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FIGURE 30-4 The circuitry used to detect objects using infrared LEDs and TV remote controls operates on the same theory as sonar; signals sent out from the LED reflect off of objects and are detected using the TV remote control.
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Surface IR Light Reflects from LED to Receiver
OBJECT DETECTION
lated signal is only active for a few hundred milliseconds to prevent the receiver from treating the signal as part of the background noise that it must filter out leaving the signal active for more than a few milliseconds will cause the receiver to filter out the signal and look for other more transient signals. The opaque barrier is to ensure there is not a direct path from the IR LED to the TV remote control. As with a simple IR LED and IR phototransistor, a block of wood drilled with two holes, one for the LED and one for the phototransistor, can be used for this purpose. Other materials such as electrical tape or sheet metal can also be used. Many materials that you would think are opaque to infrared light such as cardboard actually are not and you should test the different material you are planning on using in your robot. Probably the most common and easy to use opaque barrier is 5 mm black heat-shrink tubing, cut to 3 4 to 1 in long and slipped over the IR LED; this prevents a direct path for the IR light from the LED to the TV remote control receiver and directs the IR light into a fairly narrow beam, allowing you to control where objects can be detected. The attenuated signal can only be recognized by the TV remote control receiver when enough energy has been reflected into it as shown in Fig. 30-5. The receiver does not immediately respond to the received signal and remains active some time after the signal
IR LED
Reflected Signal Reception
IR Receiver Waveform
FIGURE 30-5 Oscilloscope waveform of the signal sent to an IR LED and the output of the TV remote control receiver when the signal is reflected from some object.
30.2 NONCONTACT NEAR-OBJECT DETECTION
has stopped. If the LED output was at full power, then the signal would reflect off of other objects around the robot, resulting in reflections coming into the TV remote control receiver at such power that it would be recognized as a reflection. The solution to this problem is to attenuate, or lessen the ability of the TV remote control receiver to receive the incoming signals. The obvious way to lessen the power of the signal is to dim the LED by allowing less current to pass through it, but a much better way of handling the signal is to shift the modulation frequency. If you were to look at a TV remote control receiver data sheet, you would discover that the ability to detect incoming signals changes as the frequency of the modulated signal changes. The closer to the design frequency of the receiver, the more sensitivity it has. By using a different frequency from the TV remote control receiver s nominal frequency, its ability to detect modulated signals decreases. In Table 30-2, the distance a particular 38 kHz IR TV remote control receiver can receive signals reflected from other objects is given. These values are for a specific manufacturer s part number receiver; you will find that other manufacturers products will respond differently. What is interesting to note about Table 30-2 is that as the modulation frequency changes, the object detection distance decreases. As will be shown in the next section, by changing the modulation frequency of the IR LED, you can estimate the range to an object from the IR LED and TV remote control, which allows you to decide whether to take immediate evasive action or slightly alter the course of the robot. BS2 Implementation of the Proximity Sensor The BS2 does not have the capability to produce 38 or 40 kHz signals, which are the most commonly used modulation frequencies for TV remote control receivers to drive an LED for detecting objects directly. Instead, the freqout function produces a digital signal that can be filtered into an analog sine wave of a specific frequency. The mathematics behind the operation of the freqout function are not important except to note that harmonics at the IR receiver s frequency are produced which allows it to be used for detecting objects around the BS2 as well as providing a simple range estimate for them. To test the ability of the BS2 to detect objects around it the circuit shown in Fig. 30-6 was created (its parts list is Table 30-3). A modulated signal can be output on Pin 0 and after
TABLE 30-2 38 kHz TV Remote Control Receiver Object Detection Distance for Different Modulation Frequencies. Modulation Frequencies Generated by a Microchip PIC Microcontroller. MODULATION FREQUENCY 38.5 kHz 35.7 kHz 33.3 kHz 31.3 kHz OBJECT DETECTION DISTANCE 30 ft (360 in) 2 ft (24 in) 10 in 6 in
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