Transfer Function Limits

[Ironwood]

I am using the CD toolkit, simulation, in labview to program a model of an exciter. The exciter model has an integrator in the form of 1/(s+1).  To implement this integrator I am using the transfer function block. When simmulating the model the output of the transfer function goes to -inf but I need to put limits on the transfer function. I know that limits can be put on the intergtator block but I dont see that the transfer function has limits.
I also know that there is a staturation block but it will only put limits on its output while the transfer function integration goes to -inf.

Is there a way to limit a transfer function? Or is there any suggestion on how to implement a TF with limits?

[Barp]

Ironwood,

First, it is important that you provide the version of the Toolkit you are using. Notice that the Control Design Toolkit and the Simulation Module were merged into one product called "Control Design and Simulation Module 8.5". So, if you have a previous version of the toolkit, we strongly recommend to upgrade to the newest version. Please contact you sales representative since you could be entitle to a cheaper upgrade.

Now, for your specific question, if you have nonlinearities in your simulation, you should use the Simulation Loop instead of the Control Design part. The Control Design part do not handle nonlinearities in the simulation. In Simulation, the integrator (1/s) has a build-in limits. You just need to configure to you the limits that you need. You can have upper, lower or both limits and define different values for this node.

Another information: the integrator equation is (1/s). If you have 1/(s+1), then you have a filter and you could limit the output with a Coerce function in the Comparison pallette.

Hope it helps!

[Ironwood]

Thanks for the fast respone.
I am using Labview version 8.2 and it has the simulation and CD combined.  I probably was not clear but I am working with simulation. The problem we had while runing a simulation of an exciter model was that a transfer function,TF, block was running to -inf. We knew that the built in integrator, 1/s, could be bounded by set limits, so the integration itself would not be saturated at the defined limits. But with the transfer function blocks, there is no place to define limits on the TF. What we did was use the saturation block under the non-linear systems menu to limit the output of the TF. This would limit the output of TF but the TF would still go to -inf.

We decided to create a seperate vi to just test the expected results of a simple TF, 1/(xs+1). From  the analysis of this simple TF, we noticed that some cases for the value of "x" the  results  did not match our  calculations.  As it turns out when we set up the timimg parameters for the simulation loop we had a step size which was smaller than the "x" time constant. All we did was change the step size and our problem was solved.

If any newbie, like us, runs into a problem like our, make sure to check your setp size of your simulation loop.

[Barp]

Ironwood,

If you have an integrator with some other filter, for example, 1/(s*(s+1)). Then you could implement the integrator with 1/s function in series with 1/(s+1). That way, you can limit the integrator with its build-in saturation and avoid going to the maximum.

Let me know if you have more questions!

[ssaara]

hi Ironwood

i think that "control toolkit " may be useful for u to build this transfer fn

i have this toolkit if u want it .