how to create barcode in vb.net 2008 Detecting Presence with Microwave Sensors in Software

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Detecting Presence with Microwave Sensors
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A static object can be detected in the field of a microwave sensor. The purpose of this detection is to determine that the object is still in the field of interest and has not departed. This is particularly desirous in control systems where the controller is performing other tasks and then accesses the sensor to determine whether there is a sensed object at that particular time. In this situation, presence sensing is especially advantageous since the output can be verified by further interrogations to eliminate false sensing. To detect the presence of an object, a microwave sensor with a separate transmitter and receiver must be used. A transceiver in this application is not adequate, although the transmitter and the receiver can be mounted in the same enclosure. The receiver must not sense any energy unless the object is present in the field. A means to modulate the transmitter is needed, and the receiver should be narrowband to amplify and detect the modulated reflection. The sensitivity of the receiver must be adjustable to allow for ambient reflections. Microwave sensors have been extensively and successfully tested at various fast-food drive-through vending locations. Other types of sensors, such as ultrasonic and photoelectric sensors, were also tested,
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but less successfully because they were too sensitive to the environment. It was discovered that frost heaving of the ground would eventually cause their buried loop to fail, and the cost of underground excavation to replace the loop was exorbitant. Another application of the microwave sensor is the door-opening market. The microwave sensor will check, for safety reasons, the area behind a swinging door to detect whether there is an individual or an object in the path way. Ultrasonic sensors may perform the same task, yet range and environmental conditions often make a microwave sensor more desirable. A microwave sensor can check boxes to verify that objects actually have been packed therein. The sensor has the ability to see through the box itself and triggers only if an object is contained in the box. This technology relies on the sensed object being more reflective than the package, a condition that is often met.
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Measuring Velocity with Microwave Sensors
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Microwave sensors are ideally suited to measuring linear velocity. Police radar is a simple example of a Doppler-frequency-based velocity sensor. This technology can be applied wherever it is necessary to determine velocity in a noncontact manner.
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2.10.6 Detecting Direction of Motion with Microwave Sensors
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Direction of motion whether a target is moving toward or away from the microwave sensor can be determined by the use of the Doppler-frequency concept (Fig. 2.101), by adding an extra mixer
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FIGURE 2.101
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Direction of motion sensor schematic.
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Two
diode to the module. A discriminating effect is generated by the additional diode, which is located in the waveguide such that the Doppler outputs from the two mixers differ in phase by one-quarter wavelength, or 90 . These outputs will be separately amplified and converted into logic levels. The resulting signals can then be fed into a digital phase-discrimination circuit to determine the direction of motion. Such circuits are commonly found in motion control applications in conjunction with optical encoders. Figure 2.102 shows the phase relationships of the different directions. Outputs from this module can vary widely to suit the application. The simplest is two outputs, one for motion and the other for direction (toward or away). These outputs can be added to a third, which provides the velocity of the target. The combination of signals could be analyzed to provide a final output when specific amplitude, direction, distance, and velocity criteria are met (Fig. 2.101). In the door-opening field, using the amplitude, direction, distance, and velocity information reduces the number of false openings. This extends the life of the door mechanism, besides saving heat if the door is an entrance to a heated building. In this case, the measurements by circuitry indicate the following characteristics:
Characteristic Person-sized object Moving at walking pace Toward or away Specific time before opening Measurement Amplitude of return Velocity Direction Distance
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