the received light beams reflected back from an object are amplitude-modulated with a high-frequency modulation frequency f ₁ , typically in the MHz range. For this purpose, a first frequency generator is used. By means of a second frequency generator, a clock signal is generated at a frequency f ₂ , wherein this frequency is shifted with respect to the frequency f ₁ by an intermediate frequency .DELTA.f, wherein this intermediate frequency .DELTA.f is typically in the kHz range. The received signals modulated by the incident receiving light beams in the receiver and modulated with the modulation frequency f ₁ are mixed with the clock signal, thereby generating a measuring signal at the intermediate frequency Δf.

In this case, the electrical reference path essentially consists of a mixer in which a gene in the first frequency generator riertes frequency signal having the frequency f ₁ to the clock signal having the frequency f ₂ is mixed. As a result, a reference signal with the intermediate frequency .DELTA.f is generated, which is read in to the evaluation unit of the optical sensor for referencing the measurement signal.

The thus formed electrical reference path has an extremely simple and inexpensive construction. Since the reference signal is generated from the same frequencies f ₁ and f ₂ , which are also used to generate the measurement signal, also errors due to drifting in the generation of the frequencies f ₁ and f _{2 are} eliminated.

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

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

2 : Time course of the optical sensor according to 1 generated measurement signal and reference signal.

3 : Schematic representation of the determination of the phase shift of the measurement signal according to 2 ,

1 shows an embodiment of an operating according to the phase measurement principle optical sensor 1 for determining distances of objects 2 within a surveillance area. The optical sensor 1 has a transmitted light rays 3 emissive transmitter 4 and a receiving light beams 5 receiving recipient 6 on. The transmitter 4 consists of a laser diode, which is connected to a voltage U _d and _is supplied with a current I _d . The recipient 6 in the present case consists of an avalanche photodiode connected to a resistor 7 is connected and supplied with a voltage U _r .

To carry out the phase measurement becomes the transmitted light beams 3 imprinted an amplitude modulation with a frequency f ₁ . The of an object 2 reflected back receive light beams 5 have an amplitude modulation with the same frequency f ₁ . According to the object distance of the respective object 2 show those at the receiver 6 incident receiving light beams 5 a phase shift φ with respect to that of the transmitter 4 emitted transmitted light beams 3 on. From this phase shift φ, the object distance is calculated and as the output from the optical sensor 1 output.

How out 1 can be seen, the optical sensor 1 one to an oscillator 8th connected first frequency generator 9 on, the modulation unit for the modulation of the transmitted light beams 3 forms. Furthermore, to the oscillator 8th a second frequency generator 10 connected.

The first and second frequency generator 9 . 10 consist of digital components and have a substantially corresponding structure. In particular, in the frequency generators 9 . 10 Dividers integrated, by means of which by dividing the in the oscillator 8th generated fundamental frequency f ₀ defined and a unique reference to this fundamental frequency f ₀ having frequency signals are generated.

By dividing the fundamental frequency f _{0 of} the oscillator 8th is in the first frequency generator 9 generates a frequency signal with the frequency f ₁ . This frequency signal becomes the modulation of the transmitted light beams 3 over a capacitor 11 in the transmitter 4 fed.

The second frequency generator 10 is via a control line S1 with the first frequency generator 9 connected. About this control line S1 is a in the first frequency generator 9 generated frequency signal with the frequency f _V in the second frequency generator 10 read. The frequency f _V specifies a clock by means of which the frequency signal f _{1 is switched} on, as a result of which the clock signal with the frequency f _{2 is} generated. In this case, the frequency f ₂ assumes the value f ₂ = f ₁ + Δf, that is, the frequency f ₂ is shifted with respect to the frequency f ₁ of an intermediate frequency .DELTA.f.

That in the second frequency generator 10 generated clock signal with the frequency f ₂ is via another capacitor 12 in the receiver 6 coupled. In the trained as an avalanche photodiode receiver 6 Thus, a mixture of the received by the light beams 5 generated receive signals, which are modulated at the frequency f ₁ , and the clock signal at the frequency f ₂ . Through this signal mixing and subsequent filtering in a receiver 6 downstream low-pass filter 13 a low-frequency measurement signal M is generated with the intermediate frequency .DELTA.f, which is the phase shift φ of the transmitted light beams 3 and the receiving light beams 5 and thus contains the information about the object distance. To evaluate the measurement signal M, this is read into an evaluation unit via an amplifier stage (not shown). The evaluation unit consists in the present case of an analog-to-digital converter 14 and a downstream computer unit 15 , which is formed by a microcontroller or the like.

In an alternative embodiment can the recipient 6 be formed by a PIN diode. In this case, the mixing of the received signals and the clock signal takes place in a separate mixer (not shown) belonging to the receiver 6 is subordinate.

In the present case, to generate the measurement signal M, a frequency f ₁ for the modulation of the transmitted light beams 3 and an associated clock signal having the frequency f ₂ used. To increase the uniqueness range of the phase measurement and thus the distance measuring range of the optical sensor 1 can the transmitted light beams 3 be modulated with several different modulation frequencies, in which case each modulation frequency is assigned a separate clock signal.

For referencing the measurement signal M, the optical sensor 1 an electrical reference path. The electrical reference path has a mixer 16 on which in the first frequency generator 9 generated frequency signal with the frequency f ₁ and the clock signal with the frequency f ₂ are performed. By mixing these two signals in the mixer 16 and a subsequent filtering in a mixer 16 downstream low-pass filter 17 , which is also part of the electrical reference path, a reference signal R is generated at the intermediate frequency .DELTA.f. This reference signal R is used for referencing the measurement signal M in the analog-to-digital converter 14 read. The low pass filter 17 can in principle be followed by an amplifier stage, not shown, for amplifying the reference signal R.

In the analog-to-digital converter 14 there is a sampling and digital quantization of the measurement signal M and the reference signal R. This is the analog-to-digital converter 14 via a further control line S2 to the second frequency generator 10 connected. One in the second frequency generator 10 generated frequency signal with the frequency f _A is via this control line S2 in the analog-to-digital converter 14 read. The frequency f _A , in turn, from the fundamental frequency f _{0 of} the oscillator 8th is derived, predetermines the sampling frequency in the sampling of the measurement signal M and the reference signal R.

Since thus all frequencies f ₁ , f ₂ , f _V and f _A of the same fundamental frequency f _{0 of} the oscillator 8th are derived, a defined reference of the measurement signal M to the reference signal R is obtained, so that a unique reference point in the referencing of the measurement signal M is given to the reference signal R.

In the present case, the frequencies f _V and f _{A are} chosen identical. In principle, the frequency f _{A can be} an integer multiple of the frequency f _A.

2 shows the time course of the measurement signal M and the reference signal R. The measurement signal M and the reference signal R, which were generated by the mixture of signals with the frequencies f ₁ and f ₂ and subsequent low-pass filtering, each have a sinusoidal shape. As in 2 illustrated, the measurement signal M, the phase shift φ, which forms a measure of the respective object distance. The reference signal R serves as a reference point for the phase shift φ of the measuring signal M.

In the analog-to-digital converter 14 the phase shift φ of the measuring signal M is determined by multiple sampling of the amplitude curve of the sinusoidal measuring signal M, whereafter the phase shift φ in the computer unit 15 the respective distance value is calculated and via an output, not shown, of the optical sensor 1 is output as the output size.

3 schematically shows the sampling of a period of the sinusoidal measuring signal M with a sampling group with four samples. In general, the number of samples within a period of the measurement signal M is determined by the sampling frequency f _A. The right diagram shows the time course of the sinusoidal measurement signal M with the phase shift φ. The amplitude of the measurement signal M is designated by U. The left diagram represents the corresponding vector diagram for the measurement signal M in the complex plane, where Im forms the imaginary part and Re the real part of the corresponding complex function for the measurement signal M. The fact that the sinusoidal measurement signal M is shifted by the phase shift φ with respect to the zero point as a reference point, results from the same phase shift φ in the complex function Z in the vector diagram according to the relationship Z = | Z | e -jφ where j = √ -1 ,

Accordingly, the phase shift φ is given according to the relationship

In order to obtain the phase shift φ from the sinusoidal profile of the measurement signal M, this relationship is utilized and accordingly four samples offset by 90 ° in each case are defined with which the phase shift φ can be determined. As in 3 (right diagram), the four samples are represented by the amplitude values U (0), U (1), U (2), U (3) de Finished, these amplitude values are offset by 90 ° to each other within a period of the measurement signal M.

As from the comparison with the phasor diagram in 3 follows, the difference D ₁ = ½ (U (1) - U (3)) represents the real part and the difference D ₂ = ½ (U (0) - U (2)) represents the imaginary part of the complex function of the phasor diagram.

Accordingly, the phase difference φ is calculated from the samples U (0), U (1), U (2), U (3) according to the relationship φ = arc tan [(U (0) -U (2)) / (U (1) -U (3))]

in principle could the phase shift φ too only with two offset by 90 ° Samples within a period of the measurement signal M are calculated. However, four sample sampling ensures that that DC level fluctuations of the sinusoidal measurement signal M not lead to errors in the determination of the phase difference.

By the chosen one Symmetry of the four over the Period of the measurement signal M each offset by 90 ° four samples is also achieved that the phase shift φ exactly is then determined when the ideal sinusoidal measurement signal M as the fundamental wave superimposed harmonic harmonics of even order are.

This is determined by the sampling according to 3 achieved that DC level fluctuations and distortions of the measured signal M with respect to the ide alen sinusoidal course by overlays with harmonics even order not lead to a systematic measurement error in the determination of the phase shift φ.

In order to also systematically eliminate effects of harmonics of odd orders, the measurement signal M can not only be used as in 3 are scanned with a scanning group, but scanned with multiple scanning groups. In this case, the sampling frequency f _{A is} increased and is an integer multiple of the frequency f _V.

(1)

optical sensor

(2)

object

(3)

Transmitted light beams

(4)

transmitter

(5)

Receiving light rays

(6)

receiver

(7)

resistance

(8th)

oscillator

(9)

frequency generator

(10)

frequency generator

(11)

capacitor

(12)

capacitor

(13)

Low Pass Filter

(14)

Analog to digital converter

(15)

computer unit

(16)

mixer

(17)

Low Pass Filter

S1

control line

S2

control line

Claims (21)

Optical sensor for the determination of distances objects in a surveillance area, with a transmitter emitting light emitting emitter, having a modulation unit, by means of which the transmitted light beams are amplitude-modulated, with a receiver for receiving the reflected as received light beams from an object Transmitted light beams and with an evaluation unit, in which the Phase shift of the transmitted light beams and the received light beams as a measure of the object distance is determined, characterized by an electrical reference path for generating a reference phase reference for those in the evaluation unit registered phase shifts (φ). Optical sensor according to claim 1, characterized in that the modulation unit comprises a first frequency generator ( 9 ) for generating a modulation frequency (f ₁ ), with which the transmitted light beams ( 3 ) are amplitude modulated that by means of a second frequency generator ( 10 ) a clock signal having a modulation frequency (f ₂ ) is generated, which is shifted by an intermediate frequency (Δf) with respect to the modulation frequency (f ₁ ), and that by mixing the clock signal with that in the receiver ( 6 ) by the incident received light beams ( 5 ) received signals the phase shifts (φ) of the transmitted light beams ( 3 ) and the received light beams ( 5 ) containing the frequency (.DELTA.f) is generated, which is evaluated in the evaluation unit for determining the object distances. Optical sensor according to claim 2, characterized in that the electrical reference path is a mixer ( 16 ), in which a in the first frequency generator ( 9 ) generated frequency signal with the frequency (f ₁ ) and in the second frequency generator ( 10 ) Generated clock signal (f ₂ ) for generating a reference signal (R) are supplied. Optical sensor according to claim 3, characterized the reference signal (R) is used to refer the measuring signal (M) is read into the evaluation unit. Optical sensor according to one of claims 3 or 4, characterized in that the electrical Reference distance to the mixer ( 16 ) downstream low-pass filter ( 17 ) having. Optical sensor according to one of claims 2-5, characterized in that in the receiver ( 6 ) a mixture of the clock signal and the received signals for generating the measurement signal (M) takes place. Optical sensor according to claim 6, characterized in that the receiver ( 6 ) is formed by an avalanche photodiode. Optical sensor according to one of claims 2-6, characterized in that the mixture of the clock signal and the received signals for generating the measuring signal (M) in a the receiver ( 6 ) downstream mixer ( 16 ) he follows. Optical sensor according to claim 8, characterized in that the receiver ( 6 ) is formed by a PIN diode. Optical sensor according to one of claims 6-9, characterized in that the receiver ( 6 ) or the mixer ( 16 ) a low-pass filter ( 13 ) is subordinate. Optical sensor according to one of claims 2-10, characterized in that the modulation unit with the first frequency generator ( 9 ) and the second frequency generator ( 10 ) consist of digital components. Optical sensor according to one of claims 2-10, characterized in that the frequency generators ( 9 . 10 ) to an oscillator ( 8th ), and that the frequencies (f ₁ ) and (f ₂ ) from that in the oscillator ( 8th ) generated fundamental frequency (f ₀ ) are derived. Optical sensor according to claim 12, characterized in that the first and second frequency generator ( 9 . 10 ) each have at least one divider, wherein the frequencies (f ₁ ) and (f ₂ ) from the fundamental frequency (f ₀ ) are generated by means of the divider. Optical sensor according to one of claims 1-13, characterized in that the evaluation unit is an analog-to-digital converter ( 14 ) and a downstream computer unit ( 15 ) having. Optical sensor according to claim 14, characterized in that by means of the analog-to-digital converter ( 14 ) a sampling and digitization of the measurement signal (M) and the reference signal (R) takes place. Optical sensor according to one of claims 14 or 15, characterized in that the first and second frequency generator ( 9 . 10 ) and the analog-to-digital converter ( 14 ) are connected via control lines (S1, S2). Optical sensor according to claim 16, characterized in that via a first control line (S1) a frequency signal (f _V ) from the first frequency generator ( 9 ) into the second frequency generator ( 10 ) is read, wherein the frequency signal (f _V ) forms a trigger signal for generating the frequency (f ₂ ). Optical sensor according to one of claims 16 or 17, characterized in that via a second control line (S2) a frequency signal (f _A ) from the second frequency generator ( 10 ) in the analog-to-digital converter ( 14 ) is read, wherein the frequency signal (f _A ) forms a trigger signal for the sampling of the measurement signal (M) and the reference signal (R). Optical sensor according to claim 18, characterized in that the frequency signals (f _V ) and (f _A ) from the fundamental frequency (f ₀ ) of the oscillator ( 8th ) are derived. Optical sensor according to claim 19, characterized in that the frequencies (f _V ) and (f _A ) are identical. Optical sensor according to claim 19, characterized in that the frequency (f _A ) is an integer multiple of the frequency (f _V ).