{"id":174,"date":"2012-01-22T07:55:40","date_gmt":"2012-01-22T12:55:40","guid":{"rendered":"https:\/\/blogs.mathworks.com\/seth\/?p=174"},"modified":"2012-01-22T07:55:40","modified_gmt":"2012-01-22T12:55:40","slug":"advanced-s-function-techniques-scheduling-future-events","status":"publish","type":"post","link":"https:\/\/blogs.mathworks.com\/simulink\/2012\/01\/22\/advanced-s-function-techniques-scheduling-future-events\/","title":{"rendered":"Advanced S-function Techniques: Scheduling Future Events"},"content":{"rendered":"

Today I will describe how to use a variable sample time s-function to schedule events in the future. This topic is fairly advanced, so hold on tight... or as we say where I come from: Attachez vos tuques avec d'la broche<\/a><\/p>\r\n\r\n

The question<\/strong><\/p>\r\n\r\n

The question I received is:<\/p>\r\n\r\n

I have a model with a variable sample time solver. In this model, I have a signal with a continuous sample time, but discontinuous amplitude. Every time the signal changes amplitude, I need to trigger a subsystem. But I do not want to trigger the subsystem immediately. I need to trigger after fixed delay.<\/em><\/p>\r\n\r\n

\"Original<\/p>\r\n\r\n

First attempt<\/strong><\/p>\r\n\r\n

The first thing the user tried is delaying the signal using the Transport Delay<\/a> block and detecting the changes in the delayed signals.<\/p>\r\n\r\n

As I explained in a previous post<\/a>, the Transport Delay expects a smoothly changing continuous signal. When fed a signal with discontinuities, the output of the Transport Delay is not exactly the input offset by a delay. This is the situation we encounter as shown in this image below.<\/p>\r\n\r\n

\"Delaying<\/p>\r\n\r\n

Variable Sample Time<\/strong><\/p>\r\n\r\n

One way for a block to tell the Simulink solver to take steps at a specific moment in the future is to specify variable sample time<\/a>. For example, the Pulse Generator<\/a> block uses this technique to run only when needed.<\/p>\r\n\r\n

If I make a model with the Pulse Generator block configured to have a period of 5s and a pulse width of 40%, the block will tell the variable step solver to take steps at times 0s, 2s, 5s, 7s, 10s,...<\/p>\r\n\r\n

\"Pulse<\/p>\r\n\r\n

Variable Sample Time S-function<\/strong><\/p>\r\n\r\n

With an s-function, you can do the same!<\/p>\r\n\r\n

For a basic example, look at the demo model sfcndemo_vsfunc.mdl<\/tt>. This model shows how to use mdlGetTimeOfNextVarHit<\/tt><\/a> to specify the next time your block should run.<\/p>\r\n\r\n

The solution<\/strong><\/p>\r\n\r\n

Based on that, we want to write an s-function that will look at its input every major step of the simulation. When a change will be detected, we will store the time value in a buffer. Every time an event will be generated, we will look into this buffer and schedule the next event.<\/p>\r\n\r\n

In S-function specific terms, this means: <\/p>\r\n\r\n