July 2012 – RCExplorer

Let me present the next evolution in my tricopter adventure. I call it the Tricopter V2.6HV where HV stands for High Voltage. This version uses a 4S LiPo battery, shorter arm and smaller props to achieve increased stability, improved maneuverability and almost no wobbling during decent.

In this version I will present a whole new vibration dampening system, that works so well that even with relatively heavy vibrations it still produces silky smooth video.

In this build I’ll also try to hide the wiring and make the Tricopter look cleaner and more polished.

The Tricopter V2.6HV is based on the Tricopter V2.5, it uses more or less the same electronics and a lot of the build is going to be very similar. Therefore I will not go as in depth on those parts I feel I already covered in the previous build.

Enough talk, let’s build.

The same old Tricopter frame I designed back for the Tricopter V1. Still work great I haven’t felt the need to change it.
(Tip, I now sell pre-cut CNC Tricopter frames!)

One goal with the new V2.6HV Tricopter is to make it look a bit more polished. To achieve that I decided to use these hollow square carbon fiber tubes for the arms rather than the wood, this will allow me to hide the wires making it look tidier. The downside is that the carbon fiber is less crash resistant. I recommend using wood arms if you’re not already an experienced tricopter pilot.

The tricopter V2.6HV will use shorter arms than the V2.5 for a couple of reasons. It’s easier to pack in a suitcase with shorter arms, I’ll be using reflashed ESC’s which will more than compensate for the shorter arm length when it comes to stability and the carbon fiber square tubes comes in 750mm lengths and it’s convenient simply to cut them in half.

Be very careful when drilling the carbon fiber arms. Do not use any force or they will crack. Rather let the drill-bit cut through the fibers in its own pace rather than forcing it. If the arm crack it will be severely weakened.

Time to fix the ESCs.

To get the highest possible stability, I bought the Hobbyking F-20A ESC’swhich I’m going to re-flash with Simon K’s awesome firmware. The modified firmware has little to none filtering of the PPM signal which means that the throttle response is much quicker.

There are a number of different ESC’s that can be re-flashed (more info in this thread), I choose the F-20A ESC’s due to the fact that it has an external oscillator and has programming pads that are really easy to get to.

I made a super simple quick connector out of a clothespin and some rigid wire.

I simply clamp the clothespin over the programming pins and go. Barely takes a minute to flash a new ESC now.

I removed the old wires.

Then soldered on some new ones. For those wondering what heatshrink I used to cover the ESC’s, here is a link.

In this version I decided to mount the ESC’s at the tip of the arms rather than the root. This was mainly for convenience as I didn’t want to cut any extra holes in the carbon fiber and I didn’t want the ESC’s mounted on or under the frame.

The wires will be hidden in between the two frame plates.

Looks pretty neat and tidy so far.

Time to solder the power connector.

Remember to put the heatshrink on the wires before soldering.

The V2.5 tail mechanism has worked flawlessly so I’m going to use it on this build as well.

A quick spray with some black paint made it fit in better wit the black arms and frame.

Servo prepared just like in the V2.5 build

I covered the servo lead in black 3mm wire mesh to make it look nicer.

One problem with the carbon fiber arms is that the zip-ties can’t ”bite” into them preventing the motors from sliding back and forth. To fix this I simply glued a small piece of scrap carbon fiber to the bottom of the arm.

Now the motor can’t slide anywhere.

The FPV system. A 300mW 1.3Ghz transmitter with a AnyVolt micro. However I do not recommend using the AnyVolt micro on a 4S system as its not rated for 16.8V. I’ll post my permanent solution further down.

The FPV transmitter mounted on a glass fiber piece on the back boom.

Ok time for the exiting bit. The new vibration dampening system. After a lot of thought, research and testing this is what I came up with. A solid wire dampening system.

It’s made from 1.2mm thick piano wire bent in a circle and then at a 90° angle, like an L, at both ends,

To simplify mounting I cut two L shaped tracks in a 1mm thick carbon fiber plate. The piano wire is then pushed into the tracks and then sandwiched between two untouched carbon fiber plates.

Here is a shot of the complete dampener that shows the 90° bends pushed into the tracks.

I made the front dampener a little differently. I used the already existing 40mm wood piece that is mounted in between the plates in the front of the tricopter body, to hold the top of the dampener.

I drilled a small hole and inserted the wire, I then bent it at 90° so the wire was parallel to the wood and then bent it another 90° a bit further in and pushed into the wood. The important thing is that the mount is stiff and can’t flex in any direction.

I had this camera tray laying around since an old tricopter build. It’s made from 1.5mm carbon fiber.

This is how I attached the dampener. Simply glued the 1mm CF plates with the L shaped tracks in it to the underside of the camera tray.

I then glued one of the untouched 1mm carbon fiber plates on top of that. Just to make sure I wrapped a zip tie around the mount.

Now to the theory of the new solid wire dampener solution. The vibrations created by the motors and props spinning has to travel through the thin wire to get down to the camera plate. Since the camera plate is stiff the whole camera plate has to vibrate for the camera to vibrate. With the heavy 4s3000mAh battery plus camera mounted on the camera plate, a lot of energy is needed to get it moving. The thin and stiff 1mm piano wire has a pretty high resonance frequency and the vibrations will have a hard time to travel down to the camera plate. The few vibrations that does make it down has so little energy that they are absorbed by the heavy battery and camera.

Time to mount the electronics.

I still use the good old KK board. The KK2 wasn’t released when I started the build.

DT750 Motors and 9×4.7 props mounted. Ready for flight.

Front dampener. I painted the wood black to make it blend in better.

Old Man Mike optimized 1280mHz Pinwheel antenna.

Tail assembly.

The GoPro case I use is lightened by removing unnecessary material. The steel screw is replaced by a plastic one, the buttons are removed and the holes enlarged. I also removed the protective lens. Not to make the case lighter but to improve the image quality. Having the rubber band holding the camera in place instead of the plastic back allows the camera to be pushed in when something pushes on the lens, reducing the risk of scratches.

The 4s3000mAh battery mounted on the camera tray.

I tested the solid wire dampening system with unbalanced props. I used my Iphone 4 running an app called iVibroMeter. Strapped straight to the frame it recorded an average of 55mm/s2 during a hover. I then moved the phone to the back of the camera and strapped it down tightly. It recorded an average of 5mm/s2, a reduction of over 90%. Very impressive.

Ok here is my permanent solution for getting stable 12V to the FPV transmitter from the 4S main pack. A LM2940 low voltage dropout regulator. This is a linear type voltage regulator, which means that it burns off excess voltage as heat. Linear regulators aren’t as efficient as a switching type voltage regulators. So why use a linear voltage regulator? – The answer is: Noise.

A switching voltage regulator works by switching the input voltage on and off, charging an inductor. It does this at a fixed rate of 50000 to 100000 times a second. Depending on the voltage and current needed by the circuit you’re powering, the regulator will be turn on for a longer or shorter period of time. Letting more or less energy pass through. This is a very efficient way of regulating voltage as it wastes very little energy. But it also emits loads of radio frequency interference. Meaning that it actually acts as a transmitter of noise. It also produces noise in the output voltage also known as ripple.
Our analog video transmitters and cameras are quite sensitive to this ripple, which can create lines and interference in the picture. And on top of that the radio frequencies emitted by the switching voltage regulator can also reduce the range of the RC link.

A linear voltage regulator on the other hand actually reduces ripple and doesn’t emit radio frequency interference. The downside is that they reduce voltage by burning of the excess as heat. But in this application it’s not too bad, as the voltage difference between the input and output is pretty low and the current we need isn’t particularly high.

The heat (in watts) that will be generated by the linear regulator can be calculated with this simple formula: (Input voltage – output voltage) X current draw
In my case that is a fully charged 4S lipo (16.8V), minus 12V output, multiplied by 0.3A (the current draw of my FPV transmitter)
(16.8-12)*0.3=1.44
1.44W of wasted heat is not much in our type of application. To put it into perspective; If you have a flight time of 10 minutes on a 4s3000mAh battery and use a switching regulator for the FPV transmitter. By replacing that with a linear regulator, you would loose about 1 seconds of flight time. However the linear regulator usually weighs less than a switching regulator, which actually can increase the flight time compared to the switching regulator.

A problem in our application is that voltage regulators normally need a couple of volts higher input voltage than they can deliver as output voltage. This is true for both linear and switching type regulators. This is problematic as the voltage of a 4S battery varies between 16.8V when fully charged down to 12V when fully discharged. If you want to use a switching regulator you need a ”step-up”-”step-down” type which actually is two regulators in one. The first one drives up the voltage to say 50V and the second one drops it down to 12V. This type of regulator can operate at a very wide range of voltages and can even output a higher voltage than its input voltage. These regulators however generate even more noise and RFI.
When it comes to linear voltage regulators they typically need 3V higher input voltage than the voltage they output. However there are a type of linear regulators that only need 0.5V more input voltage than the output. These regulators are called LDO regulators or low dropout regulators. And it is this type of regulator I will use to power my FPV transmitter.

My choice is the LM2940CT-12 low dropout regulator, that is capable of delivering over 1A of current in the TO220 package. It has a typical dropout voltage of 0.5V at 1A and 0.1V at 0.1A. However it does have a quirks to it that need to be explained, and that is that it requires a very specific ESR value capacitor on the output to be stable. The capacitor needs to have a ESR between 0.04 and 5 ohm across its entire operating temperature and needs to be 22uF or bigger. This capacitor needs to be mounted as close to the voltage regulator as possible. Other than that its just like any other linear voltage regulator.
I rummaged through my mixed electronics box and found a Rubycon 220uf 35V capacitor that happens to have a ESR of 0.056 to 0.19 ohm. Perfect.

Thanks to the low dropout voltage the LM2940-12 is perfect for a 4S setup. It’s going to hold a perfect 12V output all the way down to about 12.5V, by that time your multicopter is probably falling like a rock due to the battery being about 99% discharged. But even if the battery goes below 12.5V the output of the regulator is simply going to output 0.5V less than the input voltage. Most video transmitters will loose video sync somewhere around 8-9V and by that time your 4S battery is permanently damaged.

I’m very particular about my video link and I tend to go overboard when it comes to filtering, but I rather have a perfect picture than not adding a gram of extra components. Therefore I added a pretty powerful LC filter to my circuit. You might skip it or make it smaller if you find it too cumbersome.

Here is how I hooked up the voltage regulator.
– L1 is a 1mH coil
– C1 is a low ESR 22uF capacitor
These two components make the LC filter and is there to remove any noise generated by the ESC’s or anything else hooked up to the main power-leads.

– T1 is the LM2940-12
– C2 is a 0.47uF Tantalum capacitor recommended to use in the LM2940 data sheet.
– C3 is the capacitor that needs to be 22uF or bigger with a ESR between 0.04 and 5 ohm

Here it is all soldered up. Not my prettiest work but its solid and compact. I’ll add some hot-glue and heat-shrink to protect it. Notice that I use a balance connector to power the FPV gear. This lets me power up the FPV gear just before I need it instead of having it on all the time producing heat and reducing its life span. Note also that I mounted the LM2940 on a small heat-sink. When sitting stationary on the ground the 1.44W of waste heat will heat up the TO220 package to a pretty toasty level. Better have a small heat-sink and be sure it never overheats.

Here is a comparison to the foxtech step-up-step-down regulator I tested out before building this regulator. It spewed out a lot of RFI and even with a hugh LC filter on the output still produced noticeable lines in the video. Also the linear regulator weighs less than half.