Honeywell KFC225 servos - operation Q

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.

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Peter wrote

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

Peter.

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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.

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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

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

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.

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.

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