Optical sensor
By utilizing a closed control loop to stabilize the pulse width and amplitude of optical pulses in optical sensors, the challenges of temperature instability and external influences are addressed, resulting in high detection sensitivity and accuracy.
Patent Information
- Application Number
- JP2024180611
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2023-10-17
- Filing Date
- 2024-10-16
- Publication Date
- 2025-04-30
AI Technical Summary
Existing optical sensors struggle to maintain high detection sensitivity and accuracy due to temperature instability and external influences, which affect the generation of short optical pulses.
The optical sensor employs a closed control loop to stabilize the pulse width and/or pulse amplitude of the optical pulses emitted by the transmitter, using the light transmitting output as the actual amount for control, and accounting for temperature effects and disturbances.
This approach ensures high detection sensitivity and reproducibility, while maintaining temperature stability and resistance to external interference, thereby enhancing the overall performance of the optical sensor.
Smart Images
Figure 2025069101000001_ABST
Abstract
Description
[Technical field]
[0001] The present invention relates to optical sensors.
[0002] Such optical sensors are typically used to detect objects in a detection area.
[0003] Such optical sensors generally include at least one transmitter for emitting a light beam and at least one receiver for receiving the light beam as sensor components. The transmitter and receiver are preferably integrated in one housing. The light beam emitted by the transmitter is reflected back from the object to the receiver. The optical sensor further includes an evaluation unit, which generates an object detection signal depending on the received signal of the receiver.
[0004] Such an optical sensor can in particular be configured as a distance sensor in which the distance measurement is performed according to the pulse transit time method.
[0005] For this purpose, a light beam in the form of light pulses is emitted by the light transmitter, which here is formed by a laser diode emitting laser light, and in order to determine the distance of the object, the light propagation time of the light pulse from the light transmitter to the object and the light pulse returning to the light receiver is determined.
[0006] To achieve high measurement accuracy, it is desirable for very short light pulses to be emitted by the light transmitter, where the pulse width, i.e. the duration of the light pulse, is preferably in the range of a few hundred picoseconds.
[0007] Such short light pulses provide particularly steep rising edges of the light pulses, which is advantageous for reducing black-white errors in object detection. Depending on the reflectivity of the object, the light pulses reflected back from it will not only have different amplitudes but also different signal shapes of the light pulses, which can lead to erroneous distance measurements. These error sources are reduced by using light pulses that are as short as possible.
[0008] Another aspect of such laser-based optical sensors is the issue of eye safety, where standard eye safety regulations dictate the maximum intensity and duration of laser light that may strike the human eye.
[0009] Here again, it is advantageous for the light emitter of the optical sensor to emit the shortest possible light pulse. The smaller the pulse amplitude of the light pulse, the greater the intensity, i.e. the pulse amplitude, of the light pulse can be without violating the standard requirements for eye safety. The greater the pulse amplitude of the light pulse, the greater the detection sensitivity of the object detection.
[0010] To generate shorter light pulses, it is known to use a laser diode as the light transmitter, which is associated with a laser driver that generates short drive control pulses.
[0011] The fundamental drawback here is that these components for generating the light pulses are not temperature stable, which means that the generation of short light pulses is not reproducible for optical sensors exposed to the effects of changing temperatures.
[0012] The object of the invention is to provide an optical sensor of the type mentioned at the outset, which has a high detection sensitivity without being subject to external influences.
[0013] To achieve this goal, the features of claim 1 are specified. Advantageous embodiments and expedient developments of the invention are set forth in the dependent claims.
[0014] The invention relates to an optical sensor with a transmitter for emitting light pulses, a receiver for receiving the light pulses and an evaluation unit, in which an object detection signal is generated as a function of the received signal of the receiver. The evaluation unit has a closed control loop in which the pulse width and / or pulse amplitude of the light pulses emitted by the transmitter are closed-loop controlled. The light transmission power of the transmitter forms the actual variable of the closed control loop, whereby the drive control pulse width and / or drive control current for the transmitter form the manipulated variable of the closed control loop.
[0015] The optical sensor according to the invention allows for high detection sensitivity during object detection, which is reproducible and independent of external influences.
[0016] This is achieved in that the transmitter emits light pulses whose characteristic quantities are preset and stabilized by a closed control loop.
[0017] These characteristics include the pulse width, ie the pulse duration, of the light pulse as well as its pulse amplitude.
[0018] In the simplest case, only the pulse width of the light pulses can be closed-loop controlled, whereby light pulses with a defined pulse width are achieved by closed-loop control.
[0019] The pulse width is advantageously in the picosecond range, in particular about 100 picoseconds.
[0020] A high detection sensitivity is ensured by the closed-loop controlled and thus stable generation of light pulses with a defined pulse width.
[0021] In principle, it is also possible to adjust only the pulse amplitude.
[0022] Particularly preferably, both the pulse width and the pulse amplitude of the light pulses are controlled in a closed control loop, so that the light emitter generates light pulses with defined pulse widths and pulse amplitudes.
[0023] The manipulated variables of the closed control loop are the drive control pulse width and the drive control current for the transmitter, which are preferably generated by a suitable drive control circuit.
[0024] According to the invention, the light output of the light transmitter forms an actual variable of a closed control loop, which actual variable is continuously detected by suitable measuring means.
[0025] The light output is a real quantity that can be measured simply, quickly and accurately, which is a great advantage over measuring the pulse width and pulse amplitude directly as real quantities, since their measurement would imply high technical costs, which in turn would lead to undesirably high manufacturing costs for the optical sensor.
[0026] In order to be able to use the light output of the light transmitter as an actual quantity, the dependence of the pulse width and pulse amplitude of the generated light pulses on the light output is known in the optical sensor, i.e. is stored in the form of a characteristic curve, etc. Here, it is also possible to store a temperature-dependent characteristic curve in the optical sensor in order to take into account temperature influences when generating the light pulses.
[0027] It is particularly advantageous if, in the preadjustment of the light emitter to a setpoint light pulse, the light emission power is measured and stored in the evaluation unit as a reference variable for a closed control loop.
[0028] The reference amount represents a target value to which the light transmission power should be adjusted.
[0029] To carry out the pre-adjustment, an optical detector and a sampling oscilloscope with a wide bandwidth can be used, which signals the light pulse of the light transmitter. Here, the light pulse is set by the driving control pulse width and the driving control current of the light transmitter, which form the manipulated variable. After setting, the light output of the light transmitter is determined and stored in the evaluation unit of the optical sensor as a reference variable for the closed control loop, i.e. as a target value.
[0030] According to an advantageous embodiment, a further actual value is formed by the temperature at the optical sensor.
[0031] For this purpose, a temperature sensor is preferably provided, by means of which the temperature in the interior space of the optical sensor is determined.
[0032] By taking into account the temperature at the optical sensor as an actual quantity, temperature influences can be detected in the control process for setting the pulse width and / or pulse amplitude, thereby ensuring a temperature-stable closed-loop control of these characteristic quantities.
[0033] Alternatively or additionally, the temperature dependence of the manipulated variables can be stored in a table in the evaluation unit.
[0034] The temperature dependence of the manipulated variable can be determined in an adjustment or learning process, which also results in a temperature-stable closed-loop control of the pulse width and / or pulse amplitude or contribution to the light pulse generated by the light transmitter.
[0035] According to an advantageous development, disturbance variables are taken into account in the closed control loop.
[0036] Disturbance quantities are external influence quantities, for example contamination of an optical sensor or the influence of external light.
[0037] By taking into account the disturbance quantities in a closed control loop, high interference immunity of the optical sensor is obtained.
[0038] According to one advantageous embodiment, the light transmitter is a laser diode.
[0039] The light transmitter is in particular a VCSEL (vertical cavity surface emitting laser) diode.
[0040] In this case, the laser power, in particular the average laser power of the laser diode, is formed by the actual quantity of the closed control loop of the optical sensor.
[0041] It is particularly advantageous for the laser diode to have a monitor diode which determines the emitted light power.
[0042] Alternatively, a separate or external diode may be provided to measure the light transmission power or laser power.
[0043] Advantageously, for a transmitter in the form of a laser diode, the drive control circuit is a laser driver.
[0044] According to an advantageous embodiment, the optical sensor is configured as a distance sensor for determining the distance of the object. In principle, the determined distance value can be output directly as the object detection signal. Alternatively, a threshold evaluation of the determined distance value can be performed, so that a switching signal indicating a switching state representing whether the object remains in the determined distance range or not can be output as the object detection signal.
[0045] The distance measurement is preferably performed according to the pulse transit time method.
[0046] According to another advantageous embodiment, the fluorescence measurement is carried out by means of an optical sensor, i.e. fluorescent objects are detected.
[0047] The light pulses emitted by the light transmitter have a particular wavelength. Reflection of the light pulses off the fluorescent object changes the wavelength. The light receiver of the optical sensor is configured to detect the light pulses having the changed wavelength.
[0048] A typical application area is the detection of fluorescent markings on objects, which can be identified by optical sensors, which in particular allows the objects to be marked in a tamper-proof manner, and also makes it possible to distinguish between fluorescent and non-fluorescent objects.
[0049] Optical sensors can generally be configured as safety sensors that can be used in safety technology applications. For this purpose, they have a fail-safe design. This can be realized by a redundant design of the evaluation unit, for example in the form of two calculation units that cyclically monitor each other.
[0050] The invention will be explained below on the basis of the drawings. [Brief description of the drawings]
[0051] [Figure 1] FIG. 2 shows an embodiment of an optical sensor according to the present invention. [Diagram 2] FIG. 2 shows a laser driver for the optical sensor according to FIG. [Diagram 3] FIG. 2 shows a closed control loop for the optical sensor according to FIG. [Figure 4] FIG. 2 shows the time course of the light pulse of the optical sensor for different temperatures.
[0052] FIG. 1 very diagrammatically shows an embodiment of an optical sensor 1 according to the invention.
[0053] The optical sensor 1 has a housing 2 in which all components of the optical sensor are integrated. The optical sensor 1 has a transmitter 4 which emits a light beam 3 and a receiver 5 which receives the light beam 3. For beam shaping of the light beam 3, transmitter 4 can be followed by a transmitter optic (not shown). For focusing the light beam 3 on the receiver 5, receiver 5 can be preceded by a receiver optic (also not shown).
[0054] A transparent windscreen 6 is incorporated in the housing 2 for directing the light beam 3 out of and into the housing 2 .
[0055] The optical sensor 1 furthermore has an evaluation unit 7 for evaluating the received signal of the optical receiver 5. As a function of this evaluation, an object detection signal is generated in the evaluation unit 7, which is output via an output 8.
[0056] The evaluation unit 7 is in the simplest case formed by a computing unit, for example a microprocessor, etc. If the optical sensor 1 is a safety sensor, the evaluation unit 7 has a redundant structure, for example in the form of two computing units which cyclically monitor each other.
[0057] Finally, a drive control circuit 9 is provided which is connected to the evaluation unit 7 and is used to control the drive of the light sender 4 .
[0058] To detect an object, the light beam 3 emitted by the transmitter 4 is directed through the windshield 6 to the detection area. An object 10 present there causes the light beam 3 to be directed back to the receiver 5.
[0059] A light beam 3 in the form of light pulses is emitted by a light transmitter 4 of the optical sensor. The light transmitter 4 is formed by a laser diode, in particular a VCSEL diode. The light receiver 5 may be formed by a photodiode, in particular an avalanche photodiode.
[0060] The optical sensor 1 can be designed, for example, to detect a fluorescent object 10. In the present embodiment, the optical sensor 1 is a distance sensor. The distance measurement is carried out according to the pulse transit time method. To determine the distance, the light transit time of the light pulse from the sensor to the object 10 and back to the receiver 5 is evaluated here. The determined distance value can be output directly as an object detection signal. Alternatively, a threshold evaluation in the evaluation unit 7 is used to determine whether the object 10 is present in the distance field, so that a binary switching signal is generated as the object detection signal.
[0061] The optical sensor 1 according to FIG. 1 emits a light beam 3 in a fixed, predefined spatial direction.
[0062] The optical sensor 1 can be designed as a scanner in general, and in particular as an area distance sensor. In this case, the light beam 3 is directed periodically onto a planar detection area. For this purpose, a deflection unit can be arranged downstream of the light transmitter 4. Alternatively, the light transmitter 4 and the light receiver 5 can be components of a rotating measuring head.
[0063] In accordance with the light transmitter 4 of the optical sensor being configured as a laser diode, the drive control circuit 9 is configured in the form of a laser driver, the structure of which is shown diagrammatically in FIG.
[0064] The laser driver comprises a controlled current source 11 at potential V and switching means 12 switchable to ground GND, which allows to generate a variable drive control pulse width for the light pulses to be generated, as shown diagrammatically in FIG. 2.
[0065] According to the invention, the evaluation unit 7 of the optical sensor has a closed control loop 13 (shown in FIG. 3), by means of which stabilized, in particular temperature stabilized, short light pulses in the picosecond range are generated.
[0066] The closed control loop 13 comprises, in a manner known per se, a controller 14, a control element 15 and a closed loop control system 16.
[0067] According to the invention, the light transmission power, i.e. the average laser power of the laser diode forming the light transmitter 4, is continuously detected as an actual variable of the closed control loop 13 by means of a sensor means 17. In this embodiment, the sensor means 17 is formed by a monitor diode of the laser diode. Generally, a separate diode can also be used for this purpose.
[0068] The actual value determined by the sensor means 17 is compared in a comparator 18 with a reference quantity supplied via an input line 19, ie a setpoint value for the closed-loop control.
[0069] Advantageously, in the pre-adjustment of the light transmitter 4 to a target light pulse, the light transmission power is measured and stored in the evaluation unit 7 as a reference quantity for the closed control loop 13 .
[0070] Furthermore, a further actual value can advantageously be formed via the temperature at the optical sensor.
[0071] Here, a temperature sensor is provided, by means of which the temperature in the inner space of the optical sensor is determined.
[0072] The comparator 18 delivers a control deviation as an output variable, which is fed to the controller 14. The controller 14 generates a control variable, which is fed to the control element 15.
[0073] In the control element 15, a control variable for the closed-loop control system 16 is generated. In this case, the control variable is a drive control pulse width and / or a drive control current for a laser driver, which causes a closed-loop controlled and stabilized light pulse to be emitted by the light emitter 4. In this case, the pulse width and / or the pulse amplitude can be closed-loop controlled during the closed-loop control.
[0074] The temperature dependence of the manipulated variables is preferably stored in a table in the evaluation unit 7 .
[0075] This allows the temperature dependence to be detected systematically during closed-loop control, thereby ensuring that the light pulses are generated in a temperature-stabilized manner.
[0076] In order to achieve a high interference immunity in the generation of the light pulses, disturbance quantities detected by measurement technology are fed via line 20 to the closed-loop control system 16 .
[0077] The result of the closed loop control according to the invention is shown in Fig. 4. Here, the time course of the light pulse emitted by the light transmitter 4 of the optical sensor is plotted with the closed loop control according to the invention. Here, the time course of the light pulse is plotted against temperatures from -40°C to 70°C. As can be seen from Fig. 4, a high temperature stability of the light pulse is achieved with the closed loop control according to the invention. [Explanation of symbols]
[0078] 1 Optical Sensor 2. Housing 3 Light Beam 4 Light transmitter 5 Receiver 6. Windshield 7 Evaluation Unit 8 Output section 9 Drive control circuit 10. Target 11 Current source 12 Switching means 13 Closing the Control Loop 14 Controller 15 Control Elements 16 Closed Loop Control Systems 17 Sensor means 18 Comparator 19 Input line 20 railroad tracks GND Earth V potential
Claims
1. 1. An optical sensor (1) comprising a transmitter (4) for emitting light pulses, a receiver (5) for receiving the light pulses, and an evaluation unit (7) for generating an object detection signal depending on a signal received by the receiver (5), characterized in that the evaluation unit (7) has a closed control loop (13) in which the pulse width and / or pulse amplitude of the light pulses emitted by the transmitter (4) are closed-loop controlled, the actual quantity of the closed control loop (13) being formed by the light transmission output of the transmitter (4), and a control pulse width and / or a control current for the transmitter (4) being formed as a manipulated variable of the closed control loop (13).
2. 2. The optical sensor (1) according to claim 1, characterized in that the light sending power is measured during pre-adjustment of the light sender (4) to a target light pulse and stored in the evaluation unit (7) as a reference quantity for the closed control loop (13).
3. 3. The optical sensor (1) according to claim 1 or 2, characterized in that a further actual quantity is formed as a function of the temperature at the optical sensor (1).
4. 4. The optical sensor (1) according to claim 1, characterized in that the temperature dependence of the manipulated variables is stored in a table in the evaluation unit (7).
5. 5. Optical sensor (1) according to claim 3 or 4, characterized in that a temperature sensor is provided, by means of which the temperature in the inner space of the optical sensor is determined.
6. 6. The optical sensor (1) according to claim 1, characterized in that a drive control circuit (9) is associated with the light emitter (4), and the manipulated variable is generated in the drive control circuit (9).
7. 7. The optical sensor (1) according to any one of the preceding claims, characterized in that the light sender (4) is a laser diode.
8. 8. The optical sensor (1) according to claim 7, characterized in that the light sender (4) is a vertical cavity surface emitting laser (VCSEL) diode.
9. 9. Optical sensor (1) according to claim 7 or 8, characterized in that the laser diode has a monitor diode determining the light transmission power or is provided with an external monitor diode determining the light transmission power of the laser diode.
10. Optical sensor (1) according to any one of claims 7 to 9, characterized in that the drive control circuit (9) is a laser driver.
11. The optical sensor (1) according to any one of the preceding claims, characterized in that the optical sensor (1) is a distance sensor.
12. 11. Optical sensor (1) according to any one of the preceding claims, characterized in that by means of the optical sensor (1) fluorescent objects (10) are detected.
13. 13. The optical sensor (1) according to claim 1, characterized in that disturbance quantities are taken into account in the closed control loop (13).
14. Optical sensor (1) according to any one of the preceding claims, characterized in that the pulse width of the light pulses is in the picosecond range.