On 29 January 2012 13:23, John Thornton <bjt...@gmail.com> wrote: > So, looking at one of the links for servo tuning is our > > FF0 = Velocity Feed Forward Gain. > FF1 = Position Feed Forward Gain. > FF2 = Acceleration Feed Forward Gain or Friction Feed Forward Gain. > > I would like to update the docs with some descriptive text in addition > to "0th order feed forward gain" as that and the other descriptions only > seem to make sense to a mathematics professor.
There is a reason that mathematicians talk the way they do, it is to be unambiguous in the general case. I think the best solution would be for all LinuxCNC users to take a maths degree, but that might be impractical. The problem with your explanation is that it assumes a position-command / position-feedback system and there are other possibilities that are not that unusual. (closed-loop spindle control is one) FF0: This term adds a value to the PID output directly proportional to the input. It is useful for any system where a steady-state output requires a non-zero input. The most likely case is a closed-loop spindle speed control, where 10V might give 1000rpm, and a FF0 gain of 0.01 would mean that the PID terms were only required as correction, not the basic output value. This term would not normally be used with a position-feedback system, except possibly to compensate for the effects of a spring counterbalance. FF1: This term adds a value to the PID output proportional to the rate of change of the input. So, if the input is position, FF1 is proportional to velocity demand. If the input is rpm then FF1 is proportional to angular acceleration/torque demand. This term can often be very useful to increase the responsiveness of a position-feedback system with velocity-control servos. FF2: This adds a value to the PID output which is proportional to the rate of change of the rate of change of the input (mathematically, the second order differential with respect to time). In a typical position-control loop this would correspond to the acceleration. (in a velocity-control system it would correspond to the "jerk"). In the position-control case this would be useful to compensate for the mass of a heavy table, for example. FF3: This value does not exist and has been inserted to see who is still paying attention. A negative FF3 term could be used to add a degree of jerk control to a position-feedback system. -- atp The idea that there is no such thing as objective truth is, quite simply, wrong. ------------------------------------------------------------------------------ Try before you buy = See our experts in action! The most comprehensive online learning library for Microsoft developers is just $99.99! Visual Studio, SharePoint, SQL - plus HTML5, CSS3, MVC3, Metro Style Apps, more. Free future releases when you subscribe now! http://p.sf.net/sfu/learndevnow-dev2 _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
