|Controls, Modeling and Simulation
Science first captured my attention at a very early age when I first experienced a hidden world beneath a microscope I
received one Christmas. Science does lead us to truthful answers about nature but for me the factors of mystery,
exploration and discovery have been equally important. Connecting mathematical models with physical reality
eventually became my main interest, and I suppose this led me to pursue my career in feedback control.
Feedback is an amazing device. It exists in nature, but can also be synthesized to meet practical needs. Engineers can
design and apply feedback to a system and incredibly influence how it will respond in terms of stability, speed and
accuracy. For me, the most interesting aspect of control engineering has been modeling and simulation of the systems
themselves. Models, expressed in terms of mathematics help explain how systems work. Modeling not only provides a
foundation and direction for control design, but also serves as the key to unlocking a fundamental nature of how
systems work. Modeling requires a firm knowledge of the underlying physics and accordingly the mathematics required
to express them, but with the power of modern simulation tools, models can be realized in ways that widen perspective
and deepen our understanding as well as offering ways to easily express their behaviors to others. With the power of
simulation we can reveal hidden worlds much like what can be explored under the microscope.
Control Systems & Simulations Specialist
Simulation and Control with VisSim
VisSim is a block-diagram based software tool for programming and simulating dynamic systems and controls. VisSim is produced and marketed by Visual
Solutions of Westford, MA. With VisSim, models can be easily and rapidly expressed as a simulation but it also provides the control engineer with an incredibly
easy to use real time interface to sensors and actuators. These two capabilities enable a low cost hardware-in-the-loop tool for rapid prototyping and feasibility
testing. VisSim helps the Control Engineer to build and visualize systems and to rapidly synthesize control solutions using a complete suite of functional
libraries. VisSim provides a full, hybrid simulation environment of continuous and discrete time components. VisSim's intuitive methods for connecting and
configuring functional block diagrams allow rapid construction of conceptual simulation experiments. Compared with other simulation tools, VisSim leads the
way in terms of usability, performance and cost. Let's look at some of the examples where I've applied VisSim.
Simulation and Control For Critical Care Ventilators
The plot to the right illustrates a simulated response of a human lung to
mechanical ventilation for critical care patients. Much of my work in
controls and simulation has been for medical devices and especially for
critical care ventilators. VisSim provides a simple simulation platform for
developing linear parameter varying systems that closely model the
nonlinear behavior of compressed gas flow in the ventilator tubing and
patient airways. Models have become so reliable that most control
solutions can be developed without actual hardware or at least to the point
where only minor adjustments are required for specifying final design.
Having worked problems in VisSim for at least several ventilator
companies, successful reports spread through the industry and VisSim
has become a leading choice for simulating lung dynamics and ventilator
In 2001 I achieved an exciting breakthrough in developing an adaptive
control method for pressure based ventilation. This development and the
hardware in the loop results were largely possible because of VisSim.
These results were published in the proceedings from the 2001 American
Control Conference meeting at Arlington, Virginia.
Programming Control Solutions by Block Diagram
VisSim provides a rapid means for prototyping, and simple debugging
by direct visualization of structures. The block diagram illustrated
above is used for real-time calculation of the covariance matrix for a
recursive least squares estimator. This structure was synthesized and
operating in a few minutes time. This diagram illustrates the
visualization benefit not offered by coding. Each wired connection in
the diagram can be 'probed' by a mouse click to evaluate operation
while the diagram runs or at any desired stopping point.
VisSim Version 7 introduced 3D plotting capability and the ability rotate and
visualize simulation results. The 3D plot to the right illustrates an isometric view
of a ballistic projectile trajectory with drag forces in a gravitational field
Logic and Timing Analyses
VisSim offers the ability to simulate hybrid systems;
systems that include both continuous and discrete
time components. The plot on the left illustrates an
example of hybrid simulation of IGBT switch states
in a 3 phase PWM power converter together with
one of the normalized phase voltages. VisSim
version 8, released in 2011 now also offers UML
state chart tools for simple synthesis of state
Simulation or Reality?
What appears to be the image of a
fern leaf was generated in VisSim
using an 'Iterated Fractal System' or
IFS of equations. Fractals have the
property of self similarity over scale,
and for the fern leaf this self similarity
is apparent as one views the leaf
segments at finer and finer
magnifications using the zoom feature
in the VisSim plot block.