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FIGURE 36.9 Ultrasonic sensors mounted on an RC servo scanner turret.
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Figs. 36.10 and 36.11 show a basic circuit you can build that provides ultrasonic proximity detection and has two parts: a transmitter and a receiver (refer to the parts list in Tables 36.2 and 36.3). The transmitter circuit works as follows: a stream of 40 kHz pulses are produced by a 555 timer wired up as an astable multivibrator. The receiving transducer is positioned two or more inches away from the transmitter transducer. For best results, you may wish to place a piece of foam between the two transducers to eliminate direct interference. The signal from the receiving transducer needs to be amplified; an op amp (such as an LM741, as shown in Fig. 36.11) is more than sufficient for the job. The amplified output of the receiver transducer is directly connected to another 741 op amp wired as a comparator. The ultrasonic receiver is sensitive only to sounds in about the 40 kHz range ( about 4 kHz). The closer the ultrasonic sensor is to an object, the stronger the reflected sound will be. (Note, too, that the strength of the reflected signal will also vary depending on the material bouncing the sound.) The output of the comparator will change between LOW and HIGH as the sensor is moved closer to or farther away from an object. Once you get the circuit debugged and working, adjust potentiometer R2, on the op amp, to vary the sensitivity of the circuit. You will find that, depending on the quality of the transducers you use, the range of this sensor is quite large. When the gain of the op amp is turned all the way up, the range may be as much as six to eight feet. (The op amp may ring, or oscillate, at very high gain levels, so use your logic probe to choose a sensitivity just below the ringing threshold.)
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c Q1 2N2222 e Ultrasonic Transducer
C2 0.1
C1 0.0033
FIGURE 36.10 Schematic diagram for a basic ultrasonic proximity transmitter.
+V +V
C1 0.01
R1 330
R2 100K +V 2 +V 7 IC1 741 3 R3 10K + 4 R8 330 2 R6 10K R7 10K 6 C2 0.01 R5 1K 3 7 IC2 741 4 6 Output
Ultrasonic Transducer
R4 10K
FIGURE 36.11 Schematic diagram for a basic ultrasonic proximity receiver.
Contact Detection
A sure way to detect objects is to make physical contact with them. Contact is perhaps the most common form of object detection and is often accomplished by using simple switches. In this section we ll review several contact methods, including soft-contact techniques where the robot can detect contact with an object using just a slight touch.
PHYSICAL CONTACT BUMPER SWITCH
An ordinary switch can be used to detect physical contact with an object. So-called bumper switches are spring-loaded push-button switches mounted on the frame of the
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TABLE 36.2
PARTS LIST FOR ULTRAPROXIMITY TRANSMITTER.
IC1 Q1 R1 R2 R3 R4 C1 C2 TR1
555 Timer IC 2N2222 NPN transistor 1K resistor 5K resistor 1.2K resistor 2.2K resistor 0.1 F ceramic capacitor 0.0033 F monolithic, mica, or ceramic capacitor Ultrasonic transmitter transducer (40 kHz nominal)
TABLE 36.3
PARTS LIST FOR ULTRASONIC PROXIMITY RECEIVER.
IC1,IC2 R1,R8 R3,R4,R6,R7 R2 R5 C1,C2 TR1
741 op amp IC 330 ohm resistor 10K resistor 100K potentiometer 1K resistor 0.01 F ceramic capacitor Ultrasonic receiver transducer (40kHz nominal)
All resistors have 5 or 10 percent tolerance, 1/4-watt; all capacitors have 10 percent tolerance, rated 35 volts or higher.
robot, as shown in Fig. 36.12. The plunger of the switch is pushed in whenever the robot collides with an object. Obviously, the plunger must extend farther than all other parts of the robot. You may need to mount the switch on a bracket to extend its reach. The surface area of most push-button switches tends to be very small. You can enlarge the contact area by attaching a metal or plastic plate or a length of wire to the switch plunger. A piece of rigid 1/16-inch thick plastic or aluminum is a good choice for bumper plates. Glue the plate onto the plunger. Low-cost push-button switches are not known for their sensitivity. The robot may have to crash into an object with a fair amount of force before the switch makes positive contact, and for most applications that s obviously not desirable. Leaf switches require only a small touch before they trigger. The plunger in a leaf switch (often referred to as a Microswitch, after the manufacturer that made them popular) is extra small and travels only a few fractions of an inch before its contacts close. A metal strip, or leaf, attached to the strip acts as a lever, further increasing sensitivity. You can mount a plastic or metal plate to the end of the leaf to increase surface area. If the leaf is wide enough, you can use miniature 4/40 or 3/38 hardware to mount the plate in place.
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