FDTD method and Figure30(c) shows the experimental result. In both simulation and experiment, the ultrasonic source was set at the left hand side of the area.
Table 2. Simulation parameters for Figure 5
Simulation area 150cm2
Distance between transducer and block
50cm
Length of block 40cm
In the simulation result, the ultrasonic wave did not pass through the block directly. So on the right hand side of the block a blank space can be found. However, ultrasonic waves can still arrive the area away from the block by diffraction and reflection. Figure 30(c) shows the experimental result. This experiment used a one square meter rubber slab as a block, and set it 50cm from the ultrasonic transmitter. Then the received power at different distances from the block was measured. The experiment result is the same as the simulation. In this result, the ultrasonic waves cannot pass through the block directly. Therefore, a blank space can be observed next to the block, both in the simulation and the experiment result. However, in the space that is away from the block, the reflection wave can be received again. So received power is very low when the receiver is near the block.
When the receiver comes away from the block, it can receive more reflected waves, so the received power is increasing. Finally, when the receiver comes too far away from the transmitter side, the received power comes down again because of the attenuation. Both the simulation and result show that ultrasonic waves can reach the place behind the block, by reflection and scattering. This experiment proved that ultrasonic waves can also be used for wireless communication when there are obstacles on the transmission path.
ultrasonic wave beam just like the smart antenna.
In the traditional antenna theory, phased array is a very useful technical.
Using the phased array, we can control the directions as required. It can also increase the transmit distance of the communication system. The basic phased array is a linear array. In a linear array, several transmitters will be arranged along one axis, and the distance between each two transmitters is the same. Every element will be connected to an independent signal path.
We can change the time delays in each path to control the different phase between these transmitters. As a result, ultrasonic waves will gather into one direction at an angle of . We can get from equation(4.4)
2 d sin
(4.4)In this equation, is the time delay, d is the distance between each two transmitters and is the wavelength of the ultrasonic wave.
The ultrasonic wave sent by each transmitter is shown in equation (4.5):
jn
S
i Ae
(4.5)Here A is the amplitude of the ultrasonic wave.
In practical application, sometimes the main transceivers need to transmit to one or more particular directions. For example, connect to some special device or suit for a special room. Maybe moving the transceiver will be an easy way to achieve this objective. However, this method cannot satisfy multiple users and it is hard to readjust. So we want to use the phased array theory to achieve this objective. In the following part, we are going to test whether this theory is suitable for the ultrasonic communication system.
0o 90o
Figure 31 Schematic diagram of two transmitters with phase shifter
a b
-90 -85 -80 -75 -70 -65 -60
0 15 30 45 60 90 75
105 120 135 150 165 180
195 210
225 240
255 270 285 300 315
330 345 -90
-85 -80 -75 -70 -65 -60
Figure 32 Simulation and experimental result of received power when using two transmitters
So in the next experiment, we used two transmitters to form a simple two element array. The distance between the two transmitters is about one wavelength, which is limited by the size of the transmitter. The phase difference between these two transmitters is about 108°. Figure31 shows the schematic diagram of this simple array.
This time, we simulated this system before experiment. In this simulation we used the ADI-FDTD method which is mentioned above. The simulation result is shown in Figure32 (a), the direction is about -22° and 18°. In the simulation, we only simulated the direction of the ultrasonic wave beam. The same to other simulations, we used normalized amplitude in this simulation. Therefore, we only compared the direction between the simulation and experiment. In the result, we can see that the ultrasonic signal has been gathered in two directions. Then we actually measured this system and the experiment result is shown in Figure32(b). We can see that maximize receive power is about -60dBm, at -20° and 25°. The direction is almost the same as the simulation result. If we use more elements in the transmitter array, we can gather ultrasonic waves more easily or direct them to more users. This will be discussed in my future work.
Figure 33 Simulation result of received power with difference phase separation
-90 -80 -70 -60
-160 -140
-120
-100 -80 -60
-40 -20
0 20 40 60 80 100 120 140 160
-90 -80 -70 -60
108 208 258
Figure 34 Experimental result of received power with difference phase separation
Then we try to adjust the phase difference between two transmitters.
According to the phased array theory, the pattern of the transmitter array will become different when it adjusts to the phase difference.
Table 3. Phase difference between two transmitters in Figure 8 Figure 8（a） 108°
Figure 8（b） 208°
Figure 8（c） 258°
We also did the simulation first this time. Figure 8 shows the simulation result with difference phase separation. When the phase difference in Figure 33(a) is 108°, the same to last simulation in Figure32. The phase difference in Figure 33(b) is 208°, in Figure 33(c) is 258°. In this result, an obvious difference can be found between each picture. The direction of the ultrasonic can be controlled by phase adjustment. Then this simulation was tested, and Figure34 shows the experiment result with the same phase separation. In the experimental result, it can also be observed that the direction of the ultrasonic wave is changing with the phase difference.
At last, we also compared the longest transmission distance between the phased array and single transmitter. When used one single transmitter, the transmission distance is about 3 meters. However, for the phased the longest transmission could reach 5 meters on the maximum direction. This is two meters longer than the single transmitter.