MD500 Tow Defender Build Page 9
Electrical Systems: Main ESC Power / Main & Aux Power for Radio and Accessories, Part 1 of 2.
I have decided to use A123 batteries in this project. I am doing this for two reasons, 1) Safety. From what I have read the A123's are much safer than Lipo's. 2) Installation. I will be building a custom battery pack layout that will be embedded into the fuselage structure. It will be hiden from view and removed only to replace the batteries. I will be charging these at a lower current than they are rated because I don't want to push my luck and have something go wrong. The main ESC battery will consist of 9 or 10 cells, giving me a max voltage of 36 volts at full charge. I will also be using a Castle Creations ESC and a lower Kv motor (still figuring that out).
For the flight electronics I will be using two A123 cells. I will also have an Aux battery pack using two A123 cells. This will power all the accessories like the rotating FLIR, cockpit and instrument lighting, landing light, all navigation lighting. I will be running my electronics somewhat directly off the A123 battery packs. More about this below. By doing this I eliminate the need for so many seperate voltage regulators, and one more failure point. The power for the flight electronics is redundant as you can see in the Electrical System Diagram below.
I will be using various electronic modules to combine power, power and signal, and control some of the above mentioned lighting systems. These were all designed by myself and are for sale if you would like to buy them. Just go to the custom electronics link on the side bar for more details.
I had planned on using a Skookum SK360 for flight stabilization and the stock Trex600 ESP gyro, but I may try to fly without any electronic assistance on the cyclic controls. (Oh Boy!!)
Here is the complete electrical system block diagram. I will describe each block in detail below. The ESC, motor,
and flight controls blocks will not be discused as these are off the shelf items, and basic components needed to fly.
The Dual Battery Power Mixer. This module is a diode isolated dual battery input, single voltage output mixer. It can be ordered with a
standard diode (~0.7 volt drop), or a Schottkey diode (~0.35 volt drop), or with one of each. Using two different voltage drops will force one
battery to supply power while the other is just used for standby incase the first battery fails for some reason. I will be using two types, one with
a standard and a Schottkey diode. This will be my main Rx power input. Under normal operation the Rx will be fed from the Schottkey diode side of
the power mixer. This will provide a typical voltage of (6.6V - 0.35V) = 6.25V, and the auxilary supply will be (6.6V - 0.7V) = 5.9V. This should provide
a very safe operating voltage range for the Rx and servos. I will also be providing power into the Rx through the Data port from the auxilary supply using
a second Dual Bettery Power Mixer board with only one standard diode installed. This can be seen in the above wiring diagram.
Below is the first test done to verify the Dual Battery Power Mixer voltage. Here I am measuring the raw battery voltage.
Here you can see the raw battery voltage of the 2S A123 pack.
Here I am measuring the voltage being fed from the Dual Battery Power Mixer.
And you can see the voltage is within 10 millivolts of 6 volts. Good enough for me, and the electronics.
The Signal / Power Splitter. This module is used for higher power servos and devices that need a receiver control signal, but draw too much power
to be fed from the receiver, or for servos and devices that you just want to control from a second power source. Using these modules is simple and easy.
You connect the signal input to which ever receiver channel you want to use for control, and connect the power input to a battery pack, voltage regulator,
what ever you are using to power your accessory device. As you can see in the above diagram, all my lighting control and my automated FLIR control is
powered through one of these splitters. The photo above shows the FLIR splitter and label for the FLIR module.
The 4 Channel Light Controller. This module can be used to turn anything on and off, lights, motors, solenoids, rocket engine ignitors. The list
goes on and on. The outputs are high power 48 volt, 2.5 Amp continuous (open Drain for those who want to know). The power for the devices being
controlled can either come from the same source that powers the controller, or from an external source with more power. The outputs are setup to
toggle on and off, and are best controlled by using the Channel Multiplier 8 sold through HobbyGuysRC, LLC only. It will work without the channel
multiplier, but it is not as easy to control devices used this way. See www.HobbyGuysRC.com website for a more detailed explaination of these controls.
The output channels are color coded for easy identification. Photo shown here is of the prototype before the color coding connectors were used.
(I am also using a Deminsion Engineering DE Light Control for nav lighting. This control works well and is easy to use. I use it in my Airwolf also.)
This is the Channel Multiplier for use with the above 4 channel light control. This little control will mount on the back of my transmitter
and connect to one of the upper channels, say 8 or 9 on my JR9303 radio. It will actually control eight devices through one transmitter
channel. They are all on/off toggle type control. Maybe this will change in the future, who knows. I think the trick will be getting used to
where the buttons are without being able to see them as I am mounting this on the back of the transmitter case.
Hope I don't shoot off a rocket instead of turning on the cockpit lights, whoa!!! look-out!!!!!
The Automated FLIR controller. This module is used to simulate the FLIR operator scanning around to see the enemy targets. It is a pretty
customized control that is purpose built. On the left side there is also an optional on/off pushbutton. Operation is simple. Just plug it in
like you would any servo, then plug the FLIR servo into it. The servo and the connector it plugs into is not shown in the photo.
If you have a need for something like this please send me an email describing what you need, and maybe it can be custom built for you.
I also built a simulated radar screen using a small pocket photo viewer. The only electronics I built for this was the voltage regulator. The display runs off a
single lipo cell so I had to make a 4 volt regulator to power this off the aux battery. The regulator can be powered from a 2S lipo, or two A123 cells.
The Sound Effects system. This module is only in the prototype stage. I have tested it and it works pretty good, but finding small
speakers that are loud is proving very hard to do. When completed the sound effects system will emulate machine guns and missiles
being fired from the helicopter. I may also add some leds to simulate the firing of the missiles or machine guns.
(Sorry no photos for this yet. On hold for now)
Main ESC battery location planning. In the photo you can see what I think will be my final battery layout for 8 of the 10 cells. There will
be four cells in front of the lower bulkhead, and four behind the bulkhead. The remaining two will be cradled straight down below the
mechanics. There is a hole in the bottom of the fuselage that I am trying not to block off as I want to use it for cooling the ESC, and to
allow airflow up through the fuse and out the top or side doors. (That's what I am hoping for anyway)
Next is the main and back-up batteries for the Flight Controls, Navigation and Cockpit instruments.
The flight controls will be powered from the main battery, and also from the back-up battery if needed.
The navigation and cockpit lighting, FLIR control, and all other electronics will only be powered from the back-up battery.
The batteries will be mounted under the cockpit floor boards. They will be mounted to a plywood base that will be
glued to the sides of the fuselage. They will be velcro strapped in place. The plywood base has one cross brace to
add strength and prevent flex while flying.
Here is the bottom view of the battery mounting tray. You can also see the cutouts
for the velcro straps to hold the batteries in place.
A view from the top down looking at the battery base during a test fitting.
Here you can see the cross bracing under the base. This should provide good support.
Battery base now has slots for the velcro straps.
One more test fitting with everything in place before getting glued in.
This is what it will look like once I glue it in. I will be building a shelf under the floor boards
to secure the electronics and diode voltage regulators.
Here you can see the electronics shelf with most of the in place. I still need to cut slots for tie wraps
to help secure everything in place. Man did it get busy under the cockpit floor in a hurry.
All the power wiring is done, and most of the signal wiring. The large connector on the left will
be the on/off switch for the two battery packs burried in all that wiring. The connectors on the right
are for power and signals to and from the receiver.
Test fitting the cockpit to make sure it still fits after cramming all the wiring under it.
Thankfully it still does. It would suck to have to redo something at this point.
My plan is to make a small box to hide the on/off connector inside. I am just not
sure what you find in the full sized version of this heli. I may just make something up.
Interconnect wiring harnesses for power, signals, and nav lights.
