In this screencast we will take a look at the process by which we determine whether or not a controller
should be direct or reverse acting. The controller action is a very important aspect of properly
integrating a controller into a control scheme. If the controller action is selected incorrectly,
regardless of the quality of the tune that has been done, the process will not respond correctly to
a change in either the set point or the presence of a disturbance. What will happen is your controller will
act the opposite of the way you want it to.
So to show this, we are going to use the example of a heat exchanger, where we are trying to cool a
process fluid by the use of cooling water.
So we are going to control the temperature of the process fluid by the flow rate of cooling water as
indicated by the diagram.
So to think about what the role of the controller is, we are going to zoom in and look at what the block
diagram for the controller aspect is.
So here will be our controller, and what will come into the controller is the error, in other words the
difference between the set point and the actual measured value,
and this error is measured typically in either units such as percent transmitter output, or in milliamps,
and what comes out of this is a signal in percent controller output, which then goes to the valves,
so basically the percent controller output gives some indication about how we want the valve to operate,
whether it be fully open, fully closed or somewhere in the middle
So controller action can be reverse acting or direct acting.
For a direct acting controller, what it means is as the value of the measured variable goes up,
the necessary signal, in other words the percent controller output that the controller will make, will go up,
as the name direct acting indicates.
And the other type of controller action is reverse acting, and unsurprisingly what that represents is
as the measured variable goes up, the percent controller output goes down.
By convention, the gain for a direct acting controller is less than zero, and the gain for a reverse acting controller
is greater than zero, which at first glance seems a bit counter intuitive because direct acting causes a negative
sign in the controller. These signs are solely an issue of sign convention, most notably the sign
convention that we give valves. Recall that fail close valves take a positive sign, and fail open valves take a negative sign,
which is relatively arbitrary
The reason why those signs are selected is because fail close valves tend to be more common place in chemical processes
Generally, although not always, a majority of controller found in practice are a reverse acting,
which is also nice in the fact that it gets the positive sign.
But lets do the analysis for this system, and to do that there are two possible methods we can use.
The first method is to evaluate a change in the measured variable.
For our heat exchanger, the value we are measuring here is the temperature of the process fluid.
If the fluid temperature goes up, in other words the process fluid temperature is too high,
what that means is the fact that we are going to want to increase the cooling water flow rate
If we want to increase the cooling water flow rate, we are going to want to open the valve more.
We need to figure out whether or not a opening of the valve causes an increase or a decrease
in the controller output, the signal of the controller.
And to do that, we need to figure out if the valve fails open or if it fails closed.
If we revisit our system here, what we see is the fact that if power goes out to this process, as a safety precaution
we are definitely going to want the cooling water to still be flowing through the heat exchanger
in the case if there is a warm fluid that is still stuck in the heat exchanger, or something related to that.
Because of that, we are going to want our valve to fail open.
So with a fail open valve, we think about this where the valve here is fully closed,
and the valve here is fully open. When the power is out or we lose electricity we want this
default to fail open, in other words we will have no signal.
So therefore here, open will represent zero percent controller output.
Similarly for the valve to be fully closed we would want the system to be at one hundred percent controller output.
So in the case of our cooling water example, we would want to go from more closed to more open,
and this change from more closed to more open leads to a decrease in the percent controller output,
thus as we have an increase in the value of the measured variable thats going to require
a decrease in the percent controller output. So what that means here is the fact that our final decision here
would be to have a reverse acting controller.
The second method we can use is we can do a signs analysis.
The signs analysis can get a little tricky if we are not dealing with a feedback only system,
so for cascades, beamform systems, this can become a little trickier, meaning that method one,
which can be used for all situations cleanly will be your better choice, but method two can be done
a little bit quicker in our analysis. So we are interested in the sign of two things,
the first is the sign of the valve gain, and the second is the sign of the process gain.
So what do here in this method is we multiply the signs of the valve gain and the process gain.
If this multiplication leads to a positive then the controller will be reverse acting,
and similarly if the signs multiplied together are negative we would select a direct acting controller.
So if we evaluate this for our process the valve gain here, it is a fail open valve, so by convention fail open valves
have a minus sign. For our process gain, remember that the gain of a process is the delta output over delta input.
Here, the value of the output variable, the variable we are trying to measure here
is the temperature of the process fluid. We are trying to adjust that by altering the flow rate of the cooling water.
So we see here is if I increase the flow rate of the cooling water
that is going to cause a decrease in the temperature of the process fluid.
Therefore, the process gain is negative. So if I use the methodology I would multiply negative by negative
and that would give me a positive value, which again would predict a reverse acting controller, the exact
same as we ended up with when we use the previous method.
So in this screencast we analyzed the importance of controller action
and how to determine the controller action for a chemical process that undergoing feedback control,
but as noted earlier this can be extended to other control schemes that are more complicated such as
cascade feedback, beamform, etc.