6.111 Final Project Morrissey, Li, Wong - 1 - Phased array pulsing: A simple digital sonar system Brian Wong, Bryan Morrissey, Zhen Li {b_wong, blmorris, zhenli} @ mit.edu 14th December 2007 MIT 6.111 Digital Systems Laboratory6.111 Final Project Morrissey, Li, Wong - 2 - Abstract For this project we designed and implemented a system that processes the information collected from ultrasonic echo detection to construct a graphical representation of its environment. This required the construction of a support structure for a set of ultrasonic transmitters and receivers, as well as circuitry for signal amplification and analog to digital conversion to interface with the labkit FPGA. All signal processing tasks beyond basic analog amplification and filtering were implemented in Verilog for execution on the 6.111 labkit’s Xilinx FPGA. These signal processing tasks include pulse generation, echo detection, time interval measurement, coincidence detection, buffering data to memory, constructing an abstract environmental representation, and interfacing with the video hardware to display this representation on a VGA monitor. Preliminary experiments on the ultrasonic devices and interface circuitry provided promising results, encouraging us to push forward with a challenging, intricate and ultimately successful design which was able in demonstrations to simultaneously track the direction and range to two separate moving objects. Sonar System Overview and Theory of Operation A sonar system locates objects by transmitting an acoustic pulse (often in frequencies well above the range of human hearing), detecting the reflected echo signal, and processing the signal to derive information about the object that reflected it. The simplest form of sonar is probably the ranging device, which simply measures the time interval between when the pulse is transmitted and when the echo signal is received. Beyond this in complexity are systems which utilize multiple receivers to determine both the range and direction to the object generating the reflection signal. We designed and built this second type of sonar for our 6.111 final project. In the context of a sonar (or radar) system, a phased array can mean two different things: manipulating the relative phase of the signal on an array of transmitters to control the direction in which the transmitted pulse is sent, and measuring the relative phase of the echo signal detected by the receivers to determine the direction from which the signal is arriving. Sophisticated radar systems will often utilize both techniques, directing their radiated energy to a specific point of interest and processing the return signal to eliminate any extraneous noise that might have been received from sources in other directions. In contrast, our sonar project focuses on the second of these strategies, transmitting a sonar pulse to a broad forward region in an effort to “illuminate” multiple objects, and processing the returned signal to extract range and distance information for as many objects as possible. This design path allowed us to eliminate much of the potential complexity from the transmit stage and concentrate instead on producing highly capable signal acquisition and data processing components. The resulting system was able to detect, discriminate and track the return signatures from at least two objects; with the further opportunity for fine-tuning we feel confident that the system has the potential to detect and track several more.6.111 Final Project Morrissey, Li, Wong - 3 - SECTION 1: HARDWARE INTERFACE & DATA ACQUISITION – BRYAN MORRISSEY Hardware Overview 1.1.1 Introduction My portion of the project consisted of designing and building the external interface hardware as well as the digital processing modules to acquire the echo signal and prepare the data for analysis by the object detection logic. The physical interface of the sonar system includes several subsystems that are implemented in hardware external to the Xilinx FPGA. These systems include the transmit pulse amplifier, the transmitter and receiver arrays, and the analog circuitry to amplify the received signal and interface with the digital control portion of the Delta-Sigma analog to digital converters. (Figure 1.1) 1.1.2 Sonar Pulse Generation and Transmission The ultrasonic transmitter and receiver devices were Kobitone parts 255-400ST16 and 255-400SR16, (datasheet available at [http://www.mouser.com/catalog/specsheets/KT-400244.pdf]) These devices have an operating frequency of 40kHz, well above the range of human hearing. The transmitters are rated to be driven at 20V RMS or higher, at which point they will produce sound pressure levels in excess of 120 dB. The acoustic output is adequate at much lower levels; with a simple bridged op-amp configuration (and a very capable LT1632 op-amp) we built a circuit that could drive 10V RMS into an array of four transmitters in parallel. This was more than adequate for our purposes and had the advantageous feature that it could be constructed on the labkit using only the available +/- 12V power supplies. (Figure 1.2) Sonar PulseSignal Generator6.111 LABKITPowerAmplifierTransmit ArrayObjectReceive Array∆−Σ A/DInterfaceCircuitrySignal Generationand AcquisitionControl Logic∆−Σ A/D Control:Syncronization,Bias Compensation,Low-Pass Filter Signal Filteringand DownsamplingPass 40 kHz,Block 1 MHzThresholdDetection12-bit by 32KDual-Port BRAMUser Inerface and Display Generator BlockSignal Analysis BlockData Acquisition BlockVGA DisplayFigure 1.1: Sonar System Overview6.111 Final Project Morrissey, Li, Wong - 4 - Figure 1.3 provides a simplified illustration of the effect that we hoped to achieve by arranging the transmitter array vertically, perpendicular to the horizontal axis of the receiver array. When several transmitters are arranged in a linear array (and driven with the same signal) their sound fields will tend to reinforce each other in coherent wave fronts directly in front of the array, and all along a plane perpendicular to the axis of the array and roughly equidistant from all of the transmitters. In our case, this means that most of the acoustic energy is transmitted into a plane that is parallel to the receiver array, which is precisely the optimal region for our system to determine the direction of a received signal. 1.1.3 Receiver Array and A/D Interface
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