Method for measuring a distance to an object using a projector device and projector apparatus

Abstract

A method for measuring a distance to an object (110) using a projector device (100) comprises emitting light (104) towards an optical element (106), wherein first partial rays (208) of the light (104) are reflected at the optical element (106) and second partial rays (212) of the light (104) are transmitted through the optical element (106) to the object (110) and reflected at the object (110). An interference pattern resulting from the light (104), the first partial rays (208) and the second partial rays (212) is detected, and a first intensity value, corresponding to the first partial rays (208) and a second intensity value, corresponding to the second partial rays (212), are determined from the interference pattern. A distance to the object (114) is determined using a ratio between the first intensity value, the second intensity value and a predetermined reference path of the first partial beams (208).

Description

State of the art The invention relates to a method for measuring a distance to an object using a projector device and to a projector device according to the preamble of the independent claims. Laser feedback interferometry can be used in many different areas, for example for data glasses, but also in conjunction with optical systems, for example in the automotive industry. Disclosure of the invention Against this background, the approach presented here introduces an improved method for measuring the distance to an object using a projector device and an improved projector device according to the main claims. The measures listed in the dependent claims enable advantageous further developments and improvements of the device specified in the independent claim. The presented approach advantageously allows for a very accurate distance measurement using a laser interferometer that is known per se. A method for measuring the distance to an object using a projector device comprising a laser interferometer with a light source, a light sensor, and an optical element is presented. The method includes an output step, a detection step, a sensing step, and a determination step. In the output step, light is emitted from the light source toward the optical element, with first partial beams of the light being reflected by the optical element and second partial beams being transmitted through the optical element to the object and reflected by the object. In the detection step, an interference pattern resulting from the light, the first partial beams, and the second partial beams is detected using the light sensor.In the detection step, a first intensity value, corresponding to the first partial beams, and a second intensity value, corresponding to the second partial beams, are determined from the interference pattern. In the determination step, a distance to the object is determined using a ratio between the first and second intensity values ​​and a predetermined reference path of the first partial beams. This method allows for high measurement accuracy in distance measurement. The laser interferometer can also be referred to as an LFI laser. This means that the light source can be a laser. The optical element can be a lens, for example. Additional optical elements can also be optionally integrated into the laser interferometer. The evaluation unit can be a photodiode, for example, which can measure the light intensity of the received light. The light can then be partially reflected by the optical element and reflected back as the first partial beams, interfering with the light emitted by the light source.A further portion of the light, not reflected by the optical element, can be reflected by the object whose distance is to be measured and reflected back as second partial beams, interfering with the light emitted by the light source. The distance between the laser interferometer and the object is not known beforehand, but the distance between the laser interferometer and the optical element is. Therefore, the distance to the object can be determined by appropriately evaluating the interference pattern resulting from the different reflections. The reference path of the first partial beams can be understood as a spatial reference distance between the laser interferometer and the optical element, or as the length of a light path that the first partial beams travel, for example, between the light source and the light sensor or between the laser interferometer and the optical element.Thus, the approach described here can utilize the different travel times of the light components reflected by the optical element and the object to measure the distance to the object very precisely. These different travel times can be represented in the interference pattern. Therefore, the first interference value can be assigned to the travel time of the light component reflected by the optical element, and the second interference value to the travel time of the light component reflected by the object. The presented approach advantageously enables the creation of a cost-effective and compact laser interferometer for distance measurement. Furthermore, components already used in laser interferometers can be adapted, allowing the number of components to remain the same as in a conventional laser interferometer. This means, for example, that monitoring a wavelength sweep can be performed without additional system components or with easily integrated components. According to one embodiment, the first and second intensity values ​​can be determined from a transformation of the interference pattern during the determination step. This allows the use of established evaluation methods. According to one embodiment, the distance to the object can be determined in the step of determining the distance, taking into account at least one temperature parameter of the light source. Advantageously, inaccuracies in the result, which may be due, for example, to thermal changes in the light source, can be reduced. Advantageously, this can compensate for nonlinear behavior of the wavelength modulation sweep, which may be due, for example, to electronic and thermal effects of the laser diode. In the determination step, the distance to the object can be calculated as the product of the reference path of the first partial beams and the quotient of the first intensity value and the second intensity value. Thus, a simple rule of three can be used to determine the distance. In the determination step, the first and second intensity values ​​can be determined as frequency values ​​of a Fourier transform of the interference pattern. Advantageously, the interference pattern can exhibit two peaks, with a first peak corresponding to the first intensity value and a second peak corresponding to the second intensity value. The peaks can, for example, correspond to the wavelengths of the partial beams. The peaks can be easily identified and used for evaluation. According to one embodiment, in the output step, the first partial beams can be guided indirectly from the optical element to the light sensor using at least one further reflective element and, additionally or alternatively, an optical fiber. The further reflective element can, for example, be a mirror arranged in the beam path of the first partial beams downstream of the optical element. The optical fiber can, for example, be arranged on the optical element, and the first partial beams can be coupled into the optical fiber upon striking the optical element. Advantageously, the transit time of the portion of the light reflected at the optical element can be extended so that it can advantageously approximate the transit time of the portion of the light reflected at the object. Alternatively, in the output step, the first partial beams can be guided directly from the optical element to the light sensor. This allows for a very simple design. Furthermore, a projector device for measuring a distance to an object is presented, wherein the projector device comprises a laser interferometer with a light source and a light sensor. The light source is configured to emit light towards an optical element of the projector device. The optical element is configured to reflect first partial beams of the light and transmit second partial beams to the object, the second partial beams being reflected by the object. The light sensor is configured to detect an interference pattern resulting from the light, the first partial beams, and the second partial beams. The projector device also includes an evaluation unit configured to determine, from the interference pattern, a first intensity value corresponding to the first partial beams and a second intensity value corresponding to the second partial beams.The evaluation unit is designed to determine the distance to the object using a ratio between the first intensity value and the second intensity value and a predetermined reference path of the first partial beams. Advantageously, the light source can be a laser light source. The optical element can, for example, be a collimating lens, allowing the light to be reflected at a surface facing the light source. A surface of the optical element facing away from the light source can, for example, have a different shape. According to one embodiment, the optical element can be configured as a collimating lens, a micro-optical element (MOE), or an optical window. An optical window can, for example, comprise plane-parallel plates, which can be round or rectangular and are typically used as protective windows. These optical windows can advantageously be very stable and do not produce magnification or reduction. Advantageously, maximum transmission can be achieved within a specific wavelength range, while reflection and absorption can be reduced. They are often used to protect optical systems and electronic sensors from the environment. Exemplary embodiments of the approach presented here are shown in the drawings and explained in more detail in the following description. Figure 1 shows a schematic representation of an exemplary embodiment of a projector device; Figure 2 shows a schematic representation of an exemplary embodiment of a projector device; Figure 3 shows a schematic representation of an exemplary embodiment of an interference pattern; Figure 4 shows a schematic representation of an exemplary embodiment of a projector device; Figure 5 shows a schematic representation of an exemplary embodiment of a projector device; Figure 6 shows a schematic representation of an exemplary embodiment of a projector device; and Figure 7 shows a flowchart of an exemplary embodiment of a method for measuring the distance to an object. In the following description of favorable embodiments of the present invention, the same or similar reference numerals are used for the elements shown in the various figures and acting similarly, without repeating these elements. Fig. 1 shows a schematic representation of a projector device 100. The projector device 100 comprises a laser interferometer 101 with a laser light source 102 for emitting light 104. Furthermore, the projector device 100 comprises an optical element 106, which is shaped to reflect a portion of the light 104, as illustrated below, for example, with reference to Fig. 2, and to transmit a portion of the light 104 to an object 110. Using the portions of the light 104 reflected by the optical element 106 and by the object 110, a distance to the object 110, for example, between a reference point of the laser interferometer 101 and the object, can be determined. For this purpose, a light sensor 108 of the laser interferometer 101 is used, which is configured to detect an interference pattern resulting from the light 104 and the reflected components of the light 104. The light sensor 108 is, for example, designed as a photodiode and is optionally integrated or integrable into the light source 102. Optical element 106, for example, is designed as a lens. The projector device 100 further comprises an evaluation unit 112 configured to determine intensity values ​​from the interference pattern that can be assigned to the reflected components of the light 104. The evaluation unit 112 is configured to determine the distance to the object 110 using a ratio between the intensity values ​​and a predetermined reference path of the component of the light 104 reflected at the optical element 106. In a simple case, the reference path corresponds to a distance between the reference point of the laser interferometer 101 and the optical element 106. The evaluation unit 112 can be designed as part of the laser interferometer 101 or as an external unit. Advantageously, the approach described here can exploit the fact that LFI (Laser Feedback Interferometry) sensors can measure the distance d and velocity v of objects, in this example object 110, based on the light backscattered from the object's surface. This is possible because the returning photons alter the electric field in the laser resonator of the laser interferometer 101, thereby increasing its optical gain and decreasing the laser threshold voltage. For example, the distance to object 110 is determined using an FMCW lidar (frequency-modulated continuous-wave lidar) by sweeping the emission wavelength of the laser from light source 102 and observing the photodiode current. For a small linear sweep, λ(t) = λ0 + αt, where λ0 is the unmodulated emission wavelength and α is the modulation bias, for example in nm / s, the resulting photodiode current and the distance can be determined if the sweep frequency of photodiode 108 is known. To achieve good distance accuracy, sound knowledge of λ0 and α is therefore required, which may be unknown or subject to shift due to aging or thermal effects. By additionally using the reflection at the optical element 106 for the distance condition, the distance to the object 110 can be determined very accurately. Fig. 2 shows a schematic representation of an embodiment of a projector device 100, which is similar to the projector device described in Fig. 1. The projector device 100 also has the light source 102, which is configured to emit the light 104 in the direction of the optical element 106. The optical element 106 is configured to reflect first partial beams 208 of the light 104 at a surface 209 facing the light source 102. The optical element 206 is further configured to transmit second partial beams 212 to the object 110, where the second partial beams 212 are reflected. As in Fig. 1, the projector device 100 also includes the light sensor 108 of the laser interferometer 101 and the evaluation unit 112, as already described in Fig. 1. Consequently, according to this embodiment, the optical element 206 is configured to guide the second partial beams 212 through the optical element 106 to the light sensor 108 and thus also to the evaluation unit 112. The evaluation unit 210 is configured to determine, from the resulting interference pattern, a first intensity value, which corresponds to the first partial beams 208, and a second intensity value, which corresponds to the second partial beams 212. Furthermore, the evaluation unit 112 is configured to determine the distance to the object 110 using a ratio between the first intensity value and the second intensity value and a predetermined reference path of the first partial beams. By way of example, the distance to the object 110 corresponds to a distance dobj between the laser interferometer 101 and the object 110, and the reference path corresponds to a known distance dref between the laser interferometer 101 and the optical element 106. The first intensity value corresponds, for example, to the transit time of the portion of the light 104 reflected by the optical element 106, and thus to the optical element 106, and the second intensity value corresponds to the portion of the light 104 reflected by the object 110, and thus to the object 110. Such a correspondence can be represented in the interference pattern, as described in more detail in Fig. 3. The evaluation unit 210 is designed to determine the distance dobj to the object 110 using a ratio between the first intensity value and the second intensity value and the predetermined reference path dref to the optical element 206. According to one embodiment, the laser interferometer 101 optionally has at least one absorption area 214 on a surface facing the optical element 106. According to this embodiment, the optical element 106 is configured as a collimating lens, a MOE, or an optical window. Optical windows are, for example, plane-parallel plates that can be round or rectangular and are typically used as protective windows. Optical windows are, for example, transparent and allow maximum transmission within a specific wavelength range. The optical element 106 is arranged in a light path of the light 104 between the laser interferometer 101 and the object 110, so that the light 104 passes through the optical element 106 on its way to the object 110. In other words, the described approach eliminates an uncertainty in wavelength scanning by integrating a reference object at a known distance in the measurement path. The optical surface 209 reflects a portion of the incident light 104, described in Fig. 2 as the first partial rays 208, back to the light source 102. Fig. 3 shows a schematic representation of an embodiment of a reference pattern 300, which resulted from a transformation of the interference pattern already mentioned with reference to Fig. 2. According to this embodiment, the reference pattern 300 is represented in the form of a diagram. An x-axis 302 of the diagram represents a frequency f and a y-axis 304 represents the magnitude of an amplitude of an intensity i. The interference pattern 300 is also referred to as a distance map, which is created, for example, by a Fourier transform of the photodiode signal output, for example, by the light sensor shown in Fig. 2. The reference pattern 300 contains two peaks 306, 308, where the first peak 306 is assigned to the first intensity value and thus to the reference surface, and the second peak 308 to the second intensity value and thus to the object. The distance dobj to the object is therefore calculated. For the known reference path of the first partial beams, which is defined here by the reference distance dref and is known after assembling a sensor package of the projector device including the light source and the optical element, the parameters lambda0 and alpha, i.e., the modulation tendency or wavelength shift over time, can be estimated on-site based on measurements of aging and temperature. If the correct distance of the reference target, here the optical element, is known, the nonlinear behavior of the wavelength modulation sweep, which is due, for example, to electronic and thermal effects of the laser diode, can be compensated for. Fig. 4 shows a schematic representation of an embodiment of a projector device 100, which is at least similar to the projector device described in at least one of Figs. 1 to 2. According to this embodiment, however, the first partial beams 208 are not reflected directly back to the laser interferometer 101. Instead, the first partial beams 208 are first directed by the optical element 106 to a further reflective element 400. Thus, the first partial beams 208 are guided indirectly from the optical element 106 to the light sensor 108, which increases the transit time compared to the direct path shown in Fig. 2. According to one embodiment, the first partial beams 208 are directed at the optical element 106 to the further reflective element 400 and reflected there in the direction of the optical element 106. Upon returning to the optical element 206, the first partial beams 208 are guided back to an interference chamber of the laser interferometer 101 to generate the interference pattern that can be detected by the light sensor 108. Alternatively, the first partial beams 208 can be returned directly to the laser interferometer 101 by the further reflection element 400, or can be returned to the laser interferometer 101 using at least a third reflection element. This is advantageous, for example, if the reference distance dref is to be increased compared to the direct spatial reference distance shown in Fig. 2. Fig. 5 shows a schematic representation of an embodiment of a projector device 100, which is at least similar to the projector device described or mentioned in at least one of Figs. 1, 2, 3 to 4. According to this embodiment, the optical element 106 has an integrated light guide 500, by means of which the first partial beams 208 are indirectly guided from the optical element 106 back to the interference chamber of the laser interferometer 101. According to this embodiment, the projector device 100 is shown in a side view. According to different embodiments, the first partial beams 208 are coupled into the optical element 106 in the light guide 500 and, after passing through the light guide, are coupled out again at the optical element 105 and then returned directly or using at least one further reflection element to the laser interferometer 101. Alternatively, the first partial beams 208 are coupled out of the optical element 500 directly towards the interference region of the laser interferometer 101. Fig. 6 shows a schematic representation of an embodiment of a projector device 100, which corresponds, for example, to the projector device described in Fig. 5. According to this embodiment, the projector device 100 is shown only from the front. Fig. 7 shows a flowchart of an embodiment of a method 700 for measuring a distance to an object using a projector device such as described or at least mentioned in at least one of Figs. 1, 2, 3, 4, 5 to 6. The method 700 comprises a step 702 of output, a step 704 of detection, a step 706 of determination, and a step 708 of determination. In step 702 of output, light is output using the light source in the direction of the optical element, wherein first partial rays of the light are reflected at the optical element and second partial rays of the light are transmitted through the optical element to the object. The second partial rays are reflected at the object. In step 704 of detection, an interference pattern resulting from the light, the first partial rays, and the second partial rays is detected using the light sensor.In step 706 of the detection process, a first intensity value, corresponding to the first partial beams, and a second intensity value, corresponding to the second partial beams, are determined from the interference pattern. In step 708 of the determination process, a distance to the object is determined using a ratio between the first and second intensity values ​​and a predetermined reference path of the first partial beams. For example, in step 702 of the output process, the light is emitted, with the first partial beams being reflected by the optical element and guided to the evaluation unit. For example, in step 702 of the output process, the first partial beams are guided directly from the optical element to the light sensor. Alternatively, in step 702 of the output process, the first partial beams are guided indirectly from the optical element to the light sensor using at least one additional reflective element and / or a light guide. Optionally, in step 706 of the determination process, the first intensity value and the second intensity value are determined from a transformation of the interference pattern, for example as frequency values ​​of a Fourier transformation of the interference pattern. Optionally, in step 708 of the determination process, the distance to the object is additionally determined taking into account at least one temperature parameter of the light source. The distance to the object corresponds, for example, to the product of the reference path of the first partial beams and a quotient of the first intensity value and the second intensity value.

Claims

Method (700) for measuring a distance to an object (110) using a projector device (100) comprising a laser interferometer (101) with a light source (102) and with a light sensor (108) and an optical element (106), wherein the method (700) comprises the following steps: Emitting (702) light (104) using the light source (102) in the direction of the optical element (106), wherein first partial beams (208) of the light (104) are reflected at the optical element (106) and wherein second partial beams (212) of the light (104) are transmitted through the optical element (106) to the object (110) and reflected at the object (110); Detecting (704) an interference pattern (300) resulting from the light (104), the first partial beams (208) and the second partial beams (212) using the Light sensor (108);Determine (706) a first intensity value, which is assigned to the first partial beams (208) and a second intensity value, which is assigned to the second partial beams (212), from the interference pattern (300); and determine (708) a distance to the object (114) using a ratio between the first intensity value and the second intensity value and a predetermined reference path of the first partial beams (208). Method (700) according to claim 1, wherein in step (704) of determining the first intensity value and the second intensity value are determined from a transformation of the interference pattern (300). Method (700) according to one of the preceding claims, wherein in step (708) of determining the distance to the object (110) is additionally determined taking into account at least one temperature parameter of the light source (102). Method (700) according to one of the preceding claims, wherein in step (708) of determining the distance to the object (110) is determined as a product of the reference path of the first partial beams (208) and a quotient of the first intensity value and the second intensity value. Method (700) according to one of the preceding claims, wherein in step (706) of determining the first intensity value and the second intensity value are determined as frequency values ​​of a Fourier transform of the interference pattern (300). Method (700) according to one of the preceding claims, wherein in step (702) of output the first partial beams (208) are guided indirectly from the optical element (106) to the light sensor (108) using at least one further reflection element (400). Method (700) according to one of the preceding claims, wherein in step (702) of output the first partial beams (208) are guided indirectly from the optical element (106) to the light sensor (108) using a light guide (500). Method (700) according to one of claims 1 to 5, wherein in step (702) of output the first partial beams (208) are guided directly from the optical element (106) to the light sensor (108). Projector device (100) for measuring a distance to an object (110), wherein the projector device (100) has the following features: a light source (102) of a laser interferometer (101), wherein the light source (102) is configured to emit the light (104) in the direction of an optical element (106); the optical element (106), which is configured to reflect first partial beams (208) of the light (104) and to transmit second partial beams (212) to the object (110), wherein the second partial beams (212) are reflected at the object (106); a light sensor (108) of the laser interferometer (101), wherein the light sensor (108) is configured to detect an interference pattern (300) resulting from the light (104), the first partial beams (208) and the second partial beams (212);and an evaluation unit (112) configured to determine from the interference pattern (300) a first intensity value, which is assigned to the first partial beams (208) and a second intensity value, which is assigned to the second partial beams (212), and configured to determine the distance to the object (110) using a ratio between the first intensity value and the second intensity value and a predetermined reference path of the first partial beams (208). Projector device (100) according to claim 9, wherein the optical element (106) is configured as a collimating lens, as an MOE or as an optical window.

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