Saturday, 1 October 2011

Leak free week and a pull test

So that's about a week now the thruster has been immersed in my "tank" without any water ingress. Every day I've spun the propeller for a few minutes before disconnecting the power and leaving it for another day.

So far so good :)

This morning I carried out a pull test with my new LiPo battery (BTW I'm quite wary of this battery as I know it can deliver a lot of energy very quickly if shorted out, hence the fuse!) and measured the supply voltage with my oscilloscope at the same time.

Below is a picture of the setup I used to measure the amount of thrust I can achieve with my thruster design.

The best figure I saw on the fish scale was 1.00kg.

Obviously as you can see from my mechanism there is a certain amount of mechanical advantage and it is actually twice the distance from the pivot point to the thruster as it is to the scale so the actual thrust is 0.5kg.

I don't think this is bad considering the water in the tank is circulating quite vigorously causing a decrease in actual thrust. I would guess without this factor the figure would be closer to 0.75kg. Also considering the amount of power going into the thruster, about 24VDC at about 3A and the propeller RPM is relatively low.

Keeping the propeller RPM as low as possible means that cavitation and damage to the propeller is less likely.

I can see some noise on the supply (oscilloscope trace) which may or may not affect the system as a whole further down the line, but we'll cross that bridge when we come to it ;)

My next job is to make a shroud/kort nozzle for my thruster and do another pull test to see what kind of difference it makes to the thrust.

Saturday, 24 September 2011

Thruster back in the water

So I'm hoping I'm now in a position to do some thrust tests with my thruster soon.

It's back in my tank, i.e. the plastic box full of water :) and it's now ready to go. I leave it in there normally for a week or so, loading up the prop every now and then just to make sure everything's working and checking for water coming out of the tube that the wires go down into. Up until now this has happened quite regularly, fingers crossed it won't be doing from now on.

Here are a few pictures of the thruster as it stands at the moment.

This one is fully assembled, with the two bars down the side fully tightened up, to compress the o-rings at either end.

This next one shows the brushless DC motor controller PCB (designed to be pressure tolerant, more on that later) fitted into the end of the thruster at the rear end of the motor and the prop removed. The control to the thruster is a speed signal (analog 0V to 5V) and a direction signal, either 0V or 5V (digital). The only other wires going down the tube to the PCB are the power supply (+24V and 0V (return)) and the returns for the two control signals. I have put a ramp in the software so that the minimum time it takes to go from full thrust in one direction to full thrust in the other direction is about a second. This puts less stress on the mechanical components of the thruster design, and eventually where the thuster mounts to the ROV.

The one below shows the plastic tubing and barb/fitting where the power and control wires enter the rear end cap of the thruster.

The whole thing is sealed with 5 o-rings in total and one shaft seal! Lots of seals in one place and I'm sure the number could be minimised, but I think it would make it harder to machine the components then. BTW the tube is from the plumbers merchants, the rest of the items are machined on my small CNC machine.

I'm intending doing some thrust tests next, to start with without a shroud but eventually with a shroud to see if it makes much of a difference, which I'm hoping/suspect it will.

Tuesday, 20 September 2011

First Post

Well, here goes, my first BLOG entry ............ :)

Up until now I've been posting on various websites regarding my progress with a project of mine relating to designing a small underwater remotely operated vehicle, or ROV, in my spare time.

I thought it was time I consolidated some of my results in a single place, so here it is.

I design large ROVs for a living but I am interested in the feasibility of producing a low cost, yet hopefully very capable ROV aimed at the hobbyist (i.e. myself at home).

I do not intend producing an open design as such at the moment, but am interested in how far a small scale setup with limited funding can get me.

It has taken me about three years to get my workshop set up so that I can do most things relating to the prototyping of small mechanical assemblies.

In three years I am only now capable of producing a reliable, leak free underwater housing for my brushless DC thruster motors (or so I think ;)) and also I am able to drive it with a custom BLDC motor controller PCB designed by my self.

I will go into more detail as time goes by but for now here is a link to a video I made of my thruster a while back where I thought I had a reliable design, but in fact I didn't, that's engineering for you ;)

I am also interested in 3D simulators for ROVs and here is a video of a 3D simulator I wrote in C# with the aid of  TV3D (a very capable 3D software library containing an equally capable physics engine called Newton).

And finally here's a video of one of the ROVs I helped design in my day job.

This will not be a swift process but I am really hoping I will learn a lot along the way, I have so far, and long may it continue.