Replies to This Discussion

Permalink Reply by Michael Pursifull on October 17, 2011 at 10:17am

You know, I have been meaning to look at this again, but I keep getting distracted. There is a discussion you might be intererested in here

 

Read the whole thread and you will have the means, if not the answer, to solve this. 

I'm not sure why the sonar readings are not always accurate; I half-suspect that different models work differently in their output. I have been meaning to setup some tests with different models, and create a better algorithm, or even a calibration routine, but I haven't gotten to it.

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DistributorPermalink Reply by Dany Thivierge on October 17, 2011 at 1:07pm

Jason, is there a way you could implement a modification that woudl read a parameter in mission planner for this? I also need a scaling factor on my sonar MB 1200 but it's not the same 1.4 as other found.... 

Would be nice to be able to "calibrate" manually this for maximum accuracy!

 

------------------- Copied from Farooq post on Mike's tread (above) ------------------ 

Ok so here are the results and modifications that I did With the code given AP_RangeFinder_MaxsonarXL.h I tested using CLI test for sonar. Since MB1260 can measure upto 1068 cm so I used a wall to measure the distance horizontally rather than vertically. I used a measuring tape laid on the ground and held the MB1260 and my laptop in hand. Tried to maintain it vertically over the exact reading I read the distance from CLI and here are the results.

 

 

Actual Distance	Measured Distance	Ratio
30	23	1.304348
40	30	1.333333
50	39	1.282051
100	75	1.333333
150	111	1.351351
200	145	1.37931
250	194	1.28866
350	263	1.330798
500	377	1.32626
750	560	1.339286
800	699	1.144492

 

 

 

 

So from here it seem that except the last value the rest of the line is pretty straight. So I did some math and used an average value of 1.33 as scaling factor. Measured the distance again found some more error and changed the scaling value to 1.4. So now finally
100cm == 100 cm meaning the sonar correctly measures the 1 meter distance. Tried it on 500m it had an error of 5 cm. But this is pretty accurate with this rough method of measurement. Now I have changed the code to following in AP_RangeFinder_MaxsonarXL.h

 

#define AP_RANGEFINDER_MAXSONARXL_MIN_DISTANCE 20
#define AP_RANGEFINDER_MAXSONARXL_MAX_DISTANCE 1068

class AP_RangeFinder_MaxsonarXL : public RangeFinder
{
 // public:
    //AP_GPS_MTK(Stream *s);
  public:
    AP_RangeFinder_MaxsonarXL(AP_ADC *adc, ModeFilter *filter);

    int convert_raw_to_distance(int _raw_value) 
    {
        if(_raw_value!= 20)
         return (_raw_value*1.4);     //Added scaling to compensate the error
         else 
             return(20); 
    }   // read value from analog port and return distance in cm
};
#endif

 This code is works quite accurately in range of 30 to 200 cm then I encountered a few cm different.

 

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Permalink Reply by John L. on October 18, 2011 at 3:54am

Would it be possible to include displaying sonar output value in the Mission Planner.

This would be a great help to all as a check on whether it is working OK.

I guess a step further is to include an offset multiplication factor for calibration.

Great work guys - Very impressive

 

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Permalink Reply by rileyporter on October 18, 2011 at 9:04am

Our findings were off by a factor of 1.4 as well.  Perhaps a drop down in the APM that lets you select your model.

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DistributorPermalink Reply by Dany Thivierge on October 19, 2011 at 9:03am

yes! Jason, if you read this please comment if it is something you can add... I really think it will be quite useful for most users! 

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Permalink Reply by agmatthews on October 30, 2011 at 7:21am

All,

I've done some analysis that may show why this compensation is needed

Read more here

-Andrew

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DeveloperPermalink Reply by Marco Robustini on December 6, 2011 at 5:41am

Hi Dany, what's your ratio wirh MB1200? I've found "0.80", good value on my system...

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Permalink Reply by Heino R. Pull on October 30, 2011 at 9:27am

There is no doubt that the sonar's absolute accuracy varies quite a bit - but on my quad I'm not too worried about it since it's only used for transmitter switch triggered alt hold. In this model you only care that the sonar's value is repeatable to the value set when the alt hold mode switch is set. In this role it doesn't matter that it isn't exactly 1 meter - but that it hovers at the same height over time that happens to read 1 meter. If you are using the sonar for "exact" navigation at low altitude then compensation would be needed. However, it does burn cpu cycles on the AVR that I personally would rather have keeping the update rate high until we transition to ARM processors.  So my suggestion is that any compensation be a compile time switch if it is implemented.

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Permalink Reply by I.S. on October 30, 2011 at 12:21pm

Agree, but sonar accuracy is needed for optical flow stabilization (in the future)

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Permalink Reply by Ruben Wijnhoven on October 30, 2011 at 12:43pm

I am having the exact same issues with my mb1200. It sometimes even causes for the quad to drop in hight. Pls fix this issue. It has been driving me nuts for the past week. The sonar should be accurate by 7,5m according to the spec sheet. If you use the sonar for more then 15 minutes it will also start to give false readings and cause the alt hold to drop to max 30cm or less. I have replicated this problem with 2 mb1200 ers.

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Permalink Reply by Ruben Wijnhoven on October 30, 2011 at 1:00pm

I noticed one other strange problem. If you go in Cli test mode and lift your copter it wil go letś say to 1.56m and then ik wil jump baci to 80cm and will climb again from that value. That also happens in flight. YOu alt hold ond lets say 1.80m then it wil fly for 2m and will al of the sudden lower to 80 cm. Again i noticed this behaviour on both mb1200. So clearly something is not going right.

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Permalink Reply by Heino R. Pull on October 30, 2011 at 1:18pm

Ruben,

The sonar seems to be one of the hardest things to get working. Many issues are related to the fact that it is a a tricky sensor to get a clean signal from. I spent 3 months trying to get mine working and it is a real challenge. The current software should work pretty well with alt hold if you have a good signal from the sonar. Watch out for field of view problems (landing gear or other objects in the the very wide view of the mb1200). Also all the things related to interference in the manual - such as too close to an ESC, wiring running near and ESC, poor power.  Having your sonar jumping to  a fixed value of 80cm or so is the problem I had for a month or more (mine displayed 90 cm continuously independent of the actual value) and was due to motor interference. If you plot the log of the sonar output you will see spikes going to 80 cm. 

 

Jason spent a bunch of time trying to code around noisy sonars but ultimately the fix for many was moving the sonar away from the ESCs. Unfortunately there are builders - even with shielding and moving the senor away still can't get it working. The logs will show spikes, and the software will cause the sudden drops or rises. Using the logs to look at the sonar output is critical and can point to what may be going on with placement or electrical/acoustic issues.

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