OPTOELECTRONIC SENSOR FOR DETECTING AND DETERMINING THE DISTANCE OF OBJECTS AND TRIGGER CIRCUIT FOR SUCH A SENSOR

Description

[0001] The invention relates to an optoelectronic sensor for detecting and determining the distance of objects in a monitoring area according to the preamble of claim 1 and a trigger circuit for such a sensor according to claim 12.

[0002] A well-known method for optical distance measurement is time-of-flight measurement. A distinction is made between pulse-based and phase-based measurements. In a pulse-of-flight method, light beams with short light pulses are emitted by a light source, for example a laser diode. The returning light pulses are received by an optical receiving element, for example a single photon avalanche diode (SPAD), and the time until the returning light pulses are received is measured. Alternatively, in a phase method, transmitted light is amplitude-modulated and a phase shift between transmitted and received light is determined, whereby the phase shift is also a measure of the light travel time. The light travel time is then converted into a distance using the speed of light. Optoelectronic sensors that use this method are often referred to as light scanners, TOF (time-of-flight) sensors or LI DAR (light detection and ranging) sensors.To expand the measuring range, the light beam can be moved, as is done in a laser scanner. A light beam generated by a laser periodically sweeps the monitored area with the aid of a deflection unit. In addition to the measured distance information, the angular position of the deflection unit is used to determine the angular position of the object, thus recording the location of an object in the monitored area in two-dimensional polar coordinates. In most laser scanners, the scanning movement is achieved by a rotating mirror. However, it is also known to rotate the entire measuring head with light transmitters and light receivers instead.

[0003] The accuracy of distance determination is directly dependent on the accuracy of the time-of-flight determination. It therefore requires the most precise knowledge possible of the transmission time, at which a light pulse leaves the sensor's light source, and the reception time, at which the returning light pulse hits the receiving element.

[0004] It is known from the prior art to send a signal to the light source, which activates the light source to emit at least one light pulse, usually by delivering a current or voltage pulse to the light source. This signal, also referred to as the activation signal below, simultaneously starts a time-of-flight measurement. The time-of-flight measurement is stopped by a receive signal generated by the optical receiving element upon reception of the returning light pulse. The activation signal and the receive signal are usually transmitted to a so-called time-to-digital converter (TDC), which determines the measurement of the time between the transmission of the trigger signal and the generation of the receive signal. Details on TDCs can be found, for example, in Henzler, Stephan. "Time-to-digital converters." Vol. 29. Springer Science & Business Media, 2010, or the 2015 Annual Report of the Circuits and Systems group at the University of Oulu by J. Kostamovaara and T.Rahkonen.

[0005] Since the delivery of the current or voltage pulse to the light source, and thus the emission of the light pulse, usually occurs with a latency after the activation signal is transmitted, calibration must first be performed by measuring the time of flight on objects with known distances. However, temperature drift, aging, or fluctuation (jitter) of electronic components between the trigger source and the light source, as well as the light source itself, can influence the time between the transmission of the activation signal and the transmission of a current or voltage pulse to the light source and thus the emission of the light pulse, thus impairing the accuracy of the distance measurement.

[0006] To improve the accuracy of distance measurement, it is also known to detect the emission of the light pulse by a light receiver, such as a photodiode, located immediately downstream of the light source, and to initiate the time-of-flight measurement upon detection of the light pulse. Such an arrangement almost completely eliminates the negative influences on the time-of-flight measurement described above. However, the disadvantage is that the arrangement is complex and, in particular, requires additional optical components with corresponding space requirements and adjustment effort.

[0007] European patent application EP 3 521 856 A1 proposes providing a calibration path in a circuit for determining the time of flight, in addition to a measurement path for measuring the time of flight, in order to improve the accuracy of the light flight determination. The calibration path can be configured, in particular, to measure a laser current from a laser light source. The laser current is compared by means of a comparator with a threshold value; if exceeded, laser radiation is usually emitted. By determining the time between activation of the light source and exceeding the threshold value of the laser current, the latency between activation of the light source and emission of the light pulse can be determined and taken into account in the time of flight calculation. Since the disclosed circuit uses a calibration path in addition to a measurement path, it is correspondingly complex.A direct use of exceeding the laser current threshold to initiate the time-of-flight measurement is not disclosed. A similar system with an additional light source for calibration is disclosed in DE4002356.

[0008] Based on this prior art, it is the object of the invention to provide an improved optoelectronic sensor for determining the time of flight of light.

[0009] This object is achieved by an optoelectronic sensor for detecting and determining the distance of objects in a monitoring area having the features of claim 1 or by a trigger circuit for such a sensor having the features of claim 12.

[0010] An optoelectronic sensor according to the invention comprises at least one light transmitter with at least one light source for emitting light pulses into a monitored area. An associated light receiver is capable of generating received signals from light pulses reflected or remitted by objects in the monitored area. A control and evaluation unit controls the light transmitter and the light receiver and can evaluate the received signals from the light receiver to obtain information about the object, such as a distance from the sensor. To determine the distance between the object and the sensor, the control and evaluation unit comprises a time measurement unit, for example a time-to-digital converter (TDC). The time measurement unit is configured to determine a light propagation time of a light pulse from a time interval between a trigger signal and a received signal from the light receiver.The trigger signal is generated by a trigger circuit that taps a voltage applied to the light source of the light transmitter and emits the trigger signal when the tapped voltage exceeds a specified threshold. Exceeding the threshold is directly related to the emission of a light pulse by the light source. This reduces the influence of latencies on the measurement of the light propagation time. These latencies occur between the emission of an activation signal by the control and evaluation unit to the light transmitter and the emission of a current or voltage pulse to the light source, and thus the emission of the light pulse by the light source.

[0011] The light transmitter can include a light source driver for powering the light source. This allows the voltage and current required by the light source to be adjusted to the light source and protects the light source from damage. The light source can preferably be a laser diode or an LED.

[0012] The trigger circuit can incorporate a voltage divider to tap the voltage applied to the light source of the light transmitter. By appropriately dimensioning the voltage divider, the load on the electronics of the light source driver and / or the light source can be reduced, and a suitable portion of the tapped voltage can be provided for subsequent evaluation electronics of the trigger circuit.

[0013] The voltage tap can preferably be made at a cathode of the light source. This has the advantage that the tapped voltage essentially represents the current through the light source. This ensures that all latencies of the components involved in providing the current to the light source, especially their thermal dependence, can be taken into account.

[0014] In one embodiment, the evaluation electronics of the trigger circuit for further processing the tapped voltage or the voltage provided by the voltage divider can include a comparator that provides an output signal as a trigger signal for the timing unit. The comparator is preferably a comparator with differential outputs, which allows for a short processing time and fast rise and fall times of the output signal. A signal converter can be connected downstream of the comparator to convert a differential output signal from the comparator into a unipolar output signal.

[0015] In a preferred embodiment, the trigger circuit can comprise a monostable flip-flop (monoflop) for further processing the tapped voltage or the voltage provided by the voltage divider, wherein the voltage tapped at the light source or the voltage provided by the voltage divider serves as an input signal for the monostable flip-flop and the trigger signal for the time measuring unit is generated on the basis of an output signal of the monostable flip-flop

[0016] The monostable multivibrator comprises a first and a second transistor. The voltage tapped at the light source or the voltage provided by the voltage divider can be applied as the input signal of the monostable multivibrator, preferably to the base of the first transistor. The collector voltage of the first transistor forms the output signal of the monostable multivibrator as a trigger signal for the time measurement unit. The first transistor of the multivibrator can be switched from the conducting to the off state by the input signal without DC voltage. This switches the second transistor on and enables the setting of a holding time of the monostable multivibrator, i.e., the output signal of the monostable multivibrator.

[0017] The collector-base path of the first transistor can have a series circuit comprising a first diode and a damping resistor. This prevents saturation of the first transistor in the conducting state, enabling rapid blocking of the first transistor. Furthermore, the first diode provides voltage negative feedback, which counteracts the temperature variation of the first transistor. This leads to operating point stabilization of the first transistor.

[0018] The base-emitter junction of the first transistor may include a second diode. This can protect the base-emitter junction of the first transistor from damaging negative voltages.

[0019] The collector voltage of the first transistor can preferably be AC-coupled in such a way that a residual voltage present between the collector and emitter due to the lack of saturation of the first transistor can be compensated in the idle state of the monostable multivibrator, i.e., when the voltage applied to the light source of the light transmitter does not exceed a predetermined threshold. Thus, the output signal of the monostable multivibrator has a voltage of 0 volts in the idle state of the monostable multivibrator.

[0020] The invention will be explained in detail below using an exemplary embodiment with reference to the drawing. The drawing shows: Figure 1 shows a schematic representation of an optoelectronic sensor according to the invention; Figure 2 shows a schematic embodiment of a trigger circuit of an optoelectronic sensor according to the invention; Figure 3 shows an exemplary signal curve of a voltage tapped at a light source of the sensor and of a trigger signal.

[0021] Figure 1shows a schematic representation of an optoelectronic sensor 10 according to the invention. The sensor 10 has a light transmitter 12, which comprises a light source driver 14 and a light source 16, for example a laser diode or a surface emitter (vertical-cavity surface-emitting laser (VCSEL)). The light source 16 emits a measuring light beam 18 with at least one light pulse 20. A collimating optic 22 for collimating the measuring light beam 18 is arranged downstream of the light source 16 in the light beam direction. The collimating optic 22 is shown here purely as an example as a biconvex lens, but can have a more complex structure, for example, be designed as an objective having multiple lenses.

[0022] The at least one light pulse 28 reflected or remitted by an object 24 in the monitoring area 26 is guided as a received light beam 30 via an (optional) optical filter 32 for suppressing stray light and a receiving optics 34 to a light receiver 36, which generates received signals 40 upon reception of the reflected or remitted light pulses 28.

[0023] The light receiver 36 is preferably designed as a photodiode, APD (Avalanche Photo Diode), or SPAD (Single-Photon Avalanche Diode), or SPAD Matrix (SPAD Array).

[0024] The sensor 10 also includes a control and evaluation unit 48, which is connected to the light transmitter 12 and the light receiver 36. The control and evaluation unit 48 comprises a light transmitter controller 50, a light receiver controller 52, a time measurement unit 54, for example, a time-to-digital converter (TDC), and an object distance estimation unit 56. These are initially only functional blocks that can also be implemented in the same hardware or in other functional units such as the light transmitter controller 50, the light receiver controller 52, or the light receiver 46. Via an interface 58, the control and evaluation unit 48 can output measurement data or, conversely, receive control and parameterization instructions.The control and evaluation unit 48 can also be arranged in the form of local evaluation structures on a chip of the light receiver 46, or interact as a partial implementation with the functions of a central evaluation unit (not shown).

[0025] The sensor 10 further comprises a trigger circuit 60. The trigger circuit 60 is also shown as a functional block and can be implemented as separate hardware or in other functional units of the sensor 10. The trigger circuit 60 taps a voltage present at the light source 16 of the light transmitter 12 and outputs a trigger signal 62 to the timing unit 54 when an amount of the tapped voltage exceeds a predetermined threshold value, wherein exceeding the threshold value represents the emission of a light pulse 20 by the light source 16. The trigger signal 62 starts a light propagation time measurement of the timing unit 54. The emitted light pulse 20 is reflected or remitted by the object 24 in the monitored area 26, wherein the light receiver 36 generates a received signal 40 upon detection of the reflected or remitted light pulse 28.The received signal 40 is transmitted to the timing unit 54, which stops the time-of-flight measurement, determines a time interval between the trigger signal 62 and the received signal 40, and uses this to determine the time-of-flight of the light pulse. The time-of-flight is transmitted to the object distance estimation unit 54, which determines the distance between the object 24 and the sensor 10 based on the time-of-flight.

[0026] Figure 2shows a schematic embodiment of a trigger circuit 60 of the sensor 10 according to the invention. At the circuit input 61 of the trigger circuit 60, a voltage UL applied to the light source 16 of the sensor 10 is tapped by a voltage divider 64. By appropriately dimensioning the voltage divider 64, any effect of the voltage measurement on the current flowing through the light source 16 is minimized. The voltage divider 64 has a first resistor R1 and a second resistor R2, the resistance ratio of which is selected such that the voltage applied to the resistor R2 lies within a permissible range for a monostable multivibrator 66 connected downstream of the voltage divider 64. The voltage applied to the resistor R2 serves as an input signal for the monostable multivibrator 66 and is present there as the base voltage UB at a first transistor 68.This input signal switches the first transistor 68 from a conducting state to a non-conductive state without DC voltage, causing a second transistor 70 to become conductive by blocking the first transistor 68. The collector voltage UK applied to the first transistor 68 forms the output signal of the monostable multivibrator 66 and serves as the basis for the trigger signal 62 for the timing unit 54.

[0027] The base-collector path 72 of the first transistor 68 has a first diode 74 in series with a damping resistor 76. This prevents saturation of the first transistor 68 if the magnitude of the voltage UL applied to the light source 16 of the sensor 10 does not exceed a predetermined threshold, thus enabling rapid blocking of the first transistor 68. Furthermore, the first diode 74 provides voltage negative feedback, which counteracts the temperature response of the first transistor 68 and contributes to the operating point stabilization of the first transistor 68. The temperature response of the second transistor 70 is negligible due to its saturation operation. The hold time and amplitude of the output signal of the monostable multivibrator 66 can be adjusted within the scope of conventional techniques. The base-emitter path of the first transistor 68 is protected from harmful negative voltages by a second diode 78.

[0028] To prevent saturation of the first transistor 68, the output signal of the monostable multivibrator 66 cannot become 0 volts in the idle state of the monostable multivibrator 66, i.e., when the voltage UL applied to the light source 16 does not exceed a predetermined threshold, but rather has a residual voltage. Therefore, an AC coupling 80 is connected downstream of the output of the monostable multivibrator 66 so that the trigger signal 62 at the circuit output 82 of the trigger circuit 60 has a voltage UT of 0 volts in the idle state. To achieve a sufficient voltage amplitude of the trigger signal 62, the collector branch of the first transistor must be supplied with a higher voltage than the signal amplitude required by the downstream electronics, in particular the time measurement unit 54.A minimum operating voltage of the collector branch of the first transistor 68 results from the collector-emitter voltage of the first transistor 68 in the idle state and the minimum voltage required by the subsequent electronics, so that the collector branch of the first transistor 66 may have to be supplied with a higher voltage than required by the subsequent electronics.

[0029] Figure 3shows an example of a time course of the voltage UL tapped at the light source 16 of the sensor (represented by a dash-dotted line) and the trigger signal 62 generated by the trigger circuit 60 (represented by a solid line) at the output of the trigger circuit 60. If the magnitude of the voltage UL applied to the light source 16 exceeds a predetermined threshold value 92, the first transistor 68 of the monostable multivibrator 60 blocks. The voltage signal at the input of the monostable multivibrator 60 has a time length of less than one nanosecond. The voltage UK applied to the collector of the first transistor 68 forms the basis for the trigger signal 62 output by the trigger circuit 60. The leading edge of the trigger signal 62 essentially corresponds in time, i.e. within the switching speed of the monostable multivibrator 60, to the falling edge of the voltage UL tapped at the light source 16 of the sensor 10.The hold time t H of the trigger signal 62 significantly exceeds the duration of the voltage signal at the input of the monostable multivibrator 60, in accordance with the requirements of the timing unit 54. The voltage rise of the trigger signal 62 is strictly monotonic, and its amplitude exceeds a detection threshold of the timing unit 54 for the entire hold time t H of the trigger signal 62.

Claims

1. Optoelectronic sensor (10) for detecting and determining the distance of an object (24) in a monitoring area (26), having a light emitter (12) which has a light source (16) and is intended for emitting at least one light pulse (20), having a light receiver (36) for generating a received signal (40) from the light pulse (28) which is remitted or reflected by the object (24), and having a control and evaluation unit (48) which is designed to control the light emitter (12) and to determine a time-of-flight of the light and, from this, to determine a distance of the object (24) from the sensor (10), the control and evaluation unit (48) having a timing unit (54) which is set up to receive the received signal (40) and a trigger signal (62) and to determine the time-of-flight of the light from a time interval between the trigger signal (62) and the receive signal (40), characterized in that the sensor (10) has a trigger circuit (60) which is set up to pick up a voltage (UL) applied to the light source (16) of the light emitter (12) and to output the trigger signal (62) when a magnitude of the picked-up voltage (UL) exceeds a predetermined threshold value (92) .

2. Sensor (10) according to claim 1, wherein the light emitter (12) comprises a light source driver (14) for supplying power to the light source (16).

3. Sensor (10) according to any one of the preceding claims, wherein the trigger circuit comprises a voltage divider (64) for picking up the voltage (UL) applied to the light source (16) of the light emitter (12).

4. Sensor (10) according to any one of the preceding claims, wherein the voltage (UL) applied to the light source (16) of the light emitter (12) is picked up at a cathode of the light source (16).

5. Sensor (10) according to any one of the preceding claims, wherein the trigger circuit (60) comprises a comparator for generating the trigger signal (62).

6. Sensor (10) according to any one of the claims 1 to 4, wherein the trigger circuit (60) comprises a monostable flip-flop (66) for generating the trigger signal (62).

7. Sensor (10) according to claim 6, wherein the monostable flip-flop (66) is arranged to transfer a first transistor (68) of the monostable flip-flop (66) from a conductive state to a blocking state when the magnitude of the voltage (UL) applied to the light source (16) of the light emitter (12) exceeds a predetermined threshold.

8. Sensor (10) according to claim 7, wherein the trigger circuit (60) is arranged to generate the trigger signal (62) based on a collector voltage (UK) of the first transistor (68).

9. Sensor (10) according to claim 7 or 8, wherein a base-collector path (72) of the first transistor (68) comprises a series connection with a first diode (74) and a damping resistor (76) for preventing saturation of the first transistor (68).

10. Sensor (10) according to any one of the claims 7 to 9, wherein a base-emitter path of the first transistor (68) comprises a second diode (78) for protecting the base-emitter path from negative voltages.

11. Sensor (10) according to any one of the claims 7 to 10, wherein an AC coupling (80) is arranged downstream of the monostable flip-flop (66).

12. Trigger circuit (60) in an optoelectronic sensor (10) according to claim 1 for generating a trigger signal (62) for starting a time-of-flight measurement, comprising - a circuit input (61) for picking up a voltage (UL) applied to the light source (16) of the sensor (10), - a monostable flip-flop (66), the monostable flip-flop (66) being arranged to generate the trigger signal (62) on the basis of the voltage (UL) applied to the light source (16) of the sensor (10), - a circuit output (82) for providing the trigger signal (62).

13. Trigger circuit (60) according to claim 12, wherein the trigger circuit (60) comprises a voltage divider (64).

14. Trigger circuit (60) according to claim 12 or 13, wherein the monostable flip-flop (66) is arranged to transfer a first transistor (68) of the mono stable flip-flop (66) from a conducting state to a blocking state when a magnitude of the voltage (UL) applied to the light source (16) of the sensor (10) exceeds a predetermined threshold.

15. Trigger circuit (60) according to claim 14, wherein a base-collector path (72) of the first transistor (68) comprises a series connection with a first diode (74) and a damping resistor (76) for preventing saturation of the first transistor (68) in the conducting state.