Re: [Emc-users] FF0, FF1, FF2

[Andy P. <an...@an...>]

On 14 September 2010 14:34, Igor Chudov <ic...@gm...> wrote:

> Could someone explain what are FF0, FF1 and FF2 parameters pertaining
> to tuning a servo loop?

Given a commanded value  "C" and a measured value "M", the output of the PID is:

P * (M-C) + I * (integral.dt)(M-C) + D * d(M-C)/dt + FF0 * C + FF1 *
dC/dt + FF2 * s2C/dt2.

So, the PID parameters control the response of the controller to the
error between commanded and actual, and the FF parameters control the
response to the actual input.
PID can be considered to be the "closed loop" part of the control, and
the FF values are open loo, sometimes called "precontrol"

As a random example, if you wanted to control spindle speed and knew
that your VFD was 2V / 1000rpm then you could use purely FF0 with a
value of 0.002 to run completely open loop. You might then add a
little bit of I on top to take out the steady-state error if the
response was not totally linear.

Whether you want an FF term and which one to use depends on the
physical relationship between the inputs and outputs. The example
above would not work at all well if you were trying to control for
spindle position rather than speed.

-- 
atp