In our previous video,
we supplied a couple of simple examples of controlled processes
and began to explain the specific terms of a PID controller.
As stated previously,
you can enable each parameter,
those being the proportional, integral, and derivative terms.
Now let’s discuss what the parameters are
and how they are used.
In the most simplistic terms,
the controller calculates the P, I, and D actions
and multiplies each parameter by the error or E,
which is equal to Set Point Minus Process Variable in direct acting,
as discussed previously.
Then, all parameter calculations are added up
to produce the Control Variable.
Unfortunately, there is no industry standard on the parameter terms.
Here are some of the uses found today.
The “proportional” term, often called “P” Constant,
can be referred to as “Proportional Gain” or just “Gain”,
which is not a unit but instead a “ratio”.
This parameter can also be called “Proportional Band”
and measured in the unit of percent.
The parameter can be called KP, Gain, or others.
This is the parameter that determines how fast the system responds.
The name by which it is referred varies with the manufacturer.
For controllers that use the term “Gain”,
adjusting this tuning parameter higher,
may cause more sensitive, less stable loops.
Conversely, on controllers with proportional band units,
decreasing this tuning parameter affects the loop in the same manner.
Keeping this in mind,
knowing the type of controller you have
is essential to ensure that you are properly adjusting your parameters.
The integral term or “I” Constant, often called “Reset”,
can be in different measurements as well.
There are “repeats per second”, “seconds per repeat”,
“repeats per minute”, and “minutes per repeat”.
Essentially, regardless of the measurement type,
the integral is the sum of all of the values reported from the signal,
captured from when you started counting to when you completed counting,
or the area under a plotted curve.
This parameter can be called Ki, Ti or others.
This parameter determines how fast the steady state error is removed.
Because of the different measurements,
this parameter may not be as intuitive to adjust.
Briefly, “smaller” values in “minutes per repeat” measurements
will create “larger” integral action
where “larger” values in “repeats per minute” measurements,
will create “larger” integral action.
Derivative or “D” Constant units are typically seconds or minutes.
The purpose of “Derivative” constant is for predicting change.
The Derivative action acts of the rate of change measured in the Process Variable.
The value of this parameter
basically means how far in the future you want to predict the rate of change.
This parameter can help to create a faster response in your loop
and a better performing loop as well.
However, since the “Derivative” term
is measuring the rate of change in the Process Variable,
the Process Variable must be a very clean signal
meaning no noise within the signal.
For that reason, Derivative terms are not often used in controls.
The most commonly used controller is the PI.
Most processes can be well served with this type of control.
P and PID controllers
are occasionally used while PD controllers are rarely used.
PID controllers are very sophisticated devices
with likely many adjustable parameters.
The process and algorithm types can also vary.
Other parameters to research for PID controllers
are “Series”, “Ideal”, and “Parallel” algorithms,
“filtering”, “scan times”, “anti-windup”,
“self-regulating” versus “integrating” processes,
“reverse acting”, “dead time”, “lag”,
“Derivative on E” or “Derivative on PV”, just to mention a few.
Because of the inherent difficulty in tuning the most basic of PID loops,
these parameters, which are more advanced than this basic tutorial,
are better left for future lessons or self-exploration.
There is a science to tuning a PID loop
but the most widely used tuning method is “trial and error”.
There are other methods that require a multistep process
to determine where your number should be.
Your goal is to control the process, when plotted in a trend,
as a nice, even trend line with minimal oscillation.
The first step in tuning your controller
is to determine just how much adjustment you can make
without serious implications to the process.
Talk to the plant personnel,
if adjusting the parameters of the PID controller will not have an adverse reaction
then you can begin your adjustments.
If the effects will be detrimental,
you must take a more measured approach.
Obviously, depending on whether this is a new installation
or an existing plant,
your approach will be different.
In existing installations,
you are just looking to tweak values
and create a more stable process.
Because the PI controller is the most widely used,
we will only be adjusting those parameters.
As we have discussed the differences in measurements in the PID terms,
for this tutorial we are going to standardize on “gain” and “repeats per minute”.
To just jump right in,
if your “Process Variable” changes quickly
and changes to your “Control Variable”,
you will start with a low gain,
possibly as low as 0.1
while adjusting your “Reset” between 1 and 10 repeats per minute.
Conversely, if the “Process Variable” changes slowly,
start with higher gains and lower resets,
somewhere between 2 and 8, and 0.05 and 0.5 respectively.
Adjust only one parameter at a time and observe the results.
Continue in this vein until you have achieved a stable process.
A more measured approach is to start with a low gain,
with integral and derivative disabled.
Watch the process and begin incrementally adjusting the gain
by doubling the value.
When the process begins to oscillate,
adjust the gain value down by 50%.
Employ a small integral value and watch the process.
Double the value incrementally
until oscillation occurs
then cut the integral by 50%.
At this point, you should be somewhere in the neighborhood
and can begin the fine-tuning process.
As you can tell, by the many parameters available in PID controllers,
there is much, much more to learn on this subject.
But to sum it up,
you can probably get away with using a “PI” controller for most of your processes.
Make sure you know the measurement type of your selected controller
and just incrementally adjust your P and I parameters
until you achieve that nice stable process.
If you prefer the more scientific approach to adjustment,
we may cover that in a future video
or feel free to search the equations and steps required for that approach.
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