Ultrasonic Radar for a Home PC System One of the fastest changing and most expensive fields, is that of technology. Our computers, printers, modems, and much more is being outdated faster than anything else in the world. Just as we buy a new computer that does what we want, the industry comes out with a new option on a smaller and better computer. There seems to be so much changing that unless we invest our life savings into technology, we are considered obsolete like our computers. What used to fill an entire room, is so small now that it can be swallowed with a glass of milk.
A computer used to be a mechanical engine that had many moving parts and was very slow. Now computers design computers that are tenfold their own power and a tenth the size, with less parts and using less power. 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 coffee-pot) to spare. Ultrasonic radar is now a small component for your computer, giving computer operators a chance to see low flying objects, household furniture, and even themselves on their PC screen.
Just to impress a neighbour or friend is reason enough to build your own ultrasonic radar station. 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 (Fig. 1) 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 (Fig. 2).
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 360o field of view. For entire assembly see Figure 3. 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 connections between the Experimenter, ranging board, and transducer are shown in Figure 4. 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.
Figure 5 shows the timing diagram of the ranging boards’s signals. 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 360o 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 hook-ups for different motors. Refer to Table 1 for the different connections of the stepper 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- …