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Drones

Over Spring break I had some time, and I was missing my MiniQuad since I never had the chance to finish testing it. Though BigQuad was nice but heavy (a few pounds!), and MiniQuad was an attempt to reduce that (~60% reduction in weight), I wondered how much I could push the scale. Obviously, there are quadrotors that scale down extremely. My goal was to get under 400 grams, and still hold a reasonable payload. And at that scale, with 10-15g motors, things get cheap. Everything is at least a third to a quarter of its large scale counterpart in pricing, which further encouraged me.

I was able to laser cut a small piece in the scraps bin that fit the dimensions for my frame. With a  200mm square profile, the frame contributes to nearly half of the weight. After my Hobbyking order arrived a few days ago, I was able to fit everything to this:

It was a modification to my original design. Though I originally planned to weigh around 400 grams, I could not find suitable propellers for the motors. These 5×3’s I believe are smaller than the recommended size, but they were the only size left that were not on back order. Nevertheless, the further reduction brings this little machine to 250 grams final weight. Probably, with some carbon fiber, I could reduce the weight to 180g or so, but the 250 grams is good for now.

Modifications from previous designs include an XBee radio over the Spectrum radio system I was using before, a digital IMU with 3-axis magnetometer, and an Arduino Nano over Uno. The switch to XBee will hopefully allow me to guide by computer, with a little FPV GUI showing important flight data stuff, and allow me to gather data remotely. The last part is crucial, as the most compelling reason I wanted to make this scaled down quadrotor was to function as a mapping drone. With (potentially) stereo cameras, its magnetometer, and a potential GPS system, this little quad will be able to determine heading and location. Some contact sensors will be placed around the quadrotor to allow the robot to bump into things and detect obstacles, and its small size will ultimately allow it to fly indoors relatively safely and navigate through halls, doorways, and windows.

I know I’ve been branching out to too many projects at this point. I have Spirit’s armor left to upgrade, MiniQuad to fly, and my robot arm left to automate. I’ve been meaning to go into more depth with bioprinting the plant cells, getting around to that drop on demand inkjet toolhead expansion, and finish upgrading the build surface. I need time to focus on a single thing. But school, and school takes priority. It’s the mindset that this coming summer will provide that extra gap of time I’ll need to get these things rolling again. The Shadow Fox project too, has been put on hold until next semester. At least temporarily, as the funds are lacking. I need to refine my design heavily before investing any time into actuator tuning and research (currently between PAMS, HAMS, and serial elastic connected motors). To aide that, I have a dozen or so sub-2$ turnigy servos to make a miniFox to experiment with serial elasticity as well as dynamic walking gaits over the summer. As for school, I’ll be busy for the next few weeks. I’m making a RoboChoir.

I was pretty happy with my previous quadrotor build, because it was good for its purpose (to provide aerial shots with the GoPro camera). It was large but stable with its long arms, but those arms also added much additional weight to the frame. Moreover, the entire thing was made out of wood and steel machine screws.

My main goal was to make it light and small enough to work as an aerial surveillance drone that would last for practical lengths of time. Since Spirit MK2 is in fact modular and can thus house a helipad, my thought at the time was to create a larger landing platform (Spirit) with its tank tread drive modules and have it carry the quadrotor to its destination. The quadrotor would then fly vertically, take a 360 panorama of the landscape and maybe fly around any areas of interest, then return to the mother-ship (turning Spirit into an aircraft carrier!).

It would be a wonderful splicing of two of my projects (a drone within a drone). As mentioned in earlier posts, I bought a very cheap video camera transmitter/receiver combination that I have housed on this quadrotor. At 40 or so grams, it certainly weighs less than the 400g weight added by the GoPro and mount. With only 150ft of transmission range, the system would greatly extend the usable distance of the quadrotor if the video receiver was mounted directly onto Spirit and later relayed back. And with that, here’s my progress on the MiniQuad so far:

The arms are now aluminum C beams, but the mounting plates are still particleboard. It seems I’ll never fully upgrade to metal until I’m able to buy a mini-lathe and a mini-mill. Fiberglass and carbon fiber were out of the question of course, since I’m limited to the selection at the Home Depot, and I was not able to find any source of significantly large pieces of styrofoam to cut. As a result you have a (proportionally) lighter and compact quadrotor. Motors are now directly mounted to the beams, and I’ve added some plastic landing gear made of Polymorph/Shapelock/Instamorph material. Those standoffs you see there are in fact wooden dowels.

The landing gear was heavy and failed often since it was a last minute add-on with zip ties on the previous quadrotor, so this new addition is a nice feature. The whole assembly is lighter, I promise! It was worth the hours spent rebuilding a smaller chassis. Here’s the MiniQuad’s frame next to the BigQuad’s frame:

The scale may not be apparent in the picture but the diameter of the Mini is less than half that of the Big. I have no idea on how this will effect its wind stability but this smaller size allows the quadrotor to sit on the central module of Spirit without any problems.

As of now, the MiniQuad is almost complete mechanically. The two standoff totem poles in the first picture will be replaced with a lighter material, and will support the canopy of the quadrotor. I’m hoping for something fitted and nicer than last time (a plate of plexiglass) to protect the electronics from dirt and flips. The rat’s nest wiring is shown in the next picture. I’ll clean it up after everything is sorted out.

Here’s the camera mount. It’s the same Polymorph material molded around the camera. This stuff is getting really useful when I don’t want to machine anything. Great for low strength, custom mounts!

My next step in this process is reconfiguring the software on the Arduino. I’ll probably do that tomorrow, but due to sudden and odd rain (Southern California) I won’t be able to do many outdoor tests. Since my brother’s room is vacant after he moved most of his stuff out to school I’ll fly it around in there. Video transmission does work, but it can only display on screens for now. I’m waiting for a to-USB adapter so I can record/view the video from my laptop.

 

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.