A number of experiments were conducted to measure the performance of the sonar detector head. The experiments described below were designed to find the following characteristics:
• operation of the four transducers
• coefficient to convert the sensor’s one byte data to centimeters • range resolution
• minimum detectable range • maximum detectable range
• angular resolution of the servo scanning mechanism • cycle time required to range all sectors
The Polaroid transducer’s acoustic beam pattern had been studied and well documented. No effort was made to duplicate this work; rather observations were made of the PIC microcontroller’s output data and the mechanical operation of the sensor head with the servo motor operating.
All of the experiments described below were conducted in the Naval Postgraduate School’s acoustic anechoic room. The room is designed for acoustic frequencies below the 49 kHz ultrasonic frequency of the Polaroid transducer. It was immediately observed that the room’s walls reflected sufficient acoustic energy to register as valid echoes even in the absence of the test targets. No objects, aside from test targets, were allowed within two meters of the center of the robot. Walls were 2.2 to 3.5 m from the sonar transducers. Test targets were Ø 0.5 inch solid steel rods, roughly 2 feet long. The targets were positioned vertically. The air temperature during the experiments was 20.5 ºC.
During the experiments the robot remained stationary, and an AC to DC power supply was used to supply DC voltage to the electronics power bus in place of the electronics battery. The first-generation five-volt bus was energized to power the sonar sensor head controller. The five-volt bus’ servo switch was energized when needed for experiments that required scanning the sonar transducers in azimuth. Stationary sonar sensor head observations were done by leaving the servo un-powered. The motor power bus remained off, and motors were not operated. The robot’s BL2600 CPU and I2C bus power remained off.
In place of the CPU, a laptop computer with an RS-232 serial port was connected via serial cable to the serial port on the sonar sensor head controller PCB. The Hyperterminal application bundled with the laptop’s Windows OS was used to observe the serial data transmissions. The researcher observed the ASCII characters transmitted from the PIC microcontroller and then translated them into deicmal integer values to interpret the TOF.
The data were one-byte values; hence one would expect valid data over the range zero to 255. The PIC, though, is programmed to send the hexadecimal value 0x02 for any received echo with integer TOF value greater than 250. This allows it to use the ASCII characters from 251 to 255 as control flags. For example, ASCII 254 and ASCII 251 indicate the start and stop, respectively, of its data word. Sonar blanking of the
transmitted pulse governs the minimum detection range, which, in turn, limits the expected minimum one-byte value. The minimum one-byte value was expected to be approximately 19.
The first observation made was a control with the sensor head in a static condition and no targets present. No objects were observed within two meters of the robot. The sonar sensor head’s TOF data byte was recorded.
Next an experiment was conducted to verify the operation of the four transducers and to determine the linear coefficient needed to convert the one-byte TOF value to a range in centimeters. The test was static, the servo left unpowered. A target was placed at 100 cm distance in the center of each transducer’s beam pattern, and the sonar sensor head controller’s output was observed. After observing the reported TOF data byte, the distances to the four targets were verified. Two targets required small distance adjustments, as they were not precisely placed. The targets were adjusted and the TOF range bytes were recorded.
A brief test was done to determine range resolution. Following the operational test above, two targets were moved to 98 cm, while two were left in their original positions as controls. The range data bytes were recorded for the four targets.
A minimum range experiment was conducted to determine the coefficient described above and the minimum detectable range. The four targets were removed and the sensor head was reoriented manually before the minimum range test was done. With the sensor head stationary, a target was placed in the center of one of the transducer’s beam patterns at 50 cm. The distance was decreased to 30 cm, 24 cm, and then 20 cm and the TOF data were recorded at each distance. At 20 cm the target’s tripod legs contacted the robot’s wheel and prevented placement closer than 20 cm.
A maximum range test was done in much the same manner as the minimum range test. The first experiment was attempted with a target at two meters and within about 30 cm of the corner of the room, but the proximity of the room’s corner to the target prompted the researcher to abort the test and place the target in the center of the anechoic room. Beginning at 2.0 m, a target was placed in the center of the same transducer used
for the minimum range test. At each distance, the range data byte was recorded. The target was repositioned to 2.2 m, 2.3 m, and 2.4 m. At the completion of the maximum range test, the target was removed, and the range byte recorded with no target present.
Two experiments were conducted to assess the mechanical scanning of the sensor head. Unlike previous experiments, the sensor head was allowed to move to the five positions described above by powering the RC servomotor. In the first, targets were placed in the centers of three adjacent sonar ranging sectors, at 100 cm range. Measuring from the robot’s Y axis, which coincides with the number one motor shaft, a target was placed at 24º, 42º, and 60º. These positions correspond to the predicted centers of three sectors, or azimuth “bins.” After observing the reported range data three times, the center target at 42º was removed and the range data were recorded another three times. The test was repeated with three targets placed -30º, -12º, and 6º relative to the robot’s Y axis at 100 cm.
The last azimuth scanning experiment consisted of placing one target at 1 m range and varying its azimuthal position to determine which sector would register the contact. The target was initially placed at -30º relative to the Y axis. Because the sonar’s beam pattern is approximately 15º, moving the target 7.5º off the centerline of the sector was expected to produce no contact. The target was repositioned to -37º, -39º, and then to -43º relative to the Y axis while maintaining range constant at 1m. The range data were recorded for each sector in the vicinity of that expected to register the target’s presence. Analysis of the sector edge experiment prompted the researcher to conduct it a second time. In the second experiment, the target was initially placed at 1 m along the -30º azimuth. It was moved counter clockwise to -39º, -43º, and -47º positions. Then it was placed at -23º and moved to -12º and 0º.
A simple experiment was conducted to verify the time required for the scanning detector head to complete a cycle of 20 sonar-ranging measurements. The time for the sensor head to return to its initial position was measured and averaged.