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?
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.
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.
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.
The measured intensity of an acoustic reflection is a combination of
the transmission strength
the receiver gain, sensitivity (resolution), and measurement range
higher gain means more resolution, but also increases the chance of clipping outside the detectable range
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
the distance the wave travels
the sound wave spreads out, and
some of it is absorbed as it travels through the water
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
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
whether the receiver is ‘visible’ to the sound wave
the pulse could be deflected away from the sonar
the pulse could be blocked by an obstacle on the way back (unlikely, given the relative speeds, unless it’s something mounted to/with the sonar)