Hello, I was brainstorming ideas for alternate underwater release mechanisms, such as the burn wires used for dropcams/ benthic landers to release weights from the system to make them buoyant. and I was thinking, if it would be possible using something such as the actuator used for the subsea gripper but instead of a claw, would have a mechanism that releases a rope or cable by retracting the metal rod from a “nook” like what is used for drone drop systems.
Regards,
Zahari
Hi @Kemist -
Great idea! Using the Newton gripper for that certainly seems possible, but it isn’t a lot of stroke to work with, and due to the control loop relying on current spikes to detect end-stops / force, the potential of a misfire and failed release seems higher than ideal… The depth rating is also 300m, which isn’t much in the grand scheme of things?
However… a Blue Robotics servo with 1000m depth rating and impressive torque output will be available in the not-too distant future - potentially late this year or early next! It is controlled via PWM, and has a 4th wire (besides +/- and control signal) that outputs position as an analog 0-3.3V signal - so a system would “know” if it didn’t travel as desired!
Yeah, the depth rating is the main hurdle for me, i may look into using a depth rated servo housing or a depth rated motor with some sort of threaded turn release mechanism but I don’t think something like the m200 would have enough torque. I may look into electrolytic corrosion based releases but they have a lot of variability
Hi @Kemist -
Blue Robotics has been using zinc releases as backups for DORIS - we’ve found them to have a variability of only a few hours, driven a bit by conditions (current and temperature.) They’ve been quite handy! We’re familiar with what model gives a 24-26 hour “burn” time - how long are you trying to stay on the bottom before releasing? The simplicity and cost of this approach are tough to beat!
There are several types of release devices. The passive galvanic ones are pretty unpredictable in their release time and you tend to have to wait many hours or days until release. But if you are working only in sea water, you can make an active galvanic mechanism that releases in a minute with only a few seconds variability. I’ve used 30 and 36 gauge Nichrome wire placed a cm or so from an aluminum plate - one lead to the wire, the other to the plate so the current goes through the water. If you put a few amps through the nichrome, it corrodes and breaks in sea water in about 30 seconds for the 36 gauge and a little over a minute for the 30 gauge. Just be sure than only a mm or two of the nichrome is exposed to the water or it will take longer. Also, the polarity has to be correct. Without the electricity flow, the nichrome won’t corrode since it is up near stainless steel in the galvanic series.
I’ve played with burn wires (meaning here wires that heat up to cut a fishing line) but have never gotten them to work in water. There is too much thermal conduction in the water. I am limited to under 5 amps by the through-hull connectors; it should work (in theory) with more power.
In the end, I don’t use either technique for active ballast drop. I just use a solenoid with the coils and leads coated in epoxy. Strong, reliable and resettable on the boat.
Thank you for the information, I have considered electrolytic erosion releases but I thought they were too variable to use initially, but as long as the release time is under ~6 minutes or so it should be fine for a standard BRUV release, it does have to hold some amount of force due to the underwater floater so I may go for a thicker gauge than 30, but I am working in sea water so it should work out fine, as long as the battery has more than enough capacity. I am curious though, if you do use solenoids as a release mechanism, do you encounter problems with voltage spikes or EM fields from the coil messing with your more sensitive electronics? I’m assuming you’re using a microcontroller/ RTC timing system in some sort of underwater housing of sorts.
Thanks in advance,
Zahari
Thank you for the information on DORIS, it’s preatty cool! The system captures around 7 hours of video and uses a pre set date/ time system for releasing. Ideally we would want to set it to 24-72 hours from when we deploy due to future weather unpredictability and so it has less chance of getting damaged while underwater, the depth rating should be somewhere from 1000-1500m at most.
Regards,
Zahari
I’ve never had a problem with the solenoid generating electronic noise. It is about half a meter from the electronics, embedded in the sea water and outside the aluminum housing protecting the electronics. The solenoid is fed by a FET, which has good isolation from the microprocessor input. There might be some EMI generated in the wires after the FET and inside the housing - I’d twist the wires as a first defense if I ever see an issue.
I don’t know how much force you are dealing with, but 30 ga. nichrome is pretty strong. I doubt I could break it just by pulling it apart. You can also use the magic of levers to release a large force with a small one. My solenoid holds and releases 5 kg (in air) of ballast which is reduced at the solenoid through a lever. Bob: A robotic drop camera
Wow that’s super cool!, what type of lever do you use with a solenoid? Is it like one where one side is bolted and the other is held by the solenoid’s pull pin? It may be worth while looking into solenoids for a 24 hr video system with some sort of redundancy due to them being easily reset. A nichrome release could also work with some sort of hook lever so the wire doesn’t experience any force bending back and forwards that could snap it. For my initial design I re- created a release from a mastersd thesis, that seemed to work well, but could use some improvements, like by swapping out some release parts for others that are more well known to work underwater or just further long term testing, and using an external PWM board to prevent glitches with the motor, but it worked decent in initial testing. I think I have a few videos of the initial testing but I would have to look for them first.
Paper: "DEMOCRATIZING DEEP-SEA TECHNOLOGY DEVELOPMENT USING RAPID PROTOTYPING " by Breanna Motsenbocker (Chapter 4)
I’m not sure if it fits your application, but you night want to look at an electromagnetic release mechanism which only requires a current pulse for release and has no moving parts:
Zubax Robotics releases FluxGrip FG40 — an electropermanent magnet for payload attachment in drones (that will obsolete existing electromechanical alternatives)
12 September, 2024
