Distance measuring device and method of measuring distance by using the same

Provided is a distance measuring device and a method of measuring a distance. The distance measuring device detects light reflected by an object, generates an electrical signal based on the detected light, detects whether the electrical signal is saturated or not by comparing the electrical signal with a reference value, controls a magnitude of the electrical signal based on whether the signal is saturated, and calculates a distance to the object using the electrical signal.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from Korean Patent Application No. 10-2017-0085926, filed on Jul. 6, 2017, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.

BACKGROUND

Apparatuses and methods consistent with exemplary embodiments relate to distance measuring devices for measuring a distance, and methods of measuring a distance by using the same.

2. Description of the Related Art

Recently, 3D cameras and light detection and ranging (LIDAR) techniques have been studied for measuring a distance to an object. One distance measuring method is a time of flight (TOF) method that measures the time required for light to travel between an object and a camera. Thus, the TOF method is used for measuring a distance between an image capturing device and an object, creating a depth image.

The TOF method includes processes of irradiating light of a specific wavelength, for example, a near-infrared ray (850 nm), onto an object, by using a light-emitting diode (LED) or a laser diode (LD); measuring or capturing an image of the light of the specific wavelength as reflected by the object by using a photodiode or a camera; and extracting a depth image from the measured or captured image. Various TOF methods have been developed utilizing optical processing, that is, a series of processes including light irradiation, reflection by the object, optical modulation, image capture, and processing. Discussions on methods of accurately measuring a distance to an object are ongoing.

SUMMARY

One or more exemplary embodiments may provide distance measuring devices configured to correctly measure a distance by using light, and methods of measuring the distance by using the distance measuring device.

According to an aspect of an exemplary embodiment, a distance measuring device includes: a light-receiver configured to detect light reflected by an object and output an electrical signal based on the detected light; a peak detector configured to detect a peak from the electrical signal; a saturation detector configured to detect whether the electrical signal is saturated or not by comparing the electrical signal with a reference value and to output a saturation detection result; and a processor configured to measure a distance to the object by using the peak and to control a magnitude of the electrical signal by using at least one of a peak detection result, the saturation detection result, and a measured distance to the object.

The processor may decrease the magnitude of the electrical signal when the magnitude of the electrical signal is greater than a reference value.

The light receiver may include a light detector that detects light while a bias voltage is applied to the light detector.

The processor may control the magnitude of the electrical signal by controlling the bias voltage.

The magnitude of the electrical signal may be proportional to the magnitude of the bias voltage.

The light detector may include an avalanche photodiode (APD) or a single-photon avalanche diode (SPAD).

The light receiver may further include an amplifier that amplifies an amplitude of the electrical signal.

The processor may control the magnitude of the electrical signal by controlling a gain of the amplifier.

The distance measuring device may further include a light source configured to irradiate light onto the object.

The processor may control the electrical signal by controlling a driving signal of the light source.

The magnitude of the electrical signal may be proportional to a magnitude of the driving signal.

The processor may increase the magnitude of the electrical signal when the peak is not detected by the peak detector for a certain period of time.

The certain period of time may be greater than an emission frequency of light of the light source.

The peak detector may detect the peak by using a constant fraction discriminator (CFD) method.

The processor may control the magnitude of the electrical signal in proportion to a calculated distance.

According to an aspect of an another exemplary embodiment, a method of calculating a distance, the method includes: detecting light reflected by an object and outputting an electrical signal based on the detected light; detecting whether the electrical signal is saturated or not by comparing the electrical signal with a reference value and outputting a saturation detection result; controlling a magnitude of the electrical signal by using the saturation detection result; and calculating a distance to the object using the electrical signal.

The controlling of the electrical signal may include decreasing the electrical signal when the magnitude of the electrical signal is greater than the reference value.

The calculating of the distance may include calculating the distance to the object by detecting a peak from the electrical signal and increasing the magnitude of the electrical signal when the peak is not detected for a certain period of time.

The magnitude of the electrical signal may be controlled by controlling a magnitude of a bias voltage applied to a light detector that detects the light.

The magnitude of the electrical signal may be controlled by controlling a magnitude of a driving signal of a light source that emits light.

DETAILED DESCRIPTION

Reference will now be made in detail to embodiments, examples of which are illustrated in the accompanying drawings. In the drawings, widths and thicknesses of layers or regions may be exaggerated or reduced for clarity and convenience of explanation. Like reference numerals refer to like elements throughout.

FIG. 1is a block diagram of a distance measuring device100according to an exemplary embodiment. Referring toFIG. 1, the distance measuring device100may include a light source110configured to emit light toward an object10, a light receiver120configured to detect the light reflected by the object10and convert the reflected light into an electrical signal, a peak detector130configured to detect a peak from the electrical signal, a saturation detector140configured to detect a saturation of the electrical signal by comparing the electrical signal with a reference value, and a processor150configured to calculate a distance to the object10by using the peak.

The light source110may be a light-emitting device. For example, the light source110may emit light having an infrared wavelength (hereinafter, infrared light). When a light source emitting infrared light is used, light from the light source may be distinguishable from natural, visible light such as sunlight. However, the light emitted from the light source110is not limited to emitting infrared light, and may alternately or additionally emit light in having any of various wavelengths. In this case, a correction may be used for removing information of mixed natural light. For example, the light source110may be a laser light source, but is not limited thereto. The light source110may be one of an edge emitting laser, a vertical-cavity surface emitting laser (VCSEL), and a distributed feedback laser. For example, the light source110may be a laser diode.

The light receiver120may convert light reflected or scattered by the object10to an electric signal, for example, a voltage. The light receiver120may include a light detector122configured to output an electrical signal, for example, a current corresponding to light, a current-voltage conversion circuit124that converts a current outputted from the light detector122into a voltage, and an amplifier126that amplifies the amplitude of the voltage. In addition to the above, the light receiver120may further include a lens that focuses light reflected by the object10and a filter, for example, a high-pass filter that filters an electrical signal of a specific frequency. The amplifier126may amplify the amplitude of the voltage, but the amplifier126is not limited thereto. The amplifier126may amplify the amplitude of the current when disposed between the light detector122and the current-voltage transformation circuit124, or may be implemented as a single circuit together with the current-voltage transformation circuit124.

The light detector122may be a light-receiving diode and may be operated in a state in which a bias voltage Vbias is applied thereto. For example, the light detector122may include an avalanche photo diode (APD) or a single photon avalanche diode (SPAD). The light detector122may be configured by a practical circuit in another way, for example, the light detector122may include an analog front end (AFE) or a time digital counter (TDC) depending on which one of the APD and the SPAD is included as a light-receiving diode in the light detector122. The configuration of the practical circuitry of the light detector122may be a well-known technique in the art, and thus, a detailed description thereof is omitted.

The peak detector130may detect a peak from an electrical signal received from the light receiver120. The peak detector130may detect a peak by detecting a central position of the electrical signal. Also, the peak detector130may detect a peak by analogically detecting a width of the electrical signal. The peak detector130may detect a peak by detecting a rising edge and a falling edge of a digital signal after converting the electrical signal to the digital signal. Also, after dividing an electrical signal into a plurality of signals and inverting and time-delaying some of the divided signals, the peak detector130may detect a peak by using a constant fraction discriminator (CFD) method in which a zero-crossing point is detected by combining the inverted and time-delayed signals and the remaining signals. A circuit that detects a peak by using the CFD may be referred to as a CFD circuit. The peak detector130may further include a comparator, and thus, may output the detected peak as a pulse signal.

The processor150may calculate a distance to the object10by using a peak detected by the peak detector130. For example, the processor150may calculate a distance to the object10by using a time of the peak detected by the peak detector130and a time of light emitted from the light source110. A method of measuring a distance by using a peak is a well-known technique in the art, and thus, a detailed description is omitted.

However, when light is irradiated onto a subject having a high refractive index or a subject located near the distance measuring device100, the magnitude of the reflected light may exceed the dynamic range of the light receiver120. Therefore, the light receiver120may output an electrical signal of a certain magnitude, that is, a saturated signal.FIG. 2Ais a graph showing an example of an unsaturated signal.FIG. 2Bis a graph showing an example of a saturated signal. As is evident, it is more difficult to obtain a peak from a saturated signal. When the light receiver120outputs a saturated signal, a certain amount of time is required to restore the original, unsaturated state of the signal, and thus, the light receiver120may output a signal having a characteristic different from that of the actual signal. Also, if the light receiver120outputs a saturated signal in some cases and outputs an unsaturated signal in other cases, an error may occur in detection of the peak.

The distance measuring device100according to the current embodiment may further include the saturation detector140that detects whether an electrical signal output from the light receiver120is saturated or not by comparing the electrical signal with a reference signal.

The saturation detector140may output an indication of a saturated signal, for example, “1”, when an electrical signal is greater than a reference signal, and may output an indication of an unsaturated signal, for example, “0”, when the electrical signal is smaller than the reference signal.

The processor150may control the magnitude of an electrical signal by using the indication output from the saturation detector140. For example, the processor150may control the magnitude of an electrical signal by controlling the magnitude of a bias voltage Vbias applied to the light receiver120, for example, to the light detector122of the light receiver120. Also, the processor150may control the magnitude of an electrical signal by controlling the magnitude of a driving signal, for example, a driving current Id applied to the light source110.

The bias voltage Vbias may be a potential difference applied to a power supply (not shown) as the light detector122is operated, and the magnitude of an electrical signal output from the light detector122may depend on the magnitude of detected light and may also depend on the magnitude of the bias voltage Vbias.

The magnitude of the bias voltage Vbias may depend on the location of the object10, the optical sensitivity of the light detector122, and the power of the distance measuring device100.FIG. 3is a graph showing an example of a magnitude of the bias voltage Vbias applied to the light detector122according to the distance to the object10. As depicted inFIG. 3, the processor150may change the magnitude of the a bias voltage Vbias being applied to the light detector122according to the distance to the object10. For example, when the object10is located within a distance of about 20 meters, the processor150may correctly measure the distance by applying a bias voltage Vbias with a magnitude in proportion to the distance to the object10. Also, when the object10is located at a distance of greater than 20 meters, the processor150may apply a maximum bias voltage Vbias, for example, 100 V, to the light detector122. The bias voltage Vbias nay be changed according to the distance to the object10, as described above, because the electrical signal output from the light detector122depends on the magnitude of the bias voltage Vbias. That is, when the magnitude of the bias voltage Vbias is small, the magnitude of the electrical signal output from the light detector122is small, and when the magnitude of the bias voltage Vbias is large, the magnitude of the electrical signal output from the light detector122is large. That is, the bias voltage Vbias controls the gain of the light detector122.

When the object10is located near the light detector122or the object10has a large refractive index, the light receiver120may output an electrical signal that exceeds a dynamic range, that is, the light receiver120may output a saturated signal, and the saturation detector140may detect the saturation of the electrical signal by comparing the electrical signal with a reference value. For example, when the saturation detector140detects a saturated signal, the saturation detector140may output a high-level pulse signal. Then, the processor150may control the magnitude of the bias voltage Vbias to be small. Thus, the light receiver120may output a signal having a small magnitude, that is, an unsaturated signal. For example, the processor150may control the magnitude of the bias voltage Vbias to be smaller than 70% of the magnitude before being controlled.

Also, the processor150may control the magnitude of the electrical signal by controlling the magnitude of a driving signal, for example, a driving current Id of the light source110. The magnitude of light reflected by the object10may be proportional to a refractive index of the object10and also proportional to the magnitude of light emitted from the light source110. An electrical signal output from the light detector122may be proportional to the magnitude of light received by the light detector122. Thus, the magnitude of an electrical signal output from the light detector122may be proportional to the magnitude of the driving signal of the light source110. When the processor150receives a saturation signal from the saturation detector140, the processor150may control the magnitude of the driving signal of the light source110to be small. Then, light having a small magnitude may be irradiated onto the object10, and accordingly, the magnitude of reflected light may also be small. Therefore, the magnitude of the electrical signal output from the light detector122becomes small, and thus, the light receiver120may output an unsaturated electrical signal. For example, the processor150may reduce the magnitude of the driving signal to be smaller than 70% of the magnitude before being controlled.

Also, the processor150may control the magnitude of the electrical signal by controlling a gain of the amplifier126. The magnitude of an electrical signal output from the light receiver120may be proportional to a gain of the amplifier126. Accordingly, when the processor150receives a saturation signal from the light receiver120, the processor150may control the magnitude of the gain with respect to the amplifier126to be small.

When the processor150receives an unsaturated signal, for example, a low-level signal from the saturation detector140, the processor150may omit additional control of the magnitude of the electrical signal. However, the current exemplary embodiment is not limited thereto. The processor150may also appropriately control the magnitude of the electrical signal according to a measured distance. That is, although the processor150receives an unsaturated signal from the saturation detector140, the processor150may control the magnitude of the electrical signal in proportion to the measured distance. For example, if the measured distance is smaller than a previously measured distance, the processor150may control the magnitude of the electrical signal to be small, and if the measured distance is larger than the previously measured distance, the processor150may control the magnitude of the electrical signal to be large. A control range of the magnitude of an electrical signal described above may be based on a look-up table that shows a relationship between distance and the magnitude of the electrical signal. The peak detector130may not detect a peak from an electrical signal for a certain period of time. Here, the certain period of time may be greater than a frequency of light emission from the light source110. For example, the certain period of time may be about two to three times greater than the frequency of light emission from the light source110. If the magnitude of an electrical signal output from the light receiver120is too small, the peak detector130may detect a peak. Then, the processor150may control the magnitude of an electrical signal to be large. The processor150may control the magnitude of the electrical signal by controlling the magnitude of the bias voltage Vbias applied to the light receiver120, for example, the light detector122, to be large. For example, the processor150may control the magnitude of the bias voltage Vbias to be 130% of the magnitude before controlling. When the magnitude of a bias voltage Vbias is large, the magnitude of an electrical signal output from the light receiver120may also be large.

Also, the processor150may control the magnitude of the electrical signal by controlling the magnitude of a driving signal, for example, the driving current Id of the light source110. For example, the processor150may control the magnitude of the driving signal to be greater than 130% of the magnitude of the driving signal applied to the processor150. If the driving signal is large, the magnitude of light emitted from the light source110is large, and the magnitude of light reflected by the object10becomes large. Also, the light receiver120may output an electrical signal having a large magnitude, and the peak detector130may detect a peak from the received electrical signal.

Also, the processor150may control the magnitude of the electrical signal to be large by controlling the magnitude of a gain of the amplifier126to be large.

FIGS. 4 and 5are each a flowchart of a method of operating a distance measuring device100, according to an exemplary embodiment.FIGS. 6A, 6B, 6C, 6D, and 6Eare diagrams for explaining a wavelength of a signal of a distance measuring device100, according to an exemplary embodiment of the inventive concept.

Referring toFIG. 4, the light receiver120receives light reflected by the object10. The light source110may emit light with a certain time interval to the object10(S410). As depicted inFIG. 6A, the light source110may emit light, for example, a laser pulse610, at a certain time interval. The light emitted from the light source110is reflected by the object10, and a portion of the reflected light may be received by the light receiver120. The light source110may be one of an edge emitting laser, a VCSEL, and a distributed feedback laser. For example, the light source110may be a laser diode.

The light receiver120may convert light reflected or scattered by the object10into an electrical signal, for example, a voltage, and may output the electrical signal (S420). Light reflected by the object10may be focused on a lens, and the light detector122may output a current corresponding to the focused light. Also, the current-voltage transformation circuit124may output a voltage by converting the current to the voltage. The light receiver120may output an electrical signal620as depicted inFIG. 6B. The light detector122may be a light-receiving diode, and may be operated in a state in which a bias voltage Vbias is applied thereto. For example, the light detector122may include an APD or a SPAD.

The saturation detector140may detect whether an electrical signal output by the light receiver120is saturated or not (S430). A portion of the electrical signal output by the light receiver120is applied to the saturation detector140, and a remaining portion of the electrical signal may be applied to the peak detector130. The saturation detector140may output a pulse wave as an indication of whether the electrical signal is saturated or not, by comparing the electrical signal with a reference value. Vt depicted inFIG. 6Bmay be a reference value. When an electrical signal620agreater than the reference value is detected, the saturation detector140may, as depictedFIG. 6C, output a saturated signal630, for example, a high-level pulse signal630.

When the electrical signal is determined to be saturated (S430-Y), the processor150may control the magnitude of an electrical signal to be small (S440). The processor150may control the magnitude of an electrical signal by controlling the magnitude of exemplary bias voltage applied to the light receiver120, for example, the light detector122. of the graph ofFIG. 6Dshows a waveform of a bias voltage. When a saturated signal630is detected, the processor150may control the magnitude of the bias voltage to be small, but the processor150is not limited thereto. The processor150may also control the driving signal of the light source110to be small.

When the electrical signal is determined to be unsaturated (S430-N), the processor150may maintain the magnitude of the electrical signal (S450), but the processor150is not limited thereto. The processor150may also appropriately control the magnitude of the electrical signal according to a measured distance to the object10.

Referring toFIG. 5, the peak detector130may detect a peak from an electrical signal output from the light receiver120(S510). The peak detector130may detect a peak by detecting the central position of an electrical signal. Also, the peak detector130may analogically detect a peak by detecting a width of an electrical signal. Also, the peak detector130may detect a peak by detecting a rising edge or a falling edge of a digital signal after converting an electrical signal to the digital signal. The peak detector130may detect a peak by using a CFD method, and may output the detected peak as a digital signal by using a comparator. of the graph ofFIG. 6Eshows a waveform of a signal output from the peak detector130. When a peak is detected, as a result, the peak detector130may output, for example, a high-level pulse650.

When a peak is detected (S510-Y), the processor150may measure a distance to the object10by using a peak detection time (S520). For example, the processor150may measure a distance to the object10by using a time difference between a peak detection time and a light emission time. The method of measurement by using a peak is well known in the art, and thus, a detailed description thereof is omitted.

When a peak is not detected (S510-N), the processor150may control the magnitude of an electrical signal (S530). When a peak is not detected for a certain period of time, the processor150may determine that the magnitude of an electrical signal is small, and thus, may control the magnitude of an electrical signal to be large. Here, the certain period of time may be greater than a frequency of light emitted from the light source110, for example, may be about two to three times greater than a driving frequency of light.

If a peak is not detected in a state in which the location etc. of the object10is not changed, this may be the reason that the magnitude of the bias voltage Vbias is too small. Accordingly, the processor150may control the magnitude of the electrical signal by controlling the magnitude of the bias voltage Vbias to be large. If a peak650ais not detected inFIG. 6E, the processor150may control a magnitude640bof the bias voltage Vbias to be large, as shown by the signal waveform ofFIG. 6D, but the processor150is not limited thereto. The processor150may control the driving signal of the light source110to be large.

FIGS. 7A, 7B, 7C, 7D, and 7Eare diagrams of a waveform of a signal that controls a driving signal of the light source110according to saturation detection, according to an exemplary embodiment.

When the saturation detector140detects a saturated signal730adepicted inFIG. 7C, the processor150may, as depicted inFIG. 7A, control a magnitude710aof the driving signal of the light source110to be small. Thus, as depicted inFIG. 7B, the magnitude of light emitted from the light source110becomes small. Thus, the light receiver120may output an electrical signal730bhaving a small magnitude due to the light, the magnitude of which is reduced. Meanwhile, as depicted inFIG. 7E, if a peak750ais not detected by the peak detector130for a certain period of time, the processor150may re-control a magnitude710bof the driving signal of the light source110to be large. For example, the processor150may control the magnitude of the driving signal to be the original magnitude.

FIGS. 8A, 8B, 8C, 8D, 9A, 9B, 9C, and 9Dare diagrams showing results of simulations of peak detections according to bias voltages. As depicted inFIG. 8A, in a state in which a bias voltage Vbias of −154 V is applied to the light detector122, the light detector122outputs a saturated signal810as depicted inFIG. 8B. A CFD circuit of the peak detector130that receives the saturation signal outputs a signal waveform as depicted inFIG. 8C, and a comparator of the peak detector130outputs a signal waveform as depicted inFIG. 8D. The signal waveform ofFIG. 8Dis output with two falling edges820, and thus, an error may occur in the processor's150determination of a peak time.

As depicted inFIG. 9A, a controlled bias voltage Vbias of −54 V is applied to the light detector122. The magnitude of the controlled bias voltage Vbias is about 36% of the magnitude of a bias voltage Vbias before being controlled. As depicted inFIG. 9B, an electrical signal910output from the light receiver120is not saturated. A CFD circuit of the peak detector130outputs a signal waveform as depicted inFIG. 9C, and a comparator of the peak detector130outputs a signal waveform as depicted inFIG. 9D. Since the signal waveform depicted inFIG. 9Dis output with only one falling edge920, the processor150may correctly determine the peak time.

As described above, since the magnitude of the electrical signal of the light receiver120is controlled so that the light receiver120outputs an unsaturated signal, an error between a saturated signal and an unsaturated signal may be reduced.

As described above, in order to control the magnitude of an electrical signal of the light receiver120, the application of a bias voltage to the light detector122or a driving signal of the light source110is controlled. However, the current exemplary embodiment is not limited thereto. That is, in order to control the magnitude of the electrical signal, the magnitude of a gain with respect to a constituent element, for example, the amplifier126, may also be controlled.

While exemplary embodiments have been shown and described, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope as defined by the appended claims. The embodiments should be considered in descriptive sense only and not for purposes of limitation. Therefore, the scope is defined by the appended claims, and all differences within the scope will be construed as being included in the inventive concept.