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Thruster for scuba-diving


(Kristiaan) #1

Can anybody advise me on building a thruster that I can use to move smoothly onderwater while filming.

I’m thinking of building a T200 in a torpedo-like design that I can either mount between my dive bottles (on/off control and maybe simple speed control via a cable to a small control box operated by my right hand) or mounted on my camera ( Panasonic GH4 with Nauticam UW housing). Autonomy of 2 hours envisaged.

Can anybody advise on how to build or buy a simple controller (on, off, half power) and if one T200 would be sufficiënt to move one divers at normal swimming speed.


(Richard) #2

Kristiaan,

The Pegasus Thruster is considered top-of-the line for Scuba divers and is built to commercial specs. It is very expensive at $4000 U.S.

Here are the specs:

12 Volt High Torque DC Motor

  • 35-45 Minute Continuous Run Time*
  • -5.1lbs. Negative Buoyancy
  • Submersible to 325 feet
  • Anodized Aluminum Structure
  • Double Sealed Housings
  • NiMH Battery
  • Conical Shroud
  • Speed up to 170 Ft. per minute*
  • approx. 35 lbs. Static Thrust
  • Dimensions: L -23.5" x W-8.5" x H-8.5 Wet Plug Connectors
  • <b>Battery Charger</b>
  • Explosion Proof Design
  • Meets New DOT Air Carrier Regulations 49 CFR Subchapters 171-175, "Shipment of Hazardous Materials" Guidelines
 

Below are the component weights:

  • Battery 5 Lb 10 Oz
  • Motor 5 Lb 8 Oz
  • Bracket 1 Lb
  • Switch 8 Oz
The Blue Robotics T200 is capable of 11 pounds thrust, roughly 1/3 of the Pegasus. The Pegasus claims a top speed for the SCUBA diver of 170 feet per minute, a little less than 2 miles per hour. Keep in mind the drag goes up as the square of speed, so to go just 1 mile per hour would require only 6 pounds of thrust. If you will only be diving in situations of less than 1 mile per hour current, and don't mind a quite slow acceleration from stop, the T200 will get the job done, sorta.

If it were me, I would use two T200’s, which will not drive up the cost very much. Building such a thruster pack for SCUBA diving would be a simple engineering task using Blue Robotics parts.

I’ll be glad to help.

Richard


(Kristiaan) #3

Dear Richard,

thank you very much for your kind advice. It makes indeed a lot of sense to use two T200’s side-by-side, and probably best to have them running in opposite rotation directions. For powering I would use two separate packs of LiFePO4 batteries. In fact it might even simplify the design since half-power could be obtained by using one side only. Battery weight isn’t an issue either since I’m diving most of the time with a drysuit and dual carbon bottles that forces a high amount of additional weight anyway. With a power density of over 85W/kg (sorry, I’m not used to US standards), each motor will have its set of 5-cell batteries at 16V. Rated at 20Ah, they should be capable of providing full power for about 1,5hours. Like for example http://www.batteryspace.com/lifepo4-prismatic-module-3-2v-20-ah-10c-rate-64-wh-6-0---un38-3-passed-dgr.aspx. A discharge rate of 1C would be very acceptable. Battery weight would be about 4Kg/package, which is reasonable. Since I’m not a software expert, in case power control is limited to on/off, I might design a very simple control circuit of an oscillator followed by a counter to generate the 3-phase pulses, and each phase pulse switching a power-FET (protected with a flyback diode because the load is slightly inductive). For the on/off control switches, I’m thinking of using small magnetic reed-switches that can be encapsulated and controlled by moving a small external magnet up or down outside the control box. The alternative, using default ESC’s, might be attractive but I have no experience in how to manage the software for controlling them. Maybe you can advice. With many thanks,


(Harold Scadden) #4

@Kristaan see the attached file. I designed a simple stupid PWM controller for pretty much what you are hunting for already.



(Kristiaan) #5

Many thanks Harold. The ‘good old 555’ , it brings back a lot of memories. We must probably be part of the last generation who knew how to make an analog design.

So I understand this oscillator will provide at the output with a continuous stream of pulses at a frequency of about 400Hz and a pulse width depending of the switch settings.

I suppose you can’t feed such PWM signal directly into an ESC to control the motor? Using standard ESC’s from Blue Robotics would of course be ideal since they are optimized for their motors and probably contain some protection circuitry too.

I thought (but again, I’m a novice on ESC’s) that an ESC needs to be programmed since it expects a series of coded commands to be activated and operated.

Best regards,

 


(Harold Scadden) #6

OMG! I just aged myself :slight_smile: Yeah the good old ‘555’ is still around because it works. Yes, this was made for the BR ESC module.


(Kristiaan) #7

Change in approach: I have to abandon the idea of using highly energy-efficient LiFePO4 batteries because they are listed by IATA as dangerous goods and cannot be shipped by passenger aircraft. (https://www.iata.org/whatwedo/cargo/dgr/Documents/lithium-battery-update.pdf). Transporting Nickel-Metal Hydride batteries on the contrary, don’t seem to pose a problem. They are less ideal though, due to lower power-per-weight ratio, higher internal resistance and they have a more critical charge cycle; on the other side, their cost is lower. (https://www.iata.org/whatwedo/cargo/dgr/Documents/guidance-document-nickel-metal-hydride-batteries-en-20140123.pdf). This is most probably the reason why Pegasus uses them in their thruster, although they are considerably less energy-efficient compared tot the most modern battery technology, LiFePO4.

Strangely, in the 787 Dreamliner Boeing uses LiFePO4 batteries for their power backup (and has a lot of problems with them such as overheating and even batteries getting on fire).