Hi all,
My project is trying to minimise the displacement of a buoy as a wave passes over it by altering its buoyancy. This means that I require a control system with an input being the displacement of the buoy, I am struggling to find a method of determining this displacement using sensors mounted on the buoy. I have looked into 6 DOF IMUs however they apparently introduce large errors due to integrating the acceleration readings twice to get displacement. Now I'm stuck for ideas!
Any help would be appreciated, thanks.
You can try using Madgwick or Kalman filters to reduce the error.
smscannell:
My project is trying to minimise the displacement of a buoy as a wave passes over it by altering its buoyancy
When I was last out sailing (admitedly it was about 6 years ago) waves passed under buoys, not over them.
Leaving aside the electronics completely, how do you plan to change the displacement?
...R
I'm not sure the premiss is correct. I'm pretty sure that reducing buoyancy will actually INCREASE the force of the wave acting upon the buoy.
I can imagine that adjusting chain length in response to changing water levels would have it's advantages. Ideally the chain holding the bouy in place would hang vertically in the water. If the chain is too short then the buoy would be held beneath the water level (and hence be hidden), If it's too long, the buoy can wander around a large area, making it's intended location, inaccurate.
With this in mind I'd assume that what you want to do is lengthen the chain as the buoy rises over the wave. and then shorten it again, once the wave has passed. I'd imagine the that feedback could be provided by measuring the tension on the chain, but should also take into account the prevailing current.
If you have a remit that demands that you adjust the buoyancy, then the ideal solution would probably be pumping water into and out of the buoy. To measure it's current buoyancy you could simply measure the water level inside the buoy. If you consider this then a project that is just measuring the water level in a tank, this has been achieved many times by using ultrasonic transducers.
OMG Don't I waffle on 
KenF:
If you have a remit that demands that you adjust the buoyancy, then the ideal solution would probably be pumping water into and out of the buoy.
I think the essential requirement is to "remove" some of the buoyancy by replacing it with water (like in a submarine). Generally this means compressing the air (rather than venting it) because you will need it again very soon. But it would take a lot of power and energy and machinery to do that - especially to do it as quickly as a wave passes by so that the wave motion is offset.
The way this is done in real life (with drilling platforms) is to have the buoyancy far below sea level where it is unaffected by the surface waves.
...R
Robin2:
I think the essential requirement is to "remove" some of the buoyancy by replacing it with water (like in a submarine). Generally this means compressing the air (rather than venting it) because you will need it again very soon. But it would take a lot of power and energy and machinery to do that - especially to do it as quickly as a wave passes by so that the wave motion is offset.
Unless, perhaps, you can get the waves energy to do the work for you. Maybe an open trough sitting just above the waterline. If breaking wave hits it, then the water gets captured in the trough. Then once the wave has passed, the trough drains through holes in the bottom.
But I really don't think this would achieve anything useful. Even if the technical problems could be solved, I think it would just prove that the whole idea is misguided.
KenF:
Then once the wave has passed, the trough drains through holes in the bottom.
It is called sinking. 
It is an effective way of escaping surface waves.
...R
Perhaps the OP could chime in and explain a bit more about his/her project, for example, "why?".
Sorry for the long wait, I've been busy trying to get a grasp on the idea behind the project iteslf! I'll give a little more insight into the project which I hope should clear some things up...
- The buoy is not attached to the seafloor, it is free to float.
- The buoyancy will be changed by pumping water into and out of the buoy which will cause it to sink and rise by a certain amount
- I'm using an arduino as a controller, the input to the system will be the displacement of the buoy relative to its position when there are no waves, the output of the system will be the water pumps
- I need a way to measure the displacement of the buoy above the position without waves
So for example: When a wave hits the buoy it will float along the wave profile, when the buoy reaches a certain height the pumps will fill the buoy with water which will reduce its height. Once buoy passes the peak of the wave and starts descending the water will be pumped back out.
Hope that helps
Where is the pumping energy going to come from?
Have you estimated what mass of water needs to be moved, and how quickly?
How accurately do you want the buoy to hold its vertical position?
I have to say I think the idea is completely impractical unless it can be connected to a riser fixed to the sea bed to provide a height reference and a means to connect up power cables.
What is the purpose of the project? Maybe there is another way.
...R
The pumping energy is coming from a 12V onboard power supply
All the necessary calculations have been done to show that it is feasable
Aiming for as accurate as possible, will only really be able to know after calibration
The purpose of the project is a cost effective way to reduce the displacement response of a buoy when it is hit by waves, causing resonance. This can be applied to floating wind turbines to ensure that the whole structure remains stable. These structures can't be rigidly attached to the sea floor as it is far too expensive for the renewable energy sector so now we are looking into active buoyancy control systems.
The main problem I am having is measuring this displacement
If you've got some substantial superstructure (such as a wind turbine) sitting on these things, then I'd suggest that the more important issue is the angle that the bouy is sitting in the water, rather than it's level.
I'd suggest some active keel arrangement would be far more effective to get the stability you're after.
smscannell:
Aiming for as accurate as possible, will only really be able to know after calibration
That is rather putting the cart before the horse.
You need to have some sense of the acceptable error in order to decide what measurement system to use.
GPS, for example, does not give accurate vertical positions.
Maybe you could design a system to minimize vertical accelerations rather than to maintain an absolute height.
I would have thought floating wind turbines would use the same technology as semi-submersible drilling platforms - buoyancy below the level of wave disturbance.
I would also be concerned that pumps would use up most of the energy produced by a wind turbine - which would not make any economic sense.
...R
SUBMERSIBLE BUOY
● Main tank holds water pump and is positioned just below the waves.
● Rigid pipe with sliding float holds sensors and electronics above surface.
● Rigid pipe is hollow, protects wiring and allows tank air intake and exhaust.
● Float moves up/down following the water surface.
● Averaged position data from float determines ideal tank position below water surface (reference plane).
● Water pump slowly adjusts water level in tank when required.
● Battery pack suspended by cable, or installed inside tank.
● Very little energy required to adjust tank position as it's already inherently stable.
For every action there's a reaction. If, instead of pumping water you had a dead weight dangling from your buoy, (say that battery) then to pull the buoy down, you just have to haul the dead weight up.
When you want to reduce the buoyancy, you allow the weight to fall again.
In the meantime you could have a small float that sits on the surface. As the small float gets lifted it will exert a tug on a sensor on the buoy. This is it's cue to start tugging on the weight. As the small float drops again, the reduction in it's tug on the sensor would be the cue to release the weight again.
dlloyd:
SUBMERSIBLE BUOY
That is essentially how a semi-submersible drilling rig works.
It raises the question, of course, "what is the purpose / value of the buoy?"
...R
That is essentially how a semi-submersible drilling rig works.
It raises the question, of course, "what is the purpose / value of the buoy?"
To use it as a probe ... collecting accurate real-time data for wave analysis, profile, reference plane determination, early warning of disturbances, rouge waves, weather / water data, etc. Data is sent to the floating systems described / required in reply #9.
Those systems would then take appropriate action(s) based on the data. For example, to react ahead of time to an incoming disturbance.
I couldn't see the purpose in having the buoy fight the waves.
dlloyd:
To use it as a probe ... collecting accurate real-time data for wave analysis, profile,
Wave data is already widely collected by, for example, the Met Office so I presume that part of the problem has already been figured out.
...R