Transducer Fundamentals Ultrasonic Transducers Exercise 2: Distance Measurement EXERCISE OBJECTIVE At the completion of this exercise, you will be able to explain and demonstrate the operation of ultrasonic measure or ruler and an oscilloscope. DISCUSSION Sound waves, including those in the ultrasonic range, can travel in virtually any medium (solid, liquid, or gas). The velocity at which the waves travel depends on, among other things, temperature and the transmission medium. You can therefore calculate the velocity at any temperature T (in C ) with the following formula: A typical value for room temperature is 25 C. Calculate the velocity of sound waves at room temperature: v = Recall Value 1) Using the conversion factor 3.28 feet = 1 m, calculate the velocity of sound in air at 25 C in feet per V fps 346 m 3.28 ft = s 1 m V fps = Recall Value 2) You can measure the time between the transmitted and received pulses to determine the distance of a target object from the transducers. Since distance is the product of velocity and time (d = v x t), you can calculate the distance by using the velocity constant and the measured time. For example, assume that you measure 2 ms between pulses. Using the velocity of sound in air, you can calculate the total distance (d T ) that the waves travel as follows: d T = v x t 1135 ft 0.002 second dt = = 2.27 feet second FACET by Lab-Volt 293
Ultrasonic Transducers Transducer Fundamentals With the transducer arrangement on your circuit board, the ultrasonic waves travel a distance d from the transmitter to the target. What is the total distance (d T ) traveled by waves sent by the transmitter and detected by the receiver? a. d b. c. 2d In the preceding example, the time between pulses represented a distance of 2.27 feet. This is the total distance that the ultrasonic waves travel from transmitter to receiver. Since d T T You can revise the basic equation to calculate the one-way distance to the target object as follows. The maximum measuring distance of the transducers on your circuit board is determined by the 9.17 ms d max d max = feet (Recall Value 3) You can also rearrange the equation to calculate the transit time from a given target distance. Calculate the transit time for a distance of 1 foot. t = ms (Recall Value 4) 294 FACET by Lab-Volt
Transducer Fundamentals Ultrasonic Transducers PROCEDURE In this PROCEDURE, you will use your oscilloscope and the ultrasonic transducers on your circuit board to measure distance. Move the base unit so that its front edge is near the edge of the bench. These steps help For best results, it is also necessary to clear away any other nearby objects that may cause false readings. Items such as scope leads, instruments, chairs, and even other students should be kept away from the measurement area. Insert a two-post connector in the BLOCK ENABLE position of the ULTRASONIC TRANSDUCERS circuit block. FACET by Lab-Volt 295
Ultrasonic Transducers Transducer Fundamentals Connect oscilloscope CH 1 to the left transmitter terminal, and CH 2 to AMP in the RECEIVER section of the circuit block. Ground the scope probes to GND. 296 FACET by Lab-Volt
Transducer Fundamentals Ultrasonic Transducers Temporarily move the base unit back from the edge of the bench. Place an object with a object should be directly in front of the ultrasonic transducers, and should be kept at a right angle to the benchtop. Adjust the scope controls to view the waveform shown. Locate the FREQ pot in the TRANSMITTER section. Rotate the pot through its entire range as you observe the effects on the AMP waveform. Adjust the pot for the maximum amplitude on the AMP waveform. a. resonant frequency. b. antiresonant frequency. Do not change the FREQ pot setting for the remainder of this PROCEDURE. FACET by Lab-Volt 297
Ultrasonic Transducers Transducer Fundamentals Put the book aside and return the base unit to its original position at the edge of the bench. Move the CH 2 probe to OUT in the RECEIVER section of the circuit block. MODE to CHOP. Make sure there is nothing in front of the transducers for a distance of about 6 feet in order to view the signals without a target object. Adjust the scope controls for the waveforms shown. If more than two pulses appear in the OUT waveform, the transducers are detecting a target object that should be removed from the measurement area. 298 FACET by Lab-Volt
Transducer Fundamentals Ultrasonic Transducers CH 1 shows the ultrasonic tone bursts that are generated by the transmitter. CH 2 displays the pulses output by the receiver. You can see that, with no target object detected, the receiver outputs a pulse for each transmitter tone burst. The receiver pulse is caused by a. an echo from a distant target object. b. the receiver picking up the transmitter pulse. c. Neither of the above FACET by Lab-Volt 299
Ultrasonic Transducers Transducer Fundamentals The period of the XDCR waveform determines the maximum distance measuring range of the transducers. As you calculated in the DISCUSSION, the period of approximately 9.17 ms corresponds to a range of about 5.2 feet. Place CM 9 in the ON position to change the maximum measuring range. The scope waveforms should appear as shown. 300 FACET by Lab-Volt
Transducer Fundamentals Ultrasonic Transducers reading). t OUT = ms (Recall Value 1) Calculate the maximum measuring distance in feet with CM 9 activated. d max t = ms (Step 18, Recall Value 1) d max = feet (Recall Value 2) Place CM 9 in the OFF position to restore the original measuring range. Return the sweep Hold the book a distance of 1 foot from the front face of the base unit, using the ruler as a reference. Make sure there are no objects in the measurement area that can cause a false reading. The book should be parallel to the front of the base unit when viewed from above and from the side. FACET by Lab-Volt 301
Ultrasonic Transducers Transducer Fundamentals The OUT waveform should now appear as shown here. If additional OUT pulses appear, check again to see that there are no other objects in the measurement area. back to the receiver. To minimize this possibility, make sure that the book is parallel to the front of the base unit when viewed from above and from the side. The second OUT pulse represents the echo back from the target. waves to travel from the transmitter to the target and back to the receiver (transit time). t (1 foot) = ms (Recall Value 3) 302 FACET by Lab-Volt
Transducer Fundamentals Ultrasonic Transducers Is your measured value of ms (Step 22, Recall Value 3) for a 1-foot distance close to the value you calculated in the DISCUSSION? a. yes b. no Increase the distance between the book and the transducers to 2 feet. Adjust the scope controls to view the waveforms shown here. If more than three pulses appear on the OUT waveform, make sure that no other objects are in the measurement area and that the book is parallel to the front of the FACET base unit. t (2 feet) = ms (Recall Value 4) Convert the transit time to distance. t (2 feet) = ms (Step 25, Recall Value 4) d = feet (Recall Value 5) FACET by Lab-Volt 303
Ultrasonic Transducers Transducer Fundamentals Increase the distance between the book and the transducers to 3 feet. Adjust the scope controls to view the waveforms shown. If more than three pulses appear on the OUT waveform, make sure that no other objects are in the measurement area and that the book is parallel to the front of the FACET base unit. t(3 feet) = ms (Recall Value 6) Convert the transit time to distance. t (3 feet) = ms (Step 29, Recall Value 6) d = feet (Recall Value 7) This table shows the distance you determined by measuring with the ruler and by calculating from the transit times at 2 and 3 feet. Distance measured with ruler Distance measured by transducer pulse time 2 feet (Step 26, Recall Value 5) 3 feet (Step 30, Recall Value 7) Are the two values about the same? a. yes b. no 304 FACET by Lab-Volt
Transducer Fundamentals Ultrasonic Transducers Slowly move the book away from the base unit as you observe the waveforms. The second pulse (the echo from the target) moves farther to the right as the distance increases. Continue to move the book away until the second pulse disappears. Why does the pulse disappear? a. The directivity angle increased. b. The next transmitter pulse was picked up before the echo from the target object. c. Both of the above Make sure all CMs are cleared (turned off) before proceeding to the next section. CONCLUSION the time it takes for transmitter pulses to echo back from the object to the receiver. In a two-transducer measuring system, your distance calculations must account for the fact that the ultrasonic waves travel twice as far as the one-way distance to the target object. The maximum measuring distance is limited by the transducer sensitivity and by the period of the clock pulse used to generate the transmitter tone bursts. REVIEW QUESTIONS 1. A book is placed 2 feet, 6 inches from the front of your FACET base unit. How long does it take for ultrasonic waves generated by the transmitter to echo off the book and be detected by the receiver? a. 3.5 ms b. 4.4 ms c. 45.8 ms d. 4.4 s 2. If you measure a transit time of 7 ms for the transducers on your circuit board to detect an object, what is the approximate distance of the object from the front of the base unit? a. 1 foot b. 2 feet c. 3 feet d. 4 feet FACET by Lab-Volt 305
Ultrasonic Transducers Transducer Fundamentals 3. Transit time is the time it takes for the ultrasonic waves to travel from the a. transmitter to the target object. b. target object to the receiver. c. target object to the transmitter d. transmitter to the target object and back to the receiver. 4. determined by the transducer sensitivity and by a. the CLK period. b. the oscillator frequency. c. room temperature. d. All of the above 5. What is the velocity of sound waves in air at 20 C? a. b. c. d. 306 FACET by Lab-Volt