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Unexpected behavior with From Workspace block in Simulink

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Thomas Marullo
Thomas Marullo 2013-5-29
The From Workspace block for Simulink is supposed to output data based on the time column in the variable. I have a model at a sample rate 5 times faster than the From Workspace, ie: 0.05 for Data, 0.001 for Model.
When I record the data from the From Workspace block at the Model sample rate of 0.001, I mostly see the data repeated 5 times but sometimes I see it repeated 6 times followed by 4 times for the next. This creates errors in my calculations because the expected data is wrong:
ie, 0 0 0 0 0 1 1 1 1 1 2 2 2 2 2 2 <- extra "2" 3 3 3 3 too little "3" 4 4 4 4 4

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Kaustubha Govind
Kaustubha Govind 2013-5-29
(I presume you meant that 0.01 is your model's sample rate, because 0.05 is 50 times 0.001, not 5 times)
This looks like an floating-point/accuracy issue - 0.05*3 is slightly larger than 0.01*15:
>> format hex
>> 0.01*15
ans =
3fc3333333333333
>> 0.05*3
ans =
3fc3333333333334
Which is why, for the 15th sample, the previous data point (=2) is used.
Perhaps you can adjust the time column of the imported variable so that it is multiples of 0.01 instead of 0.05?
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Thomas Marullo
Thomas Marullo 2013-5-29
Sorry, you are indeed correct. From Workspace block is 0.005000000000000 and model sample rate is 1.000000000000000e-03. I think your recommendation of changing the From Workspace block to be multiples of the model sample rate is a good method.
Thomas Marullo
Thomas Marullo 2013-5-29
I also tried using 0.04 as my From Workspace timing and 0.01 as my model sample rate and I had the same problems at step 40. I didn't think there would be a problem using this case since,
>> 0.01*40
ans =
3fd999999999999a
>> 0.04*10
ans =
3fd999999999999a

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