Hi all,
Who could explain me the difference between PID , PIDD and ART (active
reasoning technology) ?
These computation are used for temperatures controls.

I know what is PID but concretely I don't explain me the difference
with PIDD and ART.
Many thanks for your help.
Eric B

PID is the "proportional integral derivative" that we all know and love.
For some slow processes (like temperature), it helps a great deal to use
the acceleration as well as the speed, PIDD would certainly fit with
"proportional integral double-derivative", but I'm just guessing. A
quick web search yields damn little results, so I don't think it's a
terrifically common term.
"Active reasoning technology" sounds like a way to automatically
generate excuses. I would assume it's some controller manufacturer's
name for a heuristic-based controller that uses an expert system of some
type -- unless someone tells me I'm wrong, it may help to give some
brand names that it's attached to.

I don't know of any terminology for the PIDD either. Perhaps PIDD'. In
anycase the PID is good for tuning plants with two poles and PIDD is
good for tuning plants with 3 poles as in the repeated poles thread.
Hydraulic systems can be simply modeled as a mass between two springs
which is a pair of complex poles. There is one more pole for
integrating the velocity into a position which makes three poles. I am
assuming the valves is fast and can be ignored. A PIDD is very good
for controlling hydraulic systems.
It all depends on the number of poles in the plant, not the response of
the process unless one of the poles is so fast relative to the others
and the controller that it can be ignored.
Peter Nachtwey

Hi,
Thanks for these first answers.
1 point to clarify, PIDD is also called PID square or PID2.
I well understand that PID use the speed (D) and PIDD use the
acceleration(DD).
But I don't see exatly the which one is better and the difference for
temperature regulation process.
Ebl

First, I have never seen or heard of a need for a second derivative
feedback on a temperature control system. It would only be needed if
one could identify three poles in the plant. Since most temperature
systems are identified as having one or two poles one can get by with a
PI or PID respectively. How many gains a controller needs is dependent
on the number of poles you are trying to place. Remember, the
integrator comes with its own pole so that can be counted. See the
repeated poles thread below. One can see that tuning a system with
three real poles can be done but there is only one solution that will
provide a critically damped response. Adding the second derivative
allows making the response faster. See the .pdf in the repeated poles
thread below.
If anybody knows of a plant with three real poles I would like to hear
of it. Meanwhile if they do exist then most of them can't be tuned
easily with just the PID in a PLC. One could use a statistical method
that just reduces the tracking error. I don't get involved with
temperature control systems too often to say how this is done in
practives. It there anybody out there that has?
Peter Nachtwey

Thanks for these answers.
More information.
This is a copy of gammaflux provider.
Gammalux build hot runner controllers for injection molding machine :
http://www.gammaflux.com/en/control.php
Sense - 20 times per second, Gammaflux controllers precisely measure
the thermocouple;
Control - the proprietary self-optimizing Gammaflux PID2 control
algorithm adjusts if the actual temperature deviates 0.1º F (0.05º C)
from setpoint.
The second derivative (PID2) monitors the actual temperature rate of
change. As a result, the TTC module regulates the output to the heater
in advance of achieving setpoint to limit or eliminate over or
undershoot.
They speak well about PID2 and the second derivative.

Some systems use two thermocouples. They can tell by the temperature
difference what the rate of change is much more quickly. Without the
second thermocouple one must differentiate the the single temperature
twice. The best resolution provide a minimum detectable second
derivative of
0.1/(0.05)^ = 40.0 degree/min^2. Hum.
Since the injection machine cycles can be very fast I would think they
would have a way of measuring the temperature disturbance in a cycle
and have faster updates than every 3 seconds.

Actually, it first derivative is the rate of change.

There is nothing special about this. This should happen whenever
there is a derivative term and no over shoot is desired.

I am sure it works well if their system identification can really find
3 poles. Implementing the second derivative is easy. Identifying a
3 pole system to to make good use of it is not.
I don't normally get involved with the temperature controls in an
injection molding machine, just the hydraulic injection part. It
would seem to me the best temperature controls would know the state of
the machine and adjust a feed forward or bias according to previous
history. I wouldn't be surprised if this is done already.
Peter Nachtwey

Here is a mathematical explanation of what a poles is. Engineers use
transfer functions to describe how a system will respond. These
transfer functions have a polynomials in the numerator and
denominator. These polynomials are function of s ( Laplace
Transform ) or z ( z transform ). Values of z or s that make the
transfer function go to infinity are poles and values of z or s that
make the transfer function that go to 0 are zeros.
http://en.wikipedia.org/wiki/Pole_ (complex_analysis)
For the most part I find the above definition useless for providing an
intuitive feel for what a pole manifests itself in reality. I prefer
to think of poles as representations how energy flows to and from
devices in a system. For example a mass and spring example has two
energy storage devices. One is the mass which stores kinetic energy
and the spring store potential energy. A DC motor stores the
rotational energy and inductance of the armature store some energy.
Thermal systems must heat material so there is a pole that reflect how
the material absorbs and releases heat.
I see control systems as controlling energy. One must add or remove
energy to control something. Some of this added energy does not go to
the desired form or is lost and the controller must compensate for
that.. For instance, when adding energy to a mass and spring system,
one want to control the velocity of the mass ( kinetic energy ) but
some of the energy goes into compressing the spring ( potential
energy ). Therefor the mass will tend to fall behind the intended
position because the spring absorbs some of added energy. The
controller needs to compensate for the diversions of energy and energy
losses ( friction ) to the mass will follow the desired path.
Does anybody have a good gut feel description of a zero?
Peter Nachtwey

On Sat, 21 Jul 2007 12:00:15 -0700, Peter Nachtwey wrote:

If your heater has to warm up a column of material before your sensor
starts getting warm, or if you are heating material that flows through a
pipe before it gets to your sensor, there will be an infinite number of
poles, and sensing the second derivative of the temperature would help if
the measurement doesn't get subsumed by the noise.
As you point out elsewhere, however, one could do better by putting in two
(or perhaps more) sensors, with one close to the point you want to control
and one close to the heater. Jerry Avins has talked about adjusting the
dynamics of a furnace controller simply by moving the temperature sensor
closer to the heat source instead of increasing the derivative gain on a
controller and dealing with noise problems.
I suspect that this particular company has arbitrarily decided to only use
one sensor, and that their engineering folks have done their best within
that constraint, and that their marketing folks are now trying to
dress up a pig in a frilly dress.
But I'm getting cynical in my old age.

--
Tim Wescott
Control systems and communications consulting

It's one of main provider of hot runner controller.
I just want to know what could be really PIDD in temperature process
control.
It's the main point of this provider and I would like to know if it's
true.
I know , like all provider of HRC, that they use 1 thermocouple by
controled zone.
I haven't any special information about PIDD but I know what it's PID.
But I'm surprise that nobody know PIDD and that I don't find any
information on the web.
So if PIDD isn't a result of true calculation or reflexion , I'm
surprise that a big compagny like gammaflux use it for sales.

If you _had_ to do your control with just one sensor then using a PIDD
controller could well be better than using just a PID, as long as you took
care of noise issues. Sensing the acceleration of the temperature
increase could help you control things quickly without overshoot. I'd
still question why they couldn't use another temperature sensor -- there
may be a very good reason, or it could just be inertia.
Never assume that just because a company is big that it does things right.
Big companies survive by doing things the way they always have been done,
whether that way is correct or not. They run on inertia until they fail,
then smaller companies come in and start competing successfully with them.
At that point either the small company gets big and starts running on
inertia itself, or it gets bought by a big company and has inertia forced
on it.

--
Tim Wescott
Control systems and communications consulting

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