Sensor board for underwater ROV

Since I had bought the robotics club an I²C accelerometer and magnetometer, I decided to make a new PC board for them to mount the accelerometer, the magnetometer, and the pressure gauge on the same board.  I don’t have the SMD soldering skills to solder all the chips onto one board, and I already had breakout boards for the accelerometer and magnetometer from Sparkfun, so I decided just to put connectors for those breakout boards onto the back of the pressure sensor board.  (The back, because the pressure sensor on the front has to be stuck through a hole in the dry box and glued in place.

The new boards are tiny (1.05″ × 1.425″), so I decided to try BatchPCB (which has pricing by the square inch) rather than 4pcb.com (which has fixed pricing per board, up to a fairly large size).  The price from BatchPCB was $10 per order plus $2.50/square inch plus $0.90 for shipping, so ordering 3 copies of the board (though I only needed one), cost me $22.12, substantially less than a single board from 4pcb.com, which is $33 plus $17.30 shipping and handling per board (plus an extra $50 if your board has multiple boards on it).  The 4pcb price is lower if your board is bigger than about 15.76 square inches, so even my HexMotor boards (at 12.44 square inches) would be cheaper from BatchPCB.  If you get multiple boards from 4pcb.com on a single panel and cut them apart yourself, the breakeven point is about 35.76 square inches for a single design (so three HexMotor boards from a single 4pcb.com panel is cheaper than from BatchPCB). For multiple designs on a single panel, the 4pcb.com deal is better: for 3 different designs, a total of 27.04 square inches would make 4pcb.com the cheaper way to go.

If you want a copy of the board, you can order it from BatchPCB, or pick up the Eagle files from my web site and order copies from elsewhere.  I’ve put the HexMotor Eagle files on line also, but not put them on the BatchPCB site.  I should probably upload them there sometime.

Bottom line: BatchPCB is better for small numbers of tiny boards, but 4pcb.com is better for larger boards and multiple designs.

Bare sensor boards, as delivered by BatchPCB

Top view of the sensor board, showing the port of the pressure sensor and the 1 µF chip capacitor soldered in place.

Side view of the populated board. The pressure sensor sticks out the top, and the breakout boards for the accelerometer and magnetometer plug into sockets on the back.

The bottom of the sensor board with the Sparkfun breakout boards plugged in. Note that the x and y axes do not correspond. I would probably fix this if I made another revision of the board, though rotating one of the breakout boards could make the sensor board a bit larger.

The BatchPCB orders came back quite quickly (12 days from order to delivery by mail), though I had been worried because their design-rule check, which they say takes minutes had taken about 8 hours.  The problem was that each check takes a few minutes, but they had hundreds in the queue over the weekend, and it took a full day to clear the queue.

I had less trouble soldering the pressure gauge this time (this was my second attempt at soldering surface mount devices).  You can see in the pictures above that the results are much cleaner than in my first attempt.

The robotics club has tested the pressure sensor on the new board (using their own code on the Arduino) and it seems to work ok,  have drilled the hole in the dry box for the port, and glued the sensor board in place using superglue.  It seems to be waterproof (at least down to 1 foot—we’ve not tested in deep water yet).

Related articles

• Merged in my Arduino blog (gasstationwithoutpumps.wordpress.com)
• Learning to use I2C (gasstationwithoutpumps.wordpress.com)
• Magnetometer and accelerometer read simultaneously (gasstationwithoutpumps.wordpress.com)
• Magnetometer was not fried (gasstationwithoutpumps.wordpress.com)
• Turning a DIY Project into a Product (spectrum.ieee.org)
• Underwater ROV again this year (gasstationwithoutpumps.wordpress.com)

Underwater ROV again this year

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)

Underwater ROV contest

On 2 April 2011, MATE held the Monterey Bay Regional underwater ROV competition, one of 20 regional competitions they run (and the oldest of them!).  There were about a dozen “RANGER” teams (all high-school teams, I believe) and 30 or 40 “SCOUT” teams (mostly middle-school groups).  The team I was coaching was one of the smallest (3 boys, only 2 of whom could make it to the contest).  This was their first time doing the contest, and my first time coaching, and they started decided very late on entering the contest.  As is usual for engineers, they underestimated the amount of work that would be involved in building and debugging the design.

As a coach, I tried very hard to give them good advice but let them make all their own decisions, even when I thought they were making a mistake.  This is surprisingly hard to do—harder even than essentially the same task supervising graduate research students, as 15-year-olds are much more likely to pick up a brainstorming suggestion that is intended as a strawman argument and take it as a command.  For many of the building sessions I had to let the kids work alone, just checking on them every 15-to-20 minutes, to avoid kibbitzing.  After each coaching session I did check privately with my son to see if I had been providing the right level of support for them to keep on track but make their own decisions.

They managed to get their underwater ROV fully assembled and in the water just the day before the contest. I was not able to be present for this launching, but I understand that the original foam flotation was not sufficient, so they MacGyvered on a couple of empty soda cans, using electrical tape to seal the holes and fasten the cans to the frame.  We had still not come up with a solution to the problem of providing a pool-side TV, so they were not able to test the cameras.

The underwater ROV ashore on the day of the contest. Note the last-minute addition of soda cans for extra floatation and the very lightweight tether. There are 4 motors (two for front and back and yaw-steering, 2 for up and down and pitch). The right motor and propeller is clearly visible, but the other 3 are mostly obscured by the framework in this picture. The propellers were a good find: they cost less than $3 each from Amazon and mount directly on the motor shafts with just a setscrew (no adapter needed). They are made by Traxxas and are intended as replacement propellers for radio-controlled toy boats (their Villain EX model).

Because the team was so small, the drive down to Monterey was done in one minivan. One of the team members did not show up at the pickup point, and we had no way to contact him, so we had to leave after 20 minutes without him.  We went past our exit on Highway 1 (my fault—I should have been navigating, but I was in the back seat and not paying enough attention to the signs) and had to turn around at the next exit.

The team did have to do some repairs, as some of the tether wires had come loose from the terminal block in the controller, and there was a cold-soldered joint that had shaken loose on one switch.  They fixed these problems and got their machine set up for their first run.  We were all pleased to see that it floated.

ROV floating in the pool. All 4 motors are visible in this view, as are the two camera mounts for the navigation and tool cameras.

This was the first time the team had access to a poolside tv screen, and the monitor that had been provided at their station did not work well.  They did not know whether this was a problem with the monitor or the cameras, but eventually a new monitor was brought over, and the problem was determined to be the monitor provided by the contest.

The team switching monitors. The black-framed one worked much better than older one with the cream-colored frame. I have to find out what those black monitors cost and where we can get them—they look much more feasible for taking to pool practices than junk TVs from thrift stores, which is what we were planning to get (but had not yet gotten around to acquiring).

After switching monitors, the boys did get some flying time using the cameras.  Most of the teams had spent weeks learning to pilot their rigs and tweaking propulsion and tool systems, but this team was flying for the first time at the contest itself.  Their design was simple and robust enough that they were able to maneuver down to the props, but they did not have fine enough control to actually perform the missions with the tools they had built.  Still we were all stoked that they could see through the cameras and do some steering, even if not to the fine precision they had dreamed of.

ROV flying down to the props. Propulsion worked well, but steering was not as fine as the designers had hoped, and the camera view was somewhat confusing.

They got another half hour run later in the day, but it was pretty much a repeat of the first run, though the monitor worked this time for the whole run.  One problem the team had was that they could not tell which set of props they were looking at and wasted a lot of time trying to reach the next station over (a large identifying number on the underwater stations would have helped). A down-pointing camera so that they could see the bottom of the pool might have helped with navigation.

The boys were happy that everything worked as well as it did, given that they had not been able to test ahead of time. They did not have any illusions of winning (they knew that they were going up against teams that had been working on their vehicles for 4 or 5 years), but they were hoping that they could get a little further on the mission tasks.

The boys gave a fine engineering presentation of the features of their design, but I suspect that they lost some points for safety, as their propellers were unshrouded, and all the other designs had some sort of mesh box or ducting around the propellers to protect fingers. A very popular design (because of the ease of construction) was to put a metal mesh gutter guard around the motor and propeller fastened on with a large hose clamp.  This is something I should have coached the boys about, as safety was one of my main concerns, but I had been content with just admonishing them not to get near the motors while the power was connected. The props were all inside the frame and they had been careful to wire the motors so that they couldn’t foul the props with the motor wires. They had also been very good about disconnecting the power when doing any work on the vehicle.  Next year, they’ll have the passive safety shields around the props.

Yes, it was fun enough for them that they want to do it again next year. They were even talking about what to do on Club Day next fall to recruit a bigger team.  They are also trying to figure out how to get adequate pool access, as it was clear to them that one of their biggest constraints this year was the very limited practice and debugging time.

They are not proposing any major changes to the mechanical design (adding shields around the props, maybe moving the left and right motors further apart), but they have some major ambitions for the control system.  They want to move to electronic controls in the vehicle, so that they can use a longer lightweight tether, but not have the large resistance drops that come from having many small wires and communicating through power switching.  They want the tether to have just one pair of power conductors, the camera wires, and a few wires for a serial connection.  They got some ideas for how to make connections to a pressure-proof box (from a team that had worked on that, but not managed to finish their vehicle) and are going to spend this spring learning how to program microcontrollers to control DC motors using PWM and H-bridges.

Already in half a day, my son has a working Python program for doing serial communication with an Arduino microcontroller over a USB link from a Macintosh.  I did have to help debug, as it took us a while to realize that opening the serial connection to the Arduino did a reset and started the program on the Arduino, so we had to add a handshaking signal to the Arduino code that the Python program waited for, to make sure that the Arduino did not lose the beginning of the communication.

They’ll build a motor shield for the Arduino for controlling some small motors (since I already have a kit), but we’ll have to look for some higher-power H-bridges, since the chip used on the motor shields can only deliver 600 milliamps per motor, and they’ll need over 4 times that.  There do seem to be some reasonably priced 3 Amp H-bridges around using the LMD18200 chip, but I think it would be good for them to practice with low-power designs first.  The 3 Amp H-bridges will require some big heat sinks, and I’m not sure how they’re going to keep four of them cool enough in a watertight box.  I think that will have to wait until the summer.

Related Articles

• 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)

National Robotics Week April 9–17, 2011

IEEE Spectrum‘s DIY robotics blog posted an announcement about National Robotics Week: Clear Your Schedule: National Robotics Week is Just One Month Away.  (OK, it’s now only two weeks away, but I’ve been busy grading and not been able to keep up with my blog reading.)

The home page for National Robotics Week says the point is to

• Celebrate the US as a leader in robotics technology development
• Educate the public about how robotics technology impacts society, both now and in the future
• Advocate for increased funding for robotics technology research and development
• Inspire students of all ages to pursue careers in robotics and other Science, Technology, Engineering, and Math-related fields.

There are a lot of events listed on their events page, including several in the Bay Area, but none on our side of the Santa Cruz mountains.

I’m not going to have time to do anything for National Robotics Week, as what little time I have for the high school robotics club has been spent trying to get parts for the students to build their underwater ROV.  I don’t know if they’ll have had a chance to fly it before the contest on April 2, as we still don’t have a solution for seeing the output of the video cameras, as the first RCA-to-USB adapters we tried did not work on either Mac OS X (with freeware) Vista 7 (with the manufacturer-provided drivers).  None of the students in the robotics club has a TV at home even, and I wasn’t willing to buy one (or an expensive adapter) just for this purpose.  I suspect that the contest will be their first attempt to fly the underwater ROV using just the camera input, and it will be amusing at best.  Oh well, they started late and did not have much spare time, and the design is essentially all theirs, so if the vehicle works at all, I’ll declare it a success, even if they can’t do any of the mission tasks.  Next year, they’ll probably get very ambitious and suffer from second-system syndrome, instead of just trying to get a minimal vehicle built in time.  We’ll have to come up with a workable camera/monitor solution by then.

I’ve been trying to be good as a coach, keeping my hands off their project and only interjecting comments at rare intervals.  I do have to leave their work area once they are started and only check up on them every 10–15 minutes, though, or I’ll want to fuss at them.  Mainly I’ve been trying to anticipate what parts they’ll need and get them for them, since they’ve never left enough lead time to order parts, and the electronics stores on this side of the hill are not particularly well stocked (I’ve not been able to find the somewhat non-standard power connector that the Anaconda cameras use, for example, so we may have to cannibalize the wall warts that came with the cameras).

Waterproofing cameras for underwater ROVs

Yesterday, members of the Robotics Club at my son’s high school went down to Monterey Peninsula College for a camera-waterproofing workshop.  The cameras are  for the underwater remotely-operated vehicle contest coming up in April.  I blogged about an earlier workshop in which my son played with building an ROV—the cameras are for such a vehicle. [Note: this workshop was not an official activity of the Robotics Club, but just a couple of the members going to the workshop with their dads.  Official activities have to have one of the high school teachers present at all times.]

There are now 13 “Ranger” teams registered (who have to use cameras to navigate their vehicles) and 28  “Scout” teams (who can watch their vehicles from the pool edge and so build simpler ROVs) for the Monterey Bay regional.  The Ranger teams are mostly high school teams, and the Scout teams mostly middle school teams.  Some middle schools have 3 teams entered, but the high schools seem to be running one each.  The high school teams are coming from as far away as Stockton and Santa Maria (each about 160 miles from Monterey by car), so our little commute of 50 miles seems small.  None of the distant teams came for the camera-waterproofing workshop though—only 3 or 4 of the more local Ranger teams were there.

The whole contest started in Monterey in 2002, but has since grown to 20 regional competitions feeding into the international competition.  There are over 200 teams registered in the various regional competitions.  The contest is actually international (one regional is in Hong Kong and one is in Japan and there is a team registered from Egypt), but most of the teams are still from the US.

The camera waterproofing turns out to be surprisingly easy (though I suspect that there was a fair amount of trial and error in the development of the procedure).  There are full instructions posted on-line, so I’ll only include a few photos here of some high points.

They start with a cheap video camera (the Anaconda Color model SC18A camera with a 60-foot cord). I would have preferred starting with a USB camera (since we have laptops, but not old TV monitors with analog inputs), but the workshop cost only $25 per camera (including the camera, superglue, cups, epoxy, … ), which is an unbeatable price. There is a subsidy involved since the camera alone costs $50 on the web. I think that there is some difficulty finding USB cameras with 60-foot cords also, and we really don’t want to have to waterproof USB connections.

The first step is to remove the camera from the housing, which requires destroying the housing, since the camera is glued in.

The housing that the camera comes with needs to be removed. Some parts can be unscrewed, but part of the case has to be clipped off with diagonal cutters.

The camera, after the case has been removed, is quite small, and has rather fragile leads. One lead broke on one of the cameras, and the student had to resolder it. This was not difficult, except for the minor problem that the classroom was part of an automotive workshop and the only available soldering irons had rather large tips.

The next step is to fix the focus of the camera, since it can’t be adjusted after waterproofing.

After the camera has been focused at the desired distance, the focus is locked by adding a drop of superglue to the focus ring.

The camera is glued lens-down in the bottom of clear plastic container.

The now fixed-focus camera is glued to the bottom of a clear plastic container using superglue in a ring around the lens. The superglue forms a seal keeping the epoxy used for the waterproofing from getting in front of the lens.

A slow-curing epoxy (EnviroTex Lite pour-on high gloss finish) is used to pot the camera. Faster epoxies could be used, but the exothermic reaction of the epoxies can overheat and damage the cameras, so a slow epoxy is safer.

Mixing the epoxy with a little food coloring, so that the cameras can be color coded.

Because the epoxy takes many hours to set, the workshop organizers had us leave the cameras in the classroom and will mail us the waterproofed cameras in a few days.

Only one boy at the workshop managed to superglue his fingers together, and they had some acetone available to release him. (Luckily for our pride, it wasn’t one of our club’s members.)  The workshop was fun for the students and they got a lot of good advice on ROV building from Jeremy Herzberg, a technician at Monterey Peninsula College who has been involved in the underwater ROV competition for several years.