Category Archives: Background

State of the Machine

After some non-robot delays (got to pay the bills), intellectual work resumes.  Here is a top-level state machine for Nematon, showing the various modes of operation.  Driving around occurs during “Surface” mode, while diving operations happen during “Dive” mode.  “Remote” mode is intended for real-time control, akin to an RC car or airplane.

More software planning updates to come…

PDF Version:  nematon-state-machine-july2015

State Machine
State Machine Diagram


Formal Documentation

The Federal Communications Commission recently sent two letters indicating that the celebrations regarding Nematon I receiving an official ship station license were premature (the letters will be posted shortly).  The license has been withdrawn until further data can be provided.

In response to the FCC’s requests for additional information, the enclosed report was prepared.  It includes some of the Sketchup drawings mentioned in the last post, reference listings of the sources of various components, plus an extended (amateur) analysis of the regulatory status of watercraft like Nematon I.



Getting Serious About Mk III Design

Due to a documentation requirement (which will be explained in the next post), some detailed drawings of Nematon I had to be generated in a hurry.

Using Sketchup and its associated Engineering Toolbox, a pretty detailed drawing in 3D was created.  A couple of images are shown below.  Some work is underway to embed a 3D image in this site for viewing, but until that’s done, these will have to do.

Note the exploded XBee antenna, courtesy of ERobishaw on 3D Warehouse.


Nematon I Front View
Nematon I Front View
Nematon I Left View


The Name, Explained

As promised, here is an explanation for the Nematon I‘s unusual name.  It’s a portmanteau of “nematode” and “automaton”.

Why the change?  Poolbot Mk III was increasingly inaccurate.  The shape of Poolb… Nematon I is still based on tubes and, though an automaton,  is about as sophisticated as a roundworm.

It’s The BOM, Yo

Congratulations to the lads at Blue Robotics, who met their Kickstarter funding goal… twice over, with a little help from Poolbot.  This means that 3 T100 thrusters, with electronic speed controllers (ESCs), will arrive here within 2-3 months.

With the thrusters being an integral part of the Mk III design, it’s time to consider the actual total costs of Poolbot’s makeover.  Presenting the bill of materials (BOM)…

As seen below, the low-cost MIT SeaPerch design is now firmly in the rear-view mirror.  Fortunately, many of these items were already on-hand.  The costs of the larger PVC and sealing the solar panels against water are not shown.

Bill of Materials
Bill of Materials


Depth Sensor Arrives

good_newsA depth sensor has arrived: a Measurement Specialties 5541C, which should be good to 14 bar (at least 450 feet of depth).  It’s quite simple, featuring 8 solder points, of which only 6 are needed for normal operation.

It uses the Serial Peripheral Interface, though not by name (the documentation merely mentions a “three-wire interface”).  However, a little research  shows that removing the device select wire from standard 4-wire SPI gives you the same connections as the sensor uses.  Like the camera (next post), the sensor operates with a lower supply voltage than the Arduino, so level-shifting is needed.

SPI Connections

This should be relatively easy.  Here’s the hard part:

sensor and scale
Sensor, with scale

Mounting it to a PCB with some sort of 6-pin connector is going to be a major challenge.

Space Available

Selecting batteries or even solidifying the electronics means figuring out the maximum dimensions of any given rectangular shape that will still fit within the tube.  If only the diameter is limited, then a variety of rectangles may fit.  Time to pull out some geometry…

Geometry calculation

Thanks to a particularly good website for batteries, a complete set of dimensions for a variety of batteries is available.  The equation above can show whether a particular battery will fit in the tube or not, based on its width and length (assuming “depth” extends along the tube’s axis). Performing some calculations with the existing central tube, electronics and two batteries, some options emerge:

  • using an alternative 12 V battery, at 3.8 Ah, measuring 7.6″ x. 2.8″ x 1.85″
  • stacking the electronics within a 5″ x 2.25″ area gives 2.68″ of available height for both the electronics and ballast (not much, but usable) 
tube sizing
Tube size calculations

Motor load measurements are still needed to confirm that 3.8 Ah is sufficient for propulsion.  The Arduino Mega and Motor Shield fit within an area 5″ x 2.25″, but the connectors and DHT11 sensor need to be cleverly stacked on top.  And 2.5 lbs of ballast needs to fit under the electronics within about 2.28″ of space, total.  

It’s either this or recalculating displacement using a longer tube, plus actually building it.

Designs Today and Tomorrow

The latest Poolbot program updates have enabled XBee transmission of arbitrary strings up to the 100 byte payload limit.  In drydock, the Arduino is connected via USB to a PC, used as a “dumb terminal”; the Arduino is doing all the display work, and the XBees operate in a simple loop confirming that a communications link exists. 

The next steps are to move all the serial link data processing from the Arduino Mega 2560 to a PC.  The Arduino can then transmit compressed sensor data via XBee payloads, and let the PC locally decompress the payload and print the information in a human-readable format.  Remote control of the motors becomes possible.

Pictures sometimes explain better than words, so here’s the design today, and what’s planned for the future.

Today’s Poolbot design