Re: Honeywell KFC225 servos - operation Q

[Rich Grise]

On Wed, 09 Mar 2005 08:33:05 +0000, Peter wrote:
....
> However, superimposed onto the DC control signal (-10V to +10V or so)
> I saw a sinewave ripple, around 500Hz IIRC. The amplitude was only
> about 100mV P-P.
>
> The question is whether this is intentional, whether e.g. the servo
> uses the presence of this ripple as a confirmation that the computer
> unit is still (partially) functioning. It would be quite a clever
> trick, but it does mean that it isn't possible to fit a good filter
> onto the servo input.

It could very well be intentional - I once worked with a servo system
where they intentionally superimposed a triangle wave on the control V -
they called it "dither". It kept the hydraulic actuator from sticking in
place, so it could move smoothly.

Hope This Helps!
Rich

[Ron Natalie]

Rich Grise wrote:

>
> It could very well be intentional - I once worked with a servo system
> where they intentionally superimposed a triangle wave on the control V -
> they called it "dither". It kept the hydraulic actuator from sticking in
> place, so it could move smoothly.

Dither lets it respond more quickly. It's a common mechanical actuator
trick.

[[email protected]]

I'm sure this frequency is way beyond the bandwith of the motor
drivers.
Sounds like the switching freq on he KFC power supply.

Dithering would be a couple of orders of maginitude lower freq.

I don't think it will negatively affect anything. Could even come
from some sort of ground loop.

Bill Hale

[John Woodgate]

I read in sci.electronics.design that Peter > wrote
(in >) about 'Honeywell
KFC225 servos - operation Q', on Tue, 15 Mar 2005:

>I was just hoping somebody on here would be familiar with the design of
>this servo and would tell me (anonymously of course :)) whether the
>servo actually uses it for anything.
>
>The reason I'd like to know it is that there is likely to come a day
>when I need to get one of these repaired locally.

Is it impossible to trace the circuit of the servo? I know that you
wrote that it was impossible to get a schematic elsewhere, so DIY may be
the only option.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

[[email protected]]

KFC-225: If it uses the same servos as the KFC200 &
KFC150, etc, it is not a PWM system.

In the KFC 200: They have a bridge of power transistors in the
servo along with a couple of low level buffers. The input to them is a
linear signal on two lines: + right and + left. Only one at
a time is energized. The op-amps that
runs it are in the main box. The summation of the tachometer output
that is located in the servo actually happens in the main box.

Donno... ARE they different? Like KS-270A, etc? (The A model
servos have IC buffers instead of the discrete ones used in the
earlier servos). I have a hunch they did use the same servos...
they would have saved a ton of certification costs by doing that.

BTW: What happens to these is the brushes wear and the motor
back end gets packed with carbon. Makes them start in a jerky
manner. If enough carbon gets packed in, the bearings can fail.

Way to observe: The FD bars move, but the control wheel does
not. They are regular Globe motors, so the brushes for the same
size can be found.

Bill Hale