Javi,
One minor clarification as follows:
On Wed, Jan 28, 2015 at 05:00:35PM +0000, Javi Merino wrote:
<big cut>
> +
> +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 */
---
Eduardo Valentin
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
On Wed, Feb 25, 2015 at 02:48:49PM +0000, Javi Merino wrote:
> 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
Yes, I agree that they are constants. But if you deploy the thermal zone
with different sampling period on different devices, then the behavior
will change.
> 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.
OK. Then I would prefer to add the division, as it makes the code aligned
to the concept.
>
> Cheers,
> Javi
On Wed, Feb 25, 2015 at 07:00:38PM +0000, Eduardo Valentin wrote:
> On Wed, Feb 25, 2015 at 02:48:49PM +0000, Javi Merino wrote:
> > 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
>
> Yes, I agree that they are constants. But if you deploy the thermal zone
> with different sampling period on different devices, then the behavior
> will change.
>
> > 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.
>
> OK. Then I would prefer to add the division, as it makes the code aligned
> to the concept.
Fair enough, I've added it to the code. We will send a v2 later today
if all regressions pass.
Cheers,
Javi