Vol.3 No.4 2011

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Research paper : How car navigation systems have been put into practical use (H. Ikeda et al.)−282−Synthesiology - English edition Vol.3 No.4 (2011) 2) Sensor hardware and software conceptSensors are required to detect the travel distance and rotation angles. Requirements for use in automobiles include low cost of the order of hundredths to thousandths of the cost of sensors for submarines or aircraft, relatively good accuracy, no need for servicing, and ruggedness during the service life of the vehicle which is some ten years.Travel distance and rotation angles are determined based on the average number of revolutions of both wheels and the differences between them. Since Sumitomo Electric manufactured anti-lock braking systems (ABS), we were familiar with ABS wheel speed sensors and asked automakers to allow us to use them as travel distance and rotation angle sensors. Since rotation angle sensors are incapable of indicating the absolute direction, a magnetic field sensor was also incorporated. Wheels slip, so the number of revolutions of a wheel differs from the actual travel distance. Magnetic field sensors are subject to substantial errors depending on the location, such as a point close to a DC-driven electric train. Thus, it became critically important to develop software to correct these errors by map-matching. To test the software, we drove actual vehicles on various courses and conducted simulations using data acquired from the actual driving. Nonetheless, it had turned out through the test driving and simulation that the accuracy of the rotation angles determined from the difference between both wheels was inadequate. Eventually, we developed an optical fiber gyro[6], which will be discussed later. Other components were selected for automotive use from those used widely in automobiles and that were sufficiently reliable and durable under vibration and high and low temperature conditions. To display a map and the optimal route, we first used a six-inch CRT (some vehicles already had them installed). Maps were recorded on a CD-ROM. In 1989, a NAVS incorporating these systems was adopted for the Cedric and Cima (Fig. 2). Today, the CRT and CD-ROM have been replaced by an LCD unit and either a DVD or a hard disk.NAVS produced around 1990 used six-inch CRTs. The NAVS was positioned where audio and air-conditioner controls had been previously installed. Audio and air-conditioner control functions were included in NAVS features.3) Commercialization conceptWe used ABS sensors as wheel speed sensors for our NAVS. Accordingly, our NAVS could be installed only in vehicles that had an ABS system, while at that time, few consumer vehicles were equipped with ABS. Requirements concerning the magnetic field sensor included: erasing magnetic effects of the iron body by the automaker during production; recording sensor constants by revolving and checking the magnetic orientation; and subsequent automatic correction by wheel speed sensor readings and map-matching.In location detection by map-matching, it is basically necessary to set a starting point. In addition, the actual road may differ from the map and the vehicle location may be lost during driving. On such an occasion, it becomes necessary to re-input the starting point where the vehicle location is identifiable. Since this would have to be carried out by the user, it was necessary to reduce the frequency of such occasions and to make operation simple.To meet these requirements, it was necessary to make displayed information easy to read and the starting point setting operation simple. Consequently, in addition to the software for vehicle location detection and route navigation guidance, the software for information display and NAVS operations gained monumental importance.This NAVS system required a digital road map for map-matching in addition to a map for display. Since such a map did not exist, we decided to develop one ourselves (explained later).In addition, the NAVS display played an important role as a vehicle information display unit. Software was developed to display vehicle information. Target prices were roughly ¥50,000, ¥100,000 and ¥200,000. Using a large color display, no significant cost reduction could be expected. Basically, the NAVS was an expensive commodity. Although aftermarket units were available, their inexpensive yet small displays were subject to poor readability. They were not suitable to be marketed as original equipment manufacture (OEM) systems that needed to ensure safety, and therefore we excluded them from the options.(2) Developing orientation detection gyros1) Developing an optical fiber gyro[6]Regarding sensors used to measure vehicle rotation angles, the accuracy of determining the difference in the number of revolutions between both wheels is not precise enough. Because of this problem, early NAVS frequently got lost, with Fig. 2 Vehicle navigation system(Courtesy: Nissan Motor Co., Ltd.)

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