20
6.1.2
Proper ultrasonic signal detection
When external noise is high:
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Identify the source. Usual suspects:
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Ultrasonic based volume or movement detecting alarm systems
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Other robots using ultrasonic
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Parktronics
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Sources of very strong white or impulse noise (air guns, air press, cutters,
vacuum cleaner, etc.)
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Rotors of drones/copters
Marvelmind Indoor Navigation System uses proprietary 31kHz frequency for ultrasonic
signal and employs additional filtering to combat external noise. And it makes the
system rather immune against “usual suspects”. However, if the external noise is too
strong; or its source is too close; or it is emitting strong signal on frequencies close to
31kHz or white noise, the system functionality can be affected.
The best things to do in this case – (1) to identify the beacons that are affected. Usually,
they are those, that the closest to the source of noise; (2) manually reduce the gain of
the affected stationary beacons so that the signal from the mobile beacon has 1000-
1800 amplitude. That would give the best signal to noise ratio. Don’t make the gain too
high. The noise will be amplified, but the desired signal will be saturated and signal to
noise ratio will be poor.
The gain settings are very non-linear. 4000 to 3000 – almost no change. But around
2500 the gain starts reducing very quickly (1200 – for some HW versions). Thus, it is
possible to find the optimal gain to obtain the highest Signal/Noise ratio.
When the map is formed, only mobile beacon is emitting, whereas stationary beacons
do not. Thus, it does not matter how close the mobile beacon is to the source of noise.
But it matters how the stationary beacons are close to those sources. So, select the
positions of stationary beacons accordingly – farther from the source of noise.
