Data processing: measuring – calibrating – or testing – Measurement system
Patent
1998-08-28
2000-10-03
Hoff, Marc S.
Data processing: measuring, calibrating, or testing
Measurement system
700 31, 700 37, G05B 1304
Patent
active
061285862
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a method and device for controlling a self-regulating, delayed process, in particular for controlling temperature.
BACKGROUND OF THE INVENTION
An adaptive controller for self-regulating, delayed processes is described in German Patent No. OS 39 29 615. This adaptive controller has the ability to automatically adjust its parameters to the characteristics of a controlled system which vary over time. To do this, the process response to an abrupt variation in the setpoint is recorded, and an iterative method is used to find a PTn model which can emulate this step response as accurately as possible. Optimum parameters are those of the PTn model in which the error between the step responses of the PTn model and those of the real process, calculated by the method of least error squares, is minimal. Upon completion of process identification, a controller is designed according to the optimum absolute value using the identified process model. Single-loop control loops with PID controllers are a possible controller type. The processes to be controlled are self-regulating, which means that they represent a controlled system in which the step response enters a new steady state.
A disadvantage of this conventional method lies in the fact that, when turning on the conventional adaptive controller, it is often necessary to perform a time-consuming step change test before the controller can be optimized. This test is not completed until the controlled variable once again approaches the new steady-state limit after the setpoint step change. In doing this, the steady-state criterion must be carefully selected, since the identification process will take up too much time if the criterion is too narrowly defined, while too broad a criterion will increase the risk of incorrect identification. In addition, the control quality that can be achieved with a linear PID controller is in principle limited, since a fast start control response is achieved at the price of overshooting the controlled variable.
Another disadvantage of the conventional identification method lies in the creeping time constants, i.e. poles which are very close to the origin of the Laplace plane and can cause the process to behave very much like an integrator, so that a steady state cannot be achieved for a very long time following a step change in the manipulated variable. Consequently, the conventional identification method based on a complete step response is very time-consuming in this type of process.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method for controlling a self-regulating, delayed process, in particular for controlling temperature, which avoids the above disadvantages, as well as to provide a control device for carrying out the method.
An advantage of the present invention lies in the fact that a time-consuming step change test can be eliminated entirely when turning on a controller. A simple IT1 model approximates the process as early as the first compensation phase following a setpoint step change, and the first setpoint can be approached in a time-optimal fashion and with practically no overshooting. This is especially advantageous when adjusting temperatures to a specific setpoint with the help of an adjustable heating system in which a controller must maintain a constant temperature in spite of external disturbances and set the desired new temperature as quickly as possible when the default setpoint changes, for example during a chemical process. The adaptive controller automatically adjusts to any controlled temperature system, without any parameters being set and without prior knowledge of the process. The controller is optimized during start-up, without the operator having to make any input. As a criterion for determining when a steady state has been reached, the rate of controlled variable change is evaluated and checked when it drops below a definable limit after the manipulated variable has been reset. The process is then identified on the
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Hoff Marc S.
Miller Craig
Siemens AG
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