Saturday, October 27, 2018

Ground Station Build Notes For Converged IoT Drone

Antenna Cluster Planning

The mission scenario sketch is in the following picture. The small matchbox at the lower left corner is the base station.

The modeling is made in Blender. And, based on the mission scenario, the transmitter/FPV antennas are arranged without internal interference or obscuring/blocking signal line of sight.

Antenna/Transmitter/Receiver Setup

FrSky R9M 900mhz Long Range module is chosen as a substitute of TBS micro module to save modification in the QX7 transmitter itself. 10-pin, 1-meter ribbon cable connects TX pins to module. Breadboard jumpers tethers the module. The USB extension cable connects car cigarette power, and a retractable USB-micro cable connects the power to the FPV receiver.

The Hyper Tough cigarette power plug is specifically chosen because it has very tight fitting to fix a suspected cause of crash of pilot's knee knocking the plug during flight.

In the following picture montage, from the top right picture, clockwise, the soft, retractable USB-micro cable is wedged behind the sun shield board. 2 large paper clips are bent and coil to form hooks to hang the TX and the FPV goggles when all cables are hooked up pre-flight. The cigarette plug and USB extension cable are duct taped on the lower peddle firewall and out on to the chassis column. The USB extension cable and USB-micro cables are installed into the chassis column.

The FPV receiver based on the RX 5808 chipset is chosen because experiments on the FXT Triversity receiver had video "blueouts". The real fault point, including all previous blackouts, is the FXT Triversity receiver, not the FPV goggles.

The 3/4 inch velcro strip wraps around the lower corner LED as an anchor, then  goes over the bottom side of the receiver and up. The antenna plugs make a natural tilt angle for the velcro strip to slide down on the RF cavity cover. The video output pin 6 is soldered to the AV yellow cable for video on the PCB. The tail end of the AV cable and the row of pins are encased together by the clear mounting tape. The head end of the AV cable goes toward FPV goggles.

The right angle SMA adapter for the mushroom antenna needs to reverse the polarity with the inner core pin stem on both sides because both the mushroom antenna and the receiver side has the inner core pin hole. The patch antenna with 9.4dBi is the directional antenna projecting the craft's diving trajectory. The 45 degree SMA adapter is essential in making the near vertical radiation pattern.

The receiver antennas allow quick release folding. Just loosen up the lower antenna's 90 angle connector with 5-6 counter-clockwise twists, then fold the upper antenna backward 90 degrees and fold the the lower antenna backward 180 degrees.

The worst case noise interfering with FPV feed is with 12-16V power supply to the Caddx Turtle v1 camera in the following video if you watch the original feed video file (download it then watch it on a video player instead of watching it online with online player)

The FPV feed is clear of interference lines when 5V power is supplied. But, the voltage sensor of the camera only operates between 10.0 and 12.0 volts. Any voltage outside the range has the idle display of "12V".

The FPV goggles can be either Makerfire EV800D or Eachine EV800D. The FPV feed quality is indistinguishable between the 2 brands.

The Frsky R9M module can be set to output 500mW using Taranis transmitter's regular power pins without external power source. The flight video shows long distance dive without any transmitter RF gliches, which would manifest itself in fail safe mode level free-fall aborting diving.
This flight sequence rehearses the mission plan, diving from 333 feet east, 444 feet north, ground distance 555 feet, air distance 1000 feet, relative to ground station. 3 flips above the landing patch area. Level traversal between 333 feet east, 444 feet north, and 333 feet straight east.

In-Car Battery Charging

The EV800D can be charged directly with car 12V through the Hyper Tough power plug. It also comes with the charging adapter cable with 4-pin JST XH socket. The GoolRC charger is powered via a 5.5/2.5mm plug, which can be obtained from the NorthPada power supply cable. The plug cable leads are soldered on to the second 12-volt output. The wire leads has matching color coding, red for positive, for the GoolRC charger, as circled-highlighted in the picture. Battery should be warmed with car vent using a gardening wire to hook up the charging plug before charging in cold winter temperatures. 


The charger is placed in the glove compartment. When the car engine is offline, the car battery voltage has a slight drop, so the flight battery charger needs to be set to output 1.6A instead of the usual 2A, to prevent low voltage cut-off during charging, which can happen in the middle of a mission.

The modification on the transmitter includes peeling back the battery plug wire wraps and tap the charging wires as the above picture. Then 2 holes are drilled for the charging wires to come out. The Energizer 2300mAh NiMH are chosen for the transmitter because it can handle 0.9A charging with very moderate warming. Previous experiment with Panasonic 1900mAh batteries saw excessive heat with just 0.5A input and needed to lower the input current to 0.3A. In the above picture, each of the 2 charging wires connects to a EC2 connector core, just like charging the flight battery pack. The reason EC2's outer casing is discarded here is because it obscures the raw black and red color coding leading to more human error when attaching the connectors. Also, if the casing is installed on the flight battery pack, it increases resistance for ejection of the flight battery pack during a crash. Ejection understandably lowers damage to craft main frame by the heavy flight battery pack.

FPV Goggles Setup 

For focal point correction, a small positive or negative diopter can be added by opening the camera box and repositioning the lens. To move the lens closer to the eyes, stick 2 strips of clear mounting tape at the bottom and 2 squares on the ceiling wall, then stand the lens on the bottom strips and balance it by tilting the box until the lens is parallel to the original lens frame before closing the ceiling wall.

The AV input is connected to the RCA video cable via the yellow video adapter cable included in EV800D package.

Low Profile Radio Transmitter Antenna

TBS Crossfire Stock TX Antenna allows bending radiation pattern for better diversity. The straight polarity-reversing adapter in the part listing is needed to connect this antenna.

FPV Feed Reviewing

The SD Card needs the SD-Card-USB reader connected with OTG cable to an Android phone for craft recovery path tracking. Use the MX video player App to review and backtrack the flight path after a crash.   

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