Excel Formula Needed

Posted on 2010-01-09
Medium Priority
Last Modified: 2013-11-25

I a have a simple simulink model that

1. Reads the data 'u' from the workspace or file (an arbitrary waveform)
          easy to do in vb.net or excel

2. The data is applied to a transport delay
            Ignore for now this is easy to translate

3. The delayed data is applied to a transfer function
             k/a*s+1 (ie. 4/40*s+1)

4. The desired waveform is produced 'y_p' and compared to the reference waveform 'y'

This should be a simple translation of the transfer function
          (ie. k*(exp(-t/a)/a)
When I implement this in excel it does not come close.

What am I missing?

Question by:rwensley
Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

  • Help others & share knowledge
  • Earn cash & points
  • Learn & ask questions
  • 3
  • 2
LVL 50

Expert Comment

by:Dave Brett
ID: 26272960
 EXP in Excel is the exponential applied to the natural base e
 is this what you wanted?



Author Comment

ID: 26272981
Yes that is correct.  the LaPlace transform of k/a*s+1 is k* ((1/a)*(e^(-t/a)))
LVL 15

Expert Comment

ID: 26276468
Would you formulate what exactly is wrong? What are the curves on your Simulink figure?

If you use the same formula and values in Excel and Matlab (not Simulink), would the answer be the same? Try it, and we will understand is it the problem of Excel's calculations or your formula.
Get real performance insights from real users

Key features:
- Total Pages Views and Load times
- Top Pages Viewed and Load Times
- Real Time Site Page Build Performance
- Users’ Browser and Platform Performance
- Geographic User Breakdown
- And more


Author Comment

ID: 26278494
Looking at the excel spread sheet I attached.  Column 'u' is the input to the transfer function in simulink.  
Column 'y' is the output from the simulink transfer function.

Column 'y_p' is the time series calculation that should match column 'y'

 Yes performing the same steps in matlab, usings column 'u' as the input, produces the same column 'y' on the output.

In excel, if I apply a step input to the transfer function k*(exp(-t/a)/a, I get the expected step response.  But if I use the arbitrary input from column 'u' I do not get the expected results.


Accepted Solution

rwensley earned 0 total points
ID: 26279050
I got it.  I found a different first order formula, temp  =(C3-((1/$G$2)*F2))/($I$2) and y_p =E3+F2, where
column 'C' = u, $G$2 = the gain, $I$2 = the time constant, F2 = y_p(t-1), and E3 = temp.

Not what I was expecting to find for a formula, but it does work reasonably well.

LVL 50

Expert Comment

by:Dave Brett
ID: 26279698
No problem Ron, glad to see you got there


Featured Post

Optimize your web performance

What's in the eBook?
- Full list of reasons for poor performance
- Ultimate measures to speed things up
- Primary web monitoring types
- KPIs you should be monitoring in order to increase your ROI

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

You need to know the location of the Office templates folder, so that when you create new templates, they are saved to that location, and thus are available for selection when creating new documents.  The steps to find the Templates folder path are …
This article describes how you can use Custom Document Properties to store settings and other information in your workbook so that they will be available the next time you open the workbook.
This Micro Tutorial will demonstrate the scrolling table in Microsoft Excel using the INDEX function.
Many functions in Excel can make decisions. The most simple of these is the IF function: it returns a value depending on whether a condition you describe is true or false. Once you get the hang of using the IF function, you will find it easier to us…
Suggested Courses

770 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question