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Permalink Reply by Ritchie on March 28, 2011 at 10:29am

As no-one has responded I shall.

APM has never been used in an underwater vehicle although it has been used on water very successfully (see Harald Molle's epic blog of his adventures). The limitation is on guidance as the APM is inherently GPS based but I'm sure some dead reckoning will help out that (although I'm unsure how you will measure speed). As for control surfaces I'd assume the standard code would be pretty good although your PID values will have to be changed. As for ballast I would even know where to start but you have 4 spare PWM channels to deal with that so you should be ok.

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Permalink Reply by Todd on April 18, 2011 at 6:27am

thanks alot!

I'm using low-frequency RC to control the sub for now before I get the autonomous systems working.

I am hoping to control the ballast tank over RC too.  Basically I'm having a relay circuit for the tank empty/fill function that will require just a single square-wave pulse to activate both ways. 

How would I go about assigning one of the RC receiver channels going into the APM to a simple two-position switch on the transmitter? Then all that would be required is a single pulse from the associated servo output on APM. 

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Permalink Reply by Ritchie on April 19, 2011 at 1:11pm

As long as the two way switch is a transmitted channel and not a selector for dual rate or something you should be able to just plug it in to a channel on the APM board.

As for outputing the square wave the PWM channels on the rear may not prove to be adequate so you may wish to use a spare digital pin to produce exactly the right output. You could have a simple boolean toggle to activiate the signal.

 

I've had a thought about navigation too. The HIL setup stuff may have something of use for dead reckoning and IMU based navigation.

 

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Permalink Reply by Gerry Lichter on April 19, 2011 at 1:59pm

Being unfamiliar with it, I had to dig for the "HIL" acronym.

Hardware in loop:(gps simulation)
http://www.diydrones.com/profiles/blogs/ardupilot-successful-flight

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Permalink Reply by Todd on April 28, 2011 at 11:19pm

Thanks, I think I have it figured.  I'll just use the relay switch on the IMU shield and code it to flip when the two-way switch signal changes on one of the extra channels

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Permalink Reply by Gerry Lichter on April 19, 2011 at 12:19pm

More edits than I thought, I'll just post a link when I have a revision ready for testing.

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Permalink Reply by Gerry Lichter on April 19, 2011 at 4:50pm

Today I learned the difference between an IMU, an AHRS, and an INS.

 

The OpenPilot project project seems promising. http://wiki.openpilot.org/display/Doc/OpenPilot+INS

 

Personally, I think that the ArduIMU + compass is accurate enough to get the job done. Use of a differential pressure sensor in the nose can infer forward velocity, and a second pressure sensor can be used to read depth.

 

Thinking about the problem, I'm wondering if the refresh rate on the GPS can be slowed to a crawl with more IMU samples in between. If you were to set your destination at the surface while you had GPS, and then locked in your attitude, you may be able to get to your destination before the GPS refreshes and then surface for a saw tooth profile.

 

To start from scratch, http://diydrones.ning.com/forum/topics/how-to-extract-angular

 

Still trying to understand the difference between a Kalman filter and a DCM

http://diydrones.ning.com/profiles/blogs/dcm-imu-theory-first-draft

 

Euler angles have gimbal lock but for a submersible that doesn't seem to be a huge problem. ArduIMU can be configured to ouput data in several formats. "output.ide"

 

The whole project seems pretty doable if you lower your accuracy standards far enough. If you plan on operating in shallow water a Doppler Velocity Logger is a popular way to go. Laser scanners are popular, but I don't know if the Kinect laser is close enough to visible light to cut through water very well.

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Permalink Reply by Todd on April 28, 2011 at 11:13pm

Hello Gerry, good to hear there are other people attemping something similar with Ardupilot! Though I myself am using the ardupilot mega setup and imu shield.  I am currently in the process of figuring out how everything works in the code, as well as getting my hardware sorted.  Unfortunately, the current priority is designing components and systems so they can be manufactured or purchased (2m-long steel hull, crossplane servo linkages, ballast etc), then once all that is sorted I can get to coding. 

I will keep researching this though, and keep you updated if I make any breakthroughs.  I'll look more into the HIL setup and see if there's anything useful.

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Permalink Reply by Tony Fang on November 29, 2011 at 12:00am

I am a graduate student and we have a project that to design the control an navigation system for an torpedo shaped AUV(automonous underwater vehicle). We use the x86 processor + QNX realtime OS for the AUV, and our IMU is ADIS 16400 (three axis gero, acc and magnetometer).

We have already deisigned the three loop contrllor and undergone a PIL simulation. Now we begin to design the navigation code for the IMU. I am learning about the strawp down inertial navigation and kalman fiter, which might be used in the navigtion code design.

In my view, there is no difference between the UAV control and AUV control for they both need a three stabilization loop (pithc,roll,yaw). In fact our AUV control board is prviously used in the unmanned helicopter and works well, all you need is change the height sensor in the air to depth sensor is the water to control the depth of the AUV.

Hoping that will hlep you, my mailbox: fdybit@126.com

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Permalink Reply by Gerry Lichter on November 29, 2011 at 2:03am

Hi Tony,

Is this project for the Robosub or SAUC-E competitions?

 

UAVs and AUVs seem to be very similar for sure.

 

I'm beginning to look to surface vehicles as a transitional development platform between bench testing and completely submerged operation. The hydrodynamics are different, including behavior in pitch & roll, but overall it seems like a close enough substitute to model the more basic challenges at significantly less hassle.

 

One of the distinctions between AUVs and UAVs that has come to my attention is the difference in encountered G-Forces. The accelerometer selected would ideally have it's sensitivity range matched to the peak forces expected in the application. For AUVs, this may be much less than a typical UAV mission.

 

I don't know what your specific mission requirements are but accurately tracking distance traveled for purposes of dead reckoning seems to be a huge challenge in the Robosub competition at least. Alternatives to DVLs don't look promising. Double integration of acceleration seems to be acceptable for some applications although I've heard the error accumulates rapidly as drift.

 

Optical Flow seems like a promising alternative, if only under very narrow circumstances, but water clarity and bottom composition would play factors in that approach.

 

I hope to have some progress to share eventually myself, although right now I'm bogged down in problems of a more mechanical nature.

 

Cheers,

Gerry

GerryAUVGuy@gmail.com

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Permalink Reply by Tony Fang on November 29, 2011 at 5:57am

Hello Gerry

    The AUV designed in our lab is not for the competition that requires accurate tracking. Our goal is to design an AUV that has the ability working in the shallow water environments (wave effect should not be ingored) and finish the task by itself (such as surveying ,supervising ect.).  

    Of course,  no matter where the AUV is used , the accurate the better. So I am looking for a good navigation method that fit for the 9dof IMU (three axis gero, acc and magnetometer). There is no GPS signal in the underwater environment, so I plan to use the three axis acc to detect and compenate the pitch and roll error, and three axis magnetometer to detect and compenate the yaw error, just as  William and Paul do in http://code.google.com/p/gentlenav/downloads/detail?name=DCMDraft2....= .

    However, due to the above paper uses the GPS signal to detect and compenate the yaw angle, and has little statement on how to use the magnetometer to detect and compenate the yaw angle, I must do this work myself. If you have any  knowledge about this, could you  give me some sugestion or informaton?

    By the way, if the vehicle sails both under and above the water surface, the six dof motion model could be quite different due to the difference between hydrodynamic forces and aerodynamic forces. Double intergation of the accelermeter might yields good results in short time, however, for a long time navigation task,  another complementary position signal might be needed to revise the positon error, just as the function of  depth sensor.

     Best regards! 

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