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AC square wave from electrode

Posted 17 déc. 2020, 12:19 UTC−5 Low-Frequency Electromagnetics, General Version 5.3a 1 Reply

Ashley Cantrell
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Hi,

I am modeling a forearm with skin, muscle, fat, and nerves. I have a 9mA current delivered from an electrode with a frequency of 100Hz and pulse width of 100μs. I am using a time dependent study from 0 to 1 seconds with a time step of 0.1s. I used a global equation equal to 2pift and when I try to compute, it says I have a problem with initial values. How do I fix this problem? Should I be defining my pulse width anywhere other than global parameters?

Thank you

1 Reply Last Post 17 déc. 2020, 13:37 UTC−5
Robert Koslover Certified Consultant
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Posted: 4 years ago 17 déc. 2020, 13:37 UTC−5
Updated: 4 years ago 17 déc. 2020, 13:39 UTC−5

First, excuse me for not fully understanding your field. You mentioned a square wave. But you also mentioned "a global equation equal to 2pift." That notation would seem to imply some kind of sinusoidal wave, with an angular frequency omega = 2pif. I have no idea why that would be relevant to a square wave model. You also refer to a "frequency" of 100 Hz and pulse width of 100μs. A sinusoidal wave (which is what one would normally be talking about, if using "Hz" as a unit) of 100 Hz has a period of .01 second, which is much longer than 100μs (so one could never fit a 100 Hz sinusoidal wave into a 100μs pulse). So, I'm guessing that by "100 Hz" you actually meant to refer to a pulse repetition rate. That is, you have a train of square pulses that are each 100μs wide, at 100 pps (pps = pulses per sec). Right? Ok, let's assume that. Next, I see you are using a time-step of 0.1 sec. If so, that is potentially a big problem. After all, you can't possibly resolve (in time) any detailed physics that occurs during a 100μs pulse if using a 0.1s timestep! In general, time steps have to be short compared to the time scales of the time-dependent behaviors in your model. I suggest you post your model to the forum so that others may review it and offer more detailed assistance.

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Scientific Applications & Research Associates (SARA) Inc.
www.comsol.com/partners-consultants/certified-consultants/sara
First, excuse me for not fully understanding your field. You mentioned a square wave. But you also mentioned "a global equation equal to 2pift." That notation would seem to imply some kind of sinusoidal wave, with an angular frequency omega = 2pif. I have no idea why that would be relevant to a square wave model. You also refer to a "frequency" of 100 Hz and pulse width of 100μs. A sinusoidal wave (which is what one would normally be talking about, if using "Hz" as a unit) of 100 Hz has a period of .01 second, which is much longer than 100μs (so one could never fit a 100 Hz sinusoidal wave into a 100μs pulse). So, I'm guessing that by "100 Hz" you actually meant to refer to a *pulse repetition rate*. That is, you have a train of square pulses that are each 100μs wide, at 100 pps (pps = pulses per sec). Right? Ok, let's assume that. Next, I see you are using a time-step of 0.1 sec. If so, that is potentially a big problem. After all, you can't possibly resolve (in time) any detailed physics that occurs during a 100μs pulse if using a 0.1s timestep! In general, time steps have to be short compared to the time scales of the time-dependent behaviors in your model. I suggest you post your model to the forum so that others may review it and offer more detailed assistance.

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