I've created an I2C digital signal sonar module to avoid the noise problems I have been having with the current analog signal sonar module.
Why?
Seeing all the problems we are having with noisy sonar data got me thinking
Where does the noise come from? Is it:
- RF noise from the ESCs affecting the sonar module directly ?
- RF noise from the ESCs affecting the analog signal line between the sonar and APM?
- Noise on the DC power supply affecting the Sonar module?
- Something else?
The current popular solution of remote mounting the sonar has probably eliminated DC power as the primary source of our noise. In other words - if noisy DC power was the ource of the noise problem then no matter where you mount the sonar you would still have the noise. Regardless if DC noise were still an issue Maxbotics has a solution to DC noise in their FAQ.
Personally I've found that remote mounting the sonar module has only partially solved my noise problems. This is probably due to the Sonar power and signal cable passing within 10mm of one of the ESCs near where it plugs in to the APM on my arducopter.
(This photo shows how close my ESC is to the analog port on the IMU Shield)
So I wondered if I could get rid of the analog signal path and see how the sonar noise changed. This has led me to put together a I2C sonar module. This communicates digitally between the sonar and APM using the I2C port on the APM IMU shield. It also allows you to use the PWM output from the sonar which Maxbotics recommends as "the most accurate"
Hardware
All that is needed is an Arduino, an I2C cable and some software.
The GPS cable supplies power and connects data lines between the Arduino and APM IMU Shield (you need to cut one end off the cable)
APM IC2 Function Arduino Mini
PIN1 <-----> GND <-----> GND
PIN2 <-----> 5V <-----> VCC
PIN3 <-----> SDA <-----> A4
PIN4 <-----> SCL <-----> A5
Three Wires connect the Arduino to the Sonar
Arduino Function Sonar
GND <-----> GND <-----> GND
VCC <-----> 5V <-----> V+
D5 <-----> PWM <-----> PIN2
Here is what it looks like mounted on the sonar - with double sided tape
The sonar is on a standard jdrones sonar mount
Software
The Arduino then needs to be programmed as an I2C slave.
Here is my code Sonar_I2C_Sender_v005.zip
You must use an FTDI cable to program an Arduino Mini
The code continuously reads and stores the sonar distance as a PWM signal from the module.
It responds to any I2C request for data with a two byte integer representing the current sonar range in cm.
Additionally it has the ability to return noise filtered sonar data.
Note the MaxBotix experts strongly recommend a mode or median filter not averaging filter.
Currently the code can be requested to return spike filtered, median or mode filtered data.
I have modified the Arducopter code to use this I2C sonar and started testing
Here is my sonar library APM_I2CSonar.zip
If the testing all works OK I'll see if Chris or Jason want to put it in the code repository
Results
To put it simply the results so far are great!
Indoor and outdoor tethered tests show no noise and no data problems
Here is a graph showing Sonar data in red and baro in green.
first segment is with motors running second with motors off
Its too windy to fly here today (in the Aussie vernacular: It's strong enough to blow ya dog off its chain!)
Stay tuned!
Andrew
Comments
Hi,
I read your post on interfacing arducopter with a I2C sonar module. I am trying to interface the srf08 sonar sensor with my APM board. I tried modifying the code and so far after debugging it, I don't get any compile time errors. But I then the APM does not boot up properly... I can't figure out, I read the code again and again, but don't seem to understand how tweaking the sonar part is messing with the entire thing...
I am actually using the Gnd,SDA,SCL and VCC ports of the magnetometer, and am currently using the arducopter ver 2.3 because of the limitations of my atmega 1260.
Also, I would like to know are you able to test your sonar via the APM planner, as in does it read out the same values as with an analog sensor?
Thanks.
Allso have a look at my post about shielding LINK
Andrew,
Great detailed analysis and post.
In the "somthing else" category of possible causes, I had thought that the noise could be caused by peripheral sound interference so I bought one of these more expensive weather resistant maxbotix sensors but it has exactly the same issues.
@jason if you are still chasing it my mode filter code is in the "Sonar_I2C_Sender_v005.zip" in the blog post above. Note, it is important to have it calculate the median if a mode value does not exist
Andrew
OK, Mode filter class is coded up and running. I'll include it on the version 2.0.38
Jason
I'll remove that filter and replace it with a mode filter, no problem. Can you post your code?
I actually only mix the barometer in from 4 to 6 meters. That 1500 you see is a 15m emergency cutoff so a bad sonar doesn't send it to the stratosphere.
Jason
@chris have a read of this post from maxbotics particularly the bit about filtering.
Looking through the APM code we appear to be doing an "average filter" on the sonar in RangeFinder.cpp exactly what maxbotics tells us not to do.
I have swapped this for a Mode filter (same as Pieter recommends above) with good results
I'm currently looking at the code in update_alt() that does the blending between sonar and barometer as it seems to ignore the sonar maximum range value.
Yep
That the same template I used for my mode filter
Cleans up slightly noisy data well
Found a nice example of a maxbotix mode filter.
http://myarduinoproject.wordpress.com/2011/06/06/a-mode-filter-for-...
Have applied it to my raw sonar readings and it works great. Could be a nice improvement to APM.
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