Welcome
Username or Email:

Password:


Missing Code




[ ]
[ ]
Online
  • Guests: 89
  • Members: 0
  • Newest Member: omjtest
  • Most ever online: 396
    Guests: 396, Members: 0 on 12 Jan : 12:51
Members Birthdays:
All today's birthdays', congrats!
dan (37)
rchydro (64)
CapRack (30)


Next birthdays
11/07 Dave Marshall (40)
11/07 Worms (46)
11/08 Bert (77)
Contact
If you need assistance, please send an email to forum at 4hv dot org. To ensure your email is not marked as spam, please include the phrase "4hv help" in the subject line. You can also find assistance via IRC, at irc.shadowworld.net, room #hvcomm.
Support 4hv.org!
Donate:
4hv.org is hosted on a dedicated server. Unfortunately, this server costs and we rely on the help of site members to keep 4hv.org running. Please consider donating. We will place your name on the thanks list and you'll be helping to keep 4hv.org alive and free for everyone. Members whose names appear in red bold have donated recently. Green bold denotes those who have recently donated to keep the server carbon neutral.


Special Thanks To:
  • Aaron Holmes
  • Aaron Wheeler
  • Adam Horden
  • Alan Scrimgeour
  • Andre
  • Andrew Haynes
  • Anonymous000
  • asabase
  • Austin Weil
  • barney
  • Barry
  • Bert Hickman
  • Bill Kukowski
  • Blitzorn
  • Brandon Paradelas
  • Bruce Bowling
  • BubeeMike
  • Byong Park
  • Cesiumsponge
  • Chris F.
  • Chris Hooper
  • Corey Worthington
  • Derek Woodroffe
  • Dalus
  • Dan Strother
  • Daniel Davis
  • Daniel Uhrenholt
  • datasheetarchive
  • Dave Billington
  • Dave Marshall
  • David F.
  • Dennis Rogers
  • drelectrix
  • Dr. John Gudenas
  • Dr. Spark
  • E.TexasTesla
  • eastvoltresearch
  • Eirik Taylor
  • Erik Dyakov
  • Erlend^SE
  • Finn Hammer
  • Firebug24k
  • GalliumMan
  • Gary Peterson
  • George Slade
  • GhostNull
  • Gordon Mcknight
  • Graham Armitage
  • Grant
  • GreySoul
  • Henry H
  • IamSmooth
  • In memory of Leo Powning
  • Jacob Cash
  • James Howells
  • James Pawson
  • Jeff Greenfield
  • Jeff Thomas
  • Jesse Frost
  • Jim Mitchell
  • jlr134
  • Joe Mastroianni
  • John Forcina
  • John Oberg
  • John Willcutt
  • Jon Newcomb
  • klugesmith
  • Leslie Wright
  • Lutz Hoffman
  • Mads Barnkob
  • Martin King
  • Mats Karlsson
  • Matt Gibson
  • Matthew Guidry
  • mbd
  • Michael D'Angelo
  • Mikkel
  • mileswaldron
  • mister_rf
  • Neil Foster
  • Nick de Smith
  • Nick Soroka
  • nicklenorp
  • Nik
  • Norman Stanley
  • Patrick Coleman
  • Paul Brodie
  • Paul Jordan
  • Paul Montgomery
  • Ped
  • Peter Krogen
  • Peter Terren
  • PhilGood
  • Richard Feldman
  • Robert Bush
  • Royce Bailey
  • Scott Fusare
  • Scott Newman
  • smiffy
  • Stella
  • Steven Busic
  • Steve Conner
  • Steve Jones
  • Steve Ward
  • Sulaiman
  • Thomas Coyle
  • Thomas A. Wallace
  • Thomas W
  • Timo
  • Torch
  • Ulf Jonsson
  • vasil
  • Vaxian
  • vladi mazzilli
  • wastehl
  • Weston
  • William Kim
  • William N.
  • William Stehl
  • Wesley Venis
The aforementioned have contributed financially to the continuing triumph of 4hv.org. They are deserving of my most heartfelt thanks.
Forums
4hv.org :: Forums :: General Science and Electronics
« Previous topic | Next topic »   

Control Loops, Nodes, Poles, Phase and Gain.

1 2 
Move Thread LAN_403
Patrick
Wed Mar 04 2015, 08:04AM Print
Patrick Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Ive got a new tri-copter, and its working so well, its amazing. My previous tri's and bi's were like Spanish bulls, bucking wildly.

So, given that, im trying the typical "guess-and-fly" method and next ill try the "Z-N" method, but id like to understand the theory of control loops better. (I understand the basics of the generic PID) Ive done the basic Googl'ing; but theres alot which is still a blur. I see this as a lesson for myself and others here on the forum. Mine maybe flight related, but others here on our forum could benefit from power or mechanical feedback too, in there own projects.

Id like to ask what these terms mean in control. We often see a graph of a step change, and then a f(x) non-ideal attempt to correct. But i fail to understand how poles, nodes, gain, phase and bandwidth apply or even what they mean. PID and Kalman filters are really valuable, so im studying them thoroughly, but i get random article online.

id like to be able to explain to others and especially college professors, why i chose these particular values, instead of random guessing.


this, for the uninitiated, i found valuable.


Im also looking into a method of measuring the frequency of oscillation from a excessive "P" value. (which is a requirement for "Z-N". ) but i dont know if a strobe or oscilloscope is a good method.

Kalman:
1425457340 2431 FT1630 Basic Concept Of Kalman Filtering
So, given the above math im fearful.

Im interested in figuring out the method used in the F-117, Su-47 and X-29, to maintain stability in otherwise dynamically unstable machines.
Back to top
radiotech
Wed Mar 04 2015, 10:02AM
radiotech Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546
Graph's Law.

If the damn thing is unstable sometimes, don't mess with it.

Connect a chart recorder,and leave it running until someone can explain what the wiggly lines mean. cheesey
1425463358 2463 FT169481 Gould Recorder On Master Control
Back to top
Shrad
Wed Mar 04 2015, 10:07AM
Shrad Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780
don't forget that you can chain PIDs which is commonly done when a process needs to stabilize several parameters to control an output
Back to top
Dr. Slack
Wed Mar 04 2015, 10:18AM
Dr. Slack Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
Feedback stability is one of these instances where you get taught the simple theory on low order systems, but then find the practice is so much more complicated as the systems are necessarily much higher order. It's quite interesting to look at the history of how clever people 100+ years ago slowly moved from trial and error (hope'n'poke) to doing the maths, people like Bode, Nyquist, Routh and Horowicz.

Unless you are a maths freak (I'm not, but am in awe of folks who are) and can just look at an equation and use it, then you will have to get some feel for the underlying physics. There is no way you can simply dive into a high order system and do it right by seat of the pants. I suggest you get a simulator, find an online course on PID loops, and work through that. A pure P loop is easy, it's always stable. A pure PI, PD or PID loop will also tend to be always stable in practice. Use the language of the course and the behaviour of the simulator to understand what poles and zeroes are, and how they can be used to understand stability. Forget Kalman for the moment, it won't help you with the first steps.

Now the bad news, you will rarely meet anything as simple as a pure PID loop in the real world. Anything, any unintentional lag, any delay, any filtering, can destabilise it. In the world of flying, the order is already intentionally much higher. As control signal integrates to motor drive integrates to thrust integrates to velocity integrates to position, you start off with at least a PIIII loop, then add some sensor and calculation delays and you have some further destabilizing phase shift. The manufacturers will however have put in some phase advance to conditionally cancel some of the Is, and maybe feedback from accelerometers to give you a D term or two. The wonder is that commercial stuff is ever stable, given the range of hardware that a controller can stabilize.

The other bad news. With a high order loop, you have some quite complicated tools to predict stability. If you want to explain to your professors how your system satisfies the Routh-Horowicz criterion, then you have a lot of measurement and maths to do.

Start at the bottom. Simulate a P loop, then add an I to eliminate the DC error, then add a D to reduce overshoot. Do the maths at each stage and build up to it.
Back to top
Sulaiman
Wed Mar 04 2015, 11:08AM
Sulaiman Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
What a can of worms you are about to open!

the only coursework text book that I kept from my uni. electronics course is
Principles of automatic control by Martin Healey
(there are probably newer clearer books but this is the one that we used)
It's somewhere around the house, I'll check isbn and the correct title when I find it.
I kept this book because feedback control is applicable to so many engineering control tasks,
and I was never able to fully understand and apply it.

The F-117 type scenario is a multi-variable high order system ... brain-pain !
If we are VERY lucky one of the members here can explain such things clearly and concisely
but I doubt it because there are so many things to consider
it would certainly not be a simple PID loop !
In effect the control system needs to have a mathematical model of the controlled system
and perform predictive feedback control.
(for an idea of the scope of learning required, have a look at the Common terms and phrases
from the book that I mentioned Link2 )

i don't want to put you off this subject, just let you know what you would be getting into.
If you could master feedback control you would be very employable!

EDIT: oops! I posted without noticing the above, sorry for the overlap
ignore this and go with the above.
Back to top
Ash Small
Wed Mar 04 2015, 11:27AM
Ash Small Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
The simpler the software, the faster it runs.
Back to top
Dr. Slack
Wed Mar 04 2015, 01:28PM
Dr. Slack Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
Oh dear, I've just had a look at the first 3 minutes of the video you linked to. It's bad.

Let's look at a proportional controller? If the car's gas pedal controlled the speed of the car, and the speedo had no lag, it would be a P only system. But the gas pedal controls the torque, which integrates to speed, so it's already a PI system. Then he introduces an oscillation caused by an undefined lag, he appears to be sampling so perhaps he is hinting at the sampling latency (which is always an issue in a software-closed loop, and as Ash correctly says, minimise the lag by using fast software).

If you are going to understand the theory from the ground up, then you need to have the example match what the tutorial says it is, including the very first, simplest case.

Unfortunately, you will probably not appreciate which sources are good, and which bolox like that one, until you know as much as I do, which is still not enough to design high order flight systems systematically.

<edit> perhaps I've been guilty of confusing whether the PID is referring to the controller or the system. The problem is that the controller alone will not tell you how the whole system is going to behave. So, a P only controller, when closing the loop on the whole system that is a gas pedal, engine and speedo, will oscillate for certain gain settings because the complete system is higher order than simply P. Frequently a PID controller is not sufficiently flexible to stabilise a flight system </edit>
Back to top
Patrick
Wed Mar 04 2015, 04:32PM
Patrick Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Dr. Slack wrote ...

Unfortunately, you will probably not appreciate which sources are good, and which bolox like that one, until you know as much as I do, which is still not enough to design high order flight systems systematically.
YES! thats what i constantly worry about!

( ESC = electronic speed control )

As for the loop latency, the processor is putting out data pretty fast, but then to an "ESC," then the motor, which then changes inertia, then that causes a change in thrust. So theres a "series-circuit" like accumulation of the lag.

In terms of the ESC, many ESCs just try to increase the chopping speed - - usually to 8KHz upto 16kHz, regardless of whether this really changes the motors response time to a command. Now, several have either auto or manual set phase advance to help the motor. so given what Dr. Slack said, thats starting to make sense, though a poor solution which may not do anything.

Further, the new and best ESCs i just got are amazing. They run a program on board but between the microprocessor and 3-phase motor. this micro-ish program is a stunning difference, not sure how it works though, ill ask others in the RC forums.

Radiotech:
Absolutely agree with chart recorder, just dont know how to do it.

Dr. Slack:
yes, i see now how problems described, are just as important as a supposed explanation. that PIII comment was enlightening.

Sulaiman:
That F-117 comment scares me even more... i plan to build some of those types of systems eventually.


Back to top
radiotech
Wed Mar 04 2015, 09:34PM
radiotech Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546
In the case of the chart recorder shown, the machine had about seven DC motors,
from a few hundred to over a thousand. Each motor had a tachometer and an
ammeter. Each motor had a control amplifier, with all the elements of a PID controller.

Water and wood pulp went in one end , paper came out the other end.

Everything, from the individual dye pumps, to the steam flow rate was on one giant
loop. The idea was to keep it running.

Since everything was interlinked, speed swings, sometimes happened.One section
sped up, another tried to compensate.

The multi channel recorder, watching several parameters showed where the first action
occurred. i.e tracked the event.

Just like a flight recorder.

Here's some stuff from the era. Anything like your quadcopter ?





1425504841 2463 FT169481 Pid Example
Back to top
BigBad
Thu Mar 05 2015, 12:51AM
BigBad Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600
Basically, it's all to do with feedback loops.

You should hit wikipedia hard, go to feedback and follow the electronics, which is where the theory is the most complete.

Link2

Also read:

Link2

and you need to understand why (some small amount of) positive feedback can be stable:

Link2

check out regenerative circuits
Back to top
1 2 

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob

Go to:

Powered by e107 Forum System
 
Legal Information
This site is powered by e107, which is released under the GNU GPL License. All work on this site, except where otherwise noted, is licensed under a Creative Commons Attribution-ShareAlike 2.5 License. By submitting any information to this site, you agree that anything submitted will be so licensed. Please read our Disclaimer and Policies page for information on your rights and responsibilities regarding this site.