Monday, June 24, 2013

Dual steering servos for E-Revo VXL

Installing a second steering servo on the Traxxas Mini Revo (1/16) VXL is an easy and fairly inexpensive upgrade (about $30 in parts). The E-Revo VXL has already second servo mounting position on the chassis - it is just closed off with a blank cover.
Parts needed - Traxxas 2080 Mini Servo, Traxxas #7043 steering arm kit, link ends and some screws.
There are a few advantages to dual servo steering - the steering is more precise and it holds on better at high speeds, the extra torque provided by the second servo allows you to run much bigger tires, which could otherwise overload  and possibly damage a single servo. Additionally, this setup decreases the wear-and-tear of the servos as the steering load is evenly distributed across both servos.
The main challenge with mounting a second servo is to make sure that both servos are mechanically linked to the bell crank steering arms in such a way that their center (zero) positions (radio: 0 trim, 0 sub-trim) will match perfectly - otherwise they will work against each other!
If their center position is not exactly the same, when linked with the standard, fixed, non-adjustable links, they will "fight" for the center position "ownership", each pulling its side of the bell crank to its own center (zero) and this will cause a premature failure from the constant load (not to mention the increased electrical current).
Some drivers, who use aftermarket 3-channel radios (like the Spektrum DX3S) suggest the use of the AUX channel for the second servo control and then enable the steering (STR) channel mix to the AUX. Doing so brings some serious limitations due to the poorly designed firmware in DX3S - the steering trim on DX3S does not affect the mixed AUX channel - same thing goes for the sub-trim adjustment! If trim is added to the steering channel, AUX will still stay fixed on its own position causing servo binding. Mixing works only during steering wheel commands but not with any of the trims adjustments. AUX channel has its own servo trim function but using it means that one should adjust the steering trim first until the car drives straight with only main servo (STR) linked to the bell crank arm, then adjust AUX trim until spacing between the steering arm and the second servo horn is exactly as the fixed linked and then install the link between that servo and steering arm. This is a severe limitation - no steering trim will be possible on-the-fly without removing a mechanical linkage from one of the servos and going through this procedure again. It turns trimming of the steering into a rather slow and painful process - forget about quick trims when suspension or wheel alignment is adjusted. In addition, the "fighting servos" condition will still exist if the car (ESC) is powered on with no radio turned on to apply trims.

The other way (better IMHO) to do it is to drive both servos from the same steering channel - the original Traxxas receiver has two ports for CH1 or if using a Spektrum receiver one can use a Y-split for the steering channel. Then, the difference between servo's centers is compensated by using an adjustable length mechanical link - turnbuckle or regular screw type adjustable link will do the job. Things are never simple tho - the spacing between the steering arm and the servo horn is very small and a custom adjustable link must be fabricated to fit.

I just used two plastic toe link ends (GPM toe links), trimmed them short enough so their combined length when put back-to-back is just a tiny bit less than the original Traxxas steering link and used a M3 screw as a push rod between them (screw head removed of course)
This approach worked perfectly. I connected both servos to the STR channel on my Spektrum receiver and set the radio to 0 trim, 0 sub trim. Using the original Traxxas fixed link, I linked the servo with the best 90-degrees-servo-horn-to-body position to the steering arm and then connected the other servo with the adjustable link so there was no binding (no buzzing or humming from any of the servos). Using steering sub-trim,  I made the bell crank arm exactly perpendicular to chassis.
I also set the radio for servo travel adjustment of 116% and it seems to give me a little more steering range.
That's all there is to it - steering trim is now available as usual. Adjustable link is not needed if you are really lucky to have both servos matching their center positions, but this is very unlikely due to the way the splines are made in the servo horn - normally, the servo will take the horn exactly at 90 deg. on one side but it will be off when placed 180 degrees from that position (which is actually required for dual servo setup).
I installed metal servo horns but they have exactly the same splines as the plastic ones.
Another mod  I made to the steering system is to replace the plastic bushings in the bell crank with real bearings - Traxxas #5114 - 5mm x 8mm x 2.5mm / sealed which made movement absolutely smooth.

Thursday, June 6, 2013

Align 450 DFC - Blade grip control link mod

One disadvantage of the DFC (Direct Flight Control) Rotor Head is that it puts the blade grip control links under a lot of strain and they eventually can fail causing a crash. Align tried to solve the issue by changing the design of these links and supposedly making them stronger .
The new links are called "Main Rotor Grip Arm Integrated Control Links". These new links are made from solid aluminum and there is a pair of ball bearings at the pivot point where the link is attached to the blade grip.
The blade control links transfer the axial movement of the swashplate (when the servos move it up/down to change blade pitch angle or tilt it for aileron / elevator control) up to the blade grips while transferring down the rotational force of the blade grips to the  top (rotational) part of the swashplate.
The main issue with this design as part of a DFC rotor head is that it relies on a very stiff head dampening in order to minimize the axial movement of the feathering shaft. Any significant movement of the feathering shaft puts serious strain on the linkage, trying to "pry" it off the swashplate's linkage ball due to the created lateral force on the link arm.
By coming up with the metal "integrated arm" design, Align attempts to reinforce the linkage strength but they have focused on the top portion of the linkage. By keeping the standard for the 450 series plastic link ends on the arm, what they did is just to move the failure point down - a chain is as strong as its weakest link.
DFC pilots often see control link failures in that area, especially when flying 3D - the plastic ends sometimes break off or are being pulled off the link arm.

Point and case is this facebook post.
It is a shame when failure of a $1 plastic part causes the destruction of a few hundred dollar heli due to poor design.

To improve on the reliability of these links, what I did is to replace the lower portion of the Integrated Control Link Arm with a beefier hardware designed for Align T-Rex 550.
The part number needed for this mod is "Align T-Rex 550 Stainless Steel Linkage Rod A - HN6065A"

This my 450 PRO FBL DFC rotor head with the modified Blade Grip Control Links already fitted on.

I used a 2 mm drill bit to carefully enlarge the threaded hole on the bottom of each Integrated Arm link. The 550's Stainless steel  Linkage Rod A has a slightly larger diameter - 2 mm (actual 2.2mm) vs. 1.6 mm on the original rod. I didn't use the motor of my electric drill - I just placed the drill bit in the chuck and rotate it manually, very slowly, while making sure I drill straight along the axis, until the bit bottoms out. There was very little material that was actually removed.
To thread the new 550 linkage rods that came with the kit, I had to cut a new thread in the aluminum arm. The main problem is that the steel Linkage rod A has a square thread (it is designed to normally go into plastic link ends on both sides), not the regular screw type of thread, so using a standard 2 mm tap is of very little help (it did next to nothing).  What I ended up doing is to use one of the extra linkage rods from the kit as a tap by holding it firmly with pliers and working it in - rocking it back and forth. The rods are made from much harder stainless steel and the arm is soft aluminum so it is not that difficult to make the new thread.
Because I had to hold the rod for one of the threaded portions with my pliers I just marked it for future use as "tap" and didn't use it for actual linkage.
I took a new linkage rod, applied some RED (high-strength) thread locker and threaded the rod inside the arm until it bottomed out. All of the threaded portion of the rod should sink in.
It is very important to clean all metal shavings and debris from the hole beforehand.  If the rod is too loose because of sloppy drilling -  CA glue or even better JB Weld metal epoxy can be used instead of thread locker to permanently fix the rod - the link arm side of the rod does not need to be adjusted so permanent fix is actually a better solution.
After a few hours for the thread lock to cure, I just screwed onto the rod ends the heavy duty 550 plastic link ends that came in the package - they have exactly the same ID and fit as the standard ones for the 450 series but are much stronger with more plastic material on them. Furthermore, the rod screws much deeper into the plastic and it will require way more force to be pulled off across the thread.
The plastic link ends screw almost all the way down against the aluminum arm when setting up for 0 degree blade pitch. (on both - my 450 PRO DFC and Sport V2 DFC I had aprox. 1.5 mm gap exposing the smooth center portion of the rod - seen on the picture)
This mod is not a must if you are not a 3D pilot but at least should put some peace in your mind by taking care of a known weak point in the DFC setup.

Spektrum AR7200BX antenna repair / antenna extension

Here is a tip for the RC heli crowd on how to repair or extend the antennas of the popular AR7200BX flybarless controller.
AR7200BX is pretty much a Microbeast BeastX flybarless controller bundled with a 7 channels 2.4 GHz Spektrum DSM2/DSMX receiver. This setup is really nice because it saves a lot of wiring and cable management for those flying with Spektrum / JR radios, not to mention it saves space and weight too.
The receiver has two antennas in order to address possible polarization issues. In the GHz range proper polarization is very important and on a RC heli it is a challenge to maintain consistent Tx-Rx antenna polarization. Often, the only instance when the Heli has assumed a normal orientation (blades up, skids down)  is just before take off ( 3D pilots know exactly what i am talking about). The two antennas must be oriented in a way to cover at least two different spatial planes (X and Y and ideally, the third plane Z as well via a satellite receiver). The two AR7200BX antennas are normally placed at 90 degrees to each other. An important condition is that the antennas have to clear the carbon fiber frame (which is conductive and "lossy" when it comes to radio-waves) and at the same time stick out sufficiently so the antennas are not "radio-shadowed" by the Heli's fuselage and other parts.

I made this antenna mount for my 450 PRO using a plastic straw (salvaged from a compressed air can), a cable tie and a cable tie mount (the double-sided self-adhesive foam was removed from the mount and replaced with a single-side adhesive foam so the mount does not adhere to the FBL controller - it is held in place by the Velcro strap). It works great to maintain a good 90 degrees polarization difference. The thin, grey, 1.13 mm antenna coax can be seen, exiting the black grommet of the FBL controller.
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The two antennas are made out of a miniature coaxial cable (1.13 mm OD) and are long 110 mm and 40 mm respectively. The actual antenna element is the very end portion of the coax, where the outer jacket and the coax shield braid have been removed, leaving exposed approximately 31 mm (1/4 wavelength @ 2.4 GHz) of the insulated, center conductor only.
As per Murphy's law: one needs just a few more centimeters extra coax to clear the Heli's frame or other parts.
 Another potential problem is that these antennas can get damaged fairly easy in a crash or just by being scraped during flight by the sharp carbon fiber frame edges - such a thin coax is very fragile.

Here is my Align Trex 450 Sport V2 DFC. The AR7200BX is mounted inside the frame on the gyro plate. I wanted to have the short antenna pointing vertically downwards but alongside the plastic skid frame. This way, the frame will act as a mechanical shield protecting the antenna whip. Unfortunately, I need about 10 mm more coax on the short antenna to really clear the heli's CF frame. Another good antenna location is behind the anti-rotation bracket of the swashplate but the coax is way too short to reach there too.

After some investigation, here is what I found out: the AR7200BX is using standard IPX connector in the cable assembly for both antennas

This image is courtesy of Helifreak member sup77095. It shows both miniature IPEX / IPX  coax connectors on the AR7200BX receiver board.

As it turns out, IPEX / IPX cable assemblies are "dirt cheap" - just search on eBay for "IPX cable". They are used as interconnects for many WiFi devices, inside laptops, cell phones, etc and always come in the form of a "pigtail" (ready-made cable assembly with connectors installed).
I got two cables, completed with the connectors for just under $4 (free shipping). 
To make an antenna, just measure and cut the overall length needed for an extended antenna or repair (including the proper connector on one side and add an extra 1/4" of cable. Very carefully (!), using a sharp blade, strip only and remove the outer insulation of coax shield about 31 mm from the end. Carefully, using a needle, un-braid the exposed coax shield and trim it down to where the outer insulation begins.
(!) Be very careful not to damage the center conductor and the Teflon insulator around it - it is very easy to nick the Teflon insulation and then when bent, it will break off.
Re-measure and if needed trim down the center conductor - the goal is to have exactly 31 mm of insulated center conductor with no coax shield around it.
(Probably not need, but I'll mention anyway that such modification will void the warranty on the FBL unit and it is mostly for the brave ones)
The coaxial loss is ~3.1dB/m @ 2.4GHz or 0.031 dB per centimeter. One needs to optimize the length to the absolutely minimum needed to avoid signal strength issues in the receiver but generally up to 10 cm extension for the short antenna should be OK. 
If you fly your heli as a "dot in the sky", installing a satellite receiver is recommended anyway. 
Remember to perform Radio RANGE CHECK after doing any antenna work on your heli.