Steady-state bumpless transfer under controller uncertainty using the state/output feedback topology

Abstract

Linear quadratic (LQ) bumpless transfer design introduced recently by Turner and Walker [17] gives a very convenient and straightforward computational procedure for the steady-state bumpless transfer operator synthesis. It is, however, found to be incapable of providing convergence of the output of the offline controller to that of the online controller in several industrial applications, producing bumps in the plant output in the wake of controller transfer. An examination of this phenomenon reveals that the applications in question are characterized by a significant mismatch, further referred to as controller uncertainty, between the dynamics of the implemented controllers and their models used in the transfer operator computation. To address this problem, while retaining the convenience of the Turner and Walker design, a novel state/output feedback bumpless transfer topology is introduced that employs the nominal state of the offline controller and, through the use of an additional controller/model mismatch compensator, also the offline controller output. A corresponding steady-state bumpless transfer design procedure along with the supporting theory is developed for a large class of systems. The new technique is shown to be capable of eliminating the online/offline controller output tracking errors under significant controller uncertainty, while preserving fast convergence of Turner and Walker design. Due to these features, it is demonstrated to solve a long-standing problem of high-quality steady-state bumpless transfer from the industry standard low-order nonlinear multiloop PID-based controllers to the modern multiinput-multioutput (MIMO) robust controllers in the megawatt/throttle pressure control of a typical coal-fired boiler/turbine unit.

Hybrid dynamical systems with controlled discrete transitions

“…Switching between controllers that induces no undesirable transients in the physical plant, referred to as bumpless transfer, is an important practical problem [29]. The ideal steady-state bumpless transfer design requires that at the moment of controller switching, the values of the input and the output signals of the offline controller be, respectively, equal to those of the online controller, and that the convergence of the offline controller variables involved to those of the online controller be fast.…”

$H_{\infty}$ Bumpless Transfer Under Controller Uncertainty

“…Another case is the attainment of an improved closed-loop system performance via switching between the controllers with the complementary properties, such as the ones separately optimized for tracking and disturbance rejection ( [4]). A third case is an online performance assessment of the advanced control laws against the industry standard ones in large industrial process control units ( [5], [6]). In many of these cases, the controllers involved are multi-input-multi-output (MIMO) and of high order and implemented in software so that their states are available.…”

“…However, controller uncertainty has been reported in a number of applications [5], [6]. Indeed, perfect implementation of, say, MATLAB designed linear H1 controller in real-time software/hardware could be impossible for a variety of reasons, such as software implementation error and hardware inadequacy, e.g.…”

“…In these cases, the true off-line controller state is replaced in implementation by the nominal state, xnom, of the off-line controller that can be viewed as the state of the controller model. Hence the use of only the nominal state x nom of the controller along with its input can no longer provide its actual output, inducing bumps during controller transfer [6].…”

“…Based on this consideration, a complete design and implementation methodology for the steady-state bumpless controller transfer under controller uncertainty is presented in [6]. The state/output feedback topology proposed therein employs the nominal state of the offline controller and, through the use of an additional integral type controller/model mismatch compensator, also the actual offline controller output.…”