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MR1 Hexacopter Project 6/6/11

Posted on June 6, 2011

       Hello again everyone!! Now that I have got the MR1 on it’s feet I can tear it back down to get into the “guts” of the hexacopter :0) I need install the power distribution wiring harness and fuse block to supply power to the six ESCs (Electronic Speed Controls). The wiring harness will consist of four 12 AWG battery pigtail inputs that will feed a four position fuse block. The fuse block will in turn feed a common 120 amp power bus that will feed all six ESCs.
       I am fusing each individual battery pack at 30 amps to protect both the packs as well as the ESCs from overload conditions. The 2250 mAh 11.1 volt Li-Po battery packs I am using are capable of delivering 56 amps each which could produce over 225 amps of combined current!!! In fact these batteries could easily produce 350+ amps of instantaneous short circuit current which would ultimately end up in a molten mass of wires and smoking battery!!!
       Fusing the system will also protect against battery damage from trying to fly the hex on one pack. As soon as the ESCs pull more than 30 amps the fuse should blow and protect the pack (The hex hovers at 36 amps). I am guessing that the hex could fly on as little as two packs but at a limited duration and not at full thrust.
      
Note: Fusing of RC systems is not a common practice. In fact most series parallel systems have no over current protection except what is available at the ESC’s. This portion of the build was my own personal preference and by no means a standard practice...        

       I decided to start off with the fuse block holder bar that will eventually be positioned under the main hub of the copter. I made my fuse block holder out of a piece of 3/32” x 1/2” FRP as seen below.....

       For this build I will be using a set of 2 x 2-position (1/4” OD x 1-1/4” long type fuse) fuse blocks (McMaster-Carr Cat# 7687K12). I mounted the fuse blocks to the holder bar with some 6-32 stainless screws. These same screws will be used for the main power bus terminals.....

       The fuse holder bar was then mounted to the lower hub disk with 6-32 screws as seen below. I used some 6-32 locknuts as stand-offs to keep the bar recessed a bit into the hub disk for clearance from the battery tray....

       A couple more nylon insert locknuts were used to hold the bar in place......

       The next step was to install four support screws for the battery tray/payload deck. I used 2-1/4” long 6-32 stainless screws through 1-5/8” long 3/16” aluminum spacer tubes. The screws are held in place by a set of locknuts as seen below. These locknuts will keep the support screws from turning when the battery tray nuts are tightened over them.
       The overall length of the stand-offs including the locknuts is 1-13/16” from the lower hub disk to the top of the locknuts....

       And now to test fit the battery tray. You can see that the upper tray piece is fairly close to the fuse block when in place......

       The batteries fit beautifully!!!!!

       You can see that there is just enough room to allow for cross-ventilation for the future ESCs as well as passage for the battery pigtails....

       Before I go any further with my power harness I am going to install a Li-Po battery checker device to the battery tray. This device will notify me in-flight if my batteries are getting too low to sustain controlled flight. This is super important as if my batteries drain below the ESC’s low voltage limit they could cause the ESCs to go into cutt-off mode which will reduce power to a motor or motors which will result in a crash :0/
       The unit I am going to use is a
Integy C23212 voltage checker and warning buzzer (Tower Hobbies Cat# LXADUK). This device will monitor one of the four batteries that will be connected in parallel which in turn should represent the overall voltage of the system. The voltage checker will monitor all three cells of the one “test” battery during a flight and alert me should any of the three cells should go below a certain voltage.
       You can adjust the warning buzzer to go off from 2.7 to 3.8 volts and it will sound a loud buzzer when the pack goes below that value. I will set my buzzer to go off at 3.5 volts which should allow me some time to land the hex safely before the ESCs quit on me....

       I fabricated a small aluminum bracket to mount the Integy battery checker as seen below. I mounted the bracket so it is close to the #1 battery position so the battery balance connector can be directly used on the checker device....

       A little double sided servo tape and I’m done!! You can see the checker showing a value of 11.8 volts total (below). The checker also displays each cells voltage in order and then back to the total voltage in a continuous cycle. I should be able to hover near myself and then monitor the voltage level just like I would my fuel level on a conventional glow-fuel helicopter.
       Note: The checker I am using will only monitor one of my four packs which  leaves the other packs unmonitored. However this should be OK because the packs are in parallel and should keep within a few millivolts of each other.....

       At this point I decided to cut out a 6” disk of 1/16” FRP sheeting to be used as a protective shield for the electronics deck of the copter (top disk of the hub). I used the same length 6-32 screw setup as I used for the battery tray except that there is only three screws instead of four. The support screws were placed at 120° apart from each other as seen below.....

       While I was still in “fabrication mode” I decided to build a “tail fin” support bracket. This bracket will hold an FRP tail fin paddle that will tell me which way the copter is facing during flight (sort of like a tail rotor on a helicopter). Most hexacopter pilots prefer identifying their copters direction of flight by painting the front booms of the copter a bright color. Although I like this idea I think it will be easier to see a paddle style tail fin especially in silhouette.
       You can see below a bracket made from 1-1/4” x 1/16” aluminum angle stock. This bracket will mount directly to the #4 motor mount boom clamps......

       I think that it is worth mentioning that a tail fin on a hexacopter can have a “weather vane” effect in a cross wind which can turn the hex while hovering. I plan on making my tail fin paddle fairly small which should help reduce any wind induced yaw while hovering (later in the build).
       You can see the newly installed bracket on the #4 “tail” motor below.......

       And now back to wiring the copter!!!! I collected all six motor booms to install the motor feed wires which will extend the motor wires into the copter’s central hub.......

       I will be using 16 gauge silicone wire to extend the motor wires which should be heavy enough for these motors. (Tower Hobbies Cat# LXVHF9).....

       Using solder and colored shrink wrap I extended each motors leads down the motor booms....

       The color coded wire makes identification of each phase “leg” (A,B,C - black, red, blue) much easier than all one color wire......

       The last step was to install the 3.5 mm male “bullet” connectors on the extension wires (I reused the ones off the motors)......

       I tested out each motor with a speed control (ESC) to double check the connections (not shown). All of them checked out OK :0)

       The next step was to wire up the fuse block to the battery pigtail connectors. I chose to use prewired pigtails to save some time. For these I used W.S. Deans male pigtails (Cat# 2013) which are made with 12 gauge silicone wire. I picked them up from Tower Hobbies (Cat# LXHGM0)........

       The “negative” black wires on the pigtails were fitted with a soldered-on 12 gauge ring connector so they could be attached to the negative side of the power bus...

       The positive side of the pigtails was soldered directly to the fuse block terminals. This was done by removing the terminals from the plastic blocks and then soldering them to prevent the blocks from melting. I used shrink wrap tubing to cover the soldered joints after the fact.......

       You can see below my “American Beauty” 200 watt soldering iron that can solder anything :0) Just don’t grab it by the wrong end :0/

       You can see below how the power bus screws will channel all of the 120 amps of battery power to one set of terminals. I used aluminum tubing on the 6-32 screws to center the large ring connectors over the terminal screws. The ring connectors were then secured with a 6-32 nut as seen on the upper terminal below.
       You may also have noticed that I added some 12 gauge silicone wire to the other side of the fuse block to connect to the positive side of the power bus terminals.....

       I fitted the fuse block with some 30 amp DC quick acting automotive fuses to finish off the job. Note: A set of 30 amp DC slow-blow fuses would be more suitable for this application but I gave up on that idea when I saw the price of those fuses :OP........

       Now my power harness and fuse block systems are ready to attach to the ESCs.......

       I am in great shape for installing my ESCs and putting the copter back together again for installation of the electronics package. Please join me again for the next episode of the MR1 Hexacopter Project!!!

Till then be good to one another my friends!!!!

Don R. Giandomenico

 

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