Fig. 1. System con"guration.

JSAE Review 20 (1999) 549}554

Development of the scanning laser radar for ACC system

Keiji Osugi!, Kunihiro Miyauchi!, Nobuyuki Furui", Hironori Miyakoshi"!Body Electronics Components Engineering Department 3, DENSO CORPORATION, Showa-cho 1-1, Kariya-shi, Aichi, 448-8661 Japan

"Design Dept. No. 21, Electronics Engineering Division 2, Component System Development Center, Toyota Motor Corporation, Toyota-cho 1,Toyota-shi, Aich, 471-71 Japan

Received 26 March 1999

Abstract

In recent years, the development of the adaptive cruise control (ACC) system with the aim to improve driving convenience andcomfort, has been progressing. To put this ACC system to practical use, the elimination of unexpected acceleration or decelerationcaused by incorrect judgment is highly demanded. From 1997, the Laser Radar ACC system has been produced for the domesticversion of the Lexus LS400. It provides high recognition capability with a two-dimensional scanning technology at an a!ordableprice. This paper introduces this Laser Radar with a high-performance two-dimensional scanning technology. ( 1999 Society ofAutomotive Engineers of Japan, Inc. and Elsevier Science B.V. All rights reserved.

1. Introduction

The many functions that come with today's vehiclesincrease the burden of driving. More than ever, with theincrease of tra$c in recent years, driving conditions forcethe driver to maintain a certain distance from the vehiclein front. In this environment, the driver is placed undera great burden. Therefore, we are proposing an ACCsystem that improves driving convenience and comfortand signi"cantly decreases the driving burden for thedriver.

Accurate detection of the preceding vehicle is requiredif this system is to be put to practical use. This paperdiscusses a high-performance two-dimensional scanninglaser radar for the ACC system and introduces examplesof its application.

2. System con5guration

The system con"guration is shown in Fig. 1.The system is similar to a conventional cruise control

system, with additional products consisting of a LaserRadar, distance control ECU, headway time select switchand display. The Laser Radar is mounted under thebumper, the distance control ECU is inside the right-sidefender, the headway time select switch is on the steeringwheel, and the display is on the instrument panel. Thedisplay shows the system status, as shown in Fig. 1.

3. Overview of system action

We will now provide a simple overview of an ACCsystem action, as shown in Fig. 2. Firstly, the forwardrecognition sensor detects multiple objects in front of thevehicle, measures the distance to those objects, and calcu-lates their relative speed and position. It then determineswhether those objects are moving.

Based on the measurements provided by the sensor,the controller selects the preceding vehicle. It then con-trols to match the vehicle speed with the target headwaydistance which is selected by the driver. The success of the

0389-4304/99/$20.00 ( 1999 Society of Automotive Engineers of Japan, Inc. and Elsevier Science B.V. All rights reserved.PII: S 0 3 8 9 - 4 3 0 4 ( 9 9 ) 0 0 0 3 8 - 7 JSAE9936698

Fig. 2. System control algorithm.

Fig. 3. Structural characteristics of Japanese expressways.

Fig. 4. Tra$c characteristics of Japanese expressways.

ACC system depends on how accurately the precedingvehicle can be detected. Therefore, an accurate forwardrecognition sensor is required.

There are a number of forward recognition sensors,including laser radar, millimeter wave radar, image sen-sors and ultrasonic wave sensors. When costs, requiredperformance and technical feasibility are taken into con-sideration, the optimum sensor currently available is thescanning laser radar. Therefore, a scanning laser radarwas used in the development of this system.

4. Scanning laser radar

4.1. Specixcation background

This section explains how we determined our LaserRadar speci"cations. The ACC system will be usedprimarily on the expressway. The characteristics of ex-pressways in Japan are shown in Figs. 3 and 4.

We had to determine the azimuth and elevation angleof the detection area for the forward recognition sensor.These calculations require data on the structural charac-teristics of the expressway, such as minimum curvatureand maximum incline of the road surface. We know that90% of all Japanese expressways possess a curvature thatis larger than R350 m and uphill and downhill slopes thatare within 4%. The detection area of the radar shouldcover this area.

Next, we collected data on steady tra$c #ow, such asminimum and maximum speeds, headway time, acceler-

ation speed, and deceleration speed. The data were usedto determine the minimum detection distance for theforward recognition sensor. The following section ex-plains how the minimum detection distance was cal-culated. Refer to Fig. 4.

The legal vehicle speed is restricted to 50}100 km/h inJapanese expressways. The average headway time is ap-proximately 2 s, but occasionally increases to approxim-ately 3 s according to various situations. As shown in the`Deceleration and Accelerationa table, 90% of acceler-ation and deceleration are within $0.25 G. Therefore,to assume the use of following control in light tra$c, thedetection distance for the forward recognition sensordoes not need to be very long, approximately 100 m,which corresponds to a headway time of 3 s is su$cient.However, the structural and tra$c characteristics of ex-pressways are di!erent in each country. It is thereforenecessary to review these speci"cations for each country.

4.2. Optical design

When developing the laser radar, one of the mainrequirements besides recognition accuracy, is small size.The light collecting lens occupies the largest volumewithin the optical system. To reduce the size of this partwe had to increase the power density that returns to it.The relationships between the beam form, the light emis-sion power and the light collecting lens area in the opticalsystem are shown in Fig. 5. The receiving power relation-ships are shown next in Eqs. (1)}(3).

PR"k

EkRPE(S

TS~1T0

) (SRS~1R0

)'P0, (1)

PR"k

EkRPE(S

T/(R

2tan h

7tan h

))) (S

R/(pR

2tan2(h

5/2))), (2)

SR"P

0(k

EkRPE)~1(R4p tan2(h

5/2))tan h

7tan h

)/S

T(3)

where KR

is the reception e$ciency, KE

the emissione$ciency, P

Rthe light reception power, P

Ethe light

emission power, ST

the target area, SR

the light collectionlens area, S

T0the light emission area at target, S

R0the

light reception area at sensor, P0

the necessary light

550 K. Osugi et al. / JSAE Review 20 (1999) 549}554

Fig. 5. Principle of the laser radar.

Fig. 6. 2-dimensional scanning.

Fig. 7. Scanning method.

Fig. 8. Recognition method.

Fig. 9. Scanning data sample.

reception power, R the range to target, h7, h

)the depar-

ture angle and htthe re#ection angle of target.

Based on these equations, there are two ways toachieve this:

(1) Increasing the light emission power.(2) Narrowing the outgoing beam and increasing the

power density.

For the reason reliability, method (1) was not usedhere. As shown in Fig. 6, by two-dimensional scanning,the outgoing beam could be narrowed and power densityincreased.

4.3. Two-dimensional scanning method

Two-dimensional scanning has been achieved witha simple con"guration that uses a DC motor and poly-gon mirror. Rotation of the polygon mirror enables the

beam to scan to the left and right. The beam verticalde#ection was enabled by changing the inclination angleof each polygon mirror surface, as shown in Fig. 7.

4.4. Object recognition method

Fig. 8 shows the recognition methods. With this sen-sor, 630 points of re#ection data from the front of thevehicle can be detected reticulately (in a mesh pattern) inthe vertical and horizontal directions by the two dimen-sional scanning beams. First, the sensor classi"es there#ection data in the horizontal direction into groupswhen the data are judged as those of the same object bytheir distance and direction ("angle). Next, also in thevertical direction, this sensor classi"es the group data ineach horizontal direction into groups in the same way.After calculating the relative speed of each group, itdistinguishes whether those groups are moving objects ornot. Next it calculates the position relationship with thehost driving path that is presumed by the steering sensorangle information. These processes are in the LaserRadar, and multiple object information is transmitted tothe Distance Control ECU. In the Distance ControlECU, a target vehicle on the host path is determined bythe detected multiple object information.

An example of two-dimensional recognition is shownin Fig. 9. In this situation, data for the six horizontal faces

K. Osugi et al. / JSAE Review 20 (1999) 549}554 551

in grouped. Next, vertical data is grouped, and at thesame time the relative speed is calculated to determinewhether the preceding object is moving or not.

4.5. Circuits

Fig. 10 shows a circuit block diagram of the laserradar. The scanner rotates at a constant speed usinga DC motor. The CPU generates light emitting signal ofa laser diode according to the scanner rotation. There#ected light is collected on the photo diode using thelight collecting lens and converted into an electric signal.This signal is ampli"ed in the analog circuit and enteredinto the time measurement LSI. Based on its measureddata, the CPU calculates the distance and recognizesobjects.

The key hardware technologies in developing this laserradar were the laser diode and the time measurementLSI. Both of these parts a!ect radar performance greatly,so we developed them at DENSO.

Figs. 11 and 12 shows the outline and speci"cations ofthe laser diode and the time measurement LSI.

The laser diode which determines the object detectionperformance and quality is an important part of the laserradar. We could not obtain a Laser Diode with enoughperformance and lower cost in the market. Therefore, wedeveloped it in-house, and achieved high-power with lowcurrent, and secured long life. In addition, the devicestructure of the Laser Diode was designed to obtaina stable light emission and a beam shape easily matchedto the lens. As a result, a high-quality recognition LaserRadar was achieved.

There are a number of ways in which microscopic timecan be measured. For example, there is direct measure-ment using a super-fast counter with a resolution of 1 nsor less, Miller integration, or a method that averagesmultiple measurements using a low resolution counter.However, all these methods have problems of cost orprecision and are di$cult to adopt for mass production.

Fig. 10. Block diagram of the laser radar.

Fig. 11. Laser diode.

Fig. 12. Principle of the time interval measurement.

Therefore, we developed our own technology } a timemeasurement LSI, which uses gate delay time to obtainvery precise measurements.

Use of this LSI has improved the accuracy of distancemeasurement for each light emission and averaging hasbecome unnecessary. It has also reduced the number oflight emissions for the laser diode which gives the diodea longer life. At the same time, highly precise recognitionhas become possible.

4.6. Structure and specixcations

The internal structure of the developed sensor and itsspeci"cations are shown in Fig. 13.

4.7. The advantage of two-dimensional scanning laserradar

The reason for using two-dimensional scanning, be-sides the body size reduction, was to improve recogni-tion performance. It reduces the loss of sight of vehicles

552 K. Osugi et al. / JSAE Review 20 (1999) 549}554

Fig. 13. Construction and speci"cations of the scanning laser radar.

Fig. 14. Special feature of two-dimensional scanning laser radar.

Fig. 15. Comparison of detection error between one- and two-dimen-sional scanning.

Fig. 16. System con"guration.

due to hills and dips in the road, simpli"es identi"cationof tra$c sign boards along the road and vehicles travellingbeneath these, and excludes white line data which is easyto interpret as a moving object. These were di$cult withthe one-dimensional scanning. This is shown in Fig. 14.

Fig. 15 shows the comparison of detection error be-tween one-dimensional scanning and two-dimensionalscanning. Two dimensional scanning enabled a greatreduction in the target loss and the wrong target detec-tion under the conditions shown in Fig. 14.

5. Examples of application

To realize ACC function, only two parts, Laser radarand Distance Control ECU, are newly added to this

system. Other composition parts are used in commonwith other system's parts. For example, the steering sen-sor signal of VSC was used by the ACC system. Thetypical function outline of the ACC system is explainedas follows (Fig. 16).

Based on the curve radius which is sent from theDistance Control ECU, the laser radar calculates andsends information, for example relative position and rela-tive speed, about multiple objects in front of the vehicleto the Distance Control ECU.

The Distance Control ECU receives the steering angledata from the VSC ECU via the engine ECU and calcu-lates the curve radius. This data is then sent to the laserradar. The Distance Control ECU receives informationabout multiple preceding objects from the laser radar,and uses this information and its own vehicle speed todetermine whether there is a preceding vehicle or not.A target acceleration or deceleration level to control theHeadway Distance is calculated for the selected preced-ing vehicle and the data is sent to the engine ECU. At thesame time, a downshift timing or proximity alarm timingis calculated and each data request is sent to the engineECU.

The engine ECU acts as a cruise actuator and controlsthe electronic throttle in accordance with the target ac-celeration or deceleration data. It also controls the elec-tronic control transmission (ECT) solenoid and shifts thegears down. It communicates with the vehicle stabilitycontrol (VSC) ECU and receives steering sensor data. Atthe same time it sends proximity alarm requests from theDistance Control ECU and sounds a buzzer via theVSC-ECU.

In this ACC system, in-vehicle body LAN is used tosend the display data to the instrument cluster and toreceive switches such as the wiper switch via engine ECU.

6. Evaluation of burden level

We compared the burden level using the Flicker test,which is one of the methods to estimate human fatigue

K. Osugi et al. / JSAE Review 20 (1999) 549}554 553

Fig. 17. Comparison of fatigue level between ACC system and conven-tional cruise control system.

level by eyesight response to a #ickering light. Fig. 17shows the changes of the #ickering frequency that thedriver can detect before and after driving. We couldreduce the change of the #ickering frequency from!3.6 Hz in the conventional system to !0.4 Hz in theACC system after 300 km driving. That is because in theconventional system a driver needs to keep watching thepreceding vehicle to recognize the relative speed, but inthe ACC system the driver lets the system recognize thedetails of the situation and can a!ord to move his eyes tothe right and left. Therefore, we think that the driver willnot get tired.

7. Conclusion

We have developed a two-dimensional scanning laserradar as a forward recognition sensor for ACC systems.By using two-dimensional scanning we have increasedthe vertical detection area, and have greatly reducedwrong target detection or target loss by separating signs

and vehicles or by excluding white lines. Therefore recog-nition performance has been greatly improved. This laserradar was actually adopted in the ACC system of TOY-OTA CELSIOR (Domestic version of Lexus LS400), andwas mass produced in August 1997. This ACC systemhas gained excellent praise from the marked and theadoption for other models has been extended to theTOYOTA PROGRE'S (New model in May 1998).

We would like to continue to improve its performanceand reduce its cost for use in future ACC systems.

8. For Further Reading

The following references are also of interest to thereader: [1}3].

Acknowledgements

We would like to thank the Basic Research O$ce, theIc Division, Development Division, Electronic Techno-logy Departments 1, the Quality Control Division and allother related divisions for their cooperation.

References

[1] Furui, N., Miyakoshi, H., Noda, M., Miyaucho, K., Developmentof a scanning laser radar for ACC, SAE SP-1332, No. 980615, pp.71}76 (1998).

[2] Watanabe, T., Makino, Y., Ohtsuka, Y., Akita, S., Hattori, T.,A CMOS time-to-digital converter LSI with half-nanosecond res-olution using a ring gate delay line, IEICE, Vol. E76-C, No. 12, pp.1774}1779 (1993).

[3] Hashiguchi, M., Preview distance control, Automobile Eng. Vol.45, No. 6, pp. 84}100 (1996).

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