WO2020169845A1

WO2020169845A1 - Interferometric distance measurement on the basis of propagation time differences

Info

Publication number: WO2020169845A1
Application number: PCT/EP2020/054738
Authority: WO
Inventors: Dirk GÜNTHER
Original assignee: OptoMET GmbH
Priority date: 2019-02-22
Filing date: 2020-02-24
Publication date: 2020-08-27
Also published as: DE102019001275A1; DE112020000945A5

Abstract

The invention relates to a device and to a method for measuring a distance to a reflective object (80). The device has a radiation source for generating an object beam (12) and a reference beam (14), the object beam propagating to the object (80). The device also comprises a first modulator for periodically modulating the object beam with a first modulation frequency. The device further comprises a radiation detector at which at least part of the object beam (15) reflected by the object (80) interferes with the reference beam (14) such that an interference signal is formed. An evaluation unit (50) is designed to determine a phase difference between a signal component of the reflected object beam (15) modulated with the first modulation frequency and an umodulated signal component of the reflected object beam (15) on the basis of the interference signal at the radiation detector (40), which phase difference correlates with the distance to be measured.

Description

Interferometric distance measurement based on transit time differences Description

Technical area

The present invention relates to a device and a method for measuring a distance to a reflective object, the distance or the distance being static as well as dynamic, for example to determine a relative speed between the device and the object and / or to measure vibrations of the object is determinable.

background

With the help of an interferometer, changes in distance to an object can be measured very precisely. To do this, the phase difference between a reference beam running over a reference path and a measuring beam reflected on the object is determined. Depending on the relative phase position between the measuring beam and the reference beam, one obtains after the two beams are superimposed

different light intensities. If this interference phase is detected with a photodetector unit, it is possible to determine the shift to a fraction of the optical wavelength.

However, determining the absolute distance of the object is more complex. Since the interference phase repeats itself periodically when the object distance changes by half an optical wavelength, the uniqueness range is very small, typically a few hundred nanometers. With a simple interferometer, distances can only be determined by integrating the phase change, taking into account the period transitions.

In practical use, however, this is associated with disadvantages: the measuring device and the measuring object must either first be brought together and then separated again, or a reflector must be brought from the measuring device to the measuring point (laser tracker). If, for example, the measurement is only rougher for a short time (disturbance of the line of sight or speckle effect Measurement surface) interrupted, the integration may be faulty. Discontinuities or spatial jumps when scanning objects to be measured have a similarly negative effect.

By means of an interferometer, in particular by means of a heterodyne interferometer, relative movements between an object to be measured and a measuring device can also be measured precisely. You can also use a

Interferometer vibrations or oscillation states of the object can be measured qualitatively as well as quantitatively precisely and in real time.

For a high-precision measurement of distances, as well as movements of the object, measurement methods based on interferometry are used. If the distance between the measuring device and the object, which is typically several meters, is also to be determined, this can typically be done by means of a further contactless distance measuring system. This can be based on a completely different measuring principle. The signals of the two independently working devices intended for different areas of application for measuring the absolute or rough distance on the one hand and for high-precision

On the other hand, measurements of distances or movement states of the object must therefore be precisely coordinated with one another. In addition, it must be ensured that both independently working systems refer to one and the same point or area of the reflective object. This is associated with a comparatively high adjustment and calibration effort. In addition, the outlay on equipment is comparatively high.

The present invention is based on the object, a

to provide improved apparatus and method for measuring a distance to a reflective object. The device and the method should be characterized by a large measuring range, by a high measuring accuracy and by a low level of equipment and technical complexity. One and the same device should be able to determine, on the one hand, the absolute distance, possibly several meters, between the device and the object using a single radiation source.

At the same time, the device should have a measurement accuracy in the submicrometer range, Provide nanometer range or sub-nanometer range. Furthermore, the device should be designed to be particularly compact and have a comparatively low weight. The device should be particularly suitable for mobile use.

The device and the measuring method to be carried out with it should also be distinguished by a comparatively simple adjustment and calibration.

This object is achieved with a device for measuring a distance to a reflecting object, with a corresponding method for measuring a distance with the reflecting object and with an algorithm, implemented as a hardware circuit or in a corresponding computer program, each with the features of the independent claims. More beneficial

Refinements of the device and the method are the subject of each of the dependent claims.

To this extent, a device for measuring a distance to a reflective object is provided. The device has a radiation source for generating an object beam and a reference beam. The radiation source is typically designed to generate a transmission radiation. The object beam and the

Reference beam can typically by means of a beam splitter from the

Transmitted radiation are generated. It is advantageous if the intensities of the

The object beam and the reference beam are variably adjustable. As an alternative to this, object beams and

Reference beam can also be generated directly from the radiation source.

The object beam is directed towards the object. The device and the object can be positioned with respect to one another in such a way that the object beam propagates towards the object. The device also has at least one first modulator in the beam path of the transmitted radiation or in the beam path of the object beam. The modulator is designed to transmit the transmitted radiation or the object beam with a first

Modulation frequency to modulate periodically in time.

The device also has a radiation detector on which at least part of the object beam reflected from the object interferes with the reference beam while forming a reference signal. The object beam and the reference beam are capable of interference with one another. The device also has an electronic Evaluation unit, which is signal-coupled with the radiation detector. The electronic evaluation unit is designed to provide a distance that is correlated with the distance to be measured on the basis of the interference signal at the radiation detector

To determine the phase difference between a signal component of the reflected object beam modulated with the first modulation frequency and an unmodulated signal component of the reflected object beam. From the phase difference between the modulated and unmodulated signal component of the reflected object beam, conclusions can be drawn about the distance or the distance between the device and the reflecting object.

Instead of an interference signal, an amplitude-modulated signal can also be applied to the detector or measured. Using the phase position of this signal in relation to the phase of the modulator, the transit time and thus the distance to the object can be determined here.

The phase difference between two signal components with different frequencies corresponds to the phase of the beat signal, possibly up to an integer multiple of 2 pi. For such a difference formation the two signal components, each in the same way with a Doppler shift due to a movement of the

DUT can be acted upon, first mixed down into the baseband. Therefore, if in the following from a phase difference of two

Signal components of different frequencies for determining the distance are mentioned, so the phase determination of the

Be included beat signal. The determination takes place in both cases, both for the phase of the respective individual signal components and for the phase of the beat signal with reference to and comparison with the phase position of the modulator.

The term “unmodulated signal component” in the present context means the

Understand the signal component at the carrier frequency. The term “modulated signal component” is to be understood as meaning that or those signal components with integer multiples of the modulation frequency.

A temporally periodic modulation by means of the first modulator leads to

Frequency space for several frequency components with m = 0, m = +/- 1; m = +/- 2, ..., where m = 0 reflects the signal component at the carrier frequency, m = +/- 1 the signal component at the first harmonic component of the modulation frequency and m = +/- 2 the signal component at the second harmonic component of the

Modulation frequency reflects.

The evaluation unit and the method to be carried out with it are designed in particular to determine a transit time difference between the reference beam and the reflected object beam by measuring the phase difference between the modulated signal component and the unmodulated signal component of the object beam. The signal components of the reflected object beam have none

Runtime difference but rather a phase difference due to the modulation, which increases with the common runtime in proportion to the difference frequency of the signal components.

If there is no second modulator in the reference arm, the method described provides the transit time from the modulator via the reflecting object to the detector. With an unchangeable length of the reference arm, the transit time through the reference arm is only included as a constant, so that in this case too the transit time is determined directly from the modulator via the reflecting object to the detector. It is further or alternatively conceivable here that the object beam has only two signal components at different frequencies. Instead of modulating the object beam, two radiation sources that are capable of coherence with one another or two object beams originating from different radiation sources can be used for this purpose, each of which has a beam component that is necessary for the

different object beams a defined, i.e. system known

Have frequency shift.

Alternatively, a phase difference between a signal component of the reflected object beam modulated with the first modulation frequency and a phase position of the modulator can also be determined and used to determine the distance to the reflecting object. In particular, a

amplitude-modulated signal can be used as an object beam. The reflected object beam, in particular its phase, can then be compared with the phase position of the modulator or a signal transmitter generating the modulation. This relative phase is knowing the modulation frequency and the Propagation speed of the object beam a direct measure of the absolute distance to the object.

As a further alternative, the phase difference between a first signal component of the reflected object beam modulated with the first modulation frequency and a second signal component of the reflected object beam can also be determined, with the first and second signal components being different sidebands of the modulation or different harmonics of the

Modulation frequency are.

The comparison of the first signal component and the second signal component with the

The phase position of the modulator can be done either individually or in combination.

According to a further embodiment, the first signal component and the second signal component are mixed down to a common base frequency and each of these down-mixed signal components or the difference between the respective phases of the down-mixed signal components is compared with the phase position of the modulator. The mentioned phase difference corresponds to the phase of a beat signal with a (synthetic) wavelength which corresponds to the difference frequency of the two signal components. ,

According to a further, alternative embodiment, provision is made for the first and second signal components to be mixed multiplicatively. The mixed signal obtained in this way contains differences and with regard to the frequencies of the signal components

Sum terms. The difference terms can be filtered out by means of a downstream low-pass filtering and finally with the phase position of the

Modulator can be compared. This is how you get a

Difference signal or beat signal with a (synthetic) wavelength which corresponds to the difference frequency of the two signal components.

The (synthetic) wavelength determines the uniqueness interval of the

Positioning. A comparison with the phase position of the modulator gives the transit time to the reflecting object and back to the detector. A range of uniqueness for the distance measurement is given by the first

Modulation frequency vi determined. This distance D is measured at D = C / (2vi). With a comparatively high modulation frequency, the uniqueness range is relatively small. With a high modulation frequency, however, the resolving power of the device can be increased. At a comparatively low one

Modulation frequency, the uniqueness range for distance measurement is larger. However, this is at the expense of the measurement accuracy or at the expense of the spatial

Resolving power of the measuring device.

The radiation source is typically a monochromatic radiation source. The radiation source is typically an optical one

monochromatic radiation source implemented. The radiation source can in particular have a monochromatic laser with a comparatively large coherence length. The coherence length of the radiation source is

typically several meters, several 100 m or even several kilometers.

The device advantageously has an optical interferometer, by means of which the transmitted radiation emitted by the radiation source can be divided into an object beam and a reference beam. The reference beam is typically directed to the radiation detector via one or more deflection devices, for example via beam splitters and / or mirrors. The object beam leaves the device, propagates to the object and is reflected on the object. The object can be a retroreflective object. However, it can also be a diffusely scattering or diffusely reflecting object.

The modulator can be arranged behind the beam splitter in the direction of propagation of the transmitted radiation. In this way it is achieved that only the object beam is modulated with the first modulation frequency. The object beam thus receives at least two signal components, namely a first or basic signal component based on the carrier frequency of the transmitted radiation and a further modulated one

Signal component based on the first modulation frequency. In that both signal components propagate to the object, are reflected from the object and together with the

If the reference beam are recombined again at or in the detector, the phase difference between the modulated signal component and the unmodulated signal component can typically be measured and qualitatively determined via the interference with the reference beam. The description chosen here for the comparison of a modulated and unmodulated signal components can be generalized to two signal components that have different frequencies, typically slightly shifted from one another, so that a measurable phase difference arises when propagating over the same distance due to the mutually shifted frequencies.

The object beam can also only consist of a single oscillating or time-modulated signal. For the distance measurement to the object, only the phase of the object beam reflected by the object is then relative to the phase position of the

Compare modulator.

Assuming that the distance to the object is smaller than the uniqueness range specified by the first modulation frequency, the distance results directly from the measurable phase shift between the reflected object beam and the phase position of the modulator. From the phase difference, knowing the first modulation frequency, the transit time can be determined which the object beam needs for the propagation between the measuring device and the object. From this and with knowledge of the propagation speed, the distance or the distance can finally be calculated.

According to a further embodiment, the evaluation unit is designed to use the measured phase difference between the signal component of the reflected object beam modulated with the first modulation frequency and one or the other

unmodulated signal component of the reflected object beam to determine a transit time of the reflected object beam. In the frequency domain, the modulation gives the object beam an unmodulated signal component, typically at the carrier frequency of the radiation source, and a signal component modulated with the first modulation frequency. The unmodulated and the modulated signal component differ in the frequency space by the modulation frequency. The transit time of the reflected object beam is the time that the object beam needs to propagate from the measuring device to the object and from the object back to the measuring device.

Alternatively, the transit time of the reflected object beam from the measuring device to the object and from the object back to the measuring device can also be based on the phase difference between the reflected modulated object beam and the phase position of the

Modulator can be determined. As a result of the modulation, a beat or a further frequency-shifted signal component is generated on the object beam, which or via the

Runtime difference between the reference beam and the reflected object beam results in a measurable phase difference or phase shift between the modulated signal component and the unmodulated signal component of the reflected

Object beam leads. By measuring the phase difference between the modulated and the unmodulated signal component of the reflected object beam, knowing the modulation frequency, the transit time of the reflected object beam and thus also the transit time difference between the object beam and the reference beam can be determined. From the phase difference and / or transit time of the reflected object beam or from the transit time difference between the reflected object beam and the

Reference beam, the distance between the device and the reflecting object can be determined.

The greater the transit time difference or the greater the distance between the device and the reflecting object, the greater the phase difference between the modulated signal component and the unmodulated signal component of the reflected one

Object beam. The same applies to the phase difference between the modulated object beam and the phase position of the modulator.

According to a further embodiment, the evaluation unit is designed to determine the transit time difference between the reference beam and the reflected object beam on the basis of the above-mentioned phase difference. With the phase difference, the phase difference between the signal component of the reflected object beam modulated with the first modulation frequency and the unmodulated one can be used here

Signal component of the reflected object beam should be meant.

The uniqueness range, ie the maximum distance D to be measured with the device, is defined here by the following regularity D = C / (2vi). A desired uniqueness range can be established by a suitable choice of the first modulation frequency vi. A comparatively large uniqueness range is achieved by means of a comparatively low first modulation frequency. A low or only low modulation frequency reduces the measuring accuracy of the device. In particular, it is possible to successively and sequentially use the device with several operate larger or increasing first modulation frequencies.

In particular, it is provided here that the first modulation frequencies are selected in such a way that the uniqueness areas resulting from this have a certain overlap area. Thus, by executing a measurement method several times using the device provided here, by selecting a comparatively small modulation frequency, the distance between the device and the reflecting object can initially be determined relatively roughly and then gradually increasing finer and more precisely by increasing the first modulation frequency gradually. Here are successively or gradually larger first

Modulation frequencies used for modulating the object beam.

According to a further embodiment, it is also provided that the first modulator is designed to generate the first modulation frequency electronically. The modulator can be designed in particular to generate modulation frequencies in the kHz range, in the MHz range, in the GHz range or even in the terracotta range. It can in particular be provided that the modulator first

Can generate modulation frequencies in such a frequency range, so that the unambiguous range of the device extends into the range of half the wavelength of the carrier frequency of the radiation source. In this way, by successively varying the first modulation frequency, the measuring range and the spatial resolution of the device can be changed as required.

According to a further embodiment, the first modulator is a phase modulator, an amplitude modulator, a polarization modulator or a frequency modulator.

In particular, a phase modulator can be used in a comparatively large

Frequency range, for example in the gigahertz range, 10 GHz range or 100 GHz range, so that the measurement accuracy of the runtime-based method is better than the width of the uniqueness range of the interferometric method (typically half the wavelength of the laser light) in order to enable a transition between the two measurement methods.

The apparatus and method described here can be used in many ways

implemented in different ways. Instead of one

The phase modulator can also be an amplitude modulator in the same way Polarization modulator or a frequency modulator can be used as the first modulator. For a wide variety of measurement tasks or specifications as well as for different reflective objects, a suitable one can be found here

Modulation method can be used for the first modulator.

According to a further embodiment, the evaluation unit has a multiplicative mixer and a filter connected downstream of the mixer. The mixer can in particular be designed as a squarer. By means of the mixer and the filter, a difference signal oscillating with the first modulation frequency can be generated from the interference signal present at the detector or measurable there. By doing

Difference signal is also the information regarding the phase difference between the modulated and unmodulated signal component of the reflected object beam and thus also information regarding the transit time difference between the

Reference beam and the reflected object beam included. Ultimately, the phase difference between the modulated and unmodulated signal component and / or the transit time difference between the reference beam and the object beam can be determined computationally from the difference signal by means of suitable signal processing.

According to a further embodiment, the evaluation unit has a demodulator, by means of which by demodulating the difference signal in the first

Modulation frequency is the phase difference between the modulated signal component and the unmodulated signal component of the reflected object beam and / or by means of which the transit time difference between the reference beam and the object beam can be determined by demodulating the difference signal at the first modulation frequency.

The mixer and the demodulator can be in the form of digital

Signal processing components are present. The mixer and / or the downstream filter and / or that of the modulator can also be implemented purely in terms of software. The mixer, the demodulator, any filters, low-pass, high-pass or band-pass and any other signal processing components of the

Evaluation units can also be implemented in terms of hardware.

According to a further embodiment of the device is in the beam path of the

Reference beam or a second modulator in the beam path of the object beam arranged. This is designed to modulate the reference beam and / or the object beam with a second modulation frequency. Both the first and the second modulator can be arranged in the object beam. It is also conceivable that the first modulator is arranged in the object beam and the second modulator in the reference beam; and vice versa.

The second modulator can be a frequency modulator. The second modulator can also be designed as a so-called one-sided modulator or as a frequency shifter. The frequency in the reference beam or in the object beam can be modulated or changed by means of the second modulator. The

In this way, the interferometer arrangement of the measuring device can be designed in the manner of a heterodyne interferometer. The reference beam and / or the object beam can be provided with a second modulation frequency by means of the second modulator.

The reference beam can thus have a signal component modulated with the second modulation frequency and an unmodulated signal component. Using the

The (frequency?) modulated signal component of the reference beam can be a result of a relative movement between the object and the measuring device

Doppler shift of the reflected object beam can be measured precisely. With the frequency-modulated reference beam, based on the interference signal, both the relative speed between the reflecting object and the

Measuring device and the direction of movement of the reflective object can be determined relative to the measuring device.

The term “unmodulated signal component” used here means the signal component at the carrier frequency in the present context. The term “modulated signal component” is to be understood as meaning that or those signal components with integer multiples of the modulation frequency.

In particular, the interference signal can be subjected to a vibrometric evaluation. The measuring device can in particular be designed as a laser vibrometer which can measure high-precision changes in distance and vibrations or movements of the reflective object in real time. In parallel and quasi simultaneously with this, absolute distances or distances between the measuring device and the reflective object can be measured based on the transit time difference between the reflected object beam and the reference beam.

The simultaneous or simultaneous measurement of the absolute distance or the distance between the measuring device and the reflective object as well as the detection of vibrations, oscillation states or movements of the reflective object can also take place only by means of a modulator, namely the first modulator.

In this respect, according to a development, the electronic evaluation unit is designed to determine the speed of the object relative to the measuring device and a distance between the measuring device and the object simultaneously or simultaneously.

For the operation of a laser vibrometer, such absolute

Distance information can still be used during a scanning process that is scanning the surface of the reflecting object in order to optimize individual measurement parameters. For example, the focus of the transmitted radiation or the

The object beam can be tracked dynamically or speckle tracking can be optimized.

The same applies to other types of so-called full-field vibrometry, such as the simultaneous measurement of several measuring points on the surface of the reflective object instead of a scanning measurement. The measuring device described here can also be designed for such a simultaneous measurement of several measuring points.

The difference in transit time between the reference beam and the reflected one

The object beam is determined on the basis of the phase offset caused by the first modulation frequency. Continuous wave operation of the radiation source can be provided here. The generation of short laser pulses, as is otherwise customary for measuring time-of-flight differences, is not required for this.

Furthermore, the movement or vibration or oscillations of the reflective object and its absolute distance or distance from the measuring device can be measured precisely and virtually simultaneously with one and the same device on the basis of or by means of one and the same radiation source. According to a further embodiment of the device, the evaluation unit has a first bandpass filter and a second bandpass filter connected in parallel therewith.

The first bandpass filter is designed to extract or filter out a first signal component at the second modulation frequency from the interference signal.

The second bandpass filter is designed to use the interference signal at the difference between the first modulation frequency and the second

Modulation frequency to extract a second signal component. The second signal component contains one with the difference frequency between the first and the second

Modulation frequency oscillating signal component.

On the basis of the second signal component, in particular the previously described differential signal oscillating at the first modulation frequency can be generated. The phase difference between the modulated and unmodulated signal component of the reflected object beam and / or the transit time difference between the reference beam and the reflected one can therefore be obtained from the second signal component of the interference signal

Object beam can be determined.

On the basis of the first signal component, in particular a speed of the reflecting object relative to the measuring device can be determined and measured.

The first and / or the second band pass filter can also be based on the

Speed of the object-related Doppler shift of the reflected

Be adapted to the object beam. The filter can therefore have a bandwidth around the second modulation frequency and / or around the difference between the first and second

Have modulation frequency, which by the movement and the

Speed of the object-related Doppler shift 2v / c taken into account.

According to a further embodiment of the device, the evaluation unit has a first bandpass filter and a second bandpass filter connected in parallel therewith.

The first bandpass filter can be designed to extract or filter out a first signal component at a first harmonic of the modulation frequency from the interference signal. The second bandpass filter can be configured to convert a second signal component from the interference signal in the second

to extract or filter out harmonics of the modulation frequency. The first signal component and the second signal component quasi represent two different frequency bands of the means of the first and / or the second

Modulator modulated reference beam and / or object beam. It is conceivable that only one modulator is used and that the first and second signal components each have different frequency bands of the modulation of the

Represent the object beam. A previously described differential signal can also be generated on the basis of those first and second signal components, by means of which the absolute distance or the distance to the object can be determined. The speed of the object relative to the measuring device can also be determined on the basis of those first and second signal components.

According to a further embodiment of the device, the multiplicative mixer has a first input, which is connected for signaling purposes to an output of the first bandpass filter. The multiplicative mixer also has a second input, which is connected or coupled for signaling purposes to an output of the second bandpass filter. In this way, the first signal component of the interference signal can be multiplicatively mixed or multiplied with the second signal component of the interference signal. The individual signal components of the interference signal, which have different frequencies, can be separated from one another and separated in this way.

The mixed signal present at the output of the mixer has at least a first and a second signal component with the sum and the difference of the respective

Frequencies on. The sum term is for the consideration provided here

irrelevant. The difference term can be extracted from the mixer signal or filtered out by means of the filter connected downstream of the mixer.

That filtered mixer signal can then be fed to the demodulator, which is designed to determine or calculate the phase difference or the transit time difference through the modulation at the first modulation frequency, on the basis of which finally the distance information between the measuring device and the reflecting object can be won.

The use of the first and second band pass filters enables simpler and faster signal processing. In addition, on the basis of the first Bandpass filter extracted first signal component, the speed information of the reflective object can be determined relative to the measuring device.

According to a further embodiment of the device, the first modulator is

coupled with the evaluation unit for signaling purposes. The first modulator is also designed to generate several different first modulation frequencies. In particular, the first modulator can be controlled and / or regulated by the evaluation unit and / or by a controller. The evaluation unit can therefore have a controller or regulator by means of which the first modulator is controlled or regulated.

According to a further embodiment, the device has a beam deflection unit, by means of which the object can be scanned continuously or point by point with the object beam. In particular, by means of the beam deflection unit, the object beam can be directed to different points or points one after the other.

Regions of the object are directed in order to successively scan or scan the surface of the object facing the measuring device without contact, for example optically. In this way the topology of the object

can be measured spatially resolved. Furthermore, by means of a simultaneous

punctual speed measurement at the point of the object acted upon by the object beam, a spatially resolved speed detection or spatially resolved vibration measurement of the object over the surface of the object also take place.

As an alternative or in addition to the beam deflection unit, the measuring device can be designed in the manner of a multi-point interferometer. In this case, several, in particular a large number of object beams are directed onto separate measuring points of the object to be measured. A signal evaluation can then take place simultaneously or in parallel in time for all reflected object beams.

According to a further embodiment, the electronic evaluation unit has a logical transit time evaluation unit and a logical speed evaluation unit, the logical transit time evaluation unit for determining a distance to the object and the logical speed evaluation unit are designed to determine a speed of the object relative to the measuring device. The logical speed evaluation unit and the logical runtime evaluation unit can be implemented in a common integrated circuit or by one and the same electronic processor arrangement. The logical speed evaluation unit and logical transit time evaluation unit are typically designed as digital evaluation units.

According to a further embodiment, the electronic evaluation unit is designed to use a speed signal that can be generated by the logical speed evaluation unit to correct an absolute distance measurement from the object that can be determined by the logical transit time evaluation unit.

For the absolute distance measurement, provision is typically made for a transit time of the reflected object beam and / or the distance to the reflecting object to be determined on the basis of the phase difference. This is typically done repeatedly and successively on the basis of different first ones

Modulation frequencies of the first modulator.

The modulation frequency can be discrete, i.e. be varied step by step but also continuously or continuously changeable. In particular, the relevant modulator can be coupled to a tuning device which deterministically controls the periodic modulation of the first modulator.

If the speed or the change in position or the oscillation states of the object relative to the measuring device are determined at the same time by means of a second modulator, then during the switching of the first modulator from a first modulation frequency to a second

Modulation frequency or even during a continuous change in the modulation frequency by the continuous and uninterrupted measurement of the speed or the oscillation state of the object, any disturbance affecting the distance measurement can be detected or also compensated. A variation or tuning of different modulation frequencies is controlled by the evaluation unit or by the tuning device. With a certain delay due to the signaling and electronic processing, a new modulation frequency is impressed on the object beam or the reference beam. This delay is known throughout the system.

It can be determined and of course taken into account in the signaling evaluation. The signal-related delay can be measured once for the device for distance measurement, for example in the course of a calibration, and stored accordingly. The signaling delay can also be determined repeatedly or continuously and taken into account in the signal processing in the modulators or in the demodulators.

Since the time course of the change in the modulation frequency is also known and specified by the system, the evaluation unit in a further embodiment can be able to computationally compensate for the known modulation frequency change when determining the relative speed of the object.

In this way, even with just a single modulator, it can be achieved that a speed determination with interferometric accuracy is continued without interruption during a repetitive tuning process of the modulation frequency.

According to a further aspect, the invention also relates to a method for measuring a distance to a reflective object. The method here comprises generating an object beam and a reference beam, more typically from one

Transmission radiation by means of a beam splitter. The object beam propagates towards the object. The method further comprises modulating the object beam periodically with a first modulation frequency, typically with the aid of a first modulator.

Furthermore, an interference signal is detected on a radiation detector, on which at least part of the object beam reflected by the object with the

Reference beam interferes. In addition, there is a phase difference that correlates with the distance to be measured between the reflecting object and the measuring device between the signal component of the reflected object beam modulated with the first modulation frequency and the unmodulated signal component of the reflected object beam is determined. As an alternative to this, the phase difference correlating with the distance between the modulated reflected object beam and a phase position of the modulator can be determined.

In particular, provision is made to determine the distance to the reflective object by means of an interferometer arrangement, in particular with one described above

Device for measuring a distance to a reflective object to be determined. The method provided here for measuring a distance thus relates to a proper use of the device described above. All properties, features and advantages that are described in connection with the device also apply equally to the method provided here; and vice versa.

In particular, provision is made for the transit time difference between the reflected object beam and the reference beam to take place through a multiplicative mixing of the interference signal present at the detector. Mixing the

Interference signal can be followed by a low-pass filtering. The

low-pass filtered mixed interference signal can be demodulated by means of a demodulator at the first modulation frequency. From the demodulated signal, the transit time difference between the reflected object beam and the

Reference beam can be determined.

According to a further embodiment of the method, the first modulation frequency is determined in such a way that the distance to be measured is smaller than a uniqueness range predetermined by the first modulation frequency. The reverse is the

Modulation frequency selected to be so small that the roughly roughly estimated distance between the measuring device and the reflecting object is smaller than the uniqueness range of the measuring device predetermined by the modulation frequency.

According to a further embodiment of the method, the difference in transit time between the reference beam and the object beam is repeated and successively determined on the basis of different first modulation frequencies. Typically, the procedure starts with a comparatively small or low first Modulation frequency, so that there is a large uniqueness range for an absolute distance measurement or distance measurement between the reflecting object and the measuring device.

With a comparatively low modulation frequency, the spatial resolution or the measuring accuracy of the measuring device is comparatively low. By successively increasing the first modulation frequency, the uniqueness range can be successively reduced and the measurement accuracy or the spatial resolution can be successively increased.

The modulation frequency can be changed or tuned step by step but also continuously. Changing the modulation frequency, for example from a first modulation frequency to a second modulation frequency, takes a finite time.

During this time, however, the phase position can change due to drift or vibrations of the measurement object. It cannot be ruled out that during this time the unambiguous range required for the interferometric determination of the distance is exceeded. Then the chain of successive increases in accuracy would break.

Since the phase change or the displacement of the object, i.e., the change in phase or the displacement of the object, is now displayed in parallel and at the same time as the interferometric distance measurement, for example by means of the second modulator or with the logical speed evaluation unit. whose

Speed is continuously tracked with interferometric accuracy, this additional information can be used to take any drift or vibrations of the measurement object into account when determining the distance. In this way, the uniqueness of the distance determination can be made using the staggered or the tuning process over several modulation frequencies

guaranteed.

The use of the second modulator can be advantageous for the interferometric measurement of the speed of the object. But it is not absolutely necessary. The speed of the object can also always take place on the basis of those signal components of the interference signal at the radiation detector, which in a first and second Harmonics of the modulation frequency lie. The signal components shifted with respect to the carrier frequency are subject to the Doppler shift, which can be evaluated by means of a variant of the speed evaluation unit to determine the speed of the object, for example using PGC demodulation

The knowledge and stability of the modulation frequency required for this is basically given by using a laser as the light source. On the other hand, only moderate accuracy would initially be required for this, since during the tuning of the modulation frequency of the first modulator all that matters at first is the integer multiple of half the wavelength for the

To determine the identification of the respective uniqueness interval. A high level of accuracy is only required at the end of the tuning chain. If or as soon as an uniqueness corresponding to the light wavelength is achieved, a continuous relative interferometric distance measurement can be used.

The cyclical repetition of the method for determining the absolute distance is also advantageous in this respect, since in the case of a temporary interruption or

Blocking of the measuring beam again an absolute value for the absolute distance can be made available within a very short time.

In this respect, according to a further development of the method, a distance to the object, in particular an absolute distance to the object, as well as a speed of the object, or a movement or vibration state of the object relative to the object, is provided on the basis of the phase difference

Measuring device to measure simultaneously.

According to a further aspect, the invention also relates to a computer program for measuring a distance to a reflective object, the computer program being able to be implemented in an evaluation unit of a previously described device for distance measurement. The computer program can be carried out in or with the evaluation unit and has program means for periodically modulating an object beam of the measuring device as well as program means for detecting an interference signal on a radiation detector of the measuring device.

Furthermore, the computer program has program means for determining a the phase difference between the reference beam and the reflected object beam correlating to the distance to be measured. All properties

Features and advantages that are described in connection with the device and the method also apply equally to what is provided here

Computer program; and vice versa.

Instead of a computer program, it can also be a hardware implementation of the corresponding algorithm.

The implementation of the phase difference based distance or

Distance measurement in an interferometer also has the advantage of a quasi-optical amplification of the information-carrying signal component. The measurable intensity of the am

Radiation detector present interference signal is proportional to _A / P _m P _r , (P _m - power in the object arm, P _r - power in the reference arm). An increase in the power in the reference beam, ie in the reference arm, leads to a corresponding amplification of the signal.

The so-called shot noise also increases. However, if the power in the reference arm is selected to be sufficiently high, the system is limited by the shot noise. This means that compared to the shot noise in the detector, all other noise sources e.g. the thermal noise or the noise of the subsequent amplifier stages have a negligible influence on the signal-to-noise ratio.

As a radiation detector, a comparatively inexpensive too

implementing photodiode are used.

To get the mentioned advantages of the interferometric method for a

To use transit time measurement, it is necessary for the

Phase difference measurement or the modulated signal used for the transit time measurement, as it were, to mix up into the optical frequency range.

The information which e.g. amplitude modulated in the low electronic

Frequency range (MHz-GHz) was coded, is transferred to the phase / frequency-coded optical frequency range (100 THz). The system then works in the Continuous wave operation (cw mode). On the receiver side, ie at the detector, the information that contains the transit time must now be mixed down from the optical 100 THz frequency range to the electronically detectable and processable frequency range (MHz / GHz) with the help of the interferometer.

Mixing up and then downmixing initially appears to be unnecessary, but it offers a decisive advantage in terms of signal quality, since the optical amplification of the interferometric method can be used here. Expensive, complex and yet poorer in the signal-to-noise ratio

Avalanche photodiodes or photo multipliers are not required.

A difficulty for the interferometric transit time measurement according to the phase shift method is that, in addition to the absolute distance, the microscopic changes in distance from the order of the laser wavelength or the

Frequency shift due to the Doppler effect have effects on the phase position. Effects of the interferometric measuring principle are thus superimposed with those of the transit time measuring principle.

The phase modulation of the measuring arm is one of the possibilities

to realize harmonic signal for the use of the phase shift method in the optical frequency range. The phase-modulated laser beam with frequency w ₀ can be written as a superposition of several frequencies at a distance of the modulation frequency w ₁ ,

Here, h is the modulation index and J _n (]) is the Bessel functions of the first type. For a small modulation index, only those provide the lower orders

n ^. = o, + i, ± 2, ... _an essential contribution.

A heterodyne interferometer is a signal that is modulated with the speed of the measurement object, which also experiences a phase shift due to the movement of the measurement object. It is practically impossible to deduce the absolute distance from the phase position of the carrier signal. With the combination of at least two frequencies, for example w ₀ and w ₀ + w ₁ in the measuring beam, which the Interferometric measurement information on speed and displacement of the order of the laser wavelength are impressed, the transit time information can be separated from the speed and an absolute distance measurement can be achieved.

The technical progress with the modulators makes it possible, with the appropriate design (e.g. sufficiently high working frequency of the modulator in the object arm and sufficient power in the reference arm), to increase the resolution of the phase shift method with this structure up to the unambiguous range of the

interferometric measuring principle. With just a few additions to components, a simple interferometer can be turned into an absolute measuring device

Realize interferometer.

Brief description of the figures

Further features, advantages as well as possible applications and properties of the measuring device and the measuring method as well as the computer program are explained in the following description of an exemplary embodiment with reference to the drawings. Here show:

1 shows a schematic block diagram of the measuring device,

2 shows a flow diagram of the method for distance measurement,

3 shows a schematic block diagram of the evaluation unit of the measuring device,

4 shows a flow diagram of a further method for distance and / or

Speed measurement,

5 shows a schematic block diagram of a further embodiment of the

Measuring device and

FIG. 6 shows a schematic block diagram of the modified evaluation unit of the measuring device according to FIG. 5.

Detailed description 1 shows a schematic representation of an exemplary embodiment of the device 1 for measuring a distance to a reflective object 80. The device 1 is implemented here as an interferometer. She has a

Radiation source 10 to generate a transmission radiation 11. The radiation source 10 can have a coherent and monochromatic laser light source. In addition, the device 1 has a beam splitter 30 which, from the transmitted radiation 11, produces an object beam 12 directed towards the reflecting object 80 and an object beam

Reference beam 14 generated or extracted.

The object beam 12 also propagates through a first modulator 20 and through a further beam splitter 17. The beam splitter 17 can be designed as a polarizing beam splitter. At least a part of the object beam 15 reflected by the object 80 arrives again in the direction opposite to the object beam 12

Beam splitter 17. The reflected object beam 15 is deflected on a mirror 18 by the polarizing beam splitter 17, for example due to its rotated polarization.

From there, the reflected object beam 15 propagates in the direction of one

Radiation detector 40. The radiation detector 40 can be designed as a photodiode. A further beam splitter 19 is arranged between the mirror 18 and the radiation detector 40. In this beam splitter 19, the reflected object beam 50 and the reference beam 14 recombine. Since the transmitted radiation 11 is a

has a comparatively large coherence length, an interference signal is generated on the radiation detector 40.

The present implementation of the device 1 is based on a Mach-Zehnder interferometer. However, it can also be implemented in the form of a Michelson interferometer, for example.

A second modulator 16 is also arranged in the reference arm, that is to say between the beam splitter 30 and the beam splitter 19. The second modulator 16 is typically a frequency modulator. The radiation detector 40 is also coupled to an evaluation unit 50 to transmit signals. The evaluation unit 50 typically has an analog-digital converter 56, which consists of one of the Radiation detector 40 generated analog signal generates a digital signal sequence.

The evaluation unit 50 also has a digital signal processing component, which digitally further processes the signals available from the analog-digital converter.

The evaluation unit 50 is also connected to the first modulator 20 and to the second modulator 16 to transmit signals.

In particular, a modulation frequency of the respective first and / or second modulators 20, 16 can be set and / or changed as required by means of the evaluation unit 50. Alternatively, a separate controller or

Frequency generator may be provided, which is technically signaling both with the

Evaluation unit 50 and also with at least one of the modulators 16, 20 is coupled.

In the schematically sketched block diagram according to FIG. 3, the signal processing of the evaluation unit 50 is shown as an example in different sub-units. The evaluation unit 50 can have a signal processor 51 that is associated with the

Radiation detector 40 is connected for signaling purposes. For example, from

The signal obtainable from the radiation detector 40 can be filtered, scaled and processed for the analog-digital conversion in the signal processing unit 51. The signal conditioning 51 has an analog-digital converter 56. An output of the analog / digital converter 56 is coupled to a line separator 52 for signaling purposes. Of the

Line separator has a first bandpass 68 and a second bandpass 70. The first and second bandpass filters 68, 70 typically operate at

different or slightly different frequencies and filter out the frequency bands relevant for the signal evaluation from the signal provided by the analog-digital converter 56.

These can be frequency bands that are each different

Concern harmonics of the modulation frequency. For example, the first bandpass 68 can be used to filter the frequency component of the first harmonic, i.e. H. at m = 1 and the second bandpass 70 for filtering the frequency component at the second harmonic, d. H. be designed at m = 2.

The evaluation unit 50 also has a run time evaluation unit 53 and a speed evaluation unit 54. The run time evaluation unit 53 has a multiplicative mixer 60. A first input 61 of the mixer 60 is

signal-wise coupled to an output 69 of the first bandpass filter 68. A second input 62 of the mixer 60 is connected to an output 71 of the second

Bandpass 70 coupled. By means of the mixer 62 are those of the two

Bandpass filters 68, 70 provided and correspondingly frequency-filtered signals can be multiplied with one another. Furthermore, or alternatively, a single interference signal provided by the analog-digital converter 56 can also be squared with the mixer 60.

A low-pass filter 64 is connected downstream of the mixer 60. The combination of multiplicative mixer 60 and low-pass filter 64 makes it possible to generate a reference signal or a difference signal from the interference signal applied to detector 40. The difference signal can then be modulated at the first modulation frequency of the first modulator 20 via a modulator 66 connected to the output of the low-pass filter 64. Finally, a phase difference between a signal component of the reflected object beam 15 modulated with the first modulation frequency and an unmodulated signal component of the reflected object beam 15 is provided at the output 72 of the demodulator 66.

From this phase difference, with knowledge of the modulation frequency, the distance or the distance from the output of the first modulator 20 to the object 80 and from the object 80 to the detector 40 can finally be determined. Part of this distance, namely the optical path length from the first modulator 20 to the radiation detector 40, is constant. Varying distances between the beam splitter 17 and the

reflecting object 80 can be determined or quantitatively measured on the basis of the phase difference which can be determined by means of the transit time evaluation unit 53.

Although the logical line separator 52 is implemented in the present case by means of a multiplicative mixer and a downstream filter, its function can also be implemented in other ways, for example by separating the signal components and forming the difference using a time-frequency analysis and subsequent automated frequency peak identification.

If the object 30 is in motion with respect to the device 1, the

Measuring device 1 are preferably operated in a heterodyne mode. Of the second modulator 16, which is designed as a frequency modulator, can modulate the frequency of reference beam 14 or shift it by a second modulation frequency. With this frequency shift it is possible that the

Relative movement of the reflective object 80 and the measuring device not only to precisely measure a resulting Doppler shift of the reflected object beam 15 but also to determine the direction of the relative movement of the reflective object 80 and the measuring device 1. The speed evaluation unit 54 has a demodulator 74 which is signal-coupled to an output of the first bandpass filter 68.

The demodulator 74 performs demodulation based on the second

Modulation frequency off. An output of the demodulator 74 is signal-coupled to an input of a differentiator 76. The differentiator 76 derives the demodulated signal over time. As a result, the speed and the direction of movement of the reflective object 80 relative to the measuring device can then be provided at the output 78 of the differentiator.

The block diagram of FIG. 3 is used for illustrative purposes only. The individual components shown here must by no means be implemented as hardware components. All or only part of them can be implemented as software or as logical components.

The method for measuring a distance to the reflective object 80 and / or the simultaneously running method for measuring a speed of the object relative to the measuring device is shown schematically as a flow chart in FIG. 2. In a first step 100, an object beam 12 and a reference beam 14 are provided starting from the radiation source. This can typically be done using an im

Beam path of the transmitted radiation 11 arranged beam splitter 30 take place. The object beam 12 propagates in the direction of the reflecting object 80. In a subsequent step 102, the transmission radiation 11 or the object beam 12 is modulated with a first modulation frequency by means of the first modulator 20. In step 104, an interference signal is detected at the radiation detector 40. The

The interference signal is based on the interference of the object beam 15 reflected from the object with the reference beam 14.

On the basis of the detected interference signal, finally in step 106 a with the to be measured distance-correlating phase difference between a modulated with the first modulation frequency signal portion of the reflected object beam and an unmodulated signal portion of the reflected object beam determined. That determination is made on the basis of a freely selectable first modulation frequency of the first

Modulator 20.

The first modulation frequency is chosen so that the

The uniqueness range of the distance measurement is greater or significantly greater than a distance between the measuring device 1 and the reflected object 80.

The method can then be continued quasi iteratively. The distance measurement with a comparatively small first modulation frequency may have insufficient precision or a comparatively low spatial resolution.

The method can then continue again with step 102, but is typically increased at a different first modulation frequency, which changes by a predetermined amount compared to the previously performed distance measurement. Of the

In this way, the uniqueness range for the distance measurement is reduced in favor of a higher spatial resolution. By performing steps 102, 104 and 106 successively and repeatedly at a higher first modulation frequency in each case, the uniqueness range of the measuring arrangement can be successively reduced through successive measurements, but the resolution and measuring accuracy for the distance measurement can be increased step by step.

In parallel and at the same time, a speed measurement of the reflective object 80 can be carried out by means of the speed evaluation unit 54.

For the present device and the method it has proven to be particularly advantageous that the distance to the reflective object and the movements or the speed of the reflective object can be measured simultaneously.

One and the same hardware and one and the same signal processing can be used and used here. This clearly minimizes sources of interference and errors. Furthermore, optical amplification can be achieved through a variable division of the intensities between the object beam and the reference beam.

The intensities go into the intensity of the reference signal that can be measured at the detector 40 of the reference beam and the reflected object beam proportionally. The measurable and time-variable portion of the interference signal has an intensity proportional to the square root of the product of the intensity of the reference beam and the intensity of the reflected object beam. A so-called interferometric amplification of the interference signal at the detector 40 can be achieved by increasing the intensity of the reference beam. This is particularly advantageous for a wide variety of metrological applications, since the reference beam remains inside the measuring device. At the same time, the intensity of the object beam and the reflected object beam can be kept at a low level that is advantageous for compliance with laser protection regulations. In addition, inexpensive

Photodiodes are used as detectors.

The signal processing of the evaluation unit 50 is shown below by way of example for a measuring device 1 in which the first modulator 20 is implemented as a phase modulator. The reference beam b) is described here as b = βcos (β> ₀ + € a ₇ ) t, where the carrier frequency of the transmitted radiation 11 and w2 is the second modulation frequency of the second modulator 16. The object beam 15 reflected by the object 80 can be viewed in terms of two signal components ao and ai:

, where OJI represents the first modulation frequency, T is the delay time difference between the reference beam 14 and the reflected object beam 15, and Ao and Ai are the respective signal amplitudes. V also stands for the speed of the reflective object 80 relative to the measuring device 1, which is assumed here to be constant for the sake of simplicity, and c is the speed of propagation of the object beam 12. The individual signal components b, a0 and a1 interfere at the radiation detector 40. The measurable intensity is a time averaging of the square of the interference signal, which is represented mathematically as follows.

The DC shares, i.e. H. the temporally invariant components are filtered out in the signal conditioning 51. Terms in which individual frequencies are added to one another are so high-frequency due to the comparatively high carrier frequency that they are omitted from the time averaging. For the signal evaluation, only those terms of the interference signal are relevant for which individual

Signal components oscillate at various differential frequencies. The time-averaged intensity of the interference signal is as follows:

The signal can thus be filtered out by means of the first bandpass filter 68. The first band pass 68 extracts the signal components So at the second modulation frequency 002. The second signal components Si can be isolated or isolated with the second band pass 70.

extracted. The second bandpass operates at the difference frequency between the second modulation frequency 002 and the first modulation frequency 001.

The first and second signal components separated from the detected signal in this way are multiplied with one another in mixer 60 and then filtered via low-pass filter 64. The sum frequencies of the

Multiplication or the mixing process suppressed or filtered out, so that only the difference frequencies remain in a difference signal S _A. This

The difference signal is represented mathematically as follows

If this signal is demodulated with the first modulation frequency 001, the phase difference 001 T is obtained between the signal component modulated with the first modulation frequency 001 and the unmodulated signal component of the reflected one

Object beam 15, where T is the transit time difference between the reference beam and the object beam. The distance or the distance X to the reflective object 80 can be precisely determined via the known relationship T = 2 X / C. The factors

are very close to 1.

Alternatively, instead of the signal s _A , only the signal component s _a , which has the same spectral shape as s _A , could be used. In contrast to the latter, the latter only contains the product of the amplitudes from the reflected object beam A ₀ A _t (which can be very small depending on the reflectivity of the measurement object), while s _{A contains} the product of the amplitude of the respective component of the object beam and the reference beam ( A ₀ B or A _t B) and this brings about an optical amplification.

The speed evaluation unit 54 receives the signal So as an input signal in the demodulator 74. The demodulator 74 performs a demodulation in the second

Modulation frequency 002 off. This demodulation and the time derivative by means of the downstream differentiator 76 gives the argument V / C wo, which can be resolved according to the speed by multiplication with the term C / 2 wo.

The speed or vibration or oscillation of the object 80 can in this respect be determined at the same time.

The method according to the flowchart according to FIG. 4 is similar to that according to FIG. 3. Here too, in a first step 100, an object beam 12 and a reference beam 14 are provided starting from the radiation source. In the following step 102, the transmission radiation 11 or the object beam 12 is first Modulation frequency modulated by means of the first modulator 20. In step 104, an interference signal is detected at the radiation detector 40. The interference signal is based on the interference of the object beam 15 reflected by the object with the reference beam 14. Step 108 takes place in a manner similar to that shown in step 106 in FIG

Flowchart shows a determination of the phase difference, which correlates with the distance to be measured, between a signal component of the reflected object beam modulated with the first modulation frequency and an unmodulated signal component of the reflected object beam.

Meanwhile, with a temporal overlap or at the same time as this, the speed or the state of motion of the object relative to the measuring device is determined in step 110. The latter typically takes place by means of the second modulation of the second modulator 16 and by means of the logical speed evaluation unit 54.

While for the absolute distance measurement after step 108 the sequence of steps 102, 104 are carried out again and successively using a different modulation frequency, the frequency of the modulation frequency is tuned, so to speak, the frequency of the second modulator for the

Speed evaluation unit 54 are retained.

Thus, while changing the modulation frequency of the first modulator 20, which takes a finite time, the speed or the

The vibration state of the object 80 can be measured continuously and uninterruptedly. In the event that the uniqueness range for the absolute distance measurement should be lost during this time interval, this can possibly even be quantitatively detected by means of feedback from the logical speed evaluation unit 54 to the transit time evaluation unit 53.

In this respect, it is conceivable for this purpose and, according to one embodiment, provided that the differentiator 76 shown in FIG. 3 is connected to the demodulator 66 for signaling purposes and that an output of the differentiator 76 is data-related to a further input of the demodulator 66.

While the embodiments of the measuring device and the method according to Figures 1 to 4 have a simultaneous distance and speed or

Vibration measurements based on the use of two different Modulators 16, 20 enable only a single modulator 20 to be used in the alternative embodiment according to FIGS. 5 to 6.

The further configuration of the measuring device 1 according to FIGS. 5 and 6 is designed with only a single modulator 20. The modulator 20 is provided with a tuning device 90, by means of which the modulation frequency of the modulator 20 can be modulated or changed as required. In the embodiment according to FIG. 1, the tuning device 90 is typically integrated in the evaluation unit 50 or in a separate control for the modulator.

In the embodiment according to FIGS. 5 and 6, the logic transit time evaluation unit 53 or the line separator 52 is almost identical to that in the embodiment according to FIG. 3. In contrast to what is shown in FIG. 3, however, the first band pass 68 of the line separator 52 is decoupled from the speed evaluation unit 54. Instead, the speed evaluation unit 54 is coupled directly to the output of the analog-digital converter 56. The output of the analog-to-digital converter 56 branches, as it were, on the one hand into the line separator 52 and on the other hand into the logical speed evaluation unit 54. These are connected in parallel, so to speak, for data purposes.

The speed evaluation unit 54 according to FIG. 6, in contrast to that according to FIG. 3, has a different demodulator 94 which is coupled directly to the output of the analog-digital converter of the signal conditioning 51. The output signal of the analog / digital converter 56 contains several frequency components, including in particular a first harmonic (m = 1) and a second harmonic (m = 2) of the modulation frequency. The demodulator 94 of the logic speed evaluation unit 54 can in particular be designed as a so-called PGC demodulator (Phase Generated Carrier) modulator. By means of this, the amount and the direction of a movement of the object 80 relative to the measuring device 1 can be determined. The demodulator 94 is also coupled to the first modulator 20 in terms of signals.

In the same way as described above, the

Speed evaluation unit 54 the speed or the

State of motion of the object 80 relative to the measuring device 1 at the same time or in time determine overlapping to the absolute distance measurement of the logical transit time evaluation unit 53.

In this respect, the dashed connection between the output 78 of

Speed evaluation unit 54 and run time evaluation unit 53 in FIG. 6 show a feedback or feedback from speed evaluation unit 54 to run time evaluation unit 53. This can be used in particular to correct the position determination for those time intervals in which the position determination is too imprecise or no longer clear due to the change in the modulation frequency. The feedback or the feedback can be coupled directly to the demodulator 66 of the transit time evaluation unit 53.

Since the speed determination is basically possible with interferometric accuracy at all frequencies and possibly also takes place, the measurement of the speed can be continued without interruption during the tuning or during the change of the modulation frequency of the first and possibly only modulator and for the correction of the logical transit time - Evaluation unit 53 provided position or distance signal can be used.

In the block diagram of FIG. 5 it is also shown that the device for

Distance measurement 1 is also provided with a beam deflection unit 92, by means of which the object beam 12 can be deflected as required with regard to its transverse direction so that different surface sections of the object 80 can be scanned or scanned punctually or continuously with the object beam. This is indicated in Fig. 5 with the transversely deflected object beam 12 ‘. Of course, the device according to FIG. 1 can also have such a

Beam deflection unit 92 be equipped. A high-precision, spatially resolved measurement of the object is thus possible. The beam deflection unit 92 can have one or more adjustable mirrors as well as other beam-deflecting optical elements. Instead of or in addition to successive scanning of the surface of the object to be measured, the surface can also be measured simultaneously using several measuring beams in conjunction with one device according to the description for distance measurement (multi-point interferometry). List of reference symbols

I Device for distance measurement

10 radiation source

I I transmission radiation

12 object beam

14 reference beam

15 reflected object beam

16 modulator

17 beam splitter

18 mirrors

19 beam splitters

20 modulator

30 beam splitters

40 detector

50 evaluation unit

51 Signal conditioning

52 line separator

53 Runtime evaluation unit

54 Speed evaluation unit

56 analog-to-digital converter

60 mixers

61 input

62 entrance

64 deep bass

66 demodulator

68 band pass

69 exit

70 band pass

71 exit

72 exit

74 demodulator

76 differentiators

78 exit

80 object Tuning device Beam deflection unit Demodulator

Claims

A device for measuring a distance to a reflective object (80), comprising: a radiation source (10) for generating an object beam (12) and a reference beam (14), the object beam propagating to the object (80), at least one first modulator (20) for modulating the object beam (12), the modulator (20) being designed to periodically modulate the object beam (12) with a first modulation frequency, a radiation detector (40) on which at least part of the object ( 80) reflected object beam (15) to form a

Interference signal interferes with the reference beam (14), and with an electronic evaluation unit (50) which is coupled to the radiation detector (40) and is designed based on the

Interference signal at the radiation detector (40) has a phase difference correlating with the distance to be measured: between a signal component of the reflected object beam (15) modulated with the first modulation frequency and an unmodulated signal component of the reflected object beam (15), or between a signal component of the modulated with the first modulation frequency reflected object beam (15) and one

To determine the phase position of the modulator (20).

2. Device according to claim 1, wherein the evaluation unit (50) for this purpose

is designed, based on the phase difference, a running time of the to determine the reflected object beam (15) and / or the distance to the reflecting object (80).

3. Apparatus according to claim 1 or 2, wherein the evaluation unit (50) is designed to a delay time difference (T) between the

To determine the reference beam (14) and the reflected object beam (12) based on the phase difference.

4. Device according to one of the preceding claims, wherein the first modulator (20) is designed to generate the first modulation frequency electronically.

5. Device according to one of the preceding claims, wherein the first modulator (20) is a phase modulator, an amplitude modulator, a polarization modulator or a frequency modulator.

6. Device according to one of the preceding claims, wherein the

The evaluation unit (50) has a multiplicative mixer (60) and a filter (62) connected downstream of the mixer (60), by means of which the interference signal is converted to the first modulation frequency

oscillating difference signal can be generated.

7. The device according to claim 6, wherein the evaluation unit (50) one

Demodulator (64), by means of which the phase difference or the transit time difference can be determined by demodulating the difference signal at the first modulation frequency.

8. Device according to one of the preceding claims, wherein im

Beam path of the reference beam (14) or in the beam path of the

Object beam (12) a second modulator (16) is arranged, which is designed to modulate the reference beam (14) and / or the object beam (12) with a second modulation frequency.

9. Apparatus according to claim 8, wherein the evaluation unit (50) has a first bandpass filter (68) and a second one connected in parallel therewith Having bandpass filter (70), wherein the first bandpass filter (68) is designed to extract a first signal component at the second modulation frequency from the interference signal and wherein the second bandpass filter (70) is designed to extract from the interference signal at the difference between the first Modulation frequency and the second modulation frequency to extract a second signal component.

10. Apparatus according to claim 6 and 9, wherein the multiplicative mixer (60) has a first input (61) which is signal-connected to an output (69) of the first bandpass filter (68) and wherein the multiplicative mixer (60) has a second Has input (62) which is signal-connected to an output (71) of the second bandpass filter (70).

1 1. Device according to one of the preceding claims, wherein the

The modulator (20) is coupled to the evaluation unit (50) for signaling purposes, and the modulator (20) is designed to generate several different first modulation frequencies.

12. Device according to one of the preceding claims, further comprising a beam deflection unit (92) for continuous or point-by-point scanning of the object (80).

13. Device according to one of the preceding claims, wherein the

electronic evaluation unit (50) is designed to

To determine the speed of the object (80) relative to the measuring device and a distance between the measuring device and the object (80) simultaneously.

14. Device according to one of the preceding claims, wherein the

electronic evaluation unit (50) has a logical transit time evaluation unit (53) and a logical speed evaluation unit (54), the logical transit time evaluation unit (53) for determining a distance to the object (80) and the logical

Speed evaluation unit (54) for determining a Speed of the object (80) are formed relative to the measuring device.

15. The device according to claim 14, wherein the electronic evaluation unit (50) is designed to generate a speed signal that can be generated by the logical speed evaluation unit (54) for correcting an absolute distance measurement to the object (80) which can be determined by the logical transit time evaluation unit (53). to use.

16. Method for measuring a distance to a reflective object (80), with the steps:

Generating an object beam (12) and a reference beam (14) by means of a radiation source (10), the object beam propagating to the object (80), temporally periodic modulation of the object beam (12) with a first modulation frequency by means of a first modulator (20),

Detecting an interference signal at a radiation detector (40) on which at least part of the signal reflected from the object (80)

Object beam (15) interferes with the reference beam (14),

Determining a correlation with the distance to be measured

Phase difference between a signal component of the reflected object beam (15) modulated with the first modulation frequency and an unmodulated signal component of the reflected object beam (15) or between a signal component of the reflected object beam (15) modulated with the first modulation frequency and a phase position of the modulator (20) based on the interference signal at the radiation detector (40).

17. The method according to claim 16, wherein the first modulation frequency is determined in such a way that the distance to be measured is smaller than a uniqueness range predetermined by the first modulation frequency.

18. The method according to claim 16 or 17, wherein the phase difference or the transit time difference (T) is repeated and successively based

different first modulation frequencies is determined.

19. The method according to any one of the preceding claims 16 to 18, wherein a distance to the object (80) and a speed of the object (80) relative to the measuring device (1) are measured simultaneously on the basis of the phase difference.

20. The method according to any one of the preceding claims 16 to 19, which is carried out with a device (1) according to one of the preceding claims 1 to 15.

21. Computer program for measuring a distance to a reflective object (80), wherein the computer program can be implemented in an evaluation unit (50) of a device for distance measurement according to one of the preceding claims 1 to 15, and wherein the computer program:

Program means for modulating the object beam (12) periodically with a first modulation frequency by means of a first modulator (20),

Program means for detecting an interference signal on a radiation detector (40) on which at least part of the object beam (15) reflected by the object (80) interferes with the reference beam (14),

Program means for determining a phase difference correlating with the distance to be measured between a signal component of the reflected object beam (15) modulated with the first modulation frequency and an unmodulated signal component of the reflected object beam (15) or between a signal component of the reflected object beam (15) modulated with the first modulation frequency and a phase position of the modulator (20) based on the Having interference signal on the radiation detector (40).