Servo Frequency Question

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Servo Frequency Question

Postby synapse » Thu Feb 14, 2013 5:21 pm

This question has to do with the FlySky FS-it4 and the Servos Frequency
controls provided with that radio.

The pic displays the available frequency settings (default is 50Hz
and that option is not visible as it is scrolled up above in this view).

The Help screen in the FS-iT4 describes these optional settings as "refresh

Isn't it true that refresh rate is the same as "framerate"?


The FS-iT4 Help screen also instructs the user to refer to the specs on
a specific servo to find out the best setting to choose.

When I look at the specs on a digital servo like a HiTec HS-7555mg, there
is no frequency listed - not that I can find.


To add to my confusion, when I search HiTec's site for info on servo
frequencies, I came across info such as this:

>Q. What is the difference between an analog and digital servo?
>A. The difference is in how the servo motor is controlled via the circuit board
>(amplifier). The motor of an analog servo receives a signal from the amplifier 30
>times a second or at 30Hz. This signal allows the amplifier to update the motor
>position. Digital servos use a high frequency amplifier that updates the servo motor
>position 300 times a second or at 300Hz. By updating the motor position more often, the
>digital servo can deliver full torque from the beginning of movement and increases the
>holding power of the servo. The quick refresh also allows the digital servo to have a
>tighter deadband.

After I read this explanation and then try to make sense out of the FS-iT4's
Servos Frequency control, I think I am missing some important piece of information.
What is it???

Is there some range of frequencies being used to create the signal from the Rx to the
servo? Is this frequency different from the 30Hz vs 300Hz used by analogs and digitals
between the amplifier and the motor?

If it will help to provide a link to the FS-iT4 Instruction Manual, a copy of the
.pdf can be DL'ed at:


P.S. Could not upload a pic due to:
"Sorry, the board attachment quota has been reached."
The link to the Instruction Manual will be the only way to review the FS-iT4's Servos
Frequency controls.
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Re: Servo Frequency Question

Postby BillGriffiths100 » Thu Feb 14, 2013 8:52 pm

The setting of 50Hz represents the traditional frame rate of 20mS which is fine for analogue servos.
Higher rates will be suitable for digital servos.
Digital servos do not really require a higher frame rate the 300Hz drive to the motor comes from their drive microprocessor not the receiver.
A higher frame rate can be used to get a theoretical improvement in speed and resolution but you must not mix analogue and digital servos, at a higher frame rate nasty things will happen to the analogue servos.
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Re: Servo Frequency Question

Postby synapse » Thu Feb 14, 2013 10:20 pm

Thank you. Your answer is informative.

If you have no objection, I would like to repost your answer on Jim's RC Boat Dock
forum ( Several subscribers have been evaluating the FS-iT4 for
use in 1/4 scale gas boats. You will be respectfully quoted with credit.

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Location: CO, USA

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