Could this have been the end?
The platform with the fire brigade ship was something like OK.
The vizualization was something like OK.
For the data-collection we could have lived with that solution.
All members of the team have an engineering background (for sure not in ship building).
As an engineer you look always for better solutions (especially if you are a german one).
Grmbl...
Thinking...
Sleepless nights...
Pling!
What about catamaranes?
No bad idea!
Anyone around who knows somebody who knows someone who has ever built a cat? -nope-
So went the discussions in the weeks after the success with the fire-brigade ship.
The last days of summer went away and i was looking for new horizons.
First try with two 100mm PVC sewage-tubes, four PVC 45° fittings, a styrofoam board, meters of Gaffer-tape and countless sticks of hot-glue.
The windmill on top and here we go!
Not bad for the beginning, but looked not satisfying. No pictures left, maybe i can fake some to see, how it looked like.
Next try.
I have found a good material, while physically googling the diy-stores around my hometown: 2.5mm thick PVC hard foam boards.
They can easily be cut with sharp cutter-blades and glued with hot glue. This was the ideal material for somebody who wants to have the results of his ideas realized some miliseconds after the idea hit his brain -me-.
Building the two hulls of the cat in three hours looked quite attractive to me and it was feasible. The simplest form was a V-shaped hull with V-shaped endings.
The three hours turned to six and after that i had the two hulls manufactured. I took a 30mm styrofoam board to connect the two hulls and the platform was ready. The windmill still served as propulsion system.
This assembly will shurely not a candidate for the "Germany´s next ship model"-contest
Tadaa! The new Platform
The first course was a triangular one, starting from the the launch point, turning right in a 90° turn after 100m then 60m and then back to the launchpoint.
Looked good.
The first tries...
In the picture you can see the course of three runs, each one was started with a different launch-angle, to see, how stable the PID control loop works.
The proportional part of the PID was increased by a factor of 5 compared to the firebrigade-ship and the rudder-horn attachments were also altered to give an estimated factor of 2. So in total the proportional part was multiplied by 10! The straight-line behaviour was excellent, the cat went like it was on rails. The radius for the turn was about 10m, which is more than i had on the previous platform, but this is OK.
No oscillations!
I was happy. And it was already december.
On the morning of december the 24th we had our traditional meeting from our diving club on the lake where we drink some amount of hot wine and we have some cookies - nice event-. I wanted to test some new parameters this morning, but we had ice on the lake. Very early in this year... And the winter was long and hard, the ice lasted until mid march this year. A long time, if you are addicted to optimizing! Some years ago, a friend of mine sent me a postcard from a holiday, it was subtitled "The only ice in florida is in the drinks". I thought a lot about that in the past months.
On 25th of March i started the new season with a zigzag-course from north to south with longer legs than i did before.
The ZigZag
In this screenshot we see two trips: the green one was the first one
the blue one is the optimized one. The white is the mission planning path.
Lets have a closer look to the green one:
We can see, that there is a curve with a big radius, when heading from one point to another. Analyzis showed, that the Integrator part was the bad guy. In the past experiences we set the integrator limit to a very high level and the integrator factor (Ki) to a very low value compared to the Kp. Kp is 2.5, Ki is 0.03.
The problem came from the hard (nearly 180°) turns. While turning, the integrator adds up to very high values, because we have a big difference between the setpoint and the actual value. And with this "heritage" of integrator values, i takes a lot of time for the decay of the values.
The Integrator is needed for accuracy and i wanted to keep the ki low, to avoid oscillations.
The cure: When looking at the time, the ship needs to make a "full turn" we found about 20 seconds in every case. We already had a "integrator holdoff" function in the software that cures the NMEA parser startup bug. Increasing this time from 5s to 25s led to the blue track. (The integrator part is not needed for the turn, because it is overriden by the p-part)
This is close to being optimal. The direct straightline (a real zigzag course) is theoretical not possible because the turn itself has a fixed radius. The actual waypoint algorithm always calculates the course to the next waypoint from the actual position. If we take this into account, the result is OK. I measured the maximum lateral deviation from the straight line with about 3 meters (on a leg length of 300m).
Not bad for a pure GPS-based autopilot solution.
All for today, the weekly TV thriller starts in a few minutes and will not miss it.
Next episode will focus on the autopilot system.
Comments
I think, there a lots of things to improve in future.
One of the biggest challenges would also be, to waterproof the windmill assy, if i keeo this path of evolution.
A shroud arround the propeller may also help and in addition it will make it more secure. at the moment it is a kind of a vertical lawn mower.
Do you have motre information about that yellow trimarane-like ship?
I have already made some Tests with three different Propellers and two different motors. (you have much time on the long winter evem´nings, when the lake is frozen). It proved, that the propeller which is provided with each kit is ideally mached. I think from this side, there is no further optimization possible.
I am using relatively slow turning motors with large props with high pitches. (around 900 RPM/V). Maybe a higher turning motor with smaller props has a better efficieny.
But is till think, the shape of the hull is the point.
BTW, i am just testing DrDepth. After fiddling some hours with the user interface it looks good. It is amazing, with how less amount of data this program produces a good looking output. More about that in one of the next Episodes...
And normally i have a natural horror in writing manuals and documentations, but this is a fun project and it makes fun writing about it.
@Gary and @Tom
Yes, the current consumption is pretty high. I am curently working on that. The Fire Brigade ship consumed about the half on the same speed level. The double-hull shape is far away from being streamlined, actually we try to make more streamlined one out of styrofoam, which was designed by a "serious" CAD program.
The overall weight of the catamarane adds up to some 5.5 kgs, which is pretty much.
The thrust form the windmill assembly @9A is about 5N.
I experimented a lot with various power-levels and it seems to be a kind of "sonic wall" at 5.5kmh. Normally i would expect this coming from the "hull-speed" for monokeel hulls with an aspect ratio below 1:8, a "streched" cat-hull with higher ratios should not show this effect.
Thats theory, but practice shows, that only in theory, there is no difference between theory and practice ;-)
Am loving the posts thankyou for sharing, I think you should be asked to be on the Podcast.
Actually i get a speed of 5.4km/h at a Current consumption of about 8.5A @ about 11V.
I use 3 LiIon Battery-Packs with a total capacity of about 16 Ah. This will give a total Mission time of roughly 2hrs.
Reducing the current to 6A slightly lowers the speed (about 5.0 km/h) but the straigh line behaviour is slightly degraded. This issue is still under research.
We are also looking for better hulls. the V-shape i use is far away from beeing optimal.
stay tuned...
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