Robust model-free adaptive control

Details Robust model-free adaptive control Robust model-free adaptive control

2001-04-11

2004-01-27

Patel, Ramesh (Department: 2121)

Data processing: generic control systems or specific application

Generic control system, apparatus or process

Optimization or adaptive control

C700S040000, C700S048000, C700S052000, C700S055000, C700S037000, C700S041000, C700S042000, C700S043000, C700S039000, C700S047000, C700S049000, C700S050000, C700S054000, C700S071000, C700S072000, C700S075000, C700S076000, C700S077000, C706S014000, C706S023000, C706S016000, C706S025000

Reexamination Certificate

active

06684112

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to automatic control of simple to complex systems including industrial processes, equipment, facilities, devices, engines, robots, vehicles, and appliances, and more particularly to a method and apparatus for adaptively controlling continuous variables and providing a large user-defined robust range. The inventive controller is able to force the controlled variable to stay within a pre-defined robust range under significant system dynamic changes due to batch, load, production, configuration, or operating stage switches, and under large disturbances originating from wild product flows, plant upsets, wear and tear of sensors and valves, etc.
BACKGROUND OF THE INVENTION
The Model-Free Adaptive (MFA) control methodology described in U.S. patent application Ser. No. 08/944,450, and patent applications, Ser. No. 09/143,165 filed on Aug. 28, 1998, and Ser. No. 09/174,156 filed on Oct. 16, 1998 are able to deal with various complex systems in practice. However, MFA may be incapable of effectively controlling a system with the following situations:
a) There is a big change in the system dynamic so that a regular MFA controller is unable to provide prompt and adequate control action to meet the control performance criteria;
b) The dominant disturbance to the system cannot be economically measured so that feedforward compensation cannot be easily implemented;
c) The controller purposely de-tuned to minimize the variations in its manipulated variable may lose control when there is a large disturbance or significant dynamic behavior change; and
d) The system dynamic behavior or load change does not provide triggering information to allow the control system to switch operating modes. Otherwise, a gain-scheduling adaptive controller or the multifunction MFA control system described in patent application Ser. No. 09/174,156 would be able to switch its controller gain or control strategies accordingly based on a triggering signal.
To describe the application in more detail, a chemical reactor control problem is studied in the following. Chemical batch reactors are critical operating units in the chemical processing industry. Controlling the batch reaction temperature is always a challenge due to the complex nature of the process, large potential disturbances, interactions between key variables, and multiple operating conditions. A large percentage of batch reactors running today cannot keep the reactor temperature in automatic control through out the entire operating period thus resulting in lower efficiency, wasted manpower and materials, and inconsistent product quality.
An exothermal batch reactor process typically has 4 operating stages:
a) Startup Stage: ramps up the reactor temperature by use of steam to a pre-defined reaction temperature.
b) Reaction and Holding Stage: holds the temperature by use of cooling water while chemical reaction is taking place and heat is being generated;
c) No-reaction and Holding Stage: holds the temperature by use of steam after main chemical reaction is complete and heat is not being generated; and
d) Ending Stage: ramps down the reactor temperature for discharging the products.
During the transition period from Stage 2 to Stage 3, the reactor can change its nature rapidly from a heat-generation process to a heat-consumption process. This change happens without any triggering signal because the chemical reaction can end at anytime depending on the types of chemicals, their concentration, catalyst, and reaction temperature. Within a very short period of time, the reactor temperature can drop significantly. The control system must react quickly to cut-off the cooling water and send in a proper amount of steam to drive the reactor temperature back to normal. A regular feedback controller is not able to automatically control a batch reactor during this transition if it is tuned to control the process in Stages 1 and 2. In practice, batch reactors are usually switched to manual control and rely on well-trained operators during critical transitions. It is a tedious and nerve-wracking job that can result in low product quality and yield.
It is desirable to develop an automatic control system capable of controlling complex systems such as chemical batch reactors to improve control performance, plant efficiency, system safety, and product quality.
In this patent application, we introduce a Robust Model-Free Adaptive control system to control the problematic processes described. Robust control is usually referred to as a controller design method that focuses on the reliability (robustness) of the control algorithm. Robustness is defined as the minimum requirement a control system has to satisfy to be useful in a practical environment. Once the controller is designed, its parameters do not change and control performance is guaranteed. The control apparatus we introduce in this application is not a control system design method. We use the term ‘robust’ here because this novel control system is able to dramatically improve the control system robustness. Without the need of re-designing a controller, using feedforward compensation, or re-tuning the controller parameters, the inventive control system is able to keep the system in automatic control through normal and extreme operating conditions when there are significant disturbances or system dynamic changes.
SUMMARY OF THE INVENTION
The present invention overcomes the above-identified limitations of the prior art by using a specially designed Robust Model-Free Adaptive control system, which includes a Primary Controller and a set of Constraint Controllers. Since the additional parameters such as the Upper and Lower Bounds are easy for the user to provide, the control system is easy to setup and maintain. Robust MFA is able to dramatically improve the control system robustness. Without the need of re-designing a controller or re-tuning the controller parameters, the inventive control system is able to keep the process under automatic control during normal and extreme operating conditions when there are significant disturbances or changes in process dynamics. Because of its simplicity and capability, the control system is useful for building flexible and adaptive production systems to fulfill the on demand manufacturing needs in the new e-commerce environment.


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