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|     Written By: Scott Wielenberg     |     Date Posted: 03-12-2015     |
Sensor Noise Analysis DiagramIllustration 1: Two common shapes of corner reflectors.
Sometimes when using an ultrasonic sensor, users experience detection of unwanted objects that appear outside the expected beam pattern. These types of detections are the result of reflectors present in the environment. Corner reflectors can be surprisingly small, yet present a large reflection back to the sensor.

Certain objects are prone to causing corner reflections. A corner reflector is two flat surfaces joining together to create a 90° angle. A half circle also acts as a similar reflector. Illustration 1 shows two examples of corner reflectors.

Some examples of common corner reflectors follow: a bookshelf along a wall, a curb in a parking lot, a 1/2” wide seam in a concrete floor, internal bracing in a bin, or the inside corner of a doorway in a narrow hall. Each of these examples can create a corner reflector that may cause unwanted detections.

When using an ultrasonic sensor, the sensor transmits sound towards the target. When the sound hits the target, it is either reflected back to the sensor or reflected away from the sensor. The area where the sound reflects back to the sensor is considered a detection pattern. We display this information as beam patterns. Read more about beam patterns at this link

sensor sound areaIllustration 2: Blue shows the area covered by the sound sent from the sensor.
When the sound is sent from the sensor, it covers a much larger area than the detection pattern. The sound continues to spread the further it gets from the sensor. In Illustration 2, the blue shaded area represents where the transmitted sound from the sensor travels. When a corner reflector is placed inside the blue area and angled towards the sensor, the reflecting sound can result in an unwanted detection.

Corner reflections may amplify signal returned similar to a satellite dish. As such, these reflectors present the sensor with a target larger than the actual size.

If you are experiencing unwanted detections from a suspected corner reflector, the best testing is to place a soft material over the suspected target and see if the unwanted detection goes away. If the detection goes away, you likely have a corner reflector at that location.

Try these options to resolve issues with an unwanted corner reflector:

  • 1. The simplest is to remove the reflector. This is not always possible.
  • 2. Place a soft material over the suspected target. This is not always possible.
  • 3. Select a sensor with a narrower beam pattern,
        i.e. switching from an MB1010 to an MB1030.
        Sometimes this is not an option because sensitivity is required for the application.
  • 4. Contact the MaxBotix Inc., technical support team to see what options are available to you.
        MaxBotix Inc., may be able to meet sensor needs with a semi-custom version of the sensor.

Reflectors are an important consideration for any application design and should always be considered.

Illustration 1 as well as additional reference material is from "Corner Reflector." In Wikipedia. Retrieved on February 25, 2015.

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