Monthly Archives: July 2011

There it is. The rebirth of Spirit MK1. It is essentially a moving DDR platform box on wheels as of now. Our old solid steel frame was way too heavy and extremely overbuilt. It was good welding practice, but that old one piece frame just didn’t like to be modified. We tried to change that with our second frame iteration. This frame was made from 1″ aluminum square bar that we initially planned to braze or weld together.

The only welder we encountered was some random guy standing in line behind us at the metal supply store, and our HTS-2000 (advertised to be “stronger than the surrounding material”) failed miserably to stand up to any impact tests. and thus, the bolting began…

Six bolts per corner, three module boxes (drive left, drive right, and center) totaling hundreds of bolts. After we put it together, it started vibrating apart. Assembly number two included hundreds of tension washers. I would not be surprised if the added weight of the steel bolts has negated any weight/strength benefit from using aluminum, but at least the frame will stay shiny without sealing now. Our goal was simply to make the new config drive, which is what we did.

If you look closely you can see three cast motor mounts. Yes! They were a success. However the last mount took forever to make (too many unsuccessful pours) so we might just buy an official motor mount to compensate. On the bot is instead one of our unsuccessfully poured motor mounts. A permanent solution can wait.

Otherwise the configuration is relatively similar to Spirit MK1. We have divided the robot into functional sections. The drive modules just include the transmission. The center module will house the batteries, motors, and electronics. A separate weapons module will be installed at the front/top, while armor plating will cover the exposed sides.

We spent quite a bit of time attempting to make hubs that would effectively restrain the wheels to their axles. However, most of our attempts have been failing. For this iteration, we attempted to double epoxy two shaft collars (that conveniently are the same diameter as the wheel bore) to transmit the torque. After a successful run and rounds of cheering, the robot attempted a tank steer while beached on a rock (we built this thing with low clearance to avoid wedge attacks). The stall was enough to shear off some of our temporary wheel hubs and left the wheels to freely spin.

It will have to wait until winter break however, I’m going to start school soon.

Remember how I said I was going to make a quadrotor in the beginning of the summer? I worked on it in stealth, slowly acquiring the parts from Hong Kong dealerships (Hobbyking and other eBay sources). I got all my parts in a few weeks ago, and finished the main frame build. I foolishly didn’t take much documentation however, and my design process was pretty pathetic. It was basically just a simple CAD drawing with blocks for what I thought was important. I paid for that with the excessive weight I have packed into the frame of this thing.

With no real precision tools (mainly hand tools and a drill press), I made a fully wooden frame. It’s not even balsa wood for crying out loud. Home Depot apparently only sells dense wood, and then denser wood. So far the quadrotor looks like this:

The arms are just .75″ square pine 24″ long with 3/8″ holes drilled in a futile attempt to reduce weight. The arms have (obviously) no problem with flexing at high throttle being so overbuilt. The motors are mounted on extensions to the arms that are bolted down. At some point I will replace all the heavy steel screws in this project with lighter nylon screws if possible.

This also includes the landing gear, which I actually formed today out of Polymorph by hand. If you don’t know what Polymorph is, its a nylon-like plastic that melts at an extremely low temperature (sub-boiling water works just fine). Once you melt bits of the plastic together it becomes pliable like taffy and can easily stick to itself. It was faster to make than making some wooden structure, although I may replace the landing gear with something more effective as I go on. My handicraft turned out to be irregular, adding to the already unbalanced setup.

The main body frame is made of two pieces of particle board, which I cut out by hand with a circular saw and my friend’s coping saw using the 1:1 scale printout feature in Solidworks as a template, following the same technique as we tried for our casting attempts. Again, that too was far too heavy and overbuilt for this application. The entire setup weights in at around 3lbs, before including the weight of the GoPro Hero HD camera I plan to use for aerial video.

An Arduino Uno is used as the microcontroller for this project, with an accelerometer and a 3-axis gyro as sensors. The accelerometer will be used to maintain level flight, which I am still working on tweaking in.

As shown in the video, I need to tweak the gain values for each motor to account for the offset balance. The quadrotor lifts off at half throttle, surprising for me due to the weight of the thing. I haven’t attached the GoPro to the quad yet, but I have a feeling I’ll also need a lot of dampening to get it to ignore the heavy vibration from the motors. Notice the big bump and its effect on my *intentional* makeshift ablative landing gear. Its cheap zip-tie mountings absorb shock!

The props are mounted, by the way, like this:

Just a cap nut on a normal nut on a tension washer washer washer sandwich on the prop on another normal nut. I’m too impatient to wait two weeks for proper prop mountings so I hope I can just use the tension of the nuts to keep the props from drifting. So far at high throttle it hasn’t been bad, but the clockwise rotating motors tend to screw the propellers down towards the base of the motor drive shaft.

A few moments after I filmed the video I also happened to bump and chip a prop on the computer case you see to the left. All of this happened, by the way, in my brother’s room (bad decision) after I was too uncertain about the wind to try an outdoor flight. A word of advice: find somewhere open and closed off. You go through props very quickly. I failed to record the quad’s first actual flight outside in the wind where I managed to flip the quad over and snap a prop on a clump of dirt/grass. Use the throttle stick conservatively! (unlike me)

Electronics include a 3C lithium polymer battery, separate receiver Velcro pack, and excessive high gauge wire. To keep dirt out and to protect from potential crash damage, I installed an acrylic shield over the micro-controller. It’s pretty useless (being exposed from all other sides), but it gives a nice platform to install the video transmitter when I move to first person view.

The GoPro mounting is actually made from the case the GoPro came in. I just sawed off the sides and bolted it down to the battery plate. Since I haven’t ordered any Sorbithane for the SPIRIT MK2 shock dampeners yet, I attempted to use some neoprene in between the mountings to reduce shock. We’ll see how that goes.

I’ll be further tuning this quad throughout this week, and I’m hurrying to get my other projects finished before I have to go back to school. The 30lb combat robot probably won’t be made this year, and the walking robot may take some time before I get it battling. I’m primarily focused on SPIRIT right now, and getting my 3-axis micro-CNC going. I’m doing research at UCI Medical Center in the Department of Urology as well. Though I’m not particularly interested in urology as a field, the department focuses a lot on robotic surgery, and they have a Da Vinci surgical robot system! I have plans to make a small-scale minimally invasive surgery robot myself this coming semester, though I’ll have to learn some reverse kinematics and such. Maybe I can even find a way to mount it on the new modular SPIRIT system as a remote combat medic system. Further details will come as I make them up from thin air.

The past week we’ve been trying to get a better mold making Green Sand mixture to use. Sifting the sand and using only fine particles works pretty well, but the large particles are prone to clumping still. Part of the reason for this is probably because the “bentonite” we’ve been using is probably just clay and filler from the cat litter. Clay is still supposed to work, since it is supposed to naturally bind to sand particles. Or you could say we’ve been playing with dirt for a week. “Indian Clay” in grade school nomenclature.

Lukas purchased a coffee grinder from Walmart to try to grind the insoluble clumps in the cat litter. Not only can it grind clay, apparently it can turn sand into smaller sand dust. We are using that as our molding mixture now, but it’s still just wet sand. It is very difficult to remove the negative from the mold with the wet sand adhering to it. We managed to make a rudimentary mold that is much higher resolution than the last attempt.

The issue with removing the negative however is making an uneven mold, so a double cope and drag isn’t possible yet. The next attempt will hopefully include a casting flue and a completely enclosed casting space so the product won’t oxidize as much (opposed to the muffin tin we have now).

Melting was facilitated by simply adding a steel cap to the steel grass jelly drink can. By melting smaller bits of aluminum at a time and using the cap, we were able to melt .75 lbs of aluminum in half the amount of time as attempt #1.

Our mistake was letting the coals die down by the time we were ready to cast, so the resulting pour was only partially liquid. Since we’ll be having to cast this same piece four times, I’ll probably keep the coals fully loaded the entire time.

The pour was “too cold.”

The resulting cast looks much better than attempt #1. There is little oxidation on the surface, and little slag due to the cap on the crucible while heating. There was virtually no steam from the mold, but the backside still has the impressions of the sand from the mold.

Front Side

Back Side
Hopefully the third time around will be a charm. The two part mold and hopefully improved negative removal technique will bring this motor mount to acceptable standards for use.

Old post from 7/1/11

Motor mounts have always been a problem. SPIRIT uses two 3″ Magmotors, which have no mounting screws or protrusions. They are just plain cylinders. We attempted to make a two piece wooden mount with rubber inserts that clamp down on the face of the motor. However this approach risks the rubber being sheared off the wood by the torque of the motor.

Commercial motor mounts for this motor are generally expensive. They are just pieces of aluminum that clamp down with bolt tension to apply friction to the motors. After shipping, these mounts would total hundreds of dollars. I thought of directly casting the motor mounts as a possible method of making adequately strong motor mounts. Since the dimensions were listed online for the commercial mounts, I could just copy and mold that instead.

For our first attempt, we tried to make a sand casting rig with bentonite, sand and water. Green sand for sand casting can be made that way. Too bad the source of bentonite we used, kitty litter, had so many insoluble impurities that the mold was difficult to make even. With our lack of prime moulding materials and experience, we decided to make a muffin-tin mold and just pour from the top without a casting spout or a two part mold.

The dirt-n-brick furnace I have is pretty badly designed. It’s just a pile of cinder blocks shielded by a dirt mound. The heat efficiency is terrible, and it took a long time to melt the aluminum. However, once a puddle of liquid aluminum formed on the bottom of the crucible, melting happened very quickly. And did I mention that the crucible we used was just a tin soup can. This wasn’t an emergency casting attempt #1 for nothing. Two blow dryers were directed at the furnace opening as bellows, and paraffin tea lights were thrown in the crucible for bursts of heat (at the expense of adding impurities).

Since this was such a brash procedure, we didn’t bother to scoop off the surface slag. That was later very apparent in the quality of the final product. W also didn’t wait for the mold to dry most of its water, and we had used way too much water during the green sand mixing process. As a result the pour went as expected. At parts water spot boiled leaving large vacuums where the aluminum would not flow. The slag accumulated near the top and weakened one side of the cast. The lack of a two part mold encouraged heavy oxidation on the top side. Here’s what happened on video:

The end product was pretty disfigured, but the concept works. Had it not been for the steam bubbles, the cast would have been high enough quality to be functional as a motor mount. The resulting product was around half a pound and was heavily oxidized on one side. Regardless, the attempt was just an experiment and shows promise, so we will try again with a little more effort on the mold and get things right the second time around.