I receive an error when adding a delay block in for loop block

Hi,

I am making a for loop. To make the variation in vector values visible on the scope, I have added a transport delay block after the iterator block as seen in the image below.

The problem, is that when I run the Simulink program, I receive an error which says:

'Loops_in_Simulink/For Iterator Subsystem/Transport Delay' has sample time 0. Sample time of block must be inherited (-1) because blocks in a triggered subsystem must run only when the triggered subsystem runs.

If I double click on the transport delay block, I cannot see an option to change the sample time. This program is not a homework task. Could someone let me know how I could change the sample time? Thanks in advance.

[Stephen Eltinge]

Hi Matthew,

Can you post the full Simulink model you are working with, or at least a screenshot of the top-level model? Generally speaking, each step of the For Iterator Subsystem must finish running before the next one starts, so it's not entirely clear what purpose a Delay block would serve here.

[Fangjun Jiang]

Double click the block, click Help to bring up the document and example model.

Or, state your need and ask what block to be used.

I believe you misunderstood the "For Iterator" and "Transport Delay" block, completely.

[Matthew Tieu]

Hi @Stephen Eltinge,

The top level block diagram is attached below. I included the delay block to make each of the iterated values visible during the simulation. If I omit the delay block, I can only see the value of the last element at the display block during the simulation.

[Matthew Tieu]

Hi @Fangjun Jiang,

I know for modelling continuous systems, the transport delay block adds a specified period of zero output to the start of an input signal. I need a block that can add a specified time delay to each iteration of the for loop.

In other words, there should be a delay of T seconds before the output changes from the nth value to the (n+1)th value of the vector. Is there a block that can achieve this?

[Fangjun Jiang]

You are doing a 1:n for-loop. At the end of 1st loop, the value of output O(1) is obtained. At the end of 2nd loop, the value of output O(2) is obtained. You want "a delay of T seconds" between O(1) and O(2)??

But you should know, that in Simulink, all the for-loops, from 1 to n, are all calculated and finished in the same time step. There isn't a concept of "time delay" if your original problem is modeled and simulated this way.

If you want to insert some values between O(1) and O(2), then the "Vector Concatenate" block can be used.

But overall, I suspect that your original problem was modeled wrong. Your intended usage of the "For Iterator" block missed the mark. The output is a number (1 to N, depending on the loop) at the end of each loop. The output could be hidden (no ouptut at all). Please read the document to understand it.

Answer 1

[Stephen Eltinge]

Hi Matthew,

Fangjun is correct in the comments above. Be careful not to confuse two different concepts: the timesteps of the top-level model and the iterations of the For Iterator Subsystem. At each model timestep, the For Iterator Subsystem executes its entire contents as many times as you specify, then reports the final output. The Display block does not update during this calculation.

It seems to me that what you are trying to do is construct a model that acts element-by-element on an array and prints some result to the Display block after every step. Instead, your current model processes the entire 1x31 array at every timestep and reports the same final answer. If you want to act on one element per timestep, you need to construct a time-varying signal. See the Signal From Workspace block for a simple way to import or construct a signal from raw data.

[Matthew Tieu]

Hi @Stephen Eltinge,

Thanks for the reply, it looks like it is not possible to implement a delay per iteration directly in the for loop. I will probably use an X-Y graph block and a 'Vector Concatenate' block instead in that case.