In the blog post “Intermediate Variables in APC”, it was explained how Advanced Process Control (APC) applications can benefit from the use of intermediate variables. In this second blog, further potential for improvement is presented. If the concept of intermediate variables is new to you, check the previous blog first.
Intermediate Variables can improve APC performance
The general setting of using intermediate variables is as follows: a manipulated variable (MV) is used to control a controlled variable (CV). In the example in the previous blog, the MV was the feed flow entering a reactor and the CV was the pressure over a downstream distillation column. An intermediate variable (IV) is a measured variable that is on the one hand affected by the manipulated variable. On the other hand, the measurement of the IV can be used to improve the prediction of the controlled variable. In the example mentioned, the temperature drop over the reactor was the intermediate variable: it depends on the reactor feed flow. Moreover, its measurement can improve the predictions for the column pressure.
Using IV’s for PID deviations
The benefits of using IVs can be increased further if an IV is the PV controlled by a PID loop.
To illustrate the point, consider a different APC example in which the APC uses a furnace outlet temperature setpoint (SP) as an MV. Sometimes the outlet temperature PV deviates significantly from the setpoint, for instance due to burners being switched. Using the temperature measurement as an intermediate variable will give better predictions for some of the APC CVs than using the setpoint for the temperature alone.
When there is a difference between the temperature and its setpoint, the PID loop will take action to control the PV back to its SP. Due to the inherent slow process behaviour, this needs some time. This cannot be eliminated by tuning the PID loop.
Typically, APC will (also) react to the temperature deviation by moving one or more of its MVs. This is not desired, since clearly after some time the PID controller will bring the temperature PV back to its setpoint. The best thing the APC can do here is to just “wait until it’s over”.
The key difference between the temperature PV in this example and the temperature drop in the example in the first blog is that in the first blog, one must assume that a misprediction of the temperature drop will persist over time while in this example, a deviation between the measured PV and its prediction will be brought back to zero by the PID loop without action of the APC controller. Not only is it known that the PV will be brought back to its setpoint, APC also knows the speed with which the PID loop will achieve that from the model it has from the SP to the PV.
In INCA APC you can not only use the PV of the PID loop as an IV but also specify that it is the PV of a PID loop. In the example, predictions of CVs that are dependent on the temperature will greatly improve, and secondly, the APC application will not take action to bring the PV back to its setpoint, as it knows that the PID controller is working to make things right.
APC and base layer control have their own job
A well-tuned base layer control scheme is a prerequisite for any well-performing APC application. That applies in general, and it also applies in this setting. Marking the PV as a PID IV allows the APC to take a hands-off approach. It is not a mechanism that makes APC step in and compensate for poorly tuned PID loops!
By the way, if you are having trouble tuning your PID loops, the tuning tools in the INCA Suite may help you out. That topic is covered in other blog posts.
Choose an APC technology that allows you to get maximum performance!
Intermediate variables are a great feature to boost the performance of APC applications. INCA APC provides this capability. Moreover, by using knowledge of the base layer control structure, it further expands this capability. This is key to maximizing the return on investment when implementing APC.
Check out the intermediate variable functionality in our APC modelling tool, INCA Discovery, and how it reflects PID loops in the IVs. Use the functionality in your next online APC application with INCA MPC!
Do you want to know more about INCATools? Read more about MPC here.
