Honeywell KFC225 servos - operation Q

[Peter]

Hi,

I've had a lot of these pack up. Honeywell were at one stage blaming
it on RF pickup on the input cable, so I did some tests, connecting
into the cable and checking that no noise is present.

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.

I have the full schematics of the KFC225 computer unit and it is
evident where this ripple comes from: there is a lowpass filter on the
output, which converts a PWM waveform into the servo control voltage.
This filter allows a little bit of the PWM signal to feed through.

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.

I have been quite unable to obtain the schematics of the KFC225 servos
- they appear to be closely guarded The amazing thing is that they
pack up so often... I mean how long have people been making little
boxes with a $50 Portescap servo motor, a gearbox, and a PCB with a
few chips and a few MOSFETs on it??


Peter.

 

[Peter]

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.

Correction (just located the test results): 140mV P-P at 1.4kHz


Peter.

 

[Rich Grise]

.

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]

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.

 

[Peter]

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.

Yes, I am aware that dither is used to overcome stiction, but I would
be suprised if it was generated by the computer unit in the instrument
panel, fed along the wires with the control signal, and then
deliberately passed through the servo amplifier to the motor. It would
make a lot more sense to generate it locally.

Anyway, 1.4kHz is awfully fast for this purpose - about 100 times too
fast.

Also, there is a tacho on the motor (on the roll servo; the pitch
servo apparently uses back EMF to measure the motor speed) so there is
local motor speed feedback.

 

[[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

 

[Peter]

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

The ripple comes from the KFC225 computer unit. This has a circuit
that generates a PWM signal and then uses a lowpass filter to convert
it into DC. So there is no doubt as to where this ripple comes from.

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.


Peter.

 

[John Woodgate]

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.

 

[[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

 

[Peter]

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

Bill - I've replied by email this time with more details. I am sure
this is a PWM design as there is no heatsinking.

And yes the servos do fail in the way described but not due to a motor
failure


Peter.