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DeveloperPermalink Reply by R_Lefebvre on October 26, 2011 at 5:59pm

I would assume the AC program looks at signals from the IMU as well as the sonar to determine height.  I know it does with the magnetometer.  While testing spin the magnetometer and/or the IMU seperately and watch the heading output.

 

The same could be, or should be, done with the sonar.

 

Or, instead of a moving average filter, just simply throw out any values that are not plausible upon a comparison between dH/dT and the Z axis accelerometer.  The aircraft can't plummet without something showing up on the accelerometer.

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Permalink Reply by Allen Babb on October 25, 2011 at 10:38am

It looks like there's a few I2C SRF's out there already: http://www.acroname.com/robotics/parts/R287-SRF02.html

 

This should help minimize the noise on the signal when it's getting transferred from the sensor to the APM... provided that the EMI isn't large enough to throw off the I2C signals.

 

As an after thought, one of my professors here at the college keeps mentioning the usage of differential circuitry to transmit and recieve the lines. The concept is, since the EMI is common to both transmission lines, a simple unity buffer could be used to filter out the EMI.

 

The unity buffer would then essentially be an analog op-amp with both inputs connected to both transmission lines. In your case, you might be able to plug the cable shielding into one input (I'm thinking the - side) and your analog line into the other (the + side).

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Permalink Reply by Aaron on October 25, 2011 at 2:03pm

Also... why not shield the motor leads as they are likely the cause of the noise. Even passive shielding would help I think... Wrap the booms with tin foil and give her a try.. :)

It works for my head anyway.. :)

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Permalink Reply by Allen Babb on October 25, 2011 at 2:27pm

Yeah, but you still need somewhere to ground the shielding, otherwise it might just make matters worse (see how a Yagi array works)

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Permalink Reply by agmatthews on October 25, 2011 at 4:26pm

Allen,

I've got a couple of these (SRF02 sonar) on my work bench right now and have just finished a software library to integrate them into the arducopter code for testing.

Just need some more time (& less work)

-Andrew

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Permalink Reply by Allen Babb on October 25, 2011 at 8:23pm

Nice! My parts budget ran out already, so I've got to wait before I can get my hands on one. I've been eyeing the old SRF04 to see if I might be able to hack into it at all...

 

And yeah, ( more_time & less_work ) = more_fun;

 

Sheesh, I'm beggining to talk in code now...

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Permalink Reply by Jonis Maurin Ceara on October 25, 2011 at 2:25pm

I'm not using this filter, only a shielded cable and 3 ferrites inline. This is my test for about 5 minutes holding my hexa (with props and motors running) on my room, solid ground and changing some centimeters after 2 minutes, then 1, etc...

I guess these spikes are normal, right?

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Permalink Reply by agmatthews on October 25, 2011 at 4:25pm

Jonis

Those spikes are noise - and are not good.

If you have spikes like that while flying in Alt Hold or Loiter you will find your quad dropping like a stone to compensate for the apparent sudden rise in altitude caused by the noise.

Interesting - your noise spikes go up in value while most others I've seen are downward spikes.

Andrew

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Permalink Reply by Jonis Maurin Ceara on October 25, 2011 at 5:39pm

yeah, thats what happened before i shorted my sonar cable, shielded and put a ferrite.....it was a lot worst than now!

But you're right....i almost crashed because of these noises....i bought this little spikes wouldn't be a real problem (like i said, it was worst).

I'll try this filter with caps next weekend.

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Permalink Reply by The Old Flyer on October 25, 2011 at 5:02pm

No filter, only shielding grounded at the sensor, no ferrites.

Here is a trace of the flight that included ALT_HOLD at 6' and 25' and some LOITER at 30'.

There was a bit of hunting but this was with the default PIDs.

 

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Permalink Reply by Ken on October 25, 2011 at 5:14pm

Hey guys,

 

I don't do 'copters but it looks like while some people have solved many of the problems, a few of you still need some fresh ideas, and I have gotten some mil equipment past "noise" issues in years and decades past. You've got to get your head around a couple of different ideas. Electrical noise, magnetic noise, and mechanical noise.

 

1. Electrical noise. Putting a filter on the sonar power supply will keep power line noise from going into the sonar. But if it isn't power line noise, it does you no good. In fact, that 100 Ohm resistor is scary. Remember that when the sonar sends a pulse, it will draw a pretty big current. If that 100uF cap can not supply all the current needed then there will be a pretty big voltage drop on the power sonar's power line. That can adversely affect readings. You probably want a pretty big cap near the sonar, but a small, low ESR cap *right* near the sonar might be a good idea too. And as little R in series as you can get away with. A scope will help you track this and decide if you need it. (Cap type & size.) Come to think of it, it may not even be noise coming in the power line, it might just be voltage drop during transmission. Someone might try the cap alone, without any series R.

 

2. Magnetic noise. Y'all are running big and varying current through your ESC's. This creates big and varying magnetic fields. An electrical shield will do *nothing* against a magnetic field. Since you can't get rid of the magnetic field, you need to play it's own game. (This is what twisted pair is for.) With every twist of the wires, the induced voltage is the opposite of the voltage induced in the prior turn. The smaller (physical size, not strength) and closer the magnetic field, the more turns per inch (TPI) you need in your wires to cancel. So, make sure that the power and signal wires to the sonar make 2 or 3 turns per inch (or more) from the Ardu out to the sonar. Evenly spaced. Also, do your best to run those wires perpendicular to any high current wires. Do not run the sonar wires parallel to motor power wires, for example out an arm.

 

3. Mechanical noise. The piezo element in the sonar will generate voltage when shaken, just as if a sonar pulse is received. You can reduce sonar vibration not just by putting foam or absorbent material around it, but also by adding mass to it. Just like a an electrical filter -- for example the proposed one for the power supply -- has both R & C (or L and C) components, you put R (foam) between the shaker and your sensor, and you add storage (mass) out with the sensor. If you can double the mass of the sonar device you will have halved the frequency which it can be shaken at. Stick a couple ounces of lead on the back of the sonar...

 

The data output can be filtered -- I'm sure that it is in software -- to greatly reduce spikes that are sharper than any realistic acceleration of the airframe might cause. A single spike is not a worry. A series of spikes represents a different value.

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Permalink Reply by Aaron on October 25, 2011 at 5:54pm

Crap, I forgot to mention the twist thing.. well played sir, well played..

Have not even begun to solder mine together yet, but if i encounter the noise issues, I will use twist and software filters I think..

I actually saw a "How its Made" where they were making Cat5 cable and I never realized that each pair has a different twist per inch to prevent noise..

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