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Autotune In CelciuXº


Objectives
The purpose of this Technical Note is to give an overview of the use of Autotune in CelciuXº.
Introduction
Autotuning enables stable and fast acting control when precise temperatures are required in demanding applications. This technology can be particularly beneficial when used for sealing and forming applications in packaging machinery.
Click the following link to download a white paper detailing solutions for improved seal quality, higher throughput and less waste for your packaging application.  
 
 
Omron uses a well tried and tested technology for autotuning the PID parameters and commissioning a loop. This is done by applying an on-off style control strategy for a short period, measuring the response of the process and computing new PID parameters for the controller. 
 
Content
There are two main strategies used in Omron namely,
1. 100% Autotune – which cycles the output of the control between 0% and 100% for a heat only instrument
2. 40% Autotune – where initially the PV is stabilized at the setpoint which corresponds to a power level OP. Then a cyclic ON-OFF style control is employed where output power varies between OP-20% and OP+20%. Hence the total swing of OP is set to 40% by default. This value can be changed if required.
The advantage of method 1 is that as larger signals are used the response of the process can be identified much better than method 2 where temperature swing will be relatively small.

The diagram below shows the response of a typical process where 100% Autotune is used with PV starting from ambient.
 

Typical autotune process response

The purple line is the PV, blue setpoint and brown the output power requested. The resulting PID parameters were (11.8C, 48s, 8.2s) for P, I and D parameters. A similar autotune sequence with 40% method looks like below

Note that the tuner attempts to find some initial PID parameters as PV is rising and a single ON-OFF cycle is employed. Once the interim PID parameters are calculated the PV is raised to setpoint in a controlled manner. Then the smaller ON-OFF style cycles with OP varying by ±20% of the nominal. The PID parameters thus computed were (8.1C, 39s, 6.7s).

A further test was also done with a 40% autotune starting from PV at Setpoint. PID parameters were (8.1C, 41s, 7s).
 

Autotune can easily be employed in the trendview page of CX-Thermo

 
Summary

The Autotuning feature in CelciuXº is a powerful method to enable precise and responsive temperature control for demanding applications often found in the packaging and process industry. Understanding and incorporating Autotuning into temperature control applications can ensure higher quality, higher throughput and less waste.

 

 



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Created 2008-12-04
Modified 2014-03-14
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