Friday, September 5, 2014

GenCon 2014 recap

 I went to GenCon 2014 and had a great time.  I made the two-day drive out along with my wife and another friend, staying over in western PA because I'm getting a little too old and weary to drive 13 hours in one day.  Next year I might fly instead.

GenCon itself was, as always, hugely crowded (something like 56,000 attendees this year?).  Despite that the convention itself ran smoothly.  The Will Call line was amazingly fast and efficient this year, and the convention center itself seems to still have plenty of room.  The surrounding city is showing more strain, getting housing was a nightmare and getting food required waiting an hour or more in line.

Most of my time at the convention was spent gaming.  I go to GenCon for the gaming events, and this year I was fortunate enough to get into nearly all of my first choice of events.  When I wasn't at games, running between events, or waiting in line at the food trucks I did manage to get several hours of demo time in with the new little walking robot.  The robot did great - held up through the entire convention, and ran through about four of the five battery packs I had for it each day Thursday, Friday, and Saturday.  Radio reception was only a problem when the antenna came loose from the transmitter - easily fixed by screwing it back on tightly, but I might want to secure it better for next time.  It had enough traction to walk well on the convention center carpet.  Tile floors were a problem, so I avoided them, and I never got around to trying it outside.

I don't have any video of the robot at GenCon yet.  Running the robot takes two hands, and I can't work a camera and make the robot walk at the same time.  I've been looking on Youtube for uploaded videos of it, but haven't found any yet.

I did go through some spare parts.  I had two servos fail due to the robot switching on while still in my carrying bag.  I was in the habit of keeping a battery half-inserted while walking around with the robot in the bag, so it would be easier to pull out and switch on for a demo.  It turns out that if I hit the robot just right I would jostle the battery into place enough to power on the robot.  The first thing that the robot does when powered on is center all the servos, and when this happens while in a confined bag it can result in stripped servo gears.  I will be updating the software in the robot so that it doesn't send any motion commands to the servos until it receives a valid radio signal.

I had two servos in the robot destroyed when someone stepped on it.  I don't know if it was on purpose - I had originally thought not, but the person who did laughed and ran off when I confronted them, so it may have been intentional after all.  Fortunately it was easy enough to repair the damage to these and the other two failed servos.  I didn't even have to remove them from the robot, just open them up enough to swap the gears out.

One servo failed spontaneously when the feedback system failed.  It locked up at one end stop, and then when I tried to test it erratically smashed back and forth between the travel limits until the gears failed.  I haven't taken it apart enough to find out what the problem was - I suspect a broken wire or solder joint, but when the servos cost less than $3 each I'm not inclined to put much effort into troubleshooting them.  This was a slightly more difficult job than the others since I had to replace the entire servo, which also required taking the core of the robot apart to get at the wiring.  Still not a big deal, the bot was designed to be modular and I had all the tools and spare parts in my convention bag.

Late on Saturday night the transmitter failed, a broken connection in the wiring to the left-hand joystick.  I didn't have the tools with me to troubleshoot or repair the damage, and was just about ready to collapse in exhaustion anyway, so I just packed it up and was done with the robot for the convention.  The transmitter was re-used unchanged from the previous robot, and is really in need of a complete tear-down, redesign and rebuild soon anyway.  The failure only meant that I wasn't able to show the robot off on Sunday, which was not much of a loss since Sunday morning was when we were packing up and heading home anyway.

The robot was a great success at GenCon.  Up; until Sunday my demo time was only limited by the amount of free time I had between events, which admittedly wasn't much.  I never ran out of battery packs, but I did get down to my last pack for the day more than once, so I didn't have much wasted capacity.  I only had one servo fail due to non-external causes.  That last bit really surprised me - I was expecting these cheap servos to fail just from the effort of walking around, but they held up far better than I expected them to.

The big question everyone kept asking was where I got the robot - and when I told people I made it myself, was I planning to sell them?  I'm still not ready to actually sell working, finished, consumer-ready robots.  It's a huge investment in time and money to actually set up, and I really don't want to be the one doing customer support for a potentially large number of hand-made robots breaking in the field.  While the robot itself seems to work for a while without breaking, the servos are fragile and easy to break with careless handling.  Furthermore, I'd still be hand-making each robot, and even with the 3D printer it takes a while to build all the pieces for one.  That means a lot of my time invested in each sale, and I'd have to charge an unreasonable amount to make it worth my time.

I do plan on publishing the 3D files, schematics, and source code so people can make their own reproductions of the robot.  I may be willing to sell kits of parts at some point as well.  Stay tuned.

Tuesday, August 26, 2014

Building the new robot for GenCon 2014

My little walking robot, that this blog was originally created to describe, gradually beat itself to death through performing at conventions and Makerfaires over a period of several years.  Broken servos were easy (if expensive) to replace, but eventually welds cracked in the legs and the servo control board developed intermittent faults.  Fixes to the structural problems just made it heavier and pushed the already overloaded servos further.  About a year ago I gave up and declared it dead.

Earlier this year I decided to rebuild a new walking robot from scratch.  I had at one point looked at tiny inexpensive plastic-geared micro servos as a possibility for a walking robot.  Initially I thought them too weak and small for use in any walking robot, but after seeing several successful robots using them I decided to try and rebuild my walking robot based around them.

I knew from my earlier experimenting that these micro servos would burn out if I ran them at 6V like I had in the earlier robot.  Rather than drop the 8V from a two-cell LiPoly down to 5V or lower, I decided to just run the servos directly off a single cell LiPoly instead.  4V is less than the maximum these servos should be run at, but I figured that it would help them last longer without overheating.  I wasn’t going to be getting much torque out of them, so I’d need to make the robot as lightweight and small as possible.

I made as much of the structure as possible out of 3D-printed parts.  My original justification for getting the 3D printer was to print parts for robots.  I spent a few years making homemade transformers and other fun things instead, but now I’d finally be getting to making robot parts.  The printer was really great for this job, the fact that I could make parts in complex arbitrary shapes, compound curves which would be very difficult to make with sheet metal and internal stiffening ribs without welding or machining, really helped with my goal of making the bot with as few overall parts and fasteners as possible.  

Thanks to the 3D printer I was quickly able to go through multiple sets of test hardware, tweaking the design to get the clearances and geometry just right. It also meant I could fairly easily make an entire set of spare structural parts.  I didn’t expect to need them, but once I had the design finished it cost very little in materials or time to print more out.

I kept the same overall proportions as the previous robot, but scaled down everything by about five-eighths compared to the original.  I wanted to try and make the legs as short as I could get away with, reducing the torque load on the servos while still keeping the legs long enough for decent speed and mobility.  The center sphere, which in the original had been made from a three-inch copper tank float, would now be a plastic ball about two inches in diameter.  The leg span at full extension would be just about eleven inches.  I expected to be able to reduce the weight to about a quarter of the previous design. 

Sadly the printed plastic parts don’t quite have the aesthetic charm that the old polished metal ones did.  My robot’s gone from a steampunk inspired hand-made design to one that looks more like a mass-produced toy.  The final colors were somewhat accidental - I had a different color scheme in mind originally.  I printed out some test pieces in white, then switched to red and yellow, colors I don’t use much outside of test prints.  I liked the resulting look enough to make the entire robot in those colors.

I decided to switch to easily replaceable battery packs rather than attempt to cram in a single battery capable of running for an entire convention.  I got a really good deal on five single-cell 380mAh battery that would just fit inside the two-inch sphere, with enough space left over for the radio receiver. Of course, I added battery protection boards onto each of them to make sure that the robot wouldn’t go up flames in case of a short circuit.  I designed a printed shroud to go around the battery which I hoped would make it easy to swap out batteries without needing tools at the convention.  Unfortunately the plastic snaps came out stiffer than I'd have liked, and I needed to use pliers to pull the battery out at the convention.  It was still reasonably easy to swap the battery packs out.

With the battery pack and the Xbee radio taking up nearly all the space inside the robot, there wasn’t enough room for the Xbee breakout board I had used previously, let alone that and a Pololu SSC or Arduino Mini to do the serial-to-servo conversion.  I had to make my own controller board, using tiny right-angle headers mounted sideways on the board and fitting the PCB itself directly between the pin headers on the Xbee radio.  The board would have to hold a PIC16LF873A microcontroller, 3.3V regulator for the MCU and radio, clock generator and other required components, as well as the connectors for the battery and servos.  The servo leads and connectors actually took up a lot of the interior space - when you only have a 2 inch sphere to work with, trying to fit 8 3-pin headers takes up a painfully large chunk of the available real estate.

I designed the PCB using the layout tools I have available at work, and then ordered it through OSH Park.  OSH Park was quite easy to work with and helped me configure my PCB design software to produce the right output files for their process.  The cost for the three boards they sent me was amazingly low compared with other prototyping services, only $9 for three identical boards which was even less than it would have cost me to set up to etch them at home.  The quality was a lot better than I’ve ever been able to do with etching myself too.  They also had no problems with the boards being odd, non-rectangular shapes, unlike most prototyping services that require rectangles of predetermined sizes.  The only downside to OSH Park’s service is that it’s not speedy, the batch-process system they use to keep costs down means they have to wait until they get enough orders before they can send boards to be manufactured.

The radio receiver, controller board, battery, and connectors occupy a box about an inch square down the center of the two inch sphere center body.  It's really tightly packed in there.  I had to make cutouts in the PCB and battery support structure for the inner mounting tabs of the leg servos, and leave channels in the surrounding support structure wide enough to feed the servo leads through.

The firmware I wrote for the controller in the robot is really simple.  It’s essentially doing the same job as the Pololu SSC I used previously, but with a simpler protocol, lower data rate, and coarser resolution.  The receiver takes a data stream at 8929 baud (it was supposed to be 9600 baud, but a PIC16LF873A running off a 4mhz crystal can’t actually match a 9600 baud rate) with each servo encoded as a single byte of data, and outputs eight 0.524ms to 2.500ms pulses, one for each servo.  The output resolution is fairly coarse (about 0.7 degrees per LSB) but I figure that these cheap all-plastic servos are going to be lucky to be within five degrees of the commanded position anyway so it really doesn’t matter.

I used the same transmitter as on the previous version of the robot.  The only change I made was for the new serial output format for talking to my controller instead of the Pololu SSC board.  I did have some problems with the transmitter at the convention, and I’ll probably be completely redesigning it for next year.

The controller board was the long pole in the project, once it was done and programmed everything came together quickly.  There were a few design issues with the board that I had to fix with cutting traces and soldering additional parts on. For one thing, the Xbee receiver uses a lot more current than I realized, requiring me to solder a larger 3.3V regulator on.  I’ll be updating the PCB files and publishing them once I’ve cleaned up the design.

With these cheap little servos, and my previous experiences with them failing after a few seconds of running under load, I wasn’t sure that this robot would even be able to hold up its own weight without breaking.  I was therefore pleasantly surprised when the new robot not only stood but nimbly walked around, waved and rolled over just as well as the old one had.  Some quick testing showed that a single fully charged battery would be good for fifteen minutes of solid walking, and that I was able to go through several full batteries walking the robot around my living room without any servo failures.  Testing at work showed that the radio range was more than sufficient for the robot to still walk while at the other end of the longest hallway in the building, something that I had been worried about as there was no room for any kind of external antenna on the robot.

With everything tested and seeming to work, I packed the robot and all its spare parts and was off to GenCon.

Monday, August 4, 2014

Adding a second hinge point to cheap micro servos

Several years ago when looking for cheap servos to use for my walking robot I purchased a handful of HXT900 micro servos from HobbyKing.  They seemed like an incredible deal for less than $3 each, but after brief experimenting I gave up on them.  They were very jittery, not terribly strong, and when run at 6V the motor would burn out quickly if the output was stalled.  I tossed them in the junk box and forgot about them, writing them off as not usable for my robots.

Later, at Makerfaire NYC I talked to a maker who was developing kits for a hexapod robot that used 18 HXT900 servos, who had been having some success getting them to work.  I also some very impressive projects online of robots based on these servos, which was enough to make me reconsider my first impression.  My old quadruped walking robot was completely wrecked from many weekends of running demos, so I decided to build a completely new one from scratch based around the HXT900 servos.

The first thing I needed to do with these servos was add a second hinge point to them.  The knee design my robot has uses the servo case as half the hinge joint.  I'm using the same design as on the previous robot, but scaled down, using a 4-40 weld nut, 1/4" OD spacer, and 4-40 machine screw for each servo.

The first thing that needs to be done is completely removing all the stickers from the outside of each servo.  They'll prevent you from opening the case, and the tolerances on my design were tight enough that the stickers were being damaged when I installed the servos into the prototype chassis anyway.

The hole for the rear axle needs to be directly opposite the output shaft.  In theory I should have made up a jig for this, but just eyeballing it seemed to be close enough.  The hole doesn't precisely locate the axle point anyway, so it only needs to be approximate.

For plastic this thin, a drill bit would be overkill, and would probably risk shattering the case.  I used a good sharp Exacto knife to carve out the hole.  Again, the diameter of the hole is not critical, it just needs to be large enough to clear the barrel on the weld nut.

 Then glue a weld nut to the inside of each servo case.  The weld nuts I used just barely fit inside the rear of the servo case.  This helped with alignment - the sides of the servo case held the nut in place, so the position of the hole in the case didn't really matter so much.  You'll also want to make sure to remove any inspection sticker or other residue from the inside of the case before this step so the glue bonds effectively.

The spacer and screw are then temporarily put on to help clamp the weld nut against the inside of the case while the glue sets.  I'll be having to remove them later to install the servo in the knee joint.

 Finally here are two of the servos (pre-modification) installed in a test prototype for the new 3D-printed knee joint, next tot he same structure on the old robot.  You can see that the new design will be much smaller and lighter weight.  I'll also be taking a lot of advantage of the 3D printer's ability to make complex arbitrary shapes for the new robot's structure.

Monday, May 12, 2014

Gencon 2014 scheduler update

 I have updated my scheduling script for GenCon 2014. I had originally not expected to have to make any changes to the code, but GenCon this year moved the location of their event catalog files, and decided to only release the event catalogs in xlsx format instead of csv.  While you can still use last year's version of my scheduler by manually downloading and converting the catalog files, I've modified my code to use a converter utility to do the conversion.

The latest version of it can be found here.

In order to use this, you will need to install Python 2.7.  You will also need to install this xlsx to csv converter code. After installing xlsx2csv, you'll have to copy the file from the install directory into the same directory that the schedule solver python file is located.

The GenCon event registration system is also limiting event wish lists to 50 items this year, so the wishlist generator will limit itself to that number of wishlist items.

As before, this is an experimental and somewhat crude schedule optimization utility.  It allows you to search the event catalog with a text-based interface, add events to your wishlist with assigned priorities, and then generates an optimal schedule for you, as well as a wishlist to enter into the GenCon registration system.