On 1/29/2012 8:10 AM, andy pugh wrote: > 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) The problem explaining it is I don't know enough about it to even pretend to know... > > 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. Thanks for the explanations for the above... If I add FF3 to the manual then someone has to make it work right? > 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. > John
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