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Old 10-29-2004, 04:48 PM   #1
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Gyro Epiphany

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I just had a gyro epiphany. Hopefully treatment is covered by OHIP.

But seriously folks...

The gyroscopic control of the tail on helis is a negative feedback servo loop with the set point coming from the receiver and the control from the gyro sensor. The error between the two generates the correction signal. Such a feedback system becomes unstable when the proportional gain is set too high for the system characteristics. But why the "Delay" control on the gyro? What the heck does it do?

I was reading about PID controllers today. These are Proportional, Integral, Differential feedback controllers used to control a huge variety of processes. Then it hit me. The "Delay" control must be mixing in differential feedback. Mixing in some differential (ds/dt) feedback adds damping to the system, reducing the amplitude at resonance and making the system more stable - but only if it is needed and it is only needed if you have a slow servo or high inertia system increasing the system "Q" (amplitude at resonance). So, if your forego the 9253/9254 servo with your GY401 and use a standard servo, you will have to add delay to increase how much feedback you can have before the onset of oscillation.

Or I could be dead wrong....

Anywho, I think it further reinforces the benefits of using the right servo on the tail and illustrates why keeping the mass of the tail as low as possible improves tail control and precision.

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Old 10-29-2004, 04:50 PM   #2
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You just brought back some nightmares from my systems course in university...blech...

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Old 10-29-2004, 05:42 PM   #3
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Nightmares. Just in time for Halloween.

Heat, mass, momentum tranfer was my nightmare course. Well, that and chemistry. Oh and all the math courses.

Ack. I admit it. The whole darn thing was a nightmare!

Working has been better. Work is more like a bad dream...

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Old 10-30-2004, 07:35 AM   #4
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WOW!!!!!!!!!!! That's some pretty high tech stuff, I just wish I new what it meant (lol).
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Old 10-30-2004, 08:08 AM   #5
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I don't have my manual handy so I will wing this one.

I would imagine you could turn down the gain if you have a slow servo, but that defeats the whole purpose of having a sensitive gyro in the first place. By slowing the speed at which the gyro output changes you can still have a high sensitivity to motion but the command from the gyro will be closer to what the servo van actually deliver.

Yes, it is always best to match the performance of gyro and rudder servo to get the best out of the system
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Old 11-01-2004, 09:04 AM   #6
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Rob, that did bring back the nightmares from my Control System Theory lectures in University. I guess you are right. The Gain will amplify the differential feedback (ds/dt) making the gyro more sensitive, whereas the Delay will simply add a delta t to the feedback signal making the system less "Adaptive" (damping effect).

Feedback = Gain x ds/(dt + delta t)

Reasons why they have both settings on a gyro?? Lowering the gain will only lower the feedback signal amplitude and dropping some noise (thus becoming less sensitive). The actual output pulses will still be at high frequency that the actuator cannot cope with (in our case a slow servo). However giving it a delay can accomodate the deficiency of any slow actuator.
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Old 11-01-2004, 06:41 PM   #7
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I never took any courses on control systems so I'm not trained or qualified in such systems. My observations are totally based on recent investigations into how one of the products we manufacture works that I haven't worked with before (a servo controller for multi-axis industrial equipment). I did mess around with motional feedback for loudspeakers many years ago starting with displacement feedback assuming operation in the compliance-dependant region (F=kx) and soon discovered acceleration feedback was required as most loudspeaker operaton is in the mass-dependant region (F=ma).

I think that (and I could definitely be mistaken in this case) the primary gain is Proportional and that the Delay is the Derivative. The term "Delay" seems a bit of a misnomer in my view although it is a "Delay" in one sense (a topic for another day). If it is truly Derivative it is modulated by the rate of change of the Proportional correction signal. The greater the rate of change of the Proportional feedback the higher the Derivative feedback component. This Derivative feedback signal tends to fall off as the positional error decreases and hence it's effect is reduced or "damped" as the positional error approaches zero. This has the effect of reducing overshoot and oscillation in high quality factor (high Q) dynamic systems. Characteristics like slow servos with high-inertia motors and more moving mass in the tail will increase the system Q and make addition of the Delay or Derivative feedback component more necessary to damp the system.

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Old 11-01-2004, 08:01 PM   #8
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From what I can remember of my course from hell (feedback and control systems)... It is probably a derivative feedback, furthermore, to respond to quick changes (step input) it may be second or third degree. Can't really remember the details since it's been about 20yrs ago.
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Old 12-31-2004, 04:03 AM   #9
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yikes i,m getting a headache just thinking about it !!
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