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Thread: Controlling head-pressure
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08-04-2010, 03:48 PM #51
Re: Controlling head-pressure
Wish it were so easy. Actually, the simulations show that, given a random disturbance on the pressure signal, the system instability can stop on its own, given the 'correct' instantaneous value - strange as it may seem.
If the random number generator of the white noise (pressure disturbance) is changed, the correction point can move.
So, what could very well be happening is that the system is being disturbed (noise) on the pressure signal. This then, via perhaps a poorly-tuned PID controller, coupled with saturation & system local transport lags, ends up setting the system into a limit cycle for a period, whereafter it, then moves onto a stable trajectory. This is an uncontrolled system - in chaos.
This is why I ask the question as to whether, in this case, a temperature sensor may not be less noisy than a pressure sensor for such a system?
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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08-04-2010, 04:07 PM #52
Re: Controlling head-pressure
The object of PID is to minimize the offset. In the Pr chart I would expect to see a large offset decreasing in amplitude. Instead, the offset was limited compared to the later offset (after 80 minutes).
To my mind, it seems unlikely that the oscillations were caused by the water flow/pump control.
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08-04-2010, 04:12 PM #53
Re: Controlling head-pressure
Fair comments. So we are sure that pump control is not the cause of the above disturbance?
The interesting thing about this head-pressure problem is that it occurred on two tests - around 10 days apart - where the ambient air reached around 10'C, & was not seen on other tests with higher ambients.
There could be a few other issues - but, these did not show at other ambients, only at the 10'C ambient tests:
1. Reversing valve stiction, or malfuncton;
2. Cavitation in the water pipes.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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08-04-2010, 04:38 PM #54
Re: Controlling head-pressure
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08-04-2010, 09:40 PM #55
Re: Controlling head-pressure
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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09-04-2010, 12:22 AM #56
Re: Controlling head-pressure
Speculating about what might be wrong is not what I do, and in fact I am opposed to the practice. I don't like guessing. Its right up there with parts changing. I want to know what is wrong.
That said, if you don't have access to the machine, then all we can do is play guessing games.
Anything could include instrument error, pump cavitation, air bubbles... maybe even intermittant compressor flooding... and who knows what else.Last edited by Gary; 09-04-2010 at 12:36 AM.
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09-04-2010, 03:08 AM #57
Re: Controlling head-pressure
Thanks Gary. Let's see how the discussion progresses. Thanks for your help, so far.
With a PID controller, as I remember things, there is a reference junction. When this is either incorrectly configured, or even faulty, the controller may exhibit limit cycle phenomena, amongst other things e.g. drift.
This thought came off the back of Tesla's mention of referencing to chilled water temperature.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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11-04-2010, 07:19 PM #58
Re: Controlling head-pressure
It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.
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12-04-2010, 04:23 AM #59
Re: Controlling head-pressure
^ Thanks Peter. That is an elegant way to do it.
I would think that the temperature sensor may perhaps have a slower response than the pressure sensor & better internal damping - from the pipe wall and thermal mass. This could turn out to be a good thing.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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