Measurements,
Which Parameters to Identify?
Model
Calculations,
Manufacturer's Specifications
What Tests to Perform?
Parameter
ID
Physical
Physical
Math
System
Model
Model
Equation Solution:
Assumptions
Experimental
Physical Laws
and
Analysis
Engineering Judgement
and Numerical
Modify
Solution
Actual
Predicted
Dynamic
Compare
Behavior
or
Design
Design
Decisions
Dynamic
Behavior
Make
Modify
Analytical
Model Inadequate:
Model Adequate,
Model Adequate,
Performance Inadequate
Performance Adequate
Complete
Augment
Dynamic System Investigation
Mechatronics
Mathematical Modeling - General
K. Craig
1
Physical Model to Mathematical Model
• We derive a mathematical model to represent the
physical model, i.e., write down the differential
equations of motion of the physical model.
• The goal is a generalized treatment of dynamic
systems, including mechanical, electrical,
electromechanical, fluid, and thermal systems.
Mechatronics
Mathematical Modeling - General
K. Craig
2
Steps in Deriving Mathematical Model
• Define System
– Boundary
– Inputs & Outputs
• Define Variables
– Through Variables
– Across Variables
• Write System Relations
– Dynamic Equilibrium Relations
– Compatibility Relations
• Write Physical Relations for Each Element
• Combine - Generate System Differential Equations
Mechatronics
Mathematical Modeling - General
K. Craig
3
• Define System
– A system must be defined before equilibrium
and/or compatibility relations can be written.
– Unless physical boundaries of a system are clearly
specified, any equilibrium and/or compatibility
relations we may write are meaningless.
• Define Variables
– Physical Variables
• Select precise physical variables (velocity, voltage,
pressure, flow rate, etc.) with which to describe the
instantaneous state of a system, and in terms of which to
study its behavior.
Mechatronics
Mathematical Modeling - General
K. Craig
4
– Through Variables
• Through variables (one-point variables) measure
the transmission of something through an element,
e.g.,
– electric current through a resistor
– fluid flow through a duct
– force through a spring
– Across Variables
• Across variables (two-point) variables measure a
difference in state between the ends of an element,
e.g.,
– voltage drop across a resistor
– pressure drop between the ends of a duct
– difference in velocity between the ends of a damper
Mechatronics
Mathematical Modeling - General
K. Craig
5
– In addition to through and across variables, integrated
through variables (e.g., momentum) and integrated
across variables (e.g., displacement) are important.
• Write System Relations
– Dynamic Equilibrium Relations
• Write dynamic equilibrium relations to describe the
balance - of forces, of flow rates, of energy - which
must exist for the system and its subsystems.
• Equilibrium relations are always relations among
through variables, e.g.,
– Kirchhoff’s Current Law (at an electrical node)
– continuity of fluid flow
– equilibrium of forces meeting at a point
Mechatronics
Mathematical Modeling - General
K. Craig
6
– Compatibility Relations
• Write system compatibility relations to describe
how motions of the system elements are interrelated
because of the way they are interconnected.
• These are inter-element or system relations.
• Compatibility relations are always relations among
across variables, e.g.,
– Kirchhoff’s Voltage Law around a circuit
– pressure drop across all the interconnected stages of a
fluid system
– geometric compatibility in a mechanical system
Mechatronics
Mathematical Modeling - General
K. Craig
7
• Write Physical Relations for Each Element
– These relations are called constitutive physical relations
as they concern only individual elements or
constituents of the system.
– They are natural physical laws which the individual
elements of the system obey, e.g.,
•
•
•
•
mechanical relations between force and motion
electrical relations between current and voltage
electromechanical relations between force and magnetic field
thermodynamic relations between temperature, pressure, etc.
– They are relations between through and across
variables of each individual physical element.
– They may be algebraic, differential, integral, linear or
nonlinear, constant or time-varying.
Mechatronics
Mathematical Modeling - General
K. Craig
8
– They are purely empirical relations observed by
experiment and not deduced from any basic principles.
• Combine System Relations and Physical Relations
to Generate System Differential Equations
Mechatronics
Mathematical Modeling - General
K. Craig
9
Mechanical Equations of Motion
• Geometry
– Picture the system in an arbitrary configuration
(with respect to a reference configuration)
– Define coordinates and their positive directions
– Note geometric identities
– Note relations implied by geometric constraints
– Write system compatibility relation, if
advantageous
Mechatronics
Mathematical Modeling - General
K. Craig
10
• Force Equilibrium
– Write force balance relations
• Draw a free-body diagram
• Write equations of equilibrium of all forces acting
on the free body
– Write energy balance relations
• Define the system envelope
• Invoke conservation of energy for the system
• Physical Force-Geometry Relations
– Write these for the individual elements.
Mechatronics
Mathematical Modeling - General
K. Craig
11
Electrical Equations of Motion
• Network Variables
– Define variables – voltages and currents. Note
identities. Note constraints imposed by sources.
• Equilibrium or Compatibility
– Apply Kirchhoff’s Current Law (KCL) (node
analysis) or Apply Kirchhoff’s Voltage Law
(KVL) (loop analysis).
• Physical Voltage-Current Relations
– Write these for the individual elements.
Mechatronics
Mathematical Modeling - General
K. Craig
12