DISTANCE SENSOR

Description

[0001] The invention relates to a distance sensor and a method for determining the distance of objects by means of a distance sensor.

[0002] Such a distance sensor generally comprises a transmitter that emits transmitted pulses, a receiver that receives received pulses, and an evaluation unit. To determine the distance of objects in a monitored area, transmitted pulses emitted by the transmitter are directed to the respective object, which is then reflected back to the receiver as received pulses. The evaluation unit determines the travel time of the transmitted pulses to the object and back to the receiver. From this travel time, the distance from the object to the distance sensor is determined.

[0003] In order to be able to measure even small distances in the millimeter or centimeter range, it is necessary to carry out the time of flight measurement with an accuracy in the picosecond range.

[0004] One possible approach here is to work with TDC (time-to-digital converter) units. These can include delay lines, so-called TDLs (tapped delay lines). These are digital delay structures that can be used to precisely resolve the propagation time of a received pulse arriving at the receiver.

[0005] Such a distance sensor is known from DE 10 2015 103 472 A1. The distance sensor described therein comprises a transmitter for transmitting at least one transmission signal, a receiver for generating a reception signal from the transmission signals remitted in the monitoring area, a clock generator for generating a clock pulse, and an evaluation unit designed to determine the signal propagation time between the transmission of a transmission signal and the reception of a remitted transmission signal. The evaluation unit comprises a coarse measuring unit for determining the signal propagation time in whole clock pulses and a fine measuring unit with a tapped delay line for determining a residual signal propagation time within a clock pulse. The fine measuring unit has several tapped delay lines whose combined length corresponds to one clock pulse and which are controlled with a different phase of the clock pulse.

[0006] In this distance sensor, the clock signal is synchronized with the transmission of the transmitted signals. This assumes a defined, consistent relationship between the start of the clock signal and the transmission of the transmitted signal. Only a stop signal, which is generated when the received signal is registered, is measured. To precisely determine the arrival of the received signal, two tapped delay lines are used to determine the remaining signal propagation time of the received signal as accurately as possible.

[0007] EP 1 876 468 A1 (D1) relates to a device for optoelectronic, contactless distance measurement according to the time-of-flight principle, comprising a sensor unit with at least one light source for emitting light pulses, a detector for detecting light pulses, a device for splitting the light pulses into a measuring pulse and a reference pulse, measuring optics for directing the measuring pulses to an object whose distance from the sensor unit is to be measured, and for directing measuring pulses reflected from the object as echo pulses to the detector, and reference optics for directing the reference pulses towards the detector, wherein the distance of the object from the sensor unit can be determined from a time difference between a start signal and an echo signal derived from an echo pulse. An optical path of the reference pulses to the detector is shorter or longer by at least one offset distance than an optical path of the measuring pulses to the detector.

[0008] The detector is used for the alternating quantitative detection of both the echo pulses and the reference pulses, whereby a controllable pulse attenuator is arranged in the beam path of the echo pulses for the targeted attenuation of the echo pulses so that they are comparable in size to the reference pulses.

[0009] The Examining Division rightly cites D1 as the closest prior art, since it has a distance sensor with a TDC unit, which is used to determine the distance by performing a coarse measurement by counting pulses of a clock signal and two fine measurements to determine the time differences between the emission and reception of a light pulse relative to the clock of the clock signal.

[0010] Nevertheless, in our opinion, the subject matter of claim 1 of the present application is neither anticipated nor suggested by D1, since the precision measurements according to the invention differ from the precision measurements described in D1 (sections

[0069] -

[0073] ).

[0011] The first fine measurement in D1 (fine interpolator L7) is started with a start signal and not with the transmission pulse, as described in claim 1. The same applies to the second fine measurement in D1, whereby it is also not described in D1 that the second fine measuring unit stops the coarse measurement.

[0012] In CLAUDIO FAVI ET AL: "A 17ps Time-to-digital converter implemented in 65nm FPGA technology", FIELD PROGRAMMABLE GATE ARRAYS, February 22, 2009 (2009-02-22), pages 113-120, XP058287913 (D2), a time-of-flight measurement using a time-to-digital converter is described, particularly useful for time-off-flight sensors. Delay lines are used to perform the time-of-flight measurements.

[0013] In CHEN HAOCHANG ET AL: "A Low Nonlinearity, Missing-Code Free Time-to-Digital Converter Based on 28-nm FPGAs With Embedded Bin-Width Calibrations", IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, Vol. 66, No. 7, July 1, 2017 (2017-07-01), pages 1912-1921, XP011651875 (D3), a time-to-digital converter for performing time-of-flight measurements is described. The time-to-digital converter comprises a cell with multiple delay lines for which heterogeneous sampling is provided.

[0014] The invention is based on the object of providing a distance sensor and a method by means of which an accurate and reproducible determination of the distance between objects is made possible.

[0015] To achieve this object, the features of the independent claims are provided. Advantageous embodiments and expedient developments of the invention are described in the dependent claims.

[0016] The invention relates to a distance sensor for determining the distance between objects in a surveillance area, comprising a transmitter and a receiver, wherein transmission pulses emitted by the transmitter are guided to the object and reflected by the latter as a reception pulse to the receiver, and comprising an evaluation unit in which the distance between the object and the distance sensor is determined from the propagation time of the transmission pulses to the object. Two independently operating asynchronous oscillators are provided, wherein one oscillator generates pulse-shaped, periodic clock signals for transmitter operation. One oscillator forms a clock generator that generates periodic clock signals for controlling components of the evaluation unit. The clock generator generates a clock signal asynchronous to the transmission pulse. The evaluation unit comprises at least one TDC unit, which has a coarse measuring unit and two fine measuring units, each with a delay line.The coarse measuring unit is designed to count entire cycles of the clock signal. A transmit pulse starts a first fine measuring unit, which starts the coarse measuring unit and measures the time interval between the transmission of the transmit pulse and the subsequent start of a cycle of the clock signal. The receive pulse following the transmit pulse starts the second fine measuring unit, which stops the coarse measuring unit and measures the time interval between the reception of the receive pulse and the subsequent start of a cycle of the clock signal.

[0017] The invention further relates to a method for determining the distance of objects in a surveillance area.

[0018] An essential aspect of the invention is that the clock generator generates a clock signal used for propagation time measurement, which is asynchronous and thus independent of the transmission clock of the transmission pulses.

[0019] Accordingly, according to the invention, the time of flight measurement is carried out in such a way that both the time of transmission of a transmission pulse and the time of arrival of the reception pulse are measured.

[0020] The times can be determined with high precision by using the coarse measuring unit to count the number of cycles of the clock signal and also by performing fine measurements such that a first fine measuring unit determines the time interval between the transmission of the transmission pulse and the start of the next clock signal, and a second fine measuring unit determines the time interval between the reception of the reception pulse and the start of the next clock signal. This allows the times of emission of the

[0021] The transmission pulse and the reception of the reception pulse can be determined with high precision using two independently operating precision measuring units.

[0022] The control of the components for the runtime measurement is reproducible and simple in that the first fine measuring unit starts the coarse measuring unit when the transmission pulse is sent and the second fine measuring unit stops the coarse measuring unit again when the reception pulse is received.

[0023] The components of the time-of-flight measurement form a TDC (time to digital converter) unit, which can be manufactured as an integrated module.

[0024] Such a TDC unit enables highly precise time-of-flight measurement. Problems with time-of-flight measurement can only arise if a transmit or receive pulse occurs exactly at the beginning of a clock signal. In this case, a clear assignment of the transmit or receive pulse to a pulse of the clock signal is not possible.

[0025] To solve this problem, the evaluation unit has several TDC units to which the clock signal is supplied with different phases.

[0026] The measurement results determined in the TDC units are offset or compared with each other.

[0027] This means that if one of the TDC units does not provide usable measurement results, it is not used for further evaluation, thus avoiding measurement errors.

[0028] Advantageously, the or each TDC unit is implemented on an FPGA.

[0029] This results in a particularly compact and cost-effective hardware structure for the runtime measurement components.

[0030] To perform highly accurate propagation time measurements, the precision measuring units of the or each TDC unit have delay lines, i.e. digital delay structures in the form of TDL (Tapped Delay Lines).

[0031] Advantageously, the delay line of a precision measuring unit comprises a series arrangement of delay elements, wherein the propagation time of a transmitted pulse or received pulse is detected by detecting the propagation of a signal edge along the delay elements.

[0032] The delay elements have binary signal states, with each delay element being assigned a sampling element to detect the signal states.

[0033] In particular, each delay element consists of a multiplexer to which an XOR gate is assigned.

[0034] Furthermore, each scanning element is advantageously formed by a flip-flop.

[0035] When the individual delay elements are sampled by the sampling elements at a given time, a so-called thermometer code is obtained. The position of a signal pulse corresponding to a transmit pulse or receive pulse is represented by a signal edge that separates a sequence of bits with signal values ​​of 0 from a sequence of bits with signal values ​​of 1.

[0036] According to a first variant, homogeneous sampling is carried out by means of the sampling elements in such a way that each sampling element taps the signal state at the same component of a delay element.

[0037] In this case, the sampled signal sequence may be corrupted due to unavoidable hardware tolerances of the delay elements, which consist of semiconductor components. Such tolerances can shorten the hardware connections between two delay elements, so that one delay element always delivers the signal value 0 or always the signal value 1, thus corrupting the sampled signal sequence.

[0038] In order to rule out such corruptions in principle, according to a second variant, a heterogeneous sampling is carried out by means of the sampling elements in such a way that, in an alternating sequence, a first sampling element taps the signal state at a first component of a delay element and a second sampling element taps the signal state at a second component.

[0039] To further increase the measurement accuracy, each precision measuring unit advantageously has a correction unit to eliminate bubble errors.

[0040] Such bubble errors are individual corrupted bits in the signal sequence obtained during a sampling of the delay elements, which is represented as a thermometer code. These corrupted bits are corrected using the bubble error unit.

[0041] The correction unit thus formed advantageously has a dual function such that by means of the correction unit a signal sequence present on the delay line in the form of a thermometer code is implemented in a one-hot code.

[0042] Advantageously, the TDC unit has a computer unit in which output signals generated by the coarse measuring unit and the delay elements are processed.

[0043] In order to be able to easily evaluate the signal sequences obtained during the sampling of the delay elements of the delay lines in the computer unit, a converter unit is arranged downstream of the correction unit, in which a conversion of the one-hot code into a binary number representation is converted.

[0044] The measuring accuracy of the or each TDC unit is particularly advantageously increased by the fact that each precision measuring unit has a calibration unit.

[0045] The functionality is such that the calibration unit compensates for measurement inaccuracies due to different delay times of the delay elements of the delay lines.

[0046] Since the transmitter operates asynchronously to the clock generator for the or each TDC unit, it is possible to perform a statistical calibration with the calibration unit.

[0047] In this statistical calibration, a histogram is determined for the delay elements of a delay line in such a way that, given a large number of measurements with transmitted pulses, the frequency up to which delay element the signal pulse corresponding to a transmitted pulse or received pulse has reached the delay line is determined. Since the transmission time of the transmitted pulse is asynchronous to the clock of the clock generator and thus asynchronous to the time measurements in the respective delay line, the same frequency is expected for all delay elements in an ideal delay line. Due to component tolerances of the delay elements, the recorded histogram does not show an even distribution for the delay elements. From this histogram, correction tables can be created in the calibration unit, which can be used to compensate for the tolerance-related differences in the delay elements.

[0048] The distance sensor according to the invention can advantageously be designed as an optical sensor whose transmitter emits transmitted light beams.

[0049] Alternatively, the distance sensor can also emit radio radiation, radar radiation or ultrasonic waves.

[0050] According to a first variant, the transmitter of the distance sensor emits transmission pulses in a fixed direction.

[0051] Alternatively, the distance sensor can also be designed as a scanner. In this case, the transmitted pulses are periodically guided within a planar or spatial scanning area. For this purpose, a deflection unit, such as a motor-driven deflection mirror, can be assigned to the transmitter, which deflects the transmitted pulses so that they are guided within the scanning area. Alternatively, the transmitter and receiver can be arranged in a rotating measuring head.

[0052] In principle, the distance sensor can also be extended to include multiple transmitters and / or receivers.

[0053] The invention is explained below with reference to the drawings. They show: Figure 1Embodiment of the distance sensor according to the invention. Figure 2Representation of a TDC unit of the distance sensor according to Figure 1 Figure 3Representation of a precision measuring unit of the TDC unit according to Figure 2 with homogeneous sampling of a delay line. Figure 4 Representation of a precision measuring unit of the TDC unit according to Figure 2 with heterogeneous sampling of a delay line. Figure 5 Timing diagram to illustrate the measuring principle of the distance sensor according to Figure 1 Figure 6 a) Representation of a thermometer code generated in a precision measuring unit with an erroneous bit. Figure 6 b) Thermometer code according to Figure 6 a) after correction with a correction unit. Figure 6 c)From the thermometer code according to Figure 6b)generated one-hot code. Figure 7 Histogram for 6 delay elements of a delay line of a precision measurement unit a) real case b) ideal case

[0054] Figure 1 shows schematically the structure of an embodiment of the distance sensor 1 according to the invention, which in the present case is designed as an optical sensor.

[0055] The distance sensor 1 has an FPGA 2, on which the components of an evaluation unit are integrated. A transmitter 3 and a receiver 4 are also provided. To detect objects 5 in a monitored area, the transmitter 3 emits transmitted light beams 7 in the form of transmitted pulses. These are reflected by the object 5 as received light beams 8 in the form of received pulses back to the receiver 4.

[0056] Two oscillators 9, 10 are assigned to the FPGA 2, which operate independently of each other, i.e., asynchronously. Oscillators 9, 10 can generally also be implemented on the FPGA 2. An oscillator 9 forms a clock generator that generates periodic clock signals 11 for controlling components of the evaluation unit. An oscillator 10 generates pulse-shaped, periodic clock signals 12 for transmitter operation.

[0057] How Figure 1 shows, transmission signals 13 of the transmitter 3 and reception signals 14 of the receiver 4 are read into the evaluation unit.

[0058] The clock signals 12 of the oscillator 10 drive a pulse generation unit 15. This generates control pulses 16, which drive the transmitter 3 to emit a sequence of transmission pulses, which are evaluated individually.

[0059] With the distance sensor 1 according to Figure 1Distance measurement is performed using a pulse-time-of-flight method. To determine the distance to object 5, transmitter 3 emits transmission pulses, which are reflected back by object 5 as reception pulses to receiver 4. The evaluation unit determines the travel time 6 of the transmission pulses to object 5 and back to receiver 4, and from this, the distance of object 5 to distance sensor 1 is calculated.

[0060] To determine the propagation time of the transmission pulses, the embodiment according to Figure 1 Three identical TDC units 17, i.e., time-to-digital converters, are provided. Generally, a different number of TDC units 17 may also be provided, although in particular, only one TDC unit 17 may be provided.

[0061] The clock signals 11 of the oscillator 9 are fed to the individual TDC units 17 with different phase positions. The results of the runtime measurements are forwarded by the individual TDC units 17 as output signals 18 to a central unit 19, where the output signals 18 are jointly evaluated to generate a distance signal that is output by the distance sensor 1.

[0062] Figure 2 shows the structure of a TDC unit 17 of the distance sensor 1 according to Figure 1. The TDC unit 17 has a first fine measuring unit 20, to which the transmission signals 13 are fed. Furthermore, a second fine measuring unit 20' is provided, to which the reception signal 14 of the receiver 4 is fed. Furthermore, a coarse measuring unit 21 is provided, which is designed to count the clock signals 11 of the oscillator 9, i.e., the clock generator.

[0063] Detection signals 22, 23 generated in the precision measuring units 20, 20' are fed to the coarse measuring unit 21. The output signals generated in the precision measuring units 20, 20' and the coarse measuring unit 21 are fed to a computer unit 24, in which the output signal 18 is generated depending on the output signals.

[0064] The precision measuring units 20, 20' have an identical structure. Each precision measuring unit 20, 20' has an input filter 30. If the transmitter emits three multiple pulses, the input filter 30 filters out one transmitted pulse from each of these. The same applies to the received pulses.

[0065] A delay line 31 is arranged downstream of the input filter 30. Its structure is shown in the Figure 3 and 4shown in more detail. This forms a tapped delay line, i.e., digital delay structures for highly accurate propagation time measurement. The delay line 31 is followed by a first register stage 32, which is followed by a correction unit 33 for eliminating bubble errors. This is followed by a second register stage 34, which is followed by a converter unit 35 designed to convert a thermometer code into a one-hot code. This is followed by a further register stage 36 and a calibration unit 37. Finally, a unit 38 for detecting valid measurements is provided, which is connected to the first register stage 32 of the converter unit 35 and the calibration unit 37.

[0066] The measuring principle of the TDC unit 17 according to Figure 1 Runtime 6 is in Figure 5 illustrated. In Figure 5The clock signal 11 of the oscillator 9 forming the clock generator is shown. The clock signal 11 consists of a periodic sequence of clock pulses a. If a transmission pulse is emitted by the transmitter 3, the transmission signal 13 corresponding to the transmission pulse is transmitted in the form of a signal pulse (in Figure 5 (denoted by S) is fed to the first precision measuring unit 20. The precision measuring unit 20 then starts the coarse measuring unit 21 with the detection signal 22, whereupon the coarse measuring unit 21 counts the clock pulses a of the clock signal 11. The precision measuring unit 20 detects the propagation time 6 from the occurrence of the signal pulse to the beginning of the next clock pulse by propagating the signal pulse S in the delay line 31.

[0067] As soon as the transmitted pulse reflected by object 5 reaches the receiver 4 as a received pulse, the signal pulse E is fed to the second precision measuring unit 20' as the received signal 14, corresponding to the received pulse. The precision measuring unit 20' then stops the coarse measuring unit 21 with the detection signal 23. In parallel, the precision measuring unit 20' uses the delay line 31 to measure the time interval c between the occurrence of the received pulse and the beginning of the next cycle of the clock signal.

[0068] The transit times b and c measured with the precision measuring units 20, 20' as well as the count of the coarse measuring unit 21 are fed as output signals to the computer unit 24. There, the transit time 6 between the transmitted pulse and the received pulse is calculated from these measured values ​​and fed as output signal 18 to the central unit 19.

[0069] Such a transit time measurement is only not possible unambiguously if the transmit pulse or receive pulse, i.e. one of the signal pulses, coincides with the beginning of a clock a.

[0070] To avoid such problems, the evaluation unit of the distance sensor 1 has three TDC units 17, to which the clock signal 11 is supplied with different phases. If, during the propagation time measurement in one of the TDC units 17, the transmit pulse or receive pulse coincides with the beginning of a clock pulse a, this is definitely not the case for the other two TDC units 17, so that they deliver correct propagation time values.

[0071] By jointly evaluating the output signals 18 of the TDC units 17, for example by a majority decision, the correct transit time value is determined in the central unit 19, which is then converted into a distance value and output as an output signal of the distance sensor 1.

[0072] Figure 3shows a first embodiment of a delay line 31 with an associated register stage 32 and a correction unit 33.

[0073] The delay line 31 consists of a series arrangement of delay elements 40 in the form of a multiplexer or the like. An XOR gate 41 is connected to each delay element 40.

[0074] Register stage 32 serves to sample the current signal states of delay elements 40. For this purpose, register stage 32 has a series arrangement of flip-flops as sampling elements 32a. The outputs of the flip-flops, which are designed as D flip-flops, are fed to correction unit 33.

[0075] The delay elements 40 have binary signal groups 0 and 1 at their outputs, which are sampled by the sampling elements 32a simultaneously at a sampling time.

[0076] In the embodiment according to Figure 3homogeneous sampling is carried out such that all flip-flops sample the signal value at the output of the delay element 40.

[0077] The embodiment according to Figure 4 is with regard to the embodiment according to Figure 3 modified so that heterogeneous sampling occurs. In this heterogeneous sampling, first flip-flops sample the signal at the output of delay element 40, and second flip-flops sample the signal at XOR gate 41.

[0078] Since in homogeneous sampling according to Figure 3 all flip-flops sample the signals at the output of the respective delay element 40, a simple design of the correction unit 33 is sufficient in this case.

[0079] Since heterogeneous sampling according to Figure 4an alternating sequence of flip-flops is provided, in which first flip-flops sample the outputs of the delay elements 40 and second flip-flops sample the XOR gates 41, in this case the first register stage 32 and the correction unit 33 are implemented twice.

[0080] This scanning creates a thermometer code, as in Figure 6 shown. The signal pulse corresponds to a signal edge, i.e. a transition from signals with the signal value 1 to signals with signal values ​​0.

[0081] This signal edge travels along the individual delay elements 40 according to the delay times of the delay line 31. In the present case, the signal edge corresponding to the signal pulse has arrived at the m-th delay element 40. With the sampling, the time interval c between sampling a signal pulse S or E until the beginning of the next clock pulse a (see Figure 5) is measured by detecting how far the signal edge propagates in the delay line 31.

[0082] As from Figure 6 a) As can be seen, a bubble error is present at the n-th delay element 40, since the signal value 0 is incorrectly present there.

[0083] This bubble error is eliminated in the correction unit 33, so that the error-free thermometer code according to Figure 6b) is received.

[0084] In the correction unit 33, the thermometer code ( Figure 6b) into a one-hot code ( Figure 6c) changed.

[0085] Finally, in the converter unit 35, the one-hot code is converted into a binary number representation, which is fed as an output signal to the computer unit 24.

[0086] In the arrangement according to Figure 3During homogeneous sampling, errors may occur that are caused by component-related tolerances. For example, a delay element 40 may always have the same signal state. With homogeneous sampling, this could lead to errors in the propagation time determination. This source of error is eliminated by the embodiment according to Figure 4 , in which inhomogeneous scanning occurs, is largely eliminated.

[0087] Due to tolerance-related component variations, the individual delay elements 40 of the delay line 31 of a TDC unit 17 do not operate completely identically, which also leads to errors in the propagation time determination.

[0088] To eliminate this source of error, the calibration unit 37 is provided, in which a statistical calibration is carried out.

[0089] For this purpose, a calibration measurement is carried out in which, through a large number of distance measurements, it is determined to which delay element 40 of the delay elements 40 the individual signal pulse has reached. The histogram thus recorded shows Figure 7 a) , while Figure 7b) shows the ideal case. Since the transmit pulses and receive pulses occur completely asynchronously to the clock signal 11, the position of the transmit pulse or receive pulse relative to the clock a is completely random. Therefore, with a large number of distance measurements for ideal delay elements 40 of the delay line 31, a histogram as in Figure 7b) shown, that is, for all delay elements 40 the same frequency is expected that the signal pulse corresponding to the transmission pulse or reception pulse is registered there.

[0090] In fact, however, no equal distribution is obtained, as in Figure 7 a)illustrated, since the delay elements 40 of the delay line 31 are not completely identical.

[0091] The real, measured histogram according to Figure 7 a) is therefore used in a calibration of the calculation of correction factors for the individual delay elements 40.

[0092] To further reduce measurement errors, the Figure 2 The unit 38 shown here is used to detect valid measurements. This unit 38 checks whether the signal sequence obtained by sampling in the first register stage 32 corresponds to an expected propagation of a signal edge corresponding to a transmitted pulse or received pulse. List of reference symbols

[0093] (1)Distance sensor (2)FPGA (3)Transmitter (4)Receiver (5)Object (6)Travel time (7)Transmitted light beams (8)Received light beams (9)Oscillator (10)Oscillator (11)Clock signal (12)Clock signal (13)Transmitted signal (14)Received signal (15)Pulse generation unit (16)Control pulse (17)TDC unit (18)Output signal (19)Central unit (20)Fine measuring unit (20')Fine measuring unit (21)Coarse measuring unit (22)Detection signal (23)Detection signal (24)Computer unit (30)Input filter (31)Delay line (32)First register stage (32a)Sampling element (33)Correction unit (34)Second register stage (35)Converter unit (36)further register stage (37)calibration unit (38)unit (40)delay element (41)XOR gate (a)clock (b)propagation time (c)time interval (E)signal pulse (S)signal pulse

Claims

1. Distance sensor (1) for determining the distance of objects (5) in a monitoring area, having a transmitter (3) and a receiver (4), wherein transmission pulses emitted by the transmitter (3) are guided to the object (5) and reflected by the latter as a reception pulse to the receiver (4), and having an evaluation unit in which the distance of the object (5) to the distance sensor (1) is determined from the transit time (6) of the transmission pulses to the object (5), wherein two independently asynchronously operating oscillators (9, 10) are provided, one oscillator (10) generating pulse-shaped, periodic clock signals (12) for transmitter operation and one oscillator (9) forming a clock generator which generates periodic clock signals (11) for controlling components of the evaluation unit, wherein the clock generator generates a clock signal (11) which is asynchronous to the transmission pulse, in that the evaluation unit comprises at least one TDC unit (17), wherein this has a coarse measuring unit (21) and two fine measuring units (20, 20'), each with a delay line (31), wherein the coarse measuring unit (21) is constructed as follows counting whole clock pulses (a) of the clock signal (11), in that a first fine measuring unit (20) is started with a transmission pulse, the coarse measuring unit (21) being started with this and the time interval (c) between transmission of the transmission pulse and the subsequent start of a clock pulse (a) of the clock signal (11) being measured, in that the second fine measuring unit (20') is started with the receive pulse following the transmit pulse, the coarse measuring unit (21) being stopped with this and the time interval (c) between receiving the receive pulse and the subsequent start of a clock pulse (a) of the clock signal (11) being measured.

2. Distance sensor (1) according to claim 1, characterised in that the delay line (31) of a fine measuring unit (20, 20') has a series arrangement of delay elements (40), wherein the propagation time (6) of a transmit pulse or receive pulse is detected by detecting the propagation of a signal edge along the delay elements (40).

3. Distance sensor (1) according to claim 2, characterised in that the delay elements (40) have binary signal states, wherein a sensing element (32a) is assigned to each delay element (40) to detect the signal states.

4. Distance sensor (1) according to claim 3, characterised in that homogeneous sensing is carried out by means of the sensing elements (32a) in such a way that each sensing element (32a) taps the signal state at the same component of a delay element (40).

5. Distance sensor (1) according to claim 3, characterised in that heterogeneous sensing is carried out by means of the sensing elements (32a) in such a way that, in an alternating sequence, a first sensing element (32a) taps the signal state at a first component of a delay element (40) and a second sensing element (32a) taps the signal state at a second component.

6. Distance sensor (1) according to one of claims 2 to 5, characterised in that each delay element (40) consists of a multiplexer with which an XOR gate (41) is associated.

7. Distance sensor (1) according to claim 3, characterised in that each sensing element (32a) is formed by a flip-flop.

8. Distance sensor (1) according to one of claims 1 to 7, characterised in that each fine measuring unit (20, 20') has a correction unit (33) for eliminating bubble errors.

9. Distance sensor (1) according to claim 8, characterised in that a signal sequence present on the delay line (31) in the form of a thermometer code is carried out in a one-hot code by means of the correction unit (33).

10. Distance sensor (1) according to claim 9, characterised in that a converter unit (35) is arranged downstream of the correction unit (33), in which a conversion of the one-hot code is converted into a dual-number representation.

11. Distance sensor (1) according to one of claims 1 to 10, characterised in that each fine measuring unit (20, 20') has a calibration unit (37).

12. Distance sensor (1) according to claim 11, characterised in that measurement inaccuracies due to different delay times of the delay elements (40) of the delay lines (31) are compensated for by means of the calibration unit (37).

13. Distance sensor (1) according to one of claims 11 or 12, characterised in that a statistical calibration is carried out by means of the calibration unit (37).

14. Distance sensor (1) according to one of claims 1 to 13, characterised in that the TDC unit (17) has a computer unit (24) in which output signals generated with the coarse measuring unit (21) and the delay elements (40) are processed.

15. Distance sensor (1) according to one of claims 1 to 14, characterised in that the evaluation unit has a plurality of TDC units (17) to which the clock signal (11) is supplied with different phases.

16. Distance sensor (1) according to claim 15, characterised in that the measurement results determined in the TDC units (17) are calculated or compared with each other.

17. Distance sensor (1) according to one of claims 1 to 16, characterised in that the or each TDC unit (17) is implemented on an FPGA (2).

18. Distance sensor (1) according to one of claims 1 to 17, characterised in that this is an optical sensor whose transmitter (3) emits transmitted light beams (7).

19. Distance sensor (1) according to one of claims 1 to 17, characterised in that its transmitter (3) emits radio radiation, radar radiation or ultrasonic waves.

20. Distance sensor (1) according to one of claims 1 to 19, characterised in that it is designed as a scanner.

21. Method for determining the distance of objects (5) in a monitoring area by means of a distance sensor (1) with a transmitter (3) and a receiver (4), wherein transmission pulses emitted by the transmitter (3) are guided to the object (5) and from this are guided as reception pulses to the receiver (4), and with an evaluation unit in which the distance of the object (5) to the distance sensor (1) is determined from the transit time (6) of the transmission pulses to the object (5), wherein two independently asynchronously operating oscillators (9, 10) are provided, one oscillator (10) producing pulsed, periodic clock signals (12) for transmitter operation and an oscillator (9) forms a clock generator which generates periodic clock signals (11) for controlling components of the evaluation unit, the clock generator generating a clock signal (11) which is asynchronous to the transmission pulses, in that the evaluation unit comprises at least one TDC unit (17), the latter having a coarse measuring unit (21) and two fine measuring units (20, 20') each with a delay line (31), the coarse measuring unit (21) being designed to count whole clock pulses (a) of the clock signal (11), in that a first fine measuring unit (20) is started with a transmission pulse, the coarse measuring unit (21) being started therewith and the time interval (c) between transmission of the transmission pulse and the subsequent start of a clock pulse (a) of the clock signal (11) being measured, in that the second fine measuring unit (20') is started with the receive pulse following the transmit pulse, the coarse measuring unit (21) being stopped with this and the time interval (c) between receiving the receive pulse and the subsequent start of a clock pulse (a) of the clock signal (11) being measured.