Methods for determining a distance

Abstract

Sensor (10) for determining a transit time, in particular a distance according to a pulse-based transit time method, comprising a transmitter (12) for emitting a signal pulse (14) triggered by a transmit pulse, a receiver (22) for generating a receive pulse from the signal pulse (20) reflected or remitted by an object (18a-b) and received again, and a control and evaluation unit (24) configured to determine a transit time from the transmit pulse and the receive pulse and, over a measurement period (28), to trigger or transmit a further signal pulse (14) upon receipt of a receive pulse, such that the received signal pulse immediately triggers the next emitted signal pulse, so that during the measurement period (28) more signal pulses (14, 20) travel back and forth for a nearby object (18a) than for a distant object (18b).and wherein the runtime is determined directly or indirectly from the ratio of a counting information and the measurement duration, wherein the counting information is proportional to the number of signal pulses triggered or passed through the measuring section during the measurement duration.

Description

[0001] The invention relates to a sensor and a method for determining a distance using a pulse-based time-of-flight method according to the preamble of claim 1 and 10, respectively.

[0002] Time-of-flight (TOF) distance measurement is known for various signals, such as microwaves, light, or ultrasound. Many sensor types exist specifically for optical distance measurement, including single-beam rangefinders, laser scanners, and TOF 3D cameras. Such sensors can be used, for example, in vehicle safety, logistics, or factory automation. A particular area of ​​application is safety technology, where the sensor monitors the area around a hazard and, upon detection of an unauthorized object, initiates a safety measure, such as an emergency stop.

[0003] In pulse detection, a pulse is emitted and reflected by the object being measured. The time elapsed between the transmitted and received pulse must be measured very precisely. This is a demanding task and places very high demands on the hardware. If the distance measurement resolution is to achieve an accuracy of even a few tens of millimeters, the signal propagation time must be determined to within a hundred picoseconds. Typically, the timing measurement is based on a Time-to-Digital Converter (TDC), which essentially acts as a stopwatch to measure the time interval between a transmission and a reception. A relatively simple timing measurement can be achieved using a counter with a reference clock. However, this is no longer feasible for fine timing resolutions, as, for example, a resolution of 100 ps already requires a clock speed of 10 GHz.Therefore, the signal is refined in further stages, and the basic accuracy of the counter is increased using clock delay methods. Such TDCs become complex and require elaborate structures. Further details on TDCs can be found, for example, in Henzler, Stephan. “Time-to-digital converters.” Vol. 29. Springer Science & Business Media, 2010, or in the 2015 annual report of the Circuits and Systems group at the University of Oulu by J. Kostamovaara and T. Rahkonen.

[0004] Another analog timing method involves charging a capacitor with a defined current starting from a start signal, ending the charging process with a stop signal, and reading the capacitor voltage. With such an analog method, it is very difficult to control the component properties and environmental influences to such an extent that extremely high measurement accuracy can actually be achieved.

[0005] Another conventional approach to time-of-flight measurement uses so-called tapped delay lines (TDLs). These are digital delay structures (delay lines) consisting of multiple repetitions of a simple logic circuit. In a register parallel to the delay elements, the transmit pulse generates ones as long as an enable signal is present that ends with the receive pulse. The desired time of flight thus corresponds to the position of the transition from one to zero in the register. Such distance-measuring sensors are presented, for example, in DE 10 2015 103 472 A1 and EP 3 502 734 A1. The implementation, preferably on a programmable component such as an FPGA (Field Programmable Gate Array), is still relatively complex. Furthermore, the time resolution is limited to that of the delay elements, so that very long TDLs, and therefore complex structures, are required for very high measurement accuracy.

[0006] DE 10 2017 127 505 A1 discloses a pulse-generating device that can generate extremely short pulses via a delay element and a reflector. While such pulses can be used for a time-of-flight method, DE 10 2017 127 505 A1 does not elaborate on this.

[0007] German patent DE 10 2018 127 635 A1 describes an optoelectronic sensor that, upon receiving a light pulse, triggers the emission of another light pulse. This is intended to mix up the transmission times, thus providing a kind of coding that prevents similar systems from interfering with each other.

[0008] US patent 2018 / 0284278 A1 deals with an adaptive pulse rate in a lidar system. By transmitting a new pulse immediately upon receiving the previous one, the pulse rate can be accelerated.

[0009] From the subsequently published DE10 2020 100 451 A1, a time-of-flight measurement is known which feeds a delay line with the transmit pulse and the receive pulse at both ends. The time of flight can be deduced from the delay element where the transmit and receive pulses meet.

[0010] EP 3 614 175 A1 discloses an optical distance measurement which emits a multitude of measurement pulses in a predetermined sequence and temporally localizes this sequence on the receiving side using an optimal filter.

[0011] From DE 10 2016 224 509 A1 a receiver arrangement for receiving at least one light pulse is known, which uses two evaluation devices with two different evaluation methods to determine a flight time, namely a time-to-digital conversion and a time-correlated photon counting.

[0012] US 2020 / 0137373A1 describes a three-dimensional image sensor with pixels that register incident photons in Geiger mode and accumulate the times of these detection events in a histogram.

[0013] It is therefore the object of the invention to improve the determination of a running time.

[0014] This problem is solved by a sensor and a method for determining a distance using a pulse-based time-of-flight method according to claims 1 and 10, respectively. The sensor operates using a pulse-based time-of-flight method in which a signal pulse is emitted and received. Measurements are usually taken using a sampling principle, where the transmitter and receiver are located at the same location, i.e., a remitted pulse is received. However, time-of-flight measurement with spatially separated transmitter and receiver is also conceivable, for example, for material determination. The signal pulse is the pulse on the measuring path, while the transmitted and received pulses are electronic, internal pulses. A time offset between the electronic pulses and the signal pulses can be compensated for by reference measurement or calibration.A transit-time measurement unit determines the transit time of the signal pulse, which can then be converted into the desired distance using the speed of light (half the speed of light in a scanning principle due to the signal pulse's round trip). If the signal pulse travels through a medium other than air, the speed of light changes accordingly, and this effect can be used conversely for material detection when the signal path is known. A new signal pulse is emitted after a signal pulse has been received and a receiving pulse has been generated.

[0015] The invention is based on the fundamental idea of ​​determining the transit time based on the number of signal pulses emitted within a fixed measurement period. Accordingly, as many signal pulses are emitted and received as possible within the measurement period. The number of signal pulses triggered during the measurement period can be directly or indirectly incorporated as measurement information into the determination of the transit time.

[0016] The invention has the advantage that transit-time measurement becomes possible without actually having to measure the transit times of individual pulses. This eliminates the need for complex evaluation electronics such as time-delay data centers (TDCs) and allows for a compact and cost-effective implementation. The transit-time measurement according to the invention can also be combined with other transit-time measurement methods, particularly other pulse methods. This can increase measurement accuracy or enable mutual monitoring, thus facilitating applications in safety engineering, for example.

[0017] The control and evaluation unit is designed to determine the runtime from the ratio of a counting value to the measurement duration, where the counting value is proportional to the number of signal pulses triggered or passed through the measuring section during the measurement duration. In one embodiment, the measurement value is directly related to the number of signal pulses during the measurement duration. Embodiments described later utilize measurement value that is indirectly influenced by the number of signal pulses.

[0018] The control and evaluation unit is preferably designed to count or integrate the signal pulses. Counting signal pulses is the most direct embodiment. For an object at a short distance, many signal pulses can be transmitted and received within the measurement period; for an object at a greater distance, fewer. The number of these back-and-forth signal pulses is inversely proportional to the distance of the object, so the propagation time is determined by the ratio of the number of pulses to the measurement period. In another embodiment, instead of counting transmitted pulses, they are integrated. This also yields measurement information proportional to the number of transmitted pulses, which can be easily acquired analogously, for example, using a capacitor. When integrating, additional constants are added to the pure number of transmitted pulses.With appropriate calibration or consideration of these constants, the ratio of measurement information to measurement duration still determines the runtime.

[0019] The control and evaluation unit preferably comprises a delay line with a plurality of delay elements connected in series and nodes between the delay elements. It is configured to feed the transmit pulse into the delay line at one end and the receive pulse at the other, and to determine the propagation delay based on the at least one node with an amplitude corresponding to the superposition of the transmit and receive pulses. The delay elements are preferably identical or at least produce the same delay, but neither is a necessary condition. The delay line is fed with the transmit pulse and the receive pulse from both ends. Thus, the transmit and receive pulses travel towards each other in the delay line and superimpose at a specific position within the delay line.At the node corresponding to this position, the signal amplitude is therefore greater than both the transmitted and received pulses, which is how the node can be identified. The position of this node within the delay line indicates how far the transmitted pulse could travel before the superposition occurred, and this is proportional to the desired propagation delay. It is conceivable that the superposition affects two or more adjacent nodes if the transmitted pulse, and consequently also the received pulse, have a sufficiently large pulse width.

[0020] Such a time-of-flight measurement using a delay line is the subject of the previously mentioned, subsequently published DE 10 2020 100 451 A1. This can be advantageously combined with the determination according to the invention and will therefore be explained in more detail below before the combination is discussed further:

[0021] The transmit and receive pulses are preferably normalized to the same amplitude. The transmit pulse can be directly fed internally and is therefore subject to few changes. The signal amplitude of the receive pulse, on the other hand, depends on the specific measurement, for example, the reflectivity of the emitting object and the distance. These effects are preferably eliminated by normalization. Furthermore, normalization reduces the superposition to a pulse of twice the amplitude, making it particularly easy to detect, as, for example, the required position of a threshold can be determined very simply.

[0022] The amplitudes at the nodes are preferably compared to a threshold value corresponding to the amplitude of the transmit or receive pulse. The superposition results in a larger amplitude. Therefore, a threshold value that lies above the amplitude of the individual pulses allows differentiation between the transmit and receive pulses on the one hand and the superposition on the other. The threshold value is preferably based on the stronger of the two pulses, whereby the transmit and receive pulses are preferably brought to the same amplitude during the initial input anyway.

[0023] Preferably, each node is connected to a diode that only allows a signal with an amplitude at least equal to the threshold value to pass through. This allows very simple circuit elements to ensure that only the superposition is considered for further evaluation. The diode blocks both the transmit and receive pulses individually. Alternatively, threshold evaluation using another circuit element, such as a comparator, is conceivable.

[0024] A storage element or integrator for detecting a superposition of transmitted and received pulses is preferably connected to the nodes. The storage element, particularly in the form of a capacitor, temporarily stores the superposition pulse. Subsequently, it is determined at which node the storage element detected a superposition pulse, and possibly also with what amplitude. Preferably, the storage element is only indirectly connected to the node via the aforementioned threshold comparison or the diode, so that only the superposition reaches a storage element. The storage element is erased after a measurement, for example by a parallel resistor through which the charge flows away.

[0025] The propagation delay is preferably determined from the position of the node within the delay line where a superposition of the transmit and receive pulses occurs. This is a type of gridded or digital evaluation with a time resolution corresponding to the delay of the individual delay elements. It is conceivable that a superposition was registered at several adjacent nodes, and then, for example, their time center is used as a measure of the desired propagation delay.

[0026] The propagation delay is preferably determined by comparing the amplitudes at several nodes where the transmit and receive pulses overlap. This allows for even more precise timing measurements with a resolution better than that achieved by simply delaying a delay element. The weighting of the amplitudes determines the respective fractions during which the overlap occurred in one delay element and in the other. This requires not only identifying the nodes where overlap occurred, as in the digital evaluation described in the previous paragraph, but also determining the amplitude, for example, through analog-to-digital conversion.

[0027] The transmit pulse is preferably wider than the delay of a delay element. This automatically applies to the receive pulse as well, which is essentially an echo of the transmit pulse. Sufficient pulse width prevents the transmit and receive pulses from completely overlapping within a delay element and not being registered at any node. Furthermore, a wider transmit pulse facilitates and improves the resolution increase just described by comparing the amplitudes at the nodes affected by the superposition. A portion of the rising and falling edges of the transmit pulse can be included in the pulse width, specifically to the extent that the edge amplitude is high enough to produce a detectable superposition.

[0028] Time-of-flight measurement using a delay line has the disadvantage of requiring a considerable amount of installation space to accommodate delays over typical distance measurement ranges. Therefore, the control and evaluation unit is preferably designed to inject another transmit pulse into the delay line after a transmit pulse has passed through it, but to trigger a signal pulse only when the transmit and receive pulses have superimposed themselves on the delay line. Whether this superposition has occurred can be determined directly or indirectly. The delay line, which is inherently too short for the required range, is traversed once or multiple times.In this system, either the transmitted pulse passing through the delay line simultaneously triggers the next transmitted pulse, which then passes through the delay line again, or a repetition frequency of the transmitted pulses is set according to the length of the delay line. Only when the transmitted pulse encounters the received pulse is another signal pulse emitted onto the measuring section. This allows the number of signal pulses and transmitted pulses to differ from other embodiments: If the delay line has to be traversed multiple times, there are fewer signal pulses than transmitted pulses. The inventive evaluation of measurement information, which depends on the number of signal pulses emitted and received within a measurement period, is used to restore unambiguous results despite the multiple traversals.

[0029] The control and evaluation unit is preferably designed to determine the ratio of transmitted pulses to received pulses as measurement information. As just explained, at greater object distances, multiple passes through the delay line occur, and therefore the number of transmitted pulses is only a fraction of the number of received pulses. This ratio thus indicates how many times the delay line was traversed before a superposition of transmitted and received pulses occurred. This measurement information can be obtained, for example, from the integrated level of the superpositions during the measurement period.This is intuitively understandable, because if each transmitted pulse generates a signal pulse at a short object distance, there are more encounters between the transmitted pulse and the received pulse in the delay line within the measurement period than at a greater object distance, where transmitted pulses cyclically pass through the delay line at least once without encountering the received pulse.

[0030] Preferably, an energy storage unit is associated with the transmitter, and the measurement information is determined from the energy stored in the energy storage unit after or during the measurement period. In this embodiment, the signal pulses are powered by an energy storage device whose stored energy decreases, at least temporarily, with each signal pulse. The energy stored after the measurement period is therefore a measure of the object distance. The shorter the distance to the object, the more signal pulses are transmitted and received within the measurement period. This also means that more energy is available per signal pulse at greater object distances. This at least partially compensates for the decreasing received signal level with increasing object distance. Therefore, favorable dynamic range compression is also achieved over the measurement range.

[0031] The energy storage unit preferably comprises a capacitor, and the control and evaluation unit measures the stored energy by means of a voltage across the capacitor. This is a very cost-effective implementation of an energy storage unit, and the voltage measurement allows very easy access to the remaining stored energy after or even during the measurement period.

[0032] The sensor is preferably designed as an optoelectronic sensor, where the transmitter is a light emitter, the receiver is a light receiver, and the signal pulse is a light pulse. The distance is thus measured optically using a time-of-flight method. Such an optoelectronic sensor can also be understood as a distance-measuring module from which various sensors, such as photoelectric sensors, light grids, or laser scanners, can be constructed.

[0033] The method according to the invention can be further developed in a similar manner and exhibits similar advantages. Such advantageous features are described by way of example, but not exhaustively, in the dependent claims following the independent claims.

[0034] The invention is further explained below with regard to additional features and advantages by way of example embodiments and with reference to the accompanying drawing. The illustrations in the drawing show: Fig. 1 a schematic representation of a sensor for distance measurement using a time-of-flight method; Fig. 2 a block diagram of an embodiment of a time-of-flight measurement by counting or integrating signal pulses; Fig. 3 a schematic representation of the signal pulses running back and forth within a measurement period for different object distances; Fig. 4. Another representation of the signal pulses running back and forth within a measurement period, now as signal amplitude or number of pulses as a function of time; Fig. 5 a schematic representation of a delay line for determining a runtime; Fig. 6 a representation of transmit and receive pulses propagating towards each other from both sides in the delay line; Fig. 7 a block diagram of another embodiment of a time-of-flight measurement by repeatedly passing through a delay line; Fig. 8 a block diagram of a further embodiment of a time-of-flight measurement by determining the remaining energy of an energy storage device supplying the transmitter for the generation of signal pulses; and Fig. 9 A representation of the pulse amplitudes and the remaining energy in the energy storage as a function of time during the measurement period.

[0035] Fig. Figure 1 shows a schematic representation of a distance-measuring sensor 10 in an embodiment as a one-dimensional optoelectronic distance probe. A light emitter 12, for example an LED or a laser light source, emits a light pulse 14 into a monitoring area 16. The trigger for the light pulse 14 is an electronic transmit pulse (not shown). When the light pulse 14 encounters an object 18, part of the light pulse 14 is remitted or reflected and returns as a remitted light pulse 20 to a light receiver 22. There, an electronic receive pulse (also not shown) is generated. In a practical embodiment, the sensor 10 has further elements, in particular transmitting and receiving optics and connections, which are not discussed here for the sake of simplicity.

[0036] A control and evaluation unit 24 controls the light transmitter 12 with the transmit pulse and evaluates the received pulse of the light receiver 22. The term control and evaluation unit 24 refers to analog circuit elements and / or digital components that measure the transit time between the transmit pulse and the received pulse for distance measurement to the object 18 and that may also be responsible for other sensor functions. This will be discussed later with reference to the Fig. 2, Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7, Fig. 8 to Fig. 9 explained in more detail. Constant components of the measured transit time due to internal signal propagation delays and the like can be compensated for by a reference measurement or calibration, also taking into account the current temperature. The possibly adjusted transit time corresponds to the desired distance of the object 18, where the scaling factor is half the speed of light. The halving accounts for the outward and return journey of the light pulse 14, 20.

[0037] The in Fig. The one-dimensional optoelectronic distance sensor shown in Figure 1 is only one embodiment of a sensor 10 according to the invention. For example, the sensor 10 can be coaxially constructed, be a multidimensional system such as a laser scanner or a 3D time-of-flight camera, or operate with entirely different electromagnetic signals, such as a TDR level sensor or radar. In principle, non-electromagnetic signals such as ultrasound are also conceivable, in which case the speed of sound would be used instead of the speed of light, thus mitigating the problem of extremely fast time measurement.

[0038] Fig. Figure 2 shows a block diagram of an embodiment of the time-of-flight measurement. The basic idea is to repeatedly transmit a signal pulse from the sensor 10 to the object 18 and receive the signal pulse again. Each received signal pulse triggers the next transmitted signal pulse. For a nearby object 18, many signal pulses are thus transmitted and received within a fixed measurement period, and fewer signal pulses are transmitted as the distance of the object 18 increases. The number of signal pulses within the measurement period is therefore a measure of the distance. Depending on the embodiment, either the number of signal pulses or a measured quantity influenced by them is evaluated directly.

[0039] A measurement in block 26 begins, for example, with a start pulse. This initiates a fixed measurement duration in a measurement duration block 28. The start pulse is also fed via one input of an OR gate 30 to an AND gate 32. A signal is present at the other input of the AND gate 32 only during the measurement duration, and in this way, measurement duration block 28 ends the measurement after the measurement duration has elapsed. At the beginning of the measurement, the measurement duration has not yet expired, and therefore the start pulse is fed from the OR gate 30 via the AND gate 32 as a transmit pulse to transmitter 12. There, a signal pulse is generated that travels along the measuring path 16 to the object 18 and back. The received pulse generated in receiver 22 is delayed by the desired propagation time relative to the transmitted pulse due to the measuring path 16.

[0040] The received pulse is fed into the other input of the OR gate 30 via a feedback loop. This generates another transmitted pulse, provided the measurement time has not yet elapsed. Simultaneously, the received pulse is counted or integrated in an evaluation unit 34. At the end of the measurement period, the evaluation unit 34 therefore provides the number of transmitted and received signal pulses, or their integral, as proportional measurement information. The ratio of measurement time to number of pulses is an estimate of the propagation time.

[0041] The implementation shown is only an example. In particular, specifying a measurement duration is also possible using methods other than the OR gate 30 and the AND gate 32, for example, by means of diodes or in a completely different implementation. In an optoelectronic sensor 10 as in Fig. The signal pulses are the emitted light pulse 14 and the reflected light pulse 20, and these are to be distinguished from the internal electronic transmit and receive pulses. A light transmitter 12 is particularly suitable for very short signal pulses, such as that of DE 10 2017 127 505 A1 mentioned in the introduction. The light receiver 22, on the other hand, preferably has at least one APD (Avalanche Photodiode) or SPAD (Single-Photon APD) that generates short pulses as receive events.

[0042] Fig. Figure 3 illustrates the measurement principle once again using a nearby object 18a with a solid line and a distant object 18b with a dashed line. The measurement duration t runs from top to bottom. Mess . At the beginning, a signal pulse is emitted, which returns from object 18a-b to sensor 10 and immediately triggers the next signal pulse. During the fixed measurement duration t MessFor a nearby object 18a, inversely proportionally more signal pulses travel back and forth than for a distant object 18b.

[0043] Fig. Figure 4 illustrates the measurement principle in a further representation. Here, the signal pulses themselves are shown in the upper part, and their number or integral over time is shown in the lower part. A solid line represents a closer object 18a with a travel time of 1.5 ns, and a dashed line represents a more distant object 18b with a travel time of 2 ns. The signal pulses for the closer object 18a are closer together, and consequently, a higher number of pulses is recorded after the measurement period.

[0044] The method has the property that propagation times to nearby objects 18a are determined more accurately than to distant objects 18b. Furthermore, the susceptibility to errors due to lost or externally added signal pulses is lower for closer objects 18a. This becomes particularly clear when comparing situations where the measurement duration is sufficient, compared to the propagation time, to receive an initial signal pulse and just barely receive, or just barely miss, a second signal pulse. The actual propagation time differs only minimally here, but the measured propagation time differs by a factor of two. The measurement duration should therefore be chosen so that several signal pulses are received over the entire range. However, this only mitigates the effects; it does not eliminate the discretization error.

[0045] A more precise measurement is possible in an embodiment that combines the method of the previously mentioned, subsequently published DE 10 2020 100 451 A1. Its delay-line-based method will therefore now be described in an insert using the Fig. 5 and Fig. 6 will be explained again.

[0046] Fig. Figure 5 is a schematic representation of the delay line 36, which in the aforementioned combined embodiment is part of the control and evaluation unit 24. Essentially, it is a series connection of a plurality of delay elements 38, with each transition being designated as a node 40 between two delay elements 38. It is assumed, for example, that the delay elements 38 are identical in construction or at least each produce the same delay of, for example, 200 ps. This numerical value can be varied and represents a basic time resolution for the transit-time measurement, which can be refined by measures to be presented later. A shorter delay means an overall longer delay line 36 with more delay elements 38, or alternatively, a shorter range for the transit-time measurement, so these requirements must be weighed against each other.

[0047] The transmit pulse and, upon its arrival, the receive pulse are injected at both ends of the delay line 36. An impedance matching circuit 42a-b is provided there to adapt the pulses to the impedance of, for example, 50 ohms. The impedance matching circuit 42a-b can consist of simple resistors. Preferably, normalization also takes place, which brings the amplitude of the transmit and receive pulses to the same value. A pulse weighting circuit 44 is provided at each node 40.

[0048] Fig. Figure 6 illustrates the time measurement in the delay line 36. At various times t=1, ..., 10, the position of the light-hatched transmit pulse and the dark-hatched receive pulse is shown, with the scale divisions each representing a node 40. The time steps t are given as multiples of the delay of a delay element 38. Arrows indicate the opposite direction of propagation of the pulses fed in from both sides at several points.

[0049] At the start of the time measurement at time t=1, the transmit pulse is injected at the left end of the delay line 36 and then advances by one node 40 with each time step. The start of the time measurement here refers to the one with the delay line 36; this should not be confused with the measurement duration of, for example, the measurement duration block 28. Fig. 1. At time t=3, the received pulse has arrived and is fed in from the right side, so that the two pulses in the delay line 36 approach each other. At t=6, they meet, so that their amplitudes add up. Assuming equal amplitudes of the transmitted and received pulses, this superposition has twice the amplitude. Otherwise, it is always larger than the larger of the two individual amplitudes, and the time t=6 can be determined by this criterion. In the subsequent time steps after t=6, the pulses drift apart, but this is no longer relevant for the time measurement, since no further superposition with an increased amplitude occurs.

[0050] To find node 40, where the superposition occurred, a threshold comparison can be made with a threshold that lies slightly above the amplitude of a simple transmit or receive pulse. This threshold is only exceeded by the superposition. A concrete implementation is conceivable, for example, with diodes and a downstream capacitor as a pulse storage device. In the case of repeated measurements, the capacitor also acts as an integrator. Since the superposition can only occur at the earliest in the middle of the delay line, it makes sense to first delay the transmit pulse by half the total delay and only then to provide finer delay elements 38 and pulse weightings 44. The desired propagation delay can be deduced from node 40 of the superposition.It should also be noted that the transmit and receive pulses converge in the delay line 36, and therefore the measured delay must be doubled: If the superposition occurs, for example, at a node 40 that lies after 300 ps in the delay line 36, then the measured propagation time is 600 ps.

[0051] One way to refine the measurement resolution is by using shorter delay elements 38. Another possibility is to locate a superposition between nodes 40. For this purpose, a larger pulse width is chosen compared to the individual delay of a delay element 38. Then, interpolation can be performed between the nodes 40 based on the measured amplitudes of the superposition: Superpositions occur at several adjacent nodes 40, and the ratio of the amplitudes in the associated pulse weights 44 is proportional to the finely resolved pulse position between the delay elements 38.

[0052] This concludes the section on the procedure according to the subsequently published DE 10 2020 100 451 A1. Time measurement using a delay line 36 has the disadvantage that the hardware requirements increase linearly with increasing range, while maintaining the same measurement accuracy.

[0053] Fig. Figure 7 shows a block diagram of an embodiment in which the delay line 36 is traversed multiple times. Based on the number of signal pulses within a predetermined measurement period, it is determined in which of the multiple traversals a superposition between the transmitted pulse and the received pulse occurred.

[0054] In a transmit pulse block 46, a transmit pulse is initially generated. This transmit pulse is fed on one side to the delay line 36 and on the other side to a signal pulse enable 48. The signal pulse enable 48 allows the first transmit pulse to pass through to the measuring section 16 between transmitter 12 and receiver 22. When an object 18 is nearby, the received pulse in the delay line 36 overlaps with the transmit pulse. However, the delay line 36 is too short to cover the range of the sensor 10. Therefore, when an object 18 is farther away, the transmit pulse passes through the delay line 36 without overlapping.

[0055] The transmit pulse block 46 generates further transmit pulses at a fixed frequency corresponding to the length of the delay line 36. Alternatively, a transmit pulse that has traveled through the delay line 36 can be fed back and itself form the next transmit pulse. This repeats as long as the measurement duration in the measurement duration block 28 specifies, which is shown here only in general terms and without specific logic circuitry, as also shown in Fig. 2 was only an example. The signal pulse release 48 only allows further transmit pulses to pass through the measuring section 16 if a received pulse has been registered beforehand. This ensures that a superposition between the transmit pulse and the received pulse has occurred in the delay line 36.

[0056] If the time of flight to be measured is longer than can be represented by the delay line 36, then several transmit pulses travel through the delay line 36 before another signal pulse is sent to the measuring section 16. The ratio of signal pulses to transmit pulses indicates on which passage through the delay line 36 the superposition between the transmit and receive pulses occurs. For example, if there are as many signal pulses as transmit pulses, then the delay line 36 was long enough to detect the then nearby object 18. If there are only half as many signal pulses as transmit pulses, the superposition occurred on the second passage through the delay line 36.

[0057] The ratio of signal pulses to transmitted pulses is derived from the integrated signal of the pulse weighting unit 44. The level of this unit is higher the more frequently a transmitted pulse and a received pulse overlapped within the measurement period. Thus, signal pulses are effectively counted here. The number of transmitted pulses is known from the repetition frequency of the transmitted pulse block 46 and also from the ratio between the measurement duration and the length of the delay line 36.

[0058] In this combined embodiment, measurement information is also incorporated into the transit-time determination. This information relates to the number of signal pulses that can be transmitted and received within a predetermined measurement duration, given that each subsequent signal pulse is triggered by the reception of the preceding signal pulse. Here, this measurement information is used to determine in which pass through the delay line 36 a superposition of the transmitted and received pulses occurs. The fine measurement of the transit time is performed by the delay line 36 itself, as described in the Fig. 5 and Fig. 6 was explained.

[0059] Fig. Figure 8 shows a block diagram of another embodiment of a time-of-flight measurement, in which the evaluation takes place at the transmitter side. The basic principle is the same as already explained several times. In a transmit pulse block 46, a transmit pulse is generated during a measurement duration defined by a measurement duration block 28 and sent by the transmitter 12 as a signal pulse over the measuring path 16. The registration of a received pulse in the receiver 22 triggers the next transmit pulse. The number of transmitted and received signal pulses within the measurement duration is then a measure of the desired time of flight.

[0060] In this embodiment, the transmitter 12 is powered by an energy storage device 50. Each emitted signal pulse initially reduces the remaining energy in the energy storage device 50. A transmitter-side evaluation 52 can therefore deduce the transit time based on the remaining energy determined at the end of the measurement period or during the measurement period. An example of the energy storage device 50 is a capacitive battery.

[0061] In an advantageous embodiment, the capacitor is continuously charged. The charging current depends on the capacitor's voltage and increases as the voltage decreases. With each emitted pulse, the voltage drops and then recovers to a specific level until the next pulse. An equilibrium is thus established with the voltage-dependent charging current. This equilibrium still exhibits a sawtooth pattern between pulses, but this can be smoothed out by low-pass filtering. This constant voltage value is proportional to the pulse rate and, therefore, considering the measurement duration, is the desired measure of the propagation time. Movements of the object 18 can also be detected if they are slower than the settling time or if the settling time is compensated.

[0062] Alternatively, the capacitor could be charged before the measurement. The corresponding voltage would then decrease during the measurement according to the number of emitted signal pulses, without reaching an equilibrium. The remaining voltage, taking the measurement duration into account, would then be the desired measure of the propagation time. However, in this case, the pulse amplitude decreases with each pulse.

[0063] Fig. Figure 9 shows, by way of example, the pulse amplitude in the upper part and the current energy or the corresponding voltage of the energy storage device 50 in the lower part as a function of time in an embodiment with continuous charging of the energy storage device 50. A distant object 18 is represented by a darker line and a closer object 18 by a lighter line, for example with transit times of 40 ns and 20 ns, respectively. The transient phase is shown, in which the equilibrium between discharging and charging is established from about the second half of the time. The voltage in the lower part still exhibits a certain sawtooth behavior corresponding to the pulses, which could be suppressed by stronger low-pass filtering or other averaging. The pulse amplitudes and the residual energy decrease to a significantly lower equilibrium level for the closer object 18.This is because there are more cycles of emitted and received signal pulses within the measurement period, and consequently, more signal pulses are supplied from the energy storage device 50. The evaluation unit 52 can evaluate this curve or, by extension, the state at the end of the measurement period. There is an inversely proportional relationship between the pulse amplitude or residual energy remaining at the end of the measurement period and the distance of the object 18.

[0064] The described procedure has a further advantage. More distant objects 18 are measured with fewer, but stronger signal pulses. This at least partially compensates for the range-related level loss. In this way, a desired dynamic compression is achieved across the range.

[0065] Particularly advantageous is the one that refers to the Fig. 8 and Fig. The embodiment described in section 9 is combined with short transmission pulses from a pulse generation device according to DE 102017 127 505 A1 mentioned in the introduction. Furthermore, a combination of the transmission-side evaluation according to Fig. 8 and Fig. 9 with a receiving-side evaluation according to Fig. 2 and / or a delay line 36 according to Fig. 7 are conceivable.

Claims

[1] Sensor (10) for determining a transit time, in particular a distance according to a pulse-based transit time method, comprising a transmitter (12) for emitting a signal pulse (14) triggered by a transmit pulse, a receiver (22) for generating a receive pulse from the signal pulse (20) reflected or remitted by an object (18a-b) and received again, and a control and evaluation unit (24) configured to determine a transit time from the transmit pulse and the receive pulse and, over a measurement period (28), to trigger or allow a further signal pulse (14) to pass through to a measuring section (16) each time a receive pulse is received, such that the received signal pulse immediately triggers the next emitted signal pulse, so that during the measurement period (28) more signal pulses (14, 20) travel back and forth for a nearby object (18a) than for a distant object (18b),and wherein the runtime is determined directly or indirectly from the ratio of a counting information and the measurement duration, wherein the counting information is proportional to the number of signal pulses triggered or passed through the measuring section during the measurement duration. [2] Sensor (10) according to claim 1, wherein the control and evaluation unit (24) is configured to count or integrate the signal pulses (14). [3] Sensor (10) according to claim 1 or 2, wherein the control and evaluation unit (24) has a delay line (36) with a plurality of delay elements (38) connected in series and nodes (40) between the delay elements (38) and is configured to feed the transmit pulse into the delay line (36) at one end and the receive pulse into the other end and to determine the transit time based on the at least one node (40) with an amplitude corresponding to a superposition of transmit pulse and receive pulse. [4] Sensor (10) according to claim 3, wherein the control and evaluation unit (24) is configured to feed a further transmit pulse into the delay line (36) after a transmit pulse has passed through the delay line (36), but to trigger a signal pulse only when the transmit pulse and receive pulse have superimposed each other in the delay line (36). [5] Sensor (10) according to claim 4, wherein the control and evaluation unit (24) is configured to determine the ratio of transmit pulses to signal pulses as measurement information. [6] Sensor (10) according to one of the preceding claims, wherein the transmitter (12) is associated with an energy storage unit (50) and the measurement information is determined from the energy stored in the energy storage unit (50) after or during the measurement period (28). [7] Sensor (10) according to claim 6, wherein the energy storage unit (50) has a capacitor and the control and evaluation unit (24, 52) measures the stored energy by means of a voltage across the capacitor. [8] Sensor (10) according to one of the preceding claims, which is designed as an optoelectronic sensor, wherein the transmitter (12) is a light transmitter, the receiver (22) is a light receiver and the signal pulse (14, 20) is a light pulse. [9] Method for determining a distance using a pulse-based time-of-flight method, in which a signal pulse (14) is emitted triggered by a transmit pulse, and a received pulse is generated from the signal pulse (20) reflected or remitted and received again by an object (18a-b), and a time of flight is determined from the transmit pulse and the received pulse, wherein, over a measurement period (28), each time a received pulse is received, another signal pulse (14) is triggered or allowed to pass through to a measuring section (16) such that the received signal pulse immediately triggers the next emitted signal pulse, so that during the measurement period (28) more signal pulses (14, 20) travel back and forth for a nearby object (18a) than for a distant object (18b), and wherein the time of flight is determined directly or indirectly from the ratio of a number of information and the measurement period.where the number of signal pulses is proportional to the number of signal pulses triggered or passed through the measuring section during the measurement period.

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