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UNCLASSIFIED SECURITY CLASSIFICATION OF THIS PAGE (WTien D«ra Enlrred) 1. REPORT NUMBER G-TR-0001 REPORT DOCUMENTATION PAGE T GOVT ACCESSION NO «. TITLE (and Subtitle) A MULTI-ELEMENT ULTRASONIC RANGING ARRAY NAVSEA CASE AUTHORfs; LCD^ Bart Everett, USN 9. PERFORMING ORGANIZATION NAME AND ADDRESS Office of Robotics & Autoncmous Systems Naval Sea Systems Comand (SEA 90G) Washington, D.C It. 'QONTROLLING OFFICE NAME AND ADDRESS Office of Robotics & Autonomous Systems Naval Sea Systans Ccrrtnand (SEA 90G) Washington, D.C «. MONITORING AGENCY NAME ADDRESS^/ dlllertrt Irom Controllint Oltice) READ INSTRUCTIONS BEFORE COMPLETING FORM 3. RECIPIENT'S CATALOG NUMBER 5. TYPE OF REPORT & PERIOD COVERED TECHNICAL REPORT-FINAL Jan laas- 6. PERFORMING ORG. REPORT NUMBER 8. CONTRACT OR GRANT NUMBERfsJ 10. PROGRAM ELEMENT, PROJECT, TASK AREA a WORK UNIT NUMBERS 12. REPORT DATE Jan NUMBER OF PAGES SECURITY CLASS, (ol this report) UNCLASSIFIED 15a DECLASSIFICATION'DOWNGRADING SCHEDULE 16. DISTRIBUTION STATEMENT (ol thit Report) /^proved for public release; distribution unlimited BUTION STATEMENT (of the tb,tr,ct ermered Ir, Block 20. It dule,er,t Irom Report) 17. DISTRI Approved for public release; distribution unlimited 18. SUPPLEMENTARY NOTES 19. KEY WORDS (Continue on reverse tide il nece«««ry and Idenllly by block nurrber) Sentry Robot Sensors Rangefinding Ultrasonic 20. ABSTRACT (Continue on reverie tide il neceeeary and Idenllly by block number) A Multi-Elarent Ultrasonic Ranging Array describes the use of an array of multiple transducers catibined with dedicated microprocessor capabilities in the development of a high resolution ultrasonic rangefinding system. The system design utilizes Texas Instrumsnts' Ultrasonic Ranging Modules and Polaroid Electrostatic Transducers interfaced through a specially designed multiplexer to a controlling microprocessor. The system generates a series of pulses, monitors for echo detection, and DD 1j N* EDITION OF 1 NOV 65 IS OBSOLETE UNCLASSIFIED SECURITY CLASSIFICATION OF THIS PAGE (When Dele Eutere,' RESo'V^C!) REPORTS DIVISION NAVAL POSTGRADJAiE SCh'OOL MONTEREY, CAUfORNIA NAVSEA TECHNICAL REPORT No. ^ TR-OOOl A MULTI-ELEMENT ULTRASONIC RANGING ARRAY NAVSEA CASE 8A-1235 PREPARED BY: LCDR BART EVERETT, USN DIRECTOR, OFFICE OF ROBOTICS AWD AUTONOMOUS SYSTEMS (SEA 906) JANUARY 1985 Approved for public release; distribution is unlimited,, NAVAL SEA SYSTEMS COMMAND WASHINGTON, D.C m NAVSEA TECHNICAL REPORT No G-TR-0001 A MULTI-ELEMENT ULTRA-SONIC RANGING ARRAY NAVSEA CASE Approved SEA 00 J i3 ^ ^^' Date APPROVED FOR PUBLIC RELEASE; DISTRIBUTION UNLIMITED NAVAL SEA SYSTEMS COMMAND WASHINGTON, DC A Multi-Element Ultrasonic Ranging Array One of the first issues for concern in the evolution of a mobile robot design is the need to provide the system with sufficient environmental awareness so as to make possible intelligent movement. The first step towards this end consists of the acquisition of appropriate information regarding ranges and berarings to nearby objects, and the subsequent interpretation of that data. Several methods for approaching this problem have been proposed and investigated by numerous researchers, and can be broken down into two broad categories: passive devices, such as stereoscopic vision and swept-focus ranging sye^tems, and active devices, such as laser and ultrasonic rangefinding systems. Th3S article? deelcribes the mast widely used ultrasonic ranging system employed today for this particular application, discusses some of the problems associated with its use, and then presents one method for overcoming some of these problems through the use of multiple transducers arranged in a sequentially fired array (F-iqu'-e 1), with temperature compensation. The ranging modules employed on ROEiAFrr II ( A Second Generation Autonomous Sentry Robot , ROBOTICS AGEI, ; ;Kx ;; ; ; ;M; ;- S5) were made by Texas Instrumients (Figure 2) for use with the Polaroid electrostatic ultrasonic transducer, and were selected due to their low cost, high reliability, and ease of interface. An alternative system made by Massa F-'roducts Corporation riode?l E-2M»0) was evaluated but not se^lected because the unit cost (over S160) made this choice impractical for a mu.l t i-el ement array Pi,.r. 1. Front We. P-too. prototype.entry rb,^^. Showing location of censors in five ^^^^^^^ '/^ replaced by two i^s :rs;;tcti:^n'p;;r.. ;^ P.oto%ir.r..y^:? N... B...=. Weapons Center, White Da^t, MD. ) Figure 2. Line drawing of the Texas Instruments Ultrasonic Ranging Module made -for use with the Polaroid electrostatic transducer. Seven such boards are interfaced through a specially designed multiplexer to a single parallel por t on the controlling microprocessor. requiring several modules. By comparison, Polaroid Corporation offers both the transducer and ranging module circuit board for only $35 a set when purchased'in quantities of ten. An improved version of the circuit board, the SN2SS27, is now available from Te ;as Instruments which greatly reduces the parts count and power consump'ti on, as well as simplifying computer interface requirements ( An Ultrasonic Ranging System , BYTE, October S4). The Polaroid ranging module is an active time of fiiqht device developed for automatic cs-.mers. focusing, and determines the range to target by measuring elapsed time between the transmission of a chirp of pulses and the detected echo. The chirp is of one millisecond duration and consists of four discrete frequencies transmitted back-to-back: 8 cycles at 60k:H2, S cycles at 56kH2, Ivt. cycles at 52.5kHz, and 24 cycles at 49.41t.Hr. This technique is employed to increase the probability of signal reflection from the target, since certain surface characteristics could' in fact cancel a single-frequency wavefoi-m, preventing detection. It should be recognized, however, that the one millisecond length of the chirp is a significant source of potential error., in that, sound travels roughly 1 ICHJ feet per second at sea level, which equates to about 13 inches per millisecond. The uncertainty and hence error arise5^ from the fact that it is not known which of the four frequencies making up the chirp actually returned to trigger the receiver, but timing the echo always begins at the start of the chirp. A second very important characteristic of the Polaroid system is the use of a stepped gain control in the receiver section, where both the qain and the D of the amplifier B.r^ increased as a -function of time following chirp transmission. This ensures a high siqnal-to-noise ratio while matching the relative amplification level to the strength of the returned echo, which decays rapidly as a function of distance and hence time). This becomes an im.pdrtant factor in the design of an Brray of sequential emitters, where residual or multiple echos could easily confuse the ne;-;t element in the B.rray. A faint residual echo generated by a previous chirp of another sensor would be in all probability too weak for detection by the now active ranqefinder since its own gain had not yet been increased to the required level. To understand the advantages of the sequential array it is necessary to have a good feel for the strengths and weaknesses of ultrasonic ranging in general, keeping in mind that the ultimate goal IS to be able to repeatedly obtain accurate range information on objects surrounding a mobile platform. This dictates that power consumption be kept to a minimum and that the system be capable of operating in real time, where real time depends to some e ;tent on how fast the robot travels. These two constraints make a mechanically positioned sensor less than debirable, in that precious time and energy are wasted while the sensor is being repositioned to tat::e ranges in a new direction. The ideal solution would be to employ a multitude of prepositioned transducers that could be individually selected at will, thus enabling the robot to get range information in any given direction at any particular time. Since in reality there is associated with each sensor some overhead in terms of physical Long Range Proximity Detector Ultrasonic Transducers #0, #1 Smoke Detector Ambient Temperature Sensor Speaker Passive Infrared Sensors Ultrasonic Transducers #5, #6 Passive Infrared Sensor ##i Ultrasonic Transducers #2, #3, #4 Tactile Bumpers Figure 3. Front; view line dr-awinq depicting the Ic'cation of various sensors which provide the robot with information on its en-'i ronment. Temperature sensor at right in diagram on icm't access door provides information to correct for the temperature dependence of the speed of sound in air. space requirements, power consumption, interface circuitry, and acquisition cost, an array size of five transducers was chosen for implementation on ROBART II (Figure 3 . In addition, two more sensors were mounted on the robot's head, which is positionable up to 100 degrees either side of centerline. This configuration complements the fi.vied array for rangefinding outside its area of coverage. For any ultrasonic ranging system there exists a multitude of error sources that must be understood and taken into account. In Figure 4 it is shown that the speed of sound in air is proportional to the square root of temperature in degrees Rank:ine, which for the temperature variations likely to be encountered in this application, results in a significant effect even considering the short ranges involved. Temperature variations over the span of 60 to 80 degrees F can produce a range error as large as 7,8 inches at a distance of 35 feet. Fortunately, th:s situation is easily remedied through the use of a correction factor based upon the actual room temperature, available to F.'DBART II with an accuracy of 0.5 degrees F from an e.-;ter-nel sensor mounted on the left access door. This sensor (Industrial Computer Designs, Remote Temperature Sensor RTS-l) produces an output voltage which varies from.80 to 4.80 volts over the temperature range of 20 to 120 degrees F, and is interfaced to the system through an eight-bit analog-to-digital converter (Figure 5 . The ranging units are calibrated at standard room temperature (70 degrees F), and then the correction factor is applied to adjust for actual conditions. The formula is simply: actual range equals measured range times the correction Speed of Sound = v^ g^ k R T Where; c = speed of sound (feet/second) 9c - oravhational constant k = ratio of specific heats (for air = 1.4) R = gas constant for a specific gas T = temperature (degrees Rankine) Substituting in appropriate values for air yields: c = V (32.3) (1.4) (53.3) T = stt ft/sec Which says the speed of sound in air is proportional to the square root of local tehiperature, in degrees Rankine (degrees Farenheit degrees). At 70 degrees F: c = V ^ = 1128 ft/sec At 30 degrees F: c = V = 1085 ft/sec Distance d traveled in feet over time t seconds is given by: d = ct which yields: where subscripts S and A d/\ / c/x = t = ds / cs denote standard and actual conditions, respectively. Thus the formula for the actual distance measured by an ultrasonic ranging unit calibrated at standard temperature Ts is: da = (ds) (ca/cs) = ds V TA / Ts As an example, for a system calibrated at 80 F operating at an actual temperature of 60 F, a measured range of 35 feet corresponds to an actual range of da = 35 \ / For an = feet error of inches Figure 4. Derivation of the temperature dependence of the spet^d of sound in air. The effects of humidity on k and R B.re considered insignificant for this discussion. 8 SYSTEM ARCHITECTURE - ROBART 11 I CPU #6 VISION (8 BIT) T STEREO VISION TELEMETRY CPU #7 SYNTHESIS 18 BIT) I CPU #5 SPEECH (8 BIT) I CPU #0 PLANNER (16 BIT) i CPU #1 SCHEDULER {8 BIT) =3 CPU #4 DRIVE (8 BIT) 1 T PWM CPU #8 RECOGNITION (8 BIT) ENCODERS INTERRUPT SOURCES DIAGNOSTIC INPUTS TRANSDUCERS Figure 5. Control hierarchy for RDBART II. The ambient temperature sensor is interfaced to CPU #2 via a 16 channel A/D converter. The 7 ranging modules are inter-faced to CPU #3 through a special multiplexing circuit which allows them to be individually activated in sequence upon Scheduler. command -from the factor, where the correction factor is the square root of the ratio of actual temperature to etandard temperature, in degrees Rankine. The possibility does still exist, however, for temperature gradients between the sensor and the target to introduce range errors, in that the correction factor is based on the actual temperature near the sensor only. All other sources of error can be attributed to properties of the target itself, the transducer, or the timing and processing circuitry and software. Previously it was mentioned that the one millisecond length of the transmitted chirp introduced an uncertainty into the timing process. In addition, random electrical or ultrasonic noise, if not properly discriminated by the receiver circuitry, can lead to erroneous information. But for the most part it can be shown that the more significant errors arise from the various ways the ultrasonic beam emitted by the transducer interacts with the target, as discussed below. The width of the beam is determined by the transducer diameter and the operating frequency. The higher the frequency of the emitted energy, the narroirfer and more directional the beam, and hence the higher the angular resolution. Unfortunately, an increase in frequency also causes a corresponding increase in signal attenuation in air, and decreases the maximum range of the system. For the Polaroid transducers the chosen frequencies which make up the chirp result in a beam width of approx i mate;! y thirty degrees. Best results s.re obtained when the beam centerline is maintained normal to the target surface. As the 10 angle of incidence varies from the perpendicular, however, note that the range actually being measured does not always correspond to that associated with the beam centerline, as shown in Figure 6. The beam is reflected first from that portion of the target that., is closest to the sensor. In fact, at a distance of 15 feet from a flat target, with an angle of incidence of 70 degrees, the theoretical error could be as much as 10 inches, in that the actual line of measurement intersects the target surface at point B' as opposed to point A. The problem is further complicated for surfaces of irregular shape. The width of the beam introduces an uncertainty in the perceived distance to an object from the sensor, but an even greater uncertainty in the angular resolution of the ob.iect^s position. A very narrow vertical target such as a long wooden dowel maintained perpendicular to the floor would have associated with it a relatively large region of floor space that would essentially appear to the sensor to be obstructed. Worse yet, an opening such as a doorway may not be discernable at all to the robot when only six feet away, simply because at that distance the beam is wider than the door opening. In fact, using a one inch diameter vertical dowel as a target, the effective beam width of the Polaroid system was found to be 36 inches at a distance of only 6 feet from the sensor. The doorway detection problem is illustrated in Figures 10 and 11. Another significant error occurs when the angle of incidence of the beam decreases below a certain critical angle, and the reflected energy does not strike the transducer (Figure If. This 11 / TRANSDUCER Figure 6. Due to beam divergence, ultrasonic ranging works best when the beam centerline is maintained normal to the target's surface. For off normal conditions, the range measured does not always correspond to that associated with the beam centerline. 12 TRANSDUCER Figure 7. As the anqle of incidence decreases below a certain critical angle, the reflected energy will not be detected by the transducer, resulting in erroneous range information. For specular reflection from smooth surfaces, the angle of reflection /? is equal to the angle of incidence oc. 13 occurs because most targets are specular in nature with respect to the relative^ly long wavelength (roughly 1/4 inch) of ultrasonic energy, as opposed to being dif-fuse. In the case of?cular reflection, the angle of reflection is equal to the spec IS in diffuse reflection energy is angle of incidence, wherea; scattered in various directions, caused by surface irregularities equal to or larger than the wavelenth of incident radiation. The critical angle is thus a function of the operating frequency chosen, and topographical characteristics of the target. For the sensors used on ROBART II this angle turns out to be approximately 65 degrees for a flat target surface made up of unfinished plywood. In Figure 8 the ranging system would not see the target and indicate instead maximum range, whereas in Figure 9 the range reported wou]d reflect the total roundtrip through points A, B, and C as opposed to just A and B. The relatively long range capability (approximately 35 feet) of the Polaroid system ma :es it well suited for gathering range ^^^^^ xpp both navigational planning and collision avoidance. Navigational planning involves making a determination of where the robot is, and in addition its particular orientation in that ^.-,r-,t a-. well as the subseguent calculation of appropriate commands to move the robot to a new location and orientation.. The simplest case reduces the problem to two dimensions with a priori knowledge of the surroundings in the form of a memory map, or world model. The task becomes one of trying to correlate a reb.]- world sensor-generated image to the model, and extracting position and orientation accordingly. Several factors complicate tl'ie problem. 14 F-aure 8. For smooth surfaces, the ranging system will ill n not see the Ln ahead of the robot, and will erroneously m di cate fnakiftium range instead. ^//z/////////;;?^^//^/^///^^^^^^ 11 _^xi=f-+- the round trip Fiaure 9. The measured range will reflect tne r ^^^^ distance through points A, B, and C as opposed to the distance from A to B. 15 For one thing, the real environment is three-di mensi one],, and although the model represents each object as its projection on the X-Y plane, the sensor may see things differently, which complicates the task of correlation. Secondly, the computational resources required are large, and the process is time consuming, requiring the robot at times to stop and think . Also, the acquisition of the data itself can take several seconds using ultrasonic ranging techniques, due to the relatively slow velocity of soundwaves in air. More importantly for the purposes of this discussion, however, ^re the effects of the various error sources previously described, which can collectively impede a solution altogether. Figure 10 depicts the results of 256 range values taken by a single sensor mounted on the head of ROBART II, with the robot situated approximately 5 feet from the wall as shown. The data took approximately 7 seconds to collect as the head was mechanically repositioned between rangings. The process could have been speeded up to some extent by reducing the number of range readings taken while the head was scanning. Note, how?ver, in Figure 10 that only two positions of the head allowed the beam to pass through the doorway. Had the number of positions been reduced from 256 to 100, it is possible that the doorwav would have escaped detection altogether. The resulting plot is of exceptional quality primarily due to the nature of the walls themselves, which were located in a basement room with exposed studs, thereby providing excellent beam return properties. The proper identification of the open 16 Y-AXIS (FEET) 10.0 SONAR PLOT 2 - BASEMENT - 29 MARCH _ X-AXIS (FEET) 8.0 Figure 10- Flot. o-f 256 range readings taken by s^ eincile mechanically positioned sensor mounted on the head of ROBHRT II. An open doorway is detected in the wall approximately 5 feet directly
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