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The IMC (Model Corrector) Closed Loop Controller is used for controlling a process that has serious delays in the PV response and is extremely difficult or impossible to control using standard PID Process Controllers. The IMC can also be used to control non-linear processes.
The IMC EFB - Extruder Temperature Control Example Application was developed to demonstrate a solution for controlling Extruder Temperature Control Loops. The output requirements include Split Range and PWM for the Heating and Cooling of the Extruder Zones. Extruder temperature control loops typically have a very slow PV Response and are therefore well suited for the IMC Controller. Here are images of what is included in the IMC EFB - Extruder Temperature Control Example Application .
Here is a simplified diagram of an Extruder Barrel Zone. More general information related to Plastics Extrusion is here.
In addition to the requirement of simply controlling the temperature, the End User required Autotuning of the control loop. The first attempt to meet the requirements used the PIDFF and Autotune EFBs however, Autotuning the loop terminated unsuccessfully due to either the 'actuating pulse too small' or the 'reaction of the control process being incomprehensible'. This is to be expected when standard PID controllers are used to control processes that have serious delays in the PV response. The PV response boundaries of the Unity Pro Autotune EFB for use with the PI_B and PIDFF EFBs are shown here.
In addition to the requirement of simply controlling the temperature, the End User required Autotuning of the control loop. The first attempt to meet the requirements used the PIDFF and Autotune EFBs however, Autotuning the loop terminated unsuccessfully due to either the 'actuating pulse too small' or the 'reaction of the control process being incomprehensible'. This is to be expected when standard PID controllers are used to control processes that have serious delays in the PV response. The PV response boundaries of the Unity Pro Autotune EFB for use with the PI_B and PIDFF EFBs are shown here.
Determining it impossible (or impractical) to define or search for the obscure combination of Step Amplitude (step_ampl) and Duration of the Actuating Pulse (tmax) to meet the boundaries of the Autotune EFB, an Open Loop response test was performed. The procedure for this test is: (1) achieving a stable PV by manually adjusting the output (2) manually set the output to 100% and hold it there until a PV rise of > 2% is achieved (3) manually set the output to a neutral value which does not heat nor cool the load (4) allow the natural load of the process to reduce the PV until it is back to the original value. The PV and Output values must be recorded during this procedure for analysis. Here are the results of the testing procedure when applied to the Extruder Zone.
Referring to the Extruder Barrel Zone diagram and the Open Loop Test Data, it is easy to see why the PV Response Delay exists and how getting heat through the Extruder Barrel to the Product results in the over storage of BTUs in the Extruder Barrel which causes the temperature to continue to rise long after the heating is removed.
From the Open Loop Test Data, a Unity Pro DFB was created to model the Extruder Zone Heat Load. This DFB is included in the IMC (Model Corrector) Example Application and its performance matches the empirical data.
With the heat load model added into the code, an IMC controller was substituted for the PIDFF EFB and an IMC Autotune DFB was created (as shown here). The IMC Autotune DFB functions similarly to the native Unity Pro Autotune EFB by taking control of the IMC Output, generating a Pulse and monitoring the PV response to calculate the IMC tuning Parameters. Here are the details of this process.
At the completion of an Autotune cycle, the IMC Controllers tuning parameters are updated and the previous values are available to be restored. Here are the Public Variables of the IMC Autotune DFB, where the status and collected data is stored.
The result is a solution that can be used to analyze and predict the Extruder Temperature Control Loop performance. This solution can also be adapted for other process control loops with inherent PV response delays.
All of the DFBs contained in the Example Application (also available in U2V DFB Mngr Demo LIbrary V7.4) are Unity Hardware Platform independent.
There are Help files for all of the DFBs. To obtain these, download the U2V DFB Mngr Demo LIbrary V7.4. The U2V DFB Mngr is also recommended and it can be downloaded here. The U2V DFB Mngr will install the DFB Library into the Unity Pro Types Library and install the Help files.
- It is very important to understand that the example application is provided and built for use in the Unity Pro Simulator, and while this use can provide insight into how the IMC Controller can perform, the results from use in the Simulator cannot be compared to implementation in an actual MODICON PAC. The Unity Pro Simulator has execution timing constraints imposed by the host PC (as documented in the Unity Pro Help), therefore any application using the example code must be tested in the real world using hardware. These differences are illustrated in the following comparison of similar execution cycles of operation in the Simulator versus a live PLC. Note the changes of the IMC Output in the Simulator Trend are greater as is the deviation of the PV from the SP.
Also, referring to the above illustration, it should be noted that this application requires 30 minutes of stabilization following a Cold Start and each Autotune cycle.
Systems of this nature should not have step changes to the Setpoint. Setpoint changes should be slowly ramped at a rate that takes into account the PV reaction delay. This also applies to Load changes (or Production Rate changes).
