JP2007132848A - Optical distance measuring device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To realize high ranging accuracy even under such the environment that the intensity of background light is high. <P>SOLUTION: An accumulation differential signal of an Ach signal from a first accumulating element 22 and a Bch signal from a second accumulating element 23 is obtained by using a differential operation section 24. Thus, carrying out the differential operation of the Ach signal and the Bch signal enables noise components such as the background light and the like to be removed appropriately and only signals which are necessary to calculate a distance from a measurement object 14, to be extract and accumulated. Results of above differential operations are integrated two or more times, thereby enabling a sufficient amount of charge for calculating the distance to be accumulated on the differential operation section 24. Accordingly for example, a round-trip time of light to the measurement object 14 is detected from a sufficient amount of signal component, and the calculation of the distance can be carried out precisely, even under the environment such as outdoors and the like in which the intensity of the background light is very high. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an optical distance measuring device that measures the travel time of light from when light is emitted until it is reflected by a measurement object and detected by a light receiving element, and detects the distance to the measurement object. .

Conventionally, a so-called TOF (Time Of Flight) method, which is a method of calculating a distance to a measurement object by measuring a round trip time of light, is widely known as a distance measurement method. In this distance measuring method, since the speed of light c is known as 3.0 × 10 ⁸ m / s, the distance L to the object is calculated by the following equation (1) by measuring the round-trip time t1. It is a method to do.
L = (c · t1) / 2 (1)

  Various specific signal processing methods in the TOF method have been proposed. For example, in a distance measuring device disclosed in Japanese Patent Laid-Open No. 6-18665 (Patent Document 1), a start pulse (synchronized with a light emitting element) is used as a start signal. The charge is accumulated (or discharged) in the integrator until a stop pulse (light reception signal) is detected, and the round-trip time of light is detected from the increase (or decrease) amount. As described above, as a measuring method for measuring the time between the start pulse and the stop pulse, for example, as in the distance measuring device disclosed in JP-A-7-294642 (Patent Document 2), the start pulse At the same time, there is one that starts counting the number of pulses of the reference CLK and obtains the light round-trip time based on the number of pulses when the stop pulse is detected.

  However, in any of these methods, signal processing is performed by converting a current signal detected by a light receiving element into a pulse (voltage) signal and giving time information to the pulse waveform. In general, the measurement object is not specified, the dynamic range of the amount of light reflected from the measurement object or the like is very large, and the noise component due to background light such as natural light is often larger. In such a situation, it is very difficult to appropriately extract signal light pulses by removing noise due to background light. In addition, the voltage waveform easily causes phase delay due to the influence of the environment (mainly temperature). Therefore, the variation on the time axis becomes very large and some time correction means is required. In that case, the circuit configuration becomes very complicated, resulting in an increase in manufacturing cost.

  On the other hand, R.Miyagawa et al. Can obtain distance information by processing photocurrent before voltage conversion of the received light signal by using a photogate having a general CCD (charge coupled device) structure. ("CCD-Based Range-Finding Sensor" IEEE Transactions on Electron Devices, Vol. 44, No. 10, October, 1997, p1648-1652 (Non-patent Document 1)). FIG. 6 is a schematic cross-sectional view showing an example of a light receiver having a photogate structure proposed by R. Miyagawa et al. FIG. 7 is a timing chart showing the operation of the photogate structure.

  In FIG. 6, 1 is a P-type semiconductor substrate, 2 is an n-type semiconductor layer constituting a light receiving portion together with the P-type semiconductor substrate 1, 3 is an n-type semiconductor layer constituting an Ach (A channel) charge storage portion, This is an n-type semiconductor layer constituting a Bch charge storage section. Reference numerals 5 and 6 denote gates having a MOS (metal oxide semiconductor) structure, and the n-type semiconductor layer 2, the charge storage unit 3 and the gate 5 form an Ach-side switching MOS transistor 7. Similarly, the n-type semiconductor layer 2, the charge storage section 4, and the gate 6 constitute a Bch side switching MOS transistor 8.

A light emitting element (not shown) irradiates the measurement object with light according to the timing shown in FIG. The optical signal reflected by the object to be measured is detected by the light receiving section constituted by the P-type semiconductor substrate 1 and the n-type semiconductor layer 2 in FIG. 6, and becomes a light reception signal as shown in FIG. In this case, the phase relationship between FIG. 7A and FIG. 7B is that the received light signal is delayed by the time (t1) in which the light travels back and forth the distance to the measurement object. Here, the gate 5 in the switching MOS transistor 7 on the Ach side is turned on / off in synchronization with the light emission signal, and the gate 6 in the switching MOS transistor 8 on the Bch side is turned on at the same time as the gate 5 is turned off. . In this case, the level “H” duration in the gate signals G _A and G _B input to the gates 5 and 6 is the same as the level “H” duration t 0 in the light emission signal.

  By performing the switching operation by the switching MOS transistors 7 and 8 at the timing as described above, the n-type semiconductor layer corresponding to the time (t0-tl) shown in FIG. 2 is accumulated, and the charge from the n-type semiconductor layer 2 corresponding to (tl) is accumulated in the Bch charge accumulation section 4. Then, this operation is repeated a plurality of times to accumulate charges in the charge accumulating units 3 and 4 to increase the signal component (that is, accumulated charges), and then read out the signals of both channels, for example, the ratio of both signals The distance to the measurement object can be measured by calculating

  According to the photogate structure shown in FIG. 6, information on the phase delay amount corresponding to the round-trip time of light is processed as the accumulated charge amount (intensity). Therefore, for example, even if there is a temperature change, it is not necessary to consider phase variations in signal processing. Therefore, stable distance measurement is possible.

  In a general environment, there is some background light such as sunlight or illumination (fluorescent lamps, etc.). And when background light exists, background light is superimposed on the received light signal waveform shown in FIG. The modulation frequency of background light varies from DC (in the case of sunlight) to several tens of kilohertz (in the case of inverter lamps), but is at most a kHz level in a general living environment. On the other hand, since the TOF method is a delay time measurement using the speed of light, its frequency is high and it is common to use a level of several tens of MHz. Therefore, the frequency of the background light is sufficiently lower than the pulse waveform of the received light signal, and can be regarded as DC within one period of the pulse waveform. FIG. 8 shows a timing chart when there is background light. As shown in FIGS. 8 (e) and 8 (f), when there is background light, the amounts of charges accumulated in the Ach charge accumulation unit 3 and the Bch charge accumulation unit 4 are as follows. It is increased by the amount of background light in the duration of on. For this reason, the delay time t1 cannot be obtained using the amount of charge stored in the Ach charge storage unit 3 and the Bch charge storage unit 4.

In order to solve such a problem, in the distance image sensor disclosed in Japanese Patent Application Laid-Open No. 2004-294420 (Patent Document 3), in addition to the same structure as that in Non-Patent Document 1, the charge accumulating units 3 and 4 are further provided. A charge storage unit (not shown) different from the above is provided, and only the reflected light is extracted from the outputs of Ach and Bch by monitoring only the background light in the third time zone. FIG. 9 shows the timing chart. As shown in FIG. 9, following the gate signal G _B of the gate signals G _A and Bch of Ach, the switching MOS transistor of Cch having a gate which is turned on by the gate signal G _C having the same pulse width t0 (not shown) (Not shown) is provided around the light receiving unit. In this case, since the pulse signal based on the reflected light in a time zone where the gate signal G _C is turned on is not present, charges due to only the background light is accumulated, the background light intensity is monitored. Therefore, the distance to the measurement object can be obtained from these three accumulated carriers (intensities) using the following formula (2) even in an environment where there is background light. In Equation (2), A is the amount of charge stored in the Ach charge storage unit 3, B is the amount of charge stored in the Bch charge storage unit 4, and C is Cch charge storage (for background light). Charge amount accumulated in the portion.

  However, the distance image sensor disclosed in Patent Document 3 has the following problems. That is, the background light as described above reaches several hundred thousand lux under outdoor sunlight, for example, and has a brightness of several thousand lux even in a relatively bright indoor such as an office. Due to such strong background light, for example, when a normal photodiode is used as the light receiving element, the obtained photocurrent is generally higher than the mA order, although it depends on the optical system and the light receiving area. Easy to calculate. On the other hand, the amount of light reflected and returned from the measurement object depends largely on the state of reflection on the surface of the measurement object and the distance to the measurement object.For example, a high-power laser diode ( Even if (LD) (several hundred mW) is used, the light quantity incident on the light receiving element may be reduced to about nW if there is a distance of several meters to the object.

Under such an environment, the S / N ratio of the charges accumulated in the accumulation units 3 and 4 in FIG. 6 is very low, and a minute signal component exists in most noise components. Since the capacity of the storage units 3 and 4 with respect to the electric charge is limited, the number of repetitions when the electric charge increased by the noise component is repeatedly stored is limited. The lower the S / N ratio, the more the measurement distance error becomes. It gets bigger.
Japanese Patent Laid-Open No. 6-18665 JP-A-7-294642 JP 2004-294420 A “CCD-Based Range-Finding Sensor” IEEE Transactions on Electron Devices, Vol.44, No10, October, 1997, p1648-1652

  Accordingly, an object of the present invention is to provide an optical distance measuring device with high distance measuring accuracy even in an environment with strong background light.

In order to solve the above problems, the optical distance measuring device of the present invention is:
A transmitter for transmitting light in synchronization with a modulation signal having a repetition frequency;
A receiver that receives the light transmitted from the transmitter and reflected by the measurement object, and outputs a signal corresponding to the received optical signal;
A signal processing unit for processing the signal output from the receiver,
The receiver is
A light receiving element that converts the received optical signal into an electrical signal;
A switch that switches an electrical signal from the light receiving element to at least two paths at a predetermined timing synchronized with the modulation signal;
A plurality of storage elements that are arranged in each of the paths and store electrical signals switched to the paths;
The signal processor is
A calculation unit for calculating the difference between the electrical signals for each path accumulated in the respective storage elements;
An accumulation time determination unit that determines an accumulation time of the accumulation element based on a calculation result by the calculation unit;
A distance determination unit that determines a distance to the measurement object using a calculation result of the electric signal stored in each storage element by the storage time determined by the storage time determination unit using the calculation result by the calculation unit. It is characterized by that.

  According to the above configuration, since the calculation unit calculates the difference between the electrical signals stored in the respective storage elements arranged in each of the two paths switched by the switch, from the calculation result by the calculation unit, the background light Such noise components are appropriately removed. Therefore, only the signal component due to the reflected light reflected by the measurement object is extracted and stored in the calculation result. Further, since the accumulation time by each of the storage elements is determined based on the calculation result by the storage time determining unit, only the signal component after the background light removal can be accumulated until the amount is sufficient for distance measurement. .

  From the above, even if the background light is very strong, such as outdoors, the storage element does not saturate with noise components, and light is transmitted from the transmitter based on the sufficiently accumulated signal components. It is possible to accurately detect the elapsed time from when the light reflected by the measurement object is received by the receiver, and to determine the distance to the measurement object with high accuracy by the distance determination unit. it can.

In the optical distance measuring device of one embodiment,
The accumulation time determination unit is configured to determine the accumulation time by comparing a calculation result obtained by the calculation unit with a threshold value.

  According to this embodiment, since the storage time by each storage element is determined by comparing the calculation result by the calculation unit and a threshold value, the storage time determination unit is easily configured by a comparator such as a comparator, for example. In addition, the accumulation time can be reliably determined.

In the optical distance measuring device of one embodiment,
The accumulation time determination unit sets the accumulation time Tsum when the time until the calculation result by the calculation unit reaches the threshold is t, the repetition frequency of the modulation signal is T, and the accumulation time is Tsum. A value that satisfies the following relationship is determined.
Tsum = m × T
Where m is the smallest integer satisfying m> t / T

  According to this embodiment, since the accumulation time Tsum is determined to be the minimum value satisfying the relationship, the calculation result is set to the threshold value while the electrical signal for one period of the modulation signal is being accumulated. Even in such a case, the electric signal can be stored in units of one period of the modulated signal. Therefore, it is not necessary to interrupt the operation of the storage element and the calculation unit in an incomplete state, and the distance to the measurement object can be accurately determined.

In the optical distance measuring device of one embodiment,
The receiver and the signal processing unit include two units including a set of the light receiving element, the switch, the storage element, and the arithmetic unit,
The switching interval of the switch in the first unit that is one of the two units is at least twice the switching interval of the switch in the second unit that is the other of the two units,
The storage time determination unit determines the storage time of the storage element of the first unit and the storage element of the second unit by comparing the calculation result of the calculation unit of the first unit with a threshold value. ,
The distance determination unit determines a distance to the measurement object using a calculation result by the calculation unit of the first unit and a calculation result by the calculation unit of the second unit.

  According to this embodiment, the switching interval of the switch in the first unit is set to the switching interval of the switch in the second unit among the two units including the set of the light receiving element, the switch, the storage element, and the arithmetic unit. It is over 2 times. Therefore, if the switching interval of the switch in the second unit is made the same as the repetition period of the modulation signal, the electric signal accumulated in the accumulation element of each path in the first unit is reflected by the measurement object. The entire received light signal is included. As a result, in the first unit, it is possible to monitor the intensity of the received light signal by performing the calculation of the difference between the electric signals stored in the storage elements in the paths by the calculation unit.

  That is, according to the present invention, the intensity of the received light signal of the reflected light is unknown based on the calculation result by the calculation unit of the first unit and the calculation result by the calculation unit of the second unit, and Even in an environment with strong background light, the distance to the measurement object can be accurately determined.

  Furthermore, by monitoring the intensity of the received light signal of the reflected light, the calculation result can be accumulated until a signal necessary for measuring the distance to the measurement object is sufficiently accumulated. Can be determined more accurately.

In the optical distance measuring device of one embodiment,
The storage time in the storage element of the first unit is equal to the storage time in the storage element of the second unit.

  According to this embodiment, since the storage times of the storage elements of the first unit and the second unit are the same, it is easy to calculate and determine the distance by the distance determination unit performed later. Can do.

In the optical distance measuring device of one embodiment,
A control unit for controlling the switching timing of the path by the switch and erasing the electrical signal stored in each storage element,
The control unit
Causing the switch to switch the path according to the first timing;
When the storage time of the storage element is determined by the storage time determination unit, after erasing the electrical signal stored in each storage element, the switch is switched the path by the second timing, An electrical signal is accumulated in each of the storage elements for a time based on the determined storage time,
The distance determination unit includes a calculation result by the calculation unit when the switch switches the route at the first timing, and a result when the switch switches the route at the second timing. The distance to the measurement object is determined using the calculation result by the calculation unit.

  According to this embodiment, in the first time zone, the switch is operated at the first timing, the storage time determination unit determines the storage time of the storage element, and the storage time determination unit determines the storage time based on the storage time. The time length of time zone 2 is determined. Then, in the second time zone, the switch is operated at a second timing, and the calculation result by the calculation unit in the first time zone and the second time zone is obtained by the distance determination unit. Used to determine the distance to the measurement object. Therefore, the switching interval of the switch in the first time zone is set to be twice or more the switching interval of the switch in the second time zone, and the switching interval of the switch in the second time zone is set to the modulation signal. In the first period, the electrical signal stored in the storage element of each path includes the entire received light signal of the reflected light from the measurement object. As a result, in the first time zone, it is possible to monitor the intensity of the received light signal by performing the calculation of the difference between the electrical signals stored in the storage elements in the paths by the calculation unit. Become.

  That is, in this embodiment, the intensity of the received light signal of the reflected light is based on the calculation result by the calculation unit in the first time zone and the calculation result by the calculation unit in the second time zone. Even in an unknown case and in an environment with strong background light, the distance to the measurement object can be accurately determined.

  In other words, according to this embodiment, the same effect as that of the optical distance measuring device including two units each including a set of the light receiving element, the switch, the storage element, and the calculation unit is obtained with the configuration including one unit. Can be achieved. Therefore, it is possible to reduce the size of the optical distance measuring device including two units.

In the optical distance measuring device of one embodiment,
The accumulation time determination unit makes the accumulation time undecidable when the calculation result by the calculation unit does not reach the threshold value within a first predetermined time,
The distance determination unit is configured to be unable to determine the distance to the measurement object in response to the determination of the accumulation time by the accumulation time determination unit.

  When the calculation result by the calculation unit does not reach the threshold within a first predetermined time, the measurement object is sufficiently far away, the reflectance of the measurement object is extremely low, It is assumed that the amount of reflected light received by the light receiving element is too weak because, for example, the reflected light is too strong because the surface of the measurement object is in a mirror state.

  According to this embodiment, when the calculation result by the calculation unit does not reach the threshold value within a first predetermined time, the accumulation time cannot be determined and the distance to the measurement object cannot be determined. Therefore, it is possible to prevent erroneous determination of the determined distance when the amount of reflected light received by the light receiving element is too weak due to the reasons described above.

In the optical distance measuring device of one embodiment,
The accumulation time determination unit makes the accumulation time undecidable when the calculation result by the calculation unit reaches the threshold value within a second predetermined time,
The distance determination unit is configured to be unable to determine the distance to the measurement object in response to the determination of the accumulation time by the accumulation time determination unit.

  When the calculation result by the calculation unit reaches the threshold value within a second predetermined time shorter than the first predetermined time, the measurement object is near or the surface of the measurement object Since the reflected light is too strong because of the specular state, the number of electrical signals stored in the storage element is reduced, so that a sufficient averaging effect cannot be obtained, resulting in a large measurement error per measurement. It comes to influence.

  According to this embodiment, when the calculation result by the calculation unit reaches the threshold value within a second predetermined time, the accumulation time cannot be determined and the distance to the measurement object cannot be determined. Therefore, in the case where a sufficient averaging effect cannot be obtained as described above and a measurement error per time greatly affects the measurement result, erroneous determination of distance by the distance determination unit can be prevented. it can.

  As is clear from the above, the optical distance measuring device of the present invention performs the calculation of the difference between the electric signals stored in the storage elements arranged in the two paths switched by the switch by the calculation unit. Noise components such as background light can be appropriately removed from the calculation result by the calculation unit. Therefore, only the signal component due to the reflected light reflected by the measurement object can be extracted and stored in the calculation result. Furthermore, since the storage time by each storage element is determined by the storage time determination unit based on the calculation result, only the signal component after background light removal can be stored until the amount is sufficient for distance measurement.

  That is, according to the present invention, the storage element does not saturate with a noise component even in an environment where the background light is very strong, such as outdoors, and the transmitter is based on a sufficiently accumulated signal component. The elapsed time from when the light is transmitted to when the reflected light from the measurement object is received by the receiver can be detected with high accuracy, and the distance to the measurement object is determined with high accuracy by the distance determination unit. be able to.

  Hereinafter, the present invention will be described in detail with reference to the illustrated embodiments.

First Embodiment FIG. 1 is a block diagram of an optical distance measuring device according to the present embodiment. FIG. 2 is a timing chart showing the operation of the optical distance measuring device shown in FIG. The optical distance measuring device according to the present embodiment will be described below with reference to FIGS.

  As shown in FIG. 1, the optical distance measuring device includes a transmitter 11 that emits a light beam 17 toward a measurement object 14 and a receiver that detects a light beam 18 reflected by the measurement object 14. 12 and a signal processing circuit unit 13 for processing a detection signal from the receiver 12.

  In the transmitter 11, a light beam 17 is emitted toward the measurement object 14 by the light emitting element 16 in synchronization with the modulation signal from the modulation signal generator 15. Here, as shown in FIG. 2A, the modulated signal is a pulse wave that repeats a pulse having a pulse width t0 at a constant repetition frequency. However, the modulation signal is not limited to a pulse wave, and the function of the optical distance measuring device can be obtained as long as the signal has a waveform such as a triangular wave or a sawtooth wave as a function. The following description will be made assuming that the pulse wave. Details of each modulation signal will be described later.

  The light beam 18 reflected by the measurement object 14 is detected by the light receiving element 19 in the receiver 12. In this case, as shown in FIG. 2B, the light beam 18 is detected with a phase delay from the modulation signal by the time (t1) in which the light beams 17 and 18 reciprocate the distance to the measurement object 14. . Here, “Ip” in FIG. 2B represents the intensity of a current signal (hereinafter referred to as a pulse signal) generated by the light beam 18 of reflected light generated by photoelectric conversion by the light receiving element 19, and “Ib” represents background. It represents the intensity of a current signal (noise signal) due to light.

  Assuming that the distance measurement range is 7.5 m, it can be easily understood from the above formula (1) that the pulse width t0 of the light emission signal needs 50 nsec. Furthermore, since the background light is at most about several tens of kHz, the period is about several tens of μsec, which is sufficiently large for the pulse width of 50 nsec. Therefore, as shown in FIG. 2, the background light can be regarded as DC light in the time length of the pulse width t0 of the light emission signal.

Thereafter, the detection signal (light reception signal = pulse signal + noise signal) from the light receiving element 19 is switched between the first path (Ach) and the second path (Bch) by the first switch 20 and the second switch 21. The received light signal flowing from the light receiving element 19 to the first path (Ach) side is input to the first storage element 22. On the other hand, the light receiving signal that has flowed from the light receiving element 19 to the second path (Bch) is input to the second storage element 23. Here, as shown in FIGS. 2C and 2D, the first switch 20 and the second switch 21 are ON / OFF controlled at the same timing as the light emission signal. 2C and 2D, the first opening / closing signal S _A is a control signal for the first switch 20, and the second opening / closing signal S _B is a control signal for the second switch 21. Both are supplied from the modulation signal generator 15 of the transmitter 11. Then, as shown in FIG. 2 (e), the first storage element 22 of the first path (Ach) has (Ip (t0−tl) + Ib · t0 per period of the modulation signal shown in FIG. 2 (a). ) Charge is accumulated. Similarly, as shown in FIG. 2 (f), the second storage element 23 in the second path (Bch) accumulates charges of (Ip · tl + Ib · t0) per cycle of the modulation signal.

  That is, the light receiving element 19 constituting the receiver 12 specifically corresponds to, for example, the P-type semiconductor substrate 1 and the n-type semiconductor layer 2 in FIG. 6, and the first switch 20 and the second switch 21 are illustrated in FIG. 6 corresponds to the MOS transistors 7 and 8, the first storage element 22 corresponds to the charge storage section 3 in FIG. 6, and the second storage element 23 corresponds to the charge storage section 4 in FIG.

The charges stored in the first storage element 22 and the second storage element 23 in this way are input to the differential operation section 24 as the operation section constituting the signal processing circuit section 13 as an Ach signal and a Bch signal. Then, the differential calculation unit 24 calculates a difference (differential calculation) between the Ach signal from the first storage element 22 and the Bch signal from the second storage element 23. Then, an accumulated differential signal as shown in FIG. 2 (g) is obtained. Here, the accumulated differential signal can be expressed by Expression (3) using the accumulation count N.
Accumulated differential signal = N. [Ip (t0-t1) + Ib.t0] -N. (Ip.tl + Ib.t0)
= N.Ip (t0-2t1) (3)

  Note that “N” in the above equation (3) is determined as follows. FIG. 3 is a diagram showing the light emission signal shown in FIG. 2A and the accumulated differential signal shown in FIG. 2G over a plurality of cycles in the timing chart shown in FIG. As shown in FIG. 3, the differential signal corresponding to the duration t0 of the light emission signal is integrated every period and crosses a preset threshold value -Vth at a certain time t2. In this case, the number of times of accumulation (the number of cycles of the light emission signal) is determined as the number of times of accumulation N. Incidentally, in FIG. 3, the case where the accumulated differential signal becomes negative is illustrated as an example, but the sign of the accumulated differential signal is determined by the relationship between the pulse width t0 and the delay time t1. However, since the differential signal corresponding to the pulse width t0 becomes 0 when the relationship t1 = t0 / 2 is established, the differential signal is not accumulated even if the integration of the differential signal is repeated. In this case, it is necessary to limit the measurable range to t0 / 2.

As shown in FIG. 3, in order to detect time t2 when the value of the accumulated differential signal reaches a preset threshold value (± Vth), output is performed when the accumulated differential signal value reaches the threshold value. It is preferable to use a comparator (comparator) whose level is inverted. In the present embodiment, as shown in FIG. 1, an output signal (stored differential signal) from the differential operation unit 24 is input to the comparator 25. Then, the comparator 25 compares the value of the accumulated differential signal with the threshold (± Vth), and | the value of the accumulated differential signal |> Vth, and the level of the output signal from the comparator 25 is “ When inverted from “H” to “L”, the first open / close signal S _A and the second open / close signal S _B are always set to “L” by the output signal from the comparator 25, and the first switch 20 and the second switch 21 is stopped and the measurement is terminated.

  Here, the timing at which the measurement is terminated by the output signals from the comparator 25 to the first switch 20 and the second switch 21 is the time t2 when the value of the accumulated differential signal reaches the threshold as shown in FIG. It is desirable to set it to the shortest cycle time t3 of the modulated signal including. By doing so, the measurement is not terminated while charges are accumulated in the first accumulation element 22 and the second accumulation element 23, and can be terminated after the charges are completely accumulated.

  Further, the output signal (accumulated differential signal) from the differential calculation unit 24 is also input to the distance determination unit 26. Then, the distance determination unit 26 obtains the number N of accumulations from the time t2 when the value of the accumulated differential signal reaches the threshold value, and the light beams 17 and 18 reciprocate to the measurement object 14 using the above equation (3). The time t1 is calculated, and the distance L to the measurement object 14 is further calculated using the above equation (1).

  As described above, in the present embodiment, the differential calculation unit 24 stores the signal according to the equation (3) based on the Ach signal from the first storage element 22 and the Bch signal from the second storage element 23. A differential signal is obtained. In this way, by performing a differential operation between the Ach signal and the Bch signal, noise components such as background light are appropriately removed, and only signal components necessary for calculating the distance to the measurement object 14 are extracted and stored. be able to. Therefore, by accumulating the results of the differential calculation a plurality of times, the differential calculation unit 24 can accumulate a sufficient amount of charge for distance calculation.

  Further, since the accumulated number N in the case of obtaining the accumulated differential signal is determined by a comparison result between the accumulated differential calculation result and a preset threshold value (± Vth), noise such as background light is removed. The accumulated amount of only the subsequent signal component can be accumulated until the amount is sufficient to obtain the distance to the measurement object 14. Therefore, the first storage element 22 and the second storage element 23 do not saturate with noise components even in an environment where the background light is very strong, such as outdoors, for example, from a sufficient amount of signal components to the measurement object 14. It is possible to calculate the distance with high accuracy by detecting the round trip time of the light.

  By the way, the time t1 when the light beams 17 and 18 reciprocate the distance to the measurement object 14 can be calculated using the above equation (3) when the received light signal intensity Ip is known. For example, when using a measurement system in which the transmitter and the receiver are opposed to each other, the light energy is output without being dispersed using coherent light such as a laser, and the light receiving element directly receives light. Limited. In such a case, since the emission energy and the light reception energy are equal, by measuring the light reception signal intensity Ip in advance, the laser is emitted from the light emitting element using the equation (3) and then received by the light receiving element. It is possible to calculate the time t1 until the reception is performed, and calculate the distance 2L to the receiver using the equation (1) based on the obtained time t1.

  Thus, the time t1 can be obtained using only the above equation (3) only in a peculiar case. However, in a general optical distance measuring device, the transmitter 11 and the receiver 12 are disposed at substantially the same position, and the reflected light from the measurement object 14 is detected and the distance to the measurement object 14 is reduced. In most cases, the reciprocation time t1 is measured, and the light reception signal intensity Ip is unknown, so that the reciprocation time t1 to the measurement object 14 cannot be calculated using only equation (3). However, it goes without saying that the round trip time t1 can be calculated when the value of the received light signal intensity Ip can be obtained by some method. Further, as described above, since there is a dead zone when t1 = t0 / 2, it is necessary to limit the range of application and usage.

Second Embodiment This embodiment relates to an optical distance measuring device that can measure the distance to the measurement object even if the received light signal intensity Ip of the reflected light from the measurement object is unknown. In this optical distance measuring device, by providing two sets of units including a receiver and a differential operation unit, it is possible to detect the reflected light from the measurement object and measure the distance to the measurement object. Yes. Hereinafter, the optical distance measuring device will be described in detail.

  FIG. 4 is a block diagram of the optical distance measuring device in the present embodiment. FIG. 5 is a timing chart showing the operation of the optical distance measuring device shown in FIG. Hereinafter, the optical distance measuring device according to the present embodiment will be described with reference to FIGS. 4 and 5.

  In FIG. 4, the transmitter 11, the measurement object 14, the modulation signal generator 15, the light emitting element 16, and the light beams 17 and 18 are the same as those of the optical distance measuring device shown in FIG. 1 in the first embodiment. Therefore, explanation is omitted. Further, the first light receiving element 19, the first switch 20, the second switch 21, the first storage element 22 and the second storage element 23 constituting the receiver 31 are the same as those in the case of the optical distance measuring device shown in FIG. Therefore, explanation is omitted. Further, the first differential calculation unit 24 constituting the signal processing circuit unit 32 is the same as that of the optical distance measuring device shown in FIG.

Here, the first light receiving element 19, the first switch 20, the second switch 21, the first storage element 22, the second storage element 23, and the first differential operation unit 24 are shown in FIGS. As shown in the timing chart of g), in the first embodiment, the light receiving element 19, the first switch 20, the second switch 21, and the first accumulation shown in the timing charts of FIGS. 2 (a) to 2 (g). The operation is the same as in the case of the element 22, the second storage element 23, and the differential operation unit 24. Accordingly, the first accumulation differential signal output from the first differential operation unit 24 can be expressed using the accumulated number of times N ₁ as for formula (3) in the first embodiment (Equation (4 )).
First accumulated differential signal = N ₁ · [Ip (t0−t1) + Ib · t0] −N ₁ · (Ip · tl + Ib · t0)
= N ₁ · Ip (t0-2t1) (4)

As shown in FIG. 4, the receiver 31 further includes a second light receiving element 33, a third switch 34, a fourth switch 35, a third storage element 36 and a fourth storage element 37. The light beam 18 reflected by the measurement object 14 is detected by the second light receiving element 33. As shown in FIG. 5 (h) and FIG. 5 (i), the detection signal from the second light receiving element 33 includes the third open / close signal S _C having the pulse width 2t0 synchronized with the light emission signal shown in FIG. Switching between the third path (Cch) and the fourth path (Dch) is performed by the third switch 34 and the fourth switch 35, which are controlled to be turned on / off by the 4 open / close signal _SD .

  The light reception signal that has flowed from the second light receiving element 33 toward the third path (Cch) is input to the third storage element 36. On the other hand, the light reception signal that flows from the second light receiving element 33 to the fourth path (Dch) side is input to the fourth storage element 37. Then, as shown in FIG. 5 (j), the charge of (Ip · t0 + Ib · 2t0) per two periods of the modulation signal shown in FIG. Accumulated. Similarly, as shown in FIG. 5 (k), the fourth storage element 37 in the fourth path (Dch) stores (Ib · 2t0) charges per two periods of the modulation signal.

  That is, the receiver 31 in the present embodiment specifically corresponds to, for example, the first light receiving element 19, the first switch 20, the second switch 21, the first storage element 22, and the second storage element 23. In addition to a unit composed of a P-type semiconductor substrate, an n-type semiconductor layer, a MOS transistor, and a charge storage unit, a second light receiving element 33, a third switch 34, a fourth switch 35, a third storage element 36, and a fourth storage element 37 A unit composed of a P-type semiconductor substrate, an n-type semiconductor layer, a MOS transistor, and a charge storage portion corresponding to is arranged side by side so that the light beam 18 can be incident on both n-type semiconductor layers simultaneously.

The charges stored in the third storage element 36 and the fourth storage element 37 in this way are input to the second differential operation section 38 constituting the signal processing circuit section 32 as a Cch signal and a Dch signal. Then, the second differential operation unit 38 performs the above differential operation between the Cch signal from the third storage element 36 and the Dch signal from the fourth storage element 37. Then, a second accumulated differential signal as shown in FIG. 5 (l) is obtained. Here, the second accumulated differential signal can be expressed by Equation (5) using the accumulation number N ₂ .
Second accumulated differential signal = N ₂ · [Ip · t 0 + Ib · 2t 0] −N ₂ · (Ib · 2t 0)
= N ₂ · I p · t 0 (5)

  As described above, in the present embodiment, the switching time between the third switch 34 and the fourth switch 35 related to the third route and the fourth route is set as the first switch 20 related to the first route and the second route. The switching time with the second switch 21 is doubled. By doing so, the value of the second accumulated differential signal takes a form depending on the received light signal intensity Ip because the switching time t0 is known.

Therefore, in the first embodiment, as in the case of determining “N” in the above equation (3), the second differential signal corresponding to twice the duration t0 of the pulse signal is converted into two cycles of the light emission signal. Integration is performed every time, and the number of times of accumulation when a preset threshold value is reached is determined as the number of times of accumulation N ₂ .

In this case, the second difference is detected in the same manner as in the first embodiment in order to detect the time when the value of the second accumulated differential signal reaches the preset threshold value (that is, the accumulation number N ₂ ). An output signal (second accumulated differential signal) from the dynamic calculation unit 38 is input to the comparator 39. Then, the comparator 39 compares the value of the second accumulated differential signal with the threshold value, and | the value of the second accumulated differential signal |> the threshold value, and the level of the output signal from the comparator 39 is When inverted, the output signal from the comparator 39 stops the first switch 20, the second switch 21, the third switch 34, and the fourth switch 35, and the measurement is finished.

  In this case, as is clear from the above equation (5), in the second accumulated differential signal related to the third path and the fourth path, noise components such as background light are removed to detect the received light signal intensity Ip itself. Therefore, it is possible to accumulate signals until the amount of received light signal becomes sufficiently large. In addition, the intensity of the received light signal based on the reflected light from the measurement object 14 is monitored by the second accumulated differential signal through the third path and the fourth path. Therefore, when the relationship between the delay time t1 with respect to the light emission signal of the light reception signal and the pulse width t0 of the light emission signal is “t1 = t0 / 2”, the first accumulation difference due to the first path and the second path. Even if the motion signal is “0”, the distance to the measurement object can be measured by appropriate processing such as amplification of the second accumulated differential signal.

  However, if the threshold value is not reached even if the result of the differential operation between the Cch signal and the Dch signal is accumulated for a certain period of time, the measurement object 14 may be far from the measurable distance range or It can be considered that the reflectance of the measurement object 14 is extremely low, or the reflected light is extremely weak because the surface of the measurement object 14 is in a mirror surface state. In those cases, it is possible to prevent measurement results with low accuracy by determining that measurement is impossible and stopping signal accumulation.

On the other hand, the accumulated value of the differential calculation result between the Cch signal and the Dch signal (that is, the value of the second accumulated differential signal) is shorter than the other predetermined time shorter than the predetermined time. In this case, it is considered that the measurement object 14 is closer to the measurable distance range or the reflected light intensity is too strong because the surface of the measurement object 14 is in a mirror state. . In those cases, the values of the number of times of accumulation N ₁ and the number of times of accumulation N _{2 in} the above formulas (4) and (5) are small, so that the measurement accuracy per measurement greatly affects the measurement results. A sufficient averaging effect cannot be obtained, resulting in a decrease in measurement accuracy. In order to prevent this, it is preferable to determine that measurement is impossible even when the value of the second accumulated differential signal reaches the threshold value within the predetermined time.

The output signal (first accumulated differential signal) from the first differential operation unit 24 and the output signal (second accumulated differential signal) from the second differential operation unit 38 are input to the distance determination unit 40. The Then, the distance determination unit 40 calculates the ratio between the first accumulated differential signal S1 and the second accumulated differential signal S2, and uses the value of this ratio to measure the measurement object 14 according to the following equation (6). Distance L is obtained.

  As described above, in the present embodiment, the receiver 31 includes the first light receiving element 19, the first switch 20, the second switch 21, the first storage element 22, and the same as in the first embodiment. In addition to the second storage element 23, a second light receiving element 33, a third switch 34, a fourth switch 35, a third storage element 36 and a fourth storage element 37 are provided. The signal processing circuit unit 32 includes a second differential operation unit 38 in addition to the same first differential operation unit 24 as that in the first embodiment.

  The first light receiving element 19, the first switch 20, the second switch 21, the first storage element 22, the second storage element 23, and the first differential operation unit 24 are the same as in the case of the first embodiment. Thus, by performing a differential operation between the Ach signal and the Bch signal, a first accumulated differential signal from which noise components such as background light are removed as shown in the above equation (4) is obtained. Further, the second light receiving element 33, the third switch 34, the fourth switch 35, the third storage element 36, the fourth storage element 37, and the second differential operation unit 38 make a Cch signal as shown in the above equation (5). And a second accumulated differential signal which is an accumulated signal of a differential operation result between the Dch signal and the Dch signal.

  At that time, the open / close time in the third switch 34 and the fourth switch 35 is set to twice the pulse width t0 of the light emission signal. Therefore, the Cch signal and the Dch signal include the entire pulse signal generated by the reflected light beam 18 and depend only on the received light signal intensity Ip by performing a differential operation between the Cch signal and the Dch signal. The second stored differential signal can be obtained. As a result, the distance determination unit 40 can obtain the distance L to the measurement object 14 using the above equation (6) based on the first accumulated differential signal and the second accumulated differential signal. is there.

  Therefore, according to the present embodiment, it is possible to accurately measure the distance L to the measurement object 14 even in an environment where the received light signal intensity Ip is unknown and the background light is strong. It is.

  Further, the integration time for obtaining the first accumulation differential signal and the second accumulation differential signal is determined by a comparison result between the result of the second accumulation differential operation and a preset threshold value. . Therefore, even when the relationship between the delay time t1 of the light emission signal to the light emission signal and the pulse width t0 of the light emission signal is "t1 = t0 / 2" and the first accumulated differential signal is "0", the measurement object It is possible to measure distances up to 14. Further, since the light reception signal intensity Ip itself is detected by the second accumulation differential signal, the differential signal can be accumulated until the amount of the light reception signal becomes sufficiently large.

  In the second embodiment, the switching time of the third switch 34 and the fourth switch 35 is set to twice the pulse width t0 of the light emission signal. However, the present invention is not limited to twice the pulse width t0, and the Cch signal and Dch signal include the entire pulse signal by the reflected light beam 18 as long as it is twice or more the pulse width t0. That is, the same effect can be obtained. However, when the switching time of the third switch 34 and the fourth switch 35 is longer than twice the pulse width t0, the charge accumulation amount of noise components such as background light only increases. Therefore, the switching time of the third switch 34 and the fourth switch 35 is most preferably twice the pulse width t0 of the light emission signal.

  In the second embodiment, two sets of units each including a receiver and a differential operation unit are provided, and on / off is controlled by the open / close signals shown in FIGS. 5 (c) and 5 (d). The first switch 20 and the second switch 21 and the third switch 34 and the fourth switch 35, which are controlled to be turned on / off by the open / close signals shown in FIGS. 5 (h) and 5 (i), in different units. I am trying to configure it. However, even with the optical distance measuring device shown in FIG. 1 having only one set of the receiver and the differential operation unit, an operation equivalent to the operation according to the timing chart shown in FIG. 5 can be executed. it can.

  In that case, in the configuration shown in FIG. 1, when the output signal from the comparator 25 is further received and the level of the output signal is inverted from “H” to “L”, the modulation signal generator 15 The cycle of the open / close signal supplied to the first switch 20 and the second switch 21 is changed to change the switching timing of each path of each switch, and the charge accumulated in the first storage element 22 and the second storage element 23 A control unit for erasing data is provided.

  The control unit first controls the modulation signal generator 15 to output the opening / closing signals shown in FIGS. 5 (h) and 5 (i), and the first switch 20 and the second switch 21 are turned on. It is turned on / off at the timing shown in FIG. 5 (h) and FIG. 5 (i). In that case, the differential operation unit 24 operates in the same manner as the second differential operation unit 38 illustrated in FIG. 4 and outputs the second accumulated differential signal represented by the above equation (5). Thus, processing in the first time zone is performed. Then, the comparator 25 inverts the level of the output signal from “H” to “L” when | the value of the second accumulated differential signal |> Vth.

Then, the control unit erases the electric charges accumulated in the first accumulation element 22 and the second accumulation element 23 and also performs the first time period from the value of the second accumulation differential signal and the threshold value Vth. The time length of the second time zone following the first time zone is determined based on the accumulation time (number of times of accumulation N ₂ ). Further, the modulation signal generator 15 is caused to output the opening / closing signals shown in FIGS. 5 (c) and 5 (d), and the first switch 20 and the second switch 21 are set in FIGS. 5 (c) and 5 (d). ) Is turned on / off at the timing shown in FIG. In that case, the differential operation unit 24 operates in the same manner as the first differential operation unit 24 shown in FIG. 4 and outputs the first accumulated differential signal shown in the above equation (4). Thus, the process in the second time zone is performed.

  Thus, when the processing of the second time period is completed, the distance determination unit 26 calculates the ratio between the first accumulated differential signal S1 and the second accumulated differential signal S2, and calculates the value of this ratio. Using this, the distance L to the measurement object 14 is obtained according to the equation (6).

  In this case, the second embodiment is such that the light receiving signal intensity Ip is unknown and the distance L to the measurement object 14 can be measured with high accuracy even in an environment with strong background light. The same effect as that of the above embodiment can be achieved by an optical distance measuring device having only one set of units including the receiver and the differential operation unit. Therefore, it is possible to reduce the size of the optical distance measuring device including the two units in the second embodiment.

  In each of the above embodiments, a pulse wave is used as the modulation signal applied to the light emitting element 16. This is because, when the modulation signal is a pulse wave, the waveform of the light reception signal is also a pulse wave, and the light reception signal during reception of the reflected signal light maintains the same light reception signal intensity (a constant value). As a result, the charge accumulation amount of each signal of Ach to Dch changes in proportion to the distance L to the measurement object 14, and the charge accumulation amount of each signal and the distance L over the entire range within the distance measurement possible range. It exhibits linearity. Therefore, the resolution can be made constant over the entire distance measurement range.

  On the other hand, when a triangular wave or a sawtooth wave is used as the modulation signal, the waveform of the light reception signal is a linear function of time, and the charge accumulation amount of each signal of Ach to Dch is a quadratic function of time. . As a result, the resolution becomes coarse and dense in the distance measurement range. Therefore, it is preferable to properly use a pulse wave, a triangular wave or a saw wave as the modulation signal depending on the application.

It is a block diagram in the optical distance measuring device of this invention. It is a timing chart which shows operation | movement of the optical distance measuring device shown in FIG. It is the figure which showed the light emission signal shown to Fig.2 (a), and the accumulation | storage differential signal shown in FIG.2 (g) over several periods. FIG. 2 is a block diagram of an optical distance measuring device different from FIG. 1. 5 is a timing chart showing the operation of the optical distance measuring device shown in FIG. 4. It is a schematic cross section which shows an example of the light receiver which has a photogate structure. 7 is a timing chart showing the operation of the photogate shown in FIG. 6. 8 is a timing chart when there is background light in FIG. 7. It is a timing chart which shows operation | movement of the conventional distance image sensor.

Explanation of symbols

11 ... Transmitter,
12, 31 ... receiver,
13, 32 ... signal processing circuit section,
14 ... measurement object,
15 ... modulation signal generator,
16 ... light emitting element,
17, 18 ... light beam,
19, 33 ... light receiving element,
20, 21, 34, 35 ... switch,
22, 23, 36, 37 ... storage element,
24, 38 ... differential operation unit,
25,39 ... comparator,
26, 40 ... Distance determination unit.

Claims (8)

A transmitter for transmitting light in synchronization with a modulation signal having a repetition frequency;
A receiver that receives the light transmitted from the transmitter and reflected by the measurement object, and outputs a signal corresponding to the received optical signal;
A signal processing unit for processing the signal output from the receiver,
The receiver is
A light receiving element that converts the received optical signal into an electrical signal;
A switch that switches an electrical signal from the light receiving element to at least two paths at a predetermined timing synchronized with the modulation signal;
A plurality of storage elements that are arranged in each of the paths and store electrical signals switched to the paths;
The signal processor is
A calculation unit for calculating the difference between the electrical signals for each path accumulated in the respective storage elements;
An accumulation time determination unit that determines an accumulation time of the accumulation element based on a calculation result by the calculation unit;
A distance determination unit that determines a distance to the measurement object using a calculation result of the electric signal stored in each storage element by the storage time determined by the storage time determination unit using the calculation result by the calculation unit. An optical distance measuring device. The optical distance measuring device according to claim 1.
The optical distance measuring device, wherein the accumulation time determination unit determines the accumulation time by comparing a calculation result of the calculation unit with a threshold value. The optical distance measuring device according to claim 2.
The accumulation time determination unit sets the accumulation time Tsum when the time until the calculation result by the calculation unit reaches the threshold is t, the repetition frequency of the modulation signal is T, and the accumulation time is Tsum. An optical distance measuring device characterized in that the value is determined to satisfy the following relationship.
Tsum = m × T
Where m is the smallest integer satisfying m> t / T The optical distance measuring device according to claim 2,
The receiver and the signal processing unit include two units including a set of the light receiving element, the switch, the storage element, and the arithmetic unit,
The switching interval of the switch in the first unit that is one of the two units is at least twice the switching interval of the switch in the second unit that is the other of the two units,
The storage time determination unit determines the storage time of the storage element of the first unit and the storage element of the second unit by comparing the calculation result of the calculation unit of the first unit with a threshold value. ,
The distance determination unit is configured to determine a distance to the measurement object using a calculation result of the calculation unit of the first unit and a calculation result of the calculation unit of the second unit. Optical distance measuring device. The optical distance measuring device according to claim 4,
An optical distance measuring device characterized in that an accumulation time in the storage element of the first unit is equal to an accumulation time in the storage element of the second unit. The optical distance measuring device according to claim 1.
A control unit for controlling the switching timing of the path by the switch and erasing the electrical signal stored in each storage element,
The control unit
Causing the switch to switch the path according to the first timing;
When the storage time of the storage element is determined by the storage time determination unit, the electrical signal stored in each storage unit is erased, and then the switch is switched by the second timing. The electric signal is accumulated in each accumulator for a time based on the determined accumulation time,
The distance determination unit includes a calculation result by the calculation unit when the switch switches the route at the first timing, and a result when the switch switches the route at the second timing. An optical distance measuring device, wherein a distance to the measurement object is determined using a calculation result obtained by the calculation unit. The optical distance measuring device according to claim 2,
The accumulation time determination unit makes the accumulation time undecidable when the calculation result by the calculation unit does not reach the threshold value within a first predetermined time,
The optical distance measuring device, wherein the distance determination unit is configured to be unable to determine the distance to the measurement object in response to the determination of the accumulation time by the accumulation time determination unit. The optical distance measuring device according to claim 2,
The accumulation time determination unit makes the accumulation time undecidable when the calculation result by the calculation unit reaches the threshold value within a second predetermined time,
The optical distance measuring device, wherein the distance determination unit is configured to be unable to determine the distance to the measurement object in response to the determination of the accumulation time by the accumulation time determination unit.

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