Optoelectronic system for the bidirectional measurement of optical pulse propagation time and distance

Abstract

A system for measuring the propagation time of optical pulses between at least two units. The presented system further enables the calculation of the distance between at least two units (101a, 101b), and the successive derivatives of said distance with respect to time, in various configurations. The bidirectional optical time transfer between two units is used for the calculation of the propagation time of optical pulses, which are generated by the optical source of each unit.

Description

DescriptionTitle of Invention: Optoelectronic system for the bidirectional measurement of optical pulse propagation time and distance Technical Field

[0001] The present invention relates to a system for measuring the propagation time of electromagnetic waves between at least two units. The presented system also enables the calculation of the distance between at least two units.Background Art

[0002] The propagation time of electromagnetic waves between two units and the distance between said units are linked by the propagation speed of the electromagnetic waves. Therefore, the two measurements are closely related.

[0003] Systems in which there is an active unit and a unit acting as a passive reflector are classically used to measure the distance between two specific units. In this way the measurement system is configured as a radar or a lidar which detect the object to be identified, characterized by a high reflectivity.

[0004] In radars, this kind of measurement is performed using radiofrequencies.

[0005] At radiofrequencies the "Symmetric double-sided two-way ranging" method is sometimes used, as defined in the IEEE 802.15.4a standard, characterized by sending a request by one unit and generating a response by another unit.

[0006] Moreover, in EP2602636A1, a method is described which uses radiofrequency signals, between two separate active terminals of the kind of Frequency Modulated Continuous Wave radars, to identify the distance between the two terminals.

[0007] The use of radiofrequency signals leads to several problems. Firstly, radiofrequencies, especially those at higher wavelengths, show lower spatial resolution, making it difficult to detect small objects or minor details.

[0008] Another disadvantage of the use of radiofrequencies in the measurements is due to the necessity of permissions for their usage, because the use of radiofrequencies is strictly regulated by government authorities in several countries. Indeed, some frequency bandwidths need specific licenses, and their use is improper and can lead to legal issues, making their implementation more complex and increasing the operating costs.

[0009] An alternative solution to the use of radiofrequencies for measurements is to use optical signals, because they enable better results in terms of precision, resolution and possibility of working at longer distances.

[0010] Solutions have been proposed which enable the calculation of the propagation time between two units using the bidirectional optical time transfer, by means ofoptical components having as inputs both the sent electromagnetic wave and the electromagnetic wave received by another unit. These solutions are presented in the journal article Giorgetta, F. R., Swann, IE C., Sinclair, L. C, Baumann, E., Coddington, I., & Newbury, N. R. (2013). Optical two-way time and frequency transfer over free space. In Nature Photonics (VOL 7, issue 6, pp. 434-438). Springer Science and Business Media LLC and in the conference article Prochazka , I., Blazej, J., Flekova, T., & Kodet, J. (2020). Laser-based Time Transfer through Eree-space L inks. In P. Helfenstein (A C. di), EP J Web of Conferences (Vol. 243, p. 09001 ). EDP Sciences.

[0011] In the context of optical solutions, the patent EP3165943 Al (claiming the priority of the German patent DE102015221836) describes a conveyor provided with a communication system allowing the communication between at least a first transmitting and receiving unit and a second transmitting and receiving unit, each provided with a transmitter, a receiver and a control unit. The first transmitting and receiving unit records the total time period dT between the sending of its own package of data and the subsequent reception of the answer package of data from the second transmitting and receiving unit. The second transmitting and receiving unit determines the time period dt between the reception of the package of data sent by the first unit and the subsequent sending of the answer package of data, and transmits this period of time recorded as a piece of information to the first transmitting and receiving unit, using the same optical channel that is used to send the package necessary to the measurement of dT and dt. The patent EP3165943 Al further proposes the usage of a device on each unit to measure the change of distance.

[0012] Specifically, the units make use of an optical free beam communication system to exchange data packages. The optical free beam optical connection implies, according to the common knowledge, that the optical signal directly travels along a non-guided beam along the transmitting and receiving units. This means that there are not objects, like optical fibers or waveguides, guiding the optical signals along a specific path. Instead, the signal is directly transmitted in the free space between the transmitter and the receiver along a predetermined direction. In this kind of application, indeed, the transmitter and the receiver should be accurately aligned, so that the optical signal can travel along the predefined direction without dispersion or attenuation.

[0013] This implies that the devices should be positioned such that the optical path among them is obstacle-free and accurately aligned. The alignment can be done manually or using external automatic alignment systems, especially in application where high precision is required or in presence of variable weather conditions. In other practical situations, in which the position among the units is not constrained by a conveyor, however, it is not always possible predetermining the alignment direction, especiallyon long distances or in dynamic environments where the devices, for example drones, can move in a higher number of degrees of freedom higher compared to the ones limited by a conveyor.

[0014] Moreover, this kind of solution shows other issues, related to the time necessary to evaluate distances, that is high, because of the large amount of information sent using the transmitters and the receivers of the units, which is related to the measurements taken and is useful for the distance calculation.

[0015] The transmission of a large amount of information related to measurements, together with the use, for said transmission, of the transmitters and receivers of the units, leads also to energetic inefficiencies.

[0016] So, identifying a system able to make the distance measurement between two or more units more efficient is a technical problem.Purpose of Invention

[0017] One purpose of the invention is to provide a solution for calculating the propagation time of optical pulses between at least two units, useful, for example, for synchronizing the clocks of two units or for calculating the distance between two units, with the position in space of at least one of said units being unknown.

[0018] The present invention utilizes the high radial resolution offered by the optical frequencies, using said optical frequencies for the transmission of pulses no longer necessarily along a previously known direction, as it happens in the cited patent EP3165943A1, but in the space surrounding each unit.

[0019] Moreover, the use of electromagnetic waves at optical frequencies makes it possible to collimate the optical radiation by means of lenses and to direct the optical beam by means of movable mirrors, thus also making it possible to obtain information on the relative angular position between the units, so enabling relative three-dimensional localization between the devices.

[0020] Moreover, in the system of the present invention, each unit comprises a communication device which allows wireless sharing of information with at least one other unit.

[0021] The present invention can be applied in industrial fields including but not limited to indoor and outdoor localization systems, autonomous driving, triangulation, remote surgical operations and clock synchronization.Summary of Invention

[0022] The subject of the present invention is a system for measuring the propagation time of optical pulses in free space between at least two units 101.

[0023] The system of the present invention has the features of independent claim 1, the content of which is an integral part of the present description. Some further featuresand embodiments of the invention are described by the following claims, the contents of which are part of the present description.

[0024] In particular, the present invention discloses a system, comprising at least two units 101, capable of evaluating the propagation time of optical pulses in the space between at least two of the aforesaid units 101, each of the units 101 comprising: a. an optical source 103; b. an electrical circuit 105 to drive the aforesaid optical source 103 by means of an electrical signal; c. an optical detector 107 to detect at least one optical pulse generated by the optical source 103 of at least one other unit 101; d. a measuring apparatus 109 enabling the analysis of two temporal events, respectively associated with:- an impulsive variation of an electrical quantity related to the optical source 103;- an impulsive variation of an electrical quantity related to the optical detector 107; e. a processing device 111; f. a communication device 113, characterized in that each of the optical sources 103 in at least two units 101 generates at least one optical pulse, and in that the propagation time of optical pulses in the space between two units 101 is calculated by the processing device 111 of at least one unit 101 by processing the following data: a. temporal difference between the aforesaid temporal events related to one unit 101; b. temporal difference between the aforesaid temporal events related to another unit 101, and further characterized in that at least one unit 101 shares information on the temporal events analyzed by the aforesaid measuring apparatus 109 with at least one other unit 101, by means of the communication device 113.

[0025] Moreover, the present invention proposes a system further characterized in that the processing device 111 of at least one unit 101 processes the propagation time of optical pulses in the space between two units 101 and the propagation speed of the optical pulses in the space between said units 101 to calculate the distance between the above units 101.

[0026] The present invention further proposes a system characterized in that the processing device 111 of at least one unit 101 calculates at least one of the successive derivatives with respect to time of the distance between two units 101 of the system.

[0027] One embodiment of the invention is characterized in that the measuring apparatus 109 of at least one of the units 101 comprises: a. at least one time-to-digital converter 114; b. a first electrical circuit 115 for reading the electrical quantity related to the optical source 103; c. a second electrical circuit 116 for reading the electrical quantity related to the optical detector 107.

[0028] A possible further embodiment of the invention, alternative to the previous one, is characterized in that the measuring apparatus 109 of at least one of the units 101 comprises: a. at least one analog-to-digital converter 117; b. a first electrical circuit 115 for reading the electrical quantity related to the optical source 103; c. a second electrical circuit 116 for reading the electrical quantity related to the optical detector 107.

[0029] The invention further proposes the optional use of an optical lens, included within at least one unit 101, for collimating the optical pulse generated by at least one unit.

[0030] The invention further proposes the optional use of a rotating mirror, included within at least the unit 101 which further comprises the optical lens, for directing the collimated optical pulse to different angular positions.

[0031] An advantageous embodiment of the invention provides that the system includes at least three units 101, and that the distances between units belonging to distinct pairs of units 101 are processed by means of geometric procedures in order to determine at least one of the following quantities: a. the position of at least one specific unit 101 in space; b. at least one of the successive derivatives with respect to time of said position.

[0032] The invention further proposes a system comprising at least three units 101 and in which at least two units 101 are relatively positioned at fixed and known distances and angles, thus realizing at least one rigid subsystem, and in which it is possible to calculate at least one of the following quantities by means of geometric procedures: a. the position in space of at least one unit 101 belonging to one rigid subsystem; b. at least one of the successive derivatives with respect to the time of the position in space of at least one unit 101 belonging to one rigid subsystem; c. the orientation in space of at least one rigid subsystem; d. at least one of the successive derivatives with respect to the time of the orientation in space of at least one rigid subsystem.

[0033] The invention also provides for the possibility of a plurality of optical sources 103.

[0034] The invention also provides for the possibility of a plurality of optical detectors 107.Brief Description of Drawings

[0035] The description of the preferred embodiments is provided using the figures attached to this description, wherein:

[0036] [Fig-1] is the schematic representation of the system comprising two units 101, labelled as 101a and 101b, where the optical source 103 is represented with the symbol of a diode laser, the optical detector 107 is represented with the symbol of a photodiode and the communication device 113 is indicated with the common symbol of the wireless communication.

[0037] [Fig.2] is a representation of the explanation of the calculation principle used to evaluate the propagation time between the two units 101.

[0038] [Fig.3] is a schematic representation illustrating a unit 101 of the system according to a preferred embodiment of the invention which provides for the use of a time-to-digital converter 114.

[0039] [Fig.4] is a schematic representation of the input signals to the time-to-digital converter 114 with two stop inputs, according to a preferred embodiment of the invention.

[0040] [Fig.5] is a schematic representation illustrating a unit 101 of the system according to a further preferred embodiment of the invention which provides for the use of an analog-to-digital converter 117.

[0041] [Fig.6] is the schematic representation of the system comprising a rigid subsystem.Description of Embodiments

[0042] The present description provides non-limiting examples of the embodiment of the invention, as preferred at the time of filing of the patent application. However, they are not limiting with respect to other post-filing embodiments, which are alternative to the preferred embodiments, although they are covered by the claims of the present invention.

[0043] In the description of the preferred embodiments, only two units 101 are considered, indicated below as numbers 101a and 101b, to better clarify some examples.

[0044] In particular, the present description shows the invention in the case in which exactly two units 101 are present, labelled as unit 101a and unit 101b, as shown in [Fig.l]. However, it is emphasized that the system achieves the purpose of the invention also when it includes more than two units 101. Each of the two units 101 considered comprises: a. an optical source 103, preferably a pulsed laser; b. electrical circuit 105 to drive the optical source 103;c. an optical detector 107 to detect at least one optical pulse generated by the optical source 103 of at least the other of the two units 101; d. a measuring apparatus 109 enabling the analysis of two temporal events, respectively associated with: i. an impulsive variation of the electrical quantity related to the optical source 103; ii. an impulsive variation of the electrical quantity related to the optical detector 107; e. a processing device 111; f. a communication device 113.

[0045] In the context of the presented example, the pulsed laser is a device using the light amplification by stimulated emission of radiation, able to generate optical electromagnetic waves with rapid rise-time and high peak power. They are often used in flash lidars, as described in the papers Marino, R. M., Stephens, T., Hatch, R. E., McLaughlin, J. L., Mooney, J. G., O’Brien, M. E., Rowe, G. S., Adams, J. S., Skelly, L., Knowlton, R. C., Forman, S. E., & Davis, W. R. (2003). A compact 3D imaging laser radar system using Geiger-mode APD arrays: system and measurements. In G. W. Kamerman (Ed.), SPIE Proceedings. SPIE and Hao, Q., Tao, Y., Cao, J., & Cheng, Y. (2021 ). Development of pulsed - laser three - dimensional imaging flash lidar using APD arrays. In Microwave and Optical Technology Letters (Vol. 63, Issue 10, pp. 2492-2509). Wiley.

[0046] The described system is characterized by the fact that: a. each of the optical sources 103 in at least two units 101 generates at least one optical pulse; b. the propagation time of optical pulses in the space between two units 101 is calculated by the processing device 111 of at least one unit by processing the following data: i. temporal difference between the aforesaid temporal events related to one unit 101; ii. temporal difference between the aforesaid temporal events related to another unit 101; c. at least one unit 101 shares information related to the temporal events analyzed by the aforesaid measuring apparatus 109 with at least one other unit 101, by means of the communication device 113.

[0047] So, each optical source 103, driven by the corresponding electrical circuit 105, can generate an optical pulse that can propagate in the space surrounding the unit, within a sufficiently large solid angle, so that the optical pulse generated by each unit can reach the other unit, even in case in which the relative direction between the two unitsis unknown. In order to achieve this, it is clear that the optical pulse should preferably be not collimated but divergent.

[0048] The idea of using the same optical pulses in a space not limited to a single dimension, using the system described with the system in the present patent application, overcomes the technical problems of the prior art, especially of the patent EP3165943A1, previously cited, because the expert in the field, aware of the patent EP3165943A1, in the unlikely event that he or she had tried to extend the application of the patent EP3165943A1, he or she would have clearly used well-known common techniques enabling the modification of the divergence of the optical signals, like the usage of divergent lenses. However, this solution would have led to low efficiency in terms of measurement speed and power consumption, mainly due to the fact that in the patent EP3165943 Al the same optical transmitter is used both for the sending of optical pulses useful for the measurement of time difference and for the sending of optical information useful for the calculation of optical distances. Indeed, in this hypothesis, optical transmitters with high power would have been necessary, to be radiated over a wide solid angle, that usually have much lower pulse rate, with respect to low power optical transmitters.

[0049] On the contrary, the present invention: a. does not necessarily assume the existence of a preferred direction of emission of optical signals and works even sending one single optical pulse, thus leading to power reduction; b. uses a communication device 113, dedicated to the sharing of pieces of information necessary for the calculation of propagation time. So, said pieces of information are not shared using the optical source 103 and the optical detector 107, thus reducing the signals transferred among them. The presence of a communication device 113, separated by the optical source 103 and by the optical receiver 107, makes it possible to both speed up and improve the energy efficiency of the sending and reception of optical pulses of the optical source 103 and the optical receiver 107, and, consequently, to make measurements in a faster and more energy-efficient way. It is clarified that the communication device 113 is not restricted to use of optical signals; rather it is more efficient from an energetic point of view when it makes use of signals in different bandwidths, for example, among the ones typically used to transfer data in local networks, for example Bluetooth® or Wi-Fi®, since the signal is transmitted using a channel different from the one used by the optical source 103.

[0050] Each of the units 101 is provided also with an optical detector 107 that detects the optical pulses coming from the other optical sources 103. Specifically, the unit 101auses the optical detector 107 to detect at least one optical pulse generated by the optical source 103 of the unit 101b, whereas the unit 101b uses the optical detector 107 to detect at least the optical pulse generated by the optical source 103 of the unit 101a.

[0051] According to a preferred embodiment of the invention, the processing device 111 of each unit transmits start signals to the electrical circuit 105 that drives the pulsed laser, in the provided example. So, the electrical circuit 105 of each unit 101 drives the pulsed laser so that it generates an optical pulse. The optical pulse generated by the pulsed laser of each unit 101 propagates through the three-dimensional space surrounding each of the two units 101 and reaches the other unit 101.

[0052] The generation of an optical pulse by the optical source 103 is assumed to be simultaneous with the impulsive variation of an electrical quantity of the optical source 103, which is the current flowing through the optical source 103, according to a preferred embodiment. The optical detector 107 of each unit detects the optical pulse coming from the other unit 101. The detection event of an optical pulse leads to an impulsive variation, considered to be simultaneous with the optical pulse, of an electrical quantity of the optical detector 107, which is the current generated by the optical detector 107, according to a preferred embodiment.

[0053] The measuring apparatus 109, connected both to the optical source 103 and to the optical detector 107, enables the identification of two events associated with an impulsive variation of the current flowing through the optical source 103 and with an impulsive variation of the current generated by the optical detector 107, respectively. Further details related to the measuring apparatus 109 in a preferred embodiment of the invention will be specified later.

[0054] The processing device 111 is also connected to the measuring apparatus 109 and to the communication device 113. The processing device 111 of unit 101a communicates wirelessly with the processing device 111 of unit 101b.

[0055] According to a preferred embodiment of the invention, the communication device 113 of each unit comprises a radiofrequency antenna.

[0056] The processing device 111 of at least one of the two units 101, for example the unit 101b, sends the information related to the temporal difference between the two events, identified by the measuring apparatus 109, to the processing device 111 of the other unit 101, in this case the unit 101a, by means of the communication device 113. The processing device 111 of the other unit 101, in this case the unit 101a, receives the information related to the temporal difference between the events identified by the unit 101b, by means of the communication device 113 The processing device 111 of unit 101a calculates the propagation time of optical pulses between the two units 101a, 101b as half the absolute value of the sum of the following terms: a. temporal difference between the two events identified by the unit 101a;b. temporal difference between the two events identified by the unit 101b.

[0057] It is observed that the temporal differences between the two events related to the unit 101a and between the two events related to the unit 101b can have opposite signs.

[0058] In order to clarify the operating principle of the method, [Fig.2] is used. [Fig.2] is a representation of the explanation of the principle of calculation of the propagation time. In particular, the optical pulses generated and received by each unit 101 are represented. In details, in [Fig.2] a first graph and a second graph are shown are shown, the first graph, on top, related to the unit 101a and the second graph, at the bottom, related to the unit 101b, in which the vertical axis of each unit 101 represents an electrical quantity labelled as "which in the preferred embodiment is a current, and the horizontal axis of each unit 101 represents the time axis labelled as "t". In order to understand the operating principle, a common time axis is considered for the two units 101. However, it will be shown that synchronization of the clocks between the two units 101 is not necessary.

[0059] The solid line 207 of the first graph shows the current flowing through the optical source 103, for example a pulsed laser, of the unit 101a. The dashed line 208 of the first graph shows the current generated by the optical detector of the unit 101a. The solid line of the second graph 209 shows the current flowing through the optical source 103 of the unit 101b. The dashed line of the second graph 210 shows the current generated by the optical detector of the unit 101b.

[0060] In the first graph, the instant of time when an impulsive variation of the current flowing through the optical source 103 of the unit 101a occurs is labelled as tALand the instant of time when an impulsive variation of the current generated by the optical detector 107 of the unit 101a occurs is labelled as tAR. In this exemplifying explanation, tALand tARidentify the peak of the curve to which they are referred.

[0061] In the second graph, the instant of time when an impulsive variation of the current flowing through the optical source 103 of the unit 101b occurs is labelled as tBL and the instant of time when an impulsive variation of the current generated by the optical detector 107 of the unit 101b occurs is labelled as tBR. In particular, in this exemplifying explanation, tBLand tBRidentify the peak of the electrical quantity to which they are referred.

[0062] In the first graph the propagation time from the unit 101a to the unit 101b and the propagation time from the unit 101b to the unit 101a, equal to each other, are labelled as tp.

[0063] By looking at the graph, it is possible to write:

[0064] [Math.l]

[0065] It should be noted that the quantities (tAL- tAR) and (tBL- tBR) can assume both positive and negative values. It should be also noted that the absolute value is necessary, when it is not known a priori which unit 101 generates the first pulse. Since the magnitude (tAL- tAR) can be measured by the unit 101a, and the magnitude (tBL- t BR) can be measured by the unit 101b, it is not necessary for the clocks of the two units 101 to be synchronized. The propagation time is calculated as:

[0067] or with equivalent formulas. Advantageously, the processing device 111 of at least one unit 101 can process the propagation time of optical pulses in the space between two units 101 and the propagation speed of optical pulses in the space between said units 101 to calculate the distance between the units 101.

[0068] So, it is possible to further calculate the distance between two units 101 by multiplying the propagation time by the propagation speed.

[0069] Furthermore, advantageously, the processing device 111 of at least one unit 101 can calculate at least one of the successive derivatives with respect to time of the distance between the two units 101 of the system, for example the speed and the acceleration, which can be calculated starting from the information about the distance.

[0070] Below, several preferred embodiments will be provided in detail.

[0071] Embodiment using a time-to-digital converter 114

[0072] In the following an embodiment of the invention will be explained more closely, providing further details on the measurement process and the measuring apparatus 109 present in each unit 101.

[0073] According to a preferred embodiment of the invention, [Fig.3] provides a schematic representation of one generic unit 101 of the two units 101 of the system of the present invention, said unit 101 comprising:- an optical source 103, such as a pulsed laser;- an electrical circuit 105 for driving the optical source 103;- an optical detector 107;- a measuring apparatus 109 composed of: a. a time-to-digital converter 114 with at least two stop inputs; b. a first electrical circuit 115 for reading an electrical quantity related to optical source 103; c. a second electrical circuit 116 for reading an electrical quantity related to the optical detector 107.- a processing device 111;- a communication device 113.

[0074] It should be noted that the time-to-digital converter 114 is a digital measuring instrument used to measure temporal intervals between events of various kinds. These temporal intervals are identified by a start signal and at least one stop signal present at the input of the time-to-digital converter 114.

[0075] Here, the operation of each unit 101 is illustrated in the described preferred embodiment. In the described embodiment, the first electrical circuit 115 for the readout of the pulsed laser or the generic optical source 103 is characterized in that it enables the conversion of the current flowing through the pulsed laser into a voltage proportional to it and to compare this voltage to a reference voltage. This first electrical circuit 115 for the readout of the pulsed laser is realized, in a preferred, nonlimiting embodiment, with a resistor connected in series to the pulsed laser, placed between the pulsed laser and a ground node, and a comparator which compares the voltage at the common node between the resistor and the pulsed laser with a threshold voltage.

[0076] In the described preferred embodiment, the second electrical circuit 116 for the readout of the optical detector 107 is characterized in that it enables the conversion of the current generated by the optical detector 107 into a voltage proportional to it and to compare this voltage with a threshold voltage value. In a described embodiment, this second electrical circuit 116 for the readout of the optical detector is realized, in a nonlimiting way, with a transimpedance amplifier connected to the optical detector 107 and with a comparator connected to the output of the transimpedance amplifier.

[0077] According to a preferred embodiment of the invention, the time-to-digital converter 114 has at least one start input and at least two stop inputs. The processing device 111 is connected to one start input of the time-to-digital converter 114. The output of the first electrical circuit for the readout of the pulsed laser is connected to a stop input of the time-to-digital converter 114. The output of the second electrical circuit for the readout of the optical detector 107 is connected to another stop input of the time- to-digital converter 114. At least one output of the time-to-digital converter 114 is connected to the processing device 111 to communicate the results of the digital time measurements to it.

[0078] In the described preferred embodiment of the invention, the processing device 111 in each unit 101 sends a start signal to the start input of the time-to-digital converter 114 which starts the time-to-digital conversion process. A digital signal, delayed with respect to the start signal, is sent by the processing device 111 of each unit 101 to the electrical circuit 105 for driving the optical source 103, and the electrical circuit 105 for driving the optical source 103 consequently generates a current which drives the optical source 103, that is, in this example the pulsed laser.

[0079] When the current pulse flows through the pulsed laser, simultaneous to the optical pulse generated by the pulsed laser, the first electrical circuit 115 for the readout of the pulsed laser generates a stop signal at the first stop input of the time-to-digital converter 114.

[0080] When the optical pulse is received by the optical detector 107 of each unit 101, the second electrical circuit 116 for the readout of the optical detector 107 generates a stop signal at the second stop input of the time-to-digital converter 114 of each unit 101.

[0081] To better illustrate the operation of the preferred embodiment of the described invention, reference is also made to [Fig.4], which shows two graphs related to the unit 101a, on the top, and to the unit 101b, at the bottom . In each graph the vertical axis represents the voltage, labelled as " V" and the horizontal axis the time, labelled as "t".

[0082] In particular, [Fig.4] shows, in each of the two graphs:- the voltages at the start input 403, 404 of the time-to-digital converter 114 of each unit 101;- the start instants (startA,startB) of the time-to-digital converters 114 of the units 101a, 101b;- the voltages at the first stop input 405, 406 of the time-to-digital converter 114 of each unit 101;- the stop instants (stopLA,stopLB) corresponding to the first stop input of the time-to-digital converters 114- the voltages at the second stop input 407,408 of the time-to-digital converter 114 of each unit;- the stop instants (stopRA,stopRB) corresponding to the second stop input of the time-to-digital converters 114.

[0083] Once the stop time measurements (stopLA, stopRA) are read by the processing device 111 of the unit 101a, connected to the corresponding time-to-digital converter 114, and the stop time measurements (stopLB, stopRB) are read by the processing device 111 of the unit 101b, connected to the corresponding time-to-digital converter 114, the processing device 111 of the unit 101b communicates the measurement (stopLB - stopRB) to the processing device 111 of the unit 101a, by means of the communication device 113 of the unit 101b, and the processing device 111 of the unit 101a receives such communication by means of the communication device 113 of the unit 101a, and calculates the propagation time, tp, as:

[0084] [Math.3]Zp= j(stopLA - stopRA) + ( stopLB - stopRB) |.

[0085] In the described preferred embodiment of the invention, the communication devicedevice 113 in a different preferred embodiment may also include a calculation unit, separate from the processing device 111 of the corresponding unit 101.

[0086] According to a preferred embodiment, the measurement process initiated by sending the start signal by the processing device 111 of each unit to the time-to-digital converter 114 of each unit 101 is periodically repeated by the processing device 111 of each unit 101, in order to acquire several measurements to calculate statistical parameters such as mean and variance. In this case, each unit 101 generates and detects sequences of optical pulses.

[0087] In a further preferred not limiting embodiment of the invention, two time-to-digital converters can be used in each unit 101, instead of a single time-to-digital converter 114 with two stop inputs. In this case, in the described embodiment the two converters receive the same start signal from the processing device 111.

[0088] The embodiment just described is not limiting for the realization of the system subject of the invention. For example, in a different embodiment of the invention, the start signal in each unit 101 (startA,startB) can be internally generated by the time- to-digital converter 114 and the signal delayed with respect to the start signal can be generated by the time-to-digital converter 114 and sent to the electrical circuit 105 that drives the optical source 103.

[0089] Embodiment with the use of an analog-to-digital converter 117

[0090] [Fig.5] schematically represents each of the two units 101 of the system, unit 101a and unit 101b, according to a preferred embodiment of the invention, providing details about the measuring apparatus 109 of each unit 101.

[0091] In particular, each unit 101 comprises:- a pulsed laser, that is an optical source 103;- an electrical circuit 105 for driving the pulsed laser;- an optical detector 107;- a measuring apparatus 109 composed of: a. an analog-to-digital converter 117 with at least two input channels; b. a first electrical circuit 115 for reading an electrical quantity related to the pulsed laser; c. a second electrical circuit 116 for reading an electrical quantity related to the optical detector 107.- a processing device 111;- a communication device 113.

[0092] The operation of each unit 101 is illustrated below according to the described embodiment.

[0093] In the preferred embodiment of the invention described here, the first electrical circuit 115 for the readout of the pulsed laser is characterized in that it enables theconversion of the current flowing through the pulsed laser into a voltage proportional to it. The first electrical circuit 115 for the readout of the pulsed laser is realized, in a preferred, non-limiting embodiment, with a resistor connected in series to the pulsed laser, between the pulsed laser and a ground node, and with a voltage amplifier which amplifies the voltage across the resistor.

[0094] In the described embodiment, the second electrical circuit 116 for the readout of the optical detector 107 is characterized in that it enables the conversion of the current generated by the optical detector 107 into a voltage proportional to it. In the described embodiment, this second electrical circuit 116 is provided, in a non-limiting way, with a transimpedance amplifier whose input is connected to the detector.

[0095] The processing device 111 of each unit 101 starts the measurement with a start signal sent to the electrical circuit 105 that drives the pulsed laser.

[0096] The two input channels of the analog-to-digital converter 117 enable the sampling and the digital conversion of the voltage at the output of the first electrical circuit 115 for the readout of the pulsed laser and the voltage at the output of the second electrical circuit 116 for the readout of the optical detector 107. In this embodiment, the analog-to-digital converter 117, connected to the processing device 111, provides data resulting from the digitization of the input signals to the processing device 111 .

[0097] For measuring the temporal difference between the arrival time of the pulse measured by one channel of the analog-to-digital converter 117 and of the pulse measured by the other channel of the analog-to-digital converter 117, the processing device 111 uses known processing techniques, such as the cross -correlation between the two digitized signals, or it evaluates the arrival instant of each pulse converted to digital with a digital threshold.

[0098] The processing device 111 of one unit 101, for example 101b, communicates the temporal difference between the identified events to the processing device 111 of the other unit 101, for example 101a, by means of the communication devices 113 of each unit 101. The processing device 111 of the unit 101a calculates the propagation time as half the absolute value of the sum of the following terms: a. temporal difference between the events identified by the unit 101a; b. temporal difference between the events identified by unit 101b.

[0099] Embodiment which includes an optical lens and, optionally, a rotating mirror

[0100] In a further embodiment, at least one unit 101 comprises an optical lens for collimating the optical pulse generated by the optical source 103 and, optionally, a rotating mirror, used to direct the collimated beam into different angular position with respect to a predefined reference system fixed to the unit 101.

[0101] So, according to the present embodiment, in an operating configuration of said at least one unit 101, the rotating mirror is used to sequentially direct the optical pulsecollimated by the optical lens to different directions in three-dimensional space that can be explored by the rotating mirror itself.

[0102] So, the distance measurement can happen only when the collimated beam of said one unit 101a is oriented by means of the movements enabled by said rotating mirror to a direction almost parallel to the direction connecting the said unit 101a to another unit 101b, whose distance and orientation with respect to the unit 101a need to be known.

[0103] When the collimated optical pulse of the unit 101a is directed toward a direction approximately parallel to the direction between said unit 101a with unit 101b, the latter is able to receive the collimated optical pulse emitted by the other unit 101a and to send to the same unit 101a information about the taken measurement of time difference.

[0104] The processing device 111 is thus able to obtain information also related to the relative angular orientation of the other unit 101b, by knowing the angular position adopted by the rotating mirror at the time of the measurement.

[0105] An embodiment providing for a plurality of sources or detectors

[0106] In case one unit 101 is provided with one single optical source 103 and / or one single optical detector 107, said unit 101 could be unable to send and / or receive optical pulses to and / or from all the angular directions in the surrounding space. So, in such a case, it is advantageous that the same unit 101 is provided with more than one optical source 103 and / or with more than one optical detector 107, such that optical pulses transmitted and / or received could cover a wider solid angle.

[0107] This makes it possible to increase the reliability of the system that is the subject of the present invention, by redundancy, and to maximize the observation domain in terms of solid angle.

[0108] Said more than one optical source 103 could be driven by the electrical circuit 105 in unit 101 as if they were a unique optical source 103, or with a plurality of different electrical circuits 105, operating simultaneously.

[0109] Each optical detector 107 can be, instead, connected to the same measuring apparatus 109, that will identify the first of the received pulses from one of said optical detectors belonging to the same unit 101. In case the fields of view of optical detectors 107 do not overlap, only one of the optical detectors 107 will identify the pulse coming from another unit 101.

[0110] Method of realization for triangulation or trilateration

[0111] According to an embodiment, subject of claim 8, the distances obtained between the at least three different units 101 can be used within geometric and topological procedures (such as triangulation or trilateration) to know the position of one of the above units 101 and further topological information of the system composed of the different units 101.

[0112] As an example, in a particularly advantageous embodiment, the system in composed of four units 101, of which three units 101 are fixed and one unit 101 mobile in a two- dimensional space, so that the distances measured between said mobile unit 101 and each of said fixed units 101 enables the univocal identification of the position of the mobile unit 101 in the two-dimensional space.

[0113] In details, it should be noted that three units are sufficient only in case the mobile unit moves in a one-dimensional space, for which one mobile unit in the onedimensional space and two fixed units are necessary.

[0114] Similarly, four units, three of which are fixed and one is movable, are necessary if the movable unit can move in the two-dimensional space, whereas five units, four of which are fixed and one is mobile, are necessary in the three-dimensional space.

[0115] An embodiment for detecting different degrees of freedom of rigid subsystems

[0116] According to an embodiment of the invention, subject of claim 9, some units 101 of the system subject of the present invention are relatively positioned at fixed distances and angles, in order to realize at least one rigid subsystem, that is a subsystem in which at least two units 101 have fixed distances and orientations among each other. In this way, the knowledge of the distances between each of the aforesaid units 101 belonging to the rigid subsystem with respect to at least one unit 101 not belonging to said subsystem, together with the knowledge of the relative positions of the units 101 belonging to the rigid subsystem, enables the detection of the translational and rotational degrees of freedom of the aforesaid rigid subsystem with respect to said at least one unit 101 not belonging to the subsystem.

[0117] Indeed, since the relative positions of units 101 realizing the rigid subsystem are known, when one unit 101 not belonging to the rigid subsystem identifies, for example, the distances from each of two units 101 belonging to the subsystem, it is possible to infer from said distances the orientation of the rigid subsystem, in case that the rigid subsystem has only one rotational degree of freedom and no translational degree of freedom. In the presence of two units 101 not belonging to the rigid subsystem it is possible to infer the position of the rigid subsystem with respect to the two units 101 not belonging to the subsystem, in case that the rigid subsystem has only one translational degree of freedom.

[0118] In the presence of more units 101 belonging to the rigid subsystem and / or more units 101 not belonging to the subsystem it is possible to infer more rotational and / or translational degrees of freedom of the rigid subsystem with respect to the at least one unit 101 not belonging to the rigid subsystem.

[0119] For example, a rigid subsystem can be rigidly connected to a moving body for which it is necessary to know its location and orientation in space during a time evolution.

[0120] With illustrative purposes, [Fig.6] shows a system comprising five units 101a, 101b, 101c, lOld, lOle, comprising a rigid subsystem composed of four units 101a, 101b, 101c, lOld. In this rigid subsystem, the relative position between the four units 101a, 101b, 101c and lOld are known. The unit lOle does not belong to the rigid subsystem.

[0121] In a preferred embodiment, the units 101a, 101b, 101c and 10 Id belonging to the rigid subsystem are activated, meaning that they send optical pulses, in a sequential order, whereas the unit lOle is always active.

Claims

Claims

1. A system, comprising at least two units (101), capable of evaluating the propagation time of optical pulses in the space between at least two of the aforesaid units (101), each of the units (101) comprising: a. an optical source (103); b. an electrical circuit (105) to drive the aforesaid optical source (103) by means of an electrical signal; c. an optical detector (107) to detect at least one optical pulse generated by the optical source (103) of at least one other unit (101); d. a measuring apparatus (109) enabling the analysis of two temporal events, respectively associated with: i. an impulsive variation of an electrical quantity related to the optical source (103); ii. an impulsive variation of an electrical quantity related to the optical detector (107); e. a proces sing device (111); f. a communication device (113), characterized in that each of the optical sources (103) in at least two units (101) generates at least one optical pulse, and in that the propagation time of optical pulses in the space between two units (101) is calculated by the processing device (111) of at least one unit (101) by processing the following data: a. temporal difference between the aforesaid temporal events related to one unit (101); b. temporal difference between the aforesaid temporal events related to another unit (101), and further characterized in that at least one unit (101) shares information on the temporal events analyzed by the aforesaid measuring apparatus (109) with at least another unit (101), by means of the communication device (113).

2. System according to claim 1, further characterized in that the processing device (111) of at least one unit (101) processes the propagation time of optical pulses in the space between two units (101) and the speed of propagation of the optical pulses in thespace between the aforesaid units (101) to calculate the distance between the aforesaid units (101).

3. System according to claim 2, further characterized in that the processing device (111) of at least one unit (101) calculates at least one of the successive derivatives with respect to time of the distance between two units (101) of the system.

4. System according to any one of the previous claims, characterized in that the measuring apparatus (109) of at least one of the units (101) comprises: a. at least one time-to-digital converter (114); b. a first electrical circuit (115) for reading the electrical quantity related to the optical source (103); c. a second electrical circuit (116) for reading the electrical quantity related to the optical detector (107).

5. System according to any one of the claims between 1 and 3, in which the measuring apparatus (109) of at least one unit (101) comprises: a. at least one analog-to-digital converter (117); b. a first electrical circuit (115) for reading the electrical quantity related to the optical source (103); c. a second electrical circuit (116) for reading the electrical quantity related to the optical detector (107).

6. System according to any one of the previous claims, characterized in that at least one unit (101) further comprises an optical lens to collimate the optical pulse generated by the optical source (103).

7. System according to claim 6, characterized in that at least one unit(101) further comprises a rotating mirror to direct the collimated optical pulse according to different angular positions.

8. System according to claim 2, comprising at least three units(101), in which the distances between units (101) belonging to distinct pairs of units (101) are processed by means of geometric procedures in order to determine at least one among the following quantities: a. the position of at least one specific unit (101) in space; b. at least one of the successive derivatives with respect to time of said position.

9. System according to claim 2, comprising at least three units(101), in which at least two units (101) are relatively positioned at fixed and known distances and angles, in order to realize a rigid subsystem, and in which it is possible to calculate at least one of the following quantities by means of geometric procedures : a. the position in space of at least one unit (101) belonging to one rigid subsystem; b. at least one of the successive derivatives with respect to time of the position in space of at least one unit (101) belonging to one rigid subsystem; c. the orientation in space of at least one rigid subsystem; d. at least one of the successive derivatives with respect to time of the orientation in space of at least one rigid subsystem.

10. System according to any of the previous claims, characterized in that each unit (101) comprises a plurality of optical sources (103).

11. System according to any of the previous claims, characterized in that each unit (101) comprises a plurality of optical detectors (107).

Images