Perspective on Process Control James J. Downs Advanced Controls Technology Eastman Chemical Company
Apr 01, 2015
Perspective on Process Control
James J. DownsAdvanced Controls TechnologyEastman Chemical Company
Early History
Ktesibios (270 BC)
Water Clock
Early History
Edmund Lee (1745)
Windmill Fantail
Industrial Age
James Watt (1788) Centrifugal governor patented for the steam engine
Mathematical Age
James Clerk Maxwell (1831-1879)
On Governors. Proc. Roy. Soc. 16 (1868) 270-283.
• Stability Concept• Simple Mathematical Models• Importance of integral action• Linearization
Mathematical AgeStability Concept
"It will be seen that the motion of a machine with its governor consist in general of a uniform motion, combined with a disturbance which may be expressed as the sum of several component motions. These components may be of four different kinds:
1. The disturbance may continually increase. 2. It may continually diminish. 3. It may be an oscillation of continually increasing amplitude. 4. It may be an oscillation of continually decreasing amplitude.
The first and third cases are evidently inconsistent with the stability of the motion: and the second and fourth alone are admissible in a good governor. This condition is mathematically equivalent to the condition that all the possible roots, and all the possible parts of the impossible roots of a certain equation shall be negative."
Mathematical Age
Proportional Action Concept
"Most governors depend on the centrifugal force of a piece connected with a shaft of the machine. When the velocity increases, this force increases, and either increases the pressure of the piece against a surface or moves the piece, and so acts on a break or a valve.
In one class of regulators of machinery, which we may call moderators, the resistance is increased by a quantity depending on the velocity."
Mathematical Age
Integral Action Concept
"But if the part acted on by centrifugal force, instead of acting directly on the machine, sets in motion a contrivance which continually increases the resistance as long as the velocity is above its normal value, and reverses its action when the velocity is below that value, the governor will bring the velocity to the same normal value whatever variation (within the working limits of the machine) be made in the driving-power or the resistance."
Mathematical Age
1877 Vishnegradsky – stability1893 Lyaponov – stability of nonlinear differential equations1895 Heaviside – transient behavior of systems
1927 Black – negative feedback1932 Nyquist – design of stable amplifiers1938 Bode – frequency response
(Bell Telephone Laboratories)
Time Domain 𝑑𝑥𝑑𝑡
= 𝑓 (𝑥 ,𝑡 )
Frequency Domain L [𝑦 (𝑡 ) ]=𝑌 (𝑠)
Hardware Age
Hardware Age
Control Implementation Technology
• Pneumatic single loop controllers (3-15 psig standard)
• Electronic controllers (4-20 ma standard)
Hardware Age
Control Implementation Technology
• Pneumatic ingenuity
7216 Variable Ratio Regulators are used with nozzle-mix burners to achieve temperature uniformity while using minimum excess air. … A high quality stainless steel spring is used to bias 7216 Regulator air/gas ratio. As air rate is turned down towards low fire, gas rate drops faster, giving increasing percentages of excess air …
Migration to the Computer Age
Control Implementation Technology
• Block logic (PID, summer, selectors, digital points, ratio, cascade, etc.
• Control logic programs
• Distributed vs. Centralized Architecture
Computer Age
Control Strategy Technology
• What should be controlled ?• Simple variables, measurements• Computed combinations of measurements• Inferred measurements• Laboratory measurements
• How to control process variables• Single loop control• Cascade, override, multivariable, feedforward, etc.• Tuning, performance considerations
• Control Objectives• Migration from "hold constant" to "find optimum"
Today's Age
Control to Maximize Profit
• Control Objectives• Measures of profit• Real time changes to the profit function (pricing, demand, etc.)
• Process Constraints• Measureable/predictable, unpredictable• Current time or future time• Operation at process constraints
• Changing Targets• Maintaining good control when the target is always moving• Interface with supply chain management
• Managing Process Variability• Control strategy design
Today's Age
Models
A model in this context is a mathematical description of a process
• Linear / Nonlinear• Continuous / Discrete• Deterministic / Stochastic• First principle / Empirical
ProcessModel
Inputs: valve positions flow rates energy inputs
Outputs: temperature composition properties
Parameters
Today's Age
Models
Example: ( linear, no dynamics, continuous )
Process y = au + b
Inputs: u
Outputs: y
Parameters:a, b
b
Today's Age
Models
Example: ( linear, dynamic, continuous )
ProcessInputs:
Outputs:
Parameters: A, B, C
𝑑𝒙𝑑𝑡
=𝐴 𝒙+𝐵𝒖
𝒚=𝐶 𝒙
Today's Age
Models
Example: ( linear, dynamic, continuous, uncertainty descriptions )
ProcessInputs:
Outputs:
Parameters:A, B, C, N(µ, σ)
𝑑𝒙𝑑𝑡
=𝐴 𝒙+𝐵𝒖+𝐺𝒘
+
Coming Age
Models
Example: ( nonlinear, dynamic, continuous, known uncertainty description )
ProcessInputs:
Outputs:
Parameters: a, b, c, …
𝑑𝒙𝑑𝑡
= 𝑓 (𝒙 ,𝒖 ,𝒘 )
𝒚=𝑔 (𝒙 ,𝒗 )
Coming Age
Control of Uncertain Processes
Characterization of Uncertainty
• Mean, standard deviation (normal distribution)• Other distributions• Non stationary distributions
Characterization of Risk
• Determination of the cost of risk• Deciding the risk/reward tradeoff point
Coming Age
The PSUADE Uncertainty Quantification Project
Uncertainty quantification is defined as the
• identification (Where are the uncertainties?), • characterization (What form they are in?), • propagation (How they evolve during the simulation?), • analysis (What are their impacts?), and • reduction of ALL uncertainties in simulation models.
Coming AgeModels
Current research:
• Improved, more accurate models• System identification methods to develop models• Model reduction techniques• Mathematical methods to drive models to optimum points in an
optimum manner (given a mathematical description of 'optimum').
Optimizers exploit model weaknesses and can find poor,
undesirable answers
As solution techniques get
stronger we need better models
Coming Age
Process Uncertainty
• Models provide a template upon which to overlay process data
• Uncertainty descriptions coupled with models can suggest "best" answers in a probabilistic sense.
• Techniques for handling model size, complexity, unknown parameters, probability distributions, initialization, etc. are barriers
DeterministicWorld
ProbabilisticWorld
Coming Age
Process
ModelStructure
Extracted DataFit to Model
Structure
Uncharacterized Process
Information
Coming Age
Process Inputs
RefineParameterize
AnalyzeOptimize
Strategize InvertFilter
Add LogicClampLimit
etc., etc.
Coming Age
How should we describe the remainder of the process information ?Other process
information/behavior we don't know about
… 0100101110100110 …
µ,σ
∑𝑛=1
∞
( 𝑎𝑛 cos 𝑛𝜋 𝑥𝐿 +𝑏
𝑛 sin 𝑛𝜋 𝑥𝐿 )
Process information we
are not measuring.
Coming Age
• Measure of risk• Probabilistic answers• Stochastic/chaotic• Random behavior
Process Inputs
Other process information/behavior we don't know about
Final Thoughts
ComputeModel
InverseProcess
Model
DesiredState
ProcessOutput
Inputs: Outputs:
Estimate
Applications
Biological Systems• Diabetes• Drug dosing strategies• Virus spread• Predator/prey dynamics• Population demographics
Economics• Stock market• Fiscal policy• Supply/demand• Financial modeling
Questions ?
Final Thoughts
ComputeProcessInputs
ProcessDesiredState
ProcessOutput
Inputs: Outputs:
Final Thoughts
ComputeModel
InverseProcess
Model
DesiredState
ProcessOutput
Inputs: Outputs:
Estimate
Final Thoughts
ComputeModel
InverseProcess
Model
DesiredState
ProcessOutput
Inputs: Outputs:
Estimate
Final Thoughts
ComputeModel
InverseProcess
Model
DesiredState
ProcessOutput
Inputs: Outputs:
Estimate