DEVICE FOR OPERATING A LIGHT SOURCE FOR OPTICAL TIME-OF-MOUNT MEASUREMENT

Description

[0001] The present invention relates generally to a device for operating a light source for optical time-of-flight measurement.

[0002] In general, various methods for optical time-of-flight measurement are known, which can be based on the so-called time-of-flight principle, in which the travel time of a light signal emitted and reflected by an object is measured in order to determine the distance to the object based on the travel time.

[0003] Particularly in the automotive sector, sensors based on the so-called LIDAR principle (Light Detection and Ranging) are used, in which pulses are periodically emitted to scan the environment and the reflected pulses are detected. A corresponding method and device are known, for example, from WO 2017 / 081294.

[0004] US 8,497,478 B2 describes a high-voltage power supply circuit for driving a light source in an optical sensor system.

[0005] WO 2018 / 050897 A1 describes an electronic circuit for a time-of-flight sensor.

[0006] US 2005 / 0201435 A1 discloses an object detection unit.

[0007] Although solutions for optical distance measurement are known from the prior art, one object of the present invention is to provide a device for operating a light source for optical time-of-flight measurement.

[0008] This task is solved by the device according to claim 1.

[0009] According to a first aspect, the present invention provides a device for operating a light source for optical time-of-flight measurement, comprising: a light source configured to emit light pulses according to a pulse signal sequence; and a monitoring circuit for monitoring the light output emitted by the light source based on a current and / or voltage signal from the light source.

[0010] As mentioned, exemplary embodiments relate to a device for operating a light source for optical time-of-flight measurement, comprising: a light source configured to emit light pulses according to a pulse signal sequence; and a monitoring circuit for monitoring the light output emitted by the light source based on a current and / or voltage signal from the light source.

[0011] The device can generally be used in a LiDAR system or the like, and, for example, in the automotive sector, without the present invention being limited to these cases. Consequently, in some embodiments, the device also includes a suitable detector or sensor, for example, based on SPAD (Single Avalanche Photodiode) technology, CAPD (Current Assisted Photodiode) technology, CMOS (Complementary Metal Oxide Semiconductor) technology, or the like, for detecting light pulses emitted by the light source and reflected by an object. Furthermore, the device can be configured to determine the travel time of the emitted light pulses and, based on this, to determine, for example, the distance between the device and the object, a three-dimensional image of the object, or the like.

[0012] The light source can include one or more laser elements, e.g., laser diodes, VCSEL (Vertical Surface Emitting Laser) or the like, or it can also be based on LED (Light Emitting Diode) technology or the like.

[0013] In some embodiments, the light source is intended to be eye-safe, as may be required for some light sources, especially lasers. For example, eye safety regulations may stipulate that the emitted average power must not exceed certain values ​​over different timescales, such as one millisecond, ten seconds, or the like.

[0014] According to the invention, the monitoring circuit monitors the light output emitted by the light source, wherein in exemplary embodiments the light source is deactivated or not even activated in the first place if the emitted light output exceeds a threshold at which, for example, eye safety is no longer guaranteed.

[0015] Since the light source emits light pulses, in some embodiments it is necessary to integrate or sum the emitted light energy in order to monitor the emitted light power.

[0016] Accordingly, in some embodiments it is necessary not only to monitor the on-time of the light source, but also the current or voltage that the light source needs to operate.

[0017] In some embodiments, the current pulses used to operate the light source based on the pulse sequence (e.g., a laser diode) are in the range of 2 to 10 nanoseconds in length, without limiting the present invention to such durations. It has also been recognized that a conventional analog-to-digital converter or transformer in the 1 GHz to 5 GHz range would be required to digitize the current signal. Such converters are typically expensive and require high power (e.g., greater than 500 milliwatts, or 0.5 watts). Therefore, in some embodiments, other means are employed, as explained further below.

[0018] In some embodiments, the determination of the distance is based on the so-called TCSPC (time correlated single photon counting) measurement principle, especially in embodiments which are based on LIDAR.

[0019] In some embodiments, the light source can periodically emit light pulses, e.g. at a high frequency of every two microseconds in a 300 meter range, without limiting the present invention to this specific example.

[0020] The device may include a (start) pulse generator to generate a pulse signal, whereby the pulse signal can serve as a start pulse for the measurements and also as a basis for generating the pulse signal sequence.

[0021] As mentioned, light pulses can have a length of five to twenty nanoseconds for measurement, and in some embodiments, the pulse sequence represents a bit sequence, e.g., a 16-bit pseudo-random bit sequence, so that, for example, a pulse sequence with a length of five nanoseconds represents 16 bits, where, for example, each bit with the value "1" means that the light source is active and "0" that the light source is inactive (or vice versa).

[0022] In some embodiments, the monitoring circuit includes an on-time monitor configured to track whether the continuous on-time of the light source is less than a predefined on-time threshold. For this purpose, the on-time monitor can, for example, analyze a voltage signal from the light source, which has a specific value when the light source is active and no value or a lower value when the light source is inactive. The on-time can be proportional to the emitted luminous energy or luminous power.

[0023] In some embodiments, the monitoring circuit includes a duty cycle monitor configured to monitor whether the duty cycle of the light pulses (e.g., based on the pulse signal and the current / voltage signal) is less than a predefined duty cycle threshold. The duty cycle monitor can, for example, integrate a train of light pulses generated from the pulse sequence using the current / voltage signal, ensuring that all active light pulses in the train are taken into account. This can be achieved by identifying all bits in the pulse sequence where the light source is active (e.g., all bits with the value "1" or "0"). The duty cycle threshold can be chosen, for example, to specify that the light source was active for no more than 8 bit periods (e.g., 80 ns).

[0024] In some embodiments, the monitoring circuit includes a window monitor configured to monitor whether the light source is activated outside of the pulse sequence. This allows, for example, the detection of a malfunction of the light source if, for instance, the light source is not deactivated after the pulse sequence or no longer responds correctly to it.

[0025] In some embodiments, the device includes a pulse sequence generator configured to generate the pulse signal sequence, e.g., based on the pulse signal.

[0026] The device can also include a pulse window generator that generates a pulse window based on the pulse signal. For example, the start of the pulse window is generated upon receipt of the pulse signal, and the end of the pulse window is generated after the pulse train has finished. The pulse window itself can be represented by a corresponding signal and represents the active time of the light source (relative to one pulse train).

[0027] According to the invention, the monitoring circuit comprises a first converter for time-correlated sampling of the current signal from the light source (and for outputting a corresponding sampled and digitized current signal).

[0028] Fast time-to-digital converters are generally known and can, for example, have a time resolution of better than 500 picoseconds.

[0029] This allows the power signal from the light source to be digitized cost-effectively and with high temporal resolution.

[0030] According to the invention, the monitoring circuit comprises a second converter for time-correlated sampling of the voltage signal from the light source (and for outputting a corresponding sampled and digitized voltage signal).

[0031] The analog waveform of the current signal or the voltage signal can be sampled sequentially, since in some embodiments it is synchronous and periodic to the TCSPC measurement cycle.

[0032] In some embodiments, the monitoring circuit includes an energy calculator configured to calculate an electrical energy value, and consequently a total luminous power or pulse energy, based on the current and / or voltage signal of the light source (e.g., based on the current or voltage signal sampled by the first or second converter). The energy calculator can, for example, simply calculate the energy by multiplying the voltage value U by the current value I, where the current value I is determined by integrating the current signal from the first converter, and where the voltage value U can be a model-based value (e.g.,determined based on a linear relationship with the current value I or based on a monotonically increasing function that represents a unique function for the relationship between voltage value U and current value I), can be a predetermined constant or can be determined by integrating the voltage signal from the second converter.

[0033] In some embodiments, the device further includes a temperature compensator configured to correct the emitted luminous power based on the electrical energy value and an operating temperature value of the light source. Generally, the emitted luminous power can correlate with temperature, with efficiency decreasing as the temperature increases, meaning that a higher current flowing through the light source does not necessarily result in higher emitted optical energy. In other words, the electrical energy required to produce the same emitted luminous energy can increase in correlation with the temperature increase. The temperature compensator can account for this effect by, for example, correcting the luminous power output, which is based on the current and / or voltage signal, according to the current temperature, or by correcting a reference value, such as...The electrical energy calculated by the energy calculator is compared.

[0034] Accordingly, in some embodiments, the light source includes a temperature sensor that outputs the operating temperature value of the light source to the temperature compensator.

[0035] In some embodiments, the device includes a measuring resistor (e.g., a shunt resistor) that outputs a voltage sampling signal based on a current signal from the light source. A differential amplifier may also be included to amplify the voltage sampling signal from the measuring resistor. Furthermore, the device may include a comparator that compares the voltage sampling signal with a reference value and, based on this comparison, outputs a status signal from the light source to the monitoring circuit. This status signal could, for example, be a "light source on" signal indicating that the light source is active.

[0036] In some embodiments, the monitoring circuit includes a fault logic designed to deactivate the light source based on monitoring the light output emitted by the light source.

[0037] The device discussed herein can be integrated into a time-of-flight measuring device, such as a LiDAR measuring device, which in turn may be integrated into or intended for use in a motor vehicle or other device. The described device can also be used in an autonomously operated (motor) vehicle.

[0038] The procedural steps described above or herein may also be the subject of a procedure for operating a light source for time-of-flight measurement (or for operating a LIDAR measuring device or the like), which is carried out, for example, by a device described herein.

[0039] In some embodiments, monitoring of critical eye safety parameters is provided. The light source can be switched off by a corresponding signal if a violation of a parameter relevant to eye safety is detected. Furthermore, a start pulse signal can be used to monitor the on-time of the light source (e.g., the laser). In some embodiments, the light source is switched off if a maximum on-time ("t_on_max" of, for example, seven or five nanosecond pulses) is exceeded. The light source can also be switched off if it is active outside the pulse window and / or if a pulse sequence exceeds a duty cycle threshold. In some embodiments, the average and peak current values ​​of the light source are also monitored, e.g.,based on a TDC circuit using a histogram, where current values ​​are filled into a histogram to obtain a corresponding time-based current value profile.

[0040] Exemplary embodiments are now described by way of example and with reference to the accompanying drawing, wherein the first exemplary embodiment is not covered by the claims, but serves for a better understanding of the invention, and the second exemplary embodiment is according to the invention in which: Fig. 1 A first embodiment of a device for operating a light source for optical time-of-flight measurement is illustrated; and Fig. 2 A second embodiment of a device for operating a light source for optical time-of-flight measurement is illustrated.

[0041] Fig. 1 Figure 1 illustrates a circuit diagram of a first embodiment of a device 1 for operating a laser diode 2 for optical time-of-flight measurement with a monitoring circuit 3 for monitoring the eye safety of the operation of the laser diode 2.

[0042] A start pulse generator 4 of the facility 1 outputs periodic trigger signals to start a LIDAR pulse measurement, e.g. a single measurement within a TCSPC cycle.

[0043] The periodic trigger signals or pulse signals are received by a pulse sequence generator 5, which in response generates a pulse sequence by transforming the periodic pulse signals into a pulse sequence. This sequence can be, for example, a 16-bit sequence of "0" and "1", where a "1" causes the laser (laser diode) 2 to be switched on or activated, so that the laser emits 2 light pulses according to the pulse sequence. The pulse sequence is serialized with a 10 ns bit period. In the present embodiment, the length of a pulse train of light signals is 160 ns.

[0044] The pulse train can, in principle, have any combination of "1" and "0" with a certain boundary condition for the "1", since this determines the on-time of laser 2, so that the total number of "1"s together with the pulse frequency defines the average laser power. In other embodiments, the pulse train can also consist of only a single light pulse.

[0045] The pulse sequence generator 5 outputs the pulse sequence to a laser driver 6, which converts the pulse sequence into high-current signals to operate the laser diode 2.

[0046] The laser driver 6 is connected to a laser switch 7, to which a laser supply voltage +V ls is applied, which is supplied to the laser driver 6 when the laser switch 7 is switched on.

[0047] The laser switch 7 can switch off the laser diode 2 if, for example, the laser driver 6 malfunctions, is defective, or a short circuit occurs, causing the laser diode 2 to emit light (pulses) continuously, which would be critical with regard to eye safety. It also provides a redundant control path, as the laser switch 7 can switch off the laser diode in addition to the laser driver 6. Furthermore, the laser switch 7 can receive a corresponding switch-off signal from the monitoring circuit 3, as will be explained in more detail below.

[0048] The current flowing through laser diode 2 is measured by the voltage drop across a shunt resistor 8 connected to laser diode 2. The sampled voltage is supplied to a differential amplifier 9, which amplifies the small sampling voltage signal and outputs it to a comparator 10.

[0049] The comparator 10 compares the amplified voltage sampling signal with a reference value and outputs a corresponding laser status signal, which indicates whether the laser diode is on (voltage sampling signal greater than the reference value) or off (voltage sampling signal less than the reference value).

[0050] The signal output by the comparator 10 is, in the present embodiment, a laser-on signal, and the first rising edge of this signal can be used to start the TDCs for the time-of-flight measurement.

[0051] The start pulse generator 4 also supplies the pulse signal to a pulse window generator 11, so that the start pulse generator 4 triggers the pulse window generator 11. The pulse window generator outputs a window signal, which is set "high" (start of the window) as soon as the start pulse has been received and is set to "low" (end of the window) after the maximum time of the pulse train, i.e., for example, after 160 ns, which corresponds to the length of the pulse train in this embodiment.

[0052] The laser-on signal of the comparator 10 is supplied to the monitoring circuit 3, which has three monitors, namely an on-time monitor 12, a duty cycle monitor 13 and a window monitor 14.

[0053] The on-time monitor 12 checks whether the continuous on-time of the laser diode 2 (without interruption) exceeds a predefined on-time threshold. For example, the laser diode 2 must not be activated for longer than two consecutive pulses, i.e., two consecutive "1"s of the pulse sequence. Accordingly, the on-time monitor 12 outputs an error signal to an error logic 15 if the on-time or activation time in this embodiment is above an on-time threshold of 22 ns, where the 22 ns is derived from the on-time of 20 ns for two pulses with an added tolerance of 2 ns.

[0054] The duty cycle monitor 13 accumulates or integrates the on-times of the laser diode 2 within the period defined by the pulse train window supplied by the pulse window generator 11. In this embodiment, the duty cycle monitor 13 outputs an error to the error logic 15 if the laser diode 2 is activated for more than 8 bit periods (= 80 ns = duty cycle threshold) within a pulse sequence (without limiting the present invention to this specific example).

[0055] Window monitor 14 verifies that laser diode 2 is not operating or activated outside the pulse sequence window (window signal) supplied by pulse window generator 11. This allows system malfunctions to be detected, for example, if laser driver 6 malfunctions, is defective, or similarly fails to switch off at the end of a pulse sequence. In this case, window monitor 14 also outputs an error signal to error logic 15.

[0056] The fault logic 15 combines the fault signals it receives from monitors 12 to 14 and, in the event of a fault, sends a corresponding shutdown signal to the laser switch 7, which, in response to the shutdown signal, switches off the laser diode 2. The fault logic 15 can also send a corresponding fault report to a higher-level controller.

[0057] Fig. 2Figure 1 illustrates a circuit diagram of a second embodiment of a device 20 for operating a laser diode 21 for optical time-of-flight measurement with a monitoring circuit 22 for monitoring the eye safety of the operation of the laser diode 21.

[0058] This embodiment refers to the TCSP LIDAR measurement principle and involves emitting at least two pulses or pulse sequences consecutively without modification.

[0059] A start pulse generator 23 of the device 20 outputs periodic trigger signals to start a LIDAR pulse measurement, e.g. a single measurement within a TCSPC cycle.

[0060] The periodic trigger signals or pulse signals are received by a pulse sequence generator 24, which in response generates a pulse sequence by transforming the periodic pulse signals into the pulse sequence. This sequence can be, for example, a 16-bit sequence of "0" and "1", where a "1" causes the laser (laser diode) 21 to be switched on or activated, so that the laser 21 emits light pulses according to the pulse sequence. The pulse sequence is serialized with a 10 ns bit period. In the present embodiment, the length of a pulse train of light signals is 160 ns.

[0061] The pulse train can basically have any combination of "1" and "0" with a certain boundary condition for the "1", since this determines the on-time of the laser 21, so that the total number of "1" together with the pulse frequency defines the average laser power.

[0062] The pulse sequence generator 24 outputs the pulse sequence to a laser driver 6, which converts the pulse sequence into high signals (current signals) to operate the laser diode 21.

[0063] The laser driver 25 is connected to a laser switch 26, to which a laser supply voltage +V ls is applied, which is supplied to the laser driver 25 when the laser switch 26 is switched on.

[0064] The laser switch 26 can switch off the laser diode 21 if, for example, the laser driver 25 malfunctions, is defective, or a short circuit occurs, causing the laser diode 21 to emit light (pulses) continuously, which would be critical with regard to eye safety. It also provides a redundant control path, as the laser switch 26 can switch off the laser diode 21 in addition to the laser driver 25. Furthermore, the laser switch 26 can receive a corresponding switch-off signal from the monitoring circuit 22, as will be explained in more detail below.

[0065] The current flowing through the laser diode 21 is measured by the voltage drop across a shunt resistor 27 connected to the laser diode 21. The sampled voltage is supplied to a differential amplifier 28, which amplifies the small sampling voltage signal and outputs it to a comparator 29.

[0066] The comparator 29 compares the amplified voltage sampling signal with a reference value and outputs a corresponding laser status signal, which indicates whether the laser diode 21 is on (voltage sampling signal greater than the reference value) or off (voltage sampling signal less than the reference value).

[0067] In this embodiment, the signal output by comparator 29 is a laser-on signal, and the first rising edge of this signal can be used to start the TDCs for time-of-flight measurement and also to start the two TCADCs 30 and 31 (see also below) of the monitoring circuit 22. Alternatively, the start pulse of the start pulse generator 23 can also be used for this purpose, whereby the output signal of comparator 29 is slightly delayed and is not affected by jitter from the laser driver 25. TCADCs 30 and 31 can, for example, be configured as disclosed in German patent application no. 102018220688.0.

[0068] In the present embodiment, the monitoring circuit has a first TCADC 30 (time correlated analog-to-digital converter), also called first converter 30, and a second TCADC 31, also called second converter 31.

[0069] The first TCADC 30 samples the analog current signal supplied by the differential amplifier 28 in a time-correlated manner and, after one TCSPC cycle, e.g., one hundred pulse repetitions, provides a sampled current signal for further processing, which is supplied to an energy calculator 32.

[0070] In the present embodiment, the second TCADC 31 samples the analog voltage signal of the "high side" of the laser diode 21 in a time-correlated manner, and after one TCSPC cycle or, for example, one hundred pulse repetitions, the second TCADC 31 provides a sampled voltage signal for further processing, which is also supplied to the energy calculator 32.

[0071] The energy calculator 32 calculates, as soon as available (e.g. after one hundred pulse repetitions), based on the digitized current signal from the first TCADC 30 and the digitized voltage signal of the second TCADC 31, the total pulse energy (total electrical energy that has flowed through the laser diode 21 and corresponds to an emitted light power) by multiplying the integral over the voltage signal with the integral over the current signal.

[0072] In embodiments where the second TCADC 31 is not provided, the voltage value can be assumed to be a constant or, for example, derived via a linear (or, as above, monotonic) relationship with the current signal.

[0073] The value of the total pulse energy (here correspondingly the total electrical energy) is supplied to a comparator 33, which compares the value of the total pulse energy with a reference value and, if the reference value is exceeded, outputs a corresponding error signal to an error logic 34. In response to the error signal, the error logic 34 outputs a corresponding shutdown signal to the laser switch 26, which then switches off the laser diode 21. The error logic 34 can also receive other error signals.

[0074] In the present embodiment, a temperature sensor 35 is provided on the laser diode 21, which determines the operating temperature of the laser diode 21 and supplies a corresponding temperature signal to a temperature compensator 36 of the monitoring circuit 22.

[0075] The temperature compensator 36 monitors the temperature of the laser diode 21 based on the received temperature signal. The measured electrical energy (by sampling the TCADCS 30 and 31 and calculated by the energy calculator 32) consumed by the laser diode 21 is related to the emitted light power via the optical efficiency of the laser diode 21, whereby the efficiency of the laser diode 21 is temperature-dependent, such that the efficiency decreases with increasing temperature, particularly above 60 °C in this embodiment.

[0076] However, for eye safety, the emitted light energy or light power is not relevant, so that at high operating temperatures of the laser diode 21 the electrical energy can also be increased without the emitted light power being too high in terms of eye safety.

[0077] The temperature compensator 36 has stored in its memory a relationship between the emitted optical light power and the operating temperature and can adjust the reference value stored there based on the current operating temperature by communicating with the comparator 33, so that at higher operating temperatures the reference value for the permissible pulse energy (which is based on the electrical energy, as explained above) is increased accordingly. Reference sign

[0078] 1 Device for operating a laser diode for optical time-of-flight measurement 2 Laser diode (light source) 3 Monitoring circuit 4 Start pulse generator 5 Pulse sequence generator 6 Laser driver 7 Laser switch 8 Shunt resistor (measuring resistor) 9 Differential amplifier 10 Comparator 11 Pulse window generator 12 On-time monitor 13 Duty cycle monitor 14 Window monitor 15 Fault logic 20 Device for operating a laser diode for optical time-of-flight measurement 21 Laser diode (light source) 22 Monitoring circuit 23 Start pulse generator 24 Pulse sequence generator 25 Laser driver 26 Laser switch 27 Shunt resistor (measuring resistor) 28 Differential amplifier 29 Comparator 30 First TCADC (converter) 31 Second TCADC (converter) 32 Energy calculator 33 Comparator 34 Error logic 35 Temperature sensor 36 Temperature compensator

Claims

1. Device (1, 20) for operating a light source (2, 21) for optical time-of-flight measurement, comprising: a light source (2, 21) which is configured to emit light pulses according to a pulse signal sequence; and a monitoring circuit (3, 22) for monitoring light output emitted by the light source (2, 21) based on a current signal and / or voltage signal of the light source (2, 21), wherein the monitoring circuit (3, 22) is configured to deactivate or not activate the light source (2, 21) when the emitted light output exceeds a threshold value, characterized in that the monitoring circuit (22) comprises a first converter (30) for time-correlated sampling of the current signal from the light source (21) and a second converter (31) for time-correlated sampling of the voltage signal from the light source (21).

2. Device according to claim 1, characterized in that the monitoring circuit (3) comprises an on-time monitor (12) which is configured to monitor whether the continuous on-time of the light source (2) is less than a predetermined on-time threshold.

3. Device according to claim 1 or 2, characterized in that the monitoring circuit (3) comprises a duty cycle monitor (13) which is configured to monitor whether a duty cycle of the light pulses is smaller than a predetermined duty cycle threshold value.

4. Device according to one of the preceding claims, characterized in that the monitoring circuit (3) comprises a window monitor (14) which is configured to monitor whether the light source (2) is activated outside the pulse sequence.

5. Device according to one of the preceding claims, further comprising a pulse sequence generator (5, 24) which is configured to generate the pulse signal sequence.

6. Device according to claim 5, further comprising a pulse window generator (11) which is configured to generate a pulse window based on a pulse signal.

7. Device according to claim 1, characterized in that the monitoring circuit (22) comprises an energy calculator (32) which is configured to calculate an electrical energy value based on the current signal and / or voltage signal of the light source (21).

8. Device according to claim 7, further comprising a temperature compensator (36) which is configured to correct the emitted light output based on the electrical energy value and an operating temperature value of the light source (21).

9. Device according to claim 8, wherein the light source (21) comprises a temperature sensor (35) that outputs the operating temperature value of the light source (21) to the temperature compensator (36).

10. Device according to one of the preceding claims, further comprising a measuring resistor (8, 27) that outputs a voltage sampling signal based on a current signal of the light source (2, 21).

11. Device according to claim 10, further comprising a differential amplifier (9, 28) that amplifies the voltage sampling signal of the measuring resistor (8, 27).

12. Device according to claim 11, further comprising a comparator (10, 29) which compares the voltage sampling signal with a reference value and, based thereon, outputs a status signal of the light source (2, 21) to the monitoring circuit (3, 22).

13. Device according to one of the preceding claims, characterized in that the monitoring circuit (3, 22) comprises an error logic (15, 34) which is configured to deactivate the light source (2, 21) based on the monitoring of the light output emitted by the light source (2, 21).