Hi Eduardo, On Tue, Feb 24, 2015 at 06:21:26PM +0000, Eduardo Valentin wrote: > On Wed, Jan 28, 2015 at 05:00:35PM +0000, Javi Merino wrote: > > + > > +k_d > > +--- > > + > > +`k_d` configures the PID loop's derivative term constant. It's > > +recommended to leave it as the default: 0. > > + > > I know we are considering K_d = 0. However, ... > > <yet another big cut> > > > + /* > > + * Calculate the derivative term > > + * > > + * We do err - prev_err, so with a positive k_d, a decreasing > > + * error (i.e. driving closer to the line) results in less > > + * power being applied, slowing down the controller) > > + */ > > + d = mul_frac(tz->tzp->k_d, err - params->prev_err); > > > ... Shouldn't the above d component consider the rate of changes over time of > the error? > > I would expect you should do: > d = k_d * (dE / dt) > > or > > d = K_d * ((err - params->prev_err) / sampling_period) > > in plain C: > > + d = mul_frac(tz->tzp->k_d, err - params->prev_err); > + d /= tz->passive_polling; /* might require fixed point division */
Could do. To be honest, both k_d and passive_polling are constants so I don't think you get anything by doing this other than the added complexity of the fixed point division. As you said, the default k_d is 0, so I'm not strongly against it. Cheers, Javi -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
