Optoelectronic time-of-flight measuring device

Abstract

Optoelectronic time-of-flight measuring device (1) for determining a measured value (M) that is proportional to the distance (Δs) of objects (2) within a monitoring area (8), consisting of: - a first circuit (3) which has at least one light source (7) emitting light rays (9), the pulsed light rays (9) of which follow a predetermined beam path (16), - a second circuit (4) which is connected in parallel to the first circuit (3) and is formed from a series circuit of at least one sensor (10), a receiver amplifier (12) and a time-of-flight meter (13), which detects the light rays (9) reflected by the object (2) and by which the measured value (M) proportional to the distance (Δs) is generated wherein the first and second circuits (3, 4) are supplied independently of a power source (5, 5'), - from a differentiating circuit (11) arranged in the second circuit (4), which is electrically connected downstream of the receiving amplifier (12) and which provides a correction signal (u K ) generates a change proportional to the change over time (you A / dt) of the output signal (u A ) of the receiving amplifier (12) is, characterized by that the differentiating circuit (11) has a peak-value rectifier (15) and that the differentiating circuit (11) is formed from at least two capacitors (18, 20) and a resistor circuit (19), wherein one of the capacitors (20) is a resistor that contributes to the change in the output signal (du A / dt) proportional correction voltage (v K ) generated.

Description

[0001] The invention relates to an optoelectronic time-of-flight measuring device for determining a measured value according to the preamble of patent claim 1.

[0002] Such an optoelectronic time-of-flight measuring device is previously known from DE 10 2006 049 935 A1. Reference is also made to US 2008 / 0068584 A1.

[0003] Such optoelectronic time-of-flight measuring devices are proven and versatile, and essentially consist of two circuits, each connected to a power source by an evaluation unit. The power source supplies the two circuits with power independently of each other. The first circuit consists of a light source that emits the required light beams in pulses to determine the distance using the time-of-flight measurement. The second circuit consists of a sensor that detects the light beams reflected by the object located in the monitored area. The sensor is designed as an avalanche photodiode (APD) or a multipixel photon counter (MPPC) and can be continuously operated with different high voltages to ensure optimal sensor performance.When a light beam reflected from an object in the monitored area hits the sensor, a photocurrent is generated, and the light beam's travel time can be measured. The light beam's travel time can then be converted into a distance. From the measured value, the distance between the device and the object can be calculated using the physical formula 2s = c * t, since the emitted light sources travel through space at a constant speed of light.

[0004] Such a measuring device can be found in EP 2 071 356 A2, but it has been found that such time-of-flight measuring devices often determine distances extremely inaccurately and with errors. These measurement errors arise because the different reflection properties of the objects mean that the brightness of the reflected light waves lies within a certain tolerance range, which leads to measurement inaccuracies or errors. The light rays are reflected with different brightnesses depending on the reflective surfaces of the objects. The reaction speed of the receiving circuit depends significantly on the amount of light received by the sensor and thus leads to delayed detection of the reflected light beam in the case of a dark object with low remission. This erroneous time measurement leads to the aforementioned measurement errors.

[0005] From DE 10 2015 000 941 B3, a method for operating such a time-of-flight measuring device has become known, wherein the high voltage of the MPPC diode is regulated in such a way that the internal gain of the receiving diode is adjusted in such a way that the same signal amplitude is permanently set at the output of the receiver circuit, regardless of whether a light or a dark object is located in front of the time-of-flight measuring device.

[0006] Such a time-of-flight measuring device thus measures the distance regardless of the brightness of the object's surface, resulting in only minor measurement errors. However, it has proven disadvantageous that regulating the sensor's high voltage is complex and has a limited response time and dynamic range. This makes it impossible to perform distance measurements on dynamically moving objects with varying brightness.

[0007] Another method for operating such a time-of-flight measuring device can be found in DE 10 2012 211 222 A1. The time-of-flight measuring device is comprised of a plurality of receiver circuits with varying levels of amplification. After a reflected light beam has been detected, an evaluation device selects the sensor whose received signal falls within a sensitivity range valid for the current received signal.

[0008] The disadvantage of this state of the art is the high complexity due to the large number of receiver circuits and, in addition, the measurement error that occurs when the received signal is between two of the optimal sensitivities of the amplifiers.

[0009] Optoelectronic time-of-flight measuring devices have become known from DE 10 2014 005 521, DE 10 2014 106 463 A1 and DE 10 2014 106 465 B3, in which a receiving amplifier and one or more comparators are connected downstream of the sensor, which first amplify the photocurrent and then determine the signal level at which the received signal is accepted.

[0010] A disadvantage of this state-of-the-art technology is that such time-of-flight measuring devices only provide reliable measurements as long as the receiver amplifier is in the analog range. For very bright objects, the receiver amplifier overloads, making it impossible to adjust the comparator threshold to compensate for the measurement error.

[0011] The object of the invention is therefore to further develop an optoelectronic time-of-flight measuring device of the type mentioned above in such a way that it reliably and quickly determines the distance of even dynamically moving objects, regardless of the brightness of the light beam reflected by the object. Furthermore, such a time-of-flight measuring device should be able to generate a correction signal so that the measured value, which is proportional to the distance of the object, can be corrected and thus is independent of the brightness of the light beam remitted by the object. Furthermore, such an optoelectronic time-of-flight measuring device should provide reliable and reproducible measured values ​​and be cost-effective to manufacture.

[0012] This object is achieved according to the invention by the characterizing part of patent claim 1. Further advantageous developments of the invention emerge from the subclaims.

[0013] Due to the fact that in the second circuit (4) arranged differentiating circuit (11), which is electrically connected downstream of the receiving amplifier (12) and which generates a correction signal (u K ) which is proportional to the change in time (du A / dt) of the output signal (u A ) of the receiving amplifier (12), so that the correction signal is proportional to the slope, i.e., the change in the output signal of the photocurrent amplified by the receiving amplifier. The slope depends on the brightness of the object, and thus the measurement error can be compensated.

[0014] Furthermore, it is particularly advantageous if a correction circuit is electrically connected downstream of the time-of-flight meter and the differentiating circuit, which mathematically calculates the correction signal with the measured value of the time-of-flight meter. The signal output by the time-of-flight meter and the differentiating circuit can be a voltage or a digital signal, which are calculated or mathematically correlated with each other in the correction circuit.

[0015] Furthermore, it has proven advantageous if the correction circuit has a microcontroller that calculates the output signal of the runtime meter using the correction signal and other stored characteristic maps or polynomials.

[0016] Furthermore, it has proven advantageous to use an LED, a laser diode, or a vertically imaginable laser diode (VCSEL) as the light source. The sensor can also be configured as an avalanche photodiode or MPPC diode, an array of CMOS photodiodes operating in Geiger mode.

[0017] The drawing shows an embodiment of an optoelectronic time-of-flight measuring device according to the invention, which is explained in more detail below.

[0018] In detail: Fig. 1 an optoelectronic time-of-flight measuring device, consisting of a first circuit having a transmitting unit that emits a pulsed light beam and a second circuit formed by a pure circuit of a sensor, an amplifier and a time-of-flight meter, wherein a differentiating circuit is arranged parallel to the time-of-flight meter, which generates a correction signal that is offset against the output signal of the time-of-flight meter, Fig. 2 the output signal of the receiving amplifier and Fig. 3 the output signal of the differentiating circuit.

[0019] In the Fig. Figure 1 shows the schematic structure of an optoelectronic time-of-flight measuring device 1 for detecting the distance Δs between the object and an approaching object 2. The optoelectronic time-of-flight measuring device 1 is formed from a first circuit 3 and a second circuit 4, which are independently supplied with a voltage from one or more power sources 5. The first circuit 3 is formed by a pulse generator 6 and a light source 7, wherein the pulse generator 6 generates the electrical signal, the pulse signal u E , through which a pulsed light beam 9 is emitted by the light source 7 into a monitoring area 8 along a predetermined beam path 16.

[0020] The light source 7 can be designed as an LED, a laser diode or as a vertically emitting laser diode or VCSEL laser diode.

[0021] The second circuit 4 is essentially formed by a series connection of a sensor 10, an amplifier 12, and a time-of-flight meter 13. The sensor 10 serves to detect the light beam 9 remitted by the object 2 and is designed as an avalanche photodiode, MPPC diode, or an array of CMOS photodiodes operated in the Geiger mode. Upon detecting a remitted light beam 9, the sensor 10 generates a photocurrent u PH , which is amplified by the downstream amplifier 12.

[0022] The output signal u A of the amplifier 12 thus corresponds to the multiple of the photocurrent u PH . This output signal u A is passed on to the transit time meter 13. The transit time of the light beam 9 is then determined by comparing the pulse signal u E and the output signal u A of the receiving amplifier 12 and an erroneous measured value is formed.

[0023] In particular, the Fig. 2 is the time course of the output signal of the amplifier 12 for different photocurrents u PH The amplifier 12 is designed as a transimpedance amplifier 12, so that the photocurrent to be measured u PH a voltage drop is generated. The brighter the object 2 in the monitoring area 8, the more photons hit the sensor 10 and thus a higher photocurrent u PH generated than with low-reflecting objects 2. The output voltage v A of the output signal u A at the amplifier 12 decreases comparatively significantly more for the bright object 2 than for the dark object 2.

[0024] The curve of the output voltage v A of the output signal u A in the Fig. 2 shows that with increasing photocurrent u PH the slope, so (you A / dt) takes on a larger value. It has proven advantageous that the output voltage v A of the output signal u A is limited to a predetermined voltage, since such amplifiers 12 typically overload when receiving particularly strong signals.

[0025] The receiver amplifier 12 is followed by the time-of-flight meter 13. The time-of-flight meter 13 is a time / voltage converter which, when a threshold value vs of the output signal u is reached, A of the amplifier 12, the runtime measurement is terminated and a measurement signal u M The measurement signal u M corresponds to a voltage whose amplitude depends on the travel time of the light beam 9.

[0026] For this purpose, the transit time meter 13 is coupled to the first circuit 3 and the amplifier 12, so that on the one hand the pulse signal u E and on the other hand the output signal u Aof the amplifier 12. The propagation time corresponds to the time offset of the pulse signal u E and the output signal u A of the amplifier 12.

[0027] Due to the different photocurrents and PH the time at which the threshold value vs is reached is different, since, for example, the output signal of the amplifier 12 at a high photocurrent u PH - so with a bright object 2 - a large slope you A / dt and the threshold value vs is already reached at time t1, whereas for the dark object 2, the threshold value vs is only reached at time t2. Due to this time difference (t2 - t1), both measurement signals are error-prone. Consequently, at a constant distance Δs, the error-prone measurement signal u M for the dark object 2 is lower than the measurement signal u M for the bright object 2.

[0028] A differentiating circuit 11 is electrically arranged in parallel with the transit time meter 13, which produces a correction signal u K The correction signal u K the differentiating circuit 11 is proportional to the differential of the output signal u A of the amplifier 12 and thus describes the temporal change of the output signal u A of the amplifier 12, the slope (you A / dt).

[0029] The differentiating circuit 11 comprises a first capacitor 18, a resistance circuit 19, a peak value rectifier 15 and a second capacitor 20. The edge steepness of the output signal u Aof the amplifier 12 generates a current in the first capacitor 18, which in turn generates a negative voltage pulse in the resistance circuit 19, which is proportional to the edge steepness. The peak value rectifier 15 and the second capacitor 20 rectify the pulse-shaped voltage waveform, whereby a constant correction voltage v K is output from the second capacitor 20, which is proportional to this edge steepness. The curve of the generated correction voltage v K on the second capacitor 20 is the Fig. 3. The correction voltage v K corresponds to the correction signal u K .

[0030] The correction signal u K the differentiating circuit 11 is transferred to a correction circuit 14, which is connected downstream of the differentiating circuit 11 and the transit time meter 13, and the corrected measured value M is determined by a compensation of the measuring signal u Mof the runtime meter 13 with the correction signal u K The measured value M is thus exclusively proportional to the distance Δs of the object 2 and is now independent of its brightness.

[0031] The compensation in the correction circuit 14 is carried out by subtracting the voltage of the measuring signal u M and the voltage of the correction signal u K Alternatively, the correction signal can be K the differentiating circuit 11 and the measuring signal u M of the runtime meter 13. The correction circuit 14 can calculate or mathematically correlate these digital signals with each other in a microprocessor and perform the compensation using stored characteristic maps or polynomials and output the corrected measured value M.

[0032] Furthermore, the sensor 10 can be operated with a constant high voltage with temperature compensation. The temperature compensation compensates for any temperature drift of the breakdown voltage of the sensor 10, which is operated in Geiger mode.

[0033] The voltage supply of the sensor 10 can also be provided with a noise control that always sets the operating point of the sensor 10 to a maximum gain.

Claims

[1] Optoelectronic time-of-flight measuring device (1) for determining a measured value (M) which is proportional to the distance (Δs) of objects (2) within a monitoring area (8), consisting of: - a first circuit (3) having at least one light source (7) emitting light beams (9), the pulsed light beams (9) of which follow a predetermined beam path (16), - a second circuit (4) which is connected in parallel to the first circuit (3) and is formed from a series circuit of at least one sensor (10), a receiving amplifier (12) and a transit time meter (13), which detects the light rays (9) reflected by the object (2) and by which the measured value (M) proportional to the distance (Δs) is generated wherein the first and second circuits (3, 4) are supplied independently from a power source (5, 5'), - from a differentiating circuit (11) arranged in the second circuit (4), which is electrically connected downstream of the receiving amplifier (12) and which generates a correction signal (u K ) which is proportional to the change in time (du A / dt) of the output signal (u A ) of the receiving amplifier (12), characterized by , that the differentiating circuit (11) has a peak value rectifier (15) and that the differentiating circuit (11) is formed from at least two capacitors (18, 20) and a resistance circuit (19), wherein one of the capacitors (20) has a value which is related to the change in the output signal (du A / dt) proportional correction voltage (v K ) is generated. [2] Device (1) according to claim 1 characterized bythat a correction circuit (14) is electrically connected downstream of the transit time meter (13) and the differentiating circuit (11) and that the correction circuit (14) compares the measured value (U) of the transit time meter (13) to the correction signal (u K ) are mathematically correlated. [3] Device (1) according to claim 2 characterized by that the transit time meter (13) and the differentiating circuit (11) each produce a voltage (v M , v K ), or that the transit time meter (13) and the differentiating circuit (11) output a digital signal. [4] Device (1) according to claim 2 or 3 characterized by that the correction circuit (11) comprises a microcontroller, and that the correction circuit (11) compensates the output signal (u M ) of the runtime meter (13) with the correction signal (u K ) and stored characteristic maps or polynomials. [5] Device (1) according to one of the preceding claims, characterized by, the light source (7) is an LED, a laser diode or a vertically imitating laser diode or VCSEL laser diode. [6] Device (1) according to one of the preceding claims, characterized by that the sensor (10) as an avalanche photodiode or MPPC diode, is an array of CMOS photodiodes operated in the Geiger mode. [7] Device (1) according to one of the preceding claims, characterized by that the sensor is operated with a constant high voltage with temperature compensation, and that the temperature compensation compensates for the temperature impact of the breakdown voltage of the diode. [8] Device (1) according to claim 7, characterized by , the high voltage of the sensor (10) is provided with a noise control and that the noise control sets the maximum gain at the operating point of the sensor (10).

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