# Open-loop mechanisms; Too bad; Bad idea

**URL:** <http://discourse.iapct.org/t/open-loop-mechanisms-too-bad-bad-idea/7990>\
**Category:** CSG1993\
**Created:** [November 8, 1993, 5:04pm UTC](http://discourse.iapct.org/t/open-loop-mechanisms-too-bad-bad-idea/7990 "1993-11-08T17:04:49Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![William\_T\_Powers2](http://discourse.iapct.org/letter_avatar_proxy/v4/letter/w/c67d28/32.png) [@William\_T\_Powers2](http://discourse.iapct.org/u/William_T_Powers2)\
**Post date:** [November 8, 1993, 5:04pm UTC](http://discourse.iapct.org/t/open-loop-mechanisms-too-bad-bad-idea/7990/1 "1993-11-08T17:04:49Z")

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[From Bill Powers (931108.0900 MST)]

Chuck Tucker (931107) --

> I certainly believe it is possible to construct a machine which  
> has both open and closed looped parts to it.

ALL machines, and people too, are constructed of components which  
individually operate open-loop. Think of a muscle: signals in,  
contractile forces out. Or a sensory nerve-ending: stimulation  
in, perceptual signals out. Even when the components may have  
internal control loops (biochemical, for example) that shape  
their responses, we treat them as input-output devices at a  
certain level of analysis. We break the whole complex interactive  
system down until we reach a level of detail at which each  
component can be described as a function making an output depend  
on one or more inputs in a certain predictable way. Once the  
components are all described in this way, we can begin putting  
them back together by specifying which inputs come from which  
outputs of other functions. This leads to a system of  
interrelated components, the interrelations being now a system of  
equations. When we solve those equations analytically or by  
running models in simulation, and when the resulting system  
produces behavior like that of the real system, we can say we  
understand how the real system works. And we can predict its  
behavior in a lot of new circumstances we would never have  
thought of otherwise. We can also deduce what parts of the system  
have failed when its overall behavior develops some sort of  
problem.

> My favorite example recently are the windshield wipers which  
> appear to be closed looped but are not at least on my car.

Why do the wipers appear closed-loop? When you turn them on, they  
go back and forth. They will go back and forth even if the rubber  
blade falls out. The shaft on which they are mounted will rotate  
left and right even if the whole blade assembly is removed. Such  
disturbances have no effect at all on the active part of the  
system, the motor and circuits that create the oscillating  
movement.

There's an obvious closed-loop aspect to the wipers which is easy  
to overlook when you're focusing on the wipers instead of your  
purpose in using them. If you turn the stalk or press the button  
and the wipers don't go into motion, you get a great big error  
signal. You try to turn the stalk farther or press the button  
harder, until you figure out that the mechanism is broken. If the  
wiper blade drops off, you immediately change your action to  
avoid scratching the windshield.

Note how the language changes. When you're talking about the  
mechanism itself, you say "it" moves the wiper assembly back and  
forth across the windshield. But if the wiper blade drops off and  
you don't immediately turn the wiper off, YOU would be scratching  
the windshield. The wiper mechanism returns to its proper place  
as a means by which you can achieve or avoid going against your  
own purposes.

When you're trying to decide between open loop and closed loop  
descriptions, remember to think about the effects of disturbances  
of the output effect. If the device just keeps on blindly  
producing the same action despite a drastic change in the effects  
of the action, it's most probably open-loop.

> **···**
>
> --------------------------------------------------------------  
> Hugh Petrie (931108) --
> 
> Fooey. Welcome to the club.
> 
> Your protest letter is strong enough to bother a reasonable  
> person. I think it's very appropriate to protest. In my long  
> years of experience with referees, my two protest letters have  
> succeeded in reversing a verdict 1-1/2 times. The half time  
> resulted in a new review but another rejection. The whole time  
> resulted in publication (the 1971 rat paper).
> 
> The main thing is to keep up the effort and not let the bastards  
> wear you down.  
> --------------------------------------------------------------  
> Rick Marken (phone call yesterday) --
> 
> Thanks for bringing me down to earth. In my enthusiasm for my own  
> ideas I didn't bother to check out my brilliant new conception of  
> how a control system might switch automatically from closed-loop  
> to feedforward operation. The one factor I would have seen  
> immediately if I had actually tried a model (as you did) is that  
> with nothing but inhibitory reference signals, there is nothing  
> to cause any output or perceptual signal to appear, except  
> external disturbances. Some source of positive nonzero outputs  
> has to exist if this is going to work -- an equivalent of control  
> of muscle tone in systems that use opposing muscles to control an  
> arm.
> 
> With positive (excitatory) reference signals using opposed one-  
> way systems, everything works fine: the system can be turned off  
> just by setting both reference signals to zero. The inhibitory  
> perceptual feedback signals can't cause any output by themselves.  
> But unexpected loss of the perceptual signals during operation  
> does not lead to "nice" behavior.
> 
> Somewhere in there may still be a workable solution, but whatever  
> it is (if it exists) it isn't the one I proposed.
> 
> This makes me wonder whether the general concepts others have  
> proposed for this situation have actually been checked out with  
> running models. If so, I'd like to see an example. As I can  
> attest, some ideas just sound so self-evidently workable that one  
> doesn't bother to try them out. I can also attest that not trying  
> them out is a mistake.  
> ---------------------------------------------------------------  
> isaac kurtzer (931107.1530) --
> 
> Some computing centers use weekend hours to shut down various  
> functions for maintenance. That can lead to messages being  
> delayed until the system comes on line again. It could be your  
> system, or it could be some other system enroute between the U.  
> of IL computer and yours -- or even the U. of IL computer itself.  
> The internet keeps trying until it succeeds, giving up only after  
> (I think) three days.
> 
> I got about 5 messages from CSG-L this weekend.  
> --------------------------------------------------------------  
> Best to all,
> 
> Bill P.