Autotuning

Astrbm and Hagglund (Proceedings of the 1983 IF AC Conference, San Francisco) suggested an autotune procedure that is a very attractive technique for determining the ultimate frequency and ultimate gain. We call this method ATV (autotune variation. The acronym also stands for all-terrain vehicle which makes it easy to remember and is not completely inappropriate since ATV does provide a useful tool for the rough and rocky road of process identification. 

ATV is illustrated in Fig. 14.8. A relay of height h is inserted as a feedback controller. The manipulated variable m is increased by h above the steadystate value. When the controlled variable x crosses the setpoint, the relay reduces m to a value k below the steadystate value. The system will respond to this bang-bang control by producing a limit cycle, provided the system phase angle drops below —180°, which is true for all real processes. 

The period of the limit cycle is the ultimate period (PJ for the transfer function relating the controlled variable x and the manipulated variable m. So the ultimate frequency is 

The ultimate gain of the same transfer function is given by 

It should be noted that Eqs. (14.38) and (14.39) give approximate values for 0) and X. because the relay feedback introduces a nonlinearity into the system. However, for most systems, the approximation is close enough for engineering purposes. 

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Astrom s autotune method has several distinct advantages over openloop pulse testing ... 

From the autotune test, the frequency (uj and the argument of (A) and the magnitude of (Af) are known at this frequency. 

This method can be extended to evaluate models with more parameters and with diHerent kinds of transfer functions (e.g., underdamped second-order lag) by using hysteresis in the relay feedback or by inserting an additional dead time in the loop to produce a limit cycle with a different frequency. The two autotune tests give four equations so four parameters can be evaluated. 

Y. Cheng-Ching. Autotuning of PID Controllers Relay Feedback Approach (Advances in Industrial Control). Springer-Verlag, London, 1999. 

The XTC/2 and XTC/C group can control vacuum coating sources and up to three different coatings of a process (not to be confused w/ith nine different coating programs). In the case of XTM/2, XTC/2 and XTC/C units, the AutoZero and AutoTune functions are not available, and measurement with several sensors simultaneously as well as simultaneous control of two vacuum coating sources are not possible. 

However, the IC/5 offers all comfort functions available today measurement with up to eight sensors with AutoZero and AutoTune as virell as capability of simultaneous control of two evaporator sources. Moreover, it offers 24 material programs, with which 250 coatings in 50 processes can be programmed. To simplify operation and avoid errors, the unit also has a diskette drive. All types of crystal holders can be connected here. The thickness resolution is around 1 A, the rate resolution for rates between 0 and 99.9 A/s around 0.1 A/s and for rates between 100 and 999 A/s around 1 A/s. A particularly attractive option offered by the IC/5 is a microbalance... 

V. Saraf, F. Zhao, B.W. Bequette, Relay autotuning of cascaded-controlled open-loop unstable reactors, Ind. Eng. Chem. Res. 42 (2003) 4488 1494. 

W. Tang, S. Shi, M. Wang, Simple autotuning PID control for large time-delay processes and its application to paper basis weight control, Ind. Eng. Chem. Res. 41 (2002) 4318-4327. 

There has been signihcant effort and progress by the mass spectrometer vendors in the last decade to create software that is user-friendly and automates many of the routine funchons. One particular example is the use of autotuning and calibrahon scripts that are used to tune and caUbrate against a series of reference peaks to be found in a tuning standard. 

The ATV (autotune variation) method determines the ultimate gain and period in a manner similar to the ultimate method, but ATV tests can be implemented without unduly upsetting the process. Controller settings can be calculated, and the controller can then be tuned on-line to meet the selected dynamic performance. 

Simulate several first-order lag plus deadtime processes on a digital computer with a relay feedback. Compare the ultimate gains and frequencies obtained by the autOtune method with the real values of and Ku obtained from the transfer functions. 

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