[HN Gopher] Control theory for fun and profit
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Control theory for fun and profit
Author : evanweaver
Score : 111 points
Date : 2020-06-19 16:51 UTC (6 hours ago)
(HTM) web link (fauna.com)
(TXT) w3m dump (fauna.com)
| Scarbutt wrote:
| What's the catch with this database? skimming the website, it
| seems to promise too much. Impressive if all those features work
| well together as advertised.
| thelittlenag wrote:
| At work we have a problem in a different domain that I had
| planned to use an approach like this on. The extra resources that
| folks have linked to or provided are quite helpful. Thank you!
| carapace wrote:
| I can recommend "Feedback Control for Computer Systems,
| Introducing Control Theory to Enterprise Programmers" by Philipp
| Janert
| rland wrote:
| For a deeper dive, I can recommend the Astrom and Murray book.
| It's free:
|
| http://www.cds.caltech.edu/~murray/amwiki/index.php?title=Se...
|
| This subject is tightly coupled with system modeling, and can
| touch many different domains: Cruise Control
| Bicycle Dynamics Operational Amplifier Circuits
| Computing Systems and Networks Atomic Force Microscopy
| Drug Administration Population Dynamics
|
| Cheap computers are making it easy to make controllers that can
| reliably control systems with hundreds or thousands of variables
| and unknown dynamics.
| ithkuil wrote:
| Another trick for reducing tail latency with minimal utilization
| increase is to fire the second call earlier but send a
| cancellation signal when the first response arrives (e.g. a udp
| packet is usually good enough).
|
| This works when the cost of managing the request queue is low
| relative to the coat of serving the requests themselves (usually
| true whenever there is some IO involved).
| occamrazor wrote:
| They say they use a PID controller, but don't write about the
| next step: how to choose the parameters for the controller?
| orbital-decay wrote:
| Fair warning: this is a rabbit hole reaching the Earth's core.
| lmilcin wrote:
| I like me the most popular answer which is: Have a little fun.
|
| I had the same idea (ie PID loop to control system behaviour)
| some time ago to control traffic to external system so that it
| is just at the limit of throughput of that external system. The
| external system is really badly implemented and we had to put
| much effort into maximizing throughput.
|
| In the end we are replacing HTTP requests with Kafka topics
| which let's the other system process as fast as possible
| without destabilizing. On our end we continuously monitor
| latency and have flow control based on consumer behaviour.
| russh wrote:
| Play around with this a bit to get a feel for it:
| http://codinglab.blogspot.com/2016/04/online-pdi-trainer.htm...
| wenc wrote:
| There are at least 6 classes of approaches [1] for controller
| tuning. But they all involve the following: (1) starting with
| some initial guesses for tuning parameters (2) fine-tuning them
| in real-life.
|
| If you have a (rough) model of the system e.g. a transfer
| function, you can invert the model to get some initial
| parameters, and then fine-tune by hand.
|
| If you don't have a model but are able to run tests [0] to
| collect data, you can apply any number of heuristic methods to
| _systematically_ perturb the system to collect data to aid in
| tuning. Then fine-tune by hand.
|
| Difficult loops e.g. MIMO (multiple input multiple output)
| loops that are larger than 2x2 where there's interaction
| between the variables (i.e. you move one variable, others move
| as well) require more analysis with loop-pairing techniques
| like RGA (relative gain analysis).[3]
|
| For really difficult loops (or loops that have degraded over
| time), you can use software like Loop Pro [2]. This only makes
| sense if you're controlling something valuable and where loop
| degradation could lead to safety or performance issues. You
| probably don't need software to tune the PID in your espresso
| maker.
|
| --
|
| [0] Some systems are "open-loop unstable", which means they can
| go haywire if you perturb too much in the wrong direction.
| Simply blindly perturbing these types of systems is
| inadvisable. Process understanding/analysis is needed to avoid
| unstable situations.
|
| [1]
| https://sites.chemengr.ucsb.edu/~ceweb/faculty/seborg/teachi...
|
| [2] https://controlstation.com/pid-tuning/
|
| [3] For really large MIMO systems, PID is no longer the right
| control mechanism. Advanced control techniques like MPC are
| used, where a computer repeatedly solves an numerical
| optimization problem at some frequency to calculate the next
| control moves. MPC is usually used to control chemical plants
| with hundreds/thousands of control variables. PID is used in
| local loops, whereas MPC sits on top as an optimizing layer.
| fauna-ben wrote:
| That's fair! We didn't use any fancy process for loop tuning. I
| basically played around with it until we were getting decent
| convergence.
| jcims wrote:
| Brian Douglas is the one delivering that MatLab video linked
| at the bottom of the article. He's got a ton of deep control
| theory lectures on his own channel, too -
| https://www.youtube.com/user/ControlLectures/videos
| vladTheInhaler wrote:
| Steve Brunton also has a wealth of incredibly well-produced
| content on controls. Here is his control bootcamp series:
| https://www.youtube.com/playlist?list=PLMrJAkhIeNNR20Mz-
| Vpzg...
| jcims wrote:
| Wow that looks like a great series, thanks for the heads
| up. I'm just starting to dip my toes into basic PID
| control and am astounded by how deep (and predictive) the
| math can get in this domain.
|
| Edit: Also, I learned a lesson about closed-loop systems
| with the launch (well, EDL) of the Curiosity rover on
| MARS. It's mentioned in this article [1] but the trigger
| to start lowering the rover on the crane wasn't some
| process monitoring distance from the ground...it actually
| had no direct notion of that. It just watched the
| throttle setting on the retro-rockets and started
| unwinding the winch when the throttles reached a reduced
| steady state setting...something expected once the
| descent stage had reached its hover altitude.
|
| [1] (search for throttle) -
| https://www.planetary.org/blogs/emily-
| lakdawalla/2012/070607...
| activatedgeek wrote:
| I have seen these videos and highly recommend them
| especially for people like me who want to get a sense of
| the traditional control theory literature, have seen
| similar concepts elsewhere and are trying to map the
| motivations and notations.
| fmakunbound wrote:
| Guess and "suck it and see" as old professor of mine in control
| theory would tell the class.
| wenc wrote:
| That would not be the way to tune the control loops on a
| nuclear reactor.
| solumos wrote:
| I'm always curious about delving deeper into the history of
| control theory. Specifically, the work pertaining to Cybernetics
| in the late 40s/50s. Has anyone gone down that rabbit hole and
| found anything interesting?
| rozgo wrote:
| Control theory for fun can be really fun. Using control systems
| for solving problems you are familiar with can help develop an
| intuition, that you can then map to other problem domains. Years
| ago a peer at LucasArts taught me how to use PIDs for game
| cameras, and I've used them everywhere since.
|
| Very simple rules can create lifelike emergent behavior. This is
| an example of 2 simple motors using PID to try to target a red
| dot, in 3D, with gravity on:
| https://www.youtube.com/watch?v=8hfKs02QHR4&list=PLpDgOnZpfh...
|
| I can watch control systems all day.
| quickthrower2 wrote:
| That video reminds me of a puppy or dog when you hold food
| above it's mouth.
| in9 wrote:
| are took longer
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