Digital servos

Someone on another forum I frequent posed the question " are digital servos worth the extra cost compared with their analogue counterparts?" Then someone else posted this useful link to a PDF file which explains the differences http://www.futaba-rc.com/servos/digitalservos.pdf I found it interesting hope you do too

Useful info, nice quick guide.

The screeching noise from digital servos in bigger models is so dfistinctive. What makes the noise?

Im not sure thats the servos, I always assumed it was the control surface/hinge line/airflow pressure thingy ??

It's the motor that makes the sound. to keep it short, it is the sound of the motor being switched on and off very quickly by the control electronics, to maintain the position demanded....

Andy

I think the noise Paul means is the screech found on composite models which is air being forced over the open end of the composite ailerons on high deflections, you still get it with analogue servos

Yahooo - I was nearly right...more righter than Andy for sure

I blame an ambiguous question, and the thread starting off being about digital servos........

The whisltely noise you get on composite aircraft is mainly due to the hollowness of the wings around the hingeline. Because most composite hinges are "skin" hinges, or "live" hinges, where the wingskin itself acts as the hinge (Usually on the top surface of the wing), when the control surface is deflected upwards, the bottom wingskin is opened up to the airflow. The opening and chamber now act exactly the same as a whistle, resonating at a frequency that is dependant on the size of the chamber (and a few other minor things).

Smaller composite models do it as well, it is just that the smaller sized chamber has a much higher resonant frequency, and therefore the whistle pitch is much higher pitched. Also the smaller size is not as loud....

The fact that the wings are usually hollow moulded, means they alos act as giant soundboxes, effectively amplifying the sound, which makes it so noticeable on the bigger models.

Andy

Thanks Andy, I think your first explanation was nearly right. I wasn't thinking of composites and the whistling musical wings you describe, but the the bigger built up models I've seen at the shows which squeal violently at every flick and twist.
I did find though this web page just now
http://www.servocity.com/html/digital_servo_faq.html

In particular it shows
Are there disadvantages to digital servos?
A-1. If there is any disadvantage to using digital servos it is power consumption. To obtain the high standing torque and quick response, digital servos require more from the power source than standard servos. By using a larger capacity battery and heavy duty harnesses, you will ensure proper servo performance.

A-2. Digital servos also create a high frequency pitch or squeal.
That's nearer to being the answer that matches the question I arsked.(posh for asked)

I was also surprised to find that conventional pots and brushed motors are used in digital servos. It's the Mosfet and high speed processor that makes all the the difference.

PDQ wrote:
I was also surprised to find that conventional pots and brushed motors are used in digital servos. It's the Mosfet and high speed processor that makes all the the difference.

That's the important bit. Just because servos are "digital" doesn't automatically mean they are good. Generally, you get what you pay for with servos, and there are old style analogue servos out there, that are much better quality (and performance) than some digitals. It's about the whole package, not just the control electronics inside

Speaking of conventional pots, Hitec have got an encoder based servo listed on their website now. Which is interesting (and bound to be damn expensive!)....

Andy

I guess your talking about the one with the magnetic position sensor. IMHO its about time the historical servo cct was scraped and the digital positional info sent by the Tx (as a digital number) was used directly and not converted back to a pulse width time devision multiplexed information stream. 'Discuss'

I have come across just about every type of digital servo of all types at shows and comps and have yet to hear one emit a screeching noise audible from the ground, the only such noises I've heard are from the ailerons of composite models be they gliders or power models. Richie's Yak does it as does my Wasabi glider

You can hear the wing servos chuntering away on my big glider on a slow and low pass.... That's with the motor off mind....

Andy

Ah but chuntering is not screaming like the noise under discussion.

That's only because the motor is off though. Motor off, they chunter with content, but they start screaming in pain when I open the throttle.....

Andy

Come to mention it, I dont think I have ever heard a chunter screaming anyway - which raises the question.... can they scream at all? - or indeed just a few
I better go now.