PID Control Auto-Tuning Research
A PID control autotune technique, developed empirically at Scientific Instruments (SI) (Riviera Beach Florida) for temperature control systems operating in the Cryogenic regime (that is between 4-100K) has been successfully deployed by SI in hundreds, if not thousands, of systems all over the world. The control method is still an active research area.
The PID parameters tuning is based on a sequence of closed loop step response tests utilizing the measured peak overshoot values and the time between the first peak and first dip of the step response.
The first paper (FCRAR 2004) describes the structure of typical cryogenic temperature control systems dynamic models, including typical nonlinear effects (such as actuator’s saturation and uni-directional operation). It then outlines the step-by-step tuning method. The tuning method produces satisfactory results for set-point changes of up to 10K over the entire cryogenic temperature range. The paper does not present Controls Systems theoretical justification for the tuning method. The paper also includes the results of a set of open-loop step response tests done at different temperature levels. The tests provide small-signal modeling data, that allows in principle the creation of a dynamic simulator, featuring the process gain, time constant and pure delay time, all as functions of the temperature.
The second paper (FCRAR 2005) focuses on simulation studies designed to assess more deeply the performance of the SI method, explore its validity for a large range of system parameters and configurations. The main finding of these studies is that this novel PID control tuning method appears to be limited in scope to processes characterized by one slow and exactly two fast time constants. The algorithm fails to produce satisfactory results if the number of short-duration modes exceeds two. The simulations explore corrective means to extend the SI PID autotuning method to a larger set of process control scenarios. The full set of MATLAB simulator m files is tacked to the paper pdf and can be downloaded together with the paper.