Priority 2 Use of Ultrasonic Signal Coding and PIR Sensors to Enhance the Sensing Reliability of an Embedded Surveillance System

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  Use of Ultrasonic Signal Coding and PIR Sensors to Enhance the Sensing Reliability of an Embedded Surveillance System Ying-Wen Bai, Chen-Chien Cheng and Zi-Li Xie Department of Electrical Engineering, Fu Jen Catholic University,  New Taipei City, Taiwan, 242, R.O.C.  Abstract—  In this paper we design and implement an embedded surveillance system by use of ultrasonic signal coding of ultrasonic sensors with multiple pyroelectric infrared sensors (PIR) to detect an intruder in a home or a storehouse. The PIR sensors are placed on the ceiling, and the ultrasonic sensor module consists of a transmitter and a receiver which are placed in a line direction; however, ultrasonic sensors with the same frequency are subject to interference by crosstalk with each other and have a high miss rate. To overcome these disadvantages of the ultrasonic sensor, our design reduces the miss rate from the environmental interference by using an ultrasonic coding signal. Both ultrasonic sensors and PIR sensors are managed by the majority voting mechanism (MVM).  Keywords—Embedded Surveillance System; Majority Voting  Mechanism; PIR Sensor; Ultrasonic Sensor I.   I  NTRODUCTION Recently surveillance systems have become more important for everyone’s security. The embedded surveillance system, frequently used in a home, an office or a factory [1-3], uses a sensor triggered to turn on a camera [4-5]. Some designs use different types of sensors to achieve reliability by means of the different features of each sensor [6-7]. In this paper we extend our previous design not only by using both multiple PIR sensors and ultrasonic sensors as a sensor group, but also by using the MVM. Ultrasonic receivers and transmitters are located at opposite ends [8-9]. However, to reduce the interference from other frequencies in ultrasonic signals, we use a coding signal to enhance the ability to distinguish the random interference [10]. To enhance system reliability in the experiment, we focus on how to improve the shortcomings of the ultrasonic sensor. Some research explores the influence of attenuation in air and crosstalk of ultrasonic signals by using a coding signal [11-12], while some provides improvement of the ultrasonic signal by using different coding signal types [13-14]. Other research uses the application of a coding signal to increase resolution and contrast of images [15]. Yet another approach build a 3D image with an ultrasonic sensor in the PN code that solves the problem with time delay [16]. To enhance the reliability of the ultrasonic sensors group, we propose adding to the number of bits with coding to reduce the  probability of code breaking. II.   S YSTEM A RCHITECTURE  Fig. 1 shows our design which contains several ultrasonic and PIR sensor groups. In the modules of the ultrasonic sensor groups the transmitter and the receiver are separated. The transmitter circuit generates a multi-frequency square waveform, and the receiver circuit amplifies the received signals and filters out any noise. When a transmitter transmits an ultrasonic coding signal, the ultrasonic receiver determines whether there is an intruder passing through the sensing area. If there is no intruder, the MCU (Micro Controller Unit) will use the predefined ultrasonic signal pattern to decode the received signal. Use of relay stations and frequency conversion extends the sensing range. Our design reduces the environmental interference with the ultrasonic signal. All sensing signals are input to the embedded surveillance system  by the GPIO (General purpose input and output), and the MVM program counts the number of sensing states to determine whether to adopt the MVM or not. The PIR sensor groups obtain the sensing signals from human temperature. If the voting results of ultrasonic and PIR sensor groups pass the criteria, the embedded surveillance system starts the Web camera to capture images. Figure 1. Embedded surveillance system with ultrasonic signal coding. 978-1-4673-3108-1/13/$31.00 ©2013 IEEE   A.   Software modules We choose Embedded Linux as our operating system. The  program of the majority voting mechanism contains a detection of the GPIO function, a counting and majority voting function, an image captured function and a Web server. Fig. 2 shows the detection software flowchart of the embedded system. The embedded system scans the GPIO sockets, which are connected to external PIR sensors and ultrasonic sensors. To verify the state of each PIR and ultrasonic sensor, the embedded system reads the voltage levels of the GPIO sockets. When the system reads 5V from a GPIO socket, we learn that the ultrasonic sensors or the PIR sensors have been triggered and will execute the majority voting program by counting the state of each ultrasonic and PIR sensor. Majority voting is achieved by the sensor groups of the different GPIO sockets. The embedded system, when interrupted by the detection  procedure, starts the Web camera to capture images. When this is finished, the embedded system starts the detection procedure over again. If the intruder is still in the monitoring area, the count of the GPIO sockets’ voltage levels continues the majority voting mechanism, and the embedded system again starts the Web camera to capture images. The embedded system uploads the captured images by means of both the Web server and the streaming server through the Internet. Enter the detection modeScan GPIO socketsDetect intruder or notAdopt the MVM or notTurn on the camera and capture imagesUpload images on the internetDetect intruder or notENDAYes NoYes NoA   Figure 2. Detection software flowchart of embedded system.  B.    Hardware modules We use two groups of the external hardware circuits, the PIR and the ultrasonic sensor group. As the PIR sensor  produces a weak voltage, we input the sensed signal to a two-stage OP amplifier to amplify the weak voltage by about 1000 times. Since the amplified signal changes between  positive and negative voltage, we input this signal to the absolute value circuit, and then we input it to the adjustable comparator to compare the sensing voltage and the reference voltage which are set according to the environment temperature. Fig. 2 shows the block diagram of the PIR module. Figure 3. Block diagram of PIR module. Fig. 3 shows the ultrasonic transmitter circuit which uses a  pulse width modulation (PWM) function in the MCU to send out the desired frequency of the ultrasonic signal. The ultrasonic transducer transforms the voltage waveform into an ultrasonic transmission, and the transducer of the receiver transforms the ultrasonic transmission into the voltage waveform. Figure 4. Ultrasonic transmitter circuit. Fig. 4 shows the ultrasonic receiver circuit. We use a two-stage amplifier to enlarge the ultrasonic waveform, and the filter suppresses any undesired frequency. The MCU comparator determines the level of the ultrasonic waveform, which is the method used for the ultrasonic transmission, whether blocked or not. When there is no intruder, the MCU changes the transmitting frequency of the ultrasonic signal to avoid any crosstalk for the next stage. We also design several ultrasonic receivers to receive the ultrasonic transmission and to activate the majority voting mechanism (MVM) to improve the sensing reliability in long distance sensing [6]. The receiving states of all receivers are input to the embedded home surveillance system, which, depending on the results from the ultrasonic receivers, uses the MVM. Figure 5. Ultrasonic receiver circuit. Table I shows our comparison of the sensing characteristics of both the ultrasonic sensor and the PIR sensor [6]. We have found that the ultrasonic sensor and the PIR sensor can both compensate each other and enhance the overall sensing  probability in our design. TABLE I. C OMPARISON OF C HARACTERISTICS OF PIR    S ENSOR AND U LTRASONIC S ENSOR   Sensor Condition for trigger Influence of environment temperature Sensing type Disadvantages Ultrasonic Moving  block Independence Line direction  Noise interference PIR Temperature change Dependence Projection area Slow speed and heat insulation  III.   U LTRASONIC I  NTERFERENCE  Ultrasonic transmission is a sound transmission which is easily interfered with by other frequencies from the random ultrasonic signals. The signal interference causes difficulties in setting up the reference voltage. Therefore we propose the coding signal to increase the reliability of the system. According to the features of the coding signal our design enhances the ultrasonic signal pattern to distinguish the random ultrasonic interferences. IV.   F EATURES OF C ODING S IGNAL  In this paper we use a coding signal to increase the reliability of the ultrasonic sensor group. Equation (1) is the function of probability of code breaking. Equation (2) is the function of reliability. Fig. 5 shows the results, when the number of bits increases, the reliability approaches 1. According to Eqs. (1) and (2) we know that with one bit if the  probability of code breaking is 0.5, the reliability would be 0.5. To increase the number of bits of the ultrasonic signal code is to increase the reliability. P     (1) R  1 P  (2)   Figure 6. Relationship between reliability and number of bits of the ultrasonic signal code. In our design we use an 8-bit number coding signal. Using (1) and (2), we know that if the number of bits is n  8,  the probability of code breaking is P     .  The reliability is equal to   1 P  0.996.  V.   I MPLEMENTATION AND E XPERIMENT R  ESULTS  In the experiment results we found that an ultrasonic signal would be affected by environment sounds and the amplitude of the reference voltage. Those factors affect the transmission distance and the error rate of detecting. We therefore put the transmitter and the receiver on both ends of the sensing area and make sure the intruder passes through if the outside group has detected an individual. Fig. 6 shows an ultrasonic signal being interfered with by another ultrasonic frequency. As ultrasonic signal interference causes difficulties in setting up the amplitude of the reference voltage of the circuit, therefore we use the coding signal to reduce ultrasonic signal interference based on the characteristics of the ultrasonic signal. Figure 7. Ultrasonic signal interfered by another frequency. In Fig. 8 we see that the signal has been coded by our system. The coded signal is not affected by another frequency  because our design receives a signal through the code instead of through its signal’s frequency. Figure 8. Our ultrasonic coding signal in the scope. Fig. 9 shows the signal judgment of our experiment. When  judging a coding signal, one method of our design counts the rising edge number. If the rising edge number is equal to two, it means the signal is correct. Lines A and B of Figure 9 show a coding interval. Figure 9. Judgement from received ultrasonic coding signal.    Fig. 10 shows our design that consists of the internal software module and the home embedded system software module. When an intruder has been detected, the MCU wakes up the majority decision to test the threshold and then turns on the power supply for the indoor sensors. If the indoor sensors detect no intruder when the outdoor sensors are misjudging, the MCU turns off the power of the indoor sensors and goes  back to the alert state. If the indoor sensors detect an intruder, the MCU turns on the Web camera to capture images in keeping with the decision of the MVM. Figure 10. Software flowchart of embedded surveillance system [11]. After the intruder has been detected outdoors, the MCU software submodule of the ultrasonic coding signal is executed as shown in Fig. 11. Our design transmits the ultrasonic coding signal, and the receiver checks whether the coding signal is correct or not. If the coding signal is correct, the ultrasonic sensor group continues for some seconds to make sure that there is no intruder. If the coding signal is not correct, our design also starts the majority voting mechanism (MVM) to make sure whether there is any detection. Figure 11. Flowchart of detection by ultrasonic coding signal. In the experiment the ultrasonic signal of the receiver has the same direction as the transmitter, and we find that if the amplitude of the voltage waveform has been reduced to approximately the same as the comparator's reference voltage our design can work normally. The scattering causes the amplitude of the voltage waveform to become gradually lower than the reference voltage. To increase the amplitude of the voltage waveform we place a PET bottle at the front end for focusing. Fig. 11 shows the distribution of scattering after adding a PET bottle focus. We have found that the ultrasonic signal increases at the central point and achieves the focusing effect. 0 10 20 30 40 50 60 70 80 90 10000.   7m6m5m4m3m   Figure 12. Curves showing distribution of scattering after adding PET  bottle focus. Fig. 12 shows the arrangement of our experimental environment that detect intruders in a suitable place. We place the PIR sensor on the ceiling or above the detection area. Transmitter and receiver of the ultrasonic sensor module are  placed in a line direction. When an intruder enters the detection area, the ultrasonic coding signal will be blocked and the PIR sensors will detect temperature changes. Figure 13. Arrangement of our experimental environment. Table II compares our coding signal and noncoding signal. The coding signal is not interfered with by other frequencies unless their patterns are similar. It is easier to break a noncoding signal than a coding signal. When we add to the  bits of the ultrasonic coding signal, the message type rise with 2  . A noncoding signal transmits just two types of messages, 0 and 1. N means number of bits. With more message types,
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