Process Identification and PID Control

Part Four

Process Activation

The tuning of a PID controller or the design of an advanced model-based controller goes through the following steps. Step 1, activate the process with a test signal generator. Step 2, estimate the process model using the process identification algorithms. Step 3, tune the PID controller or design the advanced model-based controller. Step 2 and Step 3 are described in Part Three and Part Two respectively. This chapter talks about the test signal generator in Step 1. If the process is activated too aggressively, then the quality of the products from the process may not be acceptable and the safety of the process is not guaranteed. Meanwhile, if activation is not enough, then an accurate process model cannot be obtained because the information included in the activated data is limited and the uncertainties, such as measurement noise and disturbances, become dominant. So, the goals in Step 1 are activating the process in as short a time as possible and activating the dynamic information (frequency components) as much as possible. One of the most efficient methods for process activation is the relay feedback method. If the process is activated with the relay feedback method, then it is straightforward to detect the time-scale (ultimate period) of the process and what frequency components are included in the activated data. Then, it is easy to determine the termination time for the process activation and the design parameters (for example, the maximum frequency, ...