4 ms·

One of my peeves with control theory, at last the way it's used by proponents I've met, is that it's designed to react to the smallest input. This makes intuiti

by kqr 4y ago

One of my peeves with control theory, at last the way it's used by proponents I've met, is that it's designed to react to the smallest input. This makes intuitive sense when trying to optimise performance, but in a system with natural variations around the mean (noise), this can easily lead to amplifying variation and worsening performance. The alternative is statistical process control. In practise, it performs about the same for basic tasks, but it gives you qualitative insights into many other things too: (registration-walled) https://www.qualitydigest.com/inside/operations-column/process-monitor-charts-071116.html https://www.qualitydigest.com/inside/operations-column/proce...

xvedejas 4y ago
I think you just haven't dived deep enough into control theory. Your criticism should be addressed, for instance, by introducing an appropriate Kalman filter. Yes, much of the truly interesting and powerful aspects of control theory are in its interaction with statistics.
- kqr 4y ago
  The Kalman filter also assumes every data point conveys a signal of change, however small. That approach inevitably leads to reading tea leaves (interpreting noise as signal). It is more economical, in my experience, to start from the idea that unless there's been an actual consistent change in the process, any data you're getting reflects noise around the process mean. SPC is designed to tell those two cases apart.
  - deleted 4y ago
    [deleted]
  - krapht 4y ago
    I'm really struggling to follow along. Control theory is nothing but a set of math tools to derive controllers and filters. Just because most people stick to linear systems with gaussian noise doesn't mean you're limited to that. I'm not sure how a quality control methodology has anything to do with control theory in general.
elromulous 4y ago
Deadbands are used fairly often. I'll agree that they're suboptimal in practice though
omnicognate 4y ago
Knowing absolutely nothing about either, and looking at the wiki articles [1] and [2], Statistical Process Control sounds like something specific to quality control rather than a general alternative to control theory. Is that the SPC you're talking about? Is control theory used in quality control? Does SPC have wider applications? [1] https://en.m.wikipedia.org/wiki/Statistical_process_control https://en.m.wikipedia.org/wiki/Statistical_process_control [2] https://en.m.wikipedia.org/wiki/Control_theory https://en.m.wikipedia.org/wiki/Control_theory

by kqr 4y ago

xvedejas 4y ago

I think you just haven't dived deep enough into control theory. Your criticism should be addressed, for instance, by introducing an appropriate Kalman filter. Yes, much of the truly interesting and powerful aspects of control theory are in its interaction with statistics.

kqr 4y ago
The Kalman filter also assumes every data point conveys a signal of change, however small. That approach inevitably leads to reading tea leaves (interpreting noise as signal). It is more economical, in my experience, to start from the idea that unless there's been an actual consistent change in the process, any data you're getting reflects noise around the process mean. SPC is designed to tell those two cases apart.
- deleted 4y ago
  [deleted]
- krapht 4y ago
  I'm really struggling to follow along. Control theory is nothing but a set of math tools to derive controllers and filters. Just because most people stick to linear systems with gaussian noise doesn't mean you're limited to that. I'm not sure how a quality control methodology has anything to do with control theory in general.

elromulous 4y ago

Deadbands are used fairly often. I'll agree that they're suboptimal in practice though

omnicognate 4y ago

Knowing absolutely nothing about either, and looking at the wiki articles [1] and [2], Statistical Process Control sounds like something specific to quality control rather than a general alternative to control theory. Is that the SPC you're talking about? Is control theory used in quality control? Does SPC have wider applications? [1] https://en.m.wikipedia.org/wiki/Statistical_process_control https://en.m.wikipedia.org/wiki/Statistical_process_control [2] https://en.m.wikipedia.org/wiki/Control_theory https://en.m.wikipedia.org/wiki/Control_theory