This is where you can try your hand at a bit of journalism if
you feel like writing up the construction of a kit or reviewing some new gadget
that you've tried and tested. Articles about any aspect of model-making,
accessories or general interest relating to model flight will be welcome. We're
not necessarily looking for hi-tech stuff either, but good, practical and
easy-to-understand items that will be of interest and value to the majority of
us!
Our first item is a sober one, but something we all need to
take careful note of if the integrity of our hobby is to remain intact. Our
second topic is a bit technical at first sight but is, in fact, a user-friendly
item reproduced by kind permission of Ripmax. It is a fact file
about Futaba Digital Servos and hopefully will help you understand what this
comparatively new technology in the servo field is all about.
more safety recommendations
Further to the advice given by the BMFA
concerning the use of Failsafe mode on PCM transmitters which appeared in our
last issue, the BMFA safety committee has now also addressed the question of
transmitter control at the flying field in order to try and avoid the
possibility of two transmitters being used on the same frequency, and they urge
all clubs to give the matter the same attention they have evidently been giving
the Failsafe issue. The following edited comments have been taken from the BMFA
Safety Bulletin which has just been published in the February 2000 issue of BMFA
News.
The most traditional system of transmitter
control seems to be the 'Peg Off' pegboard, where the user is required to remove
a frequency peg or marker to indicate that the frequency is in use. However,
once the peg is removed no indication is given as to who is actually using the
frequency and another flaw in this sytem is the removal of pegs from the flying
site by people forgetting to put them back on the board when they leave. This
normally results in a duplicate peg being produced and if the original returns,
you then have two pegs for the same channel.
The second most favoured system in use is the
'Peg On' pegboard in which the user is required to display an indicator on a
pegboard to denote that he or she is using a given frequency. This system will
eliminate the need for more than one peg to exist and overcomes the problem of
pegs disappearing home at the end of the session. If the system requires the
marker to carry some identification of the owner and this requirement is adhered
to, this system can be very good indeed, says the BMFA.
The BMFA's preferred system for suitable sites is
a combination of the two systems described above. They say that if your
infrastructure and site allows you to easily use this sytem, you should
seriously consider it. In this system, frequency markers exist as with the 'Peg
Off' system but when they are removed from the board, hook or whatever, they are
replaced with an indicator of a distinctly different design bearing the
identification of the person using the frequency. The psychological reassurance
of being in 'control' of the peg in the 'Peg Off' system is considered to be
very strong and thus it is retained with this system. If a peg does go home with
the user, they will probably have left their own named marker in its place. A
new indicator can be made when it is clear that the original is irrecoverable,
and a temporary 'Missing Peg' indicator can be displayed if the original is
expected back. A simple way to avoid the markers going home with the user is to
remove the requirement for them to be displayed on the user's transmitter or
placed in his/her pocket. Users can be requested to place the marker alongside
their flight box in their personal pit area or even on the flightline. If the
frequency is cleared after every flight, which it should be, the marker will be
back on the board anyway. [12.02.00]
Futaba digital
FET servos
the significant operational advantages of a
digital servo
OVER THE LAST FEW YEARS, servos have changed
tremendously, with size, rotational speeds and torque ever improving. The latest
development, known as the 'digital servo', is yet another step forward. Digital
servos have significant operational advantages over standard servos, even
coreless versions, but with these advantages also come minor disadvantages, and
this fact file will try, in simplified terms, to explain the positives and
negatives of digital servos. It will also dispel some myths.
The
standard servo (left) has custom logic chip and timing
components with standard 30-strand lead.
The digital servo (right) has a Quartz crystal controlled
microprocessor, FET amplifier and heavy-duty 50-strand lead.
To start with, a digital servo is the same as a
standard servo, except for a microprocessor which analyses the incoming receiver
signals and controls the motor. It is incorrect to believe that digital servos
differ drastically in physical design to standard ones. Digital servos have the
same motors, gears and cases as standard servos and they also, most importantly,
have a Feedback Potentiometer (Pot) just like their standard counterparts.
Where a digital servo differs, is in the way it
processes the incoming receiver information, and in turn controls the initial
power to the servomotor, reducing the 'deadband' - explained below - increasing
the resolution and generating tremendous holding power.
In a conventional servo at idle, no power is
being sent to the servomotor. When a signal is then received for the servo to
move, or pressure is applied to the output arm, the servo responds by sending
power/voltage to the servomotor. This power, which is in fact the maximum
voltage, is pulsed or switched On/Off at a fixed rate of 50 cycles per second,
creating small 'blips' of power. By increasing the length of each pulse/blip of
power, a speed controller effect is created, until full power/voltage is applied
to the motor, accelerating the servo arm towards its new position.
In turn, as the servo positioning pot tells the
servo's electronics it is reaching its required position, the power blips are
reduced in length to slow it down, until no power is supplied and the servomotor
stops.
As you can imagine, a quick 'blip' of power 'On',
followed by a pause, does not give the motor much incentive to turn, whereas
leaving the power 'On' for a longer period of time does. This means that a small
control movement, which in turn sends small initial pulses to the motor, is
very ineffective, and that is why there is what is termed a 'deadband', i.e.,
sluggish or virtually no movement around the centre of a standard servo, in
relation to a small transmitter stick movement.
the distinct advantages of a digital servo
First, it is able, via its microprocessor, to
receive the incoming signal and apply preset parameters to that signal before
sending its pulses of power to the servomotor. This means the length of the
power pulse/blip, and therefore the amount of power sent out to activate the
motor, can be adjusted by the microprocessor's program to match its function
requirements and therefore optimize the servo's performance.
Second, is that a digital servo sends pulses to
the motor at a significantly higher frequency. This means that, as opposed to
the motor receiving 50 pulses/second, it now receives 300. Although the length
of the pulses is reduced in a direct ratio to the higher frequency, because the
power is being turned on/off more frequently, the motor has more incentive to
turn. This also means that not only does the servomotor respond faster to the
commands, increases or decreases in power for acceleration/deceleration are able
to be transmitted to the servomotor far more frequently. This gives a digital
servo an improved deadband, a faster response, quicker and smoother
acceleration/deceleration, and better resolution and holding power.
just one disadvantage
The downside to these significant advantages -
'well, there's got to be one' - is power consumption. Naturally, with power
being transmitted to the servomotor more frequently, together with increases in
power being supplied to the motor earlier, the overall power consumption must go
up.
However, with batteries in general gaining
monthly in capacity for the same size and weight, increased current drain as a
trade off for significantly better performance is no longer a problem. The key
point to remember with digital servos is to instal the largest capacity battery
that space/weight will allow. Always instal a battery monitor to check the
operational capacity and, wherever possible, top up the charge before flight,
just to be sure.
Digital servos are the future for model control,
and everyone who has used them says the difference is so significant that they
would never return to standard servos if there is a digital one available to fit
the application. To quote turbine display pilot Steve Elias, 'Digital servo
response and precision is like flying on rails. After flying digital servos,
analogue versions are like controlling custard'.
So, Futaba tells us, if we need
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