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FACTS AND FIGURES
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First some statistics. At sea level, sound travels at a speed of about 1130 feet per second (about 344 meters per second) or 13,560 inches per second. This time varies depending on atmospheric conditions, including air pressure (which varies by altitude), temperature, and humidity. The time it takes for the echo to be received is in microseconds if the object is within a few inches or even a few feet of the robot. The short duration is really no problem, however, for fast-acting CMOS and TTL ICs. The overall time between transmit pulse and echo is divided by two to compensate for the round-trip travel time between the robot and the object. Given a travel time of 13,560 inches per second for sound, it takes just 73.7 microseconds (0.0000737 seconds) for sound to travel one inch. With this figure in the back of our minds, let s consider how the Polaroid ranging system works. The Ranging Module is connected to a computer or microcontroller using only two wires: INIT (for INITiate) and ECHO. INIT is an output, and ECHO is an input. The Ranging Module contains other I/O connections, such as BLNK and BINH, but these are not strictly required when you are determining distance to a single object, and so they will not be discussed here. To trigger the Ranging Module and have it send out a burst of ultrasonic sound, the computer or microcontroller brings the INIT line HIGH. The computer-microcontroller then waits for the ECHO line to change from LOW to HIGH. The time difference, in microseconds, is divided in two, and that gives you distance. To measure the time between the INIT pulse and the return ECHO, the computer or microcontroller uses a timer to precisely count the time interval. Different timing-counting approaches are used depending on the computer or microcontroller you are using. For example, with the Basic Stamp or BasicX microcontrollers (see s 31 and 32, respectively), you might use the RCTime function, which is normally used to time how long it takes for a capacitor to discharge. There is no capacitor to discharge in the Ranging Module, but the overall timing technique is still the same. With the OOPic microcontroller (see 33), you might use its oTimer object. Let s suppose you re using the BasicX microcontroller. The short bit of code in Listing 38.1, which is taken from the BasicX application note on ultrasonic ranging, uses pins 15 and 16 of the chip to connect to the ECHO and INIT lines, respectively, of the Polaroid Ranging Module. The lines of the Ranging Module are connected as shown in Fig. 38.13. Note that the BLNK and BINH lines are held LOW and that the power supply to the Polaroid Ranging Module must not come from the on-board regulator of the BasicX. The Polaroid Ranging Module needs a far more robust power supply that is capable of delivering an amp or two of current for a brief period of time. Four AA batteries connected in series will suffice. Connect the ground from the 6-vdc battery pack to the ground points of Polaroid Ranging Module and the BasicX.
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LISTING 38.1.
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' Connect pin 15 of BasicX to ECHO, pin 16 to INIT Private Const EchoPin As Byte = 15 Private Const InitPin As Byte = 16
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ULTRASONIC DISTANCE MEASUREMENT
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Sonar power
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Sonar module INIT ECHO Gnd
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470
15 BasicX Gnd
FIGURE 38.13 Basic sonar connection diagram for use with the BasicX microcontroller. Note the separate 6 vdc power supply for the sonar module (you can use a 5 7 vdc power supply that delivers 1 2 amps peak for a short period of time).
' Echo delay (floating-point variable) Dim EchoDelay As Single ' Speed of sound at room temperature (meters per second) Const SpeedOfSound As Single = 344.0 ' Take INIT HIGH and make a ping on the transducer Call PutPin(InitPin, bxOutputHigh) ' Wait for echo to be returned Call RCTime(EchoPin, 0, EchoDelay) ' Take INIT line LOW Call PutPin(InitPin, bxOutputLow) ' If no echo RCTime overflows and returns 0.0 If (EchoDelay = 0.0) Then Range = 11.0 Else Range = (EchoDelay / 2.0) * SpeedOfSound End If
When INIT is taken HIGH, the Polaroid Ranging Module emits a short burst of ~50 kHz sound from the transducer. The module then waits for a period of 2.38 milliseconds for the transducer to stop ringing. This is the period of time it takes for the sonar ping to travel about 32 inches. Considering round-trip time, this equates to the 1.3-foot minimum imposed by the system. After this so-called blanking period, the Polaroid Ranging Module listens for the return ECHO. When an echo is detected, the ECHO line goes HIGH. Note that the module itself does not do any timing; this is the domain of the microcontroller that is connected to the module.
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