In this screencast we will talk about feedforward control and its role in developing appropriate
control schemes. So before we can really talk about where feedforward control can be used
we have to first focus on where feedback control fails. Because a large majority of control
processes are of a pure feedback only control scheme, so therefore in order to deviate from
those schemes there have to be reasons why, and there are a number of reasons why, but
here we'll focus on three of them. The first is that perfect control cannot occur. A disturbance
happens, but the controlled variable does not feel it, so in other words a disturbance
to the variable will happen, but the control process will already handle it. Because feedback
control does not work until there is a deviation to the controlled variable, it is impossible
for a disturbance to have no effect on this variable, which connects number two, because
you are only measuring the controlled variable at the end of the process, there is no way
to predict what's going to happen to this variable. An additional problem is that processes
are slow or have large dead time can lead to control, which can take a while. Why? Because
if a system has a very large dead time, for example, a disturbance will occur, the process
won't feel it for a while, but once it feels it, it's going to take a while to actually
act on it because of the slowness of the process. So feedforward control is a way to handle
all three of these drawbacks. So to try to represent this we are going to look at a somewhat
rudimentary example of a blending tank, and in our blending tank we are trying to control
the output concentration, and we have two streams, so therefore an example of a feedback
only scheme would be one where we would try to directly control the composition coming
out by the flow rate of the first one. However, one example here would be let's say that the
flow rate of the first scheme is known to fluctuate a bit, so therefore if that is a
potential disturbance our process won't recognize that disturbance, and therefore if we see
a change in this flow rate that will then cause a change in the composition, which isn't
necessarily good when we are trying to control this outlet composition. So what we can do
is develop a control scheme where we combine this feedback control with a feedforward controller,
which would be able to better control this process. Why? Well the reason why is that
if I see a change in the flow what I can do is have this feedforward controller send a
direct signal to the valve saying that a change is going to happen, therefore the flowrate
of the manipulated stream can be adjusted in advance notice of the fact that this flow
is going to be changing. So now what we need to do is discuss some of the things about
feedforward control. So the first is that the variable, or variables, being used for
feed forward control must actually vary to some degree, because if they are not fluctuating
at all there would be no point to have a controller for it, and if they're only fluctuating in
a small amount it might not be worth it to introduce additional equipment and what not,
which would introduce costs, and also potentials for equipment for breaking and what not, so
therefore you want to make sure these variables actually can vary in some way. The second
part of interest here is that a model must exist that can relate the feed forward variable
or variables to the controlled variable. So the beauty of what feedforward control does
is if I know the fact that by changing my flow rate by 1 gallon per minute of this stream
will cause my composition to go down by 0.02, let's say mole fraction. What I can do is
I know how to adjust the other stream, the second stream where the valve is, the manipulated
variable, in order to adjust for that change, so therefore you must be able to have a model
with some reasonable accuracy, otherwise if you don't know how the variables are related
you're really not going to gain anything from this, so therefore having a model is important.
The third obviously is that the variable, or variables of interest must be measurable. Depending
on the process this may not be as easy as it sounds, particularly if you're in a process
with high pressure, high temperature, highly corrosive materials, etc., it may not be as
easy to measure these, but in order for feed forward control to properly work you need
to actually measure these variables to see how much they are actually fluctuating from their
assumed steady state value. So this process here represents a feedback/feedforward controller,
because the part in blue is indicative of the feed forward process, where as the part of
the control scheme in black represents the feedback processing. With the feedforward
control scheme the variable of interest, the controlled variable, is not measured. So therefore
this feedforward controller here is conducting analysis without knowing whether or not the
composition here is too high, too low, or exactly where it should be, so that's kind
of an important aspect here. By putting these together the question is from a block diagram
standpoint what does this look like, and as a start, we'll start with the part in black
which is the feedback scheme, and really the feedback scheme is no different than what
we saw before where we had the transmitter, we'd have a set point with an appropriate
gain. It would then go to the controller, the valve, the process, which here is the
tank, and then out would come the composition. So therefore this part has been maintained
by this process. However, the question to ask ourselves is how does the feedforward
controller fit in, and the way we can gain some insight on this is kind of looking at
the diagram. What we notice here is that right before the valve instead of it receiving one
controller signal it now receives two. Both sum together, and the other one is the feedforward
controller. The feedforward controller gets the signal from the flow transmitter, which
means the fact the flow transmitter has to take into account the measurement of the flow
rate of that stream, which we'll call here F1. By adjusting F1 I am also adjusting the
composition, so therefore there is a second tank transfer function here that represents
the relationship between F1, the disturbance variable, and the composition coming out of
the tank, so therefore we have another block which shows that relationship, so therefore
the composition here changes because of two things, the change of the flow rate of the
first stream, and the flow rate of the second stream, which is being manipulated by the
valve. This relationship would come out through a material balance. Now that we have developed
our feedback and feedforward control scheme in a block diagram, and therefore we could
conduct a good amount of analysis or put this into Simulink to kind of see how the processes
act together. So to leave this we'll have a few parting shots here of well why not have
just a feedforward only control scheme, and in general feedforward only control schemes
are not very beneficial by themselves, the major reason why is something that was noted
earlier is that there is no measurement of the controlled variable, so therefore the
feedforward controller is acting the same way regardless of whether or not your desired
variable is way too high to little too high, way too low, a little too low, or just right,
and there's no way for the feedforward part to know. Therefore feedforward often works
very well with feedback, because feedback is measuring that value and just making sure
that your scheme is online with what you're expecting it too be. Another issue with feedforward
only is it's very model dependent, so if the model that you have is not very strong, or
is not a great fit to the process then that means that you're going to have some problems
when it comes to actually handling the control of this process. Again feedback control can
handle that because it measures the actual value and does comparisons, so therefore if
the model is slightly off it can adjust the manipulated variable to counteract that. And
a final point, freedforward is not needed if the disturbance is minimal, or has minimal
effect. You'll just be introducing more softward/hardware to your process, which could be prone to breaking,
could more trouble than it's worth, particularly when you have a control scheme already present
from the feedback standpoint. So in this screencast we took a look at the basic idea of feedforward
control, why a process may benefit from feedforward control, and what it looks like from a block
diagram standpoint.