Sum block not summing inputs

I'm having trouble with a Sum block in my Simulink model. When I use the debugger to watch its input/output values, I see that it appears to be just passing U1 through, rather than adding the 3 values. Can anyone tell me why this might happen?

Trace: Data of 0:180 Sum block 'Robot_4_Motors/dc_motor_simplified/Sum':
U1     = [-0.064934694127110726]
U2     = [0]
U3     = [0.62636893981851649]
Y1     = [-0.064934694127110726]

[Ryan]

Did you change any settings under the 'Main' or 'Signal Attributes' sum block parameters?

[Guy Rouleau]

I would really like to see that model. Can you place it on some file sharing site so I can give a look?

At first look, this is surprising.

A few things I would try and/or like to know:

- Try is disabling optimization options like block reduction, signal reuse, etc. - Do you have hand-written s-functions in the model? If yes, try removing them from the model, and/or enable the "array out of bound exceeded" diagnostic. - Try enabling Test Points on all the input and output of the block. - Does that happen to other blocks in the model? - Is this happening for all time steps? or only once? - Always only the first input port is passed? - Are there any conditionally executed subsystem before or after? - Is there a Merge block involved?

Without the model, it is difficult to be 100% sure... so I would really like to see the model to see if there is an explanation.

[Nick Jordan]

Sorry to have asked this and then disappeared. I had selected to be notified by email when an answer was added, but apparently that doesn't work for comments. Can I be notified of comments somehow?

I monitored several sum blocks on this model, and all of the blocks with a subtraction seemed to be failing, and were all just passing U1 directly through to Y1. The one in my original post is '-+-', for example. I turned off signal storage reuse, and now it seems to be working correctly. I don't understand why this option was messing it up though.

Here is a link to the model file: https://dl.dropbox.com/u/15325551/Robot_4_Motors.mdl And to the corresponding initialization file: https://dl.dropbox.com/u/15325551/MMP30_Init.m

[Walter Roberson]

Sorry, there is no notification for comments in the current MATLAB Answers system.

Answer 1

[Nick Jordan]

Disabling Signal Reuse, as Guy Rouleau suggested, solved this problem and made the Sum block start working correctly.