A COMMUNICATION SYSTEM FOR UNDERWATER COOPERATIVE ROBOTS NSF Summer Undergraduate Fellowship in Sensor Technologies Philip Schwartz (Physics and Chemical Engineering) - University of Pennsylvania Advisors: Dr. P. Bloomfield, Drexel University, Dr. S. Frankel, University of Pennsylvania

ABSTRACT A system allowing underwater robots to communicate was developed using ultrasonic transducers sending out a signature pulse of 5 bits to enable recognition of each individual robot. Steel tubes were fabricated to house transducers for transmitting and receiving ultrasonic signals. The design affords electrical shielding of the electrical signals and the amplification electronics. The electronics were designed to send in an energizing signal and send out the amplified received signal using only one line. Power for the amplifiers and the electrical pulses that activate the transducers to emit acoustic pulses are sent in over a BNC cable. Acoustic pulses, received by the transducers, are transformed into electrical signals which are amplified and transmitted over the same BNC cable to recording electronics. The piezoelectric polyvinylidene fluoride (PVDF) transducers are backed with an inert PVDF cylinder. The outside electrode of the transducer is grounded to the steel tube and the inside electrode is connected to the wire using conductive epoxy. Signals are encoded and transmitted to the transducers using a C program and an arbitrary function generator. Signals were received from the transducers using a 20 MHz National Instruments NI5102 digital oscilloscope card for PCI bus and Virtual Bench 2.1.1 software. The hardware and software were tested to ensure capability of interpreting the received signals. The signals output by the function generator were created using Arbitrary Waveform Creation software. The electronics were designed and diagrammed in an iterative procedure using P-Spice.

1.

INTRODUCTION

The purpose of this research was to design, assemble, and test ultrasound transducers [1] in order to determine whether they would be effective at communications, to create a shielded housing to allow electronics to be in close proximity to the transducer, and to design and install electronics to transmit and receive these signals in a small underwater cooperative robot. Communication between underwater robots allows them to operate more efficiently than if they are controlled centrally. Because ultrasound travels well through the water, it was chosen to transmit the signals. Transducers were created using PVDF (polyvinylidene fluoride) piezoelectric polymer film [2] attached to a backing cylinder made of the same material as the transducer. This achieves maximum bandwidth, which allows for sharp

pulses to be sent out without much ringing [1]. Each robot would have two transducers: one that sends signals with a wide beam for broadcasting a distress signal, and one that receives information and sends signals with a narrow beam [3] for allowing two robots to locate each other precisely to enable linking up. Each robot would also have a 5-bit signature that it would send out as its signal. A robot that received this signal would then send that same signal back out and would attach its own signature at the end. This would enable the robots to know exactly whom they were communicating with [4-5]. 2.

BACKGROUND

In order to convert an electrical signal into sound and vice-versa, a transducer is needed. In last year’s SUNFEST Program (1998) the related projects, “Underwater Communication System for Cooperative Swimming Robots” and “Ultrasonic Transducers for Cooperative Swimming Robots" [4, 5], were concerned with the testing of commercially manufactured transducers which utilized piezoelectric PZT. In order to improve the transfer of acoustic pressure across the water/transducer interface we decided to use PVDF, which is a piezoelectric polymer material. When a voltage is applied across the material, it deforms, or when a stress is applied to the material, a voltage develops across it. Thus an electrical signal can be transformed into sound, then sent through the water, and finally transformed back to an electrical signal by a second transducer. Another factor influencing the type of material to be used in the transducer was how well it would transmit the sound signal into the water, which is determined by acoustic impedance matching. The acoustic impedance of a material is equal to the density of the material multiplied by the speed of sound through the material. The acoustic impedance of PVDF is close to that of water. During last year’s project [4, 5] unshielded PVDF film transducers were tested. Electrical pickup was a serious problem. The commercial transducers were installed within shielding steel housings. Thus, in this year’s project we are incorporating the PVDF transducers within steel housings. We also needed to determine at approximately what frequency we wished to operate. We wanted a frequency that was high enough to allow the robots to communicate in close proximity; however, higher frequencies attenuate faster in water. We decided to operate at around 1 MHz, which PVDF transducers are capable of doing [2]. 3.

SOFTWARE

3.1

C Program

One method for creating electrical pulses was using a C program written by Godwin Mayers (University of Pennsylvania Physics Department). This program had three variables: pulse width, number of pulses, and interval between pulses. It was capable of creating a pulse train and then repeating that train. We modified the program first by changing the variables from integers to long integers to enable a slower repetition rate (we wanted a repetition rate of approximately 1 kHz). We then changed the program to create 5-bit pulses with the first and last bits being ones and the other bits either ones or zeros (see Figure 1).

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Figure 1: Screen shot of the final version of the C program. There is a 5-bit pulse train, in this case 11011. The pulse width is the width of each individual bit and the interval is the interval between pulse trains. The numbers do not correspond to an actual time unit. 3.2

AWC Program

The AWC program is software used to program the Stanford Research Systems Model DS345 30 MHz Synthesized Function Generator. The software is capable of creating waveforms containing from 8 to 16300 points with a peak-to-peak amplitude of 10 mV to 10 volts. A waveform is created by first choosing sine, square, saw, triangle, exponential, damped sine, or pulse. Next, mathematical operations can be performed to create a waveform of any shape desired. 3.3

Oscilloscope Card Software

Once the oscilloscope card was placed inside the computer, the oscilloscope software was installed. This allowed the computer to function as an oscilloscope. The sampling rate in the normal mode is approximately 20 Megasamples/second - too slow for the waveforms we wished to view. However, the oscilloscope card’s Random Interleave Sampling (RIS) mode achieves an effective sampling rate of approximately one Gigasample/second and accurately displays high frequency functions (see Figure 2). 3.4

P-Spice Program

To aid in diagramming, designing, and testing the electronics, we used the program PSpice. This program allows the user to first create a circuit diagram and then to electrically test the diagram to debug its design before having to lay out real parts on a breadboard. We created circuit diagrams for the electronics located both inside and outside of the shielded housing (see Figures 5 and 6).

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Figure 2: Screen shot of the digital oscilloscope software. The pulse trains illustrated were generated by the C program with the values in Figure 1. 4.

HARDWARE

4.1 Steel Cylindrical Tubes as Shielded Housings for the Narrow-Beam and WideBeam Acoustic Signal Transducers For the narrow-beam transducer design we started out by taking a 24”-long circular cross-section cylindrical stainless steel tube with an outside diameter of 0.6228” and an inside diameter of 0.487” and cut it down into 2”-long pieces. Then a small end cap was welded onto one end, which was sized and threaded for a BNC connector. We lastly opened up the inside diameter to 0.5” on the other end to a 5/8” depth to create a shelf to hold the spacer and backing cylinder described in the next section (see Figure 3). The piezoelectric PVDF film will be cemented to the outward facing flat surface of the backing cylinder. For the wide-beam transducer design, we utilized a stainless steel tube with square cross-section (0.625” inside dimension). The transducer backing consisted of a machined PVDF cylinder having a square base fitting within the cylinder and its curved surface facing forward. In this case, the piezoelectric PVDF film will be adhered to this curved outwardfacing surface. Note in last year’s project unshielded rectangular PVDF films were wrapped around aluminum cylinders and their acoustic signals were received by commercial narrow field-of-view transducers.

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Figure 3: Diagram of the steel tube with the BNC connector and narrow beam transducer attached to it. 4.2

PVDF Backing Cylinders

For the narrow-beam transducer design we cut 0.5”-long pieces from a 12”-long 0.5” diameter cylindrical solid piece of PVDF. We then cut a 0.014” hole 0.125” from the center of the cylinder to allow a wire to pass through the cylinder. We then cut a small circular indent into the hole 1/32” deep and wide on the smooth side of the cylinder to prepare the surface for the transducer attachment. Since the 5/8” depth of the shelf did not match the ½”long backing cylinders, we then cut a 7/64”-long spacer out of the PVDF and cut an inside diameter of 0.135” to allow the wire to pass through. We glued the spacer to the rough end of the PVDF cylinder with two-component non-conducting epoxy. For the square crosssection tube utilized in the wide-beam transducer design a 0.625” long piece is cut from a one-inch diameter cylindrical solid piece of PVDF. Then this piece is cut parallel to and on both sides of the length symmetry axis, yielding a backing piece having a base with square cross-section and a curved outward-facing surface. A small hole is drilled through the base toward the curved surface for the same purpose as above. 4.3

0.010” Wires

We used a 0.010”-diameter wire to run through the cylinder and conductive epoxy to fill the small indent in the PVDF cylinder and attach the wire to it.

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4.4

PVDF Film

The piezoelectric PVDF film to be used as the transducer is 57 microns thick. The electrodes are nickel. After protecting the top electrode and the interior of the bottom electrode with drawn wax pencil circles, a small amount of the bottom electrode around the perimeter is removed with diluted nitric acid (10/1 water/acid by volume). A small dot of conductive epoxy was used, where the wire came up from the transducer’s backing cylinder, to afford electrical connection from the transducer’s bottom electrode to the interior electronics and BNC connector (see Figure 4). Regular epoxy is then used to connect the majority of the bottom surface of the PVDF transducer film to the PVDF cylinder. This allows a very thin glue line with no further acoustic mismatch.

Figure 4: Close up picture of the transducer, showing the PVDF film in between the two electrodes. The top electrode is grounded to the metal tube and the bottom electrode is connected to the wire. 5.

ELECTRONICS

5.1

Outside Electronics

The electronics assembled outside of the tube include an oscilloscope for reading the received signals, a voltage supply to power the amplifier inside of the tube, and a pulse generator to send in the signal to create the acoustic pulse. There are also protections to control the direction of the incoming or outgoing signals (see Figure 5). 5.2

Inside Electronics

The main component of the inside electronics, other than the transducer, is the amplifier. The amplifier must be placed so close to the transducer is in order to prevent capacitate losses caused by having a long length of coaxial cable between the transducer and the amplifier. On the inside, there are also the protective electronics and the electronics to direct and separate the incoming D.C. power and energizing signals from the outgoing received signals (see Figure 6).

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Figure 5: Diagram of the outside electronics. The voltage supply is protected by an ACblock filter. The pulse generator is protected by the diode pair. The oscilloscope is protected by a diode pair and a high pass filter.

Figure 6: Electronic components inside the tube. The main components are the two operational amplifiers that are set to amplify signals coming in from the transducer and are bypassed by signals being sent to the transducer.

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6.

SUMMARY AND CONCLUSION

More work is needed to determine that the electronics and transducer design will be practical for use in cooperative underwater robots. It appears that the transducer we created will be effective at underwater communications and we should be able to test this in the near future. We successfully created backing cylinders and shielded housings to hold the flat circular narrow-angle and curved wide-angle transducers and amplification electronics. The actual electronics are still in the design phase; however, we will be able to test the transducers before the final electronic system has been designed. Once everything has been put together, the testing will take place in a water tank with two transducers set up to send and receive the acoustic signature pulses. The second receiving transducer will send back to the first sending transducer a signal consisting of the received signature signal plus its own signature added to the end. The creation of a set of functioning cooperative underwater robots is still somewhere further in the future. 7.

ACKNOWLEDGMENTS

I would like to thank my advisors, Dr. Philip Bloomfield of Drexel University and Dr. Sherman Frankel of the University of Pennsylvania, for all of their assistance and advice. I would also like to thank Ronald Pierce, Godwin Mayers, and Mitch Newcomer for their help with our work. I also appreciate the assistance of Robert Hee and Buddy Borders in the Physics Machine shop. Finally, I would like to thank Dr. Jan Van der Spiegel and the National Science Foundation for its support of the REU grant making this program possible. 8.

REFERENCES

1. G.S. Kino, Acoustic Waves: Devices, Imaging, and Analog Signal Processing (PrenticeHall Inc., 1987). 2. H.R. Gallantree, “Review of Transducer Applications of Polyvinylidene Fluoride”, IEEE Proc., vol. 130, no. 5, pp. 219-224 (1983). 3. G. L. Gooberman, Ultrasonics: Theory and Application, p. 55-56 (Hart Publishing Co., New York, 1968). 4. C. Lundgren, “Ultrasonic Transducers for Cooperative Swimming Robots”, p. 73-87, SUNFEST 1998 Technical Report TR-CST30SEP98. 5. A Utada, “Underwater Communication System for Cooperative Swimming Robots”, p. 107-126, SUNFEST 1998 Technical Report TR-CST30SEP98.

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