I have been reading with interest the sonar usage (right sensor, etc.) because I had already purchased the previously recommended sensor (EZ0) but had yet to add it to my quad. After reading the posts by Maxbotix and others, I went ahead and took the plunge. I added a 100uF capacitor directly to the board (photo above) as recommended and put a 100ohm resistor on the +5V line.
I created a cable and hand-braided it in an attempt to eliminate as much noise as possible. I know Maxbotix recommends a shielded cable but I didn't have any. The cable is also a little too long. I also temporarily mounted it to a paint stir and put it on the opposite side of the air-frame from the receiver. But as you can see from this photo, it is darn close to one of the prop tips too. But enough of that - how did it perform?
Here is the result of a test flight in pretty high wind.
The red is the sonar height, the green is the barometer height. At no time did the actual height get above 10m during my test.
First, I am continually surprised at how accurate the barometric height is. Second, the sonar trace has some decent readings for much of the flight, but sometimes is has crazy readings. I can't tell exactly what is going on here yet, but I'm going to track it down. I know the sonar I have is ultimately too sensitive and is probably picking up things like the mailbox (which was certainly nearby). Additionally, I wonder how likely it is to be hindered by heavy winds?
I'm going to do some more testing soon. I want to shorten the boom to get it further from the prop. If anybody has any testing methods they think are good for sonar units, please let me know.
Comments
@Christian - I'm with Gary - it would be interesting to see a picture of your quad and where the sonar sensor is too.
@Gary - thanks for the link - I didn't realize the AE sensors were more noise tolerant in addition to having a different beam pattern. It makes me like the MB1000 a lot less.
Hi Scott, Christian and Hugues,
Maxbotix actually does claim greater noise rejection for the AE/XL series here: http://www.maxbotix.com/articles/012.htm
And both types have analog outputs.
And Hugues, that actually looks really good, that close to the ground I am surprised that prop wash turbulence didn't cause more spikes. The one set you did get happened when you were descending and possibly prop noise caused it. Basically looks like your good to go though.
The separation of Baro from actual altitude is very minimal considering you are reading absolute air pressure.
Especially indoors with prop wash turbulence actually affecting local air pressure.
It can easily vary far more than that from moment to moment and outside on a gusty day can change easily by 10 or more feet over a few seconds. This is the reason why pilots of real planes have to pay attention to pressure altitude.
and Christian, Your plots look a lot like mine. Those spikes are definitely caused by noise, but what kind of noise, hard to figure.
The fact that the spikes do not make up a high percentage of the total waveform means that aside from being a bit twitch it probably flies on the SONAR in Alt Hold pretty well, but the noise still needs to be addressed.
I'm guessing either EMF from your ESC's or acoustic noise / vibration from the props or airframe still getting in.
Since it doesn't seem to get worse in close proximity to the ground, in your case id doesn't seem like prop wash is the likely culprit.
A software filter could probably alleviate this level of problem.
@Hugues - OK, but take a look at my plot - the barometer looks better good and I never got above 10m too. It is interesting in any case regarding how the barometer works versus the sonar.
@Scott : the baro looks different from my sonar alt because I flew so low to the ground and below 10m high the barometer is not reliable (and not used by APM if sonar enabled). There will be always some noise left as it is impossible to totally filter out everything interferences. However look at my result versus Christian and you clearly see the positive effect of good EMI shielding , noise protection, prop wash protection!
@Gary - I would like that to be true, but I suspect the XL line needs the current to power its analog outputs.
@Hugues - overall looks very good, but then at about line 4900 I see noise on the sonar plot.
I also think it is interesting that your barometric altitude is so different than the sonar plot. I wonder if the barometer needs calibration or something?
I mounted the MB1240 today and did some test runs. They were all flown semi-indoor in a three storey high roofed area over even gravel and wood planks about 2m away from the next wall. I tried to be have to copter as stable as possible. The sonar was mounted with velcro and some plastic bolts next to the battery under the base plate. An old USB cable was used as a shielded cable.
First I thought I can be lazy and skip the RC filtering (also I forgot to connect the shielding to ground) and this were the results:
@Scott : here is a graph of the sonar and baro parameters. It is a small flight (max 2 meters high), but did not have the chance to get out yet for better flight:
Hi Scott, I hadn't seen the 2 different filters.
The LV EZ does take less current than the XL so can get by with a 100 ohm resistor.
There would probably be too much voltage drop on the XL EZ.
I do believe the XL is a bit more noise immune.
@Gary - I read that a 100ohm resistor was needed for the MB1000 so that is what I used (this is what Maxbotic recommends here: http://www.maxbotix.com/documents/MB7961_Datasheet.pdf). It isn't in the photo because I put it inline with the +5V line further down the cable.
@Hugues - select CTUN, then Sonar Alt. I like to put Baro Alt on the same graph in order to compare the two, but Sonar is all that's really needed. Thanks!
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