Issue
148 November 2002
Ultrasonic
Homing Device
Start
•
Reveiver • Ear
Mounting • The Transmitter
• Software • Options
• Sources and PDF
EAR MOUNTING
The ultrasonic transducers
should be separated by several inches on a parallel
axis, as shown in Figure 3. This is necessary to provide
enough time difference between when each ear receives
the ping. If they are too close, unless the transmitter
is way off-center, they will both hear the ping at essentially
the same time. This is especially true the farther the
transmitter is from the receiver.
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|
| Figure 3—As the transmitter distance
increases, it’s more difficult to differentiate
between positions B, C, and D. The effective range
is greater with increasing ear separation distance
(S). |
Table 1 shows the distances
from the transmitter at various points to the left ear
as dL and to the right ear as dR. The difference between
dL and dR is also shown. Note that as the transmitter
distance increases, the difference between when each
ear detects the ping decreases. Eventually, the difference
will be too small for the software to determine which
ear received the ping first.
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dT = 30"
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dT = 60"
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dT = 120"
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dL
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dR
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dL – dR
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dL
|
dR
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dL – dR
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dL
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dR
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dL – dR
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|
A
|
46.86"
|
54.92"
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8.06"
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69.97"
|
75.60"
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5.63"
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125.28"
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128.51"
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3.23"
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|
B
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30"
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31.62"
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1.62"
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60"
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60.82"
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0.82"
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120"
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120.42"
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0.42"
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|
C
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30.42"
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30.42"
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0"
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60.21"
|
60.21"
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0"
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120.10"
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120.10"
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0"
|
|
D
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31.62"
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30"
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1.62"
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60.82"
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60"
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0.82"
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120.42"
|
120"
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0.42"
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|
E
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54.92"
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46.86"
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8.06"
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75.60"
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69.97"
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5.63"
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128.51"
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125.28"
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3.23"
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| Table 1—The distance
(dX) between A and B (and D and E) is 36". Notice
that as dT increases, the difference between when
each ear detects the ping decreases. In this example,
the distance between receivers (S) is 10". |
In Figure 3, the ear separation
distance (S) is 10". As S decreases, so does the difference
between dL and dR. However, S will be dictated by the
size and shape of your robot, and will become the overriding
constraint as to how far away from the transmitter the
ears can differentiate the signal source.
The receivers (and the
transmitter for that matter) have decreasing sensitivity
the farther they are off the acousical center. Ideally,
the transmitter would be in the same plane as the receiver
so that you’re not fighting a vertical misalignment.
This does not mean they have to be at the same height,
but rather that they’re pointed at each other as shown
at points A and E (see Figure 3). In other words, if
the transmitters are located in the upper corners of
a room, the receivers should be pointed up toward them.
A means of panning the
sensors, either as a sensor unit or as the entire robot,
is necessary for the robot to localize the transmitted
signal. After one is found, the robot can home in on
the exact location and steer itself to it.
