# control system interactions

**URL:** <http://discourse.iapct.org/t/control-system-interactions/13435>\
**Category:** CSG1994\
**Created:** [July 29, 1994, 8:58pm UTC](http://discourse.iapct.org/t/control-system-interactions/13435 "1994-07-29T20:58:34Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![\_Avery\_Andrews](http://discourse.iapct.org/letter_avatar_proxy/v4/letter/_/848f3c/32.png) [@\_Avery\_Andrews](http://discourse.iapct.org/u/_Avery_Andrews)\
**Post date:** [July 29, 1994, 8:58pm UTC](http://discourse.iapct.org/t/control-system-interactions/13435/1 "1994-07-29T20:58:34Z")

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[Avery.Andrews 940729.1531]  
(Jeff Vancouver 940728)

I think what's being gotten at here is the existence of a big collection  
of hard questions about how to make control-systems interact properly -  
the 14deg.c and nu14.c programs by Bill Powers and myself are part of an  
attempt to get at some of these issues in a relatively concrete setting.

\>They just hang. Either, their gains have been turned so far down  
\>that they receive no energy, or they are given some general reference  
\>signal that is very easily matched. The former is not possible in your  
\>system diagrams and the latter requires an output from the higher-order  
\>system to change when the error signal subsides (this might have been  
\>talked about but I missed it)

I don't think either of these solutions should be rejected out hand, but  
Bill Powers has a clever third alternative: `let it be' mode wherein  
the reference signal for a control system is identical to its  
perception:

> **···**
>
> ---------  
> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;\> \> R  
> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;\> v  
> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;\> P ----- E  
> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;-----\>| C |------  
> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;\> ---- |  
> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;\> \>
> 
> Since P=R, E is zero, and no behavior is produced.
> 
> This can be seen more generally as the problem of dealing with excess  
> degrees of freedom, and it's my impression that a number of different  
> solutions are employed. If hand-orientation doesn't matter, the tendency  
> seens to be that the wrist is set at zero pitch and yaw  
> (at least by males who want to be perceived as heterosexual).
> 
> Another excess df arises from the fact that the `roll' of the wrist  
> results from adding the roll at the elbow and shoulder joints, so there  
> are an infinite number of combinations available for producing most  
> orientations (except the extremal ones), which can be observed by  
> holding a coffee cup in front of you, vertical and full, and slowly  
> moving your elbow.
> 
> It's my impression that the normal solution is to roll each joint thru  
> the same proportion of it's total range (about 180 deg for the elbow,  
> a bit more than 90 for the shoulder), so that the total roll is  
> distributed on a roughly 2:1 basis. Thus, if you hold your hand  
> out in front of you with the palm up, your elbows will tend to be  
> close to your sides, but if you roll the palm 90% (as might be  
> appropriate to holding a cup), your elbows will tend to stick out.  
> But this can be overriden, if necessary, as when at a crowded table.
> 
> So my conjecture is that there are `default' control systems, different  
> ones for different joints, etc., that can be overridden when necessary.  
> How to handle the overriding, I'm not sure. nu14 has an `elbow-stickout  
> limit' system, whose error signals switch off the gain of the the  
> roll-distribution system. But that's just the first organization  
> that I got to work reasonably well.
> 
> Avery.Andrews@anu.edu.au