Distance measuring system and distance measuring method

• US 8,508,720 B2
• Filed: 10/02/2009
• Issued: 08/13/2013
• Est. Priority Date: 10/20/2008
• Status: Active Grant

• Current Assignee: Honda Motor Co., Ltd. (Honda Motor Company)
• Original Assignee: Honda Motor Co., Ltd. (Honda Motor Company)
• Inventors: Kamiyama, Tomoyuki
• Primary Examiner(s): Alsomiri, Isam
• Assistant Examiner(s): Abraham, Samantha K
• Application Number:
  US13/123,789
• Publication Number:
  US 20110194099A1
• Time in Patent Office: 1,411 Days
• Field of Search: 356/4.01, 356/5.01, 356/5.03
• US Class Current: 356/5.03
• CPC Class Codes: G01C 3/08 Use of electric radiation detectors
• CPC Class Codes: G01S 17/10 using transmission of interrupted, pulse-modulated waves determination of distance by phase measurements G01S17/32
• CPC Class Codes: G01S 17/894 3D imaging with simultaneous measurement of time-of-flight at a 2D array of receiver pixels, e.g. time-of-flight cameras or flash lidar
• CPC Class Codes: G01S 7/4876 by removing unwanted signals G01S7/495 takes precedence

Abstract

A distance measuring system and a distance measuring method which use a time-of-flight (TOF) method. The distance measuring system obtains a reference light quantity of reflected light which is a cumulative light quantity of the reflected light during a reference period, obtains a measured light quantity of the reflected light which is a cumulative light quantity of the reflected light during a measurement period, and calculates, on the basis of a ratio of the measured light quantity of the reflected light to the reference light quantity of the reflected light and a ratio of the reflected light incident period to the reference period, a reflected light incident period that is a period which is included in the measurement period and during which the reflected light is incident upon photoelectric conversion elements of a light-receiving device. Then, the distance measuring system calculates the distance between the distance measuring system and an object on the basis of the reflected light incident period.

Claims

1. A rangefinding system comprising:

a light-emitting device for emitting pulsed light toward an object;

a light-detecting device for detecting reflected light from the pulsed light, and producing an output signal depending on the energy of reflected light that is detected;

a control device for controlling the light-emitting device and the light-detecting device; and

an arithmetic device for calculating a distance up to the object according to a time-of-flight process using the output signal from the light-detecting device;

the light-detecting device further comprising;

a photodetector for detecting the reflected light and generating photoelectrons by the detected reflected light;

first through fourth capacitors for storing the photoelectrons from the photodetector;

a photoelectron discharger for discharging the photoelectrons from the photodetector;

first through fourth gate electrodes disposed between the photodetector and the first through fourth capacitors, for allotting the photoelectrons with respect to the first through fourth capacitors in synchronism with emission of the pulsed light; and

a fifth gate electrode disposed between the photodetector and the photoelectron discharger, for controlling supply of the photoelectrons from the photodetector and the photoelectron discharger;

wherein, if it is assumed that a time at which the pulsed light starts to be emitted is referred to as time Teu,

a time at which the pulsed light stops being emitted is referred to as time Ted,

a time at which the reflected light stops being exposed to the photodetector is referred to as time Trd,

respective times at which the first through fourth gate electrodes are opened are referred to as times Tg1u, Tg2u, Tg3u, and Tg4u,

respective times at which the first through fourth gate electrodes are closed are referred to as times Tg1d, Tg2d, Tg3d, and Tg4d,

a period from the time Tg1u to the time Tg1d is referred to as period P1,

a period from the time Tg2u to the time Tg2d is referred to as period P2,

a period from the time Tg3u to the time Tg3d is referred to as period P3,

a period from the time Tg4u to the time Tg4d is referred to as period P4,

a period from the time Tg4d to the time Trd is referred to as period Psr,

a photoelectron quantity stored in the first capacitor during the period P1 is referred to as photoelectron quantity Q1,

a photoelectron quantity stored in the second capacitor during the period P2 is referred to as photoelectron quantity Q2,

a photoelectron quantity stored in the third capacitor during the period P3 is referred to as photoelectron quantity Q3,

a photoelectron quantity stored in the fourth capacitor during the period P4 is referred to as photoelectron quantity Q4,

a period during which the pulsed light is emitted, reflected by the object, and returned as the reflected light is referred to as round trip time Δ

P, and

a distance between the rangefinding system and the object is referred to as distance D,

then the control device controls emission of the pulsed light from the light-emitting device and opening and closing of the first through fourth gate electrodes so as to satisfy the following relationships;

(1) P1=P3;

(2) P2=P4; and

(3) Tg1u<

Tg1d≦

Tg2u<

Tg2d≦

Teu<

Tg3u<

Tg3d≦

Tg4u≦

Ted<

Tg4d, or Teu<

Tg3u<

Tg3d≦

Tg4u≦

Ted<

Tg4d<

Tg1u<

Tg1d≦

Tg2u<

Tg2d; and

the control device opens the fifth gate electrode to discharge the photoelectrons when all of the first through fourth gate electrodes are closed;

the arithmetic device acquires light energy information of the reflected light during the period P3 based on the difference between the photoelectron quantity Q3 stored in the third capacitor corresponding to ambient light and the reflected light, and the photoelectron quantity Q1 stored in the first capacitor corresponding to the ambient light;

the arithmetic device acquires light energy information of the reflected light during the period Psr based on the difference between the photoelectron quantity Q4 stored in the fourth capacitor corresponding to the ambient light and the reflected light, and the photoelectron quantity Q2 stored in the second capacitor corresponding to the ambient light;

the arithmetic device calculates the round trip time Δ

P based on a ratio of the light energy information of the reflected light during the period P3 and the light energy information of the reflected light during the period Psr, and a ratio of the period P3 and the period Psr; and

the arithmetic device measures the distance D based on the round trip time Δ

P.

2. The rangefinding system according to claim 1, wherein when the time Ted and the time Tg4u are equal to each other, the arithmetic device calculates the round trip time Δ

P according to the following equation (1);

Δ

P={(Q4−

Q2)/(Q3−

Q1)}×

P3 

 

(1)

and when the time Ted is subsequent to the time Tg4u, the arithmetic device calculates the round trip time Δ

P according to the following equation (2);

Δ

P=[(Q4−

Q2)/(Q3−

Q1)]×

P3−

(Ted−

Tg4u) 

 

(2)

3. The rangefinding system according to claim 1, wherein the control device controls the light-emitting device so as to emit the pulsed light a plurality of times during each measuring cycle;

and

the arithmetic device calculates the round trip time Δ

P using the photoelectron quantities Q1 through Q4, after the photoelectrons have been stored a plurality of times in each of the first through fourth capacitors.

4. The rangefinding system according to claim 1, wherein the light-emitting device sets a period during which the pulsed light is emitted to at most 1 percent of each measuring cycle;

and

the light-detecting device sets a period during which the first through fourth gate electrodes are opened to at most 1 percent of each measuring cycle.

5. The rangefinding system according to claim 1, wherein the light-detecting device further comprises first through fourth amplifiers having respective gates connected respectively to the first through fourth capacitors, for outputting voltages depending on potentials across the first through fourth capacitors.

6. The rangefinding system according to claim 1, wherein the light-detecting device further comprises a power supply and sixth gate electrodes for resetting potentials across the first through fourth capacitors.

7. The rangefinding system according to claim 1, wherein the photodetector comprises one of a photodiode, a pinned photodiode, and a photogate.

8. The rangefinding system according to claim 1, wherein each of the first through fourth capacitors comprises one of an MIM capacitor, a MOS capacitor, a pinned photodiode, and a PN junction.

9. The rangefinding system according to claim 1, wherein the light-detecting device further comprises a light shield for blocking light from entering into the first through fourth gate electrodes and the first through fourth capacitors.

10. The rangefinding system according to claim 1, wherein the light-emitting device includes a light emitter comprising one of a light-emitting diode, a laser diode, and a semiconductor laser bar.

11. The rangefinding system according to claim 1, wherein the light-emitting device includes a light emitter comprising an array of semiconductor laser bars.

12. The rangefinding system according to claim 1, wherein the light-detecting device includes one of a line sensor and an image sensor having a plurality of pixels each comprising the photodetector, the first through fourth capacitors, the photoelectron discharger, and the first through fifth gate electrodes.

13. A rangefinding method carried out by a rangefinding system including a light-emitting device for emitting pulsed light toward an object, a light-detecting device for detecting reflected light from the pulsed light and producing an output signal depending on the energy of reflected light that is detected, a control device for controlling the light-emitting device and the light-detecting device, and an arithmetic device for calculating a distance up to the object according to a time-of-flight process using the output signal from the light-detecting device, the rangefinding method comprising the steps of:

determining reflected light reference energy, which represents cumulative light energy of the reflected light during a reference period;

determining measured reflected light energy, which represents cumulative light energy of the reflected light during a measuring cycle;

calculating a reflected light incident period, which represents a period during which the reflected light is exposed to a photodetector of the light-detecting device within the measuring cycle, based on a ratio of the measured reflected light energy and the reflected light reference energy, and a ratio of the reflected light incident period and the reference period; and

calculating a distance between the rangefinding system and the object based on the reflected light incident period.

14. The rangefinding method according to claim 13, wherein the step of determining the reflected light reference energy further comprises the steps of:

determining ambient light reference energy, which represents cumulative light energy of ambient light during a first reference period;

determining combined light reference energy, which represents cumulative combined light energy of the ambient light and the reflected light during a second reference period, which has the same length as the first reference period; and

calculating the reflected light reference energy during the second reference period by subtracting the ambient light reference energy from the combined light reference energy; and

wherein the step of determining the measured reflected light energy further comprises the steps of;

determining measured ambient light energy, which represents cumulative light energy of the ambient light during a first measuring cycle;

determining measured combined light energy, which represents cumulative combined light energy of the ambient light and the reflected light during a second measuring cycle, which has the same length as the first measuring cycle; and

calculating the measured reflected light energy during the second reference period by subtracting the measured ambient light energy from the measured combined light energy.

15. The rangefinding method according to claim 14, further comprising the steps of:

setting a plurality of first reference periods, a plurality of second reference periods, a plurality of first measuring cycles, and a plurality of second measuring cycles in each measuring cycle;

determining the ambient light reference energy as a sum of cumulative light energy of the ambient light respectively during the first reference periods;

determining the combined light reference energy as a sum of cumulative combined light energy respectively during the second reference periods;

determining the measured ambient light energy as a sum of cumulative light energy of the ambient light respectively during the first measuring cycles; and

determining the measured combined light energy as a sum of cumulative combined light energy respectively during the second measuring cycles.

16. The rangefinding method according to claim 13, wherein each of a period during which the pulsed light is emitted, the reference period, and the measuring period is set to at most 1 percent of each measuring cycle.