Using Ping1D at an angle

Blue Robotics Products Ping Sonar Devices
1

Hi there,

I have been using the ping1D for a little while now and have found when its not pointed straight down to the ground the measurements are often inaccurate. For instance if I have the ping pointed 20 degrees off the downward vertical I get inaccurate readings. I wondered if anyone else has come across this problem? and if anyone has found a way to take accurate measurements while the ping is pointed at an angle?

Thank you
Ben 2

2

Hi @ben2,

A sonar is an active sensor, so it’s reliant on the pulse that it transmits getting back to it in order to estimate distances.

You can think of it like trying to take a photo of a camera in a mirror. If the camera is at an angle to the mirror that’s larger than its field of view then it won’t be able to see itself at all (get a sonar reflection) unless the ‘mirror’ is very bumpy.

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In this case, the Ping Sonar has a beam width of ~30 degrees, and you’re tilting it at 20 degrees, so the closest ‘edge’ of the beam is still 5 degrees off vertical away from the sonar.

Is there a particular reason you’re trying/wanting to use the sonar at an angle?

3

Hi,

A continuation to this question. What if the sonar was straight, but it was observing a sloped wall?

During some of my experiments in a diving pool, when the sonar was a large distance, say 7 m away, it seems to ignore the slope. However, on coming closer, at 1.5 m the readings and the confidence level reduces.

There might be some cases where a sloped wall might be inevitable, hence I am planning to form a relation between the minimum distance and the angle of the slope, using an artificial 0.5mx0.5m wall. I am planning to test it between 0-30 degrees, with 0 meaning that the wall is perpendicular to the sonar and rotating the wall clockwise when needed.

Just to make sure that no extra variables during measurement occur, do you think that the sonar behaves differently for different materials, since the artificial wall and the pool walls are of different materials.

4

Yes, that’s an equivalent scenario.

Note that some of the ‘inaccuracy’ here also comes from the fact that if some of the target is closer to the sonar than other parts, how far away is the target? The sonar makes an estimate based on where it gets a strong response that’s consistent over time, but there’s some interpretation there as to which distance is being measured, and whether that’s the distance that’s desired for a given use-case.

It’s not possible for the sonar to “ignore” acoustic physics, but depending on the reflections from the water surface and other nearby walls (and the pool base, etc) there may be several reflections measured (at different strengths) for each output pulse, and if they end up with similar intensities to the reflection from the ‘target’ wall then it makes sense that the confidence in the distance estimate would go down.

Absolutely.

The measured intensity of an acoustic reflection is a combination of

  1. the transmission strength
  2. the receiver gain, sensitivity (resolution), and measurement range
    • higher gain means more resolution, but also increases the chance of clipping outside the detectable range
  3. how well the pulse frequency resonates with the receiver
    • this is mostly relevant for systems with separate transmitters and receivers
    • it can be affected by doppler shifts if the sonar and target are moving quickly relative to each other, although it’s unlikely to make a practical difference here
  4. the distance the wave travels
    • the sound wave spreads out, and
    • some of it is absorbed as it travels through the water
  5. the relative angle between the pulse wave direction and the surface it’s hitting
    • straight on / normal / orthogonal reflections give stronger returns than reflections that are deflected away from the sonar receiver
  6. the relative angle between the returning wave direction and the sonar receiver
    • straight on returns generally resonate more strongly than those coming in from the side/behind, which does hold for our Ping Sonar
  7. whether the receiver is ‘visible’ to the sound wave
  8. the density difference between the transmission material (in our case generally water of some sort) and the target material(s)
    • large density differences make for strong reflections
    • similar densities make for weak reflections
      • in the extreme, no density difference is like travelling through the same material, and has no reflection
    • if the density changes gradually over space (relative to the sonar pulse’s frequency wavelength) then the reflection will be less strong, and will be hardly to precisely measure
      • consider how one would try to define the exact location of the material transition in some water that gets murkier and siltier until eventually it’s mud and then stone
  9. the thickness and shape of the target material(s)
    • very thin materials won’t reflect as much of the wave, and may allow a fair amount to travel through
    • some shapes (and material/density distributions) are well suited to capturing / absorbing / deflecting the wave, rather than reflecting it back at the sonar