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|     Written By: Scott Wielenberg     |     DatePosted: 11-21-2011     |
Sensor Noise Analysis Diagram
It's a little known fact that all of our MaxSonar products have been designed with a high acoustic noise tolerance. This means that the MaxSonar sensors from all of our product lines work to reduce or eliminate false detections caused by external noise sources. While acoustic noise tolerance is only a small part of our "sensor magic", that provides our users with reliable and stable range information, it is a critical factor for some of our users.

Recently, some of our users in the UAV community have been asking questions about the difference in noise tolerance between our LV‑MaxSonar‑EZ products and our XL‑MaxSonar-EZ/AE products. As a result, MaxBotix Inc., felt that it would be a good idea to provide our users with test data for each of our current sensors ability to reject acoustic noise sources.

Before Looking at the Data it is Important to Understand the Following:

First, decibels (dB) are reported on a log scale. If you are not familiar with decibels, an increase of 3dB equates to an increase to double the volume. The noise tolerance for our products fall between about 91dB and 106dB and this can seem foreign to many people. To make this report relative to human hearing, and keep things in perspective, a passing train at 30 meters is about 90dB, and a passing train at 1 meter is about 105dB. This means that both the LV and XL product lines have incredibly high acoustic noise tolerance since our noise tolerance falls within this dB range.

Second, all of our XL-MaxSonar-EZ/AE products are significantly more tolerant of acoustic noise than any of the LV-MaxSonar-EZ products, and this is shown in the data.

Lastly, sensors with a narrow beam pattern such as the EZ4 products offer greater noise tolerance than their EZ0 counterparts. As a general rule of thumb, you trade acoustic noise tolerance for the ability to detect small objects. (For example the MB1200-XL-MaxSonar-EZ0 is calibrated to detect a 0.25” (6.35 mm) diameter pole target at a distance of 3.6 meters, and the MB1240-XL-MaxSonar-EZ4 can detect this same 0.25" (6.35 mm) diameter pole target to about 1 meter.)

About the Noise Tolerance Data

The noise tolerance data shows the dB level where the noise began to affect the operation of the sensor. For this test, any noise level which caused 1 or more incorrect readings to occur out of 100 range readings was recorded.

The maximum distance to which the sensor will detect a standard target, a 9 cm (3.5 in) diameter dowel is provided for reference. This allows for easy comparison between the products for the ability of each sensor to detect small objects.

The extra acoustic noise tolerance of the XL series is readily apparent. Users who desire noise suppression along with extra sensitivity are encouraged to use the XL-MaxSonar-EZ/AE or the XL-MaxSonar-WR/WRC sensors.

LV-MaxSonar-EZ/AE Sensors Noise Tolerance Data

LV Noise Analysis Table

XL-MaxSonar-EZ/AE Sensors Noise Tolerance Data

XL Noise Analysis Table

XL-MaxSonar-WR/WRC Sensors Noise Tolerance Data

WR Noise Analysis Table

Notes About the Test

The noise threshold was set where acoustic noise caused one incorrect reading out of 100 range readings.

All of the sensors continued to function beyond the limits in the table, the data recorded is the point where sensors started to report false data. Provided the user does the appropriate noise filtering, such as a mode filter, (for a filter do not average data as this will just mix in the noisy readings), it may be possible to extend these noise tolerances.

How the Test Was Performed


MaxBotix Noise Tolerance Test

The test was designed to evaluate the real world use of the sensors, and to allow comparison between MaxBotix Inc., sensors.

A noise source was used that produced a pulsating acoustic source of 30KHz to 50KHz sweep at a 60Hz rate. This caused large spikes of noise within the sensor frequency band.

The acoustic noise source was placed about 50cm to the side of the sensor under test, with an output up to 120dB. This acoustic noise source was setup to direct the energy towards a large flat target, (which would then be reflected back to the sensor under test). A large flat target was placed 1.5 meters from the sensor. This target at 1.5 meters allowed the sensor to report 1.5 meters as a valid target.

The noise source output was increased until the noise just began to change the sensor output; the noise level was then decreased until no incorrect readings occurred, and this was recorded as the threshold.
 
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