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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.

Contents

Interesting logs

    The CAD model

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


    Videos


    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).

    This 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 or using a unique sensor array for specialised applications.

    I am looking to use the open-source Mission Planner combined with the Pixhawk autopilot platform, allowing the glider to be controlled using a standardised interface.


    Example use case

    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 and data insurance for measurement storage would remove the chance of measurement tampering, so the consumer would know both the conditions that their food grew in and exactly what they’re eating.

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

    How?

    For the glider to move, the buoyancy engine takes in water and increases the density of the glider. When the density of the glider becomes greater than that of the surrounding water, the glider descends. The wings of the glider ensure that the glider goes forwards and the angle of attack can be altered to cause different glider characteristics. When at the bottom of the descent, the buoyancy engine will expel the contained water, making the glider more buoyant, causing it to ascend, moving forward again.

    The buoyancy engine that I have designed uses an acme 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...

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    LICENSE

    Attribution-NonCommercial-ShareAlike 4.0 International

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

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    demo.ino

    Arduino demo program for gliding forwards by descending and ascending

    ino - 2.79 kB - 10/14/2017 at 23:02

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    Calibration.ino

    Arduino calibration program for calibrating the buoyancy trim of the glider

    ino - 2.51 kB - 10/14/2017 at 23:02

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    End_caps.pdf

    Endcap drilling guide

    Adobe Portable Document Format - 5.63 kB - 10/10/2017 at 22:58

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    scad - 8.97 kB - 10/14/2017 at 22:30

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    View all 54 files

    • 1 × Printed parts
    • 1 × Blue Robotics 4" tubing 1000mm The main tubing
    • 1 × MPU6050 Gyroscope/accelerometer
    • 3 × A4988 Stepper motor driver Stepper motor drivers
    • 6 × 18650 Protected cells Litium-ion batteries

    View all 73 components

    • Future testing and to-do list

      Alex Williams3 days ago 0 comments

      As the third generation of hardware is completed for the glider, it has reached a stage where it needs to undergo testing to find the capabilities of the glider. Below are a few tests that need to be performed on the glider and a brief explanation of each test:

      • Testing of the buoyancy engine system to determine a depth rating of the glider - All of the exterior components of the glider (end-caps, switches, underwater plug etc) are rated to at least a 100m depth, whereas the buoyancy engine does not currently have a rating. A pressure test of the buoyancy engine (likely destructive) will determine the overall depth rating of the glider. (The test would be to attach all the syringes to another set of syringes with a plate on top, weight would be added to the plate until the stepper motor cannot move the weight or the seals break, if the former, the stepper motor will be upgraded.)
      • Perform underwater tests with the glider running at different glide angles, used to determine the best angle of attack for different missions (steeper = faster, shallower = greater endurance)
      • Perform extended endurance/range testing as the current endurance/range of the glider is calculated by extrapolating out current data (6 hour running battery life at ~0.2m/s = 4km). Once a depth rating of the glider is achieved, the glider can glide to a greater depth which will mean that it reaches a greater speed and spends less of its time transitioning between gliding states, so the range of the glider will increase.
      • As I have only been able to test the glider in small areas of water, it has not been possible as of yet to demonstrate the turning of the glider clearly, so the glider needs to be tested in a larger body of water.

      I have also put together a to-do list to get the glider to version 3.1. This list is primarily hardware and does not include any to-do points concerning the Pixhawk and automated waypoint navigation system. An up-to-date version of this to-do list will be kept in the dropbox folder.

      Printed components

      • Change acme nut to an anti-backlash nut to prevent acme screw wear in the long term
      • Redesign wing mounts to have a slight dihedral so that the glider is more stable underwater
      • Choose a standardised micro lever switch and use screws as opposed to hot glue
      • Make the planetary gearbox thicker to reduce play/wear
      • Redesign the wiring routes past the planetary gearbox
      • Redesign the Pixhawk mounting plate so that the board can be more easily placed/removed
      • Try and fit all of the external ballast bars internally, to reduce drag etc
      • Change all roll components to a diameter of 98mm, as opposed to 100mm (to reduce friction inside the tubing)
      • Strengthen endcap mounts to prevent bowing
      • Strengthen engine bearing fastener to prevent bowing
      • Redesign motor/acme clamps to increase strength
      • Upgrade buoyancy engine motor to a NEMA 23 motor if required
      • Make pogo PCB mounting plates thicker to prevent bowing

      PCB

      • Hookup the “enable” pins of the A4988 boards, so that you can power off the stepper motors when they’re not in use to increase battery life
      • Hookup a greater number of the unused I/O pins to header pins to increase the number of additional sensors, etc. that can be used
      • Reposition the A4988 stepper motor drivers so that you can access all of the potentiometers without drilling into the aluminium tubing
      • Use the Blue Robotics switch as switch to a relay for the main power control - the switch is potentially getting to the edge of its current capability

      General

      • Think of an alternative to aluminium tubing, to reduce variability in reproduction
      • Upgrade the hotend of the printer and print all components in polycarbonate
      • Apply for Sparkfun funding for their dissolved oxygen sensor
      • Contact Onshape on their forums with review/feedback - global parameters or feature folders are the main suggestions
      • Look at using industrial pogo connector or using a plug/receptacle and signal converter
      • Look at moving the motor driver for the buoyancy engine motor towards the front (in front...
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    • Build instructions and possible kits

      Alex Williams6 days ago 0 comments

      Full instructions for the glider are now completed, with a level of detail that if you're able to assemble a RepRap kit, you should be able to assemble the glider. The only tools required are a 3D printer, soldering station, dremel and then various handtools such as hacksaw/allen keys etc.

      With the Onshape CAD model, you are able to duplicate the model and adapt the hardware for your own requirements (such as adding a front mounted camera) - this glider is designed to be a hardware platform for others to use/adapt, not a project with a fixed use case.

      If there is interest, I may look into the possibility of putting together a few kits containing all parts/3D printed components that are known to work together so you can assemble the whole glider in a couple of weekends.

    • Assembled CAD model (and onshape features)

      Alex Williams10/10/2017 at 10:45 0 comments

      I've finished assembling the Onshape CAD model for the third generation of the glider. It can be viewed here. There are a couple of features of Onshape that make visulisation of the model/sub-systems easier. You can hide objects which completely removes them from view (Useful for the tubing as it then allows you to select internal components). You can also isolate a component (or multiple) which makes all other components faded and makes it very clear as to what the individual components look like etc.

    • Final(ish) third generation hardware

      Alex Williams10/07/2017 at 16:13 0 comments


      I have gone over the glider CAD model and I have changed the parts so that no post processing is required (unless I've missed something).

      Within the Onshape model, the test prints should be printed first and they are used to select values for the component variables.

      I am aware there is currently a lack of instructions for the third generation of hardware but all the images of the assembly can be found in the dropbox folder.

      I should update the project page soon with full instructions for the assembly of the glider.

    • Ascending and descending underwater

      Alex Williams09/22/2017 at 21:54 0 comments

      Just a quick update regarding some testing of the new hardware: I recently had access to a pool and I was able to test the glider's new hardware in deeper water. There is currently no control software onboard, so the glider is dead reckoning using set timer delays (4 seconds down, 4 seconds up). The pitch mass is not varied during the duration of the filming. The angle of attack is currently quite steep, but this will be optimised as a PID algorithm becomes integrated. The tether at the back of the glider is only there for remote programming, which made it quicker to modify timings whilst the glider was in the water; the tether did not need to be attached whilst it was performing this gliding sequence.

    • Third generation hardware update

      Alex Williams09/19/2017 at 17:09 2 comments

      This week I have more or less finished the design of the third generation of the glider. I have printed all of the parts and assembled the glider. A couple of notable design changes include the larger buoyancy engine (390ml vs 150ml), quicker buoyancy engine displacement change (~6 seconds vs ~30 seconds) and various cable management improvements (e.g. pogo pins to connect the electronics of the front/back and cable chains).

      Whilst assembling, some of the parts required post processing, so I will work through the Onshape document updating all the parts with the minor changes and bar/wing mounts (I used second generation mounts as this was quicker). I also photographed the majority of the build so will be able to produce detailed build instructions.

      A quick video demonstrating the newer glider is below:

      This video is a relatively poor representation of the gliding capabilities of the glider currently as it is still changing pitch when it touches the bottom of the pond (if descending) and vice versa, so it could well reach an angle of attack which would cause it to stall, if the water were deeper. A deeper section of water is required to test the newer hardware more thoroughly.

      As stated, the components will be updated, but the current hardware set can be found at Onshape and the program that the glider is running during the video is on the GitHub page.

    • Functional peristaltic pump and why I’m not using it

      Alex Williams09/03/2017 at 18:35 5 comments

      tl;dr : Parameterised and printed a few peristaltic pumps and then designed my own peristaltic pump but they require too much torque to function with the stepper motors. Staying with a syringe design design for version 3 of the glider and will re-explore peristaltic pumps at a later date.

      Over the last few weeks I have been working on a peristaltic pump to act as a buoyancy engine. The advantages of a buoyancy engine is that it allows for a greater variable ballast within a more compact frame than the three syringe buoyancy engine that I am currently using.

      I had a look for existing printed peristaltic pump designs and I came across a parametric planetary gearbox peristaltic pump. This design was a derivative of a parametric planetary gearbox, which I was considering for the planetary gearbox for the roll mechanism as it allows people to print a gearbox with tolerances that have been adjusted for their printer. Editing the openSCAD file allowed for the addition of a motor mount and a custom exterior, allowing it to be integrated into the glider design (mounting/wiring holes). The openSCAD file is available in the developmental dropbox folder (Glider_version_3/SCAD_files) if you want to take a look/try printing it yourself.


      The first version of the planetary gearbox peristaltic pump did not work there was not enough of a gap for the peristaltic tubing to get into position and for the gears to turn. I printed a few more peristaltic pumps, changing the settings each time, but they all had issues with tolerances; if the gears are too close together, they will come out of the printer bonded together; if the gears are too far apart, there is play in the system. If there is too much play in the gearbox the amount of “squish” that the peristaltic tubing undergoes varies as the gears rotate, therefore the pump may be sealed at a certain rotation, and will hold pressure, but at another position, the tubing will be less compressed and a pressure differential cannot be maintained. Having said this, the mechanism is very pleasing and a great demonstration of the capabilities of 3D printing:

      As the planetary gearbox has herringbone gears, it is not possible for you to disassemble/assemble the gearbox, consequently it has to be printed as a whole piece, which takes ~8 hours. The slow print times meant that this was hard to iterate more than once a day, slowing development. Therefore if the peristaltic pump was redesigned to be composed of multiple parts and then assembled, you could alter individual parts and reprint single pieces (as opposed to the whole pump), reducing the time between iterations, making the development of the pump quicker. Even if I had been able to produce a version of the planetary gear pump that worked reliably, it took a long time to dial the settings in, it would not have been a viable choice for the glider as it is not easily reproducible.

      I produced a set of peristaltic pump parts within the Onshape glider document and assembled them within an Onshape assembly to check dimensions without printing. I also made the parts capable of putting the rollers at different positions, reducing revision time (disassemble/reassemble with rollers in different position vs redesign/reprint/disassemble/fit new parts).

      This new design was able to close the tubing but required more torque than the stepper motors were able to supply. I was able to run the peristaltic pump with a drill, as this had sufficient torque. (The clamps seen in the image below are because I do not have M3 bolts long enough to hand)

      I did some initial experimentation, but ran the pump a little too hard (seeing how far it could squirt water – the limit was about 1.5m) and this caused friction to cause the printed parts to heat and deform under the stresses, rendering the pump useless.

      Unfortunately, I didn’t take any footage of this initial testing so I decided to reprint the pump to take footage and do more testing (all at a much lower speed)...

      Read more »

    • PCBs have arrived

      Alex Williams08/23/2017 at 15:21 1 comment

      The PCBs have arrived from OSHpark (I was impressed at how quick they were, 9 days to the UK with the super swift service and super saver delivery) and are the very high quality you would expect with OSH park.


      Population of the board was relatively straightforward; there are a couple of SMD components and the majority of the other components are through hole headers (this version of the control board is a glorified breakout board). The new control board is only a little bit larger than the previous version, but will be mounted horizontally as opposed to vertically.


      I have also designed a mount to attach the Fathom-S board to the control board.


      The Fathom-S board mount can be exported from the Onshape CAD model (right click on a part and select "Export" to download the STL of a part)

      There is a error whilst uploading to the control board via the FTDI header ("avrdude: stk500v2_ReceiveMessage(): timeout"), but programming via the Fathom-S boards is fully functioning. (the 8 wires connecting the two Fathom-S boards can be replaced with up to 600+ metres of Fathom cabling)


      When programming over the Fathom-S boards, the glider's boards will only power on if the USB is attached to the topside board. However, if not programming, the boards can also run standalone off of batter power. I plan on using a Bulgin Standard Series buccaneer connector so that you can disconnect the Fathom cabling, making this feature is very useful.


      After attaching the Fathom-S board, I made a couple of changes to the control board (moving a couple of headers a millimeter or two and changing the silkscreens a little to make them more useful/clear) The latest board version is v0.2.1 and can be found in the development folder on Dropbox (Glider_PCBs/Control_board/v0.2.1). As with version v0.2, there is an error when uploading the .brd file to OSH Park (related to the SMD capacitors; remove them and it works, add them from the default rcl library and it breaks, I have no idea what is going on), so a zip folder containing all the gerber files is also found within the Dropbox folder.

      I also ordered a couple of PCBs to help with cable management. On the second generation glider, there was a set of cables running from the buoyancy engine (stepper motor/endstop wiring) that had to be plugged into the control board, but this would be hard to plug in and slack would occur in the wiring, interfering with the movement of the mass assembly. To fix this, I produced a pair of PCBs, one for the front end of the glider and one for the back. When the back end is slid into the tubing, pogo pins would touch against the pads on the front end PCB. One potential issue with this design is that the two PCBs will have to be within a couple of millimeters of their required positions, as the pogo pins do not compress much (2mm). Moreover, the blue robotics tubing can vary by ±3mm, so the user must be able to set the PCB positions themselves.


      The two PCBs are both within the Dropbox folder: Glider_PCBs/Buoyancy_engine_connector_female and Glider_PCBs/Buoyancy_engine_connector_male.

    • PCB development

      Alex Williams08/15/2017 at 20:24 1 comment

      I realised that the production of PCBs will take a couple of weeks, so I have prioritised the redesign of the PCBs over the last few days. This version of the PCB is designed to work in conjunction with the Fathom-S tether interface from Blue Robotics. The control board uses the serial output from the Fathom-S to program the control board and allows for remote communication. The Fathom-S board provides a serial port connection over a tether length of 600+ metres, compared to the glider's current 5 metre limit (USB communication limit). Additionally, the control board passes through the battery supply to the Fathom-S so that the control board uses the 5V supply that the Fathom-S board produces with its 7805 regulator. Programming the control board via the Fathom-S interface requires a dedicated serial port, so in order to use another serial device (compass or dissolved oxygen sensor) more dedicated serial ports are required. Therefore I am now using an ATMEGA2560 as opposed to the ATMEGA328P, providing up to 4 UART connections (and much more flash memory; 256kB compared to 32kB). Other changes include a dedicated 16MHz crystal and using the MPU6050 breakout board as opposed to the LSM9DS0 (Still out of stock until November).

      Although this control board can be used as a standalone autopilot, with the MPU6050 and external pressure sensor/compass/GPS, the board can also be used as a slave board for an external autopilot (such as Pixhawk). To allow this, there is a set of header pins towards the left of the board for analog inputs/digital outputs. The control board reads PWM signals through the analog pins from the external autopilot and converts the signal into absolute positions for the motors.

      Although I am aware that KiCad is more open-source and is completely free (no paid for features, therefore more accessible), I have decided to stick with Eagle. Given time restraints, it is practical for me to continue using a program I am familiar with. (Onshape was quick to pick up as it is very similar to Solidworks)

      Images of the schematic and board are below and you can download the Eagle design files from the Dropbox development folder (Glider_PCBs/Control_board/v0.2). I have ordered the control board from OSH Park with their Super Swift service and paid for postage to the UK so the boards should be here in about a weeks time.

    • Designing third generation hardware

      Alex Williams08/05/2017 at 21:21 0 comments

      I am using the cloud CAD service ‘Onshape’ to produce the model of the third generation of the glider. This allows the design to be more open (all files are public and viewable at any stage during the design process) and more accessible (Onshape is free and does not require an expensive license, only an internet connection).

      I'm going to produce parametric parts with dimensions that vary in accordance to local variables. The third generation of the glider will be designed so that the user can input values according to a test print and then all dimensions of the printed components would be adapted for the user’s printer, which would minimise post processing. Unfortunately Onshape does not let you specify global variables, (values that remain constant between different parts within an assembly), so I will design the glider within a single “Part studio” (A multipart assembly). If this is too complex (too many features in a single file), I will have to remove the parametric feature.

      An example test piece is below and shows how a single piece would be able to determine the best values for a person's printer for dimensions such as the size of a hole for an M3 bolt through a wall.

      A live version of the glider model can be found here and is updated automatically whenever I make an edit to the model.

    View all 27 project logs

    • 1
      Build notes
      • Given vibrations throughout the glider due to the stepper motors, I would recommend using threadlock or locknuts throughout the build.
      • Most of the parts are parameterisable and should be adjusted to your printer. However, there will be parts of the prints that require a small amount of sanding/drilling to make sure that holes are the correct size.
      • If there are any errors with the build instructions or CAD components (or if you have any CAD part name suggestions - I lost imagination after a while), leave a message and it shall be fixed as quickly as possible. All suggestions are very welcome.
      • On the Hackaday page are a set of STL files with custom values for my particular printer. While these may work for your printer, it is suggested to print the files with custom part variables for your printer, determined by printing a set of test pieces as outlined further on in the instructions.
      • As the Hackaday’s project editor is slow to work with, I made all of the instructions on a google docs documents. When I ported the instructions across all the images became slightly out of proportion and there are too many images to manually change them all. You can view the images in the correct proportions if you click on the image. You can also view all of the images on the Dropbox image link
    • 2
      Preparation of PCBs

      Some board services leave tabs on the PCBs, so you can remove these and sand the edges of the board smooth.

    • 3
      Soldering SMD components

      This step uses hotplate surface mount soldering, a more detailed example of this technique, including video, can be found here at hobbytronics.

      Using a non-food hotplate, heat the PCB slightly, this allows the solder paste to be applied more thinly as it is less viscous and comes out of the syringe more easily.

      Apply the solder paste to the SMD pads, as shown highlighted in red. Use the tip of the solder paste extruder/cotton buds/kitchen towel to remove excess solder paste from the pads. Surface tension will cause the solder to go onto the pads on the atmel chip, so don't worry about connecting them all at this stage.

      Using tweezers, place the SMD components onto the board. R1/R2/R3/R4 are 10K‎Ω resistors and C1/C2 are the 22pF capacitors. Only the atmel chip is orientation specific, so make sure that the alignment dot on the chip lines up with the star on the board. Make sure that the atmel chip's pins line up with the pads on the board.

      In order to form the solder joints, heat the hotplate up to its maximum setting and watch the pads closely. When the solder paste gets near the required temperature, it will turn into a liquid, and it will then gain the silver solder appearance as it continues. Make sure that all of the solder joints have been formed before removing the board from the heat.

      Some of the pins of the atmel chip will have bridged, so use desolder braid to remove the excess solder causing bridging. Use a multimeter continuity tester to check that no pins are connected to the pin next to them.

    View all 25 instructions

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    Discussions

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

    Alex,

    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: http://discuss.bluerobotics.com/t/open-source-3d-printed-glider-on-hackaday/1086

    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.

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    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.

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    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?

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    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.

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    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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    Vije Miller 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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