Cyrus W. Taft

Simultaneous gains tuning in boiler/turbine PID-based controller clusters using iterative feedback tuning methodology

Tuning a complex multi-loop PID based control system requires considerable experience. In todays power industry the number of available qualified tuners is dwindling and there is a great need for better tuning tools to maintain and improve the performance of complex multivariable processes. Multi-loop PID tuning is the procedure for the online tuning of a cluster of PID controllers operating in a closed loop with a multivariable process. This paper presents the first application of the simultaneous tuning technique to the multi-input-multi-output (MIMO) PID based nonlinear controller in the power plant control context, with the closed-loop system consisting of a MIMO nonlinear boiler/turbine model and a nonlinear cluster of six PID-type controllers. Although simplified, the dynamics and cross-coupling of the process and the PID cluster are similar to those used in a real power plant. The particular technique selected, iterative feedback tuning (IFT), utilizes the linearized version of the PID cluster for signal conditioning, but the data collection and tuning is carried out on the full nonlinear closed-loop system. Based on the figure of merit for the control system performance, the IFT is shown to deliver performance favorably comparable to that attained through the empirical tuning carried out by an experienced control engineer.

Full Operating Range Robust Hybrid Control of a Coal-Fired Boiler/Turbine Unit

Multi-input-multi-output robust controllers recently designed for the megawatt output/throttle pressure control in a coal-fired power plant boiler/turbine unit have demonstrated performance robustness noticeably superior to that of the currently employed nonlinear PID-based controller. These controllers, however, have been designed only for the range of 150-185 MW around the 185 MW nominal operating point, exhibiting a significant loss of performance in the lower range of 120-150 MW. Through system identification, the reason for this performance loss is demonstrated in the current work to be a pronounced dependence of the boiler/turbine unit steady state gains on the operating point. This problem is addressed via a hybrid control law consisting of two robust controllers and a robust switch between them activated by the set point change. The controllers are designed to cover the corresponding half-ranges of the full operating range. This permits attainment of the desired overall performance as well as reduction of modeling uncertainty induced by the operating point change to approximately 25% of that associated with the previous designs. Robust switching is accomplished through a novel hybrid mode of behavior-robustly controlled discrete transition.

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

Linear quadratic (LQ) bumpless transfer design introduced by Turner and Walker (2000) 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 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 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.

Bumpless transfer synthesis in the MIMO systems with a large online/offline controller mismatch using the state/output feedback topology

The simultaneous use of the state and the output of the offline controller in bumpless transfer design for multi-input-multi-output (MIMO) systems is proposed. Under a large online/offline controller mismatch, the recently introduced MIMO bumpless transfer design based on the feedback of the offline controller state and the online controller input and output is found to produce significant deviations between the output signals of the online and the offline controller. These deviations are shown to result in the plant output bumps in the wake of the controller transfer. While retaining the main features of this design, an additional internal model based controller is introduced that uses both the online and the offline controller output signals to form the novel state/output feedback bumpless transfer topology. The latter is shown to be capable of eliminating the online/offline MIMO controller output tracking errors under large mismatch of the online/offline controller dynamics, as well as to permit more flexible manipulation of the error decay rates. The new topology is demonstrated to solve a longstanding problem of the steady state bumpless transfer from standard PID-based controllers to MIMO robust controllers in the megawatt/throttle pressure control of a boiler/turbine unit. The convergence speed-up provided by the topology proposed is shown to facilitate the use of the latter in designing the agile switching controllers with an improved closed-loop performance in electrical motor control applications. The topology proposed is also demonstrated to extend the applicability of the steady state design to bumpless transfer in the oscillatory quasi-stationary regimes.

Robust controller design for simultaneous control of throttle pressure and megawatt output in a power plant unit

This paper presents an application of H/sub /spl infin// and /spl mu/-synthesis controller design methods to a coal-fired power generation unit and compares the closed loop performance and robustness of H/sub /spl infin// and /spl mu/-synthesis control laws with those of an H/sub 2/ control law. All three controller synthesis procedures are applied to a two-input two-output plant model which has time delay, differential part, colored noise output disturbance and sensor noise disturbance. Application of the procedures to the model shows that when the shape of the closed loop control signals of all three designs Is closely matched, in the low frequency range the /spl mu/-synthesis and H/sub /spl infin// control laws have robustness much better than that of H/sub 2/ control law, while providing adequate robustness in the high frequency range. H/sub /spl infin// control law gives the best performance, and H/sub 2/-the worst of the three designs, exhibiting the largest overshoot. The balancing procedure permits significant reduction of the order of the controllers without degradation in performance and robustness. The comparative evaluation of three designs shows that in power plant control problem H/sub /spl infin// and /spl mu/-synthesis designs provide much more consistent and convenient performance/robustness trade-off than H/sub 2/ design.

Level control in feedwater heater systems using nonlinear strategies

Abstract This paper deals with the level control of a horizontal closed feedwater heater in a power generation plant. Due to inherent nonlinearities in the system, conventional PI controllers are often tuned conservatively to preserve stability and hence result in suboptimal performance. In this paper, we propose an advanced control strategy that takes into account the system nonlinearities and demonstrate its ability to regulate the heater level at arbitrary setpoints in the presence of various disturbances. Dynamic response tests were carried out on a full-scale simulator of the feedwater heater in a large power plant to verify the performance of the proposed controller. The results show that a performance improvement of at least an order of magnitude can be achieved. It was also observed that the control inputs needed can be implemented on-line using available valve capabilities.

Bumpless Transfer under Controller Uncertainty: Theory and Implementation

A complete design and implementation methodology for the steady state bumpless controller transfer under controller uncertainty is presented. The off-line transfer speedup idea of Zaccarian and Teel is formalized to yield an implementation technique that completely eliminates the need for any performance/robustness trade-off in the transfer operator design and provides arbitrarily fast convergence of the offline controller output to that of the online one, removing a key obstacle to acceptance by practitioners of the technique proposed

Industrial wireless sensor standards; a user perspective

Future industrial use of wireless instrumentation will undoubtedly increase dramatically in the coming years. Deployment of such instrumentation in an industrial setting - with its security and robustness criteria that are much more stringent than residential performance criteria - hinges on user acceptance of verified performance as well as meeting cost requirements. Today, circa 2011, these industrial users are faced with many choices when specifying a wireless sensor network, including radio performance, battery life, interoperability concerns, and standards compliance. With industrial users standing on the precipice to order and deploy (literally) millions of wireless instruments, it is imperative that accurate information for applying the technology to real-world applications be available to the end-user.