Online model
Encyclopedia
An online model is a mathematical model which tracks and mirrors a plant or process in real-time, and which is implemented with some form of automatic adaptivity to compensate for model degredation over time.
An online model is related to the concept of real-time simulation
, as an online model runs in real-time by definition.
Conversely a real-time simulation
is not necessarily an online model as it by definition does not require adaptivity in either states or parameters.
While many models are adjusted to better fit historical data, traditionally this is done in campaigns or during initial design, and often this is done either manually or with a combination of mathematical and manual methods. By contrast, online models include some automatic procedure to adapt to new process data.
that deals with the use of estimation techniques to ensure that the state and parameter of the process model are as close a match as possible to the real plant.
Reasons that models may need to be taken online include that it is not possible to find offline data for the entire range of operating conditions or that the process is time-varying
.
Parameter estimation can also be used as a technique to capture the influence of effects that are no explicitly modeled, in which case parameters may need to take on values that differ from text-book or table values.
Analyzing trends of how estimated parameters and states in the updated model have changed over time may itself be useful for detecting errors or events in that have occurred during operation and influenced the plant.
.
Online models have three commonalities
One important aspect of bringing a model online is parameter estimation. By some means the parameters of the online model should match the real plant for the model to be a useful analog. System identification
and estimation theory
describe techniques to estimate values of unknown parameters.
Using observers such as the Kalman filter
or the moving-horizon estimator, it is possible to do state estimation, updating the state of the model to ensure that measured and modeled outputs remain as close as possible over time.
It is possible to combine state and parameter estimation, for instance by using an augmented Kalman filter.
Mathematical process models can be used for engineering in the design phase prior to building whole or part of a process plant, but models cannot be brought online at this stage as no process data exists to feed to the model.
However once the plant is built and in use, it is tempting to reuse the model used in the design phase for operation, control and optimization tasks. Re-using models in this manner is often term "life-cycle simulators".
An alternative way to obtain an online model is to build one from scratch for this purpose, the advantage of this approach is that issues such as complexity and simulation speed can be tailored for the needs of online use.
Relationship to other kinds of models
An online model is also sometimes referred to as an "online simulator" or "online system".An online model is related to the concept of real-time simulation
Real-time Simulation
Real-time Simulation refers to a computer model of a physical system that can execute at the same rate as actual "wall clock" time. In other words, the computer model runs at the same rate as the actual physical system...
, as an online model runs in real-time by definition.
Conversely a real-time simulation
Real-time Simulation
Real-time Simulation refers to a computer model of a physical system that can execute at the same rate as actual "wall clock" time. In other words, the computer model runs at the same rate as the actual physical system...
is not necessarily an online model as it by definition does not require adaptivity in either states or parameters.
While many models are adjusted to better fit historical data, traditionally this is done in campaigns or during initial design, and often this is done either manually or with a combination of mathematical and manual methods. By contrast, online models include some automatic procedure to adapt to new process data.
Background and context
Online models are an aspect of process simulationProcess simulation
Process simulation is used for the design, development, analysis, and optimization of technical processes and is mainly applied to chemical plants and chemical processes, but also to power stations, and similar technical facilities.- Main principle :...
that deals with the use of estimation techniques to ensure that the state and parameter of the process model are as close a match as possible to the real plant.
Reasons that models may need to be taken online include that it is not possible to find offline data for the entire range of operating conditions or that the process is time-varying
.
Parameter estimation can also be used as a technique to capture the influence of effects that are no explicitly modeled, in which case parameters may need to take on values that differ from text-book or table values.
Uses
An online model as defined here has the useful property that it closely resembles the real plant, and for this reason the online model can at any time by used to assess planned changes in operations, either for control, for optimization and for different operational tasks to be performed. It can then be used amongst other things for real-time monitoring, de-bottlenecking or plant redesign, or for "what-if" analysis.Analyzing trends of how estimated parameters and states in the updated model have changed over time may itself be useful for detecting errors or events in that have occurred during operation and influenced the plant.
Techniques
The concepts of online models have origins in control engineeringControl engineering
Control engineering or Control systems engineering is the engineering discipline that applies control theory to design systems with predictable behaviors...
.
Online models have three commonalities
- feeding the same control and disturbance signals to the model as is fed to the plant,
- synchronizing simulation clock with the real-world clock, and
- adaptivity: adjustment in states and/or parameters at regular intervals by feedback from measurements.
One important aspect of bringing a model online is parameter estimation. By some means the parameters of the online model should match the real plant for the model to be a useful analog. System identification
System identification
In control engineering, the field of system identification uses statistical methods to build mathematical models of dynamical systems from measured data...
and estimation theory
Estimation theory
Estimation theory is a branch of statistics and signal processing that deals with estimating the values of parameters based on measured/empirical data that has a random component. The parameters describe an underlying physical setting in such a way that their value affects the distribution of the...
describe techniques to estimate values of unknown parameters.
Using observers such as the Kalman filter
Kalman filter
In statistics, the Kalman filter is a mathematical method named after Rudolf E. Kálmán. Its purpose is to use measurements observed over time, containing noise and other inaccuracies, and produce values that tend to be closer to the true values of the measurements and their associated calculated...
or the moving-horizon estimator, it is possible to do state estimation, updating the state of the model to ensure that measured and modeled outputs remain as close as possible over time.
It is possible to combine state and parameter estimation, for instance by using an augmented Kalman filter.
Mathematical process models can be used for engineering in the design phase prior to building whole or part of a process plant, but models cannot be brought online at this stage as no process data exists to feed to the model.
However once the plant is built and in use, it is tempting to reuse the model used in the design phase for operation, control and optimization tasks. Re-using models in this manner is often term "life-cycle simulators".
An alternative way to obtain an online model is to build one from scratch for this purpose, the advantage of this approach is that issues such as complexity and simulation speed can be tailored for the needs of online use.
Applications
Online models are used in- weather forecasting, where techniques used are often termed data assimilationData assimilationApplications of data assimilation arise in many fields of geosciences, perhaps most importantly in weather forecasting and hydrology. Data assimilation proceeds by analysis cycles...
, - reservoir simulationReservoir simulationReservoir simulation is an area of reservoir engineering in which computer models are used to predict the flow of fluids through porous media.-Uses:...
, where techniques used are often termed (automatic) history matching, - model-based control, for instance nonlinear model predictive controlModel predictive controlModel Predictive Control, or MPC, is an advanced method of process control that has been in use in the process industries such as chemical plants and oil refineries since the 1980s...
, where adaptivity in model states and parameters are often introduced by use of an augmented Kalman FilterKalman filterIn statistics, the Kalman filter is a mathematical method named after Rudolf E. Kálmán. Its purpose is to use measurements observed over time, containing noise and other inaccuracies, and produce values that tend to be closer to the true values of the measurements and their associated calculated...
, and in - process simulationProcess simulationProcess simulation is used for the design, development, analysis, and optimization of technical processes and is mainly applied to chemical plants and chemical processes, but also to power stations, and similar technical facilities.- Main principle :...
, where such models are sometimes referred to as "lifecycle simulators"