ArduRover - In R/C boat - high frequency rudder cycling - ArduBoat User Group

Hi,

I have ArduRover v2.49 with Mission Planner 1.3.28 running with a 3DR Pixhawk with 3DR telemetry in a model boat.

I have now successfully completed a few missions albeit over small distances. While on the face of it, everything looks perfect, the boat continually wobbles (twitches) and telemetry logs indicate high frequency rudder cycling.

The telemetry replay for the 3 minute mission is on :

http://youtu.be/SgPZQHQajRg

It looks very successful apart from the Ch1out values. This is the rudder.

The attached 'Rudder and ESC.jpg' shows mission output for both rudder and ESC (motor).
I had change to manual mode just before WP3 at about 10:46:40 to avoid a collision and that explains the ESC anomoly at that time. I realize I have to adjust (reduce) the desired cruise speed and that's not a problem. But the rudder activity is a concern.

The attached 'Rudder.jpg' shows a second by by second breakdown of rudder activity over a representative interval. This shows a 20 degree to port then a 20 degree to starboard almost every second. Hence the wobbles and twitches and probably why speed isn't rising.

I know this isn't a software or firmware issue - it's down to my own incompetence / unfamiliarity.

The final 'MP config.jpg' shows what is currently set. I'm sorry to be asking stupid questions, but is this the right screen to start modifying the rudder behavior ? Does anything look obviously wrong ? Or do you have any other advice or suggestions ?.

Thanks in advance for any suggestions, help, advice or pointers.

Tony

Rudder and ESC.jpg

Rudder.jpg

Mission_Planner_Config.jpg

Hi Tony. I have attached some lovely graphs below :-)

RCOU.Ch1 is your rudder. In this graph you can clearly see the rudder going back and forth trying to compensate for the oscillation.

Now in the graph below we have zoomed in. You can see the blue line at the bottom showing the z gyro goes first when the boat starts to roll, then the controller figures out the boat is rolling (red line) and finally the rudder reacts - green line at the top.

We think what's happening is the rudder is trying to react too quickly to the roll of the boat. Instead of fighting the roll the rudder should be steering. So, currently your STEER2SRV_TCONST is 0.75 which says correct the steering within 0.75 of a second. Fine for a RC Rover on the ground with fast steering response but for a boat rudder we think higher might be better. Try to increase it to 2 or 3 to see if that improves the situation. I am aware MP says you can't set it that high but if you go to the Advanced parameter menu you should be able to make the change.

I'd love you to try it out and send another log. Let us all know!

Thanks, Grant.

Replies

Tony Holt June 3, 2015 at 2:01am

Yep it's a big log file.

I don't know how you'll use it, but if it helps, a representative part of the mission, which exhibits the behavior starts at 10:45:19 and ends at 10:48:11.

Don't think it will help in any way but (just in case) boat pics and build details are on :

http://www.holtcraft.com/mybuilds.php?item=20

Really appreciate your time and thanks for any advice or hints.

Tony
- Admin
  
  Thomas J Coyle III > Tony Holt June 6, 2015 at 9:39am
  
  @Tony,
  
  After looking at your build pictures, you appear to be steering with the propeller that is in a Kort nozzle. Is that correct?
  
  My Traxxas Villain hull steers with two subsurface propellers moved by one servo.
  
  Regards,
  
  Tom C AVD
- Tony Holt > Thomas J Coyle III June 6, 2015 at 11:41am
  
  Hi Tom and yes - Kort Nozzle around the prop is rotated by a servo to provide steering (a bit like a very slow speed jet ski).. I didn't want to confuse the thread by referring to it directly.
  
  Thanks for info on your Villain. I've Googled the base model and it looks like a hell of boat - fast and nimble. It must give you lots of fun and a few challenges. Everything is going to happen real quick with a boat like that .
  
  Mine is a real plodder by comparison, but the Kort nozzle means she turns in her own length even at slow speed. Steering starts as soon as the prop turns i.e. it doesn't rely on water flow over a rudder.
  
  Thanks again,
  
  Tony
- Admin
  
  Thomas J Coyle III > Tony Holt June 6, 2015 at 12:34pm
  
  Hi Tony,
  
  The dual propellers on my Villain help to mitigate motor torque roll because each motor is counter rotating to its twin.
  
  My lake rescue tugboat has only one propeller and therefore rolls slightly to one side (starboard) when underway. I can make fast turns to port, but have to be careful when turning to starboard.
  
  Regards,
  
  Tom C AVD
- Tony Holt > Thomas J Coyle III June 6, 2015 at 1:02pm
  
  Understood Thomas - thankyou and makes perfect sense.
  
  To be honest, in Manual mode while sailing a straight course across the pond my boat hardly rolls at all (unless she hits wash from another boat). A lack of upperworks, relatively light weight and a 5amp Lipo just above the keel make her very stable.
  
  The Kort prop is also a bit 'kinky' and aids directional stability - 4 large flat blades but low pitch and low profile.
  
  Thanks again,
  
  Tony
- David Boulanger > Tony Holt June 6, 2015 at 12:08pm
  
  Tony, Does the boat back up well with that nozzle? Every design is full of pros and cons and I am just curious.
  
  Regards,
  
  David R. Boulanger
- Tony Holt > David Boulanger June 6, 2015 at 12:50pm
  
  Hi David - It goes backwards very nicely as long as you're not too concerned about the direction of travel :)
  
  I previously built a tug with twin Korts and that boat steered backwards better than anything I've seen. But on this one, its a single Kort and steering when going astern is very tentative. I certainly wouldn't ever want to rely on much directional stability when going astern with my single Kort.
  
  As you say - pros and cons or horses for courses.
  
  Many thanks and take care,
  
  Tony
- David Boulanger > Tony Holt June 6, 2015 at 4:52pm
  
  Nice set up for a single screw boat. We don't go backwards that often.
  
  Regards,
  
  David R. Boulanger
- Developer
  
  Grant Morphett > Tony Holt June 4, 2015 at 10:02pm
  
  Hi Tony. I have attached some lovely graphs below :-)
  
  RCOU.Ch1 is your rudder. In this graph you can clearly see the rudder going back and forth trying to compensate for the oscillation.
  
  Now in the graph below we have zoomed in. You can see the blue line at the bottom showing the z gyro goes first when the boat starts to roll, then the controller figures out the boat is rolling (red line) and finally the rudder reacts - green line at the top.
  
  We think what's happening is the rudder is trying to react too quickly to the roll of the boat. Instead of fighting the roll the rudder should be steering. So, currently your STEER2SRV_TCONST is 0.75 which says correct the steering within 0.75 of a second. Fine for a RC Rover on the ground with fast steering response but for a boat rudder we think higher might be better. Try to increase it to 2 or 3 to see if that improves the situation. I am aware MP says you can't set it that high but if you go to the Advanced parameter menu you should be able to make the change.
  
  I'd love you to try it out and send another log. Let us all know!
  
  Thanks, Grant.
- Admin
  
  Thomas J Coyle III > Grant Morphett June 5, 2015 at 4:27am
  
  @Grant,
  
  Nice analysis and summary.
  
  I will keep this in mind when I run my Traxxas Villain and my Vbox in the Auto Mode.
  
  Regards,
  
  Tom C AVD

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