Simulink and Monoflops

Question

I'm using Matlab 7 and have a problem in creating a monoflop which shall raise for a specific time to "1" and after that time fall to "0". How can I do this with Matlab/Simulink 7? I don't have any other version, so I can't use the "Monostable" Block from newer versions.

Any ideas?

greets, poeschlorn

Answer 1

There are a couple of ways to do this, depending on whether or not you want the pulse (i.e. "monoflop") to occur at a predetermined time or in response to another signal (like a rising edge)...

Creating a pulse at a predetermined time:

If you want to create a single pulse that steps from 0 to 1 at time tOnset, then steps back to 0 after a time tDur has elapsed, you can do this using a Step block, a Transport Delay block, and a Sum block. Here is what the layout would look like:

You would set the Step time of the Step block to tOnset, the Time delay of the Transport Delay block to tDur, and then subtract the delayed signal from the original signal.

Creating a pulse in response to a rising edge:

This is will be a bit more complicated. It will require two Detect Increase blocks, a Relay block, a Transport Delay block, a Gain block, and a Sum block. Here's what the layout would look like:

Assuming your input signal is either a 1 or a 0, the first Detect Increase block will output a 1 when the input steps from 0 to 1. By setting the Switch on point to 0.5 and the Switch off point to -0.5 for the Relay block, this will create hysteresis in the Relay such that the output will persist in the "on" state (i.e. an output of 1) after the brief pulse that occurs when the rising edge is detected.

To get the Relay block to switch back into the "off" state (i.e. an output of 0) after a specified time tDur, you would set the Time delay of the Transport Delay block to tDur. The Detect Increase block in the feedback loop will output a 1 when the delayed signal steps from 0 to 1, and this output will then be scaled by setting the Gain of the Gain block to 2.

When subtracted from the input signal, this gain will ensure that the output from the Sum block can be pulled below -0.5 no matter what the positive input is (0 or 1), thus ensuring that the Switch off point of the Relay block is reached and its output is turned off when the delayed signal has a rising edge (i.e. after tDur has elapsed). One result of this is that any additional rising edges occurring in the model input after the first rising edge and during the time tDur will be ignored. Once the output from the model returns to 0, the next rising edge on the model input will trigger another pulse.