On Thu, 1 Nov 2012 07:09:56 -0700 (PDT), Peter C. Wallace wrote: > On Thu, 1 Nov 2012, EBo wrote: > >> Date: Thu, 01 Nov 2012 09:47:30 -0400 >> From: EBo <e...@sandien.com> >> Reply-To: EMC developers <emc-developers@lists.sourceforge.net> >> To: emc-developers@lists.sourceforge.net >> Subject: Re: [Emc-developers] ferror calculation incorrect in motion >> >> This is exactly why I asked for references so we could look at the >> mathematical definitions and either experiment with working machines >> or >> at least develop a simulation model which stress tests the various >> aspects of the two approaches. I would rather a fully principled >> approach to this (including reviewing the math) than the current >> list >> thread direction. > > This is really simple > > http://en.wikipedia.org/wiki/PID_controller > > Note that the first paragraph says: > > 'A PID controller calculates an "error" value as the difference > between a > measured process variable and a desired setpoint. The controller > attempts to > minimize the error by adjusting the process control inputs.' > > This is certainly my understanding (that the task of the PID > controller > is to minimize its input error) > > The current situation is that the PID loop and ferror have different > setpoints > but the same "process variable" so if you tune to minimize ferror you > need to > force the PID loop to minimize something different than its input > error. This > makes the integral term mostly unuseable (since the intergral term > will force > the input error to 0 and generate a ferror proportional to > velocity*sampletime)
That is the same reference I looked up yesterday, when I got into this discussion, so can we all take this as the definition we will use? Assuming this is acceptable: It makes sense to have them reference the same variables, but the question is if they the reference the same variables if they will essentially be the same thing, and if that is what needs to happen. According to the other poster, that ferror was to be forward looking, which technically is not the same thing, but maybe we are missing a modeled velocity term in the calculation. Frankly I have not crawled into the depths of the subtleties in years, but I see how if this is not done correctly it will completely hose everything downstream -- simply put, this HAS to be done correctly. So, looking back at the Wikipedia reference, there is no reference for the ferror. One interpretation of the geometric meaning of the problem is to take it from a numerical stability perspective <http://en.wikipedia.org/wiki/Numerical_stability> where we are defining an N-dimensional realization of the geometric problem (probably in Euclidean space to simplify things). If the above reference for the ferror is acceptable for general discussion, then we can can begin to analyze (formally if anyone has the training and skills) what we want to do. Intuitively, I want to minimize the ferror, and parameterize the PID accordingly. I say that because what I really care about in the end is the actual dimensions of the final piece. So, from that perspective, what is the mathematical relationship of the PID parameters to ferror in time(t)? ------------------------------------------------------------------------------ Everyone hates slow websites. So do we. Make your web apps faster with AppDynamics Download AppDynamics Lite for free today: http://p.sf.net/sfu/appdyn_sfd2d_oct _______________________________________________ Emc-developers mailing list Emc-developers@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-developers
