Anyone who has been flying multirotors knows the struggle of keeping batteries charged is real. Whether you're flying with small batteries or monster packs it comes down to charge rate. Most batteries are designed to be charged at 1C. There are endless articles online that talk about this so I will not get into too much detail here. A 1C charge rate generally takes 45 minutes to an hour. If you have a small battery such as 4500mah your 1C charge rate is 4.5 amps. If you have a larger battery like a 16,000mah your 1C charge rate is 16 amps. Generally speaking these two batteries will fully charge in the same amount of time but the larger one will require a lot more power. As you scale up your UAV and batteries you need a lot more power to charge enough batteries in a realistic timeframe. We took this problem to RotorCraftRC and they came up with a 4,000w 4x channel charger. Each channel is capable of delivering 40amps of power. This allows us to charge 8x 20,000mAh batteries in pairs at 1C. To give you an example of how this would work in the field picture this. You have a UAV coming in for a landing. Batteries come off and start a charge. By the time the second flight is done you're first batteries have been on for about 25 minutes. By the time the Third flight is done your first 4 batteries should be topping off. These battery chargers are equipped with a charge controller that uses RFID tags to automatically load the battery settings and then log the charge cycles of each battery. If your battery starts to age past its service life the charger will let you know. All the battery logs can be uploaded to dropbox, monitored on an iOS device and tracked over time to help with fleet management.
The 2400-OT was a unique project that allowed the BFD team to push the limits to make one of our largest systems yet. The 2400-OT runs 2400mm from motor to motor and swings 8x 30” propellers. Many of our systems run a over under motor configuration also known as a CoAxial configuration, however, this specific model will be used for agriculture and we wanted to have the ability to have dispensing payloads underneath the propellers if necessary. The specifications for this project were pretty open, fly a heavy payload for as long as possible and make sure it will be able to be transported in an 8’ trailer. The main goal for this aircraft is an experimental agricultural platform for dispensing a chemical payload over fields and crops. More work needs to be done on the flight controller side so this project is still ongoing. Since the hardware side has come together quite nicely, we thought it would be a good time to show you what we’ve been up to. The great thing about this project is we've learned a lot of new lessons that will be applicable to other smaller aircraft as well.
Let's take a stroll through this build and look at some of the unique characteristics. We'll start from the inside and work our way out.
1000 Amp Power Distribution Board and Center Hub
The 2400-OT utilizes the new state of the art power distribution board from Gryphon Dynamics. The board is centrally located and handles the power distribution to the motors as well as signal management for the ESCs and other low voltage systems. It was originally intended for an X8 motor arrangement so we had to do some relabeling to keep all our single wire straight.
You'll also notice the inner booms are a twin octagonal boom. Typically we use a single boom for the inner arm, however, because this aircraft could exceed 170 lbs, the double boom is a safer bet. These double booms are one piece, not simply two booms next to each other. This system is extremely strong and because the arms run all the way through the center of the aircraft the entire system is extremely sturdy even under heavy payloads.
Value Added System Integrator
After doing some flying and testing, we found some ways to improve on the aircraft. On the above picture you can see that the aircraft's original landing legs have a fairly narrow stance. As this aircraft is huge and may be landing on uneven surfaces, we decided to fabricate some bracketry to widen the leg stance. This would allow you to safely land even on a incline.
In the photo above you'll notice the Y joint in the arm is exposed. This is the stock configuration that comes from Gryphon. The arm is structurally sound but exposes some wiring to weather and debris. We improved upon this design by adding weatherproofing to these joints.
This aircraft was a great experience to further our knowledge about building and operating heavy weight aircraft. Looking forward, we'll be experimenting with more flight controller and ground controller options. The key objectives for the next version is to have a system that will automate all of the piloting. We'll be doing further testing on terrain following, spray control, package delivery and obstacle avoidance. Because we've successfully demonstrated these mission requirements on other smaller aircraft, it will be great to get all of these features into one large unit.
As UAS technology has progressed, the “weak” points of the system have shifted with time. In the early days it was payload capacity, then flight time, and then camera stability and so on. Every year or so another piece of technology comes out that raises the bar for a particular piece of the system. HD video transmission is relatively new, its only been available for UAS for a couple years. It has become much better over time but in my experience has been one of the major weak points of the whole system, particularly in film and television production. This week BFD Systems with Expressway Cinema got to preview the next step in HD video transmission. The Connex Long Range Patch antenna from Amimon.
When I found out Shahar from Amimon was going to be visiting for a test, my first thought was that we needed to test this thing in a real life situation. It’s one thing to test in a lab or the middle of nowhere but when operating on a film set, that is very rarely the case. We wanted to test the new receiver in a high RF environment. At BFD Systems we don’t operate small drones, so I figured for high RF test it would be better to car mount the camera and drive through downtown Philadelphia. So for our first test with the help of Expressway Cinema Rentals, we strapped a Movi Pro with a RED Weapon to the roof of my car. We had the receiver at the Philadelphia Art Museum steps and we drove down the parkway to city hall. The first loop we lost signal at about 3,000 feet. “Ah Ha! We’ve stumped the new system!” I thought. We pulled back around to where the ground station was set up. Shahar realized that the antennas on the Movi were still folded down from transit. We flipped them up, and on the next loop around in the car we ran out of parkway before we had any signal degradation. I did not expect that. We needed to find a longer test range.A patch antenna is a rectangular shaped antenna, and in the case of video transmission offers a much more focused beam of transmission or reception. They are not new, both Teradek and Paralinx have patch antennas for their HD Receivers. (Which use chips made by Amimon). In the case of the Teradek and Paralinx their patch antennas do not extend the range, but do a great job at blocking out unwanted RF noise from other wireless signals in the area. In the case of the Connex LR actually extends the range and helps block out unwanted RF noise. The Connex LR is actually a receiver and an antenna in one. We had hands on with the prototype which was about 8”x9”, and it worked with their existing Connex Mini transmitter. Both the Receiver and transmitter are pretty light weight and are designed for UAS. We had the patch antenna mounted on a C-stand about 8’ high to get it nice and high off the ground and the Connex Mini transmitter velcroed to the side of the camera.
All transmitters say they are rated for given distance but most often they don’t actually perform to their stated specs. Now it’s important to note that Connex LR relies on line of sight. That is to say the receiver and transmitter need to be able to see each other. The 5GHz band that the Connex uses won’t penetrate signal through buildings or obstructions, at least with their stock antennas. Youtube star and RF wizard Alex Greve (aka IBcrazy) has shown some videos where he makes his own antennas that have much greater performance in non line of sight applications. This 5GHz band is used by Connex because it offers more data throughput than 2GHz and it’s complainant with most countries frequency transmission laws. Line of sight for video transmission is also more like a cone/ donut then a direct line. Say you need to shoot video signal through a narrow space between two buildings, the signal may experience interference from the obstructions on the outer ends of the cone. The other thing is it’s absolutely crucial to make sure positions of the antennas are correct. If using linear polarized antennas (like the ones on the Connex Mini transmitter), out of phase or in an incorrect orientation of the antennas can lose more than half of your transmission strength. The difference was night and day when we repositioned the antennas on the Movi by about 90°.
For our next test we took out the drone to get some distance. We wanted to test about 3km, and turns out finding 3km line of sight in Philadelphia is a bit of a challenge. We ended up splitting into two teams. The drone team setup on top of a hill in Fairmount Park. The receiver side drove 3km away to bridge that would be free of trees and obstructions. Google maps said it was 3km but the Connex telemetry was reading 2.6km. We were out of range of the walkie-talkies so with cellphones in hand we had an open line of communication. Although we were not in a high RF environment we were by know means in the middle of nowhere either. We lifted the drone off while the visual observe with the drone team called out our altitude. As soon as we cleared the tree line the signal picked up and image was being received at full strength. It was somewhat bizarre because the drone team could not see the image because it was 3km away, and the receiving team could not see the drone, which was a 50lb, 2 meter armspan multirotor. Over the cell phone the RX team could tell us where to point the camera. At this point we had still had the RED Weapon and were shooting with an 18-35 lens set to 35mm. We spun the camera in all directions to see if there would be interference from the Movi or the drone (which is 1.4 meters wide) but there was nothing but a clear picture. Too clear even, the RX team could have been much farther away to find the limit, but we only has a short amount for time to test before Sharar had to catch his flight home back to Israel.
After this long-range test we just wanted to have some fun, so we decided to meet up with the RX team and rip around our GD-28 drone and chase some cars. For this part we could have used a regular Connex without the patch RX but it was good to test how it would work with higher speeds and more dynamic movements similar to a film set. Even with the directional Patch Antenna facing the complete wrong direction we were getting clear picture at about 300-400 meters.
The Connex line of transmitters is really designed to be used with drones, rather than ground-based cameras. It only features an HDMI input, which is painful because we love flying the Alexa Mini which only has SDI output. You can solve this with an HDMI converter. One company, Media Blackout, created an impressive solution to this problem. They have released their own version of a re-housed Connex Mini TX that has SDI out and a Lemo plug for power. Unfortunately, even with his solution, you won’t be getting time code or flags sent over the Connex. The Connex series does not have all the features of a Teradek or Paralinx. However for UAS work this is not a deal breaker. The family of Connex transmitters and receivers are a fraction of the weight and cost of other HD transmitters and generally perform much better at the main task of sending wireless HD video. It will be exciting to see what the industry comes up with and what the new use cases will be for such a long-range video transmutation system. One thing that comes to mind is a feature that allows the Connex to also be used to transmit control signal for the drone. "Whether it is used for blocking out unwanted RF on a busy film set, for flying in high traffic area, or for extremely long range flights, the Connex system has proved it can handle just about anything you can throw at it."
Thanks for reading, One last thing! Check out Expressway Cine Rental for more great blogs about camera gear, film reviews and more drone content. They rent amazing gear, but more importantly their support staff and crew are fantastic at getting you setup and checked out for your shoots.