Open source underwater glider

A versatile autonomous environmental drone using a buoyancy engine

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There has been a breakthrough with low cost autonomous drones and as this capability has matured a wide range of hobby and commercial applications have developed. There are no affordable extended duration underwater exploration platforms and this project aims to address this need.

Utilising commodity hardware, 3D printed parts and an open-source autopilot, I aim to produce a low cost and versatile underwater glider capable of extended missions of up to weeks at a time. I hope that by having this platform available, it would reduce the cost of underwater projects for all, from hobbyists, amateur scientists to seafood farmers



  1. Designing the underwater glider
  2. Printing and assembly
  3. Testing movement assemblies
  4. Component selection
  5. Schematic creation
  6. Testing specifications
  7. PCB Design
  8. PCBs and soldering
  9. Waterproofing problems
  10. Exams are over

The CAD model

The model is viewable on the Onshape online platform here (requires webGL)

Movement test video

Why a glider?

Traditional unmanned underwater vehicles depend upon active propulsion, limiting their range and runtime, making them unsuitable for long duration monitoring missions. Underwater gliders use a buoyancy engine to change the mass of the glider, allowing them to ascend and descend through the water. With power only being used to power the engine intermittently, gliders can typically run for weeks or months without recharge, making them ideal for environmental monitoring. Yet there are few commercial solutions available (and those that are available are very expensive) and even fewer hobbyist projects exist.

As underwater gliders travel slowly through the water, they disturb the surrounding water very little, allowing for accurate and reliable data recording. Underwater gliders are normally AUVs (Autonomous Underwater Vehicles) and can run a pre-determined route without requiring human interaction. Their low speeds and autonomy, combined with long battery life, make underwater gliders ideal for long duration, environmental monitoring missions, capable of recording dissolved gas levels, pH, temperature and optical sensing (for oceanic surveying and sealife recording).

The glider is open-source, with 3D printed components combined with readily available hardware, allowing it to be assembled for a low cost. Given the openness of the project, the project could be forked to produce alternative designs suited to particular scenarios. For instance; changing the tubing to aluminium to become a deep sea glider; using a unique sensor array for specialised applications; changing the buoyancy engine size to increase speed.

I am looking to use the open-source ArduSub (based upon the popular ArduPilot) autopilot platform, allowing the glider to be controlled using a standardised interface.

With this technology available, there would be a wide variety of uses. For instance, with increasing interest in product transparency and traceability, environmental monitoring is becoming increasingly important; a kelp farmer could use the glider to monitor water conditions (temperature/pH/nutrition levels/pollution) during a season of growth and push the measurements to a blockchain. The kelp/seafood could be packaged with a QR code, which would direct you to a web frontend, presenting the conditions during the season of growth. The use of the blockchain for measurement storage would remove the chance of measurement tampering, so the consumer would know both the conditions that their food grew in and what they’re eating.

Above: A block diagram outlining the how the glider could be used for product traceability


The buoyancy engine that I have designed uses a threaded rod to move the ends of the syringes when rotated by a stepper motor, causing the plungers to take in water. When water is taken in, the volume of the glider remains constant, but the overall mass increases, therefore the overall density of the glider increases and the glider becomes less buoyant.

At the centre of the glider will be a mass that controls pitch and roll. The mass shall be composed of Lithium ion batteries and a pewter casting, to form a substantial mass, so that it has sufficient control over the glider. The pewter mass shall be formed by pouring molten pewter into a printed PLA negative mould, inspired by a hackaday article. The mass shall control the pitch and roll of the glider. The pitch will be dictated by a stepper motor connected by a threaded rod to move the centre of mass forwards and backwards, thus moving the centre of gravity forwards and backwards, affecting pitch. A secondary stepper motor shall drive a planatary gearbox...

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Attribution-NonCommercial-ShareAlike 4.0 International

license - 20.36 kB - 04/26/2017 at 12:51



The eagle schematic

sch - 387.56 kB - 04/30/2017 at 08:29



The eagle board

brd - 136.03 kB - 05/18/2017 at 11:19


Standard Tesselated Geometry - 95.20 kB - 04/15/2017 at 22:29


Standard Tesselated Geometry - 1.05 MB - 04/15/2017 at 22:29


View all 39 files

  • 1 × Printed parts Print all the parts in the "files" section, print the amount indicated
  • 1 × 84mm ID 90mm Acrylic tubing 1000mm The main tubing
  • 4 × 74mm ID 5mm C/S Nitrile o-rings Seals for the central compartment of the glider
  • 1 × 49mm ID 4mm C/S Nitrite o-rings Used to seal the tail section
  • 2 × 10mm aluminium tubing (1 meter lengths) Used in general assembly

View all 48 components

  • Buoyancy engine updates

    Alex Williamsa minute ago 0 comments

    I have been able to retrofit the syringe buoyancy engine designed for the older tubing into the newer Blue Robotics tubing and this is now watertight. There are a couple of areas where the length of the buoyancy engine has increased; notably the piping has increased the length as I originally intended for each syringe to connect to the exterior, but this would have increased the number of seals to the exterior (increasing chance of leaking) so I instead joined the syringes together which increased the length of the buoyancy engine.

    The test setup:

    Testing of the buoyancy engine running underwater, first pushing out air and then taking in water:

    The current buoyancy engine has a couple of potential downsides. The current rate of change is approximately 3 grams per second with a total mass change of about 160 grams. The new tubing requires a total glider mass of approximately 11kg, so a mass change of 0.16kg gives the buoyancy engine little control authority over the glider and a greater variable mass will be more effective.

    Using a larger syringe is not possible as larger medical syringes are not produced, so a custom syringe would have to be made which would introduce the problems associated with dynamic seals. Alternatively a peristaltic pump could be used to move liquid (oil/water) from an internal bladder to an external bladder and the variable mass amount would be dictated by the bag size. A peristaltic pump based buoyancy engine would also reduce the length of the engine so it would be more compact. In order to increase the rate of change of mass on the current buoyancy engine (so the glider is more effective at shallower depths as it can transition between ascending/descending states more quickly), the motor has to rotate more quickly or the threads on the screw have to be larger, but both of these options will slightly reduce torque. I will explore the option of a peristaltic pump based buoyancy engine in the near future.

  • Exams are over

    Alex Williams07/06/2017 at 16:47 1 comment

    Now that the exam period is over I am able to start spending more time developing the glider. This is a quick update about the development over the last week or so.

    The Blue Robotics order arrived and the components are of a very high quality and a significant improvement over previous hardware. The new tubing has an internal diameter of 100mm, whereas the previous tubing had an internal diameter of 84mm. In order to produce a proof of concept quickly, I will use the internals designed for the smaller tubing and use spacers so that the internals do not move around within the new tubing. I have currently fitted the backend of the internals within the new tubing using spacers. Blue Robotics also produce endcaps which forms a watertight seal with their tubing and you can easily print components that bolt onto the endcaps.

    I uploaded the arduino bootloader to the ATMEGA328P so that it can now be programmed standalone and was able to test the stepper motor drivers on the board and they function perfectly. However, I was unable to solder another board with the IMU (LSM9DS1) attached using the hotplate method as the pins were too close together. I was going to order another IMU and solder it at a hackerspace with a hot air soldering station, however the component is out of stock until November. A workaround is using the SDA and SCL pads on the PCB to attach another IMU (MPU6050).

    I have produced a battery holder for the lithium ion batteries using battery spring contacts and printed holders which snap the batteries into place. The batteries are protected using a 3 cell protection circuit. This battery holder will also accommodate steel rods on the underside to act as the movable mass which is used to control roll and pitch.

  • ​Waterproofing problems

    Alex Williams06/09/2017 at 10:09 0 comments

    The current state of the glider is not capable of performing underwater testing (It’s likely to currently be IP66 rated without sealants, i.e. very much not sufficiently sealed). The poor performance is due to a mixture of design flaws and fabrication difficulties. For instance, the design uses multiple o-rings where fewer would suffice, increasing the chance of failure. Moreover, the dimensions of the printed o-ring grooves vary by minute amounts between test prints, making it hard to produce reliable seals as the grooves between layers allows water to pass through.

    To solve this issue, I am currently I am looking at purchasing a watertight enclosure (above, but with a custom length tube) from BlueRobotics. All of their hardware has been purposefully designed to withstand underwater conditions for ROVs and the enclosure I am looking at purchasing is rated to a 100m depth. The enclosure and associated parts are relatively affordable compared to commercial solutions, however they are expensive enough that they could put off hobbyists looking to build the glider (The order I am looking at placing is ~$500 including shipping to the UK). I feel like this is justified to produce a working prototype more quickly, with effort spent on getting to a stage where I can demonstrate underwater movement, rather than spending time on producing a watertight enclosure. In the longer term, I will revisit the printed endcaps, to reduce the cost and to increase accessibility to the design, using the Blue Robotics endcaps as a drop in option for those without access to a printer of a high enough quality to produce watertight seals or those wanting to go to a greater depth.

    Once I have recieved the parts, I will post an update showing the parts I have purchased (I’m still finalising the order) and any thoughts I have concerning the enclosure.

  • PCBs and soldering

    Alex Williams05/30/2017 at 22:07 0 comments

    I received the boards a few days ago and as I had already purchased the surface mount components, I was able to produce a partially assembled board relatively quickly.

    I soldered the surface mount components using my trusty hot plate, which seemed to do a good job on the 0.45mm pitch atmega QFN package. The power regulator, resistor and headers were hand soldered. The resistor is due to a design error using an incorrect resistor package size and will be corrected for the next board revision.

    The board pictured does not have the IMU soldered on as I wanted to test the microprocessor first (flash firmware and simple stepper motor control), as the IMUs are relatively expensive (~£6 for 1). Once I have validated the microcontroller circuit, I will assemble another board with all the components.

    This is what the board looks like with the three stepper motor drivers attached:

    The next step is flashing the atmega and porting over the control software and making the glider standalone. I also plan on publishing an update concerning water sealing soon.

  • PCB Design

    Alex Williams05/18/2017 at 11:18 0 comments

    I was able to import the glider board outline design into Eagle with relative ease using the inbuilt DXF importer. This includes mounting holes on the board to attach it to the glider.

    I have allocated spare I/O pins from the ATMEGA to a set of male headers. Battery connections are seen at the bottom of the board. The three stepper motor drivers are in a row, with each having the motor connections nearby.

    I have updated the files section to include the eagle board. I will also purchase a series of boards over the next few days, so I receive them within a few weeks.

  • Testing specifications

    Alex Williams04/30/2017 at 23:51 0 comments

    I have produced a table of various specifications that the underwater glider must pass in order to fully function as intended. The specifications listed range from absolutely necessary for the glider to glide (watertight etc.) to requirements that allow it to function as an autonomous data-logging drone (way point navigation and a functioning sensor array)

    Some requirements I am will be able to achieve relatively easily (function without external connections etc) but some are beyond my current capabilities (inertial navigation, for instance).

    Specification requirement Details How to achieve this
    Motors and movement assemblies The stepper motors should be able to control the
    movement assemblies easily
    If the motors are incapable of moving, reduce resistance. (One source of high resistance could be poorly meshing gears, which could be fixed by increasing clearance or adding
    Functions without external cables The final glider should operate underwater, without
    external cables for power or control signals
    All electronics should be inside the glider and should run off of lithium ion batteries
    Watertight No water should enter the glider whilst operating,
    otherwise this will damage electronics and cause the glider to sink
    If the glider is not watertight, change the seal dimensions and add sealant to ensure the central compartment is water
    Neutrally buoyant The glider should be able to become both positively buoyant and negatively buoyant by being neutrally buoyant when the buoyancy engine is half full Add/remove pewter ballast until the glider is neutrally buoyant when the buoyancy engine is half filled with water
    Descends and ascends through water If neutrally buoyant, the glider should descend and ascend through the water if the buoyancy engine is full/empty respectively If the glider does not change depth, look at increasing the capacity of the buoyancy engine so that there is a greater density differential
    Responds to gyroscope inputs The glider should be able to maintain a constant angle of attack whilst underwater Test the glider’s response whilst above water, the mass assembly should move to oppose a change in angle of attack, to keep the angle of attack constant whilst gliding.
    Moves forwards The glider should be able to dive forwards whilst descending, allowing it to travel between two points The center of mass should maintain a constant angle of attack and the hydrofoils will provide forward thrust
    The glider should be capable of turning whilst gliding, so that it can go between waypoints that are not in a line By rotating the central mass assembly, the glider should be capable of turning (albeit with a very large turning radius). Lowering the centre of mass of the mass assembly increases the moment and reduces the turning radius (by increasing roll)
    Waypoint navigation The glider should be capable of autonomously navigating between pre-deteremined waypoints By using a GPS module, the glider should be capable of finding location whilst at the surface (GPS signals do not penetrate water). By gliding in the correct direction, the glider will be capable of moving between waypoints
    Inertial navigation The glider should know its position whilst underwater Using the gyroscope and accelerometer inputs, the glider should be capable of calculating its position whilst underwater, between GPS readings
    Sensor array As the glider will be an autonomous environmental monitoring drone, it should be capable of recording gas levels, pH, temperature, light levels Attach the sensor array and program the microcontroller to record data inputs to non-volatile memory
    Data mapping The sensor data should overlay onto a map indicating location of the data By recording location when recording sensor readings, the data can be overlayed onto a map

  • Schematic creation

    Alex Williams04/30/2017 at 10:01 0 comments

    As I have had a small amount of prior experience using Eagle, I decided to develop the PCB within Eagle. The initial schematic was relatively quick to produce as the microcontroller and IMU both had schematics available online that were suitable. The majority of the schematic is taken up by headers to connect the FTDI/A4988s.

    There are many barebones Arduino tutorials available, so I was able to quickly produce a circuit for the atmega. Using the internal oscillator (8MHz) and only allowing it to be programmed via an FTDI cable, I was able to reduce the number of components massively.

    The LSM9DS1 IMU has a open-source breakout board produced by SparkFun which I used in tandem with the IMU datasheet to produce an sub-system circuit quickly. The IMU eagle library can be found on SparkFun's GitHub here. The ATMEGA328P-MU package is found in the library by element 14, found here. (This requires an account to download).

    I intend for the PCB to be produced on a board that fits within the glider tubing; above is a DXF board outline that will be imported into Eagle. Over the next couple of weeks I'll produce the control board and update the project as it becomes complete.

  • Component selection

    Alex Williams04/26/2017 at 15:16 0 comments

    I am now at a stage where I can start to think about the PCB/control board, and I shall use this project log to outline each major component and its reasoning.

    For the central microcontroller, I intend to use an Atmel Atmega microprocessor as it can easily be programmed with the Arduino IDE. The design of the control board should also be straightforward, as there are many tutorials on producing a barebones board. As I do not require many inputs/outputs, I have decided upon the Atmega328P (specifically the MLF package for its small footprint, which is smaller than its through-hole/TQFP counterparts).

    With regards to the gyroscope, I plan to use an IMU (inertial measurement unit) which will provide the glider with an accelerometer and magnometer, allowing for inertial navigation later on without the requirement of designing new hardware. I selected the LSM9DS1 after looking on the Sparkfun website, and I will now make use of the schematic diagrams they provide for their breakout boards; these will allow me to check my circuit against theirs to produce a board that I am sure will function. Additionally, the LSM9DS1 is an LGA package, so has a very small footprint (3.5x3mm – should be fun to solder).

    There are some components which will require a 3.3v CMOS supply, so the control board will use an LM1117 voltage regulator for the supply from the batteries. To control the stepper motors, I will be using the A4988 drivers as they are readily available and feature in many tutorials with hookup guides and programming examples.

    As I hope that the glider will be capable of waypoint navigation at some point, I will also provide pins to connect a GPS module. I found this suitable product from SparkFun at an affordable price. A tutorial for this particular module can be found here.

    After looking in the Eagle libraries, I realise I will have to create a footprint for the LSM9DS1, but the other components are relatively standard and have footprints available. I should have a schematic for the board produced within a couple of weeks or so.

  • Testing movement assemblies

    Alex Williams04/22/2017 at 11:22 2 comments

    As I have printed all of the components and assembled the glider, I was able to hook up the stepper motors to A4988 stepper motor drivers and an Arduino and test the movement assemblies of the glider.

    Below is a video showing the movement of the movement sub-systems.

    The code that was ran on the Arduino is found on the project's GitHub page here.

    Over the next couple of weeks, I'll select components to use and produce another build log outlining the rationale for selecting the components, before going on and producing the schematics/PCB.

  • Printing and assembly

    Alex Williams04/18/2017 at 15:39 3 comments

    Having produced a complete CAD model of the glider, I started printing the parts for the glider. I own a Mini Kossel printer, which is a delta style printer (as opposed to the standard cartesian printer) The components were printed at a layer height of 0.4mm, for greater structural strength and faster prints. Printing all the pieces took approximately 80 hours. I had to print some pieces multiple times to get the dimensions of some features correct (bolt and tubing holes, etc).

    The hydrofoils attach quite firmly to the glider body and are unlikely to slide off during operation.For the majority of the parts, no post processing was required, but for a few parts the printer had a printing error which caused the layers to become stepped (I have since fixed this error). As the printing error would not affect functionality, I decided to not redo the prints and would clean the affected pieces using a dremel.

    The nosecone pieces after post processing, you can see the stepping error at the join

    Printed material is less dense than water, so I made the models hollow in some areas, so that they could be filled with ballast. I am going to use a PVA/sand mixture as the ballast, as this is denser than water (~1.7g/cc depending upon the ratio of PVA to sand) and can be formed quite easily.

    The hollowed internals of the hydrofoils, ready for ballastThe planetary gearbox also required a lot of work regarding the tolerances of the gears. Although it is not perfect and there is still a slight amount of slip, I am happy with it to a sufficient degree for a proof-of-concept.

    I have hooked the stepper motors up to an Arduino and over the next week or so, I will test the movement sub-systems and post a build update showing basic functionality.

View all 11 project logs

  • 1
    Step 1

    Print all the pieces listed in the "files" section, print 1 unless indicated otherwise.

    Collect all the components listed in the "components" section.

    The only tools required for assembly are hex keys and a cross head for the screws. A drill is optional, but useful to bore out any incorrectly sized holes.

  • 2
    Step 2

    Using a hacksaw cut 9 lengths of aluminium, 3x 6cm, 3x 13cm, 3x 28.5cm

    Drill 2 holes in the 6cm pieces, each 5mm from each end on the same face.

    Drill 2 holes in the 13cm pieces, one 5mm from an end and the other 9cm from the same end. Both holes are on the same face.

    Drill 3 holes in the 28.5cm lengths, one 5mm from an end, another 245mm from the same end on the same face, the final hole 178mm from the same end on the other face.

    Make sure to go all the way through both sides of the bar for all holes.

  • 3
    Step 3

    Using 2 30mm M4 bolts and nuts per wing, construct the wings.

View all 38 instructions

Enjoy this project?



Kevin Klemens wrote 05/21/2017 at 03:18 point


This is an awesome project and i am very excited to get my materials all together and start printing (I have a Taz6). Thanks for making the files available and providing instructions, that is huge for the marine robotics hobbyists. I started a thread on the BlueRobotics forums if you wanted to join in:

I've been working on really long range communications with a friend, so give me a ring when you get to that part. We have Wifi and 4G working on a Pixhawk 2.1 and we are pretty close to getting Iridium satellite comms working for our boats, but I always wanted to put the circuits on a UUV.

  Are you sure? yes | no

Alex Williams wrote 05/22/2017 at 17:57 point

Thank you for your interest in the glider. I am currently looking at using the 4” tubing from Blue Robotics as the current design may not be watertight, due to a mixture of the endcap design and the limits of my printer. The Blue Robotics’ tubing would increase reliability of the seals and easier to produce a working prototype. I hope to have the parts purchased within a few days and will publish an update outlining the parts. Given the comments of others, I am also going to look at using a bladder based buoyancy engine, giving me a greater engine volume and reducing complexity. I will overhaul the design once my school exams have finished (a couple of months).

Once I have the glider functioning underwater, I am interested in adding an autopilot for running predetermined routes and a sensor array for data collection. I am also interested in long range communication for longer missions, using either 4G or the Iridium communication module (I was initially put off by the price though) and any help in that department would be greatly appreciated.

  Are you sure? yes | no

Kevin Klemens wrote 05/22/2017 at 19:57 point

I concur with your move to the BR 4" WTC. I've been looking at that for the hull of my vectored thrust UUV (Design idea right now). Any commonality with parts for your glider and BR products would be a good thing to make for an easier entry for people. I've physically been there to help with the WTC depth tests, so I can vouch for their numbers.

I read the below comments on oil bladders, and while a good idea, I think you're going to have more fine tuned buoyancy control with the piston style ballast tanks, but that is just my two cents. It would be interesting to see dive results using both methods.

I'll send you a PM on the command and control aspect and we can collaborate on that.

  Are you sure? yes | no

Modzer0 wrote 05/25/2017 at 15:00 point

On the oil based bladders below. The reason oil is attractive is for deeper water. When you're diving an air based system you're doubling the compression every 10m which causes changes in volume and with changes in volume come changes in buoyancy. With an external bladder which is commonly used for deep diving such as ARGO oil is the only way to go.

An alternative is to use front part of the hull itself as big buoyancy engine cylinder.

  Are you sure? yes | no

Kevin Klemens wrote 05/25/2017 at 17:46 point

I agree, if this were to be going really deep >100m I'd say oil and hydraulics would be the way to go. That's what the commercial deeper diving gliders use. The 200m Slocum looks to be using piston tanks though.

However, the real limit here is going to be the WTC. The 4" acrylic ones should be good down to 100m, but there isn't a customizeable 4" aluminum one yet, unless Alex finds one. So 100m will probably be the depth rating, which would be a good number for an inexpensive glider like this. Material cost goes up exponentially the deeper you need to go.

The reason I'm a bit against oil based buoyancy is because I've seen a few DIY oil compensation experiments (lights and servos) and none of them worked well.  It only managed to make a mess of electronics when they leaked. All the oil pumps I've seen for gliders look to be piston based with a three way valve.

The ROUGHIE glider uses piston tanks and trimmable pitch and roll and seems to have a very nice flight path.

  Are you sure? yes | no

Mike Yurick wrote 05/13/2017 at 17:37 point

Nice project, wish I had some water handy to do something like that.

Any plans for a nosecone camera?

  Are you sure? yes | no

Alex Williams wrote 05/14/2017 at 13:51 point

I am considering using the 4” tubing/seals from Blue Robotics, as this would make it far easier to make watertight (I am going to release a project log concerning this). They have a dome endcap specifically for camera use and I was planning on purchasing one to have it available for experimentation later down the line.

  Are you sure? yes | no

Modzer0 wrote 05/12/2017 at 20:13 point

An operational tip that will help you save power is to not rely on the weight mass for pitch control. When the glider is neutrally buoyant with the pistons at 50%, or 60% if you want a bit of positive reserve in case of leaks. The larger the variable ballast the larger the reserve. There will be leaks when you least desire them. Once you trim at level , and at the neutral buoyancy position of the piston rely on the piston to control the pitch. With it being forward mounted taking in ballast will naturally put it at negative pitch and the opposite is true as well.  Brute force isn't needed with the motors, slow everything down, and go for maximum power efficiency over speed.

  Are you sure? yes | no

Alex Williams wrote 05/14/2017 at 13:53 point

(I'm replying to both of your comments to make replying easier)

Many thanks for your informed comments. I am looking to make this a low cost platform that people are able to use as they wish, including to record the temperature/dissolved gas levels of lakes/still water. Hopefully, this will be the use case I demonstrate. It may only have quite limited utility in readily accessible water bodies particular any that are fast flowing or with tidal currents. Thank you for the comment about thermal layers in lakes, that would have been tricky to identify/figure out otherwise.

Having seen yours and others’ comments, I had another look for bladders and I found that water pouches could be viable. Using a pair and an oil ballast, this would give me a greater variable ballast (150g for 3 syringes vs 600g bladder in the same space) and hence a larger reserve. Due to your mention, I am looking at a small peristaltic pump that I could use to control the bladder and to adjust both buoyancy and pitch. However, I am likely to use the movable mass to control pitch when starting out, as I feel like this will be easier than using the buoyancy engine to control both buoyancy and pitch and will be sufficient for proof of concept.

In addition to having a reserve in the buoyancy engine in the case of a leak, I am also contemplating having some masses on the exterior of the glider, attached to the tubing by strong magnets (on the interior). In the case of a leak, an electromagnet ring would pulse on and repel the masses and making the glider positively buoyant. I have yet to work out the feasibility of this type of leak emergency system.

  Are you sure? yes | no

Modzer0 wrote 05/15/2017 at 22:26 point

If you try to maintain pitch with your movable mass system trying to hold it at  specific angle you're going to be fighting your buoyancy engine. Trim it level at neutral buoyancy. When you take in ballast you're going to naturally make the front heavier which will give you negative pitch. The more negative the buoyancy the greater the pitch. The same with positive buoyancy, the more, the greater the up angle. You get it for free with forward mounted variable ballast so use it to your advantage.

For a submerged object you have the center of gravity (G) which in your case is altered by the weight mass. Then you have the center of buoyancy (B). At rest B will come to rest directly over G.  When you decrease buoyancy forward B will shift aft. When you increase it B will shift forward which will alter pitch as a result. There's a lot of math but you can skip it because your center of gravity is variable and can be adjusted with input from gyros. If you had a fixed ballast mass then the math would be more important.  The buoyancy engine is at the front so changes will result in larger shifts in B.

The droppable weights are possible, but add a bit of complexity and unless they're secured mechanically they can fall off at the worst moments.

  Are you sure? yes | no

Alex Williams wrote 05/30/2017 at 21:28 point

(I am replying to this comment as hackaday does not allow you to reply to a third level comment, so I cannot reply to the intended comment.)

As I understand it, the variable ballast at the front of the glider would control pitch as desired if the oil is less dense than the surrounding water. Therefore the pitch can also be correctly controlled by the buoyancy engine if it is mounted at the back of the glider, by using oil that is more dense than the surrounding water. Other than less dense oil typically being less viscous (and more available), would there be any major advantages with mounting the variable ballast at the front of the glider, than at the back?

You were also talking about using the oil bladder to achieve greater depths than with the syringes, but the peristaltic tubings that I have come across are typically only rated to 4 bar or so (30m), which isn’t close to the 100m figure that was floating around elsewhere in the comments. Is there a way to use the lower pressure tubing to achieve those greater depths or would I need to find tubing with a higher pressure rating or use a different mechanism altogether?

  Are you sure? yes | no

Modzer0 wrote 05/30/2017 at 21:33 point

You still mount the variable ballast forward. That's where it needs to be to take advantage of the change in buoyancy for pitch. When you start going deeper you're going to need your buoyancy engine and hoses to take the pressure so aluminum and heavy duty hoses will be needed.

  Are you sure? yes | no

Modzer0 wrote 05/03/2017 at 16:24 point

As someone who has built AUVs for the US Navy I have to say this is pretty cool. The downfall of underwater gliders is they need a fairly large and deep body of water. In the open ocean they have plenty of room. It's a bit more difficult in small bodies of water because there's not really much of a power budget for any kind of depth sounder. Something that would be very useful for environmental studies is a low cost version of the Argo buoy that can be used in lakes to profile temperature, O2, and CO2.

One thing you might run into with neutral buoyancy manipulation when diving in something like lakes is a sharp thermal layer. To save power you want to manipulate the ballast as little as possible and very slowly drift down this creates a situation where you'll have it 'float' on top of the denser thermocline. You can of course just take in more ballast and power through but if you do so too quickly you may experience what we have called 'Operation Seadart' where your vehicle does an imitation of a lawn dart and doesn't have the buoyancy reserve to free itself from the bottom. It's a fine balance to dive slowly enough to get a good sensor trace, but quickly enough to gain the desired amount of propulsion, which isn't a lot.  If you want to simplify ballast control use a bladder and a peristaltic pump. It'll reduce the number of moving parts making construction simpler and give you more ballast reserve.

There are a number of ballast control methods but one you may want to consider is oil bladders, one internal and one external in a free flood area. It has the advantage of not being as compressible as air so your buoyancy doesn't change with depth using an external bladder.

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ActualDragon wrote 04/30/2017 at 01:44 point

in the movement video, the green weights are to move it forward or whatever. it looks the same size as a AA battery. if the final goal is to go untethered, couldn't you replace those with batteries?

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Alex Williams wrote 04/30/2017 at 08:17 point

The green weights in the video are actually 18650 cells, which are lithium ion batteries that is a little larger than an AA batteries, hold more energy and are rechargeable. In the video they are not powering anything and they're just there to show where they will go.

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ActualDragon wrote 04/30/2017 at 12:57 point

oh ok. nice project btw!

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VijeMiller wrote 04/30/2017 at 00:31 point

Sexy! I mean of course in an engineering sort of way and not bcz it's phallic shap--uh, never mind. Now, to 3D print the largest bath tub so to properly enjoy this submarine.

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Legrange wrote 04/10/2017 at 10:16 point

This is cool, I have an affinity for things that go under water.

Curious as to how precise your syringe control is and how precise do you need it to be to function properly? Have you done any real life experiments on the control system, if not, how soon before you try things out? How long before you have a workable prototype of the whole thing?

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Alex Williams wrote 04/12/2017 at 10:26 point

I have finished printing about half of the components, and the other half should be quicker to print (I printed the larger components first). I will then be able to assemble the glider and see if anything stands out as being incorrect. Following that, I will design the control circuits and control software.

The capacity of the syringes (180ml) compared to the volume of the glider (~6000ml) is 0.03, whereas the Slocum glider's volume change (450cc) compared to volume (52000cc) is 0.009. From these figures, it seems that the change in density will be sufficient. That being said, I am still sceptical of my glider, as the Slocum glider is clearly much larger and I am unsure as to what degree that will affect the required volume change.

The threaded bolt used to drive the syringes allows for a very precise control over the volume of water taken in; hooking up a stepper motor to the syringes makes it look like repeatable volume change of 0.1cc is possible.

I am currently in the lead up to exams, so will be able to put minimal time into the project over the next 2/3 months. However, following that, I have about 3 months free in which I will do a large amount of development and I hope to come to the point where I am able to perform real life tests on the control systems (simple ascending/descending and pitch control).

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Legrange wrote 04/12/2017 at 19:35 point

Thanks for your reply. It's good you're keeping grounded and sceptical. I look forward to hearing more about this project as you get the time to work on it.

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Peter McCloud wrote 04/04/2017 at 04:06 point

Great looking CAD model

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Alex Williams wrote 04/04/2017 at 09:25 point

Thank you. Functionally, the model has a way to go, but it's more or less there visually

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