National Instruments PID Control Toolkit 371685C-01 User Manual

National Instruments PID Control Toolkit 371685C-01 User Manual Manual pdf 15 page image

Chapter 2 PID AlgorithmsLabWindows/CVI PID Control Toolkit User Manual 2-6 ni.comFigure 2-2 illustrates the autotuning procedure excited by the setpoint relay experiment,which connects a relay and an extra feedback signal with the SP. Notice that the PID Libraryautotuning functions directly implement this process. The existing controller remains inthe loop.Figure 2-2. Process under PID Control with Setpoint RelayFor most systems, the nonlinear relay characteristic generates a limiting cycle from which theautotuning algorithm identifies the relevant information needed for PID tuning. If the existingcontroller is proportional only, the autotuning algorithm identifies the ultimate gain K u andultimate period T u. If the existing model is PI or PID, the autotuning algorithm identifies thedead time and time constant Tp, which are two parameters in the integral-plus-deadtimemodel, as follows:Tuning FormulasThis package uses Ziegler and Nichols’ heuristic methods for determining the parameters ofa PID controller. When you autotune, select one of the following types of loop performance:fast (1/4 damping ratio), normal (some overshoot), or slow (little overshoot). Refer to thefollowing tuning formula tables for each type of loop performance.Table 2-1. Tuning Formula under P-Only Control (Fast)Controller Kc T i T dP 0.5K u — —PI 0.4K u 0.8T u —PID 0.6K u 0.5T u 0.12T uSP e+– P(I) Controller ProcessPVRelay+–τG P s( ) = e τs–T p s--------