Whats needed to run digi servos?

[robgt]

My understanding from Futaba is that both types of servos use the same external interface

The plug, yes.

its the internal control pulses sent to the motor that are of different width

Exactly.

You seem to know the answer already, but still aren't grapsing it?
I'm well confused.
Cheers,
Rob

[matthew2456]

The plug, yes.


You seem to know the answer already, but still aren't grapsing it?
Rob

I meant electronically not mechanically.


It's confusing because people have been trying to explain things that they do not fully understand.

From here, http://www.futaba-rc.com/servos/digitalservos.pdf , it clearly states that "digital" servos send a higher frequency signal to the motor (irrelevant of the signal to the servo (RX/Gyro) which I am talking about).

What nobody has actually backed up with any evidence yet is the difference between driving a servo from an rx or a gyro with digital mode.

[moyesboy]

With an analogue servo when you push the servo away from where it should be the torque to the motor gradually increases to try and return the servo to the desired position. so you don't get the full torque until you have some noticeable position error.

With a digital servo if you try to move it away from where it wants to be, as soon as the error is greater than its programmed acceptable dead band, it puts full power on the motor to restore the position. This leads to the servos buzzing at rest, and when you pick the model up.

That may not be precisely how it works but to you and me that is the basic difference.

So the digital servo is more precise, the downside is it uses more power and if you work it hard it tends to get fairly hot.

Regarding the digital setting on the 401.
Many digital servos are capable of responding to a higher frequency of pulses than what is given out as std by the receiver. Thus if you use a digital tail servo with the 401 you can switch the digital function on and it gets more performance out of the servo than the receiver could. The CSM gyros do the same thing - they call it super servo support.

Use a normal servo on super servo (401 DS) setting and it could well burn the servo out.

There are also dedicated digital tail servos that are matched to gyros and will not work directly off the receiver - such as the 9256 and BLS251 that will only work with the GY611 or gyro with similar output signal (A 401 will not drive a 9256, but the spartan760, csm720 will).

hope that answers your questions.

[robgt]

It's hard to understand what the question is - seems to go around the houses??

the difference between driving a servo from an rx or a gyro with digital mode

You can plug any type of servo into a receiver, analogue or digital, and it will work fine. Digital servos have a higher resolution to them making them more accurate.
A gyro will send very high frequency pulses to a servo in digital mode - which can burn out an analogue servo. If the gyro can be set to non-digital, then it will send it's pulses to the servo at a lower rate that will not kill the servo.
This means that in non-digital mode you lose resolution on the rudder servo (driven by the gyro, not the receiver) making it less effective.

What sort of "evidence" are you looking for?
Cheers,
Rob

[marktigere1]

It's confusing because people have been trying to explain things that they do not fully understand.

Thanks for that.

[matthew2456]

Do you lose performance from a digi servo when driven direct from the rx?

[robgt]

Lose performance compared to what?
Cheers,
Rob

[matthew2456]

High speed mode on a 401?

[robgt]

If you drive the rudder servo direct from the receiver, the gyro doesn't have any input on it's performance.
Cheers,
Rob

[Ashley Davis]

Rx drive servos at the default speed of 250 FPS. So 250 pulses of positioning data are sent to the servo every second. All servos are capable of running at this speed, including digital ones.

Some digital servos are capable of running at 285 FPS and some 333 FPS. In this instance the servo is updated on where it should be much more quickly. RX do not drive servos at this frame rate and are not switchable to this frame rate. Gyros on the other hand do support this.

So in answer to your question, when you connect a digital servo to an RX it is driven at 250 FPS, ie. it is told to update it's servo position 250 times every second. When you connect a digital servo to a gyro AND that servo supports a higher frame rate AND that gyro supports a higher frame rate then you can get better performance. This better performance is based on the fact that the servo is being told to update it's position more quickly due to the higher frame rate. This is only really useful on a gyro where the gyro can tell the servo automatically to update it's position very very quickly in reaction to torque changes etc etc. On an RX (cyclic) this is pointless as the servo is capable of updating itself far more quickly (250 FPS) than a human would be able to notice differences in heli attitude and command a servo response.

[Andy from Sandy]

Which receivers drive the servos at 250 frames per second? A PPM RX only drives them at 50 fps, as the tx sends the channels and then a pause every 20 milli seconds if I remember correctly.

[Ashley Davis]

Andy, you may be right, it may only be 50 FPS, either way the argument is exactly the same, you only gain benefit of faster frame rates on an automated device like the gyro. Others have highlighted the benfits of the digital servo being it's accuracy and reluctance to move from a commanded position rather than frame rates (in a cyclic usage situation)

[Andy from Sandy]

This looks a nice little article explaining differences between digital and analogue servos.

www.futaba-rc.com/servos/digitalservos.pdf

[robgt]

This was referenced earlier in the thread
Heh heh

Cheers,
Rob

[Andy from Sandy]

And if ever you are lost for what servo to choose here is a short list that might help you out!!

http://www.fatlion.com/sailplanes/servochart.html