Monroe Wyatt Pattillo, Jr. - Quadrotors

I've been fascinated by the potential of autonomous quad propeller helicopters. While most are hundreds to thousands of dollars I was looking for one in the less than $100 range that included 6-axis stabilization (3 gyros X-Y-Z, 3 accelerometers X-Y-Z) and 2.4GHz at least 4 channel radio control. I found the UDIRC U816A quadrotor from HobbyWow for under $50 and was hooked. I bought 4 of them (2 for me, 1 active, 1 for disassembly and enhancement, 2 for friends). I couldn't wait for them to be available from a reseller in the US. I ordered them direct from China. If you want to find out more about the stock model of the UDIRC U816A simply do an Internet search on UDI U816A and you'll find more information, reviews, and flight videos. While stable flight took a little longer to conquer than anticipated I did manage it. The two secrets are to...
  1. Synchronize the remote control to the quadrotor while on a flat, perfectly level platform and then launching from there. If it is not perfectly flat then the quadrotor will take off in the direction of the lower side because it thinks you've told it what level is and it is trying to keep to that level.
  2. Realize that the quadrotor has inertia and momentum. It doesn't respond instantaneously when you command it to do something, like throttle changes or turns, and once you get it moving in a direction particularly downward it needs more control influence longer to counteract it, but once you counteract it, kick it back in the opposite direction briefly to recover from the action you have induced. An analogy is a hovercraft. If this particular lesson is not learned you will get a lot of short flights ending in crashes into all sorts of things. Luckily the styrofoam outer ring can take lots of punishment without damage to the propellers, struts, and the core containing the battery and electronics.
I then set out to enhancing the product... The increased battery selection took some learning about the letter C in battery specifications - the maximum discharge rate and the maximum instanteous discharge amount. The maximum instanteous discharge amount is the battery capacity rating times the discharge rate (C). The stock power source on the quadrotor is a 3.7V 250mAHr 15C Lithium-Polymer (LiPo) battery. Thus the maximum amount of current the four motors can instantaneously draw from the battery is 15 times 250 or 3,750mA or 3.75A (Amperes). Motors are designed to provide a maximum rotation rate given a specific voltage and current. I did not have the specifications on the motors so I didn't know their maximum current draw. I wanted to keep the maximum instaneous discharge amount under control as I didn't want to fry the four motors by overpowering them. I tried to find a battery with the same or slightly lower discharge rate (C) with increased capacity (more than the stock 250mAHr - MilliampHours). I couldn't find a less than 15C higher capacity battery. I settled on a 400mAHr 15C battery, even though the discharge rate is the same the maximum instaneous discharge amount will be noticeably higher (6A versus 3.75A) due to the higher current capacity (400mA versus 250mA).

After speaking with someone skilled in electronics I've realized that while it is possible to use a larger battery and make more current available to a motor, the motor will not attempt to consume more current than its design permits. Therefore I've ordered some 600mAHr 15C batteries. Not that battery duration has so far been an issue given the short durations of my flights. I just don't know if the design of the UDI816A uses the smaller 250mAHr to reduce weight to improve flight dynamics or to reduce costs or because a higher current battery will run the motors up to a rate above which the onboard controller can control them, e.g. it is deliberately underpowered.

Given the available instantaneous current and the current capacity of the 15C 600mAHr battery, I'll be foregoing the solar cell add-on since the larger battery will provide a better improvement in flight duration than the amount from the solar cell.

For the DVR I found very tiny units for very little money, but I had to purchase a MicroSD card for them. I found <$10 8GB class 4 MicroSD cards at Wal-Mart. I chose two tiny DVR units, one from Mini-Gadgets, Inc., their KC-640SD which shoots Standard Definition (SD) video at 720x640 resolution. The other one is a generic unit, typically referred to as an SC-508 super mini camera, the core of which is the same as the DVR 520 5.0 Megapixel covert camera DVR, which shoots High Definition (HD) video at 1920x720 resolution (yes, I know, not real HD at 1920x1080p), but once stripped down DVR 520 is actually a couple of grams less in weight than the KC-640SD unit.

I've switched from the KC-640SD to the DVR 520 (SC-508). The resolution has gone up. The electronic noise in the video image when revving the motors is gone. The camera imager vibration is gone, because the imager is fastened to the camera circuit board, e.g. not free floating at the end of a piece of flexible printed circuit material like on the KC-640SD. Also as it turns out the DVR 520 (SC-508) weighs slightly less than the KC-640SD. Having had the camera now twice separate from the quatrotor on me (once with the KC-640SD and once with the DVR 520) I'm now securing the camera to the quatrotor with a tiny zip tie.

Speaking of weight I had to monitor the weight of everything very closely so I ordered a digital scale that offers a maximum weight of 1,000g (grams) or 1Kilogram, with a resolution of 0.1g (one-tenth of a gram). The stock weight of the quadrotor with its stock battery is only 650g. I wanted to keep the total weight under 700g. The digital scale came in handy. I could weigh things in pieces as I stripped them off the devices.

Next I had to attach the DVR to the quadrotor. A piece of 3M clear mounting square worked - very light, easily removable, and cheap [but as it turns out not reliable as the camera tends to separate from the quadrotor after a few crashes, leaving it laying somewhere on the ground for me to go and find - now using a tiny zip tie with improved attachment reliability, still using 3M adhesive square to attach camera battery - the larger flat surface area of the battery against the styrofoam shell provides sufficient adhesion].

Here are pictures of the quadrotor with stripped down DVRs attached to the bottom of the battery cage...

Shows bottom of quadrotor with KC-640SD DVR attached
Closeup of attachment of KC-640SD DVR to bottom of battery cage of quadrotor
Closeup of attachment of DVR 520 DVR to bottom of battery cage of quadrotor using tiny zip tie

I did a couple of test flights with the DVR attached to check out the flight dynamics. While the battery time is obviously less due to the extra weight (the larger batteries have not yet arrived), the quadrotor was actually more stable with the DVR as a pendulum weight on the bottom of it.

Here's the first video (for download, not streaming) shot with the camera attached and recording. I bounced it off the front of the roof of my house and then crashed it on the roof of the back of my house when it went out of sight over the house from my front yard and headed for the street behind my house. I edited out about 8 minutes of blured roof top video of it just sitting there as I went to get the ladder to retrieve it from the roof. By the way the DVRs record sound as well as video so you get to hear the the buzzing of the motors and you can tell when I over powered them down. This is with the KC-640SD DVR attached.

You'll note a couple of issues with the above videos. The camera gets distorted when the motors are run up quickly. Not much I can do about this since I discarded their RF shielding cases to save on weight. The second is vibration of the image. This I may be able to do something about. The DVR imaging element is essentially floating free on the end of a multi-conductor flexible printed circuit tape. I can put a small piece of low density foam between the quadrotor body and the back of the imager to reduce the free floating vibration. This will also keep the imager pointed downwards, which is the intent.

Here's videos flying the DVR 520 aka SC-508 instead of the KC-640SD. Note the lack of electronic noise and the imager stability (no longer vibrating). Also note the flight duration and stabilty increasing (at least by my prior records). I'm not happy with the level of clarity and the degree of compression atrifacts in the details like grass, also that I had to resize the image to get 1080p out of the 720p it actually shoots.

On 20130107 the wind at my house was sufficiently calm for me to exceed the altitude radio range of the remote control. While I don't know the exact altitude, it seemed to be about 200 feet or so, certainly more than 10 times the peak of the roof of my single story house which is 12 feet, combined with the time it took to get there being about 8 seconds, and given that on a full throttle launch the quadrotor rises from the ground to over the peak of my house roof in just over one second. I do know that did go out of range and then come back into range all by itself. Once out of range the quadrotor automatically throttled down slightly, descended slowly for less than two seconds, came back into radio range, and automatically resynchronized (I know because it played the synchronization melody on the remote) - amazing!

Given that I'm now sufficiently skilly with this quadrotor to get it into this predicament I'll increase the priority on my enhancement to replace the fake stub antenna with a real removable high dBi 2.4GHz Wi-Fi antenna to get a better signal over a farther distance. Here's hoping the transceiver circuit in the RF remote control was designed with sufficient RF power capacity to power the load of the larger antenna.

Monroe Wyatt Pattillo, Jr. (MWP)
monroe.pattillo@monroepattillo.com
Site installed: 09 Jan 97 by MWP
Site updated: Tuesday, 08-Jan-2013 18:59:49 EST by MWP