汽車倒車時防撞自動測距警報系統設計畢業論文(編輯修改稿)

　

【文章內容簡介】 nic sensor systems generally calculate distance using the timeofflight (TOF) method. The distance l to a reflected object is calculated by l =c t/2。 (1) where c is the speed of sound, and t is the roundtrip timeofflight (Fig. 1). The TOF method produces a range value when the echo amplitude first exceeds the threshold level after transmitting. In spite of the simple method like this, information obtained by the ultrasonic sensor is influenced by the characteristics of the sensing system, it of environment and so on.In this paper, in order to investigate the influence of the sensor system, we constructed two ultrasonic ranging systems of which characteristics differs from each other and examined their performance such as obstacle detectability and resultant sonar map.In section II, we introduce two ultrasonic ranging systems which we developed. Their availability for obstacle detection is examined in section III, sonar map making using them in section IV. Finally, the conclusions are presented in section V. Fig. 1. The principles of the timeofflight (TOF) methodII. Two Ultrasonic Ranging SystemsFig. 2 shows a model of reflected waves, where there are two objects in a field of view. As an ultrasonic wave attenuates and spreads, the echo amplitude reflected off farther object is smaller (even from the same object[2]). Because we use piezoelectric ultrasonic sensors, we use a transmitter and a receiver separately. So, the received waves include the direct wave from the transmitter which must be neglected.We have developed a ultrasonic range finding system A in which the following fundamental method is employed[4][3]. Ultrasonic waves are discharged from a transmitter given paratively long burst waves in order to vibrate its piezoelectric vibrator fully. A range value is calculated by TOF method using the amplified echo and a threshold level. The threshold level to detect echoes is constant in order to simplify the circuit. The concept of each signal for System A is shown in Fig. 3. However, this system has some problems. First, the rise of echo signal is slow, since the intensity of the echo signal is not so large. So measuring errors result. To reduce such measuring errors, the rise of ultrasonic waves must be quickened. Next, the measuring range is limited by a fixed threshold level. It is impossible to detect far distance objects when the level is set higher, or it is likely to detect noise from near distance objects when the level is set lower. Also, horns are attached to System A to increase the intensity of waves. However, it makes the directivity narrow. Fig. 2. A model of reflected waves.To solve the problems of System A, we improved the transmit and receive circuit, and developed a new ultrasonic sensor system B. The concept of each signal for System B is shown in Fig. 4. In order to enlarge the intensity of echo signal and quicken the rise of it, a high voltage single pulse is employed for transmitting. The peak voltage is about 720V, while it is about 12V in System A. This method has the following benefits. First, themasking time can be shortened by shortening the transmitting time. Thus it is possible to measure near distances. Second, the rise time of transmitting waves is shortened byusing a high voltage for discharging ultrasonic pulses. Therefore the measuring errors can be reduced. For receiving, the threshold level is decreased with time, and is gradually adjusted to the echo amplitude decreasing with the distance. We call this method timethresholdcontrol. This method is robust to noises from close objects, and makes it possible to measure far distance object. The Polaroid ultrasonic range sensor[1] solves this problem by changing the amplification factor with time. This is called timegaincontrol. But the circuit is plicated. To solve this problem with a simple circuit, not th