Objectives
Introduction
Content
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.
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.
