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Ultrasonic Radar For A Home PC System
An airport or an army base used to have huge structures that could send out signals to find out if any aircraft were approaching. This technology is now offered to people who have a computer with Microsoft's quick basic, or a Macintosh, and space (equivalent to that of a coffeepot) to spare. Ultrasonic radar is now a small component for a computer, giving computer operators a chance to see low flying objects, household furniture, and even themselves on their PC screen. Similar to that of a Polaroid, ultrasonic transducers are used in this type of radar. A rangefinder emits a brief pulse of high frequency sound that produces an echo when it hits an object. This echo returns to the emitter where the time delay is measured and thus the result is displayed. The Polaroid rangefinder is composed of two different parts. The transducer acts as a microphone and a speaker. It emits an ultrasonic pulse then waits for the echo to return. The ranging board is the second part This board provides the high voltages required for the transducer, sensitive amplifiers, and control logic. Since R1 is variable it controls the sensitivity of the echo detector. A stepper motor rotates the transducer to get a 360 degree field of view. An Experimenter is hooked up to the ranging board to control the ranging board and to measure the round trip time of pulses. It also controls the stepper motor and communicates with the control computer. The ranging board's power requirements are usually under a 100 mA, but at peak transmission the circuit can draw up to 2 Amps of current. Power passes from GND (pin 1) and V+ (pin 9). To avoid malfunction a 300mF or greater should be connected between pin 1 and pin 9 (or alternately pin 16 and pin 5). Another 300mF resistor should be added to the Experimenter end of the cable. It takes about 360 microseconds to transmit the pulses. The transmitter waits 1 millisecond for the pulse transmission and transducer to complete it's task. Then the experimenter waits for the pulse echo to return. If a pulse is detected the board sets ECHO at high. The Experimenter times the difference between BINH going high to ECHO going high. The experimenter sets INIT to low, waits 0.5 seconds for the echo, if no echo is heard the experimenter cancels the measurement. The measured time is sent to the computer which then calculates, at thousands of calculations per second, the distance based on the speed of sound (1100 feet per second). With a program called DISTANCE.BAS the exact speed of sound can be calculated according to the local weather conditions. The stepper motor is used to rotate the radar so it can scan 360 degrees around the room. An ordinary DC motor would not do for such a project. The rotation must coincide with the emissions and the receptions of the echoes. In a DC motor the armature rotates and the brushes connect successive commuter bars to windings to provide the torque. The speed of this motor depends heavily on how much load there is and how much voltage is applied. A stepper motor has different wires to each winding. By energizing a winding the armature rotates slightly, usually a few degrees. By sequentially charging one winding after another the armature can rotate completely around. By controlling the windings energized, the operator (in this case the Experimenter board) can control exactly how many degrees the motor turns and at a precisely controlled speed. In this case a stepper motor is used because it gives a precise motor-shaft location for the Experimenter board to follow. In a DC motor the board wouldn't know shaft position and it would not be possible for the computer to take the distance readings at evenly spaced intervals. With the control of the stepper we can control the number of steps and the step rate required between each transmission. The Experimenter will control all this. There are many types of stepper motors available. These motors have either two coils, three coils, two coils with center taps, or four separate coils. These are low-cost, light- duty motors that the Experimenter can drive. The Experimenter board can control any stepper motor with drive voltages from 4.5 - 36 volts and currents up to one Amp. The Experimenter has different hookups for different motors. While all the stepper motors will operate the radar system, it is imperative that the different advantages and disadvantages of each be considered. The motor's power consumption, torque, and resolution are all factors that must be considered when choosing the appropriate motor. A unipolar stepper motor with its common leads connected to the positive power supply can be driven in modes 7, 9, 11. In mode 7 (also called the one-phase drive) the stepper motor minimizes power consumption, because only one coil is activated at any one time. This mode has very little torque. Mode 9 (also called the two-phase drive) runs two coils at the same time. This provides maximum torque, although the power consumption is doubled. Mode 11 (called the half-step drive) uses one coil, then two coils, alternating between modes 7 and 9. This doubles the number of steps per revolution. If a stepper motor of twelve volts or less (indicated on the motor, along with maximum current, coil resistance and step angle) is used it is possible to run both the stepper and the Experimenter from the same power supply. It may be more economical to use a rechargeable power supply as an alternative to a small power supply. If the ranging board, Experimenter, and stepper are run off the same power supply, it is necessary to know that the boards use about 100 mA each. If a 9 v, 500 mA supply is used the two boards would use about 200 mA combined. The motor thus has 300 mA for it's own power consumption. Depending on the stepper it must be calculated how much current is available per coil. If we were to use a two coil stepper that would be 150 mA per coil. At this low current the voltage drop would be about 0.7 volts per coil for a total drop of 1.4 volts. The new resistance can be calculated and installed in the wiring grid on the Experimenter. In this hypothetical case the resistance value would be 48 ohms. To be sure of the power rating on the circuit, the equation P = I2R should be used and the proper wattage value should be placed on the resistors. On the Experimenter power can be drawn from +A drive on driver A. In building this unit two electrical contacts must be maintained as the transducer is turning. This is done using a brass tube three inches long which will provide the ground connection between the ranging board and the ultrasonic transducer. One end must be insulted with electrical tape and covered with a larger 0.5 inch long brass tube (so the two tubes don't touch). A hole drilled in the upper (longer) tube provides a space where a wire can be fed through the tube and used as one of the leads for the transducer. The other end of this wire must be soldered to the small brass ring over the insulation. The other lead of the transducer may be connected onto the top of the longer brass tube. The outer ring will be the positive (+) lead and the inner will be the negative (-) lead of the transducer (which can be connected immediately). The longer tube can be glued to the shaft of the motor. A plastic cap has been placed on the back of the transducer for appearance. Automotive alternator brushes can be used as contact leads for the brass tubes. The negative lead (from E2 on the ranging board) must be connected with the brush to the upper (inner) brass tube. The positive lead (from E1 on the ranging board) must be connected to the lower (outer) tube. This assembly can be mounted with the aid of two non-conducting blocks (ie. wood or rubber). To operate this device one company has taken the initiative to create software programs for the PC despite there being no ready made radar kit on the market today. "Fascinating Electronics" has written a radar control program to work with the Experimenter board. The programs are written in Quickbasic called EXPER1.EXE, to operate the radar and DISTANCE.BAS to measure distances and the speed of sound. If these programs were not available, any computer hacker with the knowledge of the Experimenter board would be able to write a simple version of such a program in several hours. The DISTANCE.BAS program pulses the rangefinder several times per second to measure within 0.01-inch resolution over a range of 6 inches to thirty five feet. To calibrate the radar system a flat unit like a box can be placed at a measured distance and picked up on the radar. When the program is run, it will report the distance of the box it has measured. If this measurement is wrong the program can be calibrated for the weather conditions. The program assumes the speed of sound is 1100 feet per second. This can be calibrated by pressing "4" to increase the speed by 10 feet per second, "3" to increase the speed by 1 foot per second, "2" to decrease the speed by 10 feet per second, and "1" to decrease the speed by 1 foot per second. This new speed of sound will be incorporated into your results by a RADAR.DAT file. To achieve color graphical results the computer must have EGA, or VGA displays. If the computer only has CGA the results will be in black or white Each rangefinder distance is plotted in real-time. This provides scale information with bars of different colors to and lengths drawn along the axes. Tens of feet are marked by long green bars; five foot marks are red; the one foot marks are shorter green bars; half- foot markers are black bars; and green dots indicate quarter-foot measurements. To the left of the display, the program reports the range values and the number of scanning points in each rotation of the transducer. The distance and bearing are updated with each revolution. By pressing "L" the displayed range increases (up to 35 feet). By pressing "S" the displayed range decreases (down to 5 feet). Pressing "M" will result in scan more points per revolution (up to the resolution of the stepper motor used). "F" is used to decrease the points scanned per revolution. With any text file RADAR.DAT can be altered to change the parameters. This screen can be printed in the monochrome mode (CGA). This radar assembly is a very interesting project. It can be costly, but for the enjoyment and learning experiences it can be an asset. It will one day come in a package at one tenth the cost of the parts (about $250.00 today). Although its range is restricted, the transducer can be changed and amplified to increase to range. This radar assembly can open the gates as monitoring equipment and perhaps one day as a property monitoring alarm system on your own PC at very little cost. This radar assembly has a great potential.


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