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What is VisSim?VisSim helps you model linear and non-linear
dynamic systems - "anything that moves"
Getting Startedwith VisSim
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How does it work?A VisSim simulation is constructed from three
"layers":
1. Blocks
2. The "wires" connecting those blocks
3. Simulation parameters (simulation time step, numerical
integration method etc)
BlocksBlocks are placed on the worksheet from the Blocks drop
down menu
Blocks can generally be divided into three categories...
Blocks that produce signals that "travel" through the system
Blocks that consume signals - these are used to display the
results of your simulation
And everything else. These are typically used to transforms
signals from one form to another, create animations, or read in
external data. At their simplest level, they might add two signals
together. At their most complex level, they might be used to
numerically integrate a signal over time or represent a transfer
function
1 Getting Started with VisSim
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Hello WorldLet's try the VisSim equivalent of "Hello World". We
can't get much simpler than adding two numbers together!
STEP 1
STEP 2
Place another const block on the worksheet, together with a
summingJunction (from the Arithmetic menu) and a displayblock (from
the Signal Consumer menu)
STEP 3
STEP 4
Wire the other blocks together.
STEP 5
STEP 6
Select const from the Signal Producer menu with aleft-click.
Left -click at this point and keep themouse button held
down.
Drag a wire to a connection port onthe summingJunction
Release the mouse button to place thewire
Move the mouse pointer over the topconst block
Right-click to fire up the const Propertiesmenu. Change its
value to 2.
Click OK to get back to the simulation.
Select Go from the Simulate menu (or press F5 or click the
button)
Eureka!
Move the mouse overthe worksheet
Left -click toplace the block
Move the mouse point over one of thebranches of the
summingJunction
Hold down the Ctrl key and press the right-mouse button to
change the sign of the branchQuick Tip...
2Getting Started with VisSim
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Creating Compound BlocksCompound blocks are a vital part of
organising your simulations - they hide deeper levels of local
complexity.Let's try creating a compound block that can be used for
a common operation - finding the derivative of a signal.
STEP 1
Assemble the following blocks. The centre portion (between the
sinusoid and the plot block) is the derivative operation.
STEP 2
STEP 3
Drag a selection box around the following blocks Releasing the
mouse button gives the following
Select Create Compound Block from the Edit menu andmake the
following changes
Clicking OK creates the compound block in yourworksheet
13 Getting Started with VisSim
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Simulating a Spring-Mass Damper ArmLet's try something a little
less trivial - a classical spring-mass damper arm.
Where K = Spring ConstantB = Constant DampingM = Massx =
Vertical Displacement
From Newton's Second Law, the equation of motion for the damped
harmonic oscillation is
Integral equations are more numerically stable than differential
equations. The first step is to isolate thederivate with the
highest degree on the LHS:
Integrating the acceleration gives the velocity. Integrating the
velocity gives the position. Within VisSim, thiswould look
like:
The initial condition (x(0) = 3) is set in the second integrator
block (by right-clicking on the block to bring upthe Integrator
Properties menu).
To form the whole equation in VisSim we need to 1) Multiply the
position by K2) Multiply the velocity by B3) Add these two
quantities together with a summingJunction4) Multiply the sum by
-1/M with a gain block5) Wire the output of the gain block to the
input of the acceleration block
The completed simulation would look like:
In the simulation above we set the initial conditions for the
position inside the integrator block. However, initial conditions
can also be set within theactual simulation environment itself. For
example, let represent velocity at time t, with x(0) = 3. In
VisSim, the position x(t) is given by
Quick Tip...
4Getting Started with VisSim
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Optimising a PID Control LoopIntroduction
We'll now develop a model of a classical PID Control Loop and
optimise the gains so that we minimise rise-time and overshoot.
The model will consist of three sections: 1) the control loop,
2) a "cost" function that measures how close weare to our
optimisation goal, and 3) the parameters that we want to vary to
minimize the cost function.
1. The Control LoopAssemble the following blocks to form the
control loop
2. The Cost FunctionThe cost functions measures how far we are
from our stated goal of reducing steady state error and the timeto
set-point.
Algorithmically, it can be thought of as:1) Find the difference
between the Input and the Output2) Square it (in case the
difference is negative).3) Integrate this error over time to find
the total error.
3. The Parameters.We use parameterUnkown blocks to specify what
parameters we want to vary to minimise the cost function
5 Getting Started with VisSim
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The completed simulation should look like this.
Under Simulation>Simulation Properties set the following
options.
Under Simulate>Optimization Properties, select the following
options are selected.
Run the simulation and your plot block should look something
like this
6Getting Started with VisSim
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Optimising a PID Control Loop continued...
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There's plenty of overshoot. Let's penalise overshoot by
modifying the cost function.
This can be thought of as:
1) Find the difference between the input and the output2) If the
difference is negative (i.e. overshoot) multiply the error by 10.
If the difference is positive,
let the signal pass through without modification3) Square the
error (in case it is negative)4) Integrate the error over time to
find the total error.
Running the simulation gives this response
