NTRODUCTION

The GOduino II is a self-contained

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Last year I blogged about MATE ‘s Monterey Bay Regional underwater ROV competition in Underwater ROV contest.  I coached a robotics club at my son’s high school, and they built a small vehicle which they barely got into the water the day before the contest, because we started so late.

Although my son is being home-schooled this year, we have kept the team going, losing one member and picking up another (so we still have only 3).  The robotics club is no longer affiliated with a school, but that makes no difference, as all the school provided us was an inconvenient time to meet (lunch on Tuesdays) and 15 minutes in the pool once.  We meet for 3 hours on Sundays at my house, which gives them enough time to get something done.

I’m still paying the expenses out of my own pocket, and we now have the construction of the vehicle taking up about a third of our living room (the benchtop drill press is the first thing you see on entering the house, unless it is on the floor to make room on the robotics table for the scroll saw).

The students have made considerable progress since last year, having replaced the high-resistance tether and switch box with a low-resistance power wire and a dry box that will house an Arduino microprocessor with H-bridge chips.  They’ve made the tether with a waterproof disconnect and have tested everything for water tightness (though only at bathtub depth, not 10 feet deep).  I think that they’ll have the basic vehicle and electronics finished by the end of January, leaving some time for designing and building mission-specific tools, programming the Arduino and the laptop GUI, and learning to pilot the vehicle.  Now we just have to find a pool to practice in.

The club members have gotten much more independent this year, so my coaching involves my making some suggestions about what they should work on at the beginning of the meeting, checking to see how they are doing about once an hour, and having a discussion with them about what they’ll need to design or build next over snacks near the end of the meeting.  I also try to get them to give me specific parts to buy, but I usually end up having to find and select parts for them.  If the group were bigger (and my wallet more able to tolerate mistakes), I could have the students doing more of the purchasing.

The challenges for this year have been published, and they are in the usual verbose style.  I may have a hard time getting all the students to read the specs carefully, since the specs go on and on with irrelevant “color” hiding the nuggets of critical information.  I’m not looking forward to making the items needed for practicing the missions this year, since they are described in the same wordy way as last year (which I found difficult to follow in several places), and they haven’t even released photos or drawings of what the objects are supposed to look like.  Trying to re-create the objects from the turgid assembly directions without pictures is going to be a nightmare.

The contest does not have any tasks this year that need a depth gauge (but I bought a pressure sensor, so the students will measure depth!), but they will need to determine compass headings, so we’re trying to decide whether to get a cheap compass and put it in the camera view, or add an electronic compass module to the electronics in the dry box.  An electronic compass is definitely cooler, but we may be running out of pins on the Arduino.

Related Articles

• Underwater ROV contest (gasstationwithoutpumps.wordpress.com)
• High school Robotics Club started (gasstationwithoutpumps.wordpress.com)
• Waterproofing cameras for underwater ROVs (gasstationwithoutpumps.wordpress.com)
• National Robotics Week April 9 – 17, 2011 (gasstationwithoutpumps.wordpress.com)

I’ve just sent a new set of boards for fab.  The HexMotor rev2.3 boards have several new features: LEDs for +5v and +6.25v, a reset button, 16-bit shift register instead of 8-bit, servo outputs connected to pins 13, 7, 2, 9, 10 (rather than to the pins used for PWM).  The new board should be able to do either 6 PWM motors or 4 PWM motors, 5 servos, and 2 non-modulated reversible motors.

I’m also making some breakout boards for the MPXHZ6250A pressure sensors from Freescale Semiconductor, which will require doing some SMD soldering.  At least the design uses gull-wing pins, which can (supposedly) be hand soldered.  One of the breakout boards also has a place for mounting an ADXL335 accelerometer, which may be more difficult to solder.  I don’t think I want to spend the money for a hot-air rework station, and I’m a bit dubious about my ability to solder using a toaster oven.

The pressure sensors are tiny!  My original suggestion to the robotics club was to drill a hole in the dry box and superglue the pressure sensor to the inside of the box (after the pressure sensor had been attached to the breakout board, of course).  Now I’m not so sure that there will be enough glue area to hold firmly enough.  Perhaps a dab of some sealant on the outside of the box might help, if we can keep from plugging the hole in the sensor.

The breakout board that I think that the robotics club will end up using puts a pressure sensor on one side and headers for a piggyback ADXL335 breakout board on the back.  that way there only need to be one set of wires for connecting the analog inputs and power to the sensors.

One limitation of the Arduino for use with this combination of sensors is that the accelerometer is a 3v part and the pressure sensor is a 5v part. We’ll have to set up the analog-to-digital converter on the Arduino to have a 5v range, which reduces the precision of the acceleration readings.

I’ve also bought some other sensors (not for the underwater vehicle, but for physics class and dry robotics): a couple of ultrasonic rangefinders.  More on those in a separate post, after I’ve had a chance to play with them.

Top view of the HexMotor rev 1.3 board fully populated.

I got the board fully populated yesterday, plus I made a heat sink out of a piece of ¼” × ¾” aluminum bar stock.  The heat sink makes an enormous difference.  Before using it, running a small motor at low power for 20–30 seconds was enough to make the H-bridge uncomfortably hot.  Now running the same 12v motor stalled at full current (3A) for a full minute raises the temperature only to 100ºF.

The motor got warmer than that, and I think I burned it out, since it no longer runs and has a 400kΩ resistance.  I’m not really surprised—it was a cheap door-lock actuator, and only intended to be used with fraction of a second pulses.  Further testing will require a more robust motor.

As you can see from the photo, the screw terminal for motor 4 (second from the bottom) is a bit crooked—I’ll have to unsolder it and straighten it.  For the HexMotor 2 board, I’ll use slightly smaller holes so alignment is easier.

The thermal grease I used (Cooler Master IceFusion High Performance Thermal Compound 40G RG-ICF-CWR2-GP) was more liquid than I expected, especially since it comes with a little spatula for applying it.  I had to put on a fairly thick layer, because the aluminum extrusion was not very smooth, and when I tightened the bolts the stuff oozed out making a sticky mess. Next time, I’ll sand the aluminum smooth first and use much less thermal grease.

Closeup, showing the excess thermal grease puddling under the H-bridges, where it is very difficult to wipe off.

This closeup photo, in addition to showing the pooled excess thermal grease, shows the header pins with shorting jumpers to configure the H-bridges for either lock antiphase or sign-magnitude control.  (Because of the last-minute change from TLE-5205 to TLE-5206 chips, the silk-screen labeling of the header pins is wrong—this board is actually configured for sign-magnitude throughout not lock antiphase.

The photos also show that I did not leave room for the heat sink between the electrolytic capacitors.  The HexMotor 2.0 board will fix this problem also.

The HexMotor software now can handle 3 different boards: the HexMotor rev 1.3 board shown here, the Adafruit Motor Shield, and the HexMotor rev 2 board, which I am just about done fussing with the design for.  I’ve only tested with an Arduino Duemilanove board, but the software should work with an Arduino Mega as well.