Comparative Study On Multivariable Control And Its Application For Benchmark Boiler

From the beginning of the industrial revolution until present day, the industrial automation control system has become more and more complex and highly sophisticated. Electricity market and chemical industry for producing steam usually use multi-type-fuel-fired drum boiler. Boiler control is one of the most important tasks in automatic control system in thermal power plants. Boiler control system is a multivariable input multivariable output (MIMO), nonlinear dynamic process with large inertia and strong coupling. Moreover, boiler control process usually works in a wide range of operating conditions. The conventional proportional-integral-derivative (PID) controllers are widely used in this process, however, conventional control method may not achieve the desired requirement and performance for a drum boiler control system.This thesis focuses on the design and performance of multivariable control techniques for a benchmark boiler control.The main contents include:1.To improve the robustness of the system against uncertainties, H∞ loop shaping approach is applied to the Benchmark boiler. To implement the resulted high-order H∞ controller, a multivariable PID controller is obtained by approximation. Anti-windup scheme is implemented to take the input constraints of the system into consideration.2. To deal with the coupling and input constraints of the system, model predictive control (MPC) is applied to the Benchmark boiler. MPC performs an optimization process at each sample, which can take the coupling and input constraints into consideration naturally.3. To improve the disturbance rejection performance, active disturbance rejection control (ADRC) is applied to the Benchmark boiler. ADRC uses an extended state observer to estimate the generalized disturbance and try to attenuate it quickly. To overcome the effects of the input constraints of the system, the error between the computed output and the real output of the actuator is fed back to the extended state observer to avoid the windup.The above-mentioned three control methods are discussed in the thesis. The controller design, tuning and simulation for Benchmark boiler are illustrated in detail. Simulation results show that the proposed multivariable control techniques can achieve better performance than the conventional PID control for the Benchmark boiler. Moreover, it is shown that H∞ loop shaping control achieves the best load rejection performance for the Benchmark boiler, but the worst decoupling performance; while MPC achieves the best decoupling performance. So H∞ loop shaping control and MPC is recommended for the Benchmark boiler to improve its control performance.