Control of inertial processes

Control of inertial thermal processes

It is necessary to explain how the task was presented at the beginning of the work.
To implement the control algorithm, a touch programmable controller SPK107 from OWEN was selected, and a programmable relay PR200 was used to process field signals.
The polymerization chamber (20m2 in volume) of powder paint is divided into two parts and can be used separately. The powder paint polymerization furnace can be fully loaded, or only one of the conditional halves can be loaded.
At the same time, the technological regime must be maintained, implying the following stages::
1. Entering the warm-up mode of the part
2. Warming up the part (time and temperature from the recipe)
3. The fastest possible exit to the baking mode
4. Baking details (time and temperature from the recipe)
5. Cooling of the part


 
It should be noted right away that the rate of development of the process in the two zones of the furnace was different. This was influenced by both the condition of the heating elements themselves and the condition of the ventilation ducts providing the coolant flow. Any error, even very insignificant in appearance, seriously affected the acceleration characteristics.
For example, the temperature from 20 ° C to 190 ° C could grow for about 30 minutes, while the temperature difference in the two zones sometimes reached 20 ° C. That is, while one of the zones was only striving to achieve the desired value, the second one already needed to be cooled (sorry, there was nothing).
An extremely important detail: according to the condition of the polymerization rules, a temperature deviation of more than 2 ° C up and 2 ° C down from the setpoint was the criterion of marriage. That is, increased requirements were placed on the quality of temperature output and temperature retention in a given echelon. The heating of the part should have been as short as possible, that is, the power of the thyristor modules should be maximum (100%), the maximum period of time.


 
Polymerization of powder paint, this is what the heating elements look like


 
This is what MEYERTEK thyristor assemblies look like, allowing you to control the load on the heating elements

Initially, commissioning was not envisaged. At the request of the Customer, testing was carried out using a stand. A relatively small box was heated, and the PID regulation coped with the control of the heating process. As it turned out later, there was a mistake in this, since it was the Comission at the facility that took the bulk of the labor costs. 
A year later, when this system was installed at the factory and required a qualified setup, it turned out that the stand model was not suitable for real conditions. PID regulation, for all its advantages, was poorly suited directly to the real process of heating and stabilization. When setting up the PID circuit, there was some temperature swing with further stabilization, although in this case any swing of the temperature graph was unacceptable.


 
After spending a day searching for the right PID coefficients, it became clear that this task would not be solved quickly. The temperature constantly flew beyond 4 ° C and it was impossible to maintain the correct value.
The idea came up to create an event algorithm that allows you to bring the temperature to the setpoint strictly from the bottom of the set value and keep it. At the same time, it was necessary to take into account the fact that if both heating elements were not turned off (converted to 0%), then the parasitic influence of the heating element, striving to develop a temperature not in its zone, which necessarily heated the zone that was already overheated, affected.


 
To solve this problem, functional blocks were created that make it possible to interpret functions such as "Value growth", "Stable", "Value drop" into bit variables. These bits were used to make decisions in the event algorithm.
 


I also had to write a function to determine the angle of inclination of the function. That is, every 500ms, a slice of data was recorded in an array and these data were analyzed among themselves, calculating the angle of inclination.


 
Thus, at the stage of heating the part to 185 ° C, when 175 ° C was reached, it was necessary to "reset" the heating element power to zero. Further, the temperature increased by inertia. After waiting for the end of the inertia action or approaching the set value from below, when the limit reached less than 5 ° C, the outputs of the PID regulators were logically connected to the power regulators. Thus, it turned out to avoid a situation with a poorly controlled "run-out" of temperature. 
The function of calculating the angle of inclination was able to determine not only the rise or fall, but also the degree of termination of the temperature rise (angle decrease). That is, as long as the temperature goes by inertia (and inertia is quite large), the values will be above 8 ° C, and if it becomes less than 8 ° C, then the inertia fades and you can safely add power, controlling, of course, the distance to the set value.


 
If any of the zones exceeded the set temperature, even if by 1 ° C, then the power of the heating elements was forcibly turned off, a timer of 15 seconds was set for the stabilization time of the process, at the end of which the temperature was analyzed. If a drop in any of the temperatures was recorded, the regulators were connected. Thus, the temperatures were self-leveled and it was possible to maintain the temperature within the set value.
 



 
In addition, work was carried out to replace the thermocouple with a temperature converter. Initially, the Customer installed sensors with the "thermocouple" type on the roof of the furnaces, as well as a 4 – 20mA converter (tablet) in the sensor itself. Thermocouples themselves are slow, and even these converters added inertia, which ultimately reduced accuracy and led to the need for software solutions to the issues of "run-out" of values. Subsequently, the temperature sensors were replaced with thermal resistances (because they passed by temperature). The sensitivity of the process in this regard has improved markedly.









#Inertialprocesses, #Camerapolymerization