The AEORC S1 Dr Servo is a tiny box using the same packaging as the clever
kV meter. It consists of a servo driver and an input for an
accelerometer that you attach to the servo’s output disk/arm. That input
measures the pulse position so is also used to test receiver servo outputs.
It also requires an external 8-12V DC supply. They suggest a 2S LiPo, but
after using it I would recommend a 3S. The lower voltage just does not seem
to have quite enough headroom to supply the 5V to drive the servo at higher
currents. Most folks will probably be using the BEC from their ESC to drive
the servos in any case – see the three way cable description later.
The inside of the accelerometer is just a sealed unit
with a part number that I could not raise on the internet with an external
diode and resistor. I suspect it is the same unit used in single axis piezo
helicopter gyros. AEORC call the device a ”gyro” and I’ll do the same for
simplicity. The only control is one of those 4-way joysticks with a centre
push that you find on every compact digital camera. Long push to move
between the 4 screens; up/down/sideways to select a value if appropriate;
short push to turn the backlight on and off or start a test. In my view it
would have been more logical to make the short push for menu change and long
push for backlight/start. The backlight keeps going on and off when you
don’t want it to and on the other hand you have to dwell on a screen to
change to the next one. This results in an unwanted test starting sometimes.
No big deal, and doesn’t detract from the functionality - just an
In use, you just plug in the external battery, connect
the servo and gyro and stick the gyro to the servo output disk or arm with
foam tape. Alternatively you can use the gyro input to test transmitter
stick range and centring. There are 4 functions on the tester:
1. Display of
receiver output signal pulse width to the servo channels in thousandths of
mS with a default neutral of 1.500mS (handy to very conveniently check how
consistent the range and neutral your transmitter channels are). A
servo-plug to servo-plug cable is supplied for this and you connect the gyro
channel into each receiver output in turn. The supplied connectors are
Futaba style and the ridges have to be cut off to allow the plugs to line up
along the edge of the S1. Minor, but irritating. This menu must also be used
to set the signal pulse width to give a 60 degree rotation (since the pulse
length to do that may vary from servo to servo).
2. A cycle
test that logs the number of repeating input cycles together with the number
of output cycles recorded by the gyro (good for testing suspect servos). It
reads up to 9999999999 cycles which for a 0.1sec servo would be a total test
time of about 31 years if I have done the math correctly!
of servo travel linearity, which is really no more than setting up 2 or 3
servos side by side and comparing them as they are driven at the same time.
of servo speed, which is the uniquely useful and important function of this
To measure servo speed, you first need to tell the S1
what pulse width gives 60 degrees rotation. It’s remarkably simple to
position the servo over the protractor they have printed in the manual and
use the joystick to move the servo from the neutral position. When the holes
in the horn line up at 60 degrees, you just read off the pulse width,
subtract the neutral value and adjust it on the measurement screen with the
joystick. It turned out to be much easier and more consistent than I
expected. My guess is it is accurate to a couple of degrees if you are
careful to align your eye with the shaft of the servo on the baseline.
Obsessive can set up a more accurate rig to measure rotation precisely. Then
you just push the button, the servos cycles a couple of times and the time
to move 60 degrees is displayed in 1/100th’s of mS (more on accuracy later).
You can do 10 readings in as many seconds. If the servo doesn’t respond, a
long push upwards on the joystick reverses the signal – neat.
This is the simple way, and uses an internal regulator in
the S1 to provide 5V at a max of 1 Amp to the servo. If you want to measure
at the “standard” 4.8V then use any of the servo connectors on the supplied
three-way splitter lead which then plugs into the S1. See attached picture TestSetup.jpg.
The lead contains a 220µF capacitor to provide some reserve for peak current
draw and a forward connected series Schottky diode in the main lead which
drops the 5V to almost exactly 4.8V and protects the S1 from external
voltages from the branch leads at the same time. Very simple and clever. If
you want to measure at a different voltage, say 6V or use your ESC’s BEC,
then just plug an external supply into one of the three servo connectors.
Results for AEORC S1 Dr Servo
Initially, no servo worked for me without an external
battery supply. Changing the 2S 1000mAh battery supply to a 9V 2.5A power
supply made no difference. I measured 0.46V at the servo supply connector.
AEORC were helpful and responsive but in the end I discovered the Schottky
diode in the harness was assembled in reverse (it’s very hard to spot the
band on the diode). Once fixed, everything worked as expected. Be worth
checking if you have the same problem.
The sensor weighs 9.5g so can be used on quite small
servos. There is no simple way to calculate what the inertial load of the
sensor is on the servo of course so it isn’t quite “unloaded”. Here are a
few sample results averaged over 6 cycles.
Using a stabilized Lab 2.5A supply:
See Table 1
Using a 40A ESC with 4S 5000mAh LiPo: BEC voltage
measured at 5.16V for same servos as above.
See Table 2
Using without any external supply gives approx 4.85V at
Results without external supply for 3x Bluebird
BMS-375DMG – 11.4g.
Claimed speed at 4.8V = 130mS. Three identical
out-of-the-box, unused servos.
See Table 3
For fun: Some old servos of late 70’s vintage
See Table 4
The Orbits were the ultimate small servo at the time,
have served me well, and cost a fortune. They still hold up not too badly 30
years later! The Charlie’s (Cannon in kit form) were the smallest servo
available and a feat in their day – but look at the response speed.
Like many digital display devices this one reads out to
unrealistic precision. The specs say accuracy is 1µS +/- 10%. “10% of what?”
is the issue. Times in µS are reasonable for measurement of signal pulse
width but not servo movement. There is no information on precisely how this
gadget works but I assume it just measures the time between start of
acceleration and end of deceleration. There is going to be some imprecision
about this no matter how good the timing circuits are. I would suggest a
basic accuracy of perhaps 1 mS for servo traverse is both reasonable and
entirely adequate, not the 10 µS that is displayed. I found variations over
10 cycles of less than 3 mS with a new Hitec servo on a 6V supply so I’m
assuming the S1 itself is probably good for at least 1 mS accuracy and
anything more is servo variation. In other words I think what the specs mean
is the READOUT is good to 1 microsecond not the accuracy. In any event this
is more than adequate. It’s pretty clear from the numbers above that the
variation between cycles and between individual servos of the same type is
significant. In general the better the quality of the servo the less
variation. Those old 70’s Charlies servos I built from a kit were a wonder
in their day but a HobbyKing $2.99 special leaves them for dead!
I did quite a number of tests on different servos using a
variety of power supplies and I came to the following conclusions:
This is a uniquely useful device. There are other servos
testers of varying complexity but all the ones I have seen are really just
servo drivers. I am unaware of any other low cost device that actually
measures the servo cycle speed. Be interested if anyone knows of another.
It is low cost and simple to use. Once you get the hang
of it you can test a bunch of servos in a few minutes. I found surprising
variation between “identical” servos and also between specified and actual
performance – particularly at the low-cost generic end of the spectrum.
I’m not into helis but I know there is a great need for
speed in the tail rotor servo in particular. This would seem to be a good