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Skookum Robotics SK-720BE Teardown

The SK-720 Hardware Overview

Over the past couple of years I've taken a large interest in radio controlled single rotor
helicopters. One of the many FBL (Flybarless controllers) on the market is the Skookum
Robotics SK-720. This unit was released about 4 years ago in 2009 to much fanfare. It promised
great self leveling bailout, as well as a future GPS attachment that finally came earlier this
year. Overall the 720 was a success, but didn't typically favor well compared to popular controllers like the
Mikado VBar. This all changed with the release of the SK-540, which finally put the company on the map
as a genuine competition grade competitor. The new SK-720BE should be no exception.

The 720 was in long need of an upgrade, however what annoyed me most was
the lack of any real information on how it was being upgraded. The BE was recently released and
I managed to get my hands on 2. After waiting a few weeks for spec details, I eventually had enough.
What you are about to see is probably the first known pictures of SK-720BE insides. Sexy.
So like Dave Jones of the EEVBlog always says; Don't turn it on, take it apart!

SK-720BE

Upon external visual inspection, there really isn't much difference apart from
the metal casing and slightly differing spacing on the servo bus. Upon opening both units
however the difference was immediately clear.

SK-720BE

SK-720BE

Redesigned and upgraded

The location of the gyroscopes were moved to the top board. The most obvious change
was the move to Silicon Sensing's Pinpoint gyroscopes. A much welcome improvement.
Extremely noticeable in flight, even my 450 Pro feels like its floating on rails now.
The SK Blue uses an integrated 2-axis InvenSense IDG500 for the FBL gyroscopes and a
single axis Analog Devices ADXRS652 for the tail gyroscope.

Fixed self-leveling drift?

Apparently there is a misnomer with the self-leveling drift issues that the accelerometer
is the cause of this drift, or drifting of said accelerometer rather. However Art of
Skookum Robotics was quoted stating that the new gyroscope modules should be basically
drift-free which will help the self leveling function drastically and pretty much eliminate
any self-leveling drift. I'm not exactly sure why only now this information about the
self leveling drifting was caused by the gyroscopes themselves, when all along everybody
thought it was a fault with the accelerometer drift. Lets hope it holds true.

SK-720BE

This ain't no Wii remote

Which brings us to the accelerometer chip itself. The original SK-720 and BE both use the
same $4 Analog Devices ADXL345 3-axis accelerometer. It is located on the bottom PCB like
the original. I personally would had thought this would had been upgraded to something a
lot newer. I'm not entirely sure if the BE would benefit from better units, but if so
something from Silicon Sensing would be a godsend, however the BE would then cost us about
$700 at retail. Which it should then handle any vibrations you could throw at it, say from
a gasser and still self-level with ease. Maybe a SK-720GE in the future with a green case
called the Gasser/Gemini Edition? LOL. Not likely.

So if Art (of Skookum Robotics) is indeed correct that the accelerometers merely assist
the gyros in providing accurate self-leveling data, then such high-end accelerometers would
be unnecessary. Time will tell of course.

The SoC (System on a chip)

SK-720BE

According to the specs of each, there isn't really that big of a deal of difference between
the Blue and BE's system on a chip featurewise, other than a major change in the ARM core from ARM7 to a Cortex M3,
and most likely a clock speed bump with it. Both actually support USB 2.0, which begs the question
why the 720 Blue and BE both keep the USB bus limited to 12Mbps 1.1 speed when reading and writing
to the internal SD card. The BE also appears to have more higher speed digital i/o pins than the Blue.
There are many other unused features within both of these units, although the
original 720 Blue has more different unused bus types on its processor. The BE's USB bus is also more
capable with full host and OTG support, which will most likely go unused which is sad given its
potential usage with the SK-720. The BE's SoC also has some thermal paste mating with the top
of the case to assist in cooling, not that I've ever seen the unit get warm however. To make a
quick comparison I would say that the original Blue unit runs warmer than the BE after being left on a while.

See:
BE: NXP LPC1768FBD100
Blue: NXP LPC2368FBD100

Other things of note include an identical power supply regulator setup, although there appears
to be an H8601 dual n-channel mosfet located on the USB input supply on the BE, most likely
to protect it from any nasty USB supply voltages or whatnot. (might have been many issues with
that on the Blue)

SK-720BE

SK720:

NXP LPC2368FBD100 (System on a Chip w/ ARM7TDMI CPU)

SK-720BE

Analog Devices ADXRS652 250 degrees/sec yaw rate Gyroscope (z-axis/yaw/tail, the square ceramic/metal package above)
Curiously, another SK-720 I have opened has an ADXRS610, which is a better unit at 300 degrees/sec. Apparently its EOL, so likely why
they used the 652 instead. These units also have an onboard temperature sensor, almost certainly used for drift compensation.

Analog Devices ADXL345 (345B) Accelerometer (the long rectanguler chip directly left of the 10 pin header above)

Analog Devices 78H90 12 Bit A/D converter (500 kilosamples/sec, its the larger chip below the ADXRS gyroscope)
Since the output of the ADXRS yaw/tail gyro is purely analog, we obviously need to convert it to digital signaling.

InvenSense IDG500 2-axis Gyroscope (x-y axis, its the chip directly to the right of the ADXRS gyroscope)
This is the chip that is said to drift over time with temperature and/or shock, which is the cause of the self-leveling drift.

IDG500
"Applications:
High performance motion sensing game controllers
Pointing devices, multimedia remotes, & computer mice"
Probably not too suited for precision GPS navigation of devices that control flying lawnmowers, heh...
For comparison, the Wii's Motion Plus attachment uses an IDG600.

SK720BE:

NXP LPC1768FBD100 (System on a Chip w/ ARM Cortex M3)

SK-720BE

Silicon Sensing CRM202 x2 Single Axis Gyroscope (x-y axis)

SK-720BE

SK-720BE

Silicon Sensing CRM102 x1 Single Axis Gyroscope (z-axis tail gyro)
Unlike the 720 Blue, we actually have precision navigation rated devices here.

SK-720BE

Analog Devices ADXL345 (345B) Accelerometer (on the BE this chip is on the blue colored PCB on the bottom left)

Overall, the BE is indeed far superior, there isn't any comparison simply due to the usage of
the 3 pinpoint gyroscopes alone. Its really amazing how much they can pack into this tiny case,
never alone the features they manage to pack into a mere 512KB of programmable flash rom space.
Some questions need to be answered about the Accelerometers however, as the general consensus
has been that the BE has a better Accelerometer chip, but that doesn't appear to be the case here.
Overall I doubt its an issue if indeed the gyros were the cause of SK Blue self-leveling drift to begin with.
The new pinpoint gyros should clean it right up, and initial SL flight tests of the BE are proving this to be the case...


Last Modified: August 25 2013 14:18