136: Data Transmission via Sound

Transcription

1 136: Data Transmission via Sound Loren Lang, Sean Keller, Richard Kueper ECE 403 Spring 2001 ABSTRACT: In this project, a wireless serial data bus was constructed for use with the Wrist Display GPS project ongoing between NDSU and Rockwell Collins. In this project, we hope to answer the following questions: At what frequency does sound best transmit through the body? What baud rate can be achieved? What are the emissions? We plan on using a transmitter / receiver pair of sound transducers that will use Sound waves On-Off Keying A high frequency carrier The advantages of this design include low electromagnetic emissions and effective communications across the body without the use of wires. The main problem with this approach was the difficulty of overcoming reflection loss at the human body interface. 1

2 Chapter 1: Introduction 1.1 Introduction The United State Department of Defense (DOD) currently operates and maintains a network of satellites and ground-based transceivers for the purpose of establishing a radio based global positioning system (GPS). This system has the capability of determining the latitude, longitude, and elevation of a receiver by calculating timing differences from RF signals broadcast by three or more satellites. The ground-based transceivers are used to calculate and correct slight errors in satellite orbit position as well as timing errors caused by atmospheric inconsistencies. Rockwell Collins is currently marketing a Precision Lightweight GPS Receiver (PLGR) for military applications. The PLGR is a handheld unit useful in applications ranging from military reconnaissance to determining proper placement of transmitters or other critical equipment. As part of an ongoing project, Rockwell Collins has requested assistance form senior level electrical engineering students from North Dakota Sate University to develop a wireless interface network that will allow a master CPU to communicate with several devices. Specifically, the master CPU should be able to request information for the PLGR and communicate that information to a wrist display unit. A wrist display unit would relieve the operator of the burden of holding the PLGR and therefore free a hand for other tasks. Previous senior design groups have successfully worked with Rockwell Collins on the development of a wrist display unit that would interface directly with the PLGR. Unfortunately, this did no provide versatility for expansion of the system. It is desirable to incorporate additional sensors and input devices, all of which communicate with one master. For this reason, Rockwell Collins has requested that this problem be approached in a divide and conquer manner. Four independent projects were defined as shown below, each component of which was an individually testable, compatible component. Bus Master Slave #0 LCD Display Slave #1 PLGR Slave #2 Heart Rate etc. The bus master receives, transmits, and processes all data. This was a PC initially, with the understanding that a future design team will develop a portable unit. A second project was the creation of a wrist display. Also required is a PLGR interface that will decode information from the PLGR and provide that information to the master via the serial bus. A third project added more 2

3 capability to this system by adding slave units which monitored the operator s heart rate and temperature. One shortcoming of this design was the use of a 3-wire serial bus (Send, receive, ground). These wires will have to run from the PLGR, temperature sensor, and other sensors tot he operator s wrist. If operated out in the field, these wires may be prone to snage and break, rednering the system nonfunctional. If these wires could be replaced with a wireless form of data communications, the GPS Wrist Display system would be more reliable and more functional. Several approaches exist for creating a wireless serial data bus. If no ground wire is used, a wave of some form must be launched. Three types of waves would be EMI waves using common-mode radiation EMI waves using differential mode radiation Sound waves A wireless link using EMI waves with common mode radiation was developed and successfully demonstrated to Rockwell Collins in the summer of While this method did transmit data via the operator s skin, the common-mode radiation was significantly above military requirements for EMI radiation: on the order of 80dB at some frequencies.. A wireless link using EMI waves with differential-mode radiation should result in lower emissions, since the emission strength should drop off as 1/r 3 for differential mode. Future senior design groups may look at this approach. Sound can also be used to launch a wave and transmit data. This is the approach to be investigated in this study. 1.2 Objective: In this study, the feasibility of transmitting serial data between master and slave units via a wireless serial data bus were investigated. Specifically, The characteristics of the human body with respect to transmitting sound were researched A device will be built to transmit serial data via sound across the body, and if possible The properties of this device, such as its emissions, will be identified and characterized. 1.3 Report Outline: This report is organized as follows: Chapter 1. Introduces the problem and describes what sort of device should result from this design process. 3

4 Chapter 2. Previous work on similar devices is presented. The specifications that this device must meet and the design approach selected by our design team is then described. Chapter 3. Each section of the design is further subdivided and described. The options considered, the option selected, and reasons for this selection are elaborated upon. Chapter 4. Circuit diagrams and flowcharts are presented for each section of the design. Chapter 5. Experimental results for each section are presented. Chapter 6. Presents the results from assembling the overall device and analysis of how well this device meets the design specifications. Chapter 7. Concludes the report and suggests future directions for follow-on design projects. References, program listings, and data sheets are then located in the appendix. 4

5 Chapter 2. Previous Work & Design Approach 2.1. Introduction A number of companies and individual have researched sound transmission and used it as a means of data transmission for various applications Previous Work Using sound for wireless data transmission is not uncharted territory. There have been several patented devices which provide for ultrasonic data transmission and there have been published articles dealing with ultrasonic data transmission. None of the devices found address the needs of this project. Most devices using of sound for wireless communication are designed for underwater applications. Other devices are very application specific and are not useful to this project. Many patents mention ultrasonic as a possible way to transmit data, but there are only a few that specify it. One such device is an underwater modem from Divecom Ltd. (US ) used for carrying data at a high rate underwater. Although this device uses ultrasonic carriers, it was designed for underwater use and has a much larger range than desired for this project. Other devices are too specific or designed with other goals in mind. The Aerospace Corporation has an ultrasonic communication device (US ), but it seeks to avoid an electrical power source for embedded sensors. This does not meet the requirements of our project. There are two helpful sources that deal with at least one of the questions this project seeks to answer. Craig Wisneski's project "Ultrasonic Local Area Communication" ( and V. Gerasimov and W. Bender's "Things That Talk" ( provide an overview of sound as a communication medium. They provide a good first glimpse at the issues this project seeks to deal with Specifications Rockwell Collins has requested that the device be wireless. It also needs to be RS-232 compatible to integrate well with the rest of the body LAN. The emissions should be low, preferably well below military standards. Also, a baud rate of at least 1200 should be achieved, although Rockwell Collins would like a baud rate of at least 9600 if possible. The product should be low power and should be feasible to manufacture and produce. 5

6 2.4. Block Diagram In order to develop the sound data transmitter and receiver pair, the design is split into 9 blocks as shown below. Block Diagram RS V Buffer Amp Transducer (speaker) Sound 9600 Baud TTL 9600 Baud TTL RS V Buffer Microprocessor Amplifier Microphone Sound The incoming +- 12V RS-232 signal is converted to a TTL +- 5V signal by the buffer. Before being transmitted, the TTL signal is first amplified. Then it is converted to sound by the speaker. This sound signal will be transmitted primarily through the human body and air. On the receiving end, the microphone will convert the sound back to an electrical signal before being amplified. The microprocessor will run an error checking algorithm to assure the integrity of the signal. Then, another buffer will convert the +- 5V TTL signal back to a +- 12V RS-232 compatible signal Time Line We divided up the work for this project as evenly as possible. The actual timeline is given in the appendix. Each person was assigned a number, and that number was then used to identify which part of the project each person would work on. 6

7 1=Rick Kueper 2=Sean Keller 3=Loren Lang 2.6. Conclusions Chapter 2. presented previews of other individual s work and described in general how this task will be divided into subtasks. Each of these subtasks will be elaborated upon in Chapter 3. 7

8 Chapter 3.Detailed Design 3.1. Introduction In this chapter, each section of the design is further subdivided and described. The options considered, the option selected, and reasons for this selection are elaborated upon. 3.2 Experiment Design The objective of our experiment was to characterize sound transmission through the human body at different frequencies and to determine the most suitable frequency or frequencies for data transmission via sound. In order to get a good picture of the body s transmission characteristics it would be ideal to do a continuous frequency sweep from 0 to about 3 MHz. Unfortunately transducers in the ultrasonic range are made with a very narrow bandwidth and are fairly expensive. To overcome this problem, we obtained ultrasonic transducers for frequencies of 25kHz, 40kHz, 92kHz, 150kHz, and 215kHz. Though this limited number of frequencies did not give us a nice continuous graph, it provided a useful indication as to what the transmission characteristics are as a whole. The first stage of our experiment was to establish a baseline, i.e. the transmission characteristics of a good sound conductor, and for this we first tried a steel bar approximately 2.5 feet in length. We were unable to receive any signal through the bar, though, due to reflections at the interface between air and steel. We therefore opted to use air as our baseline. We constructed a frame that allowed us to mount the transducers securely and move them easily for our experiment. The second stage of our experiment was the actual testing of sound transmission through the body. In order to do this we placed a transmitter on the subject s back, which is where the transmitter would most likely be placed in real life, and a receiver on the subject s hand. 3.3 Options Considered In order to characterize as much of the sound spectrum as possible, we considered as many different frequency ranges as possible for our transducers. We considered numerous audible range transducers (20 Hz 20 khz). We also considered some 25kHz and 40kHz transducers. Each transducer was tested for transmission loss through air and through the body to determine the most effective means of transmission. 3.4 Options Selected After repeated test, we were unable to determine a frequency for optimal transmission. Although data was collected for sound transmission through air, no data was received for transmission through the body. An ultrasonic frequency would be ideal for transmission. An advantage of 8

9 using an ultrasonic signal is that it will not be an audible annoyance to the person wearing the device. The ultrasonic transducer pair also will provide us with the best baud rate Conclusion In this chapter, detailed descriptions of each section of the design were presented. The circuits to implement these designs are presented in Chapter 4. 9

10 Chapter 4. Circuit Diagrams & Flow Charts Introduction: In this chapter, the transmitter and receiver circuits are examined. Circuit diagrams are given with an explanation of how the design works. 4.1 Transmitter Circuit The above circuit was used to generate the modulated sonic wave sent to the transducers. The FET acted as a switch to implement ON-OFF signaling. The input to the drain terminal of the FET is a sine wave of the frequency used for transmission. The data being transmitted is input into the gate terminal. The capacitor blocks all DC current. An op-amp amplifier circuit with a theoretical gain of 100 was used to boost the signal. In practice a gain of only 15 was achieved with this circuit due to the op amp s limited bandwidth. The sound wave was then transmitted through air and analyzed using the receiver circuit below. 4.2 Receiver Circuit 10

11 The above circuit was used to receive the modulated sonic wave using the various transducers. The received signal was amplified by the LM318 op amp. Once again, the theoretical gain was 100, but only a gain of 15 was realized in testing due to the limited bandwidth of the op amp. The diode, and RC circuit before the output act as an envelope detector. The detector follows the envelope of the modulated signal received, thus restoring the original data signal. The values of the resistor and capacitor used in the envelope detector were adjusted to suit the frequency being tested. The value of RC should be large compared to 1/(2πf c ) where f c is the frequency being tested, but should be small compared to 1/(2πB) where B is the highest frequency in the modulated signal. 1/(2πB) >> RC >> 1/(2πf c ) The output of the envelope detector should be capacitively coupled to block the DC component. For testing purposes, the output was passed to the oscilloscope for measurement and analysis. 4.3 Conclusion: In this chapter, the circuits used in the design were presented. Each part of the circuit identified and a description of its function was given. The result of testing these circuits is given in the next chapter. 11

12 Attenuation vs. Range Loss (db) Distance of Seperation (in.) Chapter 5. Experimental Results 5.1 Experimental Results As a sound wave propagates, the signal will be attenuated with distance. The amount of attenuation was measured for different frequencies. The medium used in initial testing was air. Using the body as a transmission channel was also tested. In this chapter, the results from these tests are presented Transmission Through Air The results of different frequencies in air are given in Figure 5.1 below. Figure 5.1 Figure 5.1 shows attenuation for a short range (<2 ft) that would be typical for distances used with the PLGR unit. The graph shows that attenuation increases as frequency increases. However, a more general result for signal attenuation over a larger range is given in Figure 5.2 below. 12

13 Figure 5.2 Chart 5.2 shows the relationship between signal attenuation, frequency, and range. As the chart shows, attenuation increases with frequency. This poses a problem for data transmission since higher frequencies are ideal for greater data rates. Communication at higher frequencies requires a more intense signal at the transmitter (backpack) and better detection at the receiver (wrist display) Transmission Through the Human Body All frequencies were tested with the transducers placed on the skin. However, no signal was detected at the receiver for any of the frequencies tested. This is probably due to the large amounts of reflections at the skin on the human body. The human body as a transmission channel in effect 13

14 looks like an infinite impedance to a sound wave. An infinite impedence will reflect the entire incident sound wave. The use of a coupling gel would significantly reduce losses due to reflection. Larger amounts of power at the transmitter were tested but still resulted in no signal received. A coupling gel was not tested since this would impede the maneuverability of the soldier and defeat the objectives of a wireless and non-restrictive transmission scheme. 5.2 Conclusion: In this section, the result of transducer testing was presented. Transmission characteristics of air and the human body were analyzed. The functionality of the design will be examined in the next chapter. 14

15 Chapter 6. Overall Functionality 6.1 Design Difficulties A prototype was not feasible for data transmission via sound due to many reasons. First, the coupling between the transducer and skin is extremely poor. This results in nearly all of the incident wave being lost due to reflections. Serious reflections are present at both the transmitting and receiving locations. A significant portion of the signal is reflected off the skin before it enters the body and an approximately equal portion of the signal that did penetrate the body is reflected back into the body at the receiver. Also, body tissues severely attenuate whatever signal there is inside the body, further decreasing usable signal. Therefore, no part of the original signal was received at the transmitter for any of the frequencies tested. We attempted to regain whatever signal the receiver may have been picking up with an amplifier and a high-q bandpass filter, but the results were once again negative. Another problem is the low data rate that was possible. As frequency increases, the sound attenuation and reflection increases dramatically as shown by Figures 5.1 and 5.2 (Chapter 5). To achieve a high data rate, a high frequency must be used. Ultrasonic frequencies would be ideal for a fast transmission rate, but these frequencies of sound do not transmit well through the body. Low frequency sound that would be transmitted efficiently through the body would result in data rates much too slow to be of any use to Rockwell Collins in the intended application. If an audible frequency is used, the reflection and attenuation will be less, however the signal will require large amounts of power to be detected correctly at the receiver. Using a high intensity audible signal could be annoying and uncomfortable for the user of the unit and would also be a security risk since anyone in the vicinity will be able to hear the signal as well. Another issue that caused problems is the directionality of the transducers used. With the sound transducers used, the sound was propagated in a 15 degree cone, therefore, if the receiver unit was moved slightly, the signal became more difficult to detect or even lost. 15

16 6.2 Design Solutions One possible solution would be to use the body as a waveguide. If the sound wave could be effectively transmitted into the body without much loss, the inside of the arm could be used as a waveguide. Then, assuming that it was coupled extremely well to the skin, the receiver could pick up the signal. A potential problem to this approach is that the inside of the human body is not homogenous. While in some cases the sound may propagate well, in other cases tissue or bone may reflect and absorb large amounts of the signal. Another option may be to implant transducers under the skin to eliminate the problem of skin reflection. The implants could be electromagnetically coupled to devices on the surface of the skin to interface with the transducers. This could also lend itself to the waveguide principle since the direction the sound is projected could be optimized for transmission inside the body rather than having to also optimize for penetration of the air-skin boundary. This method would clearly be costly and difficult to implement in a large number of soldiers and would obviously require minor surgery to put in place. Malfunctioning of the transducers would also require a visit to a medical facility for replacement. Another possible solution is to develop a very high power transmission scheme. Our testing was conducted with the highest power possible for the transducers and amplifiers used. However, this signal was far to weak to propagate through or around the human body. Further consideration might be given to transmitting data at an ultrasonic frequency with a very high intensity sound wave. The impedance of the human body would not be a concern since the signal would be powerful enough to diffract around the body and to the receiver without significant attenuation. Frequency must be chosen with care, though. The higher the frequency, the lower the dispersion of the sound wave from diffraction. This results in a narrower range with which to receive a signal of high intensity. As the wavelength of a sound wave becomes smaller than the obstacle that it encounters, the sound is no longer able to diffract around the obstacle, and instead the wave reflects off the obstacle. Security is once again an issue due to the ease of detection of a high intensity signal. Chapter 7. Conclusions & Future Work We have determined that using a wireless sound transmission scheme for an RS-232 compatible personal area network is not feasible due to the limitations of transmitting sound through the body. Some other approaches to be considered in the future might be: 1) Launching an electromagnetic wave 2) Use a very high power ultrasonic transmission scheme. 16

17 3) Using an embedded wire. For example, wire could be concealed in clothing. 4) Implanted transducers in the skin. 17

18 Appendices Appendix A. References Hopp, S. L., Owren, M. J., and Evens, C. S. (eds.). (1998). Animal Acoustic Communication: Sound Analysis and Research Methods. Wisneski, Craig. (1998). The Physics of Information Final Project: Ultrasonics for Local Area Communication. Retreived October 10, 2000, from the World Wide Web: Kumaresan, Swami and Yeh, Peter. (1997). Adviser: Prof. Peter J. Kindlmann Electrical Engineering Freshman Project: An Ultrasonic FSK Modem. Retreived October 10, 2000, from the World Wide Web: 18