You need to know what right-half-plane zeros are

https://jbconsulting.substack.com/p/you-need-to-know-what-right-half

by jbay808 • 4 years ago

439 277 4 years ago

jbconsulting.substack.com

This is fantastic.

lordnacho2• 4 years ago

Shouldn't there be a diagram of the complex plane so that people can see what it's the right half plane of? On top of it, there's a picture of a plane which is confusing.

Fascinating subject though, in engineering class it was quite surprising how this bunch of functions tracing lines and dots on the complex plane would be relevant to just about everything. Perhaps the first lesson is that even if you know how a system works, you can't just take the inverse function to control what comes out.

jbay80812• 4 years ago

Shouldn't there be a diagram of the complex plane so that people can see what it's the right half plane of?

Author here. Yes! Very fair criticism. I was trying to strike a balance between making the concept approachable for those who don't have a background involving complex numbers, but that certainly leaves the name of the concept more confusing. I should add it in a footnote at least.

And I did honestly not think about the potential for confusion between plane // airplane. An airplane was the most familiar example system I could think of to explain the concept. Oops!

Perhaps the first lesson is that even if you know how a system works, you can't just take the inverse function to control what comes out.

That's a great point too. It probably even deserves its own article.

lordnacho2• 4 years ago

I'd definitely read a series about control theory. The first time round was very badly presented at my university. A lot of math, motivation hidden.

Stuff like Nyquist criterion just sort of appears out of nowhere as functions.

I guess the big one is feedback being the magic, and then the complex plane tells you where that blows up.

l33t23282• 4 years ago

the potential for confusion between plane // airplane

This reminds me of the famous (possibly apocryphal) story of the algebraic geometer of middle eastern descent who was brought aside by Air Marshals for talking about how a particular problem could be solved by “blowing up points on a plane”

ehvatum• 4 years ago

If you haven’t read it, I recommend: https://www.gutenberg.org/files/6949/6949-h/6949-h.htm

yesenadam4• 4 years ago

The main thing missing for me: presumably if there's a plane, you're graphing something vs something. What are the X and Y? (or, uh, the x and y in the x+iy) Talk of a plane remains rather vague without knowing that. Maybe I missed it, but I looked twice, and read the comments on this page, and couldn't see it mentioned at all. There are a couple of graphs/planes, but they seem to be a different kind of thing.

LeonB2• 4 years ago

Really good article. Make changes and corrections to it, sure, but be careful not to over react to the criticism and make too many changes such that you then get even more critical feedback and make more corrections and get worse feedback and bigger corrections and oh god help me worse feedback and bigger changes and stop me please worse feedback and help bigger changes seriously kill me bigger feedback worse changes arggggh

levi_n• 4 years ago

Thank you for this article and the follow up. The information is fascinating, and I eagerly await your next installments.

rhn_mk1• 4 years ago

I felt that the article ended just where it was ripe to dig into the maths.

woojoo6663• 4 years ago

Not to mention, it's not just a complex plane. It's the s-domain, reached through the laplace transform.

nurettin• 4 years ago

So sometimes it is better to cut losses rather than double down because planes exist.

lb1lf• 4 years ago

My signals processing professor apparently knew one joke, and one joke only.

"A LOT (Polish airline) airplane is about to land in New York City; as they align for final approach, the first officer notifies the passengers that those seated on the right can now see the Statue of Liberty. A number of passengers get up from their seats left of the aisle and lean over the people seated on the right to get a glimpse of the statue. Plane promptly crashes.

Why? There were too many Poles in the right half of the plane.

I'll lead myself out.

l33t23282• 4 years ago

I hate to be the guy to ask you to explain the joke, but why would that make the plane crash?

I do know what a “pole” refers to, but I don’t follow the crash bit.

Cerium• 4 years ago

A control system with a pole (in the control systems sense) in the right hand half of the plane will be unstable. If the system changes during operation, the pole can move around the coordinate plane.* As it goes from the left (stable) towards the right (unstable) the system will begin to oscillate as the pole crosses the Y axis.

In the joke, a Pole (person of Polish origin) was in the left of the (air) plane. When they crossed the aisle to look out the window, the (air) plane became unstable and crashed (pole in the right hand plane).

*My control systems professor loved to explain using an example of a driver as a control system. The system (car + human driver) seeks to minimize error against the lines on the road. If the driver starts drinking, one of the system's poles will move right. The car will start overshooting first, then will start weaving, before finally crashing when the driver is too drunk.

dreamcompiler3• 4 years ago

Because if your poles are not in the left half [of the] plane, the system is unstable.

ad404b8a372f2b93• 4 years ago

The weight imbalance from everybody rushing to the right made the plane unstable.

eftychis3• 4 years ago

tl; dr: The system is unstable as it has positive feedback loops. You can "first day of class" think of it as the implied series 1/(s-p) -- p the pole, s the Laplace variable -- exists and converges. And in particular you want that series to converge on the imaginary axis which means it does not diverge in the frequency domain (Fourier transform). Essentially, that means that you have regions of frequencies where your system diverges/amplifies them excessively and thus breaks down: is unstable. Filters do the opposite.

P.S. The other note is that for real linear time invariant systems the region of convergence of the series/Laplace transform of the system must be positive for the system to be causal -- and thus real and implementable. So the joke could also have been modified to get a magical and unstable plane.

tremon• 4 years ago

A little bit broader than you asked for, but:

A dynamic control system is modeled by a set of dynamic equations, usually expressed as partial derivatives. To analyze the behaviour of such a system, the equation is solved or approximated in the complex time domain. The relevant part of the solution is where the real part of time is positive, i.e. the right-half plane.

A pole is a coordinate for which the dynamic equations have no solution (y = 1/z has a pole at z=0), which results in undefined or uncontrolled behaviour.

(also see https://en.wikipedia.org/wiki/Zeros_and_poles)

dreamcompiler3• 4 years ago

Did we have the same professor? Mine loved that joke too.

The students in the classroom next door always knew when Prof Lipovski told his joke because everybody along the hallway could hear the loud groan emanating from our room.

a9h74j• 4 years ago

Heard a varient at Berkeley from a professor of Applied Math, which you would probably no longer hear.

Generic airline, specifically Polish people were asked to move to the left side of the plane. Because "Poles in the right half plane cause instability."

(As for application in economics, economist Steve Keen, for one, explicitly models time delays and dynamics, with results worth learning from, AFAIK.)

Natsu3• 4 years ago

I think all systems professors have the same joke book and there's only one page in it :)

cperciva• 4 years ago

There was nearly a disaster earlier in that flight too, when the pilots got sick from food poisoning and the cabin crew had to ask if any of the passengers knew how to fly a plane. As it turned out, one of the passengers had flown many years previously, and offered to try.

When he reached the cockpit, however, he took one look at all of the controls on the modern aircraft and realized that he wasn't going to be able to operate it. When the chief steward asked why, he replied, "I am just a simple Pole in a complex plane".

I'll follow lb1lf out.

drekipus• 4 years ago

Now this made me laugh like crazy

danuker• 4 years ago

It has a grain of truth to it. Poland could donate MiGs to Ukraine, but other countries with US fighters could not, because Ukrainian soldiers didn't know how to fly them.

jacquesm4• 4 years ago

Why did the man from Gdansk marry the woman from Zakopane? 'Opposite Poles attract'.

ldite• 4 years ago

And you know why the contour integral of Western Europe is zero? Because all the poles are in Central Europe.

(sorry)

jimnotgym2• 4 years ago

As a resident of post-Brexit Britain, I was horrified as I read this expecting it to have the opposite punchline and to turn into an anti-immigration joke.

For those that don't know, Polish (and other Eastern European) migration was a major issue raised by some in the Brexit campaign.

samfriedman• 4 years ago

The version my professor told was, “Why is politics in Poland so volatile? Because all the Poles on the right are unstable!”

selimthegrim3• 4 years ago

The real answer is Ilyushin made terribly finicky engines and people skimped on maintenance but that’s another story.

gugagore• 4 years ago

The article saying zeros (where the transfer function is 0) when it should be saying poles (where the denominator of the rational function is zero), right?

themeiguoren• 4 years ago

Controls engineer here. The author’s control theory knowledge is correct - a RHP zero indicates that corrective action will begin in the wrong direction (which the article compares well with countersteering on a bike). The technical term is a “non minimum phase system”. It’s possible this will lead to instability, but in general the long term stability of a system is determined by the location of its poles (which is more applicable to the author’s ice cream example). Poles in the RHP will cause a system to blow up.

My gripe with the article is that the author tries to wow you with some obscure technical points about a system which is unmodeled and he does not understand, to wave his hands at a vague conclusion. If he had made the same point using common English phrases that encapsulate the idea (“positive feedback loop”, “we have to let it get worse before it gets better”, etc), then it would be a lot clearer how wispy his argument is.

chkaloon• 4 years ago

That joke is at least 38 years old, as I heard it in my controls class in 1984 :)

DeathArrow• 4 years ago

Something I don't understand: on the one hand, people on HN seems to enjoy this kind of articles. On the other hand, they upvote any article who decries interviews where candidates are asked things that involves knowing some theory.

So, if we think knowing theory is useful in cracking hard problems, why is it wrong to asses its knowledge in an interview?

CamperBob23• 4 years ago

HN's readership isn't homogeneous. There's room for both points of view.

I've done a lot of useful work in feedback systems without ever really grokking Laplacian notation and the notion of complex frequency in general. A lot of the actual numerical methods used in real life boil down to a few canned formulas. But I know enough about the underlying theory to appreciate where the canned formulas come from, and fully intend to sit down some day and go through the whole process. Articles like this are interesting if only for the occasional gems in the comments, such as John N.'s pointer to Maxwell's 'On Governors' paper that I'd never run across before.

At the same time, I don't see much upside in making hiring decisions on the basis of whether someone can regurgitate a bunch of textbook math. I'd rather spend the interview talking about control problems the candidate has dealt with personally, how they were handled, and what the candidate learned from them.

genezeta• 4 years ago

While I don't claim to have the answer to your question, I think there's a jump you're making that may not be so straightforward and somewhat explain things.

You say "people on HN seems to enjoy this kind of articles" which seems reasonable, given the comments here. But then you jump to "we think knowing theory is useful in cracking hard problems".

Going from the first to the second is not quite so clear. That is, someone may enjoy such an article and even learning some theory, but not necessarily because they think they will directly apply it. People sometimes just enjoy learning stuff or reading about it and then forgetting it.

You also make a second jump, because other factors may be involved. Maybe the theory asked in the interviews is completely unrelated to the things involved in the job. The job may not even require cracking hard problems. These are frequent occurrences -in my experience, at least-, and clearly seem compatible with thinking that knowing theory is good in general.

bee_rider• 4 years ago

Programming and engineering are neighbors, so it is interesting to see what's going on over there. But many engineering analysis tools are not really useful for normal programming jobs (unless you happen to be working on engineering software, control systems, or something like that).

djmips• 4 years ago

Because programming is rarely about cracking hard problems. That's actually why the people HN enjoy the kind of article, it takes them away from the drudgery of their job.

Animats• 4 years ago

The classic paper, Maxwell's "On governors". (1869) [1] This is the first theoretical analysis of feedback.

It will be seen that the motion of a machine with its governor consists in general of a uniform motion, combined with a disturbance which may be expressed as the sum of several component motions. These components may be of four different kinds:-

(1) The disturbance may continually increase.

(2) It may continually diminish.

(3) It may be an oscillation of continually increasing amplitude.

(4) It may be an oscillation of continually decreasing amplitude.

The first and third cases are evidently inconsistent with the stability of the motion; and the second and fourth alone are admissible in a good governor. This condition is mathematically equivalent to the condition that all the possible roots, and all the possible parts of the impossible roots, of a certain equation shall be negative.

That is, in the left half-plane.

(Terminology has changed. Maxwell says "disturbance" where today, the term "error" would be used. Today, "disturbance" means an input which disturbs stability, while error is an output.)

Maxwell got so much right in that paper, and it was a long time before anybody picked up on that result.

Now, where it looks like the author is going is into economic territory. Basic economics talks about "economic equilibrium". The concept is that restoring forces will bring supply and demand into equilibrium. But basic control theory tells us that may not happen. Any system with delay in it can potentially be unstable. Too much delay, and even simple systems will not stabilize.

[1] https://en.wikisource.org/wiki/On_Governors

WanderPanda• 4 years ago

In the real world all exponentials are sigmoids eventually, so what we actually get is is a recession with a drastic reallocation of resources (creative destruction).

duped3• 4 years ago

Not really, there are plenty that are sinusoids which are just complex exponentials.

The real trouble with sigmoids is when the saturation point is beyond physically meaningful quantities of the system. See the tacoma narrows bridge collapse.

d--b• 4 years ago

This is very well explained, but there should be a simpler name for this effect than right-half-plane zeros if the author wants to spread the concept beyond control theory.

duped3• 4 years ago

The greatest sin of undergraduate engineering education is sequestering signals and systems into electrical engineering curricula. I understand why it's done that way (I even had to fight to take the course early, sidestepping some prereqs for reasons).

But it's really so foundational to understanding concepts of stability, resonance, information/energy flow (from the conceptual perspective), and the simple analytical tools for building a solid conceptual base. It takes a semester to hammer home that step response matters, positive feedback bad, negative feedback usually good, and topologies are useful.

Agentlien2• 4 years ago

I took some electrical engineering courses but mainly software engineering with a focus on low-level details and graphics.

The more I specialize in graphics the more I realize I need much more knowledge about signal processing than we were taught at the university.

AB1908• 4 years ago

Could you demonstrate with an example?

codethief• 4 years ago

Can you recommend any good introductory books on the topics you mentioned for someone who studied (theoretical/mathematical) physics but never electrical engineering or signal processing? (Read this as: I am more or less familiar with Nyquist-Shannon's theorem but that's about it.)

lagrange775• 4 years ago

Not a book, but this series on Youtube is excellent.

https://www.youtube.com/watch?v=Pi7l8mMjYVE&list=PLMrJAkhIeN...

familiar with Nyquist-Shannon's theorem

BTW: In control theory there is also https://en.wikipedia.org/wiki/Nyquist_stability_criterion

scrlk• 4 years ago

Signals & Systems by Oppenheim is a classic introductory textbook.

duped3• 4 years ago

ime the books are very loaded on theory and it's hard to connect that to practice and concepts. I don't know of any book that touches on what poles/zeros mean or why they're useful outside what are (seemingly) contrived properties of formulae like stability. It's one of those things that's just useful to go through in a university course, even just auditing it. The practical problems and lectures are extremely useful, particularly due to the overhead of notation (the practice is relatively young and derived from a different tradition of education, and a lot of contemporary notation can seem a little foreign, or is filled with shorthand).

Applied Digital Signal Processing by Manolakis & Ingle is the book I always turn to for reference and the code examples don't suck. Oppenheim & Schafer is a classic but frankly only useful as a reference, that tome is a bit dated otherwise. The Scientists and Engineer's guide to DSP is also not bad as a practical text.

trenchgun• 4 years ago

I was already thinking of taking it this summer, but you convinced me to do it for sure.

Thank you!

nbadg3• 4 years ago

I went to school for mechanical engineering (though I now work in software). We were required to take signals and systems, but if I remember right it was a weed-out course for most MechEs (it certainly was a challenge for me, though I think that had more to do with the curriculum than the topic).

Those lessons might have been hard-won on my part, but I definitely still use them. The general concepts (feedback loops etc) are applicable basically everywhere in life, and I still find uses for literal actual math (like using a convolution kernel to do rolling window sampling in numpy).

thrwyoilarticle• 4 years ago

I can imagine it's used for car suspension?

koliber• 4 years ago

Does anyone know of any courses which could explain concepts of stability, resonance, information/energy flow and help build a solid conceptual base for managers or entrepreneurs? These concepts are crucial when making business decisions. I've been building up an understanding of this through experience. If there was a way to shortcut this and provide such education deliberately, it would allow people to become better decision makers quicker.

nabla9• 4 years ago

"signals and systems" - covering signals processing, systems and control theory is very generic subject.

You need it in economics, biology, chemistry, physics, computer science, statistics, electrical engineering, robotics , automation, logistics, ...

It should be taught like calculus or linear algebra, so that everyone gets gist of the basics before learning to apply it into their specific field.

Jach• 4 years ago

I always thought my college education was backwards, with the exception that differential equations (and laplace transforms, which can help lay the groundwork for other transforms) came early enough that I could get by -- though it'd have been better if they were earlier, like high school, and if I hadn't been able to skip two semesters of calculus thanks to my HS calculus then differential equations would have come even later in college. But as a CE student at a school nominally more about video game making education I ended up first taking as CS electives an audio processing course, and then an image processing course, before the CE side reached control systems, which I had to retake after taking the next digital signal processing course, after which control systems made a lot more sense. It was only after graduating that I felt like I had reached a level of sophistication to go back and really grok all the related theory and go deep into applications. Maybe that's how graduate students are meant to feel? But I just went into full time enterprise work and 'retired' after 6 years of that, so I've since forgotten a lot... Still, the concept of feedback has proved useful in systems analysis from time to time, and it's a framework that I think could yield many low hanging fruit in other disciplines. (The book Behavior: The control of perception applies control theory to psychology in a convincing way but it's understandably been neglected by psychologists who aren't often very sophisticated mathematically.)

LolWolf• 4 years ago

Counterpoint: no you don’t.

By the time you have a perfectly reasonable model of a system that is good enough such that computing the transfer function’s poles actually tell you something interesting about the system, there’s way more you can say about the system than “it is stable.”

There are maybe some lessons to be drawn from basic “classical” control theory, but many are better stated by just analyzing the system directly.

(As a side note, I’m not saying there’s Zero value in analyzing transfer functions, just that it’s a long way to the top from there.)

magneticnorth• 4 years ago

The follow-up article on inflation of shelter costs in response to mortgage interest rates is also fascinating and worth a read, imo [0]. The tl;dr is that in the medium and longer term, higher interest rates will decrease inflation, but there's a right-half-plane zero effect that will cause higher inflation rates in the short term in response to interest rate hikes (with a lot of simplifying assumptions).

[0] https://jbconsulting.substack.com/p/on-shelter-futures-part-...

neilwilson• 4 years ago

It's a little more complex than that because of the potential feedback loop via the main price channel.

In other words if you can have a wage/price spiral you can have a profit/price spiral and an interest/price spiral for the same reasons.

[0] https://new-wayland.com/blog/interest-price-spiral/

Once you abandon the concepts of general equilibrium and a fixed amount of money for what happens in reality, then the functional control mechanisms get a whole lot more interesting.

voldacar• 4 years ago

This is some of the best non-rigorous writing about math that I've encountered. Far superior to the awful quanta articles that sometimes get posted here.

GistNoesis• 4 years ago

Are they still important if you use something like model predictive control ?

jbay80812• 4 years ago

They are certainly something that your MPC controller will need to account for, and they will constrain the theoretical maximum performance you can get out of the system no matter how optimal your control algorithm is.

ilayn2• 4 years ago

Always appreciate control theory articles. But this one needs some editing. Take ice cream example;that is not zero dynamics that's still pretty much convolution. Now zero dynamics examples must have no effect on the system. Thus it is not a zero if you continuously chugging ice cream. It is indeed a step input that you equilibriate at a constant ice cream input and some happiness comes out constantly. The moment you stop happiness goes away. That is not related to zero.

Also a pure zero action is supposed to cancel the input completely. Not at first but completely (restricting the discussion to linear systems).

Zeros effects are not so trivial to untangle as the article suggests unfortunately but fun read anyways and very nice flow.

jbay80812• 4 years ago

Now zero dynamics examples must have no effect on the system. Thus it is not a zero if you continuously chugging ice cream

Thank you for your input! I wonder if you might have misread that example. In this system there's indeed a RHP0 in the transfer function from ice cream consumption happiness. A continuously increasing rate of ice cream intake results in exactly no effect on the output.

ilayn2• 4 years ago

Your happiness shouldn't change if ice cream input has a zero. Also it should be independent from the amount of ice cream. In other words if we have an ODE say

ddot y + 2 dot y + y = dot u - 2 u

As long as my input is pure C exp(2t) independent of C I see no happiness and it's not working on my mood. In your example input u effect is cancelled by decay of y cancelling the guilt. Making it not a zero.

In my personal case eating celery is a zero i see absolute no point eating it :) no harm and no benefit just pointless chewing

once_inc• 4 years ago

For those wondering: this article is an introduction into inflation of house prices in Canada. You can easily find the next article (it's already online) by going to the substack's index.

Very good article, and well worth your time to read.

pcthrowaway2• 4 years ago

Link, because I assumed it wasn't out yet since the author hadn't provided it: https://jbconsulting.substack.com/p/on-shelter-futures-part-...

dools• 4 years ago

It’s funny as soon as I read the ice cream but I knew where he was going with it.

The interesting thing is that his modelling concludes that raising interest rates is inflationary which is what Warren Mosler also says.

Anyone interested in what to do about inflation would enjoy the recent Macro n Cheese podcast episode featuring Randall Wray

selimthegrim3• 4 years ago

Let’s hope Erdogan doesn’t read this

eigart• 4 years ago

Great read. I’ve been thinking about how this applies to earth/climate change for a while, so I was surprised that the system he worries about is inflation/the economy.

jbay80812• 4 years ago

Thank you for reading! I agree the Earth is a much worthier system to be worried about than the economy, for sure.

parasti• 4 years ago

This is weird to say, but I've never had this happen before: as someone who's never heard of control theory, the initial illustrations and the leading icecream example seemed highly confusing and put me off from reading the rest.

tdubhro1• 4 years ago

It’s a bit disappointing that an article purporting to educate us about something we should know but may be unaware of, is completely wrong about the example of how an airplane climbs - you don’t climb by pitching up, you have to increase power.

rocqua• 4 years ago

Just pitching up will have you climb, but at the expense of losing velocity right?

jhugo• 4 years ago

Briefly.

jacquesm4• 4 years ago

Right up to the stall speed limit. You can't just keep pitching up, you'd have to apply some power too if you don't eventually want to run out of forward (air)speed.

In a glider you can't do that so there when in level flight you have a limited amount of forward momentum available to help you climb if the air itself isn't moving up, you are continuously trading altitude for speed and vv (easy to see in a dive: everybody expects you to gain speed in a dive because can all relate so something falling, it's obvious the reverse has to happen when you climb and the stall speed is a design parameter of the aircraft at a given altitude combined with a bunch of other factors).

https://aviation.stackexchange.com/questions/27693/how-does-...

jbay80812• 4 years ago

Fair. Perhaps a glider would have been a better example.

nosianu• 4 years ago

How? A glider climbs when the air it is in rises faster than the airplane sinks in that air to maintain its forward speed.

If you pitch up you can only exchange speed for a little bit of altitude, briefly.

lagrange775• 4 years ago

Great article and blog!

I would also mention, that we essentially design controllers to shift the poles and zeroes of the total system (which consists of the plant system and the controller system) to more desirable positions, than those of the plant system alone.

ali-preview• 4 years ago

I'm nX 1 too<p>;)\n

jokabrink2• 4 years ago

I am not sure what to take from this. The title certainly makes sense from a engineers perspective. But from a layman's? I don't know.

It seems, that he tries to explain an important concept in control theory but does not explain the meaning of "plane" at all. Even worse: He uses airplanes as an example.

Isn't it terribly confusing for non-control-theory people? If I don't know control theory, then how on earth would I know that he means a 2D-plane? And what's this "minimum-phase-system" he mentions once? Is a pole a number? And what about RHP poles?

I would be interested in how readers without a background in control theory and higher maths understood that article and what questions arose.

lagrange775• 4 years ago

He uses airplanes as an example.

Oh wow, i didn't notice that. It's either pretty clever, or unfortunate wording.

petesergeant• 4 years ago

I am not sure what to take from this

I think the author is using a lot of words to say "things often need to get a bit worse in order to start getting better"

nbadg3• 4 years ago

That's only half of the story. The second half of the story is "if the getting worse temporarily cancels out the getting better, stop trying to correct, or you're going to crash!"

Perseids• 4 years ago

And, critically, you potentially need to ignore the signal of "things getting worse". Otherwise you might creature a vicious cycle of more corrective action -> more things getting worse (at first) -> even more corrective action, until your system crashes and you never got into the phase where most of the desired result of the corrective action materialized.

And a second important point: You mustn't ever get into a state where the "things getting worse at first" already pushes you over a red line (see the example of the plane that loses even more altitude before it rises again. If you hit the ground in between you don't care that you theoretically would have risen later on).

Ialdaboth• 4 years ago

I think "when things get worse we tend to double down on what caused things to get worse, until everything around us crash and burn" would be more accurate, given the current state of affairs.

noduerme3• 4 years ago

It really all depends on how low to the ground you are when things begin to get worse.

For example, a virus that takes 10 IQ points off people who've been infected with it might make China more apt to consume Disney movies, while it might plow America straight into the ground.

chasd00• 4 years ago

To me, that immediately sounds like someone making excuses for their side and the midterm elections. I don’t care how much math there is.

toomanydoubts• 4 years ago

At the beginning I didn't understand what the post was about. At the ending, it felt like I was at the beginning.

loser777• 4 years ago

As someone with a rapidly (exponentially?) decaying background in engineering, all the article did was create an insufferable itch of mathematical fuzziness about why these were called zeros until I got to the footnote and the actual article began.

taneq2• 4 years ago

Thanks for the tip, the rest made way more sense after reading that. I don't think I'm particularly poorly read or mathematically versed but I've never heard a pole called a "right-half-plane zero" before, so maybe the author could have lead with that (or even just called it a pole, which I have heard of so at least I'd have been less lost.)

zasdffaa• 4 years ago

Seemed a decent if over-long article to explain something I needed to know about feedback theory. Summary: to do x you may have to do, or briefly get, anti-x which you need to be aware of and account for. Key sentence where the importance clicked for me was:

"Let’s say our airplane is running in auto-pilot. We’ve sent a request to gain altitude, so the flight controller tilts the elevators to initiate a climb. But suddenly the airplane is losing altitude, moving farther away from our target? Do we pull up even harder?"

It's ok.

padolsey• 4 years ago

So, to me, this is just calibrating against a continuum. Right? I think of this as binary-searching a kinda normal'ish distribution? I'm not good at math so I had a hard time once the OP article got into differential equations

noduerme3• 4 years ago

I mean, to a layman like myself, the takeaway is something about stateful, irreversible systems where x is on both sides of the equation in unequal or inverse measures, and how those systems (maybe? sometimes?) progress toward equilibrium by going through unexpected phases that seem to contradict what you're trying to input. I read the whole thing as a metaphor for Fed policies or how we elect Democrats every time the economy is about to crash, and Republicans every time it's just recovered.

To the degree that some field of math can be best exemplified by various types of aircraft stalls, this didn't do a great job of explaining either the types of stalls or the feedbacks (what you might call negative REPL loops) leading to them.

I agree it's ok.

jokabrink2• 4 years ago

Fair enough, but then he wouldn't have needed to use control theory and just stuck with the examples.

Edit: removed ambiguity

kubanczyk• 4 years ago

Talk about a borked example.

First of all, I'd call airplane/plane an "aircraft" instead, not because that's what aviators and Wikipedia editors do, but because it's super confusing in an article about some other thing called "plane".

And, as an aviator would probably tell you, in order to climb you should care more about the throttle (i.e. increase power) than about them elevators! So, in many cases there is absolutely no such dip.

And, I know of no aircraft capable of achieving that loopy flight path using just elevators. That would require quite a bit of kinetic energy, to say the least.

The bike counter-steering example clicked instantly with me, but I guess not many people have an intuition about that. I rode for decades before learning this and I think most other cyclists are not aware as well. (Most motorbike instructors teach that however.)

foxes• 4 years ago

As a mathematician it is just a lot of fuzzy rambling, until they have the actual definition. I really don’t know what an average person is supposed to take away, you can end up in some feedback loop, lol okay. Communicating maths to a wider audience you can still be correct but there is zero reason to make it fuzzy. It’s okay to use an analogy if there is something people can latch onto, carry it with them mentally, and then you slowly deform it into the correct statement, but you have to be careful to not be fuzzy.

If you are writing something just minimise the number of non informative sentences. They probably could have explained poles, planes, etc all in the same space instead of rambling. Cutting to the examples and then giving the definition would have been better.

kaashif3• 4 years ago

Yeah, as someone with a math degree, I already know what the right half-plane refers to, I already know what zeroes and poles are (in a complex analysis way, not a control theory way), and I am just befuddled. The article doesn't really seem to have much concrete content at all, and could be cut down by a lot. Which is an achievement considering how short it already is.

Just cutting out this junk:

Again roughly speaking, zeros describe mathematically how a system reacts to some input in the short term, while poles describe how a system reacts in the long term.

and similar waffle would be good. It's so vague as to be counter-productive.

The conclusions in the article aren't easily drawn from the vague tools we're given, in my opinion. Maybe I'm just not the target audience, but I don't really understand who that is.

makach• 4 years ago

Same. I ran to the comments to check my understanding.

jimnotgym2• 4 years ago

I never studied control theory, but I was fine with this. My pure maths education stopped at say first year undergrad. I didn't even consider that there were two kinds of plane. I thought the example was a good example of the subject.

To answer the confusion about poles...I think the teaching method of, 'here are some terms you won't understand until later' is very common, isn't it. I bet it even has a name

pizza• 4 years ago

It's a nice explanation of the phenomenon of "getting tilted" in games:

Tilt originated from Poker and it's usually a state of emotional frustration and confusion.

It's most commonly used if you're going on a losing streak and then you become so frustrated that you start playing worse because you cannot focus anymore.

Part of it, as I see it, is that you are using that frustration to fuel further efforts, which ends up in a downward spiral feedback loop

https://gaming.stackexchange.com/questions/190507/what-is-th...

gsliepen• 4 years ago

The danger of the right-half-plane zero is that it lures you into reacting to it, but that is precisely the wrong thing to do. Attempting to apply a new control input to cancel the inverse response only sets off an even worse chain of events, where the resulting secondary inverse reaction becomes even more severe, requiring even more corrective action, until finally you’ve slammed into the ground. In this situation, the flight controller’s only option is to ignore the initial misdirection and wait patiently until the airplane eventually begins to climb as intended.

Of course that is easier said than done; adding a sleep() to your control loop to ignore the initial misdirection is also very bad. The right way to solve this is to not just tell the control loop to "go up", but to plan a realistic trajectory that the control loop can execute. That way, the error between the desired trajectory and the actual trajectory will be much smaller, and the closer the error is to zero, the less chance of a control loop to go wild.

thaumaturgy• 4 years ago

The danger of "systems thinking" is that it misleads otherwise clever people into believing they've found a cheat sheet for subjects they don't have deep knowledge in. This article is a fine example of that.

I didn't know what right-half-plane zeroes are. I knew some of the examples it gave (Veritasium has a fun video on the bicycle steering phenomenon), but not that there was a category they all fit into. Neat.

But I got squinty when the author said the intended audience was everybody, and got a hunch it was going to wander into the current economy ... which it did, sort of, except that in this case, "wandering" into the subject meant, "wrap it up with a graph and then point at it and go, see! See! I can predict the economy now!"

The follow-up at https://jbconsulting.substack.com/p/on-shelter-futures-part-... goes into more detail and concludes that interest rate hikes will increase inflation over the next 5-ish years, but there's no "part 2" in the series to be found (paywalled?).

There are lots of graphs and the author tries to build a case out of several arguments, buuuuut in the end it feels like that IASIP conspiracy board meme. I smell a faint whiff of gish galloping here and there, but one thing that stands out to me is that the author chooses to normalize housing costs against inflation, but there's no mention of wage stagnation anywhere.

Rents and housing costs can't rise a whole lot more, certainly not to the extent the author seems to be predicting, because people can't afford them. This is already a conversation happening in every housing market in at least several countries. In the US, pick literally any local subreddit and ask if anyone knows of an affordable place to rent, I dare you. There's already a huge epidemic of unhoused people and van-dwelling is more popular than it has ever been, and consumers are currently getting squeezed in a lot of directions. Here, one of the things in my browser history before this article was this thread: https://old.reddit.com/r/news/comments/v8knl5/gas_prices_hit...

So, if wages don't rise to meet these costs, then something big is going to break way before the cost increases the article is doomsaying.

robertlagrant• 4 years ago

Rents and housing costs can't rise a whole lot more, certainly not to the extent the author seems to be predicting, because people can't afford them.

House prices are passively controlled because there's only so much money to spend on housing?

bambax2• 4 years ago

the first important lesson of inverse response is: don’t ride your bicycle on the edge of a cliff

I don't know if many people often ride their bicycle on cliff edges, but many plane (as in airplane) accidents occur because it's difficult / impossible to recover from a stall near the ground.

LanceH• 4 years ago

You can also get your weight to the inside of a turn for a bicycle by leaning while riding straight.

Every time this comes up there is a big debate with a bunch of people saying counter steering is required. Please, go out and try it. Drive perfectly straight along a painted line. Lean left, turn left. Lean right, turn right. There is no requirement for counter steering.

bambax2• 4 years ago

I think it's somewhat speed-dependent. I ride motorcycles and bikes. On a motorcycle at high speed it's impossible (in my experience) to take a turn without counter steering. You sometimes have to push really hard on the handle on the side where you want to go.

Also, leaning is very similar to counter steering; counter steering makes the bike "fall" on the side where you want to go.

Interestingly, you can also steer a horse without doing anything on the reins; the horse will usually go where you put your weight; I think it's because it needs to compensate for the weight differential; or maybe it takes it as a hint about which way you're looking. In any case it works.

bob1029• 4 years ago

This stuff is fun. Surprised I haven't seen direct mention of state space representation yet:

https://en.wikipedia.org/wiki/State-space_representation

I recall going through this at university and it being a bit of a struggle. Looking back at it now, this stuff makes a lot more sense. A decade+ of practical experience probably helps.

raydiatian• 4 years ago

The footnote is nonsense. Needs revision, author is currently arguing that poles are indistinguishable from zeros.

oh_my_goodness3• 4 years ago

The footnote currently appears to be correct.

jbay80812• 4 years ago

I'm not aware of what you mean.

blauditore• 4 years ago

Can someone provide a TL;DR? I don't have time to read the whole article, and I don't even understand what's the topic based on title and reading a few paragraphs, nor from googling "right-half-plane zeros".

a_imho• 4 years ago

The plane refers to the plane of complex numbers (a+b*i), the right half is where the real part (a) is positive. If the zeros of your feedback loop are on the right hand side that means it is a unstable feedback loop, if they are on the left side it is stable feedback loop (this happens because the zeros eventually end up in exponents which either tend to infinity or to zero with time).

If you squint a little bit most systems looks like a feedback loop and my not very confident interpretation is, judging from the last picture, that the author thinks we have a runaway inflation problem (zeros on the right hand side of the plane) and we might or might not have the controls to move them to the left hand side to tame it.

layer8• 4 years ago

I thought this would be something about positive zero in a 2D coordinate system, but seeing drawings of airplanes in the article I concluded “bummer, it’s not” and immediately hit backspace. Thanks for the summary. :)

woojoo6663• 4 years ago

I don't think they should use the term right-half-plane zero if they don't explain laplace transform and the s-domain. Just call it an unstable system, or unbounded growth.

oh_my_goodness3• 4 years ago

The point of the article, perhaps not stressed enough, is that there are stable systems that can be very difficult to control. Zeros in the right half-plane don't indicate instability. They indicate nulls in the transfer function for unbounded inputs.

Poles in the right half-plane indicate instability. Zeros in the right half-plane indicate something a little more subtle. I believe the article is about the latter.

woojoo6663• 4 years ago

Good point, I mixed up the two. However, the right-half plane still refers to the s-domain so it doesn't make sense to keep referring to the a right half plane without explaining what plane it is

jbay80812• 4 years ago

That would be a right-half-plane pole.

An explanation of the Laplace transform is coming! This is an article, not a book.

_moof2• 4 years ago

Really should've run this by someone who knows how airplanes work, because they don't do what these hypothetical airplanes do. First off, "elevator flaps" is a howler. Elevators and flaps are two very different things. More importantly, you don't climb with the elevator; you climb with excess thrust. You can kinda sorta climb a little bit with just elevator, but unless you're making a very minor altitude correction, you'll slow way down and your climb rate will be really anemic, if you climb at all. But most importantly, an airplane won't just keep climbing and do a loop like that (unless you've got a thrust-to-weight ratio greater than one, which, to put it simply, you don't); the wing will stall way before you get anywhere near vertical, and the airplane will just stop flying until the angle of attack is reduced.

tzs• 4 years ago

More importantly, you don't climb with the elevator; you climb with excess thrust. You can kinda sorta climb a little bit with just elevator, but unless you're making a very minor altitude correction, you'll slow way down and your climb rate will be really anemic, if you climb at all.

So what happens to something like a jet fighter that is in level flight with the engines at maximum power if the pilot uses the elevators to raise the nose?

_moof2• 4 years ago

It will climb (and slow down), but again, it's the thrust that's producing the climb, and this is an extreme example that isn't illustrative. Fighter jets produce enormous thrust for their weight with many having thrust-to-weight ratios greater than one. If they weren't air-breathing, they could literally fly into space. A normal airplane, even if it weren't air-breathing, couldn't do that.

Depending on the configuration an F-16 has a T/W a little under 1.1, while a transport category aircraft (an airliner) will have a T/W somewhere between 0.20 and 0.35. Totally different performance characteristics.

p_l• 4 years ago

Powered airplanes hide the energy-management nature of climb/descent.

Essentially, you have potential energy (altitude) and kinetic energy (speed), and you trade one for the other. For safe range of inputs, your lift is function of pitch and airspeed, with drag as result. Increasing pitch increases lift (most of the time) at expense of increased drag. Lift gets you higher raising your potential energy, which you can spend back on glide, exchanging it for kinetic energy necessary for airflow. With powered plane, you provide extra kinetic energy that can be spent on higher lift. You use your stick to manipulate energy/speed, and in fact it's common to descend while pulling the stick on purpose.

A fighter jet with TWR above 1 has enough extra energy that it's going to be able to maneuver rapidly (afterburner/reheat exists for it, even, as jet engines are slow to spool up). Plane with lower TWR, or with significant mass, is going to behave closer to glider (heavy planes like airliners make energy management a big issue)

VogonPoetry10• 4 years ago

I think this article is trying to make a political statement, "Our leaders have just done something that is going to cause a future problem, a crash".

I have only read the linked article, I think the followup might be available, but I'm not going to read it before posting this -- because this article was meant to be clickbait, to get you hooked and possibly mis-informed.

The article as it stands (incomplete) contains some classic argument fallacies.

It is an "appeal to authority", by attempting to explain control theory. This person knows something.

The examples all result in bad things, airplanes crashing, hard drives (crashing - perhaps a stretch). Bicycle at the edge of a cliff. Eating too much ice cream. Chernobyl. This is a possible "appeal to emotion".

The final issue is an "incomplete comparison". The graph at the bottom of the article shows housing prices -- all of the previous examples have mathematical models which can explain the behavior. For the last graph no model (the equations) is presented. Without a model you cannot use control theory to analyze the zeros and poles.

The last graph wants you to believe in a continuing upward trend, the fallacious argument, implies that whatever the Canadian political change that has been made is going to make this trend to zero, despite the intention to keep it trending upwards.

Background: A long time ago, as a mathematics undergraduate, I did take a course in control theory, it was run out of the engineering department. It was somewhat awkward as quite a few mathematical terms were not-quite-the-same. I get the same unease when reading this for poles, zeros and right-half-planes. Any engineering textbooks that use j for sqrt(-1) instead of i (because that is taken) is an indicator. Yup, this could be construed as an "appeal to authority".

any1• 4 years ago

Textbooks that use j instead of i are an indicator of what? Why is slightly different notation awkward? It's not as if notation is always consistent within the field of mathematics, right?

VogonPoetry10• 4 years ago

The context of the writer. Apologies if I implied that the notation semantics were important -- letter exchange is free; concept change is not. What I indented to mean was the use of the terms poles and zeros is a bit wonky. Attempting to be cute with a physicist's hat, a pole is a "black hole", an entity you must not cross because the calculus will not work. It is a discontinuity, a singular value. There are worse things in mathematical generalizations; a cut / line in a manifold. But tools to cope.

fluoridation6• 4 years ago

That's not an appeal to authority. An appeal to authority is "P is a statement in the topic T. A is an expert in T. A says P is true. Therefore P is true." If I draw a parallel between two topics and proceed to explain one of them in order to argue something about the other one, that's not an appeal to authority.

It could be that the reader fails to understand the explanation and instead treats the writer as a trustworthy authority. That would not make the argument an appeal to authority or even fallacious.

It could be that the analogy is poorly justified, which would make the argument incomplete, or it could even be that the writer failed to understand some subtle aspect of the second topic that makes the analogy inappropriate. Neither of these would be appeals to authority.

VogonPoetry10• 4 years ago

In this case the author (A) is setting themselves up as an authority in (T). The statement (P) in this article has not been revealed, but because (and I suspected separated) is in the followup (which I have now read). The followup does refer back to the "now you know about poles" -- the appeal to authority.

Victerius• 4 years ago

I call it the "Ian Malcolm syndrome", in reference to the character (a mathematician) in Michael Crichton's Jurassic Park who tries to apply mathematical concepts to the dinosaur attraction to try to show why it is doomed to collapse.

There are a lot of similar posts online about people trying to use chaos theory (or what they believe chaos theory is), "systems theory", and other obscure concepts to explain why, for example, climate change will be the end of us all, or the global economy will enter a depression, or any other extremely pessimistic macro scenario. I see three reasons why these people are doing it. (1) To draw attention to themselves as someone with a superior intellect (which they actually aren't) (2) To construct an analysis no one has constructed before, in a "all these professional economists are saying the same thing, but here's why they're all misleading themselves, thanks to my ace card of a theory" kind of way, and (3) A love for the hyperbole, for dramatic outcomes.

A classic economic prediction like "Inflation pressures will potentially slow down growth in the short term, but the behavior of the economy over the next 12 months remains unpredictable" is not enough. It MUST be "A catastrophic economic collapse is inevitable", or "We are going back to the Middle Ages by the end of the decade", or "Hundreds of millions will die in the next few years".

sebastos• 4 years ago

Er, Ian Malcolm, the character who was introduced so that he could arrogantly predict that the park would collapse, only to get his satisfying come-uppance when... he turns out to be completely correct?

Natsu3• 4 years ago

Why doesn't all the same analysis apply to your making of "Ian Malcolm syndrome" itself?

yt-sdb2• 4 years ago

I agree. The author uses technical jargon but is clearly intending the reader to be non-technical. Otherwise, footnote 1 should just be in the main text. The rhetorical effect, intended or otherwise, is an appeal to (the author's) authority. As you say, the author then switches to a topic he likely much less expertise in (housing prices, inflation, etc.) while relying on the reader to believe that this new problem shares some mathematical foundations with the original set of problems.

TimPC2• 4 years ago

I’m curious what the right half plane zero in housing is. In general, we correct inflation through interest which we raise gradually waiting a month or two to observe the effects of a 0.5% raise. I think that should be sufficient to avoid the control feedback effects he talks about in this article.

jbay80812• 4 years ago

The RHP0 in (Canadian) housing comes from including mortgage interest payments -- a term that increases in proportion to interest rates -- in the measurement of the CPI.

VogonPoetry10• 4 years ago

I have now read the followup article.

There are, to me some language red flags. The followup article now uses "we' -- suggesting this is not the work of an individual. This was not present in the initial post.

An attempt is made to build a model. Curiously a visual programming model is now used. No explicit differential equations are given. If the equations are not given there is no way to check the model. So "pole" and "zero" analysis is mute.

There are no tests to validate this model against past situations - an easy and free test, the past history is known. If the model that has been created and can predict the future, why would was it not run it against the past and show the predictive success?

The built model is only presented against the current situation -- is it circumstantial / opinion based.

My understanding of science is -- build a model that can predict the future / what happens accurately, i.e. matches the measurements. The consensus agrees that this is the (current) best model.

Prices are ultimately influenced by a sentiment factor, a value derived from a I have no idea what human feeling / property. Some power "enabled" humans have been endowed with a much higher influence on this parameter. Where is this value expressed in the predictive equations?

dukeofdoom3• 4 years ago

Canada does not have 30 year mortgages. Most home owners have mortgages with 3-5 year terms. Since 3-5 years ago rates were low, and home prices were low, people's mortgage expenses are still low. However that's about to end. As people either bought at much higher prices in the last 2 years, and/or the older mortgages are getting renewed at higher interest rates. This will result in drastic increase in what people need to pay to stay in their home.... possibly causing a market crush.

PragmaticPulp• 4 years ago

I agree. The first article was interesting when it was talking about control theory, but the hard pivot to economics and politics at the end turned the article from potentially informative to a disingenuous appeal to authority.

The author clearly knows a lot about control theory, but this seems to have led to an idea that he knows a lot about everything. This is a theme I see all too frequently in people who list “systems thinking” as one of their areas of expertise (like this author).

I have the benefit of the doubt and read the second article looking for perhaps some actual modeling and references to econ literature, but he merely refers to a couple papers and then admits that his model was home-grown and will be simpler. Some parts of it might be right, but this has too many elements of “I’m an expert in one thing therefore I’m qualified to talk about everything” for me.

kspacewalk26• 4 years ago

I'll be that guy. Please don't hate me.

So "pole" and "zero" analysis is mute.

Moot.

mellavora2• 4 years ago

There are, to me some language red flags. The followup article now uses "we' -- suggesting this is not the work of an individual. This was not present in the initial post.

First person plural ("we") is normal in technical and scientific writing.

relaxing2• 4 years ago

Thank you for taking the time to write these follow-ups. I was left with the same uneasy feeling at the end of the article, and you’ve articulated perfectly why.

starkd2• 4 years ago

I admit I was a bit disappointed by the end of the article. But I don't think it was strictly an appeal to authority. The author is evidently a very good writer, but he might have done a better job at summing up a conclusion. It's very difficult to tell just what point he was trying to make. He never even said just what his inputs and outputs are in his housing model. Even if he is saving that for a later article, he should have at least mentioned that.

CFLAddLoader• 4 years ago

The author of the article has had their understanding of systems eat their understanding of humans. Every time they make a realization about people, it immediately gets turned to getting more understanding and control over the world. Which doesn't leave any room to grow for their understanding of humans.

This makes it really hard for them to write an article for non-technical people. To do so, they would have to connect the human side of things to the systems theory side of things. Which they can't do, because they don't understand the human side of things.

They aren't using bad-faith arguments to convince you that they are correct. They simply have no idea how to talk to you. They are in fact trying to be helpful, by presenting what they think is really useful information to understand and control the world with. Their help is useless to you, but they are trying.

syntaxfree• 4 years ago

“the math that I know is THE math to explain inflation, political instability and more”

TimPC2• 4 years ago

The follow-up article tries to argue there is an over five year lag to interest correcting inflation based on undisclosed principles of a model they don’t provide equations for. Their argument is in effect that the growth of roughly 30% of the inflation basket of goods (housing) dominates the shrinking of the other 70% of the inflation basket of goods. The analysis is suspect as they don’t seem to model how much the 70% shrinks or slows in growth at all. They also aren’t very numerically precise on the interest effects on housing prices and misanalyze how mortgages work in Canada.

Their analysis argues that roughly 4% of mortgages are replaced every year while 96% retain the same rates. This makes it hard to argue interest rates have the desired effect size but that doesn’t stop them from trying.

jbay80812• 4 years ago

The analysis is suspect as they don’t seem to model how much the 70% shrinks or slows in growth at all.

It doesn't model it at all. The trajectory of ex-shelter CPI is taken as an input. For the provided scenarios, all non-shelter components are assumed (as a scenario premise, not a prediction) to instantly return to a 2% growth rate and stay there.

skybrian• 4 years ago

This seems uncharitable. The author is trying to explain how knowing a bit of control theory could be helpful for understanding economics. Unfortunately they’re having trouble writing for a general audience, perhaps due to being too comfortable with the math. This doesn’t mean anything nefarious, just that they could use an editor.

eftychis3• 4 years ago

Comment: j is used instead of i in certain contexts -- e.g. circuits -- when it is possible to confuse it with the current (same letter used). As e.g. an Electrical and Computer Engineer you just move back and forth on sqrt(-1) notation appropriately. Not sure beyond that about the not quite the same terminology -- not my experience but who knows.

The article is kind of a "layman's article" to put it politely. I am erring towards the side of hocus pocus -- if economics were that well behaving and linear and composable as the author models, it would be a solved field years now. And control theory wouldn't exist as a field for 50 years, as it would have been solved. Do the components diverge? Where in the complex plane are they defined to begin with? Anyways.

Also: if you should learn something: Central Banks have a terrible track record, and despite what they keep saying are political. The joke is that they are the blind driver with a gas and break pedal that have a 2 years delay. So good luck to us all.

iandanforth• 4 years ago

TLDR; If you want to make something go up (or down, or left or right) sometimes it goes down (or whatever the opposite is) a bit first. If you were to try harder to make it go up at that point, the problem gets worse, repeat until your system crashes. Times like these are called right-half-plane zeros and any control system you build has to be aware of them and designed to avoid causing a feedback loop.

civilized2• 4 years ago

The article claims to be for a broad audience, and contains insinuations of political relevance. But right-half-plane zeros are a purely technical concept, and they're not explained with enough technical detail to be air-dropped into the middle of a broad audience piece.

It's almost good. It would be good if it dropped the pretensions to technicality.

dhosek• 4 years ago

I lost interest before I got to his political point (about inflation), but the comments made me go back and while it’s an interesting point, it ignores the fact that the current bout of inflation is almost certainly a consequence of the Covid stimulus plus the supply chain disruptions with a dose of war in Ukraine. And it turns out that inflationary pressures seem to be declining outside of the volatile food and energy sectors (see https://jabberwocking.com/inflationary-pressure-seems-to-be-...). I have a bit more faith in domain-specific analysis than abstract mathematical approaches.

civilized2• 4 years ago

It's like "I know math so take me seriously" but then the posts don't use the math in any meaningful way.

Not that I think they should use the math. Just skip the systems lectures and the "there is math behind this" and go straight to the analysis.

dukeofdoom3• 4 years ago

If you are going slow on a bicycle, and want to turn right, you might have to turn your front wheel left and then right, just so you don't fall over and maintain upright balance. Which would be a problem if you are riding along a cliff's edge.

deepsun• 4 years ago

Countersteering on a bicycle is another example: To turn right, a cyclist will first steer slightly to the left.

It's very visible and pronounced on heavier bikes, like motorcycles. Especially if you try riding a very heavy cruiser bike -- you'll immediately notice that countersteering is the only way to turn it. No matter how you try to lean it, it won't respond and will just go straight, but it'll respond very easly to handlebar inputs.

dukeofdoom3• 4 years ago

I watched this video in preparation for a Drone license test. Really interesting stuff.

Human Factors for Drone Pilots

https://www.youtube.com/watch?v=UHYjl0u2UMA

Based on what real pilates need to know about human physiology as it relates to steering airplanes.

oh_my_goodness3• 4 years ago

OP: dy/dt = dx/dt + Ax

Comments: The OP is a troubling covert political statement. The real issue here seems to be US midterm elections / climate change / COVID / appeal to authority.

dqft• 4 years ago

Everything is fine. We are prepared and can handle the situation(s). Everything is fine. It's fine. We have it under control. Fine. We have the metrics and they're fine. Okay?

entropicgravity• 4 years ago

Not to mention poles in the unit circle.

bsedlm• 4 years ago

can anybody tell me what's f in this scenario?

dy/dt = dx/dt + Ax

is it the case that y = f(x,t)?

thanks

sbf501• 4 years ago

Odd that OP would give a grim warning without an equation.

I graduated in 1991 with BSEE and the curriculum was a rush to get us to diffeq and linear systems because 90% of the remaining three years of classes were taught using S and Z plane analysis. My engineering professors were incapable of introducing a concept without starting with a differential equation.[1]

For the author to spend so much time explaining control theory, then sort of give up on it, was disappointing. Ironically, knowledge of control systems and warnings about them should pervade the thought process of the thinker and prevent them from making grandiose claims. Perhaps OP only has a topical knowledge but can spin a good yarn.

[1] I'm a career programmer, but I always start my thoughts with a d/dt: whether it's an RTOS project, a graph GraphQL endpoint, or USB driver. I guess "if all you have is a hammer..."

mattmaroon• 4 years ago

I have a feeling this is going to be about interest rates and housing.

shreyshnaccount• 4 years ago

My dynamical systems prof loved that xD

whycombinetor7• 4 years ago

The author lost all credibility when he said I have to countersteer to turn my bike. The author is a "professional engineer and consultant, specializing in feedback systems and motors" too. There are >50 million cyclists in the US who would presumably immediately recognize this as a false statement and he didn't ask a single one of them to proofread.

Edit since some people are confused by this apparently: You can steer your bike away from the edge of a cliff by leaning to initiate the turn instead of countersteering. This makes the author's advice of "Don’t ride your bicycle on the edge of a cliff" much less critical.

mk122• 4 years ago

I'm curious how you're so confident about this. I think if you ask most people (even most cyclists) what happens when you turn the handlebars to the right, they'd say you turn to the right. I cycle almost every day and I thought this for a long time. Have you actually tried it? When I tried it, I found that it forces me into a left turn.

whycombinetor7• 4 years ago

Have you tried leaning to the left while biking in a straight line? It results in you turning left without your front wheel ever having gone to the right (i.e., turning without countersteer).

adiM• 4 years ago

One can always test the hypothesis via an experiment:

https://youtu.be/9cNmUNHSBac

dilyevsky2• 4 years ago

Heh was about to reply with the same video. It’s really counterintuitive even for people who ride a lot

whycombinetor7• 4 years ago

Indeed. I like this clip where they show bike steering to the left, without its front wheel ever going to the right side of its original path, by only leaning and no countersteering. https://youtu.be/9cNmUNHSBac?t=445

jbay80812• 4 years ago

In the interest of defending my credibility, consider the challenge of explaining some concept to an audience using concrete examples, when any example has subtle associated details and exceptions that, while fun to explore on their own, do not further the goal of explaining the original concept.

If you'd prefer, I can explain the concept of the right-half-plane zero using only abstract systems described by transfer functions. But I suspect that will be less effective from an education standpoint.

I maintain that countersteering a bicycle is an excellent example of a RHP0, and it's no coincidence that a translating inverted pendulum also has an RHP0 in its control solution.

I also am one of those cyclists and, at least prior to the pandemic, commuted by bicycle every day.

xg15• 4 years ago

As a layman, I had to think of drugs or other dopamine hits first - actions which increase well-being in the short term but are harmful in the long term.

But what he really talks about seems to be the opposite: Actions which case some mild harm in the short term but increase well-being in the long term. So I guess something like working out or going on a diet or making a downpayment for a house?

Except the failure mode is also counterintuitive: Normally, we tend to overvalue the short-term downsides of those actions and therefore shy away from them, missing out on the long-term benefits. But he talks about a situation where we overvalue the long-term benefits but ignore the short-term and overdo the action until the short-term harm becomes critical.

So, e.g. someone working out, getting muscle-ache - and then working out more to counter the ache - which will only lead to more of it until the workout actually starts to become detrimental to their health.

It's easy to see how this would trip up automated control loops, but I don't really see how this has practical application outside of control theory.

cyanydeez• 4 years ago

Economics is not a physical theory.

Full stop.

You need to know what right-half-plane zeros are

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