ULTRASONIC SENSOR INFORMATION One of the primary concerns in the development of a mobile robot design is providing the system with enough environmental awareness to make intelligent movement possible (like running slaloms or mazes). A first step toward solving this issue is to acquire appropriate information about ranges and bearings to nearby objects or obstacles and to interpret that data properly. Several ways to approach this problem have been well covered in robotic literature. They can all be grouped into one of two major categories. The first are passive devices, such as stereoscopic vision and swept-focus ranging systems. The second are active devices, such as laser, microwave and ultrasonic rangefinding systems. At the end of this article, three sources of sonar ranging devices have been provided. Our club robot, RSSCy, has been a work horse for all kind of experiments. This article will cover what was discovered from experiments that have been placed on RSSCy and those conducted by other members of RSSC. This article will describe what the results have been when acoustical devices are used as range and collision sensors. No in depth discussion of temperature or altitude density compensation will be covered. As far as temperature is concerned, a 20 degree F temperature differential only contributes a 7.8 inch error at 35 ft. If your concerned about this small error, then install a thermistor on your robot to determine local temperature and make the compensation in your computer program. The altitude density error contribution is even less than encountered by temperature swings. The ranging modules employed were made by Texas Instruments for use with the Polaroid electrostatic ultrasonic transducer and were selected for their low cost, high reliability, and ease of interface. Polaroid offers both the transducer and ranging module circuit board for only $40 (1993 pricing) a set when purchased in quantities of ten. Texas Instruments developed an improved version of the circuit board which greatly reduced the parts count and power consumption, and simplified computer interface require- ments. Texas Instruments no longer produces the chips but they can be obtained directly from Polaroid. The Polaroid ranging module is an active time-of-flight device developed for their cameras to allow automatic camera focusing. It determines the range to a target by measuring elapsed time between the transmission of a "chirp" of pulses and the detected echo. The one millisecond chirp consists of four discrete frequencies composed of 8 cycles at 60 KHz, 8 cycles at 56 KHz, 16 cycles at 52.5 KHz, and 24 cycles at 49.41 KHz. This pulse train is designed to increase the probability of signal reflection from a wide range of targets. Certain conditions and surface characteristics of the target can in fact cancel a return of a single-frequency waveform. It should be recognized that the one-millisecond length of the chirp can also be another source of potential error. Sound travels roughly 1100 feet per second at sea level or equivalent to about 13 in. per millisecond. The timing of the transmit pulse to the echo always begins at the start of the chirp. The uncertainty and thus the potential error arises from the fact that it is not known which individual pulse of the four frequencies making up the chirp is the first to return to trigger the receiver. A second important characteristic of the Polaroid system is the use of stepped gain control in the receiver section. Both the gain and the Q of the amplifier are increased as a function of time following chirp transmission. This ensures a high signal-to-noise ratio while matching relative amplification levels to the strength of the returned echo. The echo decays rapidly as a function of distance and time. SEQUENTIAL ARRAYS VS MECHANICAL POSITIONING To understand the advantages or disadvantages of the sequential array vs mechanical positioning, it is necessary to consider power consumption and timeliness of the ultrasonic ranging information. We must keep in mind that the ultimate goal is to be able to repeatedly obtain accurate range information on objects surrounding a robot. Power consumption must be considered and kept to a minimum to extend battery life. The system should also be capable of operating in real time, which depends to some extent on how fast the robot travels. The power and the real time constraints seem to make a mechanically positioned sensor the less desirable of the two concepts, because of the slew time delay and energy used while the sensor is being repositioned to provide range in a new direction. The classic solution is to use a large number of prepositioned transducers that could be individually selected. Literature indicates that a number of University and industrial robots use this scheme. This enables the robot CPU to obtain range information in any given direction at any particular time. Since each sensor is associated with some overhead in terms of physical space requirements, power consumption, interface circuitry, and acquisition cost, an array size is the designers choice. Our club robot originally had one transducer on the head (355 degree rotation) with three other fixed position transducers. One of these was straight ahead in a downlooking (don't fall in a hole) position. The other two were side looking at about a 45 degree angle to serve as collision detection units. This was all based on NEWTON equipment installed in RSSCy at that time. The next concept chosen was a six transducer array. Two sensors were mounted on the robot's rotating head platform that can be mechanically positioned up to 170 degrees either side of centerline and tilted up or down +/- 75 degrees. The mechanically rotated head sensors were installed to provide for angular beamsplitting in the array. A paralleled dual sensor set was installed on either side of RSSCy. The head configuration complemented the fixed array for range finding outside their area of coverage. Jerry Burton's new K9 (dog type robot) has the capability of driving a series of eight sonar transducers which can be sequentially sampled or read at any interval or sequence that is required by the program. Two sensors are installed on individually operated "ears" and the rest are installed one front, one rear, and two each on the sides. In order to reduce the cost of having to buy a timing board and a transducer for each look angle required in the arrays, multiplexing was considered. Using a multiplexing scheme, only one timing board is required for eight sonar transducers. The Transmit/Receive signal of the timing board was multiplexed through eight relays. Relays were chosen because the pulse voltage to the transducer is 300 volts or more. These are the small reed relays that have TTL compatible coils. Three lines from an 68HC11 are used to drive a mux chip (3 to 8) which then selects the appropriate relay and transducer. I think that a PIC could handle the specialized job of selection, timing, and I/O to the Robot Main Processor quite adequately. POTENTIAL RANGE ERRORS It should be noted that the range actually being measured does not always correspond to that associated with the beam centerline, as shown in Figure 1. The beam is reflected first from that portion of the target closest to the sensor. In fact, at a distance of 15 ft. from a flat target, with an angle of incidence of 70 degrees, the theoretical error could be as much as 10 in., since 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 between object and sensor but an even greater uncertainty in the angular resolution of the object'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 is not discernible at all to the robot when only 6 ft. away, because at that distance the beam is wider than the door opening. In fact, using a 1-in. diameter vertical dowel as a target, the effective beam width of the Polaroid system was found to be 36 in. at a distance of only 6 ft. from the sensor. The doorway detection problem is illustrated in Figures 2a and 2b. It should be realized that the width of the beam also limits the ability of the acoustic sensor to look deep into a corner. The return from the side lobes are reflected first thereby effectively reducing the range reading. FOCUSING OF THE ACOUSTICAL BEAM A search of the robotic literature indicated that some experimenters were achieving reduced beam width by focusing the acoustical transmission by use of reflectors, tubes and horns. The reflectors do work but they are sometimes ten times the diameter of the transducer. Not too useful for a small robot. It was found that tubes and horns are useful for reducing the beam spread of the acoustical transmission. We tried long horns, short horns, funnels of every appreciable size, shape, and aspect ratio. The most important facet of the exercise was to discover that to be most effective, each horn or tube must be lined with a soft felt material. This obviously absorbs or at least attenuates the side lobes of the transmitted wave. The beam width can be effectively reduced to about 10 degrees or less. REDUCE RECEIVER GAIN TO NARROW RESPONSE The beam width can also be effectively reduced or angular accuracy increased by reducing the sensitivity of the receiver circuitry. The side lobes of the transmitted wave (see fig 3) are about 30 DB down in power from the main lobe. When the receiver gain is reduced to only be responsive to returns from the main lobe, the angular accuracy can be increased by a factor of two or more. We have been toying with the idea of generating a variable gain receiver, driven from the processor. The program would allow full gain until receiving a response, then processor would reduce gain to improve on the angular accuracy of the response. The processor should be able to determine if the return was from the main beam or from a side lobe. REFLECTIVE OR SPECULAR ERRORS Another significant error occurs when the angle of incidence of the beam de- creases below a certain critical angle and the reflected energy does not return to strike the transducer (Figure 4). This occurs because most targets are specular in nature with respect to the relatively long wavelength (roughly 0.25 in.) of ultrasonic energy, as opposed to being diffuse. In specular reflection, the angle of reflection equals the angle of incidence; in diffuse reflection, energy is scattered in various directions by surface irregularities. The critical angle is thus a function of the operating frequency chosen and the topographical characteristics of the target. For the sensors used on RSSCy this angle is approximately 65 degrees for a flat target surface of unfinished plywood such as would be encountered for MAZE walls or for dividers in the wall following contests. In Figure 4a the ranging system would not see the target and would indicate instead maximum range, whereas in Figure 4b the range reported would reflect the total round trip through points A,B, and C as opposed to just A and B. RANGE DATA The relatively long-range capability (approximately 35 ft.) of the Polaroid system makes it well-suited to gathering range data for both navigational planning and collision avoidance. Navigational planning involves determining the actual location of the robot and subsequently calculate the appropriate commands to move it 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 realworld, sensor-generated image to the model and extracting position and orientation accordingly. Several factors complicate the problem. For one, the real environment is three dimensional, and although the model represents each object as its projection on the X-Y plane, the sensor may see things differently, complicating the task of correlation. Second, large computational resources are required and the process is time consuming, requiring the robot to stop and think occasionally. Also, acquiring the data can take several seconds using ultrasonic ranging techniques, due to the relatively low velocity of sound waves in air. More important for the purposes of this discussion, however, are the effects of the various error sources previously described, which can act collectively to impede a solution. Figure 2a depicts the results of 30 range values taken by a single sensor mounted on an azimuth table, with the sensor approximately 5ft. from the wall. The exceptional quality of the plot is due primarily to the nature of the walls having a pebble grain finish that provided excellent beam return properties. The proper identification of the open doorway and the excellent correlation with the actual map would provide the robot with a highly accurate "fix." It should be noted that the room was fairly uncluttered, which is not always the case presented to our robots. In Figure 2b, the sensor was repositioned 7ft. from the wall and was unable to detect the opening. For such situations, the robot needs help from other types of sensors or some type of narrowing or focussing of the beam. Collision avoidance is a little easier to address, in that angular accuracies are less important and the computational overhead nowhere near so great. The intent is simply to be aware of obstructions in time to alter course. For this application the sequential array can outperform a single sensor, in that the array permits range measurements to be made in many different directions very quickly and with minimal power consumption. BEAM SPLITTING TECHNIQUE Sequential arrays can use beam-splitting to improve the angular resolution, already shown to be some what poor for a single transducer. Beam splitting involves the use of two or more range finders with partially overlapping beam patterns. Figure 5 shows the simplest case of two transducers, twice the angular resolution can by obtained, along with a 50 percent increase in coverage area. The technique is simple. As the target is detected by both sensors A and B, then it (or at least a portion of it) must lie in the region of overlap shown by the shaded area. If detected by A but not B, the target lies in the region at the top of the figure, and so on. Increasing the number of sensors with overlapping beam patterns decreases the size of the respective regions and thus increases the angular resolution. The sensor pattern used on RSSCy allowed for an angular resolution of less than 20 degrees when locating a 1-in. vertical dowel 9 ft. from the robot, a significant improvement over the 30 degree resolution of a single transducer. It should be noted, however, that this increase in resolution is limited to the case of a single target in relatively uncluttered surroundings, such as a box in the middle of the floor. No improvement is seen for the case of an opening smaller than an individual beam width, such as the doorway illustrated in Figure 2b. The entire beam from at least one sensor must pass through the opening without striking either door post in order for the opening to be detected, and the only way to improve resolution for this case is to decrease the individual beam widths by changing transducers or through acoustical focusing, which sometimes is impractical. TIMING OPERATIONS To simplify the circuitry, all timing and time-to-distance conversions are done in software. Three control lines interface the Polaroid ultrasonic circuit board to a microprocessor. The first of these, referred to as VSW (Figure 6), initiates operation when brought high to +V. A second line, XLOG, signals the start of pulse transmission, while MFLOG line indicates detection of the first echo. The controlling microprocessor must there- fore send VSW high, monitor the state of XLOG, commence timing when transmission begins (approximately 5 ms later), and then poll MFLOG until an echo is detected or time out when sufficient time elapses to indicate the absence of an echo. The eight ultrasonic ranging units are interfaced to the microprocessor through a 3-circuit 8-channel multiplexer using single pole relays operating in the digital mode, as shown in Figure 7. This way the microprocessor "sees" only one ranging unit at a time through the multiplexer. Three I/O lines from the 68HC11 handle this enabling function. The binary number placed on these I/O lines by the microprocessor determines which channel is selected. Three other I/O lines carry the logic inputs to the microprocessor for VSW, XLOG, and MFLOP. The implementation of the sequential ranging array using a small dedicated microprocessor offers several advantages to the mobile robot design. USE OF MULTIPLE TRANSDUCERS Using RSSCy electronics as the base for the paralleling experiment, two transducers were connected in parallel to the drive electronics. No degradation of the operation was observed. This technique can be used to increase the detection coverage (ie. wider angular range) beyond that of a single transducer. Each transducer looks to the drive electronics an a capacitor. I would think that even more additional units could be driven simultaneously as long as the drive capability of the small transformer is capable of applying the 350 volt pulse to all the units. POLAROID EVALUATION KIT I suggest obtaining one of the Polaroid evaluation kits for your club if members are interested in getting into ultrasonic acoustical ranging. The kit that has been available for years is a self contained kit with the timing board, readout board, ultrasonic transducer and battery pack. The read out board provides distance information on a small 3 digit LED unit. The information books included with the kit detail the timing circuits and the readout board capability. It also gives detailed directions on modifications for changes in range and readout. Polaroid has just announced a second developers kit that contains the timing board, microprocessor board, software, ultrasonic transducer and battery pack. In place of the read out board it includes a microprocessor board with all the support software to write programs that allow control or modification of all the parameters needed in ultrasonic acoustical ranging systems. SUMMARY Several prepositioned sensors seem to be superior in performance to mechanically positioned single-sensor systems, in that they allow data to be taken at a faster rate, with less power consumption, and with fewer errors associated with actual sensor position. Improvements in angular resolution can be gained through the use of beam-splitting techniques, and temperature/altitude correction can be employed to increase range accuracy. Properly designed horns or tubes can be used to reduce the beam width of the transmission thereby increasing the angular accuracy of the return. Gain compensation (reduction) can also help increase the angular accuracy by eliminating responses to side lobe energy. The array concept exploits the properties of ultrasonic ranging for collision avoidance or object tracking, where absolute accuracy is less important than relative information. For other applications in which precision is an important factor, such as navigation and map correlation in cluttered environments, complementary sensor sets with appropriate characteristics probably should be added. The relatively long wavelength, poor angular resolution, temperature dependence, and slow speed of sound in air can be significant drawbacks in the field of precision navigation or mapping. Infrared and laserbased rangefinders could be considered as alternative or supplementary approaches. Ultrasonic sensors when carefully applied, still remain the lowest cost and simplest means of obtaining both collision detection and longer range navigation information for our robots. ----------------------------------------- SOURCES OF EQUIPMENT From Digi-Key catalog Lists Panasonic Ultrasonic Microphone three types - separate transmitter and receiver From Sensor Magazine Massa Products Corp of Hingham, MA lists narrow-beam/no side lobe ultrasonic transducers in the 50, 100, 150, and 215 kHz transducer frequencies. POLAROID ULTRASONIC COMPONENTS GROUP OEM Product List 612366 7000 Transducer 604142 Transducer- Instrument grade 607281 Environmental Transducer 607943 Environmental Housing 615077 6500 Ranging Board 614095 Coil for Ranging Board 614096 Transformer for Ranging Board 607220 Ceramic Resonator 604789 Cable Assembly 614904 Digital Chip TL851 (6" to 35') 614905 Polaroid Digital Chip (2' to 24') 614906 Polaroid Analog Chip --------------------------------------- ABOUT THE AUTHORS - Jess Jackson is a Registered Professional Engineer and is the Editor of the ROBOT BUILDER newsletter. Jerry Burton is the president of the Robot Society of Southern California (RSSC) and owner of Processing Innovations, a company manufacturing smart modems.